SDI: Technology, Survivability, and Software

May 1988

NTIS order #PB88-236245
Recommended Citation:

U.S. Congress, Office of Technology Assessment, SDI: Technology, Survivability,

and Software, OTA-ISC-353 (Washington, DC: U.S. Government Printing Office,

May 1988).

Library of Congress Catalog Card Number 87-619857

For sale by the Superintendent of Documents

U.S. Government Printing Office, Washington, DC 20402-9325

(order form can be found in the back of this report)
Foreword

In its 1985 report, New Ballistic Missile Defense Technologies, OTA attempted

to place those technologies against a useful policy background for the Congress.

While that report introduced the major subject areas of Strategic Defense Initiative research, the amount of detailed technical evaluation it could offer was limited.

The chief limitations were the relative newness of the SDI program and the lack

of specific BMD system architectures to examine. Since that report, the SDIO

has conducted enough additional research and, in particular, identified a sufficiently specific system architecture that a more detailed OTA review of the relevant technologies should be helpful to Congress.

Public Law 99-190 (continuing appropriations for fiscal year 1986) called for

the Office of Technology Assessment to conduct a”. . . comprehensive classified

study . . . together with an unclassified version . . . to determine the technological feasibility and implications, and the ability to survive and function despite

a preemptive attack by an aggressor possessing comparable technology, of the

Strategic Defense Initiative Program. ” In addition, the accompanying Conference Report specified that . . . “This study shall include an analysis of the feasibility of meeting SDI computer software requirements. ”

This unclassified report completes OTA’s response to that mandate. It puts

SDI technologies in context by reporting the kinds of ballistic missile defense

(BMD) system architectures that the SDI organization has considered for “phased

deployment. ” It reviews the status of the various SDI technologies and system

components. It analyzes the feasibility of producing dependable software of the

complexity that advanced BMD systems would require. Finally, it summarizes

what is now known—and unknown—about the probable survivability of such systems against concerted enemy attacks of various kinds.

The study found that major uncertainties remain concerning the probable cost,

effectiveness, and survivability of the kinds of BMD system (which rely on kinetic

rather than directed-energy weapons) that might be deployable in the “phase-one’

proposed for the mid to late 1990s. In addition, OTA believes several more years

of SDI research would be needed to determine whether it is feasible to construct

the kinds of directed-energy weapons contemplated as follow-ons to SDIO’s “phase

one” BMD system. The survivability of both short-term and longer-term BMD

systems would depend heavily on the outcome of a continuing competition in weapons and countermeasures between the United States and the Soviet Union. Finally, developing dependable software for advanced BMD will be a formidable

challenge because of the difficulty of testing that software realistically.

OTA gratefully thanks the hundreds of individuals whose contributions of

time and effort helped make this report possible. OTA, of course, bears the final

responsibility for the contents of the report.

Director

Ill
Advisory Panel on SDI: Technology, Survivability, and Software

H. Guyford Stever, Chairman

Foreign Secretary, National Academy of Engineering

Robert Clem

Director of Systems Sciences

Sandia National Laboratories

Malcolm Currie

Executive Vice President

Hughes Aircraft Company

Gerald P. Dinneen

Corporate Vice President for Science &

Technology

Honeywell, Inc.

Peter Franken

Professor

Optical Sciences Center

University of Arizona

John Gardner

Vice President for Engineering &

Operations

McDonnell Douglas Astronautics Co.

Richard L. Garwin

IBM fellow

IBM T. J. Watson Research Center

O’Dean Judd1

Chief Scientist for Defense Research &

Applications

Los Alamos National Laboratory

Michael M. May

Associate Director at Large

Lawrence Livermore National Laboratory

Stephen Meyer

Associate Professor

Center for International Studies

Massachusetts Institute of Technology

David Parnas

Department of Computing and Information

Science

Queens University

Charles Seitz

Professor

Computer Sciences

California Institute of Technology

John Shore

Director

Washington Research Lab

Entropic Processing, Inc.

Jeremiah D. Sullivan

Professor

Department of Physics

University of Illinois

Samuel Tennant

Vice President

The Aerospace Corporation

Victor Vyssotsky

Director

Cambridge Research Laboratory

Digital Equipment Corporation

Gerold Yonas

Vice President

The Titan Corporation

Charles A. Zraket

President

The MITRE Corporation

Invited Observer:

Chief Scientist

Strategic Defense Initiative Organization

‘Currently, Chief Scientist, Strategic Defense Initiative Organization.

NOTE: OTA appreciates and is grateful for the valuable assistance and thoughtful critiques provided by the Advisory

Panel members. The views expressed in this OTA report, however, are the sole responsibility of the Office

of Technology Assessment. Participation on the Advisory Panel does not imply endorsement of the report.

iv
OTA Project Staff—SDI: Technology, Survivability, and Software

Lionel S. Johns, Assistant Director, OTA

Energy, Materials, and International Security Division

Peter Sharfman, International Security and Commerce

Thomas H. Karas, Project Director

Program Manager

Anthony Fainberg

C. E. “Sandy” Thomas

David Weiss

Administrative Staff

Jannie Coles Cecile Parker Jackie

Acknowledgments

Robinson

The following organizations generously made their personnel available to provide OTA

with information and ideas

Aerospace Corporation

American Physical Society,

AT&T Bell Laboratories

Avco Systems/Textron

Boeing

Booz-Allen & Hamilton

Ford Aerospace and Communications

General Research Corporation

Hughes Aircraft Company

Lawrence Livermore National Laboratory

Lockheed Missiles & Space Company

Los Alamos National Laboratory

LTV

Martin-Marietta

McDonnell-Douglas

M.I.T. Lincoln Laboratory

MITRE Corporation

Nichols Research

Rockwell International

SAIC

Sandia National Laboratory

Sparta

Strategic Defense Initiative Organization

System Planning Corporation

TRW

University of Texas, Center for Electromechanical Engineering

U.S. Air Force Electronic Systems

Division

U.S. Air Force Space Division

U.S. Air Force Weapons Laboratory

U.S. Army Strategic Defense Command

U.S. Naval Research Laboratory

U.S. Naval Sea Systems Command

W. J. Schafer & Associates

Westinghouse Marine Division
Workshop on Soviet Response to SDI, January 1987

Sidney Graybeal, Chairman Robert Nurick

Vice President, System Planning Co-Director RAND-UCLA, Soviet Studies —

Corporation

Alex Gliksman, Convener/Rapporteur

Consultant

Arthur Alexander

The RAND Corporation

Mark M. Lowenthal

Director, INR/SFA

US Department of State

Arthur F. Manfredi, Jr.

Assistant National Intelligence Officer

Central Intelligence Agency

John A. Martens

ITA/Office of Foreign Availability

U.S. Department of Commerce

Stephen Meyer

Professor

Center for International Studies

Massachusetts Institute of Technology

Workshop on SDI Software, January

Larry Druffel, Chairman

Director, Software Engineering Institute

Carnegie-Mellon University

Speakers

William Ainsley

Logicon, Inc.

Mack Alford

Senior Software Scientist

General Electric Corporation

Karl Dahlke

Technical Staff

AT&T Bell Laboratories

Discussants

Bruce Arden

Dean of Engineering & Applied Science

University of Rochester

Richard Kemmerer

Department of Computer Science

University of California, Santa Barbara

Center

The RAND Corporation

Sayre Stevens

President, System Planning Corporation

Keith Taggart1

Assistant Director/Countermeasures

Strategic Defense Initiative Organization

U.S. Department of Defense

Robert D. Turnacliff

Principal Director, Threat Analysis Office

The Aerospace Corporation

Vann H. Van Diepen

INR/SFA

U.S. Department of State

‘Currently with SAIC, Inc.

1987

Butler Lampson

Corporate Consulting Engineer

Digital Equipment Corporation

Brian Reid

Consulting Engineer

Digital Equipment Corporation

Panelists

David Parnas

Professor

Department of Computing and Information

Science

Queens University

John Shore

Director, Washington Research Lab

Entropic Processing, Inc.

Victor Vyssotsky

Director, Cambridge Research Laboratory

Digital Equipment Corporation

vi
Contents

Chapter Page

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Designing a Ballistic Missile Defense (BMD) System:

Architecture and Trade-Off Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Status and Prospects of Ballistic Missile Defense Technologies,

Part I: Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Status and Prospects of Ballistic Missile Defense Technologies,

Part II: Weapons, Power, Communication, and Space Transportation .. ...105

System Development, Deployment and Suport . .......................159

System Integration and Battle Management . .........................179

Computing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........199

Software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............221

Nondestructive Countermeasures Against Ballistic Missile Defense. .. ....251

Defense Suppression and System Survivability . .......................253

Defense Suppression Initiative . . . . . . . . . . . . . . . . . . . . . .................255

Appendix A: Technology for Producing Dependable Software . . . . . . .........259

Appendix B: Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................270

vii
Preface

This report is the unclassified version of a classified document delivered to

Congress at the end of August 1987. In attempting to reach agreement with the

Department of Defense on what information could be included in an unclassified

report, OTA found the wheels of bureaucracy to turn very slowly—when they turned

at all. Only through the active intervention of the Strategic Defense Initiative

Organization, beginning in late in November 1987, and extending to the end of

March, 1988, was a partial resolution of the problem achieved.

OTA, with assistance from SDIO staff, revised the entire report to produce

a complete version that both agreed should not be considered classified. The Department of Defense concurred on all but the final three chapters. These latter

chapters deal-in a general way and without the kind of specific detail that might

be useful to an adversary-with a variety of potential countermeasures to BMD

systems. In particular, chapters 11 and 12 deal with defining and countering threats

to the survivability of space-based BMD systems.

Chapter 1 offers a brief review of the “bottom lines” of chapters 10 through

12. But apparently some in the Defense Department wish to assert that it is impossible to present an unclassified analytical discussion that would enable the

reader to understand the issues and form his own judgments. In OTA’s judgment,

this position does not deprive potential adversaries of any information they do

not already have: rather, it stifles rational public debate in the United States over

the pros and cons of proceeding with ballistic missile defense. To give the reader

at least some appreciation of the scope of the deleted material, the tables of contents of chapters 10 through 12 appear at the end of this volume. In addition,

the major conclusions of these chapters (without, of course, the supporting analysis) are summarized in chapter 1.

OTA thanks the SDIO for the additional substantive comments and information it provided on the final drafts of the report. Thus, despite the many months

of delay since original completion of the report, this unclassified version is reasonably up to date. OTA, not SD IO, is responsible for the contents and conclusions

of the report.

A further note on the subject of classified information is in order. Any report

which attempts to analyze the feasibility and survivability of prospective ballistic missile defense systems must refer to possible measures an adversary could

take to counter the system. OTA sought the views of a variety of experts on Soviet military research, development, and deployment about potential responses

to the SDI. It also sought to understand the technical feasibility of various countermeasures. It did not seek out or report on the official judgments of the U.S.

intelligence community on what countermeasures the Soviet Union would or could

take against SDI-derived systems. Therefore, nothing said in this report should

be construed as an “intelligence” judgment of Soviet intentions or capabilities.

viii
Chapter 1

Summary
CONTENTS

Principal Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Origin of This Study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Goals of the SDI.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nature of This Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

First-Phase Technologies and Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Goals of First-Phase System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Technical Feasibility of Sensors and Weapons . . . . . . . . . . . . . . . . . . . . . . . . . .

Software Feasibility . . . . . . . . . . . . . . . . . .

Survivability of a First-Phase System . . .

Second-Phase Technologies and Systems. . .

Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Technical Feasibility . . . . . . . . . . . . . . . . . .

Software Feasibility . . . . . . . . . . . . . . . . .

Phase-Two Survivability. . . . . . . . . . . . . . .

Third-Phase Technologies and Systems . . . .

Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Technical Feasibility . . . . . . . . . . . . . . . . . .

Software Feasibility . . . . . . . . . . . . . . . . . .

Phase-Three Survivability . . . . . . . . . . . . .

Important General Issues . . . . . . . . . . . . . . .

costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Timing and Evolution. . . . . . . . . . . . . . . . .

Competition in anti-Satellite Weapons . . .

B o x

Box

l-A. Adaptive Preferential Defense . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Figures

Figure No.

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.,..., . . . . . . . . . . . . . . . . . . .

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Page

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1-2.

1-3.

1-4.

1-5.

l-1. The Path to “Thoroughly Reliable” Defenses

Development Decision Content . . . . . . . . . . . .

Mission Effectiveness Improves With Phased Deployment . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

Page

OTA Understanding of Pojected Roles of BMD Deployment Phases . . . . .

SDIO Proposal for Development and Deployment . . . . . . . . . . . . . . . . . . . . .

6

7

7

12

25

Table No. Page

l-1. SDIO’s Phase One Space- and Ground-Based BMD Architecture . . . . . . . . 9

l-2. OTA’s Projections of Evolution of Ground-and Space-Based BMD

Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Chapter 1

Summary

PRINCIPAL FINDINGS

The Strategic Defense Initiative Organization (SDIO) currently advocates planning for

a three-part “phased deployment” of ballistic

missile defense (BMD) systems, with each

phase providing an increment of strategic benefits while preparing the way for the next

phase. The first phase would be intended to “

. . . compel Soviet operational adjustments

and compromises by reducing the confidence

of Soviet planners in predicting the outcome

of a ballistic missile attack. ” The second phase

would be intended to negate Soviet abilities

to destroy many strategic targets, and the

third to “eliminate the threat posed by nuclear

ballistic missiles.” The exact composition and

timing of each phase are still under study, but

some tentative system “architectures” have

undergone preliminary analysis.

Finding 1: After 30 years of BMD research,

including the first few years of the Strategic

Defense Initiative (SDI), defense scientists and

engineers have produced impressive technical

achievements, but questions remain about the

feasibility of meeting the goals of the SDI. The

SDIO has identified most of the gaps between

today’s technology and that needed for highly

effective ballistic missile defenses; it has initiated programs to address those gaps. It

should surprise no one that many technical issues remain unresolved, especially when one

considers that the SD I has so far had time and

authorization to spend only a fraction of the

money that the Fletcher Commission estimated would be necessary to assess BMD feasibility. The SDIO argues that application of

sufficient resources will resolve the outstanding issues.

Finding 2: Given optimistic assumptions

(e.g., extraordinarily fast rates of research, development, and production), the kind of firstNote: Complete definitions of acronyms and initialisms

are listed in Appendix B of this report.

phase system that SDIO is considering might

be technically deployable in the 1995-2000

period. Such a system might include:

● space-based hit-to-kill vehicles for attacking missile boosters and post-boost vehicles (PBVs) and

● ground-based rockets for attacking warheads before reentry into the atmosphere.

Depending on whether U.S. deployment

schedules could be met, the effectiveness of

countermeasures that should be available to

the Soviets in that period, the numbers of

offensive weapons they had deployed, and the

nature of the attack, such a system might destroy anywhere from a few up to a modest fraction of attacking Soviet intercontinental ballistic missile (ICBM) warheads.

Again depending on the effectiveness of Soviet countermeasures, the BMD system might

be able to carry out a strategy of “adaptive

preferential defense,” allowing it to protect

successfully a useful fraction of certain sets

of U.S. military targets.1

Additional defense capabilities would soon

be needed to sustain this level of defense

against either increased or more advanced, but

clearly feasible, Soviet offenses.

One key to sustaining and improving defense

capabilities in the 2000-10 period would be development of technologies to discriminate between missile warheads and decoys so that

ground- and satellite-based rockets could effectively attack warheads in space. Assuring

functional survivability of space-based systems would also be essential (see Finding 4).

1SDIO officials argue that denial to the Soviets of high confidence of destroying as many of these targets they would like

(as estimated by U.S. planners) would enhance deterrence of

an aggressive nuclear attack.

3
4

As the Soviets phased in faster burning, faster

weapon-dispensing ballistic missiles, it would

probably be necessary to develop and deploy

directed-energy weapons to intercept missiles

in the boost phase and post-boost phases.

Given higher annual funding levels than so

far appropriated, the SDI research and technology program might establish in the midto-late 1990s whether the components needed

for warhead/decoy discrimination in a secondphase system would be feasible for deployment

in the 2000-10 period. Also assuming higher

funding levels than in the past, by the mid-tolate 1990s the SDI may determine the technical feasibility of deploying BMD directedenergy weapons in the 2005-15 period. The cost

and survivability of such weapons will be

among the key issues.

Finding 3: A rational commitment to a “phaseone” development and deployment of BMD

before the second and third phases had been

proven feasible, affordable, and survivable

would imply: a) belief that the outstanding

technical issues will be favorably resolved

later; b) willingness to settle for interim BMD

capabilities that would decline as Soviet offenses improved; or, c) belief that U.S. efforts

will persuade the Soviets to join in reducing

offensive forces and moving toward a defensedominated world.

Finding 4: The precise degree of BMD system survivability is hard to anticipate, because

it would depend on the details of measures for

offensive attack on the BMD system and defensive countermeasures, on the tactics employed

by each side, and on the inevitable uncertainties of battle. It appears that direct-ascent

nuclear anti-satellite weapons (DANASATs)

would pose a significant threat to all three defense system phases, but particularly to the

first two. Numerous DANASATs could be

available to the Soviets in the mid-1990s (e.g.,

ballistic missiles relying on mature technology,

could probably be adapted to this role.) Such

weapons deployed in quantity, especially with

multiple decoys, would threaten to degrade severely the performance of a first- or secondphase BMD system. SDIO officials say, however, that adequate survivability measures

could meet this threat. If the Soviets chose to

attack the U.S. BMD satellites during emplacement, they might prevent full system deployment and operation altogether.

Finding 5: There has been little analysis of

any kind of space-based threats to BMD system survivability. SDIO analyses assume that

U.S. BMD technologies will remain superior

to Soviet technologies (although such superiority would not necessarily guarantee U.S.

BMD system survivability). In particular,

SDIO and its contractors have conducted no

serious study of the situation in which the

United States and the Soviet Union both occupy space with comparable BMD systems.

Such a situation could place a high premium

on striking first at the other side’s defenses.

The technical (as well as political) feasibility

of an arms control agreement to avoid such

mutual vulnerability remains uncertain.

Finding 6: The survivability of BMD systems now under consideration implies unilateral U.S. control of certain sectors of space.

Such control would be necessary to enforce

“keep-out” zones against Soviet anti-satellite

weapons or space mines during and after U.S.

BMD deployment. Most BMD weapon technologies would be useful in an anti-satellite role

before they reached the levels of power and precision needed for BMD. Thus, the Soviets

would not need to achieve BMD capabilities

to begin to challenge U.S. control of, or even

access to, space.

Finding 7: The nature of software and experience with large, complex software systems

indicate that there may always be irresolvable questions about how dependable BMD software would be and about the confidence the

United States could place in dependability estimates. Existing large software systems, such

as the long-distance telephone system, have

become highly dependable only after extensive

operational use and modification. In OTA's

judgment, there would be a significant probability (i.e., one large enough to take seriously)

that the first (and presumably only) time the

BMD system were used in a real war, it would
5

suffer a catastrophic failure.1 The complexity

of BMD software, the changing nature of system requirements, and the novelty of the technology to be controlled raise the possibility

that the system may not even be able to pass

the more realistic of the peacetime tests that

could be devised for it. The relatively slow rate

of improvement in software engineering technology makes it appear unlikely to OTA that

this situation will be substantially alleviated

in the foreseeable future. SDIO officials assert,

however, that SDI software problems will be

manageable, that adequate testing will be possible, and that previous military systems have

been deployed without complete system testing (e.g., the Minuteman missile system, the

Navy’s AEGIS ship defense system.)

Finding 8: No adequate models for the development, production, test, and maintenance

of software for full-scale BMD systems exist.

Systems such as long-distance telephone networks, early missile defense systems such as

SAFEGUARD, the AEGIS ship defense system, and air traffic control all differ significantly from full-scale BMD.

The only kind of BMD system for which the

United States has software development experi1

In ch. 9 catastrophic failure is arbitrarily defined as a decline of 90 percent or more in system performance, and there

is a discussion of alternative approaches to the concept.

ence is a terminal defense system. Incorporating a boost-phase defense would add complexity to the software and require the inclusion

of technologies hitherto untried in battle. Adding a mid-course defense would probably increase the software complexity beyond that

of any existing systems.

Experts agree that new methods for producing and safely testing the system would be

needed. Evolution would be key to system development, requiring new methods of controlling and disseminating software changes and

assuring that each change would not increase

the potential for catastrophic failure. OTA has

found little evidence of significant progress in

these areas.

Finding 9: There is broad agreement in the

technical community that significant parts of

the research being carried out under the SDI

are in the national interest. There is disagreement about whether or not this research is best

carried out within a program that is strongly

oriented toward supporting an early 1990s

BMD deployment decision, and that includes

system development as well as research elements. This question was outside the scope of

OTA’s mandate and is not addressed in this

report.

INTRODUCTION

Origin of This Study

The appropriations continuing resolution for

fiscal year 1986 (Public Law 99-190) called for

the Office of Technology Assessment to produce a “comprehensive classified study . . .

together with an unclassified version. . . to determine the technological feasibility and implications, and the ability to survive and function despite a preemptive attack by an aggressor

possessing comparable technology, of the Strategic Defense Initiative Program. ” In addition,

the conference report accompanying this legislation specified that “this study shall include

an analysis of the feasibility of meeting SDI

computer software requirements. ” This report

responds to that legislation.

After 30 years of BMD research, including

the first few years of the Strategic Defense Initiative, the dedication and ingenuity of thousands of U.S. scientists and engineers have

produced many impressive technical achievements. Such achievements may someday cumulate to form the basis for a highly effective

BMD system. For now, however, many questions remain about the feasibility of meeting

SDI goals.

Goals of the SDI

According to SDIO’s annual report to

Congress:

From the very beginning, the SDIO has

maintained the same goal-to conduct a vig-
6

orous research and technology development

program that could help to eliminate the

threat of ballistic missiles and provide increased U.S. and allied security. Within this

goal, the SDIO's task is to demonstrate SDI

technology and to provide the widest range

of defense options possible to support a decision on whether to develop and deploy strategic defenses.’

Such defenses might, to a greater or lesser degree, protect the American population from nuclear weapons. But, contrary to the perceptions

of many, SDIO has never embraced the goal

of developing a leakproof shield against an unconstrained Soviet nuclear weapon threat. It

is the position of SDIO that President Reagan has not embraced that goal either.3

Rather, the organization, in its first 4 years,

worked out a scenario that it argues could lead

to President Reagan’s stated “ultimate goal

of eliminating the threat posed by strategic

nuclear missiles . . . [which could] . . . pave the

way for arms control measures to eliminate the

weapons themselves.”4 The scenario, paraphrased from the SDIO report, is as follows:

1.

2.

3.

a research and development program continues until the early 1990s, when a decision could be made by a future President

and Congress on whether to enter into fullscale BMD engineering development;

the Defense Department begins full-scale

development of a “first-phase” system

while continuing advanced technology

work;

the United States begins “phased deployment” of defensive systems, “designed so

that each added increment of defense

would enhance deterrence and reduce the

risk of nuclear war”; although this “transition period” would preferably be jointly

managed by the United States and the Soviet Union, U.S. deployments would proceed anyway; then

‘Strategic Defense Initiative Organization, Report to the Congress on the Stzategicllefense Im”thtive (Washington, DC: April

1987), p. 11-13.

‘Lt. General James Abraharnson, personal communication to

OTA staff, July 7, 1987.

4Ronald Reagan, televised speech, Mar. 23, 1983.

4. the United States completes deployment

of “highly effective, multilayered defensive systems, ” which ‘could enhance significantly the prospects for negotiated

reductions, or even the elimination, of

offensive ballistic missiles. ”

Figures 1-1 and 1-2 are SDIO graphic representations of its development and deployment

policies. Figure 1-1 illustrates that, as time

goes on, newer, more capable BMD systems

would be necessary to respond to advanced

Soviet missile threats. Alternatively, it is argued, the prospect of such new systems might

persuade the Soviets to accept U.S. proposals

for joint reductions of offensive forces which

might, in turn, obviate the need for new systems.

Figure 1-2 lists the kinds of information

SDIO seeks to provide for BMD development

decisions. According to this figure, SDIO does

not see “complete understanding” of later system phases as prerequisite to initial commitments to develop and deploy BMD. Instead,

it proposes to seek a “partial understanding”

of the issues surrounding the follow-on phase

and provide “reasonable estimates” that the

necessary systems could be available as needed.

SDIO has affirmed the so-called “Nitze criteria” as requirements for the BMD options

it offers: that the defenses be militarily effective, adequately survivable, and “cost-effective” at the margin, that is, “able to maintain

their defensive capabilities more easily than

countermeasures could be taken to try to defeat them.’”

5

SDIOop. cit., footnote 2, p. IV-3,

Figure 1-1.—The Path to

“Thoroughly Reliable” Defenses

Time

SOURCE: Department of Defense, Strategic Defense Initiative.
7

Now

Figure 1-2.—Development Decision Content

estimates

● Major Political Decision

● First of Several

SOURCE’ Department of Defense, Strategic Defense Initiative.

The SDIO has identified three “phases” of

BMD deployments that might extend from the

mid-1990s well into the 21st century (see figure 1-3). In mid-1987, SDIO proposed to proceed with a series of “technology validation

experiments” to build and test hardware that

might demonstrate the feasibility of components of a “first-phase” system. These experiments would require SDI budgets substantially above the levels appropriated by

Congress in the first 4 years of the SDI.

In deciding about funding and directing the

SDI program, then, Congress must decide

whether to accept, modify, or reject the phased

research and deployment scenario proposed by

SDIO. Options for Congress include:

● accept the SDIO phasing scenario and

plan now to decide in the early 1990s

whether the full-scale engineering development of a first-phase system is feasible

or attractive, but with only a “reasonable

estimate” at that time of whether the second and third phases would later prove

feasible; such a decision would imply an

Figure 1.3.—Mission Effectiveness Improves

With Phased Deployment -

Phase 3

Mutually

assured

survival

Time

SOURCE: Department of Defense, Strategic Defense Initiative

intention to deploy the first phase in the

●

●

●

mid-1990s while beginning fill-scale development of the second phase, but the

actual mid-1990s decisions would depend

on the progress made;

decide soon to begin immediately to develop whatever technologies may be available for deployment in the early 1990s,

bearing in mind that space-based weapons are, in any case, unlikely to be deployable in quantity until 1995 or beyond;

plan to delay a decision on a first phase

of development and deployment until advanced research confirms that the second

and third phases would be feasible;

return to the pre-SDI BMD research program intended to hedge against technological surprise and to deter Soviet BMD

deployment, but not intended to work

toward a specific deployment scenario; or

add to the previous option a new emphasis on terminal defense systems designed

specifically to protect elements of U.S.

strategic nuclear retaliatory forces.

Nature of This Report

To assist Congress in making these choices,

this report surveys the technologies under research in the SD I and reports, as of early 1988:

● which technologies might be available for

each of the projected deployment phases;

• what is known and what remains to be

learned about the feasibility of develop-
8

ing those technologies and manufacturing and deploying weapons based on them;

• what can now be said about how survivable against enemy attack space-based

BMD systems themselves may be; and

● what can now be said about the feasibility of producing the computer software

of the requisite performance and dependability.

Most experts would agree that the technical issues for BMD present severe challenges.

Thus, in attempting to provide the above information, this report identifies numerous

demanding technical problems. The technical

challenges to the SDI have been variously interpreted:

●

●

●

From the point of view of SDI officials

and contractors, questions of feasibility

are challenges that the application of sufficient time and resources can overcome.

They are working on most, if not all, the

issues identified in this report.

In another view, the obstacles to effective

BMD are great, and may not be overcome

for several decades; nevertheless, the kind

of research SDIO is sponsoring will have

some long-term military and economic

benefits for the United States whatever

the SDI outcome. In addition research on

BMD is necessary to avoid technological

surprise and to hedge against Soviet

breakout from the Anti-Ballistic Missile

(ABM) Treaty.

From a third point of view, the obstacles

to accomplishment of the SDI’s ultimate

goals are so complex and so great that

SDIO’s goals are simply implausible.

Therefore, although the United States

should conduct some BMD research to

avoid technological surprise and to hedge

against Soviet break out from the ABM

Treaty, research needed for other military

or civilian purposes should be carried out

under other auspices.

OTA attempts in this report to present realistically the available evidence about SDI feasibility. The reader must decide how optimistic or pessimistic the evidence should lead one

to be and which approach to BMD research

would be best for the nation.

This summary organizes OTA’s findings

around the kinds of system designs, or “architectures,” for the three phases that SDIO

has recently been studying and discussing. It

should be recognized, however, that, except for

the first phase, these architectures are illustrative, not definitive. They provide a means

of thinking about and understanding how various BMD technologies might be integrated

into working systems and in what timeframes.

Only the first represents SDIO’s proposal for

actual systems to develop and deploy.

Table 1-1 outlines SDIO’s suggested first

phase of deployment; the timeframe 1995-2000

is strictly an OTA assessment of a very optimistic but arguably plausible period for the

beginning and completion of deployments of

the various elements of the system phase. Table 1-2 outlines OTA’s projections of the second and third phases of BMD deployment,

based on SDIO descriptions of the technologies

it is researching. The overlapping timeframes

(2000-10 and 2005-15) reflect-OTAassessments

of very optimistic but arguably plausible

periods for the beginning and completion of

deployments of the various elements of each

system phase.

FIRST-PHASE TECHNOLOGIES AND SYSTEMS

(OTA Estimates Approximately 1995-2000)

Goals of a First-Phase System systems that the Nation might select in the

late 1980s for initial deployments in the early

In the fall of 1986 SDIO and its contractors 1990s. OTA estimates that as a practical matbegan to study options for “first-phase” de- ter—given the development, manufacturing,

ployment of BMD. They attempted to design and space transportation needs—deployment
9

Table 1-1.—SDIO’s Phase One Space. and Ground-Based BMD Architecture

Component Number Description Function

First phase (approximately 1995-2000):

Battle Management Variable

Computers

Boost Phase Several at high altitude

Surveillance and

Tracking Satellite

Space-based Interceptor 100s at several 100s of

Carrier Satellite km altitudes

Probe 10s

or

Space Surveillance and 10s

Tracking System

or

Space-based Interceptor

Carrier Satellites loos

Exe-atmospheric 1000s on ground-based

Interceptors (ERIS) rockets

May be carried on sensor

platforms, weapon platforms,

or separate platforms; groundbased units may be mobile

Infrared sensors

Each would carry about 10 small

chemical rockets or “SBIs”;

might carry sensors for

tracking post-boost vehicles

Ground-launched rocket-borne

infrared sensors

Satellite-borne infrared sensors

Satellite-borne infrared sensors

Rocket booster, hit-to-kill

warhead with infrared seeker

Coordinate track data; control

defense assets; select

strategy; select targets;

command firing of weapons

Detect ballistic or ASAT missile

launches by observing hot

rocket plumes; pass

information to tracking

satellites

On command, launch rockets at

anti-satellite weapons

(attacking BMD system),

boosters, possibly PBVs.

Acquire RV tracks, pass on to

ERIS interceptors

Cued by satellite-borne or

rocket-borne infrared sensors,

home in on and collide with

RVs in late mid-course

SOURCE: Off Ice of Technology Assessment, 1988

of the systems discussed could not begin until 1995 or later and would probably take at

least until the end of the 1990s to complete.

The first-phase options generally exclude

space-based attack on Soviet reentry vehicles

in mid-course (see table l-l). While limiting the

effectiveness of a BMD system, this omission

eases the sensing, discrimination, and battle

management tasks.

Depending on the nature of the Soviet attack assumed, and depending on the effectiveness of Soviet countermeasures, the kind of

system described by SDIO officials system

might destroy anywhere from a few up to a

modest fraction of the (now predicted number

of) Soviet reentry vehicles in a full-scale attack.

The SDIO has suggested such a system as only

the first phase of what in the longer term would

expand to a more effective system. However,

the organization cites as “an intermediate military purpose”

. . . denying the predictability of Soviet attack outcome and. . . imposing on the Soviets

significant costs to restore their attack confidence. These first phases could severely restrict Soviet attack timing by denying them

cross-targeting flexibility, imposing launchwindow constraints, and confounding weapon-to-target assignments, particularly of their

hard-target kill capable weapons. Such results could substantially enhance the deterrence of Soviet aggression.6

SDIO officials assert that the military effectiveness of the first-phase system would be

higher than indicated by the percentages of

reentry vehicles intercepted. They envisage a

strategy of “adaptive preferential defense. ” In

this strategy, first the space-based layer of defense disrupts the structure of the Soviet attack. Then the ground-based layer defends only

those U.S. targets of the highest value and un-

‘Ibid., footnote 2, p. 11-11.
10

Table 1-2.—OTA’s Projections of Evolution of Ground- and Space-Based BMD Architecture

Component Number Description Function

Second phase (approximately 20004010) replace first-phase components and add:

Airborne Optical

. . . .

System (AOS)

10s in fright Infrared sensors

Ground-based Radars

High Endo-atmospheric

Interceptors

Space Surveillance and

Tracking Satellite

(SSTS)

Space-based interceptor

Carrier

Space-based Neutral

Particle Beam (NPB)

Detector Satellites

10s on mobile platforms

1000s

50-100 at few 1000s of

km.

1000s at 100s of km

altitudes

10s to 100s at aititude

simiiar to SSTS

100s around particle

beam altitudes

X-band imaging radar

Rocket with infrared seeker, nom

nuclear warhead

High-resolution sensors; laser

range-finder and/or imaging

radar for finer tracking of

objects;

May carry battle management

computers

Each carries about 10 small

chemical rockets or “KKVS”;

at low altitude; lighter and

faster than in phase one

Atomic particle accelerator

(perturber component of

interactive discrimination;

additional sensor satellites

may be needed)

Sensors to measure neutrons or

gamma rays from objects

bombarded by NPB; -

transmitters send data to

SSTS and/or battle

management computers

Third phase (approximately 2005-2115), replace second-phase components and add:

Ground-based Lasers, 10s of ground-based Several laser beams from each

Space-based Mirrors lasers; 10s of relay of several ground sites bounce

mirrors; 10s to 100s off relay mirrors at high

of battle mirrors altitude, directed to targets by

battle mirrors at lower

altitudes

Track RVS and decoys, pass

information to ground battle

management computers for

launch of ground-based

interceptors

Cued by AOS, track RVS as they

enter atmosphere; discriminate

from decoys, pass information

to ground battle managers

Collide with RVS inside

atmosphere, but before RV

nuclear detonation could

cause ground damage

Track launched boosters, postboost vehicles, and ground or

space-launched ASATs;

Track RVs and decoys,

discriminate RVs from decoys;

Command firing of weapons

On command, launch rockets at

anti-satellite weapons

(attacking BMD system),

boosters, PBVs, and RVs

Fire hydrogen atoms at RVs and

decoys to stimulate emission

of neutrons or gamma rays as

discriminator

Measure neutrons or gamma

rays emitted from RVs: heavier

objects emit measurable

neutrons or gamma rays,

permitting discrimination from

decoys

Attack boosters and PBVs

SOURCE: Office of Technology Assessment, 1988.

der attack by the fewest reentry vehicles remaining after the winnowing by the space-based

layer (see box l-A). In this way, a meaningful

fraction of a large set of “point targets” (e.g.,

missile silos or command posts) might be protected. Such a strategy, however, would require

successful discrimination of RVs and decoys

by the first-phase system sensors–a technology that remains to be proven. In addition, the

Soviets could counter the strategy if they could

modify their current offensive systems and deploy substantial numbers of maneuvering reentry vehicles.

Figure 1-3 presents SDIO’s description of

how the phases of SDI deployment might satisfy a spectrum of strategic goals. In evaluating the desirability of the goal of enhancing
11

Box l-A.—Adaptive Preferential Defense

The SDIO has proposed that a first-phase ballistic missile defense system (see table l-l) employ a tactic

of “adaptive preferential defense. ” If successfully executed, this tactic could give an outnumbered defense

some leverage against a large attack.

“Preferential defense” means defending only a selected set of high-value targets out of a larger number

of targets under attack, thus concentrating the defensive forces. In essence, some targets would be sacrificed

to increase the chances of survival of others.

“Adaptive preferential defense” means deciding during the course of the battle which targets to defend

by adapting to the distribution of the attacking RVs (missile warheads) that survive earlier layers of defense.

Of the high-value targets under attack, those with the fewest RVs coming at them are defended first.

Two Layers of Defense

A first-phase Strategic Defense System (SDS) would include orbiting interceptors and land-based interceptors. The orbiting interceptors would first destroy a small fraction of the rising Soviet missile boosters and

post-boost vehicles. Since the SDS could not at this stage predict the targets of the Soviet missiles, the defense

would not be preferential: instead, it would merely subtract at random some warheads from the Soviet attack.

Even if the Soviets had initially aimed the same number of RVs at each target, some would have been filtereds

out by the first layer of defense.

Land-based rockets would carry other interceptors into space to destroy RVs that survived the space-based

attack. Tracking sensors would determine the targets of the RVs to within several kilometers. Battle management computers would determine which high-value targets were under attack by only one RV and launch groundbased interceptors against them first, until all were covered, Then the computers would determine which targets were under attack by two RVs and assign interceptors to them, and so on. In this way, few interceptors

would be wasted defending targets that would later be destroyed anyway by additional, unintercepted RVs.

A Simple Example

Suppose, for example, that 2000 RVs were attacking 1000 targets, with 1 RV aimed at each of 500 targets

and 3 RVs aimed at each of another 500 targets. Assume that the defense had only 1000 interceptors (each

with a 100 percent chance of interception). If the defense assigned interceptors randomly to 1000 of the 2000

attacking RVs, about 312 targets would be expected to survive (50 percent of those under single-RV attack

and 12.5 percent of those under 3-RV attack). But if it assigned 500 interceptors to defend the targets under

a single-RV attack, and then assigned 3 interceptors each to-defend the nextmight be saved.

The SDI Case

Analysts for SDIO have concluded that a first-phase system applying

fraction of selected U.S. targets against the kind of attack the Soviets are

in the mid-1990s.

Some Qualifying Considerations

166 targets, a total of 666 targets

this tactic could protect a useful

predicted to be able to carry out

If feasible, an adaptive preferential defense would be suitable mainly for protecting fractions of redundant,

single-aimpoint targets, such as missile silos, command posts, or other isolated military installations. Largearea, soft targets (such as cities or large military installations), would present so many potential aimpoints

that defending, say, a third or a half of the aimpoints in a given area would be unlikely to assure survival

of the that area. In addition, the aimpoints that could be defended would be small enough that the blast and

fires from exploding nuclear weapons would affect neighboring “soft” target areas.

Serious questions also remain about whether SDIO’s proposed phase-one BMD system could, in fact, successfully execute a strategy of adaptive preferential defense. In particular, if the infrared sensors of the tracking system could not discriminate between Soviet RVs and decoys, many of the ground-launched interceptors

would be wasted on decoys. And if the Soviets could deploy many maneuvering reentry vehicles during the

operational period of the first-phase defense system, the targets could not be accurately predicted and defended.
12

deterrence by forcing modification of Soviet

attack plans, Congress should also be aware

of the counter-arguments to that position:

●

●

●

●

Many believe that, given the awesome

consequences of nuclear war for the Soviet Union as well as for the United States,

deterrence does not require enhancement

because the U.S. threat of nuclear retaliation is already strong enough and can be

kept so with timely strategic offensive

modernization.

Soviet military planners already face operational uncertainties, such as the unreliability of some percentage of deployed

missiles.

Other, less costly, more clearly feasible,

methods of complicating Soviet attack

plans, such as increased mobility for U.S.

strategic forces, may be available.

A corresponding Soviet deployment of

BMD would impose uncertainties and

costs on U.S. retaliatory attack plans.

The context for evaluating the goal of complicating Soviet attack plans changes, however, if one accepts the point of view that it

is only the first benefit on a long-term path

toward “mutual assured survival. ” In OTA's

view, figure 1-4 illustrates, somewhat more

realistically than figure 1-1, the relative levels

of defense capability over time to be expected

from phased BMD deployments, assuming

their feasibility. Whether or not initial capabilities could be sustained or improved upon

depends on information not likely to be available by the early 1990s.

Figure 1-4.—OTA Understanding of Projected Roles

of BMD Deployment Phases

Tim e

SOURCE’ Office of Technology Assessment, 1988

Technical Feasibility of

Sensors and Weapons

In a first-phase system, space-based interceptors (SBI), also known as “hit-to-kill” or

“kinetic kill” vehicles, would attack missile

boosters and post-boost vehicles (PBVs), but

not their dispensed reentry vehicles (RVs). The

only mid-course interception would be near the

end of that phase of missile trajectory by

ground-based, exe-atmospheric interceptors.

Boost-Phase Surveillance and Tracking System

(BSTS)

It appears feasible to develop by the mid

1990s high altitude satellites that would tell

lower altitude satellites, or possibly SBIs

themselves, where to look for rising missile

boosters. Complex communications links among

the satellites may be necessary to avoid enemy

interference.

Carrier vehicles (“garages”) for space-based

hit-to-kill interceptors could receive data from

the BSTS and track the boosters and postboost vehicles with their own infrared sensors

and laser range-finders.

Space-Based Interceptors (SBI)

A few hundred SBI carriers that would carry

a few thousand kill vehicles (rocket interceptors) might destroy a modest fraction of Soviet missile warheads in the boost and postboost phases. Such a system might be feasible to deploy starting in the projected firstphase period, but questions of engineering and

cost remain unresolved. For example, considerable miniaturization of components for propulsion, guidance, and sensors would be needed

to make a rocket fast enough to reach boosting missiles and light enough to be affordably

launched into space. Recent progress toward

such miniaturization appears promising. Substantial testing of prototype weapons would

be necessary to show system feasibility. Once

these technologies were proven, the affordable mass production of rocket-carrier vehicle

systems for space deployment maintenance

would remain a major challenge.
13

Exe-atmospheric Reentry Interceptor System

(ERIS)

The Homing Overlay Experiment of 1984

and subsequent development work suggest

that it is feasible to design a ground-launched

interceptor capable of homing in on objects in

space under favorable conditions. Such weapons could make up an Exe-atmospheric Reentry Interceptor System, or ERIS. More research, testing, and engineering remain to be

done before the United States will know if the

interceptor homing warheads can be produced

cheaply enough to be affordable in large numbers. The ERIS, however, is likely to be deployable before space-based BMD interceptors.

Under study are both space-based and groundlaunched infrared sensor systems and groundbased radars to direct ERIS interceptors to

the vicinity of their targets. Both the satellite

and ground-based systems remain to be developed, tested, and affordably produced. Upgraded versions of now existing ground-based

radars might also provide initial tracking information to the interceptors.

In this first-phase architecture, the ERIS

would rely on radars or on passive infrared

detection and tracking of potential targets.

Whether or not these sensors could adequately

discriminate between decoys and RVs disguised as decoys remains to be demonstrated.

Without such discrimination, decoys could

probably cause serious problems for this late

mid-course layer of defense. Developing a decoy

system like this is within Soviet capabilities.

Even with good discrimination by external sensors, the homing sensor on the interceptor itself would need to find the genuine RV if it

were traveling within tens of meters of other,

closely spaced objects. In general, many scientists and engineers working on the SDI have

agreed that such countermeasures may well

be feasible for the Soviets in the near term.

However, both within and outside SDIO there

is some dissent on the potential type, quality,

number, and deployment times of Soviet countermeasures.

There is widespread agreement that much

more experimentation is needed on missile

“penetration aids” such as decoys. Very little

SDI money has gone to the design, construction, and testing of penetration aids, although

a full understanding of their potential and limitations would be key to developing and evaluating the effectiveness of a BMD system.

Besides decoys, ERIS interceptors could

face many other false targets, particularly

those generated by debris from PBV activity,

from intercepts made earlier in the boost phase

by the SBIs, or from deliberate Soviet countermeasures. Warm objects in the field of view

of the ERIS interceptor’s sensors might distract it from its target RV, even if it had originally been correctly pointed toward the RV by

a probe or Space Surveillance and Tracking

System (SSTS) sensor.

Software Feasibility

In the first-phase system designs now under consideration for SDI, hundreds of satellites would have to operate automatically and,

at the same time, coordinate their actions with

those of other satellites. The battle management system would have to track hundreds of

thousands of objects and decide when and how

to attack thousands of targets with little or

no human intervention.

Among the most challenging software tasks

for such a first-phase system would be designing programs for the largely autonomous operation of hundreds of satellites. But even for

ground-based components of the system, the

number of objects, the volume of space, and

the brevity of time would preclude most human participation in battle management. Humans would decide at what alert status and

state of activation to place the system. Once

the battle began, computers would decide

which weapons to use when, and against what

targets.

A first-phase system would have the advantage of a simpler battle management problem

than that of more advanced BMD systems. In

particular, the space-based segment of the system would not attempt to track and discriminate among hundreds of thousands of mid-
14

course objects, or to assign weapons to any

of them. The distribution of SBI carrier vehicles would be so sparse that the targets within

its range would not be in the range of neighboring carrier vehicles. It could, for the most

part, safely shoot at a target within its own

range without the risk that some other vehicle had shot at the same target. Some coordination among carrier vehicles would still be

necessary because the continual relative motion of carriers and targets would leave some

ambiguities about which targets were most

appropriate for each carrier to fire interceptors

at.

Although a first-phase system would have

simpler tasks than a later system, its software

would still be extremely complex. The nature

of software and experience with large, complex

software systems, including weapon systems,

together indicate that there would always be

irresolvable questions about how dependable

BMD software was, and also about the confidence we could place in dependability estimates. Existing large, complex software systems, such as the U.S. long-distance telephone

system, have become highly dependable only

after extensive operational use and modification.

Extrapolating from past experience with

software, it appears to OTA that the complexity of BMD, the uncertainty and changeability of the requirements it must meet, and the

novelty of the technology it must control would

impose a significant probability of softwareinduced catastrophic failure in the system’s

first real battle. The issue for SDI is the degree of confidence in the system that simulations and partial testing could provide. SDIO

officials argue that such tests will permit adequate confidence and that this issue is no more

serious for the SDI than for all advanced military systems developed to date.

Computer simulations would play a key role

in all phases of a BMD system’s life cycle. Battle simulations on a scale needed to represent

realistically a full battle have not yet been attempted. Whether or not sufficiently realistic

simulations can be created is a hotly debated

question. In particular, it is difficult for OTA

to see how real-world data could be gathered

to validate simulations of the phenomena that

must be accounted for, such as multiple enemy

missile launches, nuclear explosion-induced

backgrounds, and enemy choices of countermeasures. The differences between BMD software and previous complex software that is

considered dependable suggests to some experts that BMD software might never be able

to pass even its peacetime tests. It should also

be noted, however, that both the United States

and the Soviet Union now base deterrence on

an offensive nuclear delivery system that has

never been operationally tested either.

While the United States could not be certain that a BMD system would work as intended, the Soviets could not be certain that

it would not.7 If they had at least some reason

to believe the U.S. BMD system might be effective, they might be more deterred from attacking than before. On the other hand, the

United States would not want to base a major

change in its nuclear strategy on a BMD system in which it had little confidence. In the

case of a first-phase system, whose effect on

the strategic balance would be small anyway,

the risk of software-induced system failure

might seem acceptable.

The SDIO sees software problems as challenges to be overcome rather than as insurmountable obstacles to effective BMD. It is

supporting some software research intended

to address the challenges. Others argue that

the limitations of software engineering technology and its relatively slow rate of improvement make it unlikely that dependable BMD

software could be produced in the foreseeable

future. Thus far, no new software engineering

developments have appeared to contradict the

latter view.

Survivability of a First-Phase System

The survivability of any BMD system will

not be an all-or-nothing quality. The question

7Unless they had high confidence in the potential effectiveness of a secretly deployed countermeasure (perhaps a software

bug planted by a saboteur programmer).
15

will be whether enough of a system’s assets

would survive for it to carry out its mission.

The issue would then turn on whether the defense could make attacking the BMD system

too costly for the offense, or whether the offense could make defending the BMD system

to costly for the defense. (On the other hand,

if the United States and the Soviet Union

agreed to coordinate offensive weapon reductions and defensive deployments, they might

do much to ameliorate BMD survivability

problems.)

To protect satellites, the defense might employ combinations of such techniques as evasive maneuver, tracking denial, mechanical

shielding, radiation hardening, electronic and

optical countermeasures, and shoot-back. Categorical statements that these techniques will

or will not make any BMD system adequately

and affordable survivable are not credible.

Judgments on specific cases would depend on

the details of entire offensive and defensive systems and estimates of the techniques and tactics that the opponent would employ.

Space Mines

A space mine is a satellite that would trail

another satellite and explode lethally either on

command or when itself attacked. Space mines

may or may not prove a viable threat to spacebased BMD systems. Although nuclear space

mines would be a very stressing threat, much

more analysis would be needed to clarify the

question of the viability of space mines. After

repeated attempts to locate such analysis

within the SDIO or among its contractors,

OTA concludes that it has not yet been adequately performed.

Anti-Satellite Weapons (ASATs)

There is widespread agreement among experts on Soviet military practices that the initial Soviet response to U.S. BMD deployments

would not be to try to develop and deploy systems based on similar technology. They would

instead attempt a variety of less sophisticated

countermeasures. These might include extensions of their current co-orbital, pellet-warhead

anti-satellite weapon (A SAT), or else agroundlaunched nuclear-armed ASAT (or “DANASAT,”

for “Direct Ascent Nuclear Anti-satellite”

weapon).

The susceptibility of a BMD satellite system to degradation by DANASAT attack

would depend on many complex factors, including:

● the maneuvering and decoying capabilities and the structural hardness of the

BMD satellites;

● the precision and reaction time of Soviet

space surveillance satellites; and

● the speed, numbers, decoying capabilities,

and warhead power of the DANASATs.

Depending on target hardness, the radius of

lethality of a nuclear warhead could be so great

that the ASATs might need only inertial guidance (they need not home in on or be externally

guided to the BMD asset). Thus they would

not be susceptible to electronic countermeasures against homing sensors or command guidance systems. It appears that, at practical

levels, maneuvering or radiation shielding of

low-altitude satellites would not suffice against

plausible numbers of rapidly ascending nuclear

ASATs.

There appears to be no technical reason why

the Soviets, by the mid-1990s, could not deploy DANASATs with multiple decoys among

the nuclear warheads. Multiple decoys would

likely exhaust the ability of the defenders to

shoot back at the attack—unless extremely

rapid discrimination of decoys and warheads

were possible. It would be difficult to deny

tracking of or to decoy near-earth satellites,

especially large sensor platforms, if they were

subjected to long periods of surveillance. If deployed while the satellites were under attack,

satellite decoys would frequently not have time

to lure DANASATs far enough away from the

real targets.

If several SSTS satellites were a key element

of a first-phase BMD system, they would be

the most vulnerable elements. Otherwise, the

most vulnerable elements of a first-phase BMD

system would be the carrier vehicle satellites

for the interceptors. The carrier vehicles, or

CVs, as well as sensor satellites (BSTS and
16

SSTS) might employ combinations of various

defense mechanisms against the ASAT threat.

The SDIO argues that such combinations of

measures potentially offer a high degree of survivability to space-based BMD system components.

For the near-term, however, no prototypes

exist for carrier vehicles with these characteristics; the issue for SDI is whether in the 1990s

such satellites could be developed, produced,

and deployed. The Soviets, on the other hand,

have already demonstrated the ability to field

DANASATs by deploying rapidly accelerating, nuclear-armed anti-ballistic missiles near

Moscow over 15 years ago and recently upgrading that system. Newer ballistic missiles,

relying on mature technology, might also be

adapted to this purpose. More advanced

DANASATs appear feasible for the Soviets

by the mid-1990s.

DANASATs would be a stressing threat

against first-phase BMD systems and could

probably degrade severely the performance of

such systems. The SDIO argues, however, that

strong survivability measures in the defensive

system could successfully counter this threat.

The Soviets might also consider gradual attrition of the system in “peacetime.” They

might use co-orbital, non-nuclear ASATs or

ground-based laser ASAT weapons to take

“potshots” at the carrier vehicles.

Attack During Deployment

Should the Soviets deem U.S. space-based

BMD deployments to be sufficiently threatening to their national security, they might resort to attack before the system was fully deployed. Whether they waited for full deployment

or not, in the first-phase architecture SBI carrier vehicles would be so sparse that they would

probably have only limited abilities to help defend one another, although each might to some

extent defend itself. Other survivability measures, however, might offer some protection.

Attacks on Ground-Launched Systems

Insofar as the ERIS ground-launched interceptor relied on fixed, ground-based early warning radars for launch-commit information, its

effectiveness could be greatly reduced by nuclear or jamming attacks on those radars.

Use of Comparable Technologies

Responses to threats from comparable Soviet weapon systems have not been defined by

the SDIO or its contractors. Indeed, a working assumption of SDIO research and analysis has been that the United States could and

would maintain a consistent lead over the Soviet Union in BMD technologies for the indefinite future. Because the Soviets lag in some

of the technologies required for a space-based

BMD system, it seems unlikely that they

would attempt to deploy SBIS for BMD in the

1990s. A more attractive option for them might

be to deploy kinetic-kill vehicles as a defense

suppression system rather than as a BMD

system—a less difficult task.

They could then choose orbital configurations designed to give their weapons temporary

local numerical advantages over the U.S. BMD

system. In a shoot-out between the systems,

at a time of their choosing, the Soviets might

then eliminate or exhaust those SBI carrier vehicles within range of a Soviet ICBM launch

salvo. Effective non-nuclear ASATs would,

however, require good space surveillance capabilities. If a BMD system were to cohabit

space with a competent defense suppression

system (possibly embodying a lower technical

capability), the side that struck first might

eliminate the other.

The fact that a lower level of technology

would be needed for defense suppression than

for BMD could drive a race to control access

to space as soon as possible. For example, U.S.

space-based ASATs might be needed to prevent Soviet ASAT deployments that could in

turn interfere with U.S. BMD deployments.
SECOND-PHASE TECHNOLOGIES AND SYSTEMS

(OTA Estimates Approximately 2000-10)

Goals

The goal of a phase-two system would be to

“enhance deterrence, ” first by imposing uncertainty on Soviet strategic attack plans, then

by denying the Soviets the ability to destroy

‘‘militarily significant portions of important

sets of targets (such as missile silos or command and control nodes) in the United States.

As a result, the Soviets would retain the ability to inflict massive damage on the U.S. economy and population, but would lack the ability

to accomplish certain precise military objectives. At least, such denial should decrease

whatever incentives may now exist for the

Soviets to commit nuclear aggression (though

analysts disagree on whether such incentives

do now exist); at best, the Soviets might be

induced to negotiate away their militarily obsolescent missiles.

If the Soviets believed they could restore

their compromised military capabilities at an

acceptable price, they might attempt to do so

by adding new offensive weapons and by attempting both active and passive countermeasures against the U.S. BMD system. Even if

they did not believe they could recapture lost

military capabilities, but only believed that

they were in danger of losing any credible nuclear retaliatory power against the United

States, they might still attempt to employ

BMD countermeasures. If, however, they concluded that countermeasures would be futile,

they might, as conjectured in the “SDI scenario,” agree to mutual offensive arms reductions as a way of containing the U.S. threat.

In that case, BMD combined with effective air

defenses might offer much higher levels of protection of military and even civilian targets.

Currently available BMD technology for

nuclear-armed, ground-based interceptors would

probably allow the United States to build a

system that could deny the Soviets confidence

in destroying substantial fractions of certain

sets of hardened or mobile targets.8 An SDI

“phase-one,” non-nuclear system may also be

able to provide such protection. This is more

likely to be the case if the defense could be configured to defend subsets of targets preferentially, and in such a way that the Soviets could

not detect which targets were defended more

heavily. Moreover, if the Soviets continued to

aim weapons at highly defended targets, they

would have fewer weapons left over to aim at

softer military and civilian targets.

There is less evidence that the United States

could deny the Soviets the ability to strike with

high confidence at many other kinds of militarily valuable, but more vulnerable, targets.

There are, however, many ideas and some

promising technologies for pursuing this goal.

Achieving the strategic goals of this kind

of system implies air defenses of comparable

potential. Otherwise, except for the most urgent targets, the Soviets could shift strategic

missions from ballistic to cruise missiles.

Technical Feasibility

Airborne Optical System (AOS)

An airborne infrared sensor system would

tell ground-based radars where to look for reentering objects. Such a system appears technically feasible during the 1990s. The infrared

sensors, however, might be subject to confusion by high-altitude light-scattering ice crystals created as debris reentered the atmosphere, or by nuclear detonations intended to

blind the system.

Ground-Based Radar (GBR)

Imaging radar systems would observe lighter

decoys slowing down more quickly than gen-

‘See U.S. Congress, Office of Technology Assessment, Balh%tic Missile Defense 7’echno]o@”es, OTA-ISC-254 (Washington, DC: U.S. Government Printing Office, September 1985),

pp. 33-34.
18

uine RVs.Computers using this information

would launch very high acceleration rockets

(HEDI) with infrared homing sensors toward

the RVs Tests to date indicate that such radars are feasible, but unresolved questions include their susceptibility to interference from

nuclear burst, to jamming by radio-frequency

jammers on incoming warheads, to signalprocessing overloads created by many simultaneously reentering objects, and to deception

by carefully designed RV’s and decoys.

High Endo-atmospheric Interceptor (HEDI)

A rocket-borne high endo-atmospheric defense interceptor would attack incoming RVs

after they had begun to reenter the atmosphere.

Because the rising interceptor’s friction with

the atmosphere would cause it to heat up, a

cooled crystal window would have to protect

its homing sensor. Experiments suggest that

such windows are feasible, although researchers have not yet established whether they could

be rapidly mass-produced.

Because the HEDI would have a limited “divert” capability, the sensor system would need

to give it a very accurate target track. A relatively short-range ground-based radar, using

the upper atmosphere as a discriminant against

decoys, might be the easiest way to provide

such a track. This tracking method, however,

would restrict each interceptor to protecting

a relatively small area. Intensive coverage of

all U.S. territory would demand too many thousands of missiles. Instead, the HEDI mission

would be to “mop up” small numbers of warheads leaking through the earlier defensive

layers. Thus the most useful mission for HEDI

might be to protect specific, localized targets,

such as ICBM silos.

SDIO officials point out, however, that passive infrared sensors or long-range radars may

be able to discriminate between RVs and decoys in space. Then the High Endo-Atmospheric

Interceptor could be committed earlier and

thus defend a much larger area. Nevertheless,

in order to avoid the impression of providing

a defense designed primarily to protect hardened strategic targets, rather than U.S. territory in general, the SDIO elected to omit the

HEDI and its associated sensors (AOS and a

terminal imaging radar or TIR) from its

proposals for a first-phase BMD system.9 Technically, however, initial deployments in the late

1990s period appear plausible.

SSTS and RV/Decoy Discrimination

A phase-two system would add to the firstphase architecture dozens of space-based sensors that could accurately track thousands of

RVs and decoys from the moment of their deployment from the PBVs. Such sensors would

require electro-optical focal planes of unprecedented size, or high-resolution laser radar

systems, and considerable signal processing

ability.

It seems likely that, by the time a substantial U.S. BMD system could be in place, the

Soviets could deploy many reentry vehicle decoys and RVs disguised as decoys. Unless

these RVs and decoys could be destroyed on

their boosters and post-boost vehicles, some

means of distinguishing between them would

have to be developed. Otherwise, the defense’s

ammunition would be quickly exhausted.

In the terminal, “endo-atmospheric” phase

of interception, the atmosphere might filter out

all but the heaviest and most sophisticated decoys. But too many reentering objects might

overwhelm local defensive sensors and weapons. In sum, effective discrimination in the

mid-course of ballistic missile trajectories

would be necessary to a highly effective BMD

system.

One proposed technique for RV/decoy discrimination is a laser radar system that might

observe the movements of RVs and decoys as,

or after, they were dispensed from PBVs. Subtle differences in the behaviors of the less massive decoys might give them away. Concealing deployments off PBVs or other tactics

might counter this technique, but much research both on decoy technologies and spaceborne laser radars will be needed to judge the

potential of either.

‘Lt. General James Abrahamson, personal communication to

OTA staff, July 7, 1987.
19

Various methods of passive and active discrimination have been suggested, including

multiple wavelength infrared sensors, laser radar, and microwave radar. But if the Soviets

could build sufficiently sophisticated decoys,

differentiating decoys and RVs might be impossible without some means of externally perturbing all the objects being tracked and observing differences in how they react to such

perturbations. This technique is known as “interactive discrimination. ”

So far there is no proven candidate system

for the task of interactive discrimination. The

program receiving the most funding has been

the neutral particle beam (NPB). In this concept, a space-based atomic accelerator would

fire high-energy neutral hydrogen or deuterium

atoms at suspect objects. A sensor would then

detect the neutrons or gamma rays emitted

from heavier objects struck by the hydrogen

atoms. A hundred or more NPB platforms, and

perhaps several hundred sensor satellites,

would be needed for a complete system. It may

be more appropriate to consider such a system for a phase-three, rather than phase-two,

BMD architecture.

A space test of a subscale NPB platform was

scheduled for the early 1990s, although recent

budget cutbacks have made the experiment’s

status unclear. Key issues determining the feasibility of NPB systems will include cost, the

rapid and precise ability to point the beams

at thousands of objects in a few tens of minutes, and the ability to gather and correlate

the return information.

Other interactive discrimination ideas include, for example, space-based high energy

lasers that would “tap” target objects. The

greater recoil of lightweight decoys would give

them away.

Kinetic Energy Weapons

Missile boosters that completed their boost

phase in about 120 to 140 seconds—slightly

faster than current modern ICBMs–would

greatly reduce the effectiveness of rocketpropelled SBIs in the boost phase. They could

still intercept post-boost vehicles. However,

fast RV dispensing technologies could reduce

kill in the post-boost phase. On the other hand,

if such countermeasures had forced the Soviets

to greatly reduce missile payloads, mid-course

discrimination might become easier: then the

Soviets could only afford to deploy fewer, less

sophisticated decoys. Improved SBIs, even

though ineffective against boosters, could be

useful in the mid-course. They would require

long-wave infrared sensors for homing in on

small, cold RVs. Alternatively, laser designators on sensor satellites might illuminate RVs

with light that SBI sensors could see and track.

It seems likely that by roughly the period

projected for the first phase ERIS (Exe-atmospheric Reentry Interceptor System) missiles

could be refined to the specifications now envisioned. Provided that the challenge of RVdecoy discrimination had been overcome, they

would begin to provide an important layer of

missile defense. If the discrimination problem

could not be solved, ERIS interceptors would

be of doubtful utility. If it could be solved,

ERIS effectiveness in phase two would be

much greater than in phase one.

The question for HEDI in the phase-two

period is whether the Soviets could deploy

many maneuvering reentry vehicles to evade

the system and sophisticated reentry decoys

to deceive it. The more effective the earlier

defensive layers might be, the less the Soviets

could afford to use precious missile payload

weights on heavier RVs and decoys. However,

numerous, even slightly, maneuvering reentry

vehicles, especially with depressed missile

trajectories, could probably evade HEDIs unless the interceptors were equipped with nuclear warheads.

Software Feasibility

A phase-two BMD system such as envisaged

here would need to account for hundreds of

thousands (or more) of objects as they were

dispensed into space. It would require a highly

complex communications net for keeping track

of all BMD space assets, boosters, PBVs, RVs,

decoys, and space debris, then assigning weapons to intercept the selected targets. Concepts,
20

but so far no genuine designs, exist for “partitioning” the battle space into local networks

of sensors and weapons (taking into account

that different combinations of satellites would

be constantly shifting in and out of given regions of space).

In terms of sheer computing power, continued advances seem likely to provide the

processing capacities needed for advanced

BMD. The most difficult hardware engineering task will be to combine the qualities of high

capacity and radiation hardness in space-qualified electronics.

A BMD designed for boost, post-boost, midcourse, and terminal battle is likely to be the

most complex system ever constructed. In

OTA’s judgment, there would be no precedents

for estimating the likelihood of the BMD software system’s working dependably the first

time it was used in a real battle. Moreover, no

adequate models for the development, production, test, and maintenance of software on the

scale needed currently exist. The system’s complexity, coupled with the need to automate the

use of technologies previously unused in battle, might result in unforeseen problems dominating the software life cycle. For example,

large, complex systems that undergo continuous change sometimes reach states where new

changes introduce errors at a greater rate than

they remove errors.

A BMD system–as has been the case with

other strategic nuclear systems—could be

tested only with computer simulations and

some piecemeal hardware exercises. Furthermore, no existing systems must operate autonomously (without human intervention) in

the face of deliberate enemy attempts to destroy them.

Whether the risks of catastrophic BMD failure resulting from the inevitable software errors in a system of this magnitude would be

unacceptable is a policy decision, not a technical one, that the President and the Congress

would ultimately have to make. They would

have to weigh those risks against the perceived

risks and benefits of not building a BMD system but deploying national resources elsewhere. As with a first-phase system, another

consideration would be the likelihood that the

Soviets could not be confident that the BMD

system would not work as advertised, and that

they might be deterred from trying to find out

by attacking. (On the other hand, if the Soviets

found away to break into and tamper with the

software system without U.S. knowledge, they

might be confident that they could defeat it.)

Phase-Two Survivability

More advanced BMD systems would be designed and deployed with more advanced selfprotection or survivability measures. Groundlaunched, nuclear-armed ASATs (DANASATs)

would continue to be a threat. The additional

SBI carriers available after the year 2000, however, could begin to provide mutual defense

for one another, which would not be possible

in the first-phase architecture.

By that time, on the other hand, the Soviets

could develop more advanced anti-satellite

weapons and space surveillance sensor systems. Most BMD weapon technologies for use

in space or against targets in space are likely

to achieve ASAT capabilities before they become applicable to BMD missions.

Direct-Ascent Nuclear ASATs

As with phase one, DANASATs would be

particularly threatening to a “phase-two” system. The U.S. Space Surveillance and Tracking System and any associated interactive discrimination platforms would now be primary

targets for Soviet defense suppression attacks.

Since many of these satellites would be at

higher altitudes than the SBI garages, they

would have more time to maneuver away from

attackers. But they would also be heavier and

therefore more fuel-costly to maneuver. They

would be more difficult to shield against nuclear radiation.

Space Mines

The United States would have to consider

the possibility of Soviet attempts to co-orbit

nuclear or non-nuclear space mines with these
21

platforms as they were being deployed. Such

“mining” might be carried concurrently with

the deployment of the BMD system assets.

System designers have proposed “keep-out”

zones to keep potential attacking weapons outside their lethal ranges. Whether the United

States (or any power) could achieve this kind

of dominance of near-earth space remains to

be seen. In any case, very little analysis has

as yet been carried out by the SDIO or its contractors on interim and long-term space-based

threats to BMD systems.

Comparable Technologies

If the Soviets could develop technologies

comparable to those of the United States, three

might be of special concern. One would be advanced space-based surveillance systems permitting better-timed, more accurate ASAT attacks. Second would be the development of

space-based neutral particle beam weapons,

which could be very effective anti-satellite

weapons from great range. Third, even though

laser weapons might not have achieved the

power levels necessary for the BMD missions,

laser ASATs could begin to pose substantial

threats to U.S. space assets. If only for selfdefense, the United States might have to consider deploying directed-energy ASATs in the

phase-two architecture period.

THIRD-PHASE TECHNOLOGIES AND SYSTEMS

(OTA Estimates Approximately 2005-15)

Goals

In the SDI scenario, the first goal of a phasethree BMD system would be to sustain the capabilities of the second-phase system as more

advanced Soviet countermeasures came on

line. Eventually, the system might achieve still

higher levels of protection. As originally presented by the Administration, the SD I was to

identify a path to the “assured survival” of

the U.S. population against nuclear attack. An

intermediate step on this path would be to design a BMD system that would make nuclear

ballistic missiles “impotent and obsolete.” In

this scenario, the Soviets would then be confronted with the choice of negotiating away

obsolescent missiles or engaging in a costly

defensive-offensive arms race that would

sooner or later leave their offensive missiles

unable to penetrate U.S. or allied territory. Either way, in the end few or no nuclear ballistic

missiles could reach U.S. territory.10

As with a second-phase system, extremely

effective air defenses would be an essential

complement to an extremely effective BMD

system. And, as with earlier phases, deep reductions in offensive forces (by arms control

agreement) could increase the effectiveness of

the system.

Technical Feasibility

Directed-Energy Weapons

Directed-energy weapons for boost-phase interception are still far in the future. It is unlikely that confidence in their feasibility could

be established by the early 1990s even with

requested SDIO budgets. OTA judges that experimental evidence of the feasibility of BMD

directed-energy weapons (DEW) is at least a

decade away.11 It is extremely unlikely that

confidence in DE W could be established in the

next several years, given continuation of the

actual appropriation pattern.

‘

OSDIO reports to Congress make no mention of “assured survival, ” and cite as the ultimate objective of the SDI to “secure

a defense-dominated strategic environment in which the U.S.

and its allies can deny to any aggressor the military utility of

ballistic missile attack.” SDIO, op. cit., footnote2,p.II-11. Other

SDIO documents, however, do still refer to the goal of “mutually assured survival” (see figure 1-3).

1

“A similar conclusion was reached by a committee of the

American Physical Society in 1987. Science and Technology of

Directed Energy Weapons: Report of the American Physical

Society Study Group (April 1987), p. 2.
22

Ultimately however, directed-energy weapons may be necessary to intercept long-range

ballistic missiles and direct-ascent ASAT

weapons in the boost and post-boost phases.

If the Soviets could, over 15 or 20 years, develop and begin to deploy very fast-burn, laserhardened boosters with single (or few) warheads (and associated decoys) and if they deployed those boosters at concentrated launch

sites, the burden even on directed-energy weapons would be great. In that case, the time available for attacking each booster might be so

short as to drive very high the requirements

for power levels, retargeting speed, and numbers of directed-energy weapons. (However,

PBVs would continue to be vulnerable to

DEWs.)

Fast-burning Soviet boosters appear technically plausible—the main issue would be cost.

The Soviets would have to deploy enough of

these boosters to continue to deliver hundreds

of thousands of RV decoys into the mid-course,

and they would have to be aware that, for example, if U.S. DEWs achieved significant improvements in retargeting time, they might

neutralize a good fraction of the Soviets’ expensive fast-burning fleet.

Although some work has continued on chemical lasers, and proposed future budgets would

increase the share going to them, most SDI

laser funding in 1987 went to the free electron

laser (FEL). The most likely way to deploy such

lasers would be on the ground, with orbiting

relay and battle mirrors to focus laser beams

on Soviet boosters and PBVs. Scientists have

made significant progress in FEL research, but

they are a long way from having established

the feasibility of a weapon. The SDIO has

sponsored construction of laboratory versions

of FELs and plans a major test facility at

White Sands Missile Range. Among the outstanding issues to be studied with these experimental lasers are whether FELs can be

made bright enough at useful wavelengths and

the feasibility of optical techniques for successfully passing very high energy laser beams out

of and back into the atmosphere. Other outstanding issues include: whether large, agile

beam directing optics can be affordably manufactured and reliably based in space; the cost

of building and maintaining several large laser ground station complexes; and the survivability of space mirrors and ground stations

against defense suppression attacks.

Other directed-energy concepts are under

consideration. Neutral particle beams (NPBs),

which do not penetrate the atmosphere, might

engage those missile boosters and PBVs that

operated above about 120 kilometers. Advanced booster and warhead dispensing technologies, however, might evade NPBs. (Unlike

most lasers, however, NPBs could penetrate

and destroy reentry vehicles in the mid-course.)

Another directed-energy weapon may be the

nuclear-explosion pumped x-ray laser, which

also could not penetrate far into the atmosphere. For various reasons, the x-ray laser

appears more promising as an anti-satellite

weapon than as an anti-missile weapon.

Software Feasibility

If an interactive discrimination system were

added in the phase-two architecture, the phasethree architecture would not pose significantly

different software challenges and prospects

from the second phase. The very fine pointing

and tracking needed for laser weapons could

impose significant additional computing requirements on sensors.

As time went on, Soviet defense suppression

threats-weapons aimed at the BMD system

itself-could grow more intense. The additional

burdens of self-defense for the BMD system

against advanced ASAT threats would add to

the complexity of software requirements. The

challenges to producing dependable software

cited above would persist in phase three.

Phase-Three Survivability

If large directed-energy weapon platforms

were deployed in space (whether these were laser generators with beam directors or only relay and battle mirrors for ground-based lasers),

they would themselves become prime highvalue targets for defense suppression attacks.

Unless they were powerful enough to be de-
23

ployed at rather high altitudes, they would

have a difficult time either denying tracking

to enemy sensors or maneuvering out of the

way of attacks. They would probably have to

defend themselves (and one another) as well

as depend on “escort” interceptors. Thirdphase directed-energy weapons systems could

be survivable against the current or first-phase

Soviet DANASAT threat; the question is,

would they be survivable against a later

DANASAT threat that might be in place by

the time the directed-energy weapons were deployed?

Directed-Energy ASATs

Long before directed-energy weapons such

as lasers or particle beams achieve the capabilities they would need as BMD weapons, they

could be effective anti-satellite weapons. Antisatellite laser weapons, if placed in space before more capable BMD laser weapons, might

successfully attack the latter as they were being deployed.

In some cases, such as the nuclear bombpumped x-ray laser, the most likely application of an advanced directed-energy weapon

would be as an ASAT. What little analysis has

been done so far indicates that x-ray laser

ASATs launched from the ground to fire from

the upper atmosphere would be difficult, if not

impossible, to counter. However, the feasibility of x-ray laser weapons remains to be demonstrated.

Soviet Possession of Comparable Technologies

As one attempts to project various combinations of survivability techniques and various modes of anti-satellite attack into the far

term, the situation becomes even hazier. It

does appear that two DEW ballistic missile

defense systems occupying space could pose

risks of crisis instability. The side that struck

first in a simultaneous attack on all the other’s

DEWS might seize an advantage. Much would

depend on each side’s tactics and its ability

to jam, spoof, or disable the sensors on the

other side. At best, each side might neutralize

the other’s BMD system, leaving both defenseless but with nuclear retaliatory capabilities

(as is the case today). At worst, the side striking first might unilaterally neutralize the

other’s BMD (and other military space assets),

leaving him open to nuclear blackmail. Mutual

fears of this possibility might lead to crisis instability.

On the other hand, if the two sides could define precisely balanced deployments and rules

for ensuring the mutual survivability of their

systems, and then arrive at verifiable arms

control agreements providing for them, they

might avoid such instability.

IMPORTANT GENERAL ISSUES

cost s

Some experts in space systems argue that

the major cost driver of space-based BMD

would be the manufacture of hundreds or thousands of novel, yet highly reliable, spacecraft.

The SDIO suggests that its research into new

production techniques would result in substantially reduced costs. Until such techniques

have actually been demonstrated in practice,

this suggestion will be difficult to verify.

In any case, space transportation cost would

be a major challenge. The SDIO has spoken

of ultimately requiring launch operating costs

one-tenth those existing today (not counting

the costs for development of such a system).

For the nearer term (late 1990s) the goal appears to be a threefold operating cost reduction. For the very near term, planners are being told to design systems that could evolve

into less costly ones, but there is little expectation of immediate first-phase savings.

Components today are conceptual, so reliable cost estimates are not possible. Efforts to

improve “producibility” and operations costs

for SBIs, ERIS, and HEDI are also conceptual.
24

System architects’ estimates put the costs

of designs comparable to the second-phase architecture in the low hundreds of billions of

dollars. Given that the United States would

have to engineer, build, and deploy entirely new

classes of space systems, cost estimates today

are shaky at best. For any given component,

unanticipated difficulties might increase costs,

or technical breakthroughs might decrease

costs. The SDIO has produced a rough estimate for the cost of a phase-one system: $75

billion to $150 billion.

Phase-three architectures are now so loosely

defined and understood that few if any contractor cost estimates exist.

Nobody now knows how to calculate, let

alone demonstrate to the Soviets, the costexchange ratio between offense and defense.

Detailed defensive system designs and a

thoroughly researched understanding of potential offensive countermeasures may help.

But unless the ratio appears obviously to be

much greater than one-to-one, it will be extremely difficult to determine whether the criterion of “cost-effectiveness at the margin”

has been met by any proposed BMD system.

At least in the first phase, it appears that the

Soviets would have a strong incentive to add

missiles, warheads, and countermeasures to attempt to restore their strategic nuclear capabilities. The question would be whether the

Soviets were persuaded that in the long run

the defense system would evolve into one that

cost less per Soviet RV destroyed.12

Timing and Evolution

The Strategic Defense Initiative Organization (SDIO) has not pursued the SDI as an

open-ended research program to be concluded

only when a certain level of knowledge was attained. Instead, the research has been strongly

oriented toward trying to provide the basis for

‘%s discussion does not address whether the Soviets would

accept the costiexchange ratio criterion for their own decisions

or whether they might simply do their best at improving their

offense and hope the United States might not follow the ensuing offensive-defensive arms race through to its expensive conclusion.

an ‘informed decision” on BMD full-scale engineering development by the early 1990s (the

exact year, although it appears widely in the

press, is classified). Nevertheless, implied in

the SDI program was that whatever information might be available by the early 1990s,

proposals for deployment would be offered.

Congress, however, has not funded the SDI

at the level that the SDIO asserted was necessary to permit an informed decision about

such proposals by the early 1990s. Nevertheless,

by cutting back parallel technology programs

and longer-term research while preserving programs believed to have near-term promise, the

SDIO has attempted to maintain the goal of

making detailed deployment proposals by only

1 year later than the appointed date.

In late 1986 and in 1987 the SDIO began

developing the “phase-one” BMD system architecture described above. In its 1987 annual

report to Congress, the SDIO said that its

study of the first phase of a phased deployment”. . . does not constitute a decision to deploy. Such a decision cannot be made now.”13

OTA concurs. First, the required space transportation system is unlikely to be available for

early 1990s deployment. Second, the reductions in SBI weights essential to deploying significant numbers of effective weapons are not

yet available. Third, the U.S. aerospace industry would have to engineer, mass produce, and

deploy entire new classes of satellite systems.

Fourth, cost estimates for all these steps today

are shaky at best. The SDIO does argue that

the first-phase option would lay the groundwork for the deployment of subsequent phases.

This could be true if the subsequent phases

were in fact known to be feasible, affordable,

survivable, and cost-effective at the margin—

and if the first-phase system retained some capability against a responsive Soviet threat.

Every part of the complex development, production, and deployment scheme would have

to work well and on schedule. Otherwise, the

Soviets could be well on the way to neutralizing the first-phase architecture before it was

13SDIO, op. cit., foot note 2, p. 11-10.
25

fully in place. Countermeasures could have

greatly degraded SBI capabilities. For example, as the booster rocket burning times of Soviet missiles decreased (a process already

occurring as the Soviets move to solid-fueled

boosters), fewer SBI’s could reach the boosters

before their post-boost vehicles had separated

and begun to dispense reentry vehicles and

decoys. New post-boost vehicles, which would

in any case be harder to track and hit than

boosters, could also dispense their payloads

more rapidly. Without altering their rocket

technologies, the Soviets could concentrate

their ICBM bases so that fewer SBIs would

be in range when many ICBMs were launched

at once (that is, the “absentee ratio’ would be

higher). While the Soviets would not find all

such countermeasures cheap and easy, one

should compare their cost and difficulty to

those of developing and deploying a vast new

space-based BMD system.

Adding more SBIs to the BMD constellation would allow attacks on more boosters, but

the numbers of SBIs needed would become increasingly prohibitive as the Soviet ICBM

force evolved. On the other hand, if the Soviets

Figure 1-5.—SDIO Proposal for

Concept Development

approval approval

(Milestone 1) (Milestone 2)

Phase 1

Strategic

Defense —

System (SDS)

Phase

could not soon reduce the burn-times of their

post-boost vehicles, SBI effectiveness might

endure for some time-assuming that the firstphase SBI infrared sensors could effectively

home in on the colder PBVs.

Although a phase-one architecture may be

presented to Congress as the first step of a

“phased deployment, ” research on the later

phases is far from demonstrating that those

succeeding phases will be feasible, affordable,

and compatible with first-phase systems. The

feasibility of fully trustworthy battle management software systems may never be entirely

demonstrable. The feasibility of directed-energy weapons and interactive discrimination

systems remains to be demonstrated, and persuasive evidence one way or the other will probably not be available until after 1995. The feasibility of a new, post-2005 generation of Soviet

fast-burn boosters that could stress even directed-energy weapons remains plausible and

cannot be discounted.

Thus a “phased deployment” in which only

the first phase was shown to be feasible would

not necessarily be able to evolve and adapt to

Development and Deployment

Initial

Production Operational

approval Capability

(Milestone 3) (IOC)

SOURCE Office of Technology Assessment, adopted from Department of Defense information on the Strategic Defense Initiative
26

a responsive Soviet threat. The SDIO plan calls

for completing “demonstration and validation”

of phase-two concepts before actual production and deployment of phase one. Therefore,

● commitment in the early 1990s to a phaseone development would imply confidence

that phases two and three will ultimately

prove feasible, and

● commitment in the mid-1990s to phase-one

deployment would require an act of faith

that phase three would prove feasible.

Otherwise, depending on how long deployment actually took and how effective the Soviet response was, either the first- or secondphase systems could be reduced to only modest effectiveness or impotence even before deployment was completed.

SDIO officials and contractors have surmised that the technologies needed to maintain and extend the defensive capabilities of

first- and second-phase systems into the farther term will in fact become available. If a

continuing, vigorous research and development program produced the necessary technologies, and if Soviet offensive developments

could not keep pace, the first-phase concept

might evolve into a more advanced BMD system. If the Soviets responded to the SBI system by developing faster-burning PBVs that

could carry only much reduced payloads, then

the ultimate task of discriminating RVs and

decoys in the mid-course could be greatly simplified. (This conclusion assumes that the

Soviets could not afford at the same time to

double the size of their missile fleet.) The

United States could add sophisticated SWTS

satellites and SBIs with improved sensors. If

Soviet decoys were few enough and simple

enough, the sensor satellites might be able to

track and discriminate RVs and decoys in midcourse, thus allowing improved hit-to-kill weapons to attack RVs individually after they were

dispensed. Or, interactive discrimination techniques might turn out to make RV/decoy discrimination feasible.

OTA concludes that, if shown to be technically feasible and desirable, second-phase system production and deployment could not begin until around the year 2000 or be completed

much before 2010. Soviet countermeasures

coming into deployment by then could include

more missiles, advanced RVs (possibly including maneuvering RVs or “MaRVs”) and decoys, faster rocket boosters and post-boost vehicles, concentrated launch-sites for boosters,

and advanced anti-satellite weapons. The utility of space-based SBIs for boost-phase interception would then be severely limited. Depending on whether and when the Soviets

could field faster-dispensing PBVs, the SBIs

might be of some utility for PBV interception.

Overall system effectiveness, however, would

probably depend heavily on how well the midcourse discrimination challenge had been met.

If the Soviets developed high-payload, fastdispensing PBVs, the United States might

have to add laser weapons to the defense system to increase boost- and post-boost intercepts to reduce the mid-course discrimination

burden. As is noted below, however, even this

step might not suffice.

As of 1988, three uncertainties about the viability of a second-phase system especially

stand out:

1.

2.

3.

evidence demonstrating effective and affordable technology for discriminating Soviet nuclear warheads from decoys will

probably not be available before the mid1990s, if then;

a follow-on, directed-energy BMD system

would be needed to restore or maintain defense effectiveness once faster-burning

boosters were able to evade SBIs; but

directed-energy weapons for BMD mayor

may not be technically feasible; such feasibility is very unlikely to have been determined by the early 1990s; if the Soviets

were able to field a few thousand very fastburning boosters with one warhead and

several decoys each, even directed-energy

weapons might not suffice to maintain a

high level of defense effectiveness;

the survivability of a space-based system

itself against a defense suppression attack

by Soviet weapons likely to be available

after the year 2000 may not have been determined by the early 1990s.
27

Ballistic missile defense deployments of dubious long-term effectiveness could stimulate

the Soviet Union to offensive countermeasures

and weapon deployments rather than to negotiations to reduce mutual offensive threats.

Competition in Anti-satellite Weapons

As noted above, the technologies applicable

in exo-atmospheric weapons are, in most cases,

liable to be applicable in ASAT weapons before they are applicable in BMD. Thus there

will be pressures from the military establishments on both sides to field such weapons as

they become feasible, whether or not they

prove to have BMD potential. For example,

the first mission for space-based SBIs maybe

as defensive satellites, or DSATS, to protect

the BMD system as it is being deployed. Space

lasers may be attractive ASATs and DSATs

whether they are adopted as BMD weapons

or not. Neutral particle beam discriminators

could be powerful ASAT weapons. If the nuclear-pumped x-ray laser can be developed as

a weapon—which is far from proven—its most

promising application may be as an ASAT. No

credible answer to the x-ray laser as a BMD

suppression weapon has been developed.

As the United States or the Soviet Union

began to deploy substantial numbers of BMD

weapons on the ground or in space, these weapons would greatly increase the anti-satellite

threat to the other’s space assets. (Space-based

weapons themselves would, of course, be among

those space assets.) Neither side is liable to permit the other the kind of unilateral control of

space that such unchallenged ASAT capabilities would provide. Therefore, in the absence

of arms control agreements to the contrary,

we should expect from the beginning of BMD

space deployments an intense competition between the superpowers for control of near-earth

space.

A frequently proposed survivability measure for U.S. space-based BMD assets is the enforcement of keep-out zones against any potentially threatening Soviet satellites. Whether,

when, and how the Soviets might challenge

such assertions of U.S. exclusionary zones in

space has not been analyzed by those proposing this tactic. Indeed, the whole question of

the mutual occupation of space by weapons

of comparable capability has not yet been adequately addressed by SDIO or its contractors.
Chapter 2

Introduction
CONTENTS

Organization of This Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter l: Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 2: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 3: Designing a Ballistic Missile Defense (BMD) System:

Architecture and Trade-Off Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 4: Status and Prospects of Ballistic Missile Defense

Technologies, Part I: Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 5: Status and Prospects of Ballistic Missile Defense

Technologies, Part II: Weapons, Power, Communication, and

Space Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Chapter 6: System Development, Deployment, and Support . . . . . . . . . . . 32

Chapter 7: System Integration and Battle Management . . . . . . . . . . . . . . 32

Chapter 8: Computing Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Chapter 9: Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Chapter 10: Nondestructive Countermeasures Against Ballistic

Missile Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Chapter 11: Defense Suppression and System Survivability . . . . . . . . . . . 33

Chapter 12: Defense Suppression Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . 33

The Goals of the Strategic Defense Initiative. . . . . . . . . . . . . . . . . . . . . . . . 33

Three Goals for Strategic Defense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Increase Attacker Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Deny Military Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Assured Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

The Soviet Ballistic Missile Threat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Targets To Be Defended. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

The SDI Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Current SDI Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

The Criteria of Feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Affordability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Favorable Cost-Exchange Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Survivability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Box

Box Page

2-A. Potential Targets for a Soviet Nuclear Ballistic Missile Attack. . . . . . 38

Figures

Figure No. Page

2-1. Nuclear Forces-ICMBs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

2-2. Modernization of Soviet ICBMs Warhead Mix . . . . . . . . . . . . . . . . . . . . 37

Table

Table No. Page

2-1. Strategic Defense Initiative Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Chapter 2

Introduction

ORGANIZATION

This report identifies questions to be answered before the technical feasibility of

achieving the goals set for the Strategic Defense Initiative (SDI) can be determined. The

report also offers a snapshot of how far researchers have come toward answering these

critical questions and how much remains

unknown.

Chapter 1: Summary

Chapter 1 summarizes and explains the principal findings of this OTA study.

Chapter 2: Introduction

This introductory chapter devotes considerable attention to goals for the SDI, since this

subject continues to be a source of confusion

and debate in the country. Various leaders in

the Administration and in Congress have at

one time or another emphasized different goals,

and which goals will ultimately prevail remains

uncertain. Clearly, some goals would be easier to reach than others. This discussion does

not include a critical analysis of the goals nor

does it attempt to resolve the debate about

them. Instead, this chapter tries to provide a

context for the issues of technical feasibility.

Chapter 3: Designing a Ballistic Missile

Defense (BMD) System: Architecture

and Trade-off Studies

To assess the feasibility of a potential BMD

system, the United States needs to know both

what the system’s elements and the system

as a whole might look like. To this end, the

Strategic Defense Initiative Organization

(SDIO) has awarded a series of contracts to

several teams of defense companies to try to

define some candidate “system architectures”

for BMD. Drawing on these studies, SDIO synNote: Complete definititions of acronyms and initialisms

are listed in Appendix B of this report.

OF THIS REPORT

thesized its own “reference architecture” to

help SDI researchers understand the requirements that the technologies being developed

eventually must meet.

Late in 1986 and in the first half of 1987,

system architecture analysis was in a state of

flux as SDIO instructed its contractors to conceptualize the early stages of a BMD deployment. In mid-1987, the SDIO proposed a firstphase architecture to the Defense Acquisition

Board and in September the Secretary of Defense approved a program of ‘demonstration

and validation” for this architecture. The process of evolving system architecture analysis

and design is likely to continue throughout the

life of the program and into the period during

which defenses are actually deployed, if they

are. There should be continuing feedback between system designers and technology develpers, balancing the desirable and the possible.

This chapter introduces that process, discusses

its importance, and describes where it has led

so far.

Chapter 4: Status and Prospects of

Ballistic Missile Defense Technologies,

Part I: Sensors

Chapter 5: Status and Prospects of

Ballistic Missile Defense Technologies,

Part II: Weapons, Power, Communication,

and Space Transportation

These chapters are organized as reference

works on several of the key technologies under research in the SDI program-describing

them, surveying the requirements they must

ultimately meet, and reporting their status (including key unresolved issues) as of early 1988.

The chapters also examine the requirements

for combining those technologies into working components of a BMD system, with emphasis on the kinds of components needed for

recent SDIO “reference architecture” formu31
32

lations. Chapter 4 reviews technologies for

finding, tracking, and pointing weapons at missile boosters, post-boost vehicles, and reentry

vehicles and for discriminating between genuine targets and decoys. Chapter 5 reviews the

weapon technologies for delivering lethal doses

of energy (kinetic or electromagnetic) to targets. It also addresses the key technologies of

space transportation, communication, and

power supplies for space assets.

Chapter 6: System Development,

Deployment, and Support

If BMD is to play a role in U.S. national

strategy, the technologies described in the previous chapters must be incorporated into working weapon components. Those components

must be integrated into effective weapon systems that are affordable, maintainable, and

adaptable over time to possible adversary responses. By focusing on some particularly challenging issues, such as the development and

engineering of a space-based space surveillance

system and the logistics of space transportation, chapter 6 attempts to give an appreciation of the steps involved in these processes.

Chapter 7: System Integration and

Battle Management

With variations on SDIO’s reference architecture for a BMD system as models, this

chapter shows how the various components of

such a system would have to work together

to intercept a ballistic missile attack in its several phases. The chapter attempts to give an

appreciation of the complexities of integrating BMD system components into a quickly

reacting system. It does so by presenting an

overview of the tasks a BMD system would

have to perform and examples of how it would

perform them. It also exaxnines the concept

of BMD battle management and the roles of

humans and computers in such a battle.

Chapter 8: Computing Technology

Computers would be crucial to any BMD

system, from simulation testing of theoretical designs, through operation of most of the

hardware, to management of the battle. Chapter 8 focuses on the roles of computers in BMD

and on the computation capabilities needed to

satisfy SDI requirements. Computing technology encompasses both hardware and software.

This chapter, however, emphasizes hardware

questions while chapter 9 focuses on software.

Chapter 9: Software

The legislation mandating this study instructed that it include an analysis of the feasibility of meeting SDI software requirements.

Chapter 9 examines the question of whether

the complex computer programs that BMD

will require could be made sufficiently dependable. It analyzes the concepts of software trustworthiness and reliability, as well as other

important software issues. It compares requirements and characteristics of BMD software to existing, trusted software systems.

The chapter ends with conclusions about the

prospects for producing trustworthy software

for the SDI.

NOTE: Chapters 10, 11, and 12 are now available only the classified version of this

report. The descriptions here are for

reference.

Chapter 10: Nondestructive

Countermeasures Against Ballistic

Missile Defense

Ballistic missile defense systems must be designed to cope with the kinds of countermeasures the Soviets might deploy against them.

These include modified or new ballistic missiles, devices intended to make reentry vehicles harder to find or shoot at, and weapons

that could attack the BMD system. This chapter examines the first two types of countermeasure, while chapter 11 describes the latter, or “defense suppression” technologies and

their counters. Estimates of physically possible countermeasures must be refined by estimates of what is technically, economically, and

strategically feasible for the Soviet Union. The

chapter concludes with a review of the tech-
33

nologies that might provide responses to the

potential Soviet countermeasures.

Chapter 11: Defense Suppression and

System Survivability

The legislation instructing OTA to carry out

this study placed special emphasis on the survivability of an SDI-produced BMD system

in the face of an enemy attack on the system

itself. The chapter reviews the technologies

that might be applied to defense suppression

and the technologies and tactics that might

counter them.

Chapter 12: Defense Suppression

Scenarios

In a variety of “scenarios,” chapter 12 identifies the most stressing attack threats that

various BMD elements would be likely to face

and the methods a BMD system might use to

defend itself, actively or passively.

THE GOALS OF THE STRATEGIC DEFENSE INITIATIVE

According to the Strategic Defense Initiative Organization in 1986:

The goal of the SD I is to conduct a program

of vigorous research and technology development that may lead to strategic defense options that would eliminate the threat posed by

ballistic missiles, and thereby:

● support a better basis for deterring aggression,

● strengthen strategic stability, and

. increase the security of the United States

and its Allies.

The SDI seeks, therefore, to provide the

technical knowledge required to support an informed decision in the early 1990s on whether

or not to develop and deploy a defense of the

U.S. and its Allies against ballistic missiles.’

What does the phrase, “eliminate the threat

posed by ballistic missiles, ” mean, and how

‘Strategic Defense Initiative Organization, Report to the Congress on the Strategic Defense Initiative, June 1986, p. IV-1.

In its 1987 report, SDIO dropped “in the early 1990s” from

its goal; it also dropped the “not” from the phrase “whether

or not” in the above quotation.

might doing so enhance deterrence, stability,

and security? Proponents of BMD have argued

that increasing levels of defense could offer increasing benefits. Fairly modest levels of

BMD, they say, might improve deterrence of

a Soviet nuclear attack by increasing Soviet

military planners’ uncertainty about the effectiveness of such an attack. Higher levels of defense capability might actually deny the

Soviets even the possibility of achieving whatever military goals they might have for attack.

Finally, extremely good defenses against all

types of nuclear attack–including attacks by

ballistic missiles, cruise missiles, bombers, and

other means of delivery-might essentially assure the survival of the U.S. population and

society no matter what the Soviets tried to do.

Then U.S. security would no longer rely on the

threat of retaliation to deter a nuclear attack.

SDIO officials emphasize that currently the

preponderance of their attention is focused on

systems and technologies intended to lead to

early accomplishment of the first goal of enhancing deterrence.

THREE GOALS FOR STRATEGIC DEFENSE

Increase Attacker Uncertainty tended targets, Soviet military planners can

Working with assumptions about the ac- make some predictions about Soviet ability to

curacy, explosive power, and reliability of destroy a chosen set of targets. Just how conweapons systems as well as the nature of in- fident Soviet planners would or should be
34

about the validity of their assumptions is extremely difficult for U.S. analysts to determine.

Relatively modest amounts of strategic defense, 2 some argue, might add to the uncertainties that the potential attacker already faces.3

He would be forced to make additional assumptions about how—and which-of his warheads

would be intercepted by the defenses. Insofar

as a Soviet decision to launch a nuclear attack

on the United States might depend on Soviet

confidence in their ability to destroy a given

set of targets, the protection added by modest U.S. strategic defenses might help deter

such a decision.4 Presumably, the larger factor in a Soviet decision on whether to strike

first is the current high probability that a U.S.

retaliatory attack would devastate much of the

Soviet Union.

In its 1987 report to Congress, SDIO suggested that relatively modest levels of defense

might begin to add to Soviet uncertainties by

“denying the predictability of Soviet attack

outcome . . . and imposing on the Soviets significant costs to restore their attack confidence.” 5

There are ways the Soviets might try to reduce the uncertainties added by U.S. defenses.

They might deploy offensive countermeasures

designed to restore their previous level of confidence in their weapons’ ability to reach and

destroy assigned targets. They might deploy

2This section addresses strategic defense generically -i.e.,

goals for defense against all means of delivering nuclear weapons, not just against ballistic missiles. Since the SDI is directed

at developing defenses only against ballistic missiles, we quickly

turn to that particular task for strategic defenses. Where relevant, the report will call attention to the relationships between

ballistic missile defense and other kinds of strategic defense.

3These would include uncertainties about: the accuracy of missiles over untested trajectories; the vulnerabilities of some kinds

of targets, such as command and control systems; whether the

victim of the attack would launch his own missiles “on warning, ” thus defeating the most critical objective of the attack;

and the nature and results of the retaliation carried out by

submarine-launched missiles, bombers, and cruise missiles that

escaped the attack.

4For a more detailed discussion of deterrent strategy, see U.S.

Congress, Office of Technology Assessment, Bskh”stic Missile

Defense Technologies, OTA-ISC-254 (Washington, DC: U.S.

Government Printing Office, September 1985), pp. 67-132.

5Strategic Defense Initiative Organization, Report to the Congress on the Strate&”c Defense Im”tiative, April 1987, p. II-11.

additional weapons intended just to exhaust

the defenses, assuring that some weapons face

no defensive screen. They might attempt to

circumvent the BMD system by adding more

bombers and cruise missiles to their arsenal.

On the other hand, the Soviets would have

to make new assumptions about how well these

responses would work. The Soviets might also

choose to give up some weapon capabilities to

preserve others: for example, some countermeasures intended to assure that a given number of nuclear warheads could penetrate the

defense might be traded against sacrifices in

the number, accuracy, or yield (explosive

power) of those warheads. If only because the

offensive task had become more complicated,

at least some more uncertainty would exist

than if the United States had no defenses at

all.6 Opinions vary, however, on what margin

of additional uncertainty the Soviets would

face and whether there might be other, less

costly, and earlier ways to complicate Soviet

attack problems.

Deny Military Objectives

Some analysts have argued that an increase

in attacker uncertainty as described above is

itself a sufficient enhancement of deterrence

to justify deploying ballistic missile defenses.

The SDIO, however, places a more rigorous

requirement on defense:

A defense against ballistic missiles must

be able to destroy a sufficient portion of an

aggressor’s attacking forces to deny him the

confidence that he can achieve his objectives.

In doing so, the defense should have the potential to deny that aggressor the ability to

destroy a militarily significant portion of the

target base he wishes to attack.’

The goal here is not just to reduce the attacker’s confidence in achieving some set of

goals, but to deny him any reasonable pros6Alternatively, some would argue that the Soviets might find

a secret countermeasure that they were certain was capable of

totally disabling the U.S. BMD system; if they combined this

countermeasure with expanded offensive forces, their net certainty of attack success might be increased over what it is today.

7

Strategic Defense Initiative Organization, op. cit., p. IV-2.
35

pect of doing so. Suppose, for example, that

the Soviets have set for their strategic forces

the goal of destroying 75 percent of a particular target set. A U.S. strategic defense that

could predictably allow them to destroy only

50 percent of this set would therefore deny the

Soviets their goal. If the difference between

the Soviets’ choosing to attack and refraining

from attack rested on their confidence in their

ability to destroy 75 percent of the targets,

they would be deterred.

An attack of thousands of nuclear weapons

that failed in its purely military objectives,

whatever they might be, would still wreak

great, perhaps irreparable, damage on U.S. society. Such damage would include not only the

direct effects of nuclear weapons exploding

near U.S. cities, but the longer-term effects of

nuclear fallout and economic and social disruption.8 Moreover, for purposes of intimidation

or deterrence, the Soviets might change their

target plans to retain their ability to destroy

U.S. cities intentionally. Thus we would still

need to rely on the threat of retaliation to deter Soviet or other attacks (or, perhaps more

to the point, threats of attack) on our economy

and society.

Assured Survival

In his speech of March 23, 1983, inaugurating the SDI, President Reagan set an even

higher goal for strategic defenses:

What if free people could live secure in the

knowledge that their security did not rest

upon the threat of instant U.S. retaliation to

deter a Soviet attack, that we could intercept

and destroy strategic ballistic missiles before

they reached our own soil or that of our

allies?’

This goal goes beyond denying the Soviets

an ability to destroy a “militarily significant

portion” of some target base; it would be to

8See, for example, U.S. Congress, Office of Technology Assessment, The Effects of Nuclear War, OTA-NS-89 (Washington,

DC: U.S. Government Printing Office, May 1979), esp. ch. 4,

pp. 109-118.

‘Ronald Reagan, televised speech, Mar. 23, 1983.

protect people. As the President said over 3

years later:

Our research is aimed at finding a way of

protecting people, not missiles. And that’s

my highest priority and will remain SO.

10

The goals of increasing attacker uncertainty,

denying military objectives, and assuring national survival imply progressively more capable defensive systems, and correspondingly

more difficult technical challenges. The following survey of the Soviet missile threat and the

kinds of targets the United States would need

to defend against that threat illustrates the

scope of the strategic defense problem.

The Soviet Ballistic Missile Threat

The Soviets now have about 1400 intercontinental ballistic missiles (ICBMs) carrying

about 6300 nuclear-armed re-entry vehicles

(RVs). They also have about 944 submarine

launched ballistic missiles (SLBMs) with about

2800 nuclear-armed RVs (see figure 2-l). The

Soviets also have several hundred intermediate-range ballistic missiles based in the Soviet Union that can reach all or part of Europe and Asia with about 1400 nuclear RVs—

but these are to be eliminated under the Intermediate Nuclear Forces (INF) agreement

signed in December 1987. Several hundred

shorter-range missiles can deliver single warheads from tens to hundreds of kilometers;

many are based in Soviet Bloc countries and

can reach important targets in NATO countries. Under the terms of the INF agreement,

the Soviets are also to eliminate their other

missiles with ranges above 500 km.

The composition of the Soviet ballistic missile force will change over the years during

which BMD might be developed and deployed

‘“President Ronald Reagan, “SDI: Progress and Promise, ”

briefing in Washington, D.C. on Aug. 6, 1986, Current Policy

No. 858, U.S. Department of State, Bureau of Public Affairs,

Washington, DC, p. 2. Secretary of Defense Caspar Weinberger

has said, “When the President says that we are aiming at a

strategic defense designed to protect people, that is exactly what

he means. ” Speech at Harvard University, Sept. 5,1986, quoted

by David E. Sanger, “Weinberger Denies Antimissile Shift, ’’The

New York Times, Sept. 6, 1986, p. 9.
36

Ss-11 440 SS-18

SS-13 308

60 SS-19

SS-17 360

150 SS-25 About 100 Test Center A ICBM Base ●

Ss-N-5 39

SS-N-6 272

SS-N-18 224

SS-N-20 80

292 SS-N-23 48

SOURCE

Test Center A SLBM/SSBN Port ●

U.S. Department of Defense, Soviet Military Power (Washington, DC: U.S. Government Printing Office, 1987),
37

(see figure 2-2). The changes would be more dramatic if the Soviets attempted to counter the

effectiveness of prospective U.S. defenses. Anticipating this “responsive threat” is a major

challenge for BMD planners. The SDIO has

not been assigned to address the Soviet ability, present and forecast, to deliver nuclear

weapons with aircraft and ground-, sea-, or airlaunched cruise missiles. The Air Force is conducting an “Air Defense Initiative” (ADI) that

is studying the interception of air-breating

weapons. The ADI, however, is operating at

much lower funding levels than the SDI.

Targets To Be Defended

The three goals of uncertainty, denial, and

assured survival remain abstract and ambiguous until we consider the kinds of targets to

be defended against nuclear attack. Soviets attack objectives might include four broad categories of targets:

1. strategic retaliatory forces—ICBM silos

(or, in the future, mobile ICBMs), bombers

(and refueling tankers) at their bases, submarines in port, command posts, and communications nodes;

2. other military targets-including military

headquarters, barracks, nuclear and conFigure 2-2.— Modernization of Soviet ICBMs

Warhead Mix

Ss-11 ,

SS-13,

SS-25

(mobile

Mid-1990s a

1986

aEstimates based on current trends.

SOURCE: U.S. Department of Defense, Soviet Military Power (Washington, DC:

U.S. Government Printing Office, 1987),

3.

4.

ventional ammunition dumps, supply

depots, naval ports and shipyards, airfields, and radars;

enconomic targets-industrial facilities,

fuel reserves, research centers, transportation nodes, and cities; and

political targets-non-military government facilities, and civil defense shelters.

Each of these sets of targets (for further explanation, see box 2-A) has different implications for strategic nuclear offensive and defensive operations.

Strategic Retaliatory Forces

The purpose of a Soviet nuclear attack on

U.S. strategic nuclear forces—a so-called

“counterforce” attack—would be to reduce the

ability of those forces to carry out a retaliatory nuclear attack on the Soviet Union. In

1986 the Department of Defense estimated

that by attacking each of 1000 U.S. Minuteman missile silos with two SS-18 warheads,

the Soviets could destroy about 65 to 80 percent of U.S. land-based ICBMs.11

An attack would have to succeed quickly and

destroy a high percentage of the targets. Otherwise, U.S. weapons could be launched against

the Soviet Union (assuming they had not already been launched on warning, before the

first Soviet missiles arrived). The objective of

substantially reducing the retaliatory damage

inflicted on the Soviet Union would not be met.

Thus slower bombers and cruise missiles would

be less suitable than ballistic missiles for this

kind of disarming attack.

*’U.S. Department of Defense, Soviet Military Power 1986

(Washington, DC: U.S. Government Printing Office, 1986), p.

25. The United States maintains several hundred Poseidon and

Trident missiles at sea at all times and is adding sea-launched

nuclear cruise missiles to its arsenal It also maintains bombers (many with cruise missiles) on alert for rapid escape on

warning. The President’s Commission on Strategic Forces (the

“Scowcroft Commission” argued in 1983 that, in view of overall U.S. retaliatm-y capabilities, ICBM vulnerability did not warrant ABM (anti-ballistic missile) defense of missile silos in the

near term. Some argue that future Soviet anti-submarine warfare developments might compromise the survivability of U.S.

ballistic missile submarines, and that defense of land-based missiles might compensate for that eventuality. Others argue that

if both the United States and the Soviet Union were to deploy

BMD, U.S. retaliatory missiles would be less able to fulfill their

missions, whether launched from land or sea.
38

Box 2-A.—Potential Targets for a Soviet Nuclear Ballistic Missile Attack

Strategic Retaliatory Forces

Land-based ICBMs. -The United States has about 1,000 intercontinental ballistic missiles in

hardened silos. In the 1990s it may deploy “Midgetman” missiles on road-mobile carrier vehicles.

It may deploy some MX “Peacekeeper” ICBMs on railroad cars within U.S. military lands. An attack on land-based ICBMs would have to be swift, well-coordinated, and accurate. Otherwise, many

of the missiles would remain available for striking back at the Soviets. (The Soviets would also have

to consider the risk that the United States would launch its ICBMs while they were under attack,

with many escaping destruction to retaliate against the Soviet Union.

Bomber Bases. –About 350 strategic bombers, able to carry several thousand nuclear bombs

and cruise missiles, are based at some tens of airfields around the United States. Additional aircraft

are needed to refuel the bombers in flight. Normally, a substantial number of the U.S. strategic

aircraft are on standby alert and might be expected to escape a Soviet missile attack given several

minutes of warning; in times of crisis, more bombers would be placed on alert. A Soviet attack might

try to catch as many as possible of the U.S. bombers (and their refueling tankers) on the ground

or just after take-off.

Submarine Bases. —Thirty-odd submarines with several hundred underwater-launched ballistic

missiles are based at just a few U.S. ports. By plan, in peacetime somewhat more than half these

submarines, with 2,500-3,000 nuclear warheads, are always at sea. Those in port would be easy,

inviting targets for a Soviet strategic counterforce attack. During a crisis, some of the submarines

in port could be sent to join those already at sea.

Communications, Command, and Control Facilities.- Linking the above forces to U.S. National

Command Authorities is a network of underground command posts, mobile command posts, mobile

communications (air, ground, and space) relays, and fixed communications transmitter and receiver

stations. A Soviet nuclear attack is likely to try to disrupt this network by direct nuclear destruction of the fixed land facilities or by means of nuclear-generated electromagnetic pulses intended

to interfere with the functioning of electronic devices.

Other Military Targets

Military Headquarters; Barracks, Nuclear and ConventionalAmmunition Dumps, Supply Depots,

Naval Ports and Shipyards, and Airfields.–Many other military facilities, while not directly supporting U.S. rapid-response strategic nuclear forces, would be essential to the conduct of conventional warfare or tactical nuclear warfare abroad. Many of these targets are “soft” . . . difficult to

shelter from the effects of even relatively inaccurate nuclear weapons.

Economic Targets

Factories, Power Plants, Fuel Supplies, and Transportation Nodes. -These are sometimes called

“economic recovery” targets. The military purpose of attacking them might be to eliminate the

economic base that supports U.S. military power. While the United States might be able to carry

out a strategic nuclear retaliatory attack if its cities were destroyed, it could not carry on a conventional war abroad very long.

Political Targets

Government Facilities and Civil Defense Shelters.–The Soviets might also attempt to disrupt

government to hinder economic and political recovery.
39

The purpose of a U.S. ballistic missile defense against such an attack would be to preserve enough missiles and bombers to retaliate successfully against the targets in the

Soviet Union designated by U.S. military planners.12 At a minimum, the United States might

wish defenses to add to current Soviet uncertainties about how well they could prevent

those offensive weapons from reaching the Soviet Union. If these redundant, hardened targets could be defended preferentially, that is,

if defensive resources could be devoted to protecting a sub-set of them that is unknown to

Soviet planners, then Soviet confidence in being able to destroy the whole force might be

reduced to a very low level.13

At best, we would want defenses that persuaded the Soviet Union of the certainty of failure of any preemptive attack on our strategic

forces that had the purpose of reducing significantly the damage we could do to the Soviet Union.

Other Military Targets

The purpose of attacking U.S. military targets other than those connected with strategic nuclear forces would be to weaken or eliminate the ability of the United States to project

military power abroad (to fight conventional

or limited nuclear wars in Europe, Asia, or elsewhere), or even to defend its own territory

against invasion. Unlike sheltered ICBMs,

most of these other military targets are relatively soft–each could be easily destroyed by

one or a few moderately accurate nuclear weapons. Nor must they be destroyed instantaneously, since they cannot be used for a prompt

nuclear retaliation against Soviet territory.

Since these other military targets can be destroyed more or less at leisure, strategic delivery vehicles other than ballistic missiles can

“Opinions vary greatly on how many of what kinds of targets the Soviets would have to betieve they would lose in such

a retaliation before they would be deterred from launching an

attack on the United States. See OTA, %Zh”stic Missile Defense

Technologies, op. cit., pp. 68-76.

‘sFor a more detailed explanation of the concept of preferential defense, see OTA, Ballz”stic Missile Defense Technolo~”es,

op. cit., pp. 94-98.

be used against them—bombers and cruise

missiles in particular. Therefore, a strategic

defense intended to protect these targets must

be highly effective against “air breathing”

weapons as well as against ballistic missiles.

The purpose of defending such targets would

be to decrease the probability that a nuclear

attack on them could significantly weaken our

military power; at best we would want the

Soviets to be certain that such an attack would

fail.

It is important to note that many of these

‘‘other military targets” are located in or near

urban complexes, and an attack on them might

be hard to distinguish from a punitive city attack. Fallout would reach extensive areas of

the United States and millions of people might

die.

Urban Economic and Political Targets

The main military purpose of attacking the

U.S. industrial and political infrastructures

would be to remove the base from which the

United States exerts military and economic

power abroad. Another purpose, however,

might simply be to inflict punishment. Before

a war occurred, the purpose of having such an

ability to punish would be to deter actions (e.g.,

nuclear or nonnuclear attacks) by threatening

to impose a cost higher than the expected gain

of such actions. For example, Britain and

France maintain nuclear deterrent forces that

they believe help deter the Soviet Union from

attacking them, even though the effects of

those forces on Soviet military capabilities

might be more indirect than direct.14

Even a few tens of nuclear weapons landing

on U.S. cities would cause unprecedented destruction in this country. Extensive use of civil

defense measures, if feasible, might ameliorate

the effects of such destruction (e.g., if city populations could be evacuated and sheltered from

radioactive fallout and if industrial machinery

could be sheltered). But even more so than the

141t might k now, however, that the Moscow area has mwy

military facilities; attacks on them would have widespread military as well as civilian consequences.
40

kinds of “soft” military targets described

above, cities are vulnerable to attacks over

hours and days by bombers and cruise missiles

as well as by ballistic missiles. Defending cities, then, would require extremely effective air

defenses as well as missile defenses.

The purpose of defending against attacks on

urban industrial targets would be primarily to

save lives, property, and civilized society.

Militarily, the purpose of having such defenses

would be to persuade potential attackers that

we could so limit damage to our Nation that

we would not have to constrain our own actions out of fear of the effects of an enemy nuclear attack.

From the standpoint of deterrence, various

considerations may affect just how much we

believe we need to limit damage to our Nation.

One consideration might be relative damage:

would the damage the United States is likely

to suffer in a nuclear war be more or less acceptable to us than the damage the Soviets are

likely to suffer would be to them? Another

measure might be absolute: regardless of how

much damage we could inflict on the Soviets,

under what conditions would we be willing to

accept the amount of damage they could inflict on us (and vice-versa)?

An open question is just how limited the potential damage would have to be before the

United States would decide to give up entirely

its own ability to carry out a nuclear retaliation against potential attackers. That is, at

what point would we decide to rely on defense

rather than the threat of retaliation for our own

security?

The Special Case of Defense of Allies

Part of the stated mission of the SDI is to

design defenses to protect U.S. allies against

ballistic missiles. But the purposes and technical problems of doing so differ somewhat

from those of defending the continental United

States.

In the case of North Atlantic Treaty Organization (NATO) allies, for example, the Soviet ability to deliver nuclear weapons onto

Western European soil is massive and diverse.

Besides their land- and sea-based long-range

ballistic missiles, the Soviets might use hundreds of short-range ballistic missiles (intermediate and medium-range missiles with

ranges above 500 km are to be eliminated under the terms of the INF Treaty signed in December 1987). Thousands of Soviet and Warsaw Pact tactical aircraft are credited with the

ability to strike Western Europe. Air- and

ground-launched cruise missiles are or will be

available.

The probability of being able to defend Europe’s densely populated territory against all

the potential kinds of nuclear attacks on cities and industries seems low. Therefore, most

proponents of BMD for the European theater

of war focus on the defense of what are above

called “other military targets’ ’-command

posts, communications nodes, sheltered weapons-storage sites (nuclear and nonnuclear), and

airfields. Ballistic missile defenses might at

least disrupt and reduce the effectiveness of

Soviet nuclear missile attacks on such targets

(though other means of delivery would also

need to be dealt with).

Moreover, some believe that as Soviet ballistic missile accuracies increase, the Soviets

might use those missiles to attack military targets with nonnuclear explosive or chemical

warheads. Stopping moderately high (and in

some cases even modest) percentages of the

warheads in such attacks might make a military difference.16 Others argue, however, that

the conventional tactical ballistic missile

threat, if it exists, is minor compared to others

NATO will have to contend within the future.16

Another mission for Soviet “theater” ballistic missiles might be the delivery of chemical weapons intended to incapacitate NATO

troops. Again, the interception of a significant

percentage of such missiles might make the

difference between some troops surviving a

chemical attack or not.

“see Manfred Woemer, “A Missile Defense for NATO Europe,” Strategic Review, Winter 1986, pp. 13ff.

leFor a det~ed technical analysis, see Benoit Morel ~d Theodore A. Postol, “A Technical Assessment of The Soviet TBM

Threat to NATO, ” to be published by the American Academy

of Arts and Sciences, Cambridge, MA.
47

The shorter range Soviet ballistic missiles

differ in flight characteristics from their larger

relatives: their trajectories are shorter and confined to lower altitudes. While they travel more

slowly, their shorter flight times also leave less

time for them to be intercepted. On the other

hand, because these missiles spend a greater

part of their flight time inside the atmosphere,

reentry vehicle decoys present less of a problem to the defense. Space-based BMD (especially of the kinetic kill variety) would be of

limited utility, and ground-based rocket-interceptors would be the likeliest BMD candidates.

The SDI Scenario

Various statements by Reagan Administration officials over the first 4 years of the Strategic Defense Initiative can be combined to

form a scenario about how successively more

ambitious goals for strategic defenses might

be achieved.17 The expectation of the Administration is that SDI research will show that

deployment of ballistic missile defenses is feasible and desirable. As President Reagan has

said, “When the time has come and the research is complete, yes, we’re going to deploy.’’”

In the early stages of deployment, according to the Administration scenario, Soviet attack uncertainties would increase, thus reducing the probability of a Soviet first-strike

decision (though not the damage they might

inflict should they choose to attack). At first,

minimal defense capabilities would only complicate Soviet attack plans. As strategic

defenses became more capable, the Soviets

ought to be more persuaded that the military

purposes of any attack would fail. Nevertheless, as long as a substantial number of targets in the United States were still vulnerable

to attack, we would have to continue developing and deploying offensive strategic nuclear

.—— —_.

“For a list of statements prior to August, 1985, see OTA,

Balh”stic MissiJe Defense Technolo@”es,op. cit., App. I, pp.

308-309.

“President Ronald Reagan, “SDI: Progress and Promise,”

briefing in Washington, DC, on Aug. 6, 1986, Current Policy

No. 858, U.S. Department of State, Bureau of Public Affairs,

Washington, DC, p. 2.

weapons. As Secretary of Defense Weinberger

has written:

From the outset, we have insisted that

progress toward an effective SD I will have to

proceed hand in hand with regaining an effective offensive deterrent. . .19

The Administration hopes, however, that

ultimately offensive deterrence can be

abandoned:

As the United States has repeatedly made

clear, we are moving toward a future of greater

reliance upon strategic defense. The United

States remains prepared to talk about how–

under what ground rules and process-we and

the Soviet Union can do this cooperatively.

Such strategic defenses, coupled with radical

reductions inoffensive forces, would represent

a safer balance and would give future statesmen the opportunity to move beyond it—to

the ultimate elimination of nuclear weapons

from the face of the Earth.20

The key to this ultimate goal is seen to be

the development and deployment of defenses

that are unequivocally cheaper than corresponding amounts of offense. As SDIO puts it:

We seek defensive options–as with other

military systems–that are able to maintain

capability more easily than countermeasures

could be taken to try to defeat them. This criterion is couched in terms of cost-effectiveness.

However, it is much more than an economic

concept. 21

— ----

Waspa.r W. Weinberger, “U.S. Defense Strategy, ” Foreign

Affairs, Spring 1986, p. 678.

Earlier in the same article Weinberger explained his concept

of a multi-leveled deterrent:

If the adversary calculates that his aggression is likely to fail in

its own terms, he will not attack. Further, he must know that even

if his aggression should succeed in achieving its immediate objectives, he faces the threat of escalation to hostilities that would exact a higher cost than he is willing to pay. In addition to defense

and escalation, the third layer is retaliation: if the adversary confronts a credible. threat that eg ession will trigger attacks by a

surviving U.S. retaliatory capa ilit against the attacker’s vltsl

interests that result in losses exc‘&ganypossible@n, hewfl

not attack.

Ibid., p. 678.

~O~sident Ron~d Reagm, Speech to the U.N. General Assembly, Sept. 22, 1986, reprinted in The Washington Post, Sept.

23, 1986, p. A16.

zl&ra@$c Defense Initiative Organization, fipOrt @ tie COngress on the Strategic Defense Initiative, April 1987, p. IV-3.

It should be added that not only should capability be maintainable at the margin, but that our initial acquisition of defense

capability needs to be affordable in comparison with the cost

to the Soviets of upgrading their current offensive capabilities

to counter our defenses. The offense, being already in place, has

a head start on defenses yet to be built.
42

Such a favorable “cost-exchange” ratio between defenses and offenses would be intended

to persuade the Soviets of the futility of continuing a competition in offensive arms. The

SDIO has stated that:

Program success in meeting its goal should

be measured in its ability both to counter and

discourage the Soviets from continuing the

growth of their offensive forces and to channel longstanding Soviet propensities for defenses toward more stabilizing and mutually

beneficial ends. . . It could provide new and

compelling incentives to the Soviet Union for

serious negotiations on reductions in existing

offensive nuclear arsenals. ”22

Agreements on mutual offensive reductions

could make defensive tasks easier for each side.

Thus the Soviets could be offered both a carrot (possibility of their own effective defenses)

and a stick (threat of losing an arms race between offenses and defenses) as incentives to

subscribe to the U.S. scenario.

Current SDI Goals

The scenario shown in table 2-1 for the SDI

suggests the following official attitudes toward

the three goals of uncertainty, denial, and assured survival.

Uncertainty

Imposing greater uncertainty on Soviet attack planners would bean initial benefit of deploying BMD, but, presumably is not in itself

sufficient to justify the SDI.

Denial

Denial of Soviet military objectives in a ballistic missile would, in itself, justify deploying BMD. Secretary Weinberger has said:

. . . our strategic defense need not be 100 percent leakproof in order to provide an extraordinary amount of deterrence. Even a partially

effective defense would convince Moscow that

a first-strike was futile. And once we have rendered a Soviet first-strike obsolete and unthinkable, we will have dramatically increased

Table 2=1.—Strategic Defense Initiative Scenario

Stage 1:

SDI Research

Stage 2:

Development and

production of BMD

systems

Stage 3:

Initial BMD

deployments

Stage 4:

Extensive deployment of highly effective BMD

Stage 5:

Deployment of advanced BMD systems, combined

with agreed deep

reductions in

offenses

Leads to national decision in early 1990’s to proceed to fullscale engineering development

aimed at deployment of BMD

(reference to early 1990s date

dropped by SDIO in 1987)

Preparation for deployment in

mid-to-late 1990s (earlier initial

deployments raised as possibility by Secretary Weinberger

in 1987)

Introduces uncertainty into

Soviet strategic nuclear attack

planning; deployments preferably coordinated by agreement

with Soviets on transition to

defenses, but proceeds in any

case

Denies Soviet strategic forces

ability to achieve military objectives; demonstrates to

Soviets futility of competition

in offensive strategic missiles

Deep reductions in all types of

offensive strategic nuclear

forces plus defenses allows

abandonment of threat of

nuclear retaliation for security:

assured survival achieved

SOURCE: Compiled from U.S. Department of Defense, Report to the Congress

on the Strategic Defense Inltlative, June 1986, p. IV-12 and other Administration statements.

stability and rested deterrence on a rock-solid

basis. But bear in mind that our goal remains

to make ballistic missiles-the most destabilizing and dangerous weapons known to man—

obsolete. 23

Assured Survival

The goal of assured survival may well require

Soviet cooperation in offensive nuclear disarmament. A perfect defense against all ballistic missiles may not be possible, and:

Even a thoroughly reliable shield against

ballistic missiles would still leave us vulnerable to other modes of delivery, or perhaps

even to other devices of mass destruction. De-

~~Ibid., pp. IV-1-2.

z~Remmks before the Ethics and Public Policy Center, Washington, DC, Sept. 26, 1986.
43

spite an essentially leakproof missile defense,

we might still be vulnerable to terrorist attacks against our cities. Our vision of SDI

therefore calls for a gradual transition to effective defenses, including deep reductions in

offensive nuclear weapons.24

In the expressed Administration view, then,

the SDI should aim ultimately for ballistic mis-

.- 

24—.-——

Weinberger, “U.S. Defense Strategy, ” op. cit., p. 684.

sile defense systems that are nearly leakproof.

One way of achieving assured survival might

be to build defenses so effective that they

would succeed no matter what the Soviets

might throw at them. Another way might be

to build defenses that promise to be so effective that the Soviets would prefer to negotiate offenses on both sides away rather than

embark on an offense-defense race that they

have been persuaded they would lose technically or economically.

THE CRITERIA OF FEASIBILITY

Supporters and critics of the SDI would

probably both agree that proposals for deploying ballistic missile defense should meet at

least the four following criteria:

1. effectiveness,

2. affordability,

3. favorable cost-exchange ratio, and

4. survivability.

Note that in each case, meeting the criterion

will beat least partly dependent on Soviet decisions and actions: the Soviets can make the

job harder or easier for the defense. In an unconstrained arms race, they would do what

they could to make the job harder. In a cooperative regime of mutual defensive deployments and offensive reductions and controls,

each side might make the BMD job easier for

the other.

Effectiveness

Obviously, before deciding to deploy a BMD

system we would want to be confident that it

would be effective—that it would work well

enough to achieve the goals set for it. Effectiveness needs to be evaluated on two complementary levels. One level is technical performance: how well can the proposed BMD

system perform against the missile threat expected at the time of defense deployment? On

a higher level, would such performance provide

abetter basis for deterrence, strengthen strategic stability, and increase U.S. and Allied

security—the goals stated by SD IO? This second level of analyses received considerable attention in the 1985 OTA report on Ballistic

Missile Defense Technologies, so it will receive

much less attention in this report.

On the level of technical performance, it is

difficult to decide what “effectiveness” means.

For example, one frequently used criterion of

BMD effectiveness is “leakage rate”: what percentage of a specified Soviet missile attack

would we expect to penetrate our defenses and

what percentage could we stop? Given the

enormous destructive power of nuclear weapons, though, leakage rates may only tell part

of the story. A leakage rate of 10 percent might

sound worthwhile, and for some purposes it

may be. But under an attack of 10,000 nuclear

warheads, a 10 percent leakage rate would

mean 1000 nuclear detonations on U.S. territory.

Another problem with leakage rate as a

measure of effectiveness is that it is likely to

vary with the size and nature of attack. For

example, a system that could stop only 50 percent of a massive, nearly instantaneous attack

might stop 100 percent of an attack consisting of two or three missiles. On the other hand,

a system that could stop 50 percent of an attack of a certain size might not be expandable

in such a way that it could stop 50 percent of

an expanded enemy missile force. In addition,

to maintain damage at a fixed level, the defense would have to stop, for example, 75 percent of a doubled attack.
44

A slightly better indicator of effectiveness,

then, might be the absolute number of nuclear

warheads penetrating the defense under the

severest plausible attack. Such an estimate

would give abetter indication of the maximum

damage a Soviet attack might inflict.

An even better indicator would be the numbers of different types of targets that the

United States would expect to survive a missile attack. This approach would take into account the numbers of attacking weapons, the

numbers of penetrating weapons, the numbers

and types of targets attacked, and the numbers and types of targets protected. These

numbers might be translated into percentages

of types of targets surviving-e. g., 70 percent

of the land-based missile force.25 We might

carry the analysis further by weighing the

values of different types of targets. For example, one underground strategic command post

might be worth 10 missile silos.

All of the above indicators would be difficult to apply with precision. And the more factors an indicator has to take into account, the

more imprecise it is likely to be. Indeed, there

would be no direct way to measure the potential effectiveness of a BMD system: only an

actual nuclear war would do so. Instead, we

would have to rely on estimates, based on assumptions about:

enemy offensive technical capabilities

(numbers of weapons, accuracy, explosive

yields, ability to penetrate defenses);

enemy target attack plans;

defensive technical capabilities;

vulnerability of targets defended; and

the objective and subjective relative values of targets defended.

These factors would be difficult for U.S. planners to assess. They would also be difficult for

Soviet planners to estimate. Therefore, if the

U.S. goal is mainly to introduce uncertainties

into Soviet strategic calculations, precise measures of BMD effectiveness might not be nec-

‘6Note that plannin g to penetrate defenses may require the

offense to concentrate his attacks on higher-value targets, In

that case, the targets which he no longer has enough weapons

to strike can be considered “saved” by the defense.

essary. On the other hand, if we wished to be

certain of denying Soviet attack objectives, we

might need higher confidence in our estimates.

At the same time, if the Soviets decided,

along with the United States, that defenses

were desirable, then each side could help make

them more effective by agreeing to deep cuts

in offensive weapons and to restrictions on

countermeasures against defenses.

Affordability

If and when the Department of Defense eventually presents its proposals for deploying

BMD, the country will have to decide whether

the expected benefits would be worth the expected costs. Part of the SDI research program

is to estimate costs for the proposed systems.

For various reasons, the initial cost estimates

for complex weapon systems tend to be inaccurate, and usually too low. Producing reliable cost estimates for future BMD systems will

be a challenging task.

Another part of the SDI program is to attempt to develop new, cheaper ways to manufacture weapons and to deploy them in space.26

The ultimate weighing of costs and benefits

will be a political judgment made by the President and Congress. But a critical part of the

demonstration of technical feasibility of BMD

will be that the proposed systems can be built

at a cost the country would, at least arguably,

find reasonable.

As mentioned above, Soviet actions could

make effective BMD more or less affordable.

If they chose to invest heavily in offensive

countermeasures timed to take effect about

when our defenses might be deployed, they

could make those defenses much more expensive than if they stabilized the threat they pose

at today’s levels. Alternatively, in a cooperative regime they could make defenses cheaper

by agreeing to decrease their offensive threat

~Unt~ a re-organization in 1987, the SDIO Systems Endneering Directorate was in charge of this program, among others.

The Systems Engineering program element of the SDI budget

received $20.2 million in fiscal year 1987; $39 million was requested for fiscal year 1988.
45

in exchange for reductions in the U.S. offensive threat.

Favorable Cost-Exchange Ratio

The Nation must decide not only that a particular defense system proposed at a particular time is affordable, but whether the potential long-run competition of U.S. defenses

against Soviet offenses is likely to be affordable in the future. In the absence of a long-term

U.S. commitment to sustaining defensive capabilities, the Soviets would have incentives

to stay in the “game” until the United States’

will to spend flagged.

One way to try to persuade the Soviets to

abandon efforts to maintain offensive capabilities would be to demonstrate clearly that additional increments of offense would be more

costly to the Soviets than corresponding increments of defense would be to the United

States. Therefore, a corollary goal of the SDI

is to design defenses that are cheaper “at the

margin” than offenses. If the “cost-exchange”

ratio were favorable to defenses, and if the two

sides invested equal resources in defenses and

offenses respectively, then the side investing

in offenses should find its capabilities inexorably declining.

Achieving this favorable cost-exchange ratio will be technically challenging. Accurately

estimating the costs of defensive systems

would be difficult enough. Attaining high confidence that the ratio of U.S. defensive costs

to Soviet offensive costs would be favorable,

even before the United States deployed its

defenses and before Soviet offensive countermeasures were known would be even more difficult. Neither side may actually know the relative costs of additional increments of defense

and offense until they actually buy them.27

———.

*’It might be argued that, faced with these uncertainties, the

Soviets would accede to the U.S. proposal for a negotiated transition that regulated offensive and defensive deployments. On

the other hand, drafting such an agreement that both sides would

find equitable, given the asymmetries in forces and technologies

on the two sides, would be a formidable task.

Because the United States and the Soviet

Union have such different economies, it will

be difficult to quantify the cost-exchange ratio. Moreover, the effective cost-exchange ratio may differ from the technical one. That is,

the ratio depends not only on what things cost,

but also on what people are willing to pay. If

the Soviets are willing and able to pay for an

increment of offense that is more costly than

our corresponding increment of defense, for

practical purposes the cost-exchange ratio is

at least even. The SDI objective, then, is to persuade the Soviets that the defenses we can afford will more than offset the offenses they can

afford. Thus the offense/defense cost-exchange

ratio may have to be not just 1.5:1 or 2:1, but

several-to-one.

On the other hand, if the Soviets were to

agree with the United States that a mutual reduction of offensive missile capabilities was

worthwhile and that defenses were desirable,

then the technical challenge could be reduced.

In effect, mutual political decisions could improve the cost-exchange ratio by mandating

reductions—rather than enhancements-of

offensive capabilities, along with limitations

on other offensive countermeasures.

Survivability

One of the many possible types of countermeasures against a BMD system is to attack

the system itself-which will be called “defense

suppression” in this report. Obviously, to carry

out its defensive mission, the BMD system

must survive such attacks. “Survivability”

does not mean the ability of every element–

each satellite, e.g.—to survive any attack.

Rather, it means the ability of the system as

a whole to perform acceptably despite attacks

that may disable some elements.

No BMD system will be either survivable

or not survivable. The question will be, “How

survivable, at what cost?’ The cost-exchange

ratio between defense and offense will have to

be calculated on the basis of the costs of all

kinds of offensive response, including defense

suppression, compared to the costs of all kinds

of defensive counter-countermeasures, including “survivability” measures.
46

The remainder of this report surveys what systems that would meet the effectiveness, afwas—and was not—known as of April 1988 fordability, cost-exchange, and survivability

about the potential of the SDI for developing criteria.
Chapter 3

Designing a BMD System:

Architecture and Trade-off Studies
CONTENTS

The Importance of BMD Architecture Studies. . . . . . . . . . . . . . . . . . . . . . . . 49

Overview of System Architecture Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Goal Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Threat Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Systems Designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Technological Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Operational Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

costs . . . . . . . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Nuclear Force Exchange Models: Deriving Requirements From Goals.... 53

Some Conclusions Drawn From Nuclear Exchange Models. . . . . . . . . . . . 54

Limitations of Nuclear Force Exchange Models . . . . . . . . . . . . . . . . . . . . . 58

System Designs and End-To-End Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Space- and Ground-Based Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Battle Management Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Some Important Results of the System Requirements

and Design Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Important Systems Analysis Work Remaining . . . . . . . . . . . . . . . . . . . . . . . 65

Further Strategic Nuclear Force Exchange Work . . . . . . . . . . . . . . . . . . . . 65

Further System Requirements and Design Work . . . . . . . . . . . . . . . . . . . . 68

Tables

Table No. Page

3-1. Two Perspectives on BMD Effectiveness and Strategic Goals . . . . . . . 55

3-2

3-3

3-4

3-5

3-6

Judgemental Assumptions in Nuclear Force Exchange Models ...’... 58

Phases of Ballistic Missile Trajectory. . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

SDIO’s Phase One Space- and Ground-Based BMD Architecture . . . . 60

OTA’s Projections of Evolution of Ground-and Space-Based

BMD Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Two Representative Battle Management Architectures . . . . . . . . . . . . . 62
Chapter 3

Designing a BMD System:

Architecture and Trade-off Studies

THE IMPORTANCE OF BMD ARCHITECTURE STUDIES

Researchers have performed proof-of-principle experiments for some Strategic Defense

Initiative (SDI) technologies. But many of the

basic technologies for the SDI are still in an

experimental, or even theoretical, stage. Therefore it might seem premature to be designing

full-scale ballistic missile defense (BMD) systems for deployment not only in the mid-1990s,

but in the 21st century. In fact, such designs

are key to assessing the feasibility of achieving U.S. strategic goals through ballistic missile defense. National decisionmakers can only

fully evaluate proposed systems on the merit

of system architectures, not on the promise of

one technology or another. If called upon to

appropriate funds for BMD development and deployment, Congress will be asked to decide upon

an architecture-a specific system design comprising many technologies and components.

Attempting such designs, or “system architectures, as the Strategic Defense Initiative Organization (SD IO) calls them, compels

systematic analysis of all the factors that will

affect SDI feasibility. In the near term, such

analysis helps guide the technology research

effort. In the long term, it will provide the substance of the national debate over whether to

deploy BMD.

System architecture analysis, if done well,

will provide some of the key elements of information upon which to base decisions about

whether to commit the Nation to deploying any

proposed BMD system:

● Specification of Goals. Explicit identification of the particular strategic goals that

BMD system designs will be expected to

achieve (e.g., impose uncertainty on So-

●

●

●

●

●

●

Note: Complete definitions of acronyms and initialisms

are listed in Appendix B of this report.

viet strategic planners); understanding of

those goals in the larger context of U.S.

national security; and cost-effectiveness

comparisons of alternate means, if any,

of achieving the goals.

Specification of Threat. Projections of future Soviet missile and BMD countermeasures that BMD system designs

would be expected to overcome.

System Requirements. Specification of the

missile-interception tasks and sub-tasks

that effective BMD systems would have

to perform to meet the project threats;

specification of passive and active survivability measures for the system.

System Designs. Proposals for integrating

sensors, weapons, and command and control arrangements into BMD systems that

would likely meet system requirements

and that could be practicably modified to

meet changing threats; and specification

of how technologies under research would

be incorporated into a BMD system-such

a design is called a system architecture.

Technology Requirements. Specification of

the technologies needed to build the

weapon systems required by the overall

system design, by the deployment and

maintenance plans, and by plans for

adaptive evolution of the system to meet

changes in the threat; and plan for bringing all technology developments to fruition when needed (full-scale engineering

development plan).

Manufacturing Requirements. Specification of the materials, manufacturing facilities, tools, and skilled personnel needed

to manufacture all system elements.

Deployment and Operations Analyses.

Proposals for how the designed system

can be put into place and maintained (in49
50

●

eluding space transportation requirements); and schedules for doing so.

Cost Estimates. Estimates for what development, procurement, deployment, and

operation of the proposed system design

will cost; and proposals for reducing system costs.

This chapter will focus on two particular

topics:

1. the ways in which system architects for

SDIO have related strategic goals to

BMD system performance needs, and

2. the general characteristics of the system

architectures studied for SDIO.

The concluding sections of the chapter will

identify areas of analysis within those topics

where important work remains to be done.

It would be highly unrealistic now to expect

system architecture studies to be definitive.

Each category of analysis is subject to considerable uncertainty, some of which may

never be resolved by analysis and limited experimentation. The architecture analysis will

necessarily be tentative and iterative: as new

information and ideas emerge, modifications

will be inevitable. Moreover, the findings from

anal sea in each category will and should affect

the findings in other categories. For example,

meeting a particular technology requirement

may be judged possible, but too expensive. The

system architecture design may have to be

modified to utilize another technology to carry

out the same function. On the other hand, new

technological developments may make it cheaper

to carry out a function in a way that previous

analyses had shown to be too costly. For that

reason, the system architects attempt to design “evolutionary” architectures into which

advanced technical developments could be

phased as they became available.

Even after a commitment had been made to

develop a particular technology into a weapon

system, the process of full-scale engineering

development might prove more difficult than

anticipated: alternate systems might have to

be designed and developed. Moreover, while

it is the goal of the architecture analyses to

provide options for meeting a range of potential changes in the offensive missile threat, a

fully deployed BMD system might still have

to be modified in unanticipated ways if the

Soviets were to deploy unforeseen countermeasures.

Despite the necessarily tentative nature of

system architecture analyses, they compel a

coherence in thinking about BMD that would

otherwise be missing. They also bring into the

open the assumptions implicit in the arguments for and against deploying BMD. Because these analyses will inevitably include

assumptions and projections that reasonable

people may disagree about, it is important that

they be carried out competitively, by more than

one group of analysts. Such competition will

give both the Administration and Congress a

basis for identifying the uncertainties, varying assumptions, and alternative projections

of the future that will underlie decisions about

BMD. It will also be important, when these analyses are offered in justification of major decisions, that they be independently evaluated.

Recognizing the importance of system architecture studies, SDIO late in 1984 awarded

contracts to 10 teams of military systems analysis contractors to provide competing analyses at a price of $1 million each. On the basis

of that competition, five teams were chosen

for $5-million, “Phase II” architecture studies,

which were largely completed in mid-1986. In

addition, a sixth contractor provided SDIO

with analytic support to synthesize the findings of the five competitors into a “reference

architecture” to help guide SDI research. As

of this writing, the five Phase II teams had

been awarded additional contracts to continue

some analytic work common to all and to perform some tasks unique to each. Their reports

were due at the end of January 1988.1 It had

1 Three other sets of “architecture” contracts should also be

noted. First, through the Air Force Electronic Systems Division, contracts were awarded to three firms to design battle

management and communications systems for a BMD system

with land- and space-based elements. This work necessitated

definitions of more or less complete BMD system architectures,

thus to some extent paralleling the work of the general system

architecture contractors, The SDIO has subsequently attempted
51

been planned that the five would be narrowed

to two competitors in a final phase, but that

decision was postponed through 1987. Eventually a single contractor team will be chosen

to design a BMD system in detail.2

to better coordinate the parallel work of the battle management

systems analyses and the main system architecture studies.

Second, the Army Strategic Defense Command awarded three

other contracts for study of the battle management and communications systems for BMD composed primarily of groundbased components. Third, late in 1986 SDIO awarded seven

contracts to teams composed of U.S. and European firms to

begin designs of system architectures for European theater defense against intermediate-, medium-, and short-range ballistic

missiles.

‘For the future, SDIO has proposed two new organizations

for carrying out work on system architectures. One organization would bean “SDI Institute, ” a federally (and, specifically,

SD IO) funded “think tank” to monitor the work on the actual

system architecture to be proposed for deployment by SD IO.

The Institute would be independent of particular defense contractors, thus reducing the possibility that the interest of current defense firms in selling hardware to the government would

play a role in architecture designs.

A second new organization is to be a “National Test Bed, ”

which would be a network of computers, communications links,

and some sensor hardware for simulating ballistic missile

defenses. In some cases, the simulations would be purely conceptual, creating a computer “world” of BMD systems and offensive systems, and testing various assumptions about each. In

other cases, this imaginary world might, with simulated incoming and outgoing data, test computer software actually intended

for use in a real BMD system. In yet other cases, actual BMD

This report will offer numerous examples

from the findings of the system architecture

contractors and of SDIO adaptations of such

findings. With a few exceptions, we will not

cite specific contractor sources for those examples. OTA has not undertaken a systematic

analysis and comparison of all the dozens of

documents that emerged from the several contractor studies. Therefore, a few selected citations might give an unfair impression of the

overall performance of any given contractor.

Our purpose here is to convey an understanding of the system architecture analysis process and to report some of the results—not to

conduct management oversight of any Department of Defense (DoD) contractor. In addition,

the system architecture work is continuing,

and constant revision of previous findings is

both necessary and desirable. Thus any given

conclusion might not reflect the current views

of the particular contractor.

hardware tests might be conducted, with data from the computers being fed into an actual test sensor system, and the sensor system sending processed signals back into the computer

simulation. If a full-scale BMD system were deployed, the National Test Bed might then be used for simulated battle exercises of the system.

OVERVIEW OF SYSTEMS ARCHITECTURE ANALYSES

Initially, each of the system architects undertook the same general task of designing

BMD systems whose deployment might begin in the mid-1990s and that might evolve into

more advanced systems after the year 2000.

Each group produced designs that it believed

could, when fully deployed, provide nearperfect interception of Soviet ballistic missile

reentry vehicles (RVs) forecast for deployment

in the mid-1990s.3 Each also argued, however,

3A mid-1990s threat posed against a BMD system that could

not be fully deployed until after the year 2000 is unrealistic.

Not all architects used the same threat numbers for the same

time frames. The architects did, however, project this “baseline” threat into larger numbers of reentry vehicles and decoys

for later years. They also ran “excursions” on the baseline threat

to explore the impacts of larger and smaller threats on defense

effectiveness. The excursions into larger threats, with one exception, do not generally appear in the summary documents

produced by the contractors.

that lesser percentages of interception would

achieve desirable military goals along the lines

described in chapter 2 of this report.

Goal Specification

As part of their analyses, the architects used

computerized strategic nuclear exchange

models (see next section on this topic) to simulate the numerical results of hypothetical nuclear wars between the United States and the

Soviet Union. These simulations assumed various levels of defense capability on the two sides

(in general the projected offensive capabilities

for the mid-1990s were assumed at this stage)

for the purpose of showing what differences

those defenses might make.
52

From these simulations, the analysts drew

conclusions about how defenses might contribute to the goals of security and strategic stability. In chapter 2, we described the kinds of

measures used to define BMD effectiveness.

In this chapter we will further describe some

of the assumptions that went into and conclusions that came out of these strategic exchange

simulations.

Threat Definition

A preliminary step to running the strategic

exchange simulations was to state the Soviet

offensive threat that BMD systems would be

designed to counter. The starting point was

an SDIO-supplied projection of the offensive

missile forces the Soviets might have in the

mid-1990s. From this starting point, the architects made varying “excursions,” positing

possible future Soviet missile developments

and deployments. In addition, they hypothesized various types and numbers of anti-satellite weapons that the Soviets might conceivably deploy to attack space-based components

of BMD systems.

Subsequently, and under different program

managers, SDIO began a “Red Team” program to attempt to anticipate possible Soviet

responses to U.S. BMD deployments. A major project of this program has been to bring

together groups of experts to attempt to design plausible Soviet countermeasures to the

technologies under consideration in other parts

of SD IO. These potential countermeasures are

then presented to SDIO “Blue Teams” so that

they can adapt their technology research and

system designs accordingly.

In mid-1987, SDIO presented to the Defense

Acquisition Board a proposal to proceed with

“concept demonstration and validation”

(“Milestone I“) for the first phase of a “Strategic Defense System” (BMD system) to be

deployed in the mid-1990s. This presentation

included an officially approved “threat”

description for that period.

In reviewing DoD proposals for any BMD system, Congress should understand whether the

officially assumed Soviet threat is “responsive”

—i.e., whether it reflects plausible countermeasures that the Soviets could have taken by the

time the BMD system were full deployed.

System Requirements

In showing what numbers of nuclear weapons would have to be intercepted to provide

various levels of protection for different types

of targets (cf. ch. z), the strategic exchange

models also yielded basic requirements for strategic defense system performance. Additional

“end-to-end” computer simulations helped define requirements for interception at each stage

of flight.

(In SDIO presentations accompanyingmid1987 proposals for an initial, less effective

BMD system, this process was reversed. First,

a number of warheads to intercept was established, then the strategic goals that might be

served analyzed afterward.)

Systems Designs

The system architecture contractors designed BMD systems intended to intercept a

very high percentage of the projected missile

threat. The working assumption was that early

stages of BMD deployment would be stepping

stones to the ultimate goal of protecting cities and people from nuclear ballistic missile attack. The designs were not optimized to less

ambitious goals. For example, systems that

might protect hardened missile silos but could

not serve as elements of city defenses were not

considered. Systems designed from the outset

to preserve nuclear deterrence might well look

materially different from those designed toreplace it altogether.

Each architect was asked to design:

1. a system that was both space-based and

ground-based;

2 one that was primarily ground-based; and

3. one that was intended primarily for defense of U.S. allies against intermediate

and shorter range ballistic missiles.

In the second phase of system architecture

contracts, analysts placed greatest emphasis
53

on the first type of system, somewhat less on

the second, and least on the third. Each architect considered systems that might be deployable in the mid-1990s, but each also offered concepts for more advanced systems that might

be deployed against more advanced Soviet

offensive systems out to the year 2015 or so.

For each case, analysts identified countercountermeasures intended to neutralize Soviet

attempts to penetrate or directly attack the

BMD system.

The details of the systems designs (for example, a given type and number of space-based

rocket interceptors) were built into simulation

models that expanded on the nuclear exchange

models described above. These “end-to-end”

simulations represented the details of intercepting ballistic missiles throughout all phases

of flight, from rocket boost to warhead reentry. Some of the results of these “end-to-end”

simulations are discussed below. These models

also aided “tradeoff” analyses of various types

of BMD system components arranged in various configurations. The models were also used

to evaluate excursions in the technological requirements forced by particular types of So

viet anti-BMD countermeasures.

Technological Requirements

The architects quantitatively analyzed the

relative costs and effectiveness of various approaches to each defensive task. For example,

an analysis might examine trade-offs between

highly capable missile-tracking sensors on a

few high altitude satellites and less capable

sensors on many more low-altitude satellites.

Many of these “trades” are discussed

sequent chapters of this report.

Operational Requirements

in subBecause system designs are still preliminary,

it is difficult to specify their exact operational

requirements. The system architects did attempt to estimate the continuing space transportation and maintenance requirements for

space-based systems over their lifetime. Other

SDI programs are conducting research on the

logistics of maintaining various space-based

and ground-based systems.

cost s

In general, system architects estimated

costs for their nearer-term, “interim” designs—

those not including directed-energy weapons

for boost-phase missile interception. These systems were estimated to cost on the order of

$200 billion, depending on the projected need

to respond to various types of Soviet countermeasure. Costs of complementary air defense

systems were not included. It should be recognized that, given the conceptual nature of

the architectures, accurate cost-estimating is

virtually impossible at this stage. It does appear that, with thousands of space platforms

envisaged, considerable changes would be

needed in the way such equipment is now designed and manufactured if space-based BMD

systems were ever to be affordable. In addition, a major new space transportation system

would have to be designed, developed, manufactured, and deployed.

NUCLEAR FORCE EXCHANGE MODELS:

DERIVING REQUIREMENTS FROM GOALS

The SDI system architects-and several simulation models assume various U.S. and Soother groups as well-have run several types viet offensive nuclear force levels, beginning

of strategic nuclear exchange computer simu- with U.S. Government estimates for 1995.

lations to try to show how defenses might af- Then they assume various strategic targeting

feet the U.S.-Soviet nuclear balance. These plans on the two sides and analyze how the
54

attempted execution of those plans might be

affected by various levels of defense capability on the two sides.

The intermediate measure of defense effectiveness is usually the percentage of nuclear

warheads intercepted or its complement, the

number of “leakers.” The models translate the

numbers of leakers in various cases into numbers or percentages of different types of targets surviving the attack. (For examples of

such target types, see ch. 2, box 2A.) Each

type of target, in turn, is given a different

weight based on judgments about how U.S.

and Soviet leaders might value them. Thus the

numbers of different types of targets surviving are translated into “surviving strategic

value.”4 The percentage of surviving strategic value on the two sides is then linked with

particular strategic goals. (For a discussion of

goals for BMD and ways of measuring BMD

effectiveness, see ch. 2.) In some cases, “leakage” rates were linked (via asset survival expectations) to strategic goals to show what

kind of BMD system performance would be

needed given a particular assumed level of

offensive threat (for example, see table 3-l).

Some Conclusions Drawn From

Nuclear Exchange Models

Strategic Goals and Defense Leakages

The system architects’ strategic nuclear exchange simulations provide a useful basis for

studying BMD performance goals. However,

because each architect used a different computer model and different assumptions for the

sizes and compositions of future U.S. and Soviet offensive nuclear forces, the results are

difficult to compare.

With that important qualification, here are

some conclusions drawn frequently (but not

universally) by the different system architects.

First, for a mid-1990s Soviet strategic nuclear

4

In these models the Soviets are assumed to have a larger

number of strategic targets than the United States, and the

Soviet targets are assumed to be harder to destroy. Part of the

difference is due to the existence of numerous nuclear-hardened

shelters for Soviet political leaders; see Soviet Military Power,

1987, (Washington, D, C.: Department of Defense) p. 52,

threat, a BMD system that allowed a few thousand Soviet RVs to penetrate into the United

States might complicate Soviet attack plans,

but probably would not stop them from destroying most of their chosen targets.5

In support of SDIO’s mid-1987 proposal for

an initial BMD system, other SDIO contractors argued that a strategy of “adaptive

preferential defense” might prevent the

Soviets from destroying as high a percentage

of certain sets of targets as they would wish

(as estimated by U.S. analysts).

A system that allowed fewer Soviet RVs to

leak through would begin to deny the Soviets

certainty of destroying many of the military

targets that their planners might have designated. But if the Soviets chose to concentrate

on economic targets in the United States, they

might still be able to deny the United States

the possibility of economic recovery from the

nuclear war. (Compare this finding with the

second set of projections in table 3-l.)

With yet lower leakage, the Soviets could

still inflict immense damage on the United

States. Note, for example, that 10 percent of

an attack with 10,000 nuclear weapons would

still result in 1,000 nuclear weapons exploding in the United States. But since the Soviets

could not be sure which 1,000 of the 10,000

launched would reach which targets, confidence in achieving precise attack goals on a

given set of targets would be low.

Analyses also seem to show that if the

United States had a relatively highly effective

BMD system against a mid-1990s Soviet threat

while the Soviets had no BMD, the Soviets

would improve their relative strategic situation more by adding defenses to limit damage

to themselves than by adding offensive weapons in hopes of increasing the damage they

could inflict on the United States.6 In attempt-

“–s The exact percentages in this conclusion and the others below were apparently classified by the system architecture contractors because the computer simulations from which they were

derived include classified estimates of U.S. and Soviet military

capabilities.

6 This conclusion assumes that the addition of offenses could

not improve the leakage rate—the same percentage of every

added group of warheads would be intercepted, This is not necessarily a valid assumption: much would depend on the composition of the offensive and defensive forces on the two sides.
55

Table 3-1.—Two Perspectives on BMD Effectiveness and Strategic Goals

Soviet warheads

leaking through Expected strategic consequences

A. One system architect’s strategic exchange model and conclusions

Many Increase in Soviet attack planning uncertainties. They are forced to launch all their strategic forces at

Fewer

Extremely few

Assumptions:

once or reduce their military objectives. A strategic exchange would result in more losses to Soviets

than to the United States.

The Soviets could no longer reliably achieve the military goals of a strategic nuclear attack while

maintaining a secure reserve of missiles for later attacks. Preserves full range of U.S. strategic offensive force retaliatory flexible response options. Each new Soviet ballistic missile has only a fractional

chance of being useful.

Survival of a large portion of the population and industrial base, a high proportion of military targets

other than strategic offensive forces, and sufficient strategic offensive forces to preseve full range of

U.S. retaliatory flexible response options. If Soviets attack only other military targets (not strategic

offensive forces), medium-high survival of those assets.

Would preserve the full range of U.S. “flexible response” options in war with the Soviets even if

Soviets devoted entire attack to U.S. strategic offensive forces (presumably only if Soviets do not have

comparable BMD capability —OTA).

Assured survival of the Nation as a whole: 3 to 5°/0 U.S. casualties in population attack.

Assured survival: Soviet ability to put U.S. population at risk is negligible; the United States needs no

strategic nuclear retaliatory capability.

● Mid-1990s projections of Soviet and U.S. strategic forces.

● Effectiveness of Soviet BMD not specified.

• Status of air defenses not specified.

Alternate analysis: As U.S. strategic defenses improved, an option for the Soviets would be to change their offensive target

priorities to maintain a deterrent “assured destruction’ capability. Instead of concentrating their forces on hardened missile

silos, for example, they might concentrate them on key military industries or other economic targets; they might even focus

on cities per se. Various non-SDIO analysts have previously calculated potential consequences of such nuclear attacks, as

indicated below.

B. If the Soviets retargeted to maintain assured destruction

10 ”/0

30/0

1 to 2%

The Soviets attack industries in the 71 largest U.S. urban areas; the equivalent of 500 l-megaton and

200 to 300 100-kiloton weapons get through. Of the U.S. population, 35 to 45 percent is killed or injured; 60 to 65°/0 of U.S. industry is destroyed.b

The Soviets attack industries in the 71 largest U.S. urban areas; the equivalent of 100 l-megaton and

200 to 300 100-kiloton weapons get through. From prompt blast and radiation effects, 20 to 30°/0 of

U.S. population is killed or injured; 25 to 35% of U.S. industry is destroyed.c

Case 1: The Soviets attack 77 U.S. oil refineries; the equivalent of 80 l-megaton weapons get through.

From prompt blast and radiation effects, 5 million Americans die. The U.S. economy is

crippled. d

Case 2: The Soviets attack 100 key military-industrial targets with the equivalent of 100 l-megaton

weapons. Three million die of blast and radiation effects, another 8 million from fires; dead

and injured total 10 to 16 million.e

Case 3: The Soviets attack 100 U.S. city centers with the equivalent of 100 l-megaton weapons. Fourteen million die from blast and radiation effects alone, a total of 42 million die from blast, radiation, and fires; total dead and injured are 32 to 51 million.f

● Total Soviet strategic attack of 10,000 weapons.

● Air defenses equallv effective as BMD.

Assumptions:

. .

aA@t~d from Martin Marietta Aer~~pace analyses, percentages of weapons leaking and assets SlJrviVl?19 deleted fOr SeCIJdty ClaSS\fiCatiOrl re&30nS.

bFrom us, congre~sj ~conom~c and Social Consequences of ~uclear Attacks On the (/rr/ted Sfates, A Study Prepared for the Joint Committee on Defense Production,

Published by the Committee on Banking, Housing, and Urban Affairs, U.S. Senate (Washington, D. C.: US. Government Printing Office, 1979), pp. 4-14,

Clbid.

dFrom us, congress, Office of Technology Assessment, ~tre ~ffects of Nuc/ear War(Washington, D, C.: U. S. Government printing Off iCe, May, 1979), pP 64-75. Calculations on casualties were performed for OTA by the the U.S. Defense Civil Defense Preparedness Agency. About 125 500-kiloton weapons would have the same blast

effects as 60 l-megaton weapons, but the pattern of distribution of blast might in fact do more damage.

‘William Daugherty et al., “The Consequences of ‘Limited’ Nuclear Attacks on the United States,” /rrternat/orra/ Security, spring 1966 (vol. 10, No. 4), p 5 F!ndings

based on the authors’ computer simulations. About 160 500-kiloton weapons have about the same blast effects as 100 l-megaton weapons.

‘Ibid,
56

ing to assess the effect on deterrence of various levels of defense, the strategic analysts

compared the amount of damage the Soviets

might suffer (as a weighted percentage of given

types of targets) with the amount the United

States might suffer. Differences in surviving

(value-weighted) percentages of military targets were assumed to confer strategic advantages or disadvantages that would affect Soviet decisions about how to respond to U.S.

weapon deployments, whether to go to war,

or whether to escalate a conflict to nuclear exchange.

Even very low leakage of the BMD system

(and assuming comparable leakage of airbreathing nuclear weapon delivery vehicles)

could still kill several million Americans, if that

were the Soviet objective. (Note that the alternative projections in table 3-1 suggest higher

possible casualties.) This level of protection

(given the mid-1990s projected nuclear threat)

might assure survival of the United States as

a functioning nation but would not assure survival of the whole population. Most of the system architects appeared to believe that in the

long run they could design systems capable

of keeping out a very high percentage of Soviet ballistic missile RVs (assuming the mid1990s projected threat); none appeared to believe that leakage levels compatible with “assured survival” of the U.S. population would

be possible without negotiated limitations of

Soviet offensive nuclear forces.

U.S.-Soviet Asymmetries

With varying degrees of clarity, the system

architects’ use of nuclear exchange models

brought out the current-and likely future–

asymmetries between U.S. and Soviet offensive nuclear forces. The Soviet Union has more

ballistic missile RVs than the United States.

More of the Soviet RVs are based on land than

on submarines, while the reverse is true of the

U.S. RVs. The United States has more strategic nuclear bombers and air-and sea-launched

cruise missiles than the Soviet Union, while

the Soviet Union has a more extensive air defense system than the United States.

If the Soviet Union had ballistic missile

defenses comparable to those of the United

States, the net effect of trying to defend our

land-based missiles against a Soviet strike

would be to reduce the U.S. ability to carry

out planned retaliatory missions. Here is why.

If defended, a sizable number of U.S. landbased missiles that might otherwise have been

destroyed on the ground might survive a Soviet offensive strike. On the other hand, they

would then have to survive defensive attacks

as they attempted to carry out their retaliatory missions against Soviet territory. In addition, the U.S. submarine-launched missiles

(SLBMs), which would not benefit from the defense of land-based missiles, would also have

to face Soviet defenses. Furthermore, if the intercepted SLBMs were aimed in part at Soviet air defense assets, such as radar sites, the

ability of U.S. bombers and cruise missiles to

carry out their missions might also be impaired.

Besides the asymmetries in weapons, there

are asymmetries in targets on the two sides.

The Soviet Union, for example, reportedly has

more than 1,500 hardened bomb shelters for

its political leadership. The Soviets also are

said to spend copious sums on other types of

civil defense. The combination of passive defense measures and BMD might do more to

protect valued Soviet targets than BMD alone

would to protect valued U.S. targets.

Given the asymmetries in U.S. and Soviet

weapons and defenses, then, the net effect of

mutual deployments of comparable levels of

defense could be to weaken, not strengthen

deterrence-if deterrence were still measured

primarily by the penalty that we could impose

on Soviet aggression through nuclear retaliation. (If deterrence were measured by denial

to the Soviets of some attack goals other than

reducing damage to the Soviet Union, then deterrence might be strengthened.)

The United States might compensate for

U.S.-Soviet asymmetries in three ways:

1. The United States could attempt to build

and maintain BMD that was notably su-
57

perior to that of the Soviet Union, so that

a greater proportion of the smaller U.S.

ballistic missile force could be expected

to reach its targets. This was the recommendation of at least one of the SDI system architects, who argued that until very

high defense effectiveness levels had been

reached, equal defensive capabilities on

the two sides might confer an exploitable

strategic advantage on the Soviet Union

(SDIO officials disagree with this

assessment).

2. The United States could attempt to maintain and improve the ability-of its airbreathing weapons (bombers and cruise

missiles) to penetrate Soviet air defenses

so that the loss in effectiveness of our ballistic missiles was offset by the other

means of nuclear delivery. This course was

assumed in the calculations of a second

system architect.

3. If U.S. strategic defenses against all types

of nuclear threat (air-breathing as well as

ballistic missile) could be made extremely

effective, we might not care about imbalances in punitive abilities on the two sides;

the Soviets would have little or nothing

to gain by threatening nuclear attack.

Then, even a minimally destructive retaliatory ability on the U.S. side should fully

deter the Soviets from even contemplating attack. This was the ultimate goal hypothesized by all the system architects.

(It should be noted that most, though not

all, analysts believe that this kind of deterrence now exists. If so, BMD would not

significantly reduce the risk of nuclear

war. 7)

However, some would argue that future Soviet “counterforce” capabilities, plus Soviet

civil defense and perhaps active (BMD and air

defense), could reduce prospective Soviet damage to levels acceptable to them. A U.S. BMD

system, it is argued would either maintain the

survivability of the U.S. deterrent, or equalize the prospective damage on the two sides,

or both.

In sum, the force exchange models employed

by some of the SDI system architects seem to

show that BMD performance levels must be high

to substantially alter the current U.S.-Soviet

strategic nuclear relationship:

●

●

●

●

Some increments of uncertainty could be imposed on Soviet planners by defenses able

to intercept about half the Soviet missile

force. If an “adaptive preferential defense”

strategy could be executed, significant fractions of some sets of “point” targets might

be protected.

The ability to intercept a high percentage

of all Soviet strategic nuclear weapons including air-breathing ones (assuming threats

projected for the mid-1990s) might actually

deny the Soviets the ability to destroy many

military targets.

However, at such levels of defensive capability, because of asymmetries in U.S. and

Soviet strategic postures, U.S. missile and

air defenses might have to perform conspicuously better than Soviet defenses to prevent the Soviets from holding an apparent

strategic advantage.8

The design of a system that could, in the

long term, protect U.S. cities from potential nuclear destruction seems infeasible

without sizable, presumably negotiated, reductions in Soviet offensive forces.

At the conclusion of this chapter, we return

to the subject of nuclear force exchange models

to indicate the scope of future work OTA believes should be carried out if a decision on

BMD development and deployment is to be

considered fully informed.

‘That is, given the threat of retaliatory punishment, it would

be highly irrational for the Soviets to start a nuclear war. In

this view, whatever calculations the Soviets may make about

the “military effectiveness’ of their ballistic missiles, the price

(in damage to the Soviet Union) would be too high to justify

a nuclear attack.

‘However, if the United States maintained a substantial

bomber-cruise missile threat, if Soviet air defenses were ineffective, and if the Soviets did not pose a substantial bomber-cruise

missile threat to the United States, such a Soviet advantage

might be avoided.
58

Limitations of Nuclear Force

Exchange Models

Although force exchange analysis is important, applying the results of the analyses requires extreme caution. The greatest danger

lies in accepting the numbers generated by the

computer as representing reality: they do not.

The verisimilitude of a computer simulation

can only be checked by comparisons with measured results in the real world that the model

is trying to simulate. There has never been—

and we all hope there will never be—a real nuclear war to calibrate the correctness of nuclear

force exchange models.

Instead, such models combine what is known

or estimated about the characteristics of weapons and potential targets on each side with a

myriad of personal, even if carefully considered, judgments about how nuclear attacks

would take place and what the immediate physical results might be. If national leaders are

to make wise use of the outcomes of such analytic models, they need to judge whether they

agree with the assumptions that go “into the

models (see table 3-2).

Aside from the many subjective judgments

that must go into force exchange models, there

are other aspects of the real world that cannot

be included in a quantitative computer simulation. The models generally include estimates

of prompt casualties from nuclear attacks, but

they do not even attempt to account for the

longer term medical, social, political, and ecoTable 3-2.—Judgmental Assumptions in Nuclear Force

Exchange Models

●

●

●

●

•

•

●

●

Soviet valuation of Soviet targets

Estimation of U.S. targets selected by Soviet planners

Priorities Soviets would attach to destroying particular

targets

Soviet estimates of the reliabilities and capabilities of their

weapons

Soviet estimates of the reliabilities and capabilities of U.S.

weapons

U.S. estimates of the reliabilities and capabilities of U.S.

weapons

U.S. estimates of the resistance or vulnerability to nuclear

attack of various Soviet targets

Estimates of casualties on both sides from nuclear attacks

SOURCE: Office of Technology Assessment, 1988.

nomic consequences of nuclear war. Computer

simulations also abstract strategic calculations

out of political context. We can only guess,

with varying degrees of informed judgment,

under what circumstances the Soviets would

contemplate starting or risking nuclear war.

We do not know how leaders on either side

would actually behave in a real nuclear crisis.

We do not know, in particular, how and to what

degree their decisions would be affected by military planners’ strategic exchange calculations.

In sum, nuclear force exchange models can

serve as a useful tool for thinking about the

goals we might use BMD to pursue. But they

cannot demonstrate as scientific fact that

those goals will be accomplished, nor can they

offer certainty that the effects of deploying

BMD would fulfill predictions.

SYSTEM DESIGNS AND END-TO-END MODELS

Force exchange models such as those described above can help analysts estimate how

many nuclear weapons a BMD system must

intercept to achieve various levels of protection. In this way, decisionmakers can set the

overall requirements for BMD performance.

Much more detailed analysis is needed to evaluate systems designed to meet those requirements.

This kind of analysis begins, as do force exchange analyses, with projections of the Soviet missile threat during the period for which

one expects to have BMD deployed. In this

case, however, analysts must consider more

than the destructive capabilities of the offensive missile threat. Analysts must also estimate the precise technical performance of the

missiles, the numbers of each type, and the tac-
59

tical plans under which the Soviets might

launch them. In addition, the analysis has to

include possible changes in Soviet offensive

forces 9 in response to U.S. BMD deployments.

Among the techniques used for this kind of

analysis are “end-to-end” computer simulations, which model both the offensive attack

and the roles of each type of BMD component,

from the sensor that first detects an enemy

missile launch to the last layer of interceptors

engaging reentry vehicles as they approach

their targets.

As table 3-3 indicates, an ICBM flight includes four broad phases: the boost, post-boost,

mid-course, and reentry or terminal. System

architects for SDI have proposed ways of attacking ballistic missiles in all phases.

Space- and Ground-Based Architectures

Suggested components and functions of a

multi-phase BMD system are outlined in tables 3-4 and 3-5. (Chs. 4 and 5 examine the technology for many of these components in considerable detail.) The SDI system architects

subdivided the primarily space-based architectures into nearer- and farther-term BMD sys-

‘Including offensive countermeasures such as decoys and defense suppression measures such as anti-satellite weapons.

terns, with the nearer-term systems envisaged

as evolving into the farther-term systems as

the Soviet missile threat grows and as more

advanced BMD technologies become available.

Except for the projected timing, the architecture in table 3-4 reflects SDIO’s proposal in

mid-1987 for a first-phase ‘Strategic Defense

System.” The design would also be intended

to lay the basis for expansion into phase two

and three systems.

The architectures in table 3-5 draw on information provided by SDIO, but do not constitute their-or anyone else’s—specific proposal

for what the United States should plan to deploy. Instead, the examples provide a framework for analyzing how the parts of a future

BMD system would have to fit together to try

to meet the requirements set for it. The tables

do include the leading candidates for sensors,

discrimination, and weapons described by the

system architects. The projected dates in the

tables reflect OTA rather than SDIO estimates

for the earliest plausible periods over which

each phase might be deployed if it were proven

feasible.

The SDI system architects subjected their

various BMD constructs to detailed computer

simulations. (These are called “end-to-end”

simulations because they attempt to model

Table 3-3.—Phases of Ballistic Missile Trajectory

Phase Duration Description

Boost . . . . . . . . . . . . . . . .Several 10s to 100s of secondsa Powered flight of the rocket boosters lifting the missile

payload into a ballistic trajectory

Post-boost . . . . . . . . . . . . 10s of seconds to 10s of Most ICBMs now have a “post-boost vehicle” (PBV), an

minutesb upper guided stage that ejects multiple, independently

targetable reentry vehicles (MIRVs) into routes to their

targets. If these RVs are to be accompanied by decoys to

deceive BMD systems, the PBV will dispense them as

well.

Mid-course . . . . . . . . . . . About 20 minutes (less for RVs and decoys continue along a ballistic trajectory, several

SLBMs) hundred to 1,000 kilometers up in space, toward their

targets.

Reentry . . . . . . . .......30 to 60 seconds RVs and decoys reenter the Earth’s atmosphere; lighter

decoys first slow down in the upper atmosphere, then

burn up because of friction with the air; RVs protected

from burning up in friction with the air by means of an

ablative coating; at a preset altitude, their nuclear

warheads explode.

af.Jow in the hundreds of seconds, in the future boost timeS may be 9reatlY reduced.

bpo5t.boo5t dispersal tlme~ may a150 be shodened, though perhaps with penalties in payload, numbers of mid-course decoys, and aCCUraCY,

SOURCE Office of Technology Assessment, 1988.
60

Table 3-4.—SDIO’s Phase One Space- and Ground-Based BMD Architecture

Component Number Description Function

First phase (approximately 1995-2000):

Battle Management Variable

Computers

Boost Phase Several at high altitude

Surveillance and

Tracking Satellite

Space-based Interceptor 100s at several 100s of

Carrier Satellite km altitudes

Probe 10s

or

Space Surveillance and 10s

Tracking System

or

Space-based Interceptor

Carrier Satellites loos

Exe-atmospheric 1000s on ground-based

Interceptors (ERIS) rockets

May be carried on sensor

platforms, weapon platforms,

or separate platforms; groundbased units may be mobile

Infrared sensors

Each would carry about 10 small

chemical rockets or “SBIs”;

might carry sensors for

tracking post-boost vehicles

Ground-launched rocket-borne

infrared sensors

Satellite-borne infrared sensors

Satellite-borne infrared sensors

Rocket booster, hit-to-kill

warhead with infrared seeker

Coordinate track data; control

defense assets; select

strategy; select targets;

command firing of weapons

Detect ballistic or ASAT missile

launches by observing hot

rocket plumes; pass

information to tracking

satellites

On command, launch rockets at

anti-satellite weapons

(attacking BMD system),

boosters, possibly PBVs.

Acquire RV tracks, pass on to

ERIS interceptors

Cued by satellite-borne or

rocket-borne infrared sensors,

home in on and collide with

RVs in late mid-course

SOURCE: Office of Technology Assessment, 1988.

BMD performance from booster launch to final RV interception.) Such simulations help

show the interdependence of the system components and the requirements posed for the

technologies that go into them. These analyses show that, at least in the long run, intercepting a substantial portion of the missiles

in the boost phase and early post-boost phase

would be essential to a highly effective BMD

system. This conclusion follows from the fact

that 1,000 to 2,000 boosters could dispense

hundreds of thousands of decoys that would

greatly stress mid-course interception.10

The system architects noted that this boostphase interception task would eventually (barring sizable offensive arms limitations) have

to be accomplished by means of directed-ener10SDIO officials point out that an arms control agreement reducing offensive forces would make the defensive job easier and

cheaper. On the other hand, the Soviets may not be persuaded

to enter into such an agreement unless they can be shown that

potential defensive options would make offensive countermeasures on their part futile.

gy weapons, rather than by the space based

interceptors (SBIs) envisaged for the first

stage of BMD deployments. The speed-f-light

velocity of directed energy would be needed

because the development of faster-burning

rocket boosters and faster-dispensing postboost vehicles (PBVs) would eventually permit Soviet missiles to finish their boost phases

before the space-based interceptors (SBIs)

could reach them.

The SDIO contends, however, that interception of PBVs may suffice to meet SDI goals.

Although a fast-burn booster would burn out

inside the atmosphere, the PBV must clear the

atmosphere to dispense light-weiglit decoys.

It then would be vulnerable to SBIs. If SBI

interception of PBVs were adequate, directedenergy weapons might not be necessary. If successfully developed, though, they might prove

more cost-effective.

The interplay of offensive and defensive technologies is discussed in more detail in chapters 6, 10, and 11 of this report.
61

Table 3-5.—OTA’s Projections of Evolution of Ground- and Space. Based BMD Architecture

Component Number Description Function

Second phase (approximately 2000-2010) replace first-phase components and add:

Airborne Optical 10s in flight Infrared sensors

System (AOS)

Ground-based Radars

High Endo-atmospheric

Interceptors

Space Surveillance and

Tracking Satellite

(SSTS)

Space-based Interceptor

Carrier

Space-based Neutral

Particle Beam (NPB)

Detector Satellites

10s on mobile platforms

1000s

50-100 at few 1000s of

km.

1000s at 100s of km

altitudes

10s to 100s at altitude

similar to SSTS

100s around particle

beam altitudes

X-band imaging radar

Rocket with infrared seeker, nonnuclear warhead

High-resolution sensors; laser

range-finder and/or imaging

radar for finer tracking of

objects;

May carry battle management

computers

Each carries about 10 small

chemical rockets or “KKVs”.

at low altitude; lighter and

faster than in phase one

Atomic particle accelerator

(perturber component of

interactive discrimination;

additional sensor satellites

may be needed)

Sensors to measure neutrons

gamma rays from objects

bombarded by NPB;

transmitters send data to

SSTS and/or battle

management computers

1

or

Track RVs and decoys, pass

information to ground battle

management computers for

launch of ground-based

interceptors

Cued by AOS, track RVs as they

enter atmosphere; discriminate

from decoys, pass information

to ground battle managers

Collide with RVs inside

atmosphere, but before RV

nuclear detonation could

cause ground damage

Track launched boosters, postboost vehicles, and ground or

space-launched ASATs;

Track RVs and decoys,

discriminate RVs from decoys;

Command firing of weapons

On command, launch rockets at

anti-satellite weapons

(attacking BMD system),

boosters, PBVs, and RVs

Fire hydrogen atoms at RVs and

decoys to stimulate emission

of neutrons or gamma rays as

discriminator

Measure neutrons or gamma

rays emitted from RVs: heavier

objects emit measurable

neutrons or gamma rays,

permitting discrimination from

decoys

Attack boosters and PBVs

Third phase (approximately 2005-2115), replace second-phase components and add:

Ground-based Lasers, 10s of ground-based Several laser beams from each

Space-based Mirrors lasers; 10s of relay of several ground sites bounce

mirrors; 10s to 100s off relay mirrors at high

of battle mirrors altitude, directed to targets by

battle mirrors at lower

altitudes

SOURCE: Office of Technology Assessment, 1988.

Battle Management Architecture

Specifying Battle Management Architecture

Any BMD system architecture will contain

a kind of sub-architecture, the “battle management architecture. ” The battle management

design shows how BMD system components

would be integrated into a single coordinated

operating entity. The battle management software, which would direct the battle management computers and control the actions of the

system, would carry the burden of integration.

A communications system would transmit

data and decisions among the battle management computers and between the computers

and the sensors and weapons.

The system would probably divide the volume in which the battle would be fought into

a set of smaller battle spaces. A regional or

local battle manager would consist of the bat-
62

tle management software and computer with

responsibility for controlling the resources

used to fight within a particular battle space.

The battle manager and the resources it controlled would be known as a battle group. The

battle management architecture specifies the

following:

●

●

●

●

●

●

the physical location of the battle management computers and the nodes of the

communications network;

the method for partitioning resources into

battle groups so that battle management

computers have access to and control over

appropriate numbers and kinds of sensors

and weapons;

a hierarchical organization that specifies

the authority and responsibility of the battle managers, similar to a military chainof-command;

the role of humans in the battle management hierarchy;

the method used for coordinating the actions of the battle managers through the

battle management hierarchy and across

the different battle phases so that handover of responsibility, authority, and resources between boost, post-boost, midcourse, and terminal phases would take

place smoothly and efficiently; and

the organization of and the method used

for routing data and decisions through the

communications network, probably organized as a hierarchy that would govern

how the nodes of the network were connected.

Battle management architectures proposed

so far have varied widely in their approach to

these issues. For example, some architects proposed placing their space-based battle management computers on the same satellite platforms as the Space Surveillance and Tracking

System (SSTS), some on the carrier vehicles,

and some on separate battle management platforms; some proposed that the battle managers

exchange track information only among neighbor battle managers at the same level of the

battle management hierarchy, while others

proposed that the same data also be exchanged

between upper and lower levels; some architects permitted humans to intervene in the

midst of battle to select different battle strategies while others allowed humans only to authorize weapons release.

Table 3-6 describes two different battle management architectures that are representative

of those proposed. It shows the physical locations of the battle managers, the criteria used

for partitioning resources into battle groups,

the data exchanged by the battle managers,

the methods used for coordinating responsibility and authority between phases of the battle, the degree to which human intervention

would be allowed during battle, and the structure of the communications network.

Interaction Between Battle Management

and System Architecture

Battle management architectural decisions

would strongly affect the size, complexity, and

organization of the battle management softTable 3-6.—Two Representative Battle Management Architectures

Design by

location of

battle Data exchanged by Method of coordinating Degree of Communications

managers Partitioning criterion battle managers between battle phases human intervention network organization

Design i: Local battle groups Object tracks Regional battle Humans authorize Two-tiered hierarchy

assigned to cover managers control weapons release at

SSTS specific Earth-based hand-over between start of battle; can

geographic areas phases switch strategies

during battle

Design ii: Initially geographic, Health (weapon status) All battle managers Humans authorize All nodes in line-ofthen by threat tube information use same criteria for weapons release at sight of each other

Carrier (the path along target allocation, start of battle are interconnected

vehicles which a group of taking into account

missiles travels) locations of other

battle managers

SOURCE: Office of Technology Assessment, 1988.
63

ware. Because of the close relationship between

the battle management computers and the

communications network, such decisions also

would strongly affect the software that controlled the computers forming the nodes of the

communications network. A good example of

the interaction among system architecture,

battle management architecture, and battle

management and communications software is

represented by the controversy over how

widely distributed battle management should

be. The two extremes of completely centralized and completely autonomous battle managers and a range of intermediate options are

discussed in both the Fletcher and Eastport

group reports and considered in all the architectural studies.11

Physical Organization v. Conceptual Design

Analyses often have reflected confusion between the physical organization and the conceptual organization of the battle managers.

The physical organization may be centralized

by putting all of the battle management software into one large computer system, or be distributed by having battle management computers on every carrier vehicle. Similarly, the

software may be designed as:

1. a single, central battle manager that controls the entire battle;

2. a hierarchy of battle managers, with local battle managers each responsible for

a small battle space, regional battle managers responsible for coordinating among

local battle managers, and a central battle manager coordinating the actions of

the regional battle managers; or

3. as a set of completely independent battle

managers with no coordination among

each other.

Any of these three software designs might be

implemented using either a centralized or dis-

“Report of the Study on Eliminating The Threat Posed by

Nuclear Balh”stic Missiles, Vol. V, Battle Management, Communications, and Data Processing, October 1983. This was the

only unclassified volume of the Fletcher commission report. See

also “Eastport Study Group—A Report to the Director, Strategic Defense Initiative Organization” (Eastport Study Group,

Marina Del Rey, CA, 1985).

tributed physical organization. Variations on

the three designs, e.g., introducing more levels

into the battle manager hierarchy, are possible, but infrequently considered.

The physical organization and the conceptual design would impose constraints on each

other, and factors such as survivability and

reliability would drive both. A widely distributed physical design, involving many independent computers, would impose too heavy

a synchronization and communications penalty among the physically distributed components of the software to permit use of a centralized conceptual design: the attendant

complications in the software would make the

battle manager unreliable and slow to react.

Physical distribution requires the battle management software on each computer to be relatively autonomous. A system with completely

autonomous battle managers would perform

less well than a system with communicating

battle managers. Accordingly, even a widely

distributed physical organization would likely

require some communications and synchronization among the battle managers.

A centralized physical design might not provide sufficient computer processing power for

acceptable performance, but would significantly improve communications among the

battle managers. The result might simplify the

software development, and lead to greater software reliability. On the other hand, such an

organization might result in a poorly survivable system: if the central computer were disabled, the remainder of the system could not

function.

Integrating Battle Management Architecture

With System Architecture

Since the system architecture, physical battle management organization, and battle management software design affect each other, all

should be considered together. The relationships and interfaces among the battle managers should be defined either prior to or togetherwith definition of the physical organization of

the battle managers and their requirements for

communication with each other and with sensors and weapons. As the Fletcher report
64

stated, “The battle management system and

its software must be designed as an integral

part of the BMD system as a whole, not as an

applique.“ 12

Most of the SDI architectures proposed so

far have shown little evidence of an integral

design. Software design has been largely ignored, giving way to issues such as the location of the battle management computers and

the criteria for forming battle groups. The

SDIO has reported that it is attempting to better integrate overall system architecture

studies and battle management studies in its

current phase of system architecture contracting. However, the system proposed in mid-1987

for “demonstration and validation” seemed to

reflect no such integration.

Some Important Results of the System

Requirements and Design Work

Systems analysis for SDI is still, necessarily,

at a preliminary stage. Its most valuable contribution so far has probably been the identification of key issues that research would

have to resolve satisfactorily before the Nation

could make a rational decision to proceed to

development and deployment of BMD. In particular, the analyses have shown the following:

Boost-Phase Interception

Adequate boost-phase interception of missiles is essential to make the mid-course and

terminal interception problems manageable;

otherwise, the offense has the opportunity to

deploy so many decoys and other penetration

aids that they could swamp the other defensive layers. However, an adequate boost-phase

interception may, over time, be countered by

new of ensive weapons and still have done its f

job: after deploying all the faster burning

boosters and PBVs it could afford to counter

the boost-phase defense, the offense may not

be able to deploy enough decoys to overwhelm

the mid-course defense.

12Ibid.

Ultimate Need for Directed-Energy Weapons

As a corollary to the need for effective boostphase interception, it will be important to have

a credible long-term system design which includes directed-energy weapons based in space

to carry out boost-phase interception against

boosters and PBVs that are too fast to be

reached by kinetic energy weapons. Without

such a credible plan, the boost-phase interceptors would face fairly predictable obsolescence.

(It is possible, however, to imagine the development of new SBIs able to penetrate the upper atmosphere; if launched quickly enough,

they could then reach some boosters.)

Need for Interactive Discrimination

Because of the potential for Soviet deployment of hundreds of thousands of decoys that

passive sensors may not be able to differentiate from RVs disguised as decoys (“ anti-simulation”), mid-course interception is likely to require means of perturbing RVs and decoys and

highly capable sensors to detect the differences

in the ways the two kinds of objects react. Such

means of “interactive discrimination” have

been conceived but not yet built and tested.

Interdependence of Defensive Layers

Ideally, independent layers of sensors and

weapons would carry out interception of each

phase of ballistic missile trajectory, thus eliminating common failure modes and common

nodes of vulnerability to hostile action. In fact,

for practical reasons, the system architects

generally produced designs with considerable

degrees of interdependence. In addition, as

noted above, even if the functions of each layer

were performed entirely independently, failure

in one phase of interception (the boost-phase,

for example) can severely affect the potential

performance of succeeding phases.

Importance of Integrated Battle Management

Architecture

Initially, system architecture and battle

management architecture studies were separately contracted for, producing large discrepancies among those who had studied each
65

subject the most. The two sets of studies are

apparently now being better integrated, and

presumably subsequent designs will reflect

that integration.

Distributed Battle Management

Although considerable work on designing

BMD battle management remains, analysis so

far makes clear the importance of a battle management system that make decisions in a distributed, as opposed to centralized, fashion.

Attempting to centralize the decisionmaking

would both impose excessive computing, soft- b ware engineering, and communications requirements and make the system more vulnerable

to enemy disruption.

Heavy Space Transportation Requirements

The system architecture designs now permit

better forecasts of the requirements imposed

by space-based systems for space transportation capabilities-capabilities far beyond those

the United States now possesses. (Primarily

ground-based architectures do not share this

problem.)

Requirements for Assured Survival

There appears to be general agreement on

the importance of significantly reducing offensive force developments if one hopes to provide mutual assured survival for the U.S. and

Soviet populations.

IMPORTANT SYSTEMS ANALYSIS WORK REMAINING

The SDI architecture studies have just begun to address the complex problems of designing a working, survivable BMD system

with prospects for long-term viability against

a responsive Soviet threat. Thus far, the architecture studies have served the useful purpose of helping to identify the most critical

technologies needing further development. Future system designers would have to integrate

the technologies actually available-and mass

producible–into deployable and workable

weapon systems.

Given that the system architects and SDIO

are just over 2 to 3 years into an analytic effort that will take many more years, it is not

a criticism to say that much work remains.

However, it appears to be the case that the analysis supporting the first-phase architecture that

SDIO proposed in mid-1987 simply did not address many key questions. The following are further tasks that analysts should carry out to help

both the executive and legislative branches judge

the potential effects of decisions on BMD.

Further Strategic Nuclear Force

Exchange Work

The strategic nuclear exchange modeling

done so far by the SDI system architects provides a useful beginning to the larger and

lengthier task of developing the information

that will be needed for a national decision on

whether to deploy BMD. If the limitations of

these kinds of simulations are borne carefully

in mind, they can help one to understand how

BMD might affect the calculations of U.S. and

Soviet national leaders, both indecisions about

peace and war and indecisions about long-term

strategic policies. They can also help to clarify the assumptions all participants bring to

the U.S. national debate about BMD.

Introduce Comparability Among Analyses

It is desirable to have competing sets of computer simulation models for analyzing the same

questions. In that way, decisionmakers could

compare differing conclusions and identify the

underlying assumptions of each. (Comparisons

could also uncover errors in implementation

of the models.) Analysts should run different

models using the same sets of data about the

Soviet missile threat, the same configurations

of defensive systems, and the same offensive

and defensive strategies and tactics. Thus far,

differences in these elements have made the

analyses of the system architects difficult to

compare and judge.
66

Further Analyses of Soviet

Offensive Responses

The simulations run so far have examined

only limited variations on Soviet attack plans

in the face of growing U.S. defensive capabilities: the assumption is made that the Soviets

have an inflexible list of targets. The Soviets

are assumed to optimize their exact attack plan

to destroy the highest possible number of those

targets at some level of confidence. Suppose,

however, that if defenses drastically reduced

Soviet confidence in their ability to destroy

hardened military targets, they concentrated

on softer military and economic targets.

Analysts must carry out further exploration

of this possibility if decisionmakers are to understand the full implications of BMD for all

types of deterrence (see table 3-l).

Assumptions About Deterrence

An analytic focus on an inflexible Soviet target plan seems to be related to a simplified

model of potential Soviet motives for attack.

The usual working assumption seems to be

that the Soviets would decide to launch a nuclear strike on the United States on the basis

of calculations about the probabilities of destroying certain percentages of various types

of targets. In this view, above a certain threshold for one or more of these probabilities, the

Soviets would be willing to strike, and below

it they would not because they could not accomplish their military purposes. One target

set would be the weapons and command-andcontrol facilities that would permit a U.S. nuclear retaliation. But the exact role in Soviet

decisionmaking attributed to fear of retaliation–as opposed to accomplishment of other

military objectives—remains unclear. The nuclear exchange models should make more explicit

their assumptions about the weighings given

to denial of military objectives as opposed to the

likelihood and intensity of U.S. retaliation as enforcers of deterrence.

Analysts should attempt to identify the increment of uncertainty added to the Soviet calculus of nuclear war provided by levels of defensive capability that might increase Soviet

uncertainty about achieving attack objectives,

but that could not assure denial of those objectives. Many things could go wrong with a nuclear attack precisely scheduled to achieve a

specific set of goals (such as knocking out a

given percentage of U.S. retaliatory capability). How much uncertainty would a given level

of BMD add to that which already exists?

What are the potential Soviet responses to this

additional uncertainty?13 To what extent would

the increment of uncertainty strengthen deterrence? At what cost per increment of

strengthened deterrence?

Strategic Stability Analyses

Closely related to the question of Soviet attack motivations is the question of strategic

stability. In its 1985 report on BMD, OTA emphasized the importance of exploring this question thoroughly.

A simplified approach to crisis stability is

as follows: in a military confrontation with the

United States, Soviet decisionmakers would

calculate whether or not they could achieve a

given set of military objectives by launching

a strategic nuclear first strike. If the objectives

seemed attainable, they would strike; if not,

they would refrain. The system architects have

considered this scenario.

Another possibility they should address,

however, is that Soviet perceptions of a likely

U.S. first strike might affect Soviet behavior.

System architects have been understandably

reluctant to run or to report extensively on

simulations in which the United States is assumed to strike first. Such analyses might imply to some that a change is being contemplated in U.S. policy not to launch a preemptive

strategic nuclear first strike. Nevertheless,

such analysis needs to be done, not because

the United States would launch such an attack,

but because the Soviet Union might not believe that it would not.

13A possibility suggested by one reviewer of the OTA study

is that the Soviets discover, unbeknownst to the United States,

a way of disabling the U.S. BMD system (perhaps by spoofing

its command and control system). Further, the Soviets validate

their countermeasure with undetected techniques before actually launching an attack. Certain that their technique will work,

and their offensive forces augmented in response to the U.S.

defensive deployments, the Soviets in this scenario end up more

certain about the probable success of their attack than before,
67

It is conceivable, for example, that Soviet

strategic exchange calculations could show

that a U.S. first strike, backed up by U.S.

BMD, might allow the United States to reduce

significantly the damage from a Soviet “ragged”

retaliation.14 On the other hand, a Soviet first

strike might have an analogous effect. If the

Soviets believed that the United States, expecting a Soviet strike, might strike first, then

the Soviets might try to get in the first blow.

Thus, they would not make their decision to

strike on the basis of accomplishing a clear set

of military objectives, but instead on the basis of choosing the less terrible of two catastrophic outcomes.

Even if the Soviet Union and the United States

avoided a nuclear crisis in which such calculations might play a role, the calculations could

still influence the longer-range Soviet responses

to U.S. BMD deployments. The Soviets might

decide that it was extremely important to them

to maintain a “credible” nuclear threat against

the United States, and therefore be willing to

spend more on maintaining offensive forces than

“cost-exchange” ratios would seem to justify.

Administration officials have repeatedly

stated their desire to negotiate (or find unilaterally) a “stable transition” path to a world in

which strategic defenses play a large role. Finding such a path would require careful analysis

of the incentives presented to Soviet leaders by

U.S. actions. Estimating the consequences of a

hypothetical U.S. attack is one key part of such

an analysis. Only then might U.S. analysts

identify offensive and defensive force levels

that both sides could believe served their security. Some of this analytic work has been

started, but more is necessary.

U.S. Responses to Soviet BMD

It is entirely possible that the Soviet Union

will not wait until the United States decides

whether deploying BMD is a good idea or not,

but instead will unilaterally choose to expand

14A “ragged” retaliation is one carried out after the first strike

has destroyed at least portions of the nation’s strategic forces

and possibly degraded its command and control system, resulting in a relatively unstructured, diluted counter-attack.

its own BMD system.15 The United States conducts BMD research in part to be able to respond in kind to such a decision. The system

architects for SDI have conducted simulations

to show how a responding U.S. BMD deployment might restore the U.S.-Soviet strategic

balance. Before the United States chose such

a response, however, two other kinds of analysis are desirable. First, analysts should compare the BMD option with the option of circumventing Soviet BMD by means of increasing

U.S. air-breathing, low-flying cruise missile

forces. Second, researchers should determine

the ability of U.S. technology to find adequate

offensive countermeasures to Soviet BMD.

These questions are partly amenable to the

strategic exchange modeling technique. In the

first case, the model could assume various

numbers of cruise missiles with varying levels

of probability of penetration in battle scenarios

in which Soviet BMD was degrading the ability of U.S. ballistic missiles to get through.

Analysts could compare these outcomes to

those of similar scenarios in which the U.S. deployed BMD instead of additional cruise missiles. Then they could estimate quantities of

BMD and cruise missiles required to produce

similar outcomes. This information could provide the basis for cost-effectiveness comparisons between BMD and cruise missiles once

data on the actual costs of the two types of

systems became available.

Similarly, analysts could plug into the simulations the increases in warhead penetration

of Soviet defenses caused by U.S. offensive

countermeasures. Once estimates were available for the costs of these countermeasures,

analyses could develop some idea of the relative cost-effectiveness of offense and defense.

lsAs ~fitt~ by the ABM Treaty, the Soviets have retained

a limited, nuclear-armed ballistic missile defense system in the

Moscow area; they are currently expanding the system to the

full 100 interceptors permitted by the treaty, and could conceivably replicate the system elsewhere. They have also constructed a series of phased array radars around the Soviet Union

which would provide warning and limited battle management

capabilities for such an expanded system.
68

Analysis of Alternate Defensive Measures

The lesser goals of strategic defense–that

is, enhancing deterrence by increasing Soviet

uncertainty or denial of various military

objectives—have thus far been considered as

preliminary benefits on the way toward extremely high degrees of population protection.

Therefore, alternate means of achieving the

lesser goals as ends in themselves have not

been analyzed. A few examples might clarify

this point.

Defense of Land-Based ICBMs.–If strengthening deterrence by increasing the survivability of U.S. land-based retaliatory forces, especially ICBMs were the goal of deploying BMD,

then the system designs done for the SDI

might not be optimal.18 Instead, ground-based,

low-altitude interceptors located relatively

near the missiles to be defended might be less

expensive (unlike cities, hardened missile silos

or capsules might withstand low-altitude nuclear explosions). In addition, the United

States would want to consider how it could use

various forms of mobile or deceptive basing

of ICBMsin conjunction with limited BMD

to make the enemy’s cost of attacking the missiles prohibitive.

Careful analysis of the goal of protecting

strategic bomber bases from SLBMs launched

not far off U.S. shores might also yield different BMD designs combined with different

bomber basing tactics.

Defense of Command, Control, and Communications Facilities.—Similarly the strategic

goal of increasing the survivability of the U.S.

command and control system for nuclear forces

might be achieved by some form of BMD, but

the United States should also compare the cost

and effectiveness of BMD with those of other

measures for making the system more resistant to nuclear attack. Further analysis might

show that some combination of passive survivability measures and BMD would be more

cost-effective than either alone.

Defense Against Accidental or Terrorist Missile Launches.—Protecting the country against

10 or so incoming reentry vehicles is a much

different task than protecting it against thousands. While SDI-designed systems might offer such protection as a side-benefit, if this kind

of defense were to be the major goal of deploying BMD, one would consider different, much

simpler and cheaper architectures than those designed for the SDI.17

Further System Requirements

and Design Work

Analyze Additional Threats to

BMD System Survivability

The SDI system architects recognized that

survivability would be a critical feature of any

BMD system. They devoted considerable effort and ingenuity to inventing ways to reduce

system vulnerability to Soviet attack. The

chief threat to survivability they examined was

ground-based, direct-ascent anti-satellite

weapons—rockets that the Soviets could “pop

up” from their territory to attack U.S. spacebased BMD assets with nuclear or non-nuclear

warheads. This was a reasonable first approach

to the survivability problem: such weapons

probably represent the kind of defense suppression weapon most immediately available to the

Soviets. If the defense could not counter this

threat, then there would be no point in exploring other, more sophisticated threats.

In the second round of their “horse race”

competition the system architects did very little analysis of other potential threats to BMD

system survivability, particularly longer-term

space-based threats. The threat of “space

mines, ” satellites designed specifically to

shadow and destroy the various space-based

BMD components, was not considered in

depth. Moreover, no analysis assumed that the

Soviets might deploy in space a BMD system

Wee U.S. Congress, Office of Technology Assessment, AfX

Missile Basing, OTA-ISC-140 (Washington, DC: U.S. Government Printing Office, September 1981).

“For example, a few ground-based, long-range interceptors

like the Exe-atmospheric Reentry Interceptor System (ERIS)–

see ch. 5—could cover the continental United States; existing

early-warning radars could give initial track information and

a few “pop-up” infrared sensor probes provide final track information.
69

comparable to that of the United States; thus

the potential vulnerabilities of such weapon systems to one another were not considered. Instead, it was assumed that the United States

would, for the most part, militarily dominate

near-Earth space. From the statement of work

provided to the SDI system architecture contractors late in 1986, it remained unclear

whether this assumption would be changed in

the follow-on studies to be completed early in

1988.

Develop Realistic Schedules

The system architects were originally instructed to design systems that might enter

full-scale engineering development in the early

1990s and be deployed beginning in the m.id1990s. The systems they designed would have

required challenging technical achievements

even under the originally requested SDI

budgets. For example, one system architect

pointed out that a vigorous technology program did not yet exist for an active space-based

sensor crucial to an “interim” defense intended

for deployment in the mid-1990s. Or, to take

another example, deployment in the mid-1990s

of the space-based systems identified by the

architects would require that the United States

decide almost immediately to begin acquiring

the massive space transportation system that

deployment would require.18

Given the actual levels of SDI funding appropriated by Congress thus far, mid-19% deployment of the kinds of systems initially proposed

by the system architects is clearly not feasible.

Even with the requested funding, it is unlikely

that researchers could overcome all the technological hurdles in time to permit confident

full-scale engineering decisions in the early

1990s. Nor is it clear that the full-scale engineering process, including establishment of

manufacturing capabilities for the complex

systems involved, could be completed in just

3 or 4 years. (For example, the most optimistic expert estimate OTA encountered for engineering full-scale SDI battle management soft18TheSD1O rque~$Z!jo million in SUPPleIIIWIt~ funds for

fiscal year 1987 to develop technology for low-cost space transportation.

ware was 7 years.) In short, the systems

designated as “interim” (similar to those labeled “Second Phase” in table 3-5) by the system architects would not be likely to reach full

operational capability until well after the year

2000.

Late in 1986, SDIO called on its contractors

to orient their work to a much scaled-back system architecture, with scaled-back strategic

goals (see the “First Phase” in table 3-4).

Speculations emerged in the press about “early

deployment” options under consideration.

Analysis of the “phase one” designs, however,

suggests that even they could not be ready for

initial space deployment until at least the mid1990s. Nor could they be fully in place much

before the end of the century.

In the meantime, the Soviet Union might well

deploy practical countermeasures against such

systems. Specifically, many in the defense community believe that the Soviets could deploy

decoys along with their reentry vehicles that

would greatly stress the minimal mid-course

discrimination capability of a phase-one system. In addition, the Soviets could at least begin to deploy new booster rockets that would

drastically reduce the effectiveness of spacebased interceptors (SBIs) in boost-phase defense.

Even if the United States could deploy SBIs

beginning in the mid-1990s, another question

remains: how confident do U.S. decisionmakers

wish to be in the long-term viability of BMD

before they decide to deploy such systems?

Given the state of research on directed-energy

devices for BMD, it is highly unlikely that U.S.

leaders could have sufficient information by

the early 1990s to determine whether full-scale

engineering development of phases two and

three would be feasible in the following decade. Thus, an early 1990s decision implies a commitment to a space-based BMD whose obsolescence would be made highly probable by the

prospect of faster burning Soviet missile

boosters, but whose replacement would remain

unproven.

Develop Credible Cost Estimates

The SDIO has properly pointed out that trying to estimate total life-cycle costs for an un-
70

precedented system is difficult. The aerospace

industry would have to manufacture new components and weapons in new ways. The Nation would need a new space transportation

system for a space-based system. The SDIO

has agreed to estimate “cost goals” to indicate the kind of investment that the Nation

would have to make in proposed BMD architectures. The system architects were instructed

to develop cost estimates in their 1987 studies.

Develop Methods for Estimating

Cost-Exchange Ratios Between

Defense and Offense

As this report pointed out in chapter 2, one

key criterion for the technical feasibility of the

SDI scenario of transition to a “defense-dominated’ world is that there be a favorable costexchange ratio between defense and offense.

The system architects did try to address this

issue in various ways, but there still seems to

be no systematic approach toward it. The problem will be intrinsically difficult, because estimating in advance the costs of the U.S. BMD

system will be difficult, estimating the costs

of Soviet responses will be more difficult, and

predicting Soviet estimates of these quantities will be most difficult of all. Nevertheless,

analyses should at least begin to specify what

information would permit sufficient confidence

that the defense/offense cost-exchange ratio is

high enough to justify going ahead. The system

architecture contractor teams were instructed

to address the problem in their 1987 work.

Assess the Role and Costs of

Complementary Air Defenses

The Strategic Defense Initiative Organization is specifically limited to defense against

ballistic missiles. The Air Force has undertaken an “Air Defense Initiative, ” though at

funding levels far below that of the SDI. Nevertheless, at least at the systems analysis level,

U.S. decisionmakers need an integrated understanding of the role that air defense would have

to play if ballistic missile defense were to

achieve such goals as increasing Soviet uncertainty about attack success, denying Soviet

abilities to destroy high percentages of certain

types of targets, or protecting the population

from nuclear attack. Moreover, insofar as

BMD requires air defense to accomplish its

purposes, the feasibility and affordability of air

defense against possible Soviet attempts to circumvent BMD need to be included in any ultimate analysis of the feasibility of BMD.
Chapter 4

Status and Prospects of Ballistic

Missile Defense Sensor Technology
CONTENTS

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Proposed SDI Sensor Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Sensor Technology.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Sensor Technology Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......101

Phase 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................101

Phase 2....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...............101

Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...............102

Box

Box Page

4-A. Sensor Resolution Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Figures

Figure No. Page

4-i.

4-2.

4-3.

4-4.

4-5.

4-6a

4-6b

4-7.

4-8.

Major SDI Sensors and Weapons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Relations Between Temperature and Electromagnetic Radiation . . . . 75

Scanning Pattern for Satellite Sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Illustration of Three Techniques for Estimating the ThreeDimensional Position of a Target in Space . . . . . . . . . . . . . . . . . . . . . . 83

Spectral Response of Two Objects at Different Temperatures . . . . . . 87

Diffraction-Limited Range for Ten-Meter Resolution . . . . . . . . . . . . . . 89

Range Limitedly Number of Detectors for Ten-Meter Resolution . . 89

Mirror Size Plotted v. the Operating Wavelength of a

Sensor System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Illustration of an Imaging Radar Viewing a Spinning

Conical Target . . . . . . ........ .. .. .... .... . . . . . . . . . . . . . 93

Tables

Table No. Page

4-1. Summary of Typical Sensor Requirements. . . . . . . . . . . . . . . . . . . . . . . 84

4-2. Key Issues for Passive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4-3. Key Issues for Interactive Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Chapter 4

Status and Prospects of Ballistic

Missile Defense Sensor Technology

INTRODUCTION

Much of the public debate on ballistic missile

defense (BMD) technologies centers on futuristic weapon systems such as lasers, rail guns,

and particle beams. The Strategic Defense Initiative Organization’s (SDIO) initial BMD system design, however, does not include any of

these exotic weapons.1 Rather, it calls for

space-based interceptors (SBI) to collide with

Soviet intercontinental ballistic missile (ICBM)

boosters and post-boost vehicles (PBVs), and

for high acceleration ground-based missiles to

destroy Soviet reentry vehicles (RVs) by direct impact. The sensor systems required to

detect, identify, and track up to several hundred thousand targets may be more challenging than the actual kinetic energy weapons:

it may be more difficult to track targets than

to destroy them, once tracked.

The technical feasibility of a first-phase deployment, then, may depend primarily on major technical advances in the areas of sensors

and chemically propelled rockets, and less on

the availability of rail-gun or laser weapons systems. Accordingly, this report emphasizes

these more conventional technologies.

Nonetheless, the more exotic weapons technologies could become important in second-or

*Some BMD architecture contractors did, however, call for

rather exotic beam sources for “interactive discrimination, ” in

which targets would be exposed to sub-lethal doses of particle

beams or laser beams and their reactions measured to distinguish between reentry vehicles and decoys. See section on interactive discrimination.

Recently, SDIO officials have spoken of “entry level” directedenergy weapons that might constitute part of second-phase

BMD deployments. The utility of such weapons would depend

on the pace and scope of Soviet countermeasures.

Note: Complete definitions of acronyms and initialisms

are listed in Appendix B of this report.

third-phase BMD systems deployed in response to Soviet countermeasures. For example, if the Soviet Union deployed fast-burn

boosters that burned out and deployed their

RVs (and decoys) before they could be attacked

by slow-moving chemically-propelled rockets,

then laser weapons might be essential to attack ICBMs in their boost phase. These directed-energy weapons (DEW) would require

even more accurate sensors, since their beams

would have to be directed with great precision.

Thus, the required sensor technology improvements might continue to be at least as stressing as weapons technology requirements.

Some of the major sensor and weapon components proposed by Strategic Defense Initiative (SDI) system architects for both near- and

far-term deployments are listed in figure 4-1

(also see ch. 3). This chapter describes sensors;

weapons, power systems, communications systems, and space transportation required to implement a global BMD system are described

in chapter 5. For each technology, chapters 4

and 5 discuss:

● the type of system suggested by SDI architects,

● the technical requirements,

● the basic operating principles,

● the current status, and

● the key issues for each technology.

The systems aspects of an integrated BMD

system are discussed in chapter 6. Computing

technologies are discussed in chapter 8. Technologies for offensive countermeasures and

counter-countermeasures are deferred until

chapters 10 through 12 (as of this writing,

available only in the classified version of this

report).

73
74

Figure 4-1— Major SDI Sensors and Weapons

NPB

detector

SDI sensor systems:

BSTS-Boost Surveillance and Tracking System (infrared sensors)

SSTS-Space Surveillance and Tracking System (infrared, visible, and possibly radar or laser radar sensors)

AOS-Airborne Optical System (infrared and laser sensors)

TIR-Terminal Imaging Radar (phased array radar)

NPB-Neutral Particle Beam (interactive discrimination to distinguish reentry vehicles (RV’s) from decoys; includes separate

neutron detector satellite)

SDI weapons systems:

SBI-Space-Based Interceptors or Kinetic Kill Vehicles (rocket-propelled hit to kill projectiles)

SBHEL-Space-Based High Energy Laser (chemically pumped laser)

GBFEL-Ground-Based Free Electron Laser (with space-based relay mirrors)

NPB-Neutral Particle Beam weapon

ERIS-Exoatmospheric Reentry vehicle Interceptor System (ground-based rockets)

HEDI-High Endoatmospheric Defense Interceptor (ground-based rockets)
75

SENSORS

Sensors are the eyes of a weapons system.

In the past the human eye and brain have constituted the primary military sensor system.

A soldier on the battlefield would:

●

●

●

●

●

●

●

look over the battlefield for possible

enemy action (surveillance);

note any significant object or motion (acquisition);

determine if the object was a legitimate

target (discrimination);

follow the enemy motion (tracking);

Aim his rifle (weapon direction), fire;

look to see if he had killed the target (kill

assessment); and

if not, reacquire the target (retargeting),

aim, and shoot again.

Ballistic missile defense entails these same

functions of target surveillance, acquisition,

discrimin ation, tracking, weapon direction, kill

assessment, and retargeting. BMD sensors,

however, must have capabilities of resolution,

range, spectral response, speed, and data storage and manipulation far beyond those of the

human eye-brain system.

Proposed SDI Sensor Systems

The following sections describe five representative sensor systems. Most of the five SDI

system architecture contractors (see ch. 3) recommended some variation of these sensor systems. The primary attack phase and recommended sensor platforms for each type are

summarized in tables 1-1 and 1-2.

Boost Surveillance and Tracking System

(BSTS)

The BSTS would have to detect any missile

launch, give warning, and begin to establish

track files for the individual rockets. Most system architects proposed a constellation of several satellites in high orbit.

Typical BSTS characteristics are summarized in the classified version of this report.

Each BSTS would carry a sensor suite that

would monitor infrared (IR) emissions from the

Figure 4-2. - Relatlons Between Temperature and

Electromagnetic Radiation

Target radiation bands

Temperature scales (Peak radiation wavelength)

1 1 I

6,000 Kelvin (OK) 373 273

rocket plumes (see figure 4-2). From their very

high altitude, these sensors would have relatively poor optical resolution. Track files could

be started, but the Space Surveillance and

Tracking System (SSTS) or other sensors at

lower altitude might be required to achieve the

track file accuracy needed for some BMD

functions. 2

2Space-based interceptors (SBIs), formerly called “space-based

kinetic kill vehicles” (SBKKV), which have their own horning

sensors, could operate with the resolution given by a BSTS

sensor.

77

would supply adequate coverage around the

world for submarine-launched missiles.3

Redundancy would be necessary for survivability and for stereo viewing of the targets.

These SSTS satellites might be essential for

much of the mid-course battle, so some SSTSS

must survive at most locations.4

The SSTS satellites would carry one or more

long-wave infrared (LWIR) sensors for tracking the somewhat warm PBVs and cold RVs.

These LWIR sensors could not detect RVs by

looking straight down against the relatively

warm earth background. Rather, they would

look only above the horizon, in a conical or

“coolie hat” pattern which would afford the

necessary cold space background for the IR

detectors. Thus each SSTS would monitor targets that were far from the satellite. Those targets closest to each SSTS would pass below

its sensors, undetected; they would have to be

observed by more distant SSTS satellites (see

figure 4-3). This problem could be alleviated

if sensing at other wavelengths, e.g., in the visible range, were to be feasible.

For some missions, such as cueing DEW sensors, the SSTS might include short-wave infrared (SWIR) and medium-wave infrared

(MWIR) sensors to track booster exhaust

plumes. This would duplicate to some extent

the BSTS function, but with much better resolution.5 These sensors might have limited fields

of view, so that each SSTS platform would require several IR sensors to cover all the

threats. These SWIR/MWIR sensors could

look down against the Earth background, since

they would be monitoring the hot plumes.

Several architects recommended placing laser systems (and some suggested microwave

radars) on the SSTS. Lasers might be needed

‘More recent SDI studies have recommended fewer satellites.

4Akematively, pop-up IR probes on ground-based rockets

could observe the midcourse battle. These probes would have

to be based at high latitudes to get close enough to observe

the beginning of mid-course missile flight. Otherwise, they could

be based in the northern United States to view the late midcourse.

5An SSTS could not achieve the pointing accuracy needed by

DEW satellites; each DEW platform would have to carry its

own high-resolution optical sensor. An SSTS constellation might

aid the battle manager in designating targets for DEWS.

Figure 4-3. -Scanning Pattern for Satellite Sensor

SSTS

Above-the-horizon LWIR

conical scan pattern

A

"Collie hat” above the horizon scan pattern for the LWIR sensors on the SSTS which could only detect the cold RV's against

the cold background of space. The targets labeled “A” could be

detected by this SSTS platform, whereas the closer targets labeled “B” could not be detected against the warm earth background. These “B” targets would have to be tracked by another,

more distant SSTS satellite.

to designate or illuminate targets for homing

space-based interceptors (SBIs). Laser radar

(Ladar) systems might be required for all of

the interactive discrimination systems, just to

determine the target’s position with sufficient

accuracy. This would be particularly true for

tracking cold RVs, which could be passively

detected mainly by LWIR sensors with inherently poor resolution,

6 or for discriminating

and designating an RV in the presence of

closely spaced objects (that often are decoys).

In any case, a laser radar could supply the

range to the target, which is necessary to generate three dimensional track files from a single platform.

‘The resolution angle of a sensor is directly proportional to

wavelength; long wavelengths such as LWIR produce large rese

lution spots in the sensor focal plane, or large uncertainty in

the target’s location. Therefore shorter wavelength laser radars

may be needed to accurately measure target position.
78

The SSTS might also carry some battle management computers, since the SSTSS would

be above the battle and to some extent less

vulnerable than lower altitude weapons platforms, and because they would generate most

of the track-file information essential for assigning targets to weapon platforms.

The SSTS originally conceived by the system architects for ballistic missile defense now

appear too complicated, too expensive, and

possibly too far beyond the state of the art of

sensor technology for deployment in this century. As a result, there was some discussion

in late 1986 and early 1987 of launching early

SBIs without any SSTS sensor, placing minimal sensor capability on each SBI carrier vehicle instead. There would probably be no sensor capability enabling SBIs to kill RVs in

mid-course.

The phase-one architecture submitted to the

Defense Acquisition Board in June and July

of 1987 was vague about mid-course sensors:

there was a “Midcourse Sensor” (MCS) program, but no system concept. The MCS might

consist of SSTS sensors, or ground-based surveillance and tracking (GSTS) rockets or

“probes,” or SWIR/MWIR (or other) sensors

on some of the kill vehicle carrier satellites.

These sensors would apparently locate targets

for the ground-based exe-atmospheric reentry

vehicle interceptor system (ERIS) interceptors.

More recently, an MCS study proposed a combination of the three sub-systems.

The SDIO ended development work on the

original SSTS program and let new contracts

in mid-1987 to design a less complex SSTS system. The classified version of this report contains the range of parameters specified by the

original, more comprehensive system architectures. The new designs could not by themselves

furnish precise enough data to direct SBIS to

RV targets.

Airborne Optical Adjunct (AOA)

The AOA would test technology for a new

sensor addition to terminal defensive systems.

The SAFEGUARD BMD system, operated in

partial form in the 1970s, relied exclusively on

large, phased-array radars to track incoming

warheads. There were no optical detectors, The

resolution and range of these ground-based radars was adequate (assuming they survived)

to direct nuclear-tipped Spartan and Sprint

missiles to the general vicinity of target RVs.

Such radars would not be adequate as the only

guidance for the non-nuclear, hit-to-kill vehicles proposed for SDI: these interceptors would

require on-board homing guidance systems.

The AOA would test LWIR technology similar to that in the SSTS program, but deploy

it on an aircraft flying over the northern United

States. The sensor system has been designed

and is being fabricated. Above most of the atmosphere, this sensor could look up against

the cold space background and track RVs as

they flew through mid-course. Resolution would

be relatively coarse: a follow-up system based

on this technology might eventually be able

to direct ground-based radars, which in turn

would hand target track data over to high

speed hit-to-kill projectiles. These projectiles

would derive their final target position from

on-board homing sensors. The AOA aircraft

might also include laser range-finder systems

to supply accurate estimates of the distance

to each target-and possibly to discriminate

Photo credit: Strategic Defense Initiative Organization

Airborne Optical Adjunct (AOA)

In a strategic defense system, airborne sensors might

be used to help identify and track targets and to guide

ground-based interceptors to them. The AOA will

validate the technology to acquire targets optically at

long ranges, and to track, discriminate and hand data

over to a ground-based radar. It will also provide a data

base that would support future development of airborne optical systems. Sensors have been fabricated

and tested and test flights will take place soon. The

model shows the sensor compartment on top and the

crew stations in the interior of the aircraft.
79

decoys from RVs by measuring minute velocity changes caused by drag in the upper

atmosphere.

System architecture contractors proposed

tens of AOA-like aircraft as part of a sensor

system. Some proposed rocket-borne, pop-up

probes with LWIR sensors for rapid response

in a surprise attack until the aircraft could

reach altitude.

There is some uncertainty regarding the infrared background that an airborne sensor

such as AOA would see. Sunlight scattered

from either natural or (particularly) man-made

“noctilucent clouds” might obscure the real

RV targets. These clouds form at altitudes

from 60 to 100 kilometers (km). During a battle, the particles ablating from debris reentering the atmosphere would form nucleation

centers. Long-lived ice crystals would grow at

these centers, possibly creating a noisy infrared background that would obscure the real

targets arriving later. Intentional seeding of

these clouds is also a possibility.7

Ground-Based Radar (GBR)

Large phased-array, ground-based X-band

(8-12 GHz frequency) radars might work in conjunction with optical sensors to track and discriminate incoming warheads from decoys.

These radars could receive target track data

from those sensors and then use doppler processing to create a pseudo-image of the warheads by virtue of their spinning motion. Nonrotating decoys or decoys with different shapes

or rotation rates would produce different radar signatures.

Ground-based radars would also measure the

effects of the atmosphere, identifying light decoys that would slow down more than the

heavy RVs. These radars might guide or cue

the endoatmospheric HEDI and FLAGE-like

interceptor rockets and the ERIS exoatmospheric interceptors (see ch. 5).

‘See M.T. Sandford, II, A Review of Mesospheric Cloud

Physics, Report No. LA-10866 (Los Alamos, NM: Los Alamos

National Laboratory, October 1986.)

The GBR concept very recently supplanted

the proposed Terminal Imaging Radar (TIR)

system in SDIO planning. The latter would

have had a much shorter range (thereby not

being useful for cueing the ERIS interceptor)

and much less resistance to anti-radar countermeasures, such as jamming. Some radar

concepts call for deployment on railroad cars

to evade enemy attack.

Neutral Particle Beam (NPB) Interactive

Discrimination

While several interactive discrimination

techniques have been proposed (see section be

low on interactive discrimination), the NPB approach has thus far received the most attention and development funds.

A series of full space-based tests was planned

for the early 1990s, but has been subjected to

budgetary cutbacks. A 50-MeV8 NPB source

was to be placed in orbit along with a sensor

satellite and a target satellite to measure beam

characteristics and to begin interactive tests.

The primary detection method would be to

monitor the neutrons emmitted by the target

after irradiation by the NPB, although gamma

rays, x-rays, and ultraviolet radiation might

also be useful for indicating whether targets

had been hit by the neutral particle beam. The

NPB accelerator might be located 1,000 km

from the target. The neutron detectors might

ride on separate detector satellites closer to

targets, although they could be collocated on

the NPB platform under some circumstances.

A single NPB discrimination accelerator system might weigh 50,000 to 100,000 kilograms

(kg), making it the heaviest element proposed

for a second-phase BMD.9 Over 100 NPB satellites and several hundred neutron detector

‘The energy of a beam of particles is measured in ‘‘electron

volts” or “eV,” the energy that one electron would acquire traveling through an electric field with a potential of one volt. The

energy of beam weapon particles would be so high that it is

measured in rniUions of electron volts, or “MeV.” One MeV is

equal to 1.6x1O –

‘3 joules; each particle carries this amount of

energy.

‘A far-term, robust BMD system might also include very

heavy directed-energy weapons.
80

platforms might be required for a global discrimination system.10

Sensor System Requirements

Technical requirements for BMD sensors are

discussed below for each sensor function: surveillance, target acquisition, identification,

tracking, and kill assessment.

Surveillance and Target Acquisition

Requirements

A surveillance and target acquisition system

would have to detect the launch of any missile, either ground-based or submarine-based,

and render accurate positional information to

the BMD weapon system. Some SDI weapon

systems would require very high resolution

sensors. A laser beam, for example, would have

to be focused down to a spot as small as 20

to 30 cm in diameter to produce the lethal intensity levels for projected hardened missiles.11

A DEW sensor must therefore determine the

missile location to within a few tens of cm so

as to keep the laser focused on one spot on the

target.

As an illustration of what is practical or impractical, note that if the sensor were placed

in geosynchronous orbit at 36,000 km, just a

few sensor satellites could survey the entire

earth. But at this high altitude the sensor’s

angular resolution would have to be better than

8 nanoradians, or one part in 125,000,000.12

‘“Between 100 to 200 flights of the proposed Advanced Launch

System (ALS) might be required to lift a full constellation of

100 NPB diSCIbliIl ators into space. For a discussion of the number of elements in a useful NPB system, see American Physical

Society, Su”ence and Technology of Directed Energy Weapons:

Report of the American Physical Society Study Group, April

1987, pp. 152 and 335.

1 I For e~~ple, a 90 Mw laser operating at one micrometer

(pm) wavelength would require a mirror as large as 10 m in diameter to achieve the very high brightness IOX’ W/sr) required

to destroy hardened (i.e., able to resist 20 KJ/cm2) targets. A

10 m mirror would would project a 20-cm diameter spot at 2,000

km or 40 cm at 4,000 km, which are typical ranges for the proposed directed energy platforms. See chapter 5 on directed

energy weapons for more details.

“One radian is equal to 57.3 degrees; one nanoradian is

IxIO-

’ radian or one billionth of a radian.

This high resolution is clearly beyond the realm

of practical sensor systems.13

Resolution improves directly with reduced

distance to the target. Therefore a reasonable

alternative-one being examined-would be to

place many sensor satellites at lower altitudes.

Even a constellation of sensor satellites at altitudes around 4,000 km would not be adequate

for directed energy weapons: positional uncertainties for sensor satellites combined with

vibration and jitter would preclude the transmission of target positions to weapon platforms with 10-cm accuracy. Therefore each

DEW satellite would need its own sensor to

provide the final pointing accuracy. Sensor satellites might supply broad target coordinates

to each weapon platform.

Homing kinetic energy weapons (KEW)

would require less accurate information from

a remote sensor: a homing sensor on an SBI

itself would give the fine resolution needed in

the last few seconds to approach and collide

with the target. Still, the SBI must be fired

toward a small volume in space where the intercept would occur several hundred seconds

after it had been fired. The sensor system must

locate each target in three dimensions.

Target Identification or Discrimination

Requirements

Ballistic missile defense (BMD) sensors

would not only have to detect missile launches,

but they would also have to identify targets.

Identification requirements would vary considerably during missile flight. During the

boost phase, a sensor would first distinguish

between missile exhaust plumes and other natural or man-made sources of concentrated heat.

Given adequate spatial resolution, a smart sensor with memory could separate moving missiles from stationary ground-based sources of

heat. The location of the missile launcher and

the missile’s dynamic characteristics (acceleration and burn time for each stage, pitch maISFor exmple, even ~titraviolet sensor, which would have

the best resolution due to its short wavelength, would require

a 45-m diameter mirror to achieve 8 nanoradian resolution.
81

neuvers, stage separation timing, etc.) should

permit identification of missile type and probable mission. Eventually a low altitude sensor would have to identify the booster body

(as opposed to the hot plume), either by geometric extrapolation or by generating an IR

image of the booster tank.14

The post-boost phase is more complicated.

Most missiles carry a PBV or “bus” which may

include 10 or more individual warheads in RVs.

These RVs are individually aimed at separate

targets: the PBV maneuvers and mechanically

ejects each RV, one at a time, along a different trajectory. A BMD sensor system might

detect heat from a PBV propulsion system as

it made these multiple maneuvers. However,

PBV propulsion energy is far less than main

booster engine energy, making tracking (at

least in the SWIR/MWIR range) more difficult in the post-boost phase. Once ejected, cold

RVs would be even more difficult to detect and

track. 15

This reduced signal level could be partially

offset by arranging the sensor satellite to view

its targets against the cold space background

instead of the warm and noisy Earth background, as in the boost phase. The sensors

would have to look above the horizon, generally limiting detection to distant targets over

the Earth’s limb. Since detection becomes more

difficult at longer ranges, this above-thehorizon (ATH) detection of cold RVS would be

more difficult than sensing very hot booster

plumes against the earth background.

If the United States deployed a BMD system, Soviet missiles would probably disperse

decoys along with nuclear-armed RVs. Decoys

might be simple, aluminum-covered balloons

weighing 1 kg or less, or they might be somewhat more sophisticated decoys shaped like

14A boos~r body, at 3000 K is cold compared to its hot Plume,

but it is still warmer than the cool upper atmosphere at about

2200 K. An LWIR sensor could therefore image the booster body

against the Earth background at fairly long ranges, using

wavelengths which were absorbed by the upper atmosphere.

“ICBM boosters typically radiate millions of watts per steradian (W/sr), PBVS hundreds of W/sr, and RVS a few W/sr. (A

“steradian’ is the measure of a solid angle, defined as the ratio

of the surface area subtended by a cone divided by the square

of the apex of that cone.)

an RV with similar infrared and radar signatures. Simple decoys might be tethered to an

RV within a few tens of meters: defensive sensors would then require higher resolution to

separate decoys and RVs. Alternately, an RV

could be placed inside a large balloon, a technique known as “anti-simulation”: the RV is

made to look like a decoy.

The most sophisticated decoys, called

thrusted replicas (TREPs) might even have

propulsion so they could push into the atmosphere during reentry to simulate the heavy

RV’s reentry characteristics. The total postboost and mid-course threat cloud could contain something like 10,000 RVs, hundreds of

thousands of decoys, and thousands of burntout rocket stages and PBVs, all traveling

through space at 7 km/s. In the same trajectories might be literally millions of fragments

from boosters destroyed by SBIs in the boost

and post-boost phases.16

In principle, a BMD weapons system could

fire at all of these objects, but the costs would

be prohibitive. Therefore the sensors for a

second- or third-phase BMD system with midcourse capability would have to discriminate

effectively between RVs and the many decoys

and debris.

In the post-boost phase, there would be some

basis for discrimination. A sensor could, in theory, monitor PBV motion during deployment

of RVs and decoys. Decoys would produce less

PBV motion than the heavier RVs as they were

ejected from the PBV. This distinctive motion

might be detected, assuming that the Soviets

did not cover the PBV with a shroud to conceal the dispersal of decoys, or that they did

not appropriately alter the thrust of the PBV

as its RVs dispersed.

In the mid-course phase, discrimination

would become even more difficult. All the objects would travel together in a ballistic, freefall flight. Light decoys would not be slowed

down by atmospheric friction until they descended to the 100-150 km altitude range—

the same altitude range that constrains deploy-

%ee chapter 10 for details on countermeasures to BMD.
82

Photo credit: U.S. Department of Defense

COBRA JUDY Radar

A new radar had been developed and installed on the COBRA JUDY ship. This improves the capability of the U.S. for

making measurements on reentry vehicles in flight.

ment of rising decoys in the post-boost phase.

If decoys had the same signatures or characteristics of RVs as seen by conventional infrared and radar detectors, then conventional

discrimination of RVs from decoys would become extremely difficult. Mid-course discrimination is one of the most crucial challenges facing the SDI technology development program.

The BMD sensors would also have to detect

and track defense suppression threats such as

direct-ascent anti-satellite (DAASAT) missiles

or space-based ASATs which might attack

BMD defensive assets in space. The sensors

should therefore keep track of all of the BMD

weapons platforms in a given battle space, allowing the battle manager to determine which

objects were likely targets and which weapons should engage the threat.

Target Tracking Requirements

Passive IR sensors on a single BSTS or

SSTS satellite could only measure the target

position in two angular coordinates. Each target must be located in three dimensions to allow the battle management computer to calculate the expected collision point of weapon

and target.

Three techniques could furnish three dimensional data: stereo imaging, ranging, or ballistic trajectory prediction (see figure 4-4). Two

or more separated sensor satellites could generate stereo data. This would require a computer to correlate data from multiple sensors

and could become very complicated with 40

or 50 sensors generating data from thousands

or hundreds of thousands of targets.

Alternatively, a laser range-finder and a passive IR two-dimensional imager together on

one satellite could generate three dimensional

information. A laser range-finder would determine the distance to the target. With a direct,

one-to-one correlation between two target angles from a passive sensor and a third range

coordinate from a laser, computational requirements would be reduced by eliminating the

need to correlate data from separate platforms.

Finally, for objects traveling in space on a

ballistic, free-fall trajectory, Kepler’s equations
83

Figure 4-4. - Illustration of Three Techniques for Estimating the Three-Dimensional Position of a Target in Space

Stereo viewing IR angle/angle plus range finder

IR sensor B IR sensor A

#1

#2

Ballistic trajectory estimation

(from one passive sensor)

Time T-3 Time T-2 Tme T-1

I

IR sensor

#2 .

In the first view Sensor A could not distinguish between Target # 1 and Target #2. Stereo viewing from two or more separate satellites with

passive IR sensors eliminates this ambiguity. Relatively complicated software is required to correlate data from each sensor. The other

two techniques can predict three dimensional information from one platform, eliminating the requirement for multiple satellite sensor

data correlation; a laser range finder determines the range or distance to a target by measuring the travel time for a pulse of light from the

platform to the target and back, uniquely determining position with one measurement. The ballistic trajectory prediction approach uses

only the passive IR sensor, but requires three or more measurements at different times to compute the target’s path through space.
84

of motion may be applied: a passive sensor

could determine the path of an RV in three

dimensions by measuring its two-dimensional

position three or more times. This trajectory

prediction approach requires more time (hundreds of seconds) to build up an accurate track:

this would be adequate for the mid-course

phase. It would require more data storage and

processing than the laser range-finder technique, but only one passive sensor.

Kill Assessment Requirements

Sensors would also have to determine

whether a missile or RV had been disabled or

destroyed. Missed targets would have to be

retargeted, and disabled targets should be ignored throughout the remainder of the battle.

Kill assessment should be straightforward for

most KEW projectiles, since their impact

would smash targets into thousands of pieces.

However, some SBIs might partially damage

a booster by clipping anon-critical edge, leaving the bulk of the missile intact. In this case

the sensor might judge a missile “killed” if it

veered sufficiently off-course to anon-threatening trajectory.

Damage to targets attacked by laser or particle beam weapons might be more difficult to

diagnose. A laser beam might conceivably burn

through a critical component without detectable damage, yet divert a missile from its intended course. More likely, the laser would disintegrate the missile body, which is highly

stressed during acceleration—as demonstrated

by a ground-based high-energy laser test at the

White Sands Missile Range.17

Damage due to particle beams or electron

beams might be more difficult to detect. Neutral particle beams, for example, might penetrate several cm into a missile or RV, destroy17The mid-range infrared advanced chemical laser (MIRACL)

at White Sands Missile Range in New Mexico was aimed at

a strapped-down Titan missile second stage. The missile was

mechanically loaded with 60 psi of nitrogen gas to simulate the

4-g load and propellant conditions that it would experience in

an actual flight. After approximately 2 seconds of exposure to

the laser beam, which had a power greater than 1 megawatt,

the Titan booster completely ruptured, shattering into fragments

as heating of a roughly 1 m2 area destroyed the mechanical integrity of the booster skin.

ing critical electronic components without any

apparent external damage. An RV might be

effectively “killed” with respect to its mission

at much lower particle beam energy than that

necessary to show detectable damage.

On the other hand, NPB system designers

could increase particle beam fluence to levels

that would assure electronics destruction (say

50 joules/gram (J/g) —only 10 J/g destroys

most electronics) as long as the target were

hit. Kill assessment would then become “hit

assessment”: if the beam dwelled on the target long enough to impart 50 J/g, then the electronics could be judged “killed.” With this approach, NPB weapons would be effectively

lethal at lower energy levels than that needed

for melting aluminum or causing structural

weakness (500 to 1,000 J/g). Relying on this

indirect kill assessment would require confidence that the Soviets had not shielded critical internal electronic components from NPB

radiation.

Table 4-1.—Summary of Typical Sensor Requirements

Surveillance:

Coverage . . . . . . . . . .Global

Targets . . . . . . . . . . . . ICBM’s, SLBM’s, direct ascent

ASAT’s, space mines, and one’s

own BMD assets, including all

sensor and weapons satellites

and launched SBIs

Target Discrimination:

Boost Phase . . . . . . . ICBM/SLBM/DANASAT

Post-boost & midcourse. . . . . . . . . . . PBV, RV, light decoy, replica,

thrusted replica, & debris

Terminal . . . . . . . . . . . RV & thrusted replica

Tracking:

Targets . . . . . . . . . . . . ICBM’s 1,400-2,000

SLBM’s 1,000-1,500

DANASAT’s: 1,000-16,000

PBV’s 2,400-3,000

RV’s 8,000-15,000

Decoys hundreds of

thousands

Track file . . . . . . . . . . position, velocity, & acceleration in

3-D

Kill assessment:

KEW . . . . . . . . . . . . . destruction

Laser. . . . . . . . . . . . . . destruction

NPB . . . . . . . . . . . . . . hit assessment or other

SOURCE: Office of Technology Assessment, 1988.
85

Sensor Technology

Three types of sensors might satisfy portions

of these BMD requirements: passive, active,

and interactive. Passive sensors rely on natural radiation emitted by or reflected from the

target. Active sensors, such as radars, illuminate the target with radiation and detect

the reflected signal. “Interactive sensors” (a

term unique to the SDI) would use a strong

beam of energy or cloud of dust-like particles

to perturb targets in some measurable way

(without necessarily disabling it) so that RVs

could be discriminated from decoys. For example, the cloud might slow down light decoys

much more than heavy RVs, or penetrating

particle beams might create a burst of neutrons

or gamma rays from RVs but not from balloons.

Passive Sensors

How Passive Sensors Work.—Passive sensors

detect military targets either by measuring

their natural emission, or by detecting natural light reflected from the targets. A typical

Photo credit: U.S. Department of Defense,

Strategic Defense Initiative Organization

Infrared image of the moon from SDIO’s Delta 181

experiment. That experiment took measurements of a

rocket booster and other objects in space to gather

information about the kinds of sensors that would be

needed in a space-based ballistic missile defense

system. This may be the first long-wave infrared image

acquired from a platform in space.

sensor is similar to an ordinary camera. An optical element (the lens) forms an image, and

alight sensitive surface records that image (the

film).

In BMD infrared sensors, the optical lens

would be replaced by a system of reflecting mirrors and the camera film by an array of discrete optical detectors in the focal plane which

convert the optical image into electronic signals for immediate computer processing. Many

detectors are required to record a detailed image. In a sense each detector substitutes for

one grain of photographic film. Some sensors

use a stationary two-dimensional “staring” array of detectors, in direct analogy to photographic film. Others mechanically scan the image across an array of detectors that may be

either two-dimensional or linear.

Infrared Sensors. –Ordinary photographic

cameras record the visible light reflected from

a scene. For BMD, the IR energy emitted by

the target (particularly the hot exhaust gases

ejected from a missile booster engine) is abetter source of information.

18 The sensor images

the infrared radiation from the target and background onto a photosensitive array of detectors. These detectors generate a series of electrical signals that are processed by computers

to detect and track the target.

There are three distinct target classes for the

BMD mission: missiles with their rocket engines firing, post-boost vehicles with much

lower power engines, and cold objects such as

RVs and decoys in space.” Each type of target demands different IR sensors. Hot exhaust

gas from a booster engine radiates primarily

in relatively narrow bands of short wavelength

IR. The exact wavelength of this radiation is

‘aAll objects with a temperature above absolute zero(–273

C) emit energy in the form of electromagnetic waves, such as

light waves, infrared waves, microwaves, etc. For example, the

human body continuously radiates infrared waves. To an infrared camera, we all “glow in the dark”: our bodies would be

recorded on infrared film as a group of “hot spots, even if the

picture were taken in absolute darkness. Similarly, any target

emits energy which can, in principle, be detected with appropriate sensors, provided only that the target is warmer (or colder)

than the background scene.

l~he RVS do heat up from friction as they enter the

atmosphere.
86

determined by the particular gas constituents.

The primary emission bands for gas plumes

are near the water vapor and carbon dioxide

lines at 2.7 micrometers20 (in the short wave

IR or SWIR) and at 4.26 μm (in the middle

wave IR or MWIR).21

Other specific radiation lines may help identify some Soviet booster plumes: this will be

investigated in the SDI research program.

These plumes radiate hundreds of thousands

to millions of watts per steradian (W/sr) of

energy. Post-boost vehicles also have propulsion systems, but their smaller motors radiate only hundreds of W/sr.

Reentry vehicles remain near “room temperature” (20 o C or 2930 K) in mid-course, until

they are heated by the friction of the atmosphere on reentry. The maximum radiation for

room temperature objects is near 10 μm in the

LWIR. Infrared detection of RVs is difficult

because of their low level of radiation (typically

a few W/sr) and poor contrast against the earth

background. That is, the earth is also near

‘‘room temperature, ” with strong emission in

the 10-pm band. An IR sensor cannot “see”

a red target against a red background. The sensor would generally have to wait until the target RV was above the horizon to view it against

the cold (4 o K) temperature of space. The sensor system would also have to filter out the

IR energy from planets or bright stars in the

field of view.22

The technical feasibility of detecting relatively cold RVs against a space background

was demonstrated on June 10, 1984, when an

LWIR sensor on board the Army’s Homing

Overlay Experiment (HOE) missile successfully detected a simulated RV over the Pacific

—.— ---

~oone ~crometer (w) is one millionth (10 ‘o) of a meter.

ZIA~wpheric water vapor and carbon dioxide attenuate mOSt

of the IR radiation from a missile plume in the early stages of

flight. However, the higher temperature and pressure of the

water and CO* in the plume produce a broader IR spectrum than

the atmospheric absorption bands. Infrared energy will therefore leak through on both sides of the 2.7 and 4.3pm lines, even

from rockets close to the surface of the Earth.

‘The Air Force has used a star as the “target” for tests of

the U.S. F-15 launched ASAT, which uses a LWIR sensor to

home on its target.

Ocean.23 The sensor guided the HOE projectile into a collision course, destroying a target

launched earlier from Vandenberg AFB in California. This test demonstrated an ability to detect and track a single approaching RV in space

at relatively close range. (The initial HOE missile trajectory was specified by radar signals

from Kwajalein until the missile LWIR sensor could acquire the target.)

Tracking thousands of RVs and possibly

hundreds of thousands of decoys with spacebased sensor satellites from distances of 5,000

to 10,000 km would be more challenging, particularly if the RVs were encapsulated in balloons and decoy balloons were tied (tethered)

together or to an RV.

Three-Color Infrared Sensors. –Depending

on the offense’s countermeasures, discrimination of RVs from decoys might be improved

if the object temperatures could be measured

accurately. Long-wave IR sensors that detect

one narrow wavelength band cannot determine

temperature. That is, a warm object with low

IR emissivity24 could produce the same radiance at one wavelength as a cooler object with

high emissivity, as illustrated in figure 4-5.

However, the shape of the blackbody (nonreflecting object) radiation curve as a function

of wavelength is distinct for objects at different temperatures. This suggests that two or

more LWIR sensors operating at different

wavelength bands within the 8- to 24-μm region could estimate the temperature ofμpace

objects, independent of their general emissivities.

Most SDI architects recommended threecolor LWIR detectors to measure energy in

three separate wavelength bands or “colors.”

Note that this complicates sensor design and

23To place this experiment in perspective, it should be noted

that this RV was significantly brighter than the radiance expected from current RVs, while the Soviets may take steps to

further reduce IR emissions.

24The emissivity of any object indicates its ability to radiate

energy. Emissivity is defined as the ratio of the energy radiated at any wavelength to the amount of energy radiated by

a perfect blackbody at the same temperature. (A “blackbody”

absorbs all energy reaching its surface.) Thus an object with

low emissivity will radiate less energy than a higher emissivity

object, even though they are both at the same temperature.
87

Figure 4-5.-Spectral Response of Two Objects at

Different Temperatures

I

Photo credit: U.S. Department of Defense,

Strategic Defense Initiative Organization

Cryocooler for space applications. Many of the

advanced “heat-detecting” infrared sensors necessary

to identify and track missiles and warheads in space

must be cooled to work properly. Special refrigerators

called cryocoolers would produce the needed very low

temperatures. Cryocooler life, reliability, and performance experiments designed to demonstrate the ability

to cool long-wave infrared detectors

have been conducted.

of 7 years, and at least one type of cryogenic

refrigerator has demonstrated this ability in

accelerated life tests.25

UN/Visible Sensors. —Some SDI contractors

have proposed the use of visible or even ultraviolet (UV) sensors, primarily to achieve better resolution with realistic optics dimensions. 26 For example, a 28-cm diameter UV

mirror at 0.3 pm could achieve the same resolution as a 400-cm (4-m) diameter mirror operating at 4.3 μm. However, this gain is not free:

reducing the wavelength increases the fabrication difficulty. Mirrors must be polished to

within one-tenth to one-twentieth of the operating wavelength. Thus an MWIR mirror at

4.3 μm must be polished to within at least 0.43

μm of the prescribed surface figure, while a UV

mirror must be polished to an accuracy of 0.03

μm or better.

‘

bHughes Aircraft has demonstrated operation of a magnetic

gas cooler system with an accelerated test simulating 7 years iife.

ZGThe resolution of a sensor is limited by diffraction spreading of the optical image. This diffraction spreading is proportional to the wavelength of light used to form the image; shorter

wavelengths produce less image spreading, yielding better res~

lution or sharper images.
88

Visible or UV sensors might detect energy

from rocket plumes, although the visible radiation from liquid-fueled missiles is minimal.

The atmosphere attenuates UV below an altitude of a few tens of km, but a post-boost vehicle propulsion system may generate adequate UV radiation. To see RVs, however,

these sensors would have to rely on the reflection of natural radiation (sunlight, moonlight,

or Earthlight). Alternatively, they could be

used in an active mode with a laser designator

illuminating the target (see next section).

Current Status of Passive Sensors.–Passive

infrared sensors operate today in early warning satellites. A few satellites at geosynchronous orbit, some 36,000 km above the

earth, monitor the entire globe, searching for

missile launches from the Soviet land mass or

from the oceans. Several heat-seeking tactical

missiles such as the air-to-air Sidewinder and

the ground-to-air Maverick missile also employ

infrared sensors. This same sensor technology

supplied the terminal guidance for two successful space hit-to-kill experiments: the antisatellite (ASAT) experiment in which a missile fired from an F-15 aircraft destroyed a satellite in space and the Homing Overlay Experiment.

Today’s operational infrared sensors have

relatively small optical systems, typically 20

cm or less in diameter, and focal plane arrays

of a few thousand detectors. Most detectors

are fabricated from bulk silicon and could not

survive in a nuclear environment. Relatively

few large detector arrays are built each year,

Table 4-2.—Key Issues

and the United States does not yet have the

manufacturing technology to build large arrays economically.

Key Issues for Passive Sensors.—This report

has identified five key issues for passive sensor technology development (see table 4-l).

While driven by the space-based system requirements, these same sensor functions would

be required for effective ground-launched

weapons systems. Whether the sensors rode

on airborne or space-based platforms, these issues would have to be resolved to produce a

robust BMD system.

Mirror Size. —A sensor system mirror must

be large to collect enough energy, to resolve

closely spaced objects, and to accurately direct weapons systems (see box 4-A). The mirror size needed is determined by sensor operating wavelength, distance to target, and

target positional accuracy required by the

weapon system. The resolution of any optical

system is given approximately by the wavelength divided by the diameter of the aperture

multiplied by the range.

Typical mirror sizes for adequate spot resolution from a passive sensor at 3,000 km altitude are shown in figure 4-6.27 To provide adequate aiming information to homing kinetic

energy weapons, sensor resolutions from 10 m

ZTFig. 4.6 assumes a perfect, diffraction limited OptiCd sYstem. In practice other factors-such as vibration, i.nqxwfect mirror quality, and thermal distortions-would degrade resolution.

This figure, therefore, represents the minimum allowable mirror size for a spot. Tracking resolution may only require mirrors a factor of 10 smaller, as noted in the text.

for Passive Sensors

KEW DEW Current status

Mirror size (m) . . . . . . . . . . . . . . . . . . . . . about 0.1 about 1 0.1-2.4

Number of detector elements (UV/visible)

(resolution limited)

Geo/staring . . . . . . . . . . . . . . . . . . . . . . 106

-108 N/A many tens of

thousands

Geo/scanning. . . . . . . . . . . . . . . . . . . . . 104

-100 N/A

3,000 km/staring (1 oFOV) . . . . . . . . . . . 103-105

10’

3,000 km/scanning . ................103

105-10’

Detector manufacturing capacity. . . . . . 108

-109/yr 107

-108/yr IO6/yr

Signal processing

Rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10% 1010/s several x 107

/s

Memory. . . . . . . . . . . . . . . . . . . . . .. ...1 X 107

1 x 108

8x 107

SOURCE: Office of Technology Assessment, 1988.
89

—————. . --

Box 4-A.—Sensor Resolution Limits

The resolution of any electromagnetic sensor

(or its ability to separate two closely spaced ob-

 jects) is limited by two factors: diffraction and

detector element size. The image formed by the

sensor optics cannot faithfully reproduce the actual scene. An infinitesimally small point in the

scene will have a finite size in the image due to

diffraction or spreading of the light beam. This

spreading increases with distance, so diffraction

will limit the useful range of any sensor as shown

in figure 4-6a.

The optical system projects an image of the

scene onto the detector array. The size of each

Figure 4-6a.-Diffractlon-Llmited Range

for Ten-Meter Resolution

0246 8 10 12 14 16 18 20

Mirror diameter (meters)

Sensor range as a function of mirror diameter to produce

a 10-meter resolution element at the target, for three different wavelength sensors. Two point targets separated by

10 meters at these ranges could just be resolved by mirrors of these sizes.

detector element in this array must be equal to

or preferably smaller than the optical resolution

size to preserve the diffraction-resolution of the

figure in the electronic signal. If the detector elements are too large, then they will further limit

the system resolution.

For a fixed field-of-view, as the distance between the scene and the sensor increases, then

each detector element covers a larger area in

space: the resolution decreases with range, the

same dependence as diffraction spreading of the

optical image.

Figure 4-6b. - Range Limited by Number of

Detectors for Ten-Meter Resolution

Range of LWIR sensors as limited by the number of detector elements in the focal plane array. The staring array is a fixed, two-dimensional array with a 200

field-of-view. The scanning array covers a 10° by 360°

“coolie hat” pattern, with 10 rows of elements scanning

each point in the image. Both arrays detect three different LWIR bands. The scanning array could use just one

row of detectors to sweep out the image. However, to improve signal-to-noise ratio, most designs utilize more than

one row and “time delay and integrate” (TDI) circuits to

average the signals from many rows.

up to 1 km maybe adequate, depending upon Track resolution, however, imposes a less

the sensors and the divert capability of-the in- stringent requirement than the spot resolution

terceptor. As shown in figure 4-7, mirrors of for a single “look.” Data from many “looks”

l-m diameter or less are adequate for any visi- can be combined, using statistical techniques,

ble or IR wavelength. Furthermore, a l-m mir- to achieve up to a tenfold improvement. Thereror operating at 2.7 μm would yield 10-m tar- fore, proportionately smaller mirrors are

get accuracy from 3,000 km.28 needed for predicting tracks.

———-

26The primary water vapor emission line from missile exhaust Directed-energy weapons would require

plumes is at 2.7 pm. much better resolution than SBIs, since they
90

Figure 4-7.—Mirror Size Plotted v. the Operating

Wavelength of a Sensor System

1,000

100

10

1

0.1

0.01

FEL

NPB

0.4 0.7 1.0 10.0

Wavelength (mlc’ens)

Mirror size plotted v. the operating wavelength of the sensor

system, assuming a 3,000 km range to the most distant target, for

Indicated spot resolution. Note that the tracking resolution can be

up to a factor of 10 better than the resolution calculated for one

“look,” based on diffraction limits. Therefore, the tracking may only

require mirrors up to 10 times smaller than indicated in the figure.

For homing kinetic energy weapons, moderate-sized mirrors (well

under 1 meter in diameter) would be adequate for all wavelengths.

Directed-energy weapons such as high power lasers would require

sensors with very large mirrors operating in the visible or even

ultraviolet region of the spectrum. Thus all DEWS would have to use

a Iow-resolution LWIR sensor to point a second UV/visible active

sensor or laser on each weapons platform to achieve the necessary

accuracy.

SOURCE Off Ice of Technology Assessment, 1988.

must be focused to a small spot without the

benefit of a homing sensor at close range.

LWIR sensor mirrors to direct DEWS would

have to exceed 10 m in diameter. Therefore a

DEW sensor would probably have to operate

in the SWIR or MWIR, visible, or even ultraviolet (UV) wavelengths.29 Laser beam weapons would demand the highest accuracy to take

full advantage of their small spot size and

therefore high intensity on target, typically on

the order of 30 cm at 3,000 km or 0.1 microradian. Neutral particle beams, as currently envisaged, would have about one microradian

29This might be satisfactory for boost-phase kills, but cold

RV’s in mid-course could only be detected with LWIR sensors.

Hence a future laser BMD system designed to attack RV’s would

have to use a coarse LWIR sensor for detection, then a separate laser designator at shorter wavelength to illuminate targets for tracking by a second UV or visible-light sensor. This

complexity, combined with the durability of RV’s as a result

of their ablative shield needed for reentry, makes the use of laser beams for killing RV’s in mid-course very doubtful.

divergence, producing a 3 m spot at 3,000 km,

so NPB sensors could be about 10 times less

accurate than laser beam sensors.

Number of Detector Elements per Array. –

Each passive sensor would need many detector elements for both adequate resolution and

high signal-to-noise ratios. For example, a staring array sensor on a BSTS satellite at geosynchronous orbit (36,000 km) could need well over

a million detector elements to afford coarse

resolution at the surface of the Earth. This requirement could be reduced to hundreds of

thousands of detector elements by scanning

the IR image over a smaller array of detectors,

so that each detector sampled many resolution

elements in the IR image.

Many detector elements would also be necessary to yield adequate signal-to-noise ratios:

the electrical signal produced by IR radiation

from a target would have to exceed the signal

from all sources of noise. Competing IR noise

could come from the background scene such

as the Earth or stars, from the mirrors and

housing of the sensor system, and from the internal electrical noise of the detector elements.

The signal-to-background-noise ratio could be

maximized by distributing the background

from a fixed field-of-view over many detector

elements.30 For the most stressing task of detecting cold RVs above the horizon against

atmospheric background at a tangent height

of 50 to 80 km, sensors would need at least

several hundred thousand detector elements

to generate adequate signal-to-noise ratios.31

Current IR focal plane arrays on operational

military sensors for tactical elements have up

to 180 detector elements. Some other operational systems have several thousand, and experimental arrays with many more than 10,000

——.

30Ideally, each detector element should be the same size as

that of the target image. If the elements were twice this ideal

size (half the total number of detectors in the array), then each

element would collect twice the background noise with no increase in signal: the signal-to-noise ratio would be cut in half.

For many long-range BMD missions, the detector element would

be much larger than the target image.

slThese num~rs of detectors are based on the assumption that

the sensor mirrors are cooled to the800 to 1000 K range so that

IR radiation from those mirrors does not dominate the noise,

and that the detectors are fabricated with low noise.
91

●

●

Photo credit: Genera/ Electric Company

Sensor focal plane array of 128 by 128 detector

elements. These elements convert light energy into

electrical signals. Focal plane arrays are the electrooptical equivalent of film in a camera. Some SDI

sensors may require focal planes containing hundreds

of thousands of detector elements.

elements have been fabricated. The focal plane

array (FPA) for the planned Airborne Optical

Adjunct (AOA) experiment will have a 38,400-

element three-color FPA.32 However, none of

these detectors was designed to the radiation

hardness needed for BMD sensors.

Detector Radiation Hardness.—Ballistic

missile defense sensors must withstand radiation from distant nuclear explosions. Current

detectors are fabricated from relatively thick

bulk materials such as silicon or mercury cadmium telluride (HgCdTe) which are susceptible to radiation damage. Other materials, such

as gallium arsenide or germanium, or thinner

detector structures would be needed to achieve

radiation hardness goals. Impurity band conductor (IBC) detectors, which are only 10 to

12 μm thick, can withstand 10 to 100 times

more radiation than common bulk silicon detectors. Arrays with up to 500 IBC elements

have been fabricated in the laboratory.

The electronic readout from FPAs must also

be resistant to radiation damage. In the past,

charge-coupled devices (CCD) were used to read

out large detector arrays. To reduce susceptibility to radiation damage, researchers are

butt-bonding switching metal oxide semiconductor field effect transistor (MOSFET) readouts to the detectors.

Detector Manufacturing Capacity.–Industry produces about 1 million IR detectors per

year. Many of these are small linear arrays of

16 to 180 elements each, used for tactical IR

missiles or scanning IR imaging systems. The

“Teal Ruby”33 experiment bulk-silicon array

is the largest built so far. Production would

have to increase by one or two orders of magnitude to satisfy the ambitious BMD goals:

very large, radiation-hard, low-noise arrays

would be required. For example, just one BMD

sensor would require several, perhaps up to 10,

times the current annual production capacity—

and there could be many tens of sensors in a

second-phase space-based BMD system. The

SDIO has programs underway intended to

———

‘gTeal Ruby is an experimental satellite designed to detect

aircraft from space with an LWIR detector array.

———- — - - — 32 See Aviation Week and Space Technology, Nov. 10, 1986,

P. 87.

Photo credit: U.S. Department of Defense,

Strategic Defense Initiative Organization

Impurity Band Conduction Long-Wave

Infrared Detector Array
92

achieve these improvements in manufacturing

capability.

Conversion from laboratory fabrication to

full-scale manufacturing of the new IBC detectors-assuming they continue to be the preferred detector–could limit BMD sensor deployment. Industry Performance in converting

to the manufacture of bulk silicon IR “common module” arrays in the early 1980s was

not good. Producing arrays of just 60, 120, or

180 elements once held up the completion of

M-1 tanks that use forward looking IR (FLIR)

sensors.

Manufacturing yield (the ratio of the number of acceptable arrays to the number manufactured) for IR detectors would have to be improved. The overall yield (including read-out)

for the Teal Ruby array was about 2 percent.

Since yield was so low, every element had to

be individually tested at cryogenic (10° K) temperatures: testing might be the limiting manufacturing process. The SDIO has initiated

programs to address this problem in fiscal year

1988.

Signal Processing Improvements.–Projected

signal processing rates for BMD sensors would

exceed current space-ebased operational capabilities by factors of a few hundred. Current

operational signal processors can handle up

tens of millions operations per second (MOPS),

while BMD signal processing requirements

might exceed 10 billion operations per second,

or 10 giga-OPS (GOPS).

Projected on-board memory requirements

for BMD sensors vary from 10 million to 100

million bytes of information. Reaching these

memory and processing goals by the 1990s

seems likely, given the progress in very high

speed integrated circuits (VHSIC).

Power consumption of signal processors

must be reduced. The AOA experiment will require less than 10 kilowatts (KW) of power to

drive a 15 GOP processor, or over 1.5 MOPS/W.

Hardened VHSIC technology offers the promise of many times less power consumption (40

MOPS/W) and good radiation resistance.

Active Sensors

How Active Sensors Work.—Active sensors

illuminate the target with radiation and monitor reflected energy. In general, active sensors have the advantage of adequate illumination under all conditions: they do not have

to rely on radiation from the target or favorable natural lighting conditions. They suffer the

disadvantage, under some circumstances, of

being susceptible to jamming or spoofing: the

opponent can monitor the illumination beam

and retransmit a modified beam at the same

frequency to overpower or confuse the receiver.

At the very least, the illumination beam can

alert the enemy that he is under surveillance

or attack. This might be a concern for surveillance and tracking of defense suppression

weapons such as direct-ascent or orbiting

ASATs.

Microwave radar, an active sensor used so

successfully in tracking aircraft, might support some phases of BMD, particularly for terminal defense. These ground-based radars

might use advanced data processing techniques to generate pseudo-images of RVs to

distinguish between RVs and decoys, as described below. Conventional microwave radar

has two serious limitations for most spacebased BMD functions: limited resolution and

large power requirements. Because of the large

antennae, large power requirements, and survivability issues, microwave radar is not a

prime candidate for BMD space applications.34

However, the SDIO still believes that microwave radar might be included in future BMD

systems.

SDI researchers are also investigating laser

radar or ‘ladar” for applications such as measuring the range to a target and discriminating RVs from decoys. In principle, ladar is

equivalent to radar with much shorter (opti-

—.. --.—

me SDIO had considered developing shorter millimeter wave

radar to provide better radar resolution and lower power require

ments. With reduced funding, support for millimeter radar has

been reduced. Distributed antenna arrays are also being considered to provide space surveillance of aircraft and cruise missiles for the Air Defense Initiative.
93

cal or infrared) wavelengths. With shorter

wavelength, ladars generally would give better resolution with less power and weight. Ladars cannot operate in all weather conditions

on earth. They are therefore better suited for

space applications.

Imaging Radars.– If an object is moving

relative to a radar, then the radar return signal is shifted in frequency, similar to the Doppler frequency shift of a train whistle as it

passes by a stationary observer. For objects

that rotate, such as spinning satellites or reentry vehicles, pseudo-images can be generated

by processing the doppler frequency shifts of

radar signals stored over time. This is a process similar to synthetic aperture radar, sometimes called inverse synthetic aperture radar

(ISAR). 35

Consider a conical RV spinning about its axis

(figure 4-8). The tip of the cone has no significant motion due to rotation, and little doppler

frequency shift. The back edge of the cone has

a large motion (proportional to the radius of

the cone and the angular velocity of the RV)

and a large doppler frequency shift. A plot of

range to target versus doppler frequency shift

will therefore resemble the shape of the RV for

most orientations of radar beam to spinning

RV. 36

The resolution of range/doppler pseudoimages does not depend on radar-beam spot

size. The beam floods the target area, so precise beam pointing is unnecessary. Range resolution is inversely proportional to the band-

—..—- .. —-. ~5An ~r~me synthetic aperture radar system generates ~

image of the ground by measuring the doppler frequency shifts

of ail return radar signals. Targets directly ahead of the radar

aircraft have maximum Doppler frequency shift because the

relative velocity between the ground and the aircraft is a maximum. Targets perpendicular to the aircraft flight path have no

relative motion toward the aircraft and no Doppler frequency

shift. By storing all the radar returns and processing data over

time, a pseudo image of the ground is generated.

*If ~fia~g radm were boresighted aiong the trajectory

of an RV, there would be no doppler tiquency shift and no image. Conversely, if the radar looked perpendicular to the RV

flight path, there would be no information on the length of the

RV: any range spread would be due to the radius of the cone,

independent of length. For other radar look angles between these

extremes, the doppler frequency shift would be proportional to

the sine of the look angle, and the range spread would be proportional to the cosine of that angle.

Figure W.-l Illustration of an Imaging Radar Viewing

a Spinning Conical Target

Spinning cone

in space

Radar beam

Range/doppler pseudo

image on radar screen

I

Point “A” on the base of the cone has the most motion toward the

radar, producing the largest doppler frequency shift. The echo

from this point would appear at point ‘A” on a radar-generated

plot of range versus doppler frequency shift. Point “B”, at about

the same range as point “A”, is moving perpendicular to the radar beam, and will have no doppler frequency shift; its echo

would be plotted as shown. Similarly, point “C” is moving away

from the radar, and would have a negative doppler frequency

shift. Finally, points along the cone such as point “Dn have lower

frequency shifts, since they are closer to the spinning axis. The

resulting range-doppler plot will therefore resemble the conical

target.

width of the transmitted signal. For example,

a one gigahertz37 bandwidth radar signal could

have a range resolution capability of 15 cm.

Resolution in the cross-track direction (corresponding to the radius of the spinning cone)

is limited by the minimum doppler frequency

shift that can be detected, radar wavelength

(smaller is better), and the rotation rate of the

RV (larger is better).38 For microwave radars,

typical doppler frequency shifts are in the tens

to hundreds of hertz. Many radar pulses must

be stored and analyzed to measure these low

frequencies, which requires substantial data

processing.

—..

~TGig~e~z is a unit of frequency equal to one billion cyCleS

per second.

‘aNote that doppler (cross-track) resolution of these pseudo

images is not equivalent to positiomd accuracy. Object details

on the order of a few cm may be resolved in these images, but

the cross-track position of the object will not be known to better than the radar beam width, which might be tens of kms wide.
Ladar.—The short wavelength and very

short pulse-length of a laser might prove very

useful for several BMD functions. A laser radar or ladar system would illuminate the target cloud with a pulsed beam of light. An optical receiver would detect the reflected echoes,

in direct analogy to a microwave radar. Various types of ladars could supply one-dimensional range to the target (a laser range-finder),

or they could generate 2- or 3-dimensional

images.

Several modes of imaging operation are

possible:

● Scanning beam or “angle/angle” mode: a

pulsed laser beam is focused and scanned

over the scene. A single optical detector

records the time sequence of reflections

from each returned laser pulse, and a three

dimensional map of target position is generated in computer memory. Ladar resolution would depend on the beam spot size,

which could be as small as 3 m at 3,000

km with reasonably sized optics.39 Very

short-wavelength lasers are preferred to

minimize spot size. The range resolution

would be on the order of 1.5 m with 10-

nanosecond long laser pulses, which are

commercially available.

99A 0.5 ~m laser with a 60-cm mirror would produce a

diffraction-limited spot 3 m in diameter at a distance of 3,000 km.

●

●

Focal plane array: a passive imager, similar to the IR sensors, records the scene

illuminated by a laser. The laser is the

“flash lamp”.

Doppler ladar: the optical analog of a

microwave Doppler imaging radar might

be feasible if lasers with adequate coherence could be built. Doppler resolution of

a coherent ladar could be excellent. A 30-

cm RV rotating once per second would

generate a 3.8 megahertz (million cycles/

second—MHz) frequency shift in the ladar return signal, compared to only 60

hertz for an X-band imaging radar. Since

the resolution of this pseudo-image would

be independent of spot size, there would

be no need to operate at short UV or visible wavelengths. This fine image resolution would not, however, yield good positional information. A narrow beam (short

wavelength) angle/angle ladar would be required for good angular resolution.

Active Discrimination. –A ladar might be

very useful for discriminating between RVs

and decoys as they were ejected from a PBV.

The PBV would perceptibly change its velocity as each heavy RV was discharged, but not

as light decoys were dispensed. A ladar could

be designed with the spatial resolution to resolve independently the PBV and the RV or

decoy and, in theory, to measure the differen-
95

tial velocities before and after each deployment.40

Light decoys might inflate as they left the

PBV. A high resolution imaging ladar could

in principle observe this inflation and so identify balloon decoys.41 A precision doppler ladar might also observe small vibration or nutation (wobbling) differences between an RV

and a decoy. Light decoys might vibrate at tens

to hundreds of kilohertz (kHz), heavier RVs

at less than a few kHz. Over tens of seconds,

the nose of a spinning RV also nutates a few

millimeters: a very high resolution ladar might

detect this motion, but long integration times

and high data storage rates would be necessary.

Current Status of Active Sensors.—Active

sensor technologies have been tested and deployed in some form since the radars of World

War II. Considerable development remains,

however, before active sensors will be ready

for advanced BMD systems.

Phased-array Search Radars. -Ground-based

phased-array radars are currently deployed in

both the United States and the Soviet Union

to detect objects in space and give early warning of missile attack. The “PAVE PAWS” radars now at Otis AFB on Cape Cod and at

Beale AFB near Sacramento have two large

faces each, with active areas 22 m square, providing 2400 coverage. Each face has 1,792 active antenna elements, with provisions to upgrade each face to 31 by 31 m active areas with

5,354 elements. Two additional PAVE PAWS

radars are being built in Georgia and Texas.

40Consider a PBV with 10 RVs. The PBV velocity would

change very little if a light decoy were ejected. Ejecting the

first RV, if it weighed l/15th of the remairing PBV weight, would

cause the PBV to slow by l/15th of the RV-PBV separation

velocity. That is, if the two objects were designed to move apart

at a 15cm/sec rate, then the PBV would slow down by 1 cm-dsec

and the RV would speed up by 14 cndsec after separation. Later

RV’S would cause the PBV to slow down more, as the ratio of

RV to remaining PBV weight increased. The ladar would therefore need a velocity resolution of 1 cndsec in this example.

ilone Countermeasure to block the observation of decoy inflation (as well as differential velocity detection) would be to

inflate the decoys under a long shroud, although there is some

concern that the PBV rocket plume might interfere with a

shroud. Alternatively, decoys and RVS could be tethered together so that their rotation would confuse the sensor, which

could not keep track of each object (see ch. 10.)

The United States plans to replace the three

existing Ballistic Missile Early Warning System (BMEWS) mechanically scanned radars

at Clear, Alaska; Thule, Greenland; and Fylingdales Moor in England with phased array radars. The old Distant Early Warning radars

will also be replaced by 52 new phased array

North Warning System (NWS) radars. These

radars, along with the mothballed phased array radar near Grand Forks, North Dakota,

might supply RV target coordinates to an

ERIS exoatmospheric interceptor system.42

ImagingRadars. —Several radars have been

operated in the rangedoppler imaging mode

since the early 1970s. These ground-based radars are used to image satellites, RVs, and

other space objects. MIT’s Lincoln Labs operates an L-band and an X-band imaging radar

at Millstone Hill in Massachusetts.

Ladars.—Ladar systems have not been

placed in operation, but they have been tested.

In 1981 MIT Lincoln Laboratories built the

“Firepond” CO, ladar, which had a 15 kW peak

power and 1.4 kW average power. With a one

microradian resolution, this ladar could detect

targets spaced 3 m apart at a distance of 3,000

km. This ladar has been reactivated for the SDI

program. It will be operated in the range-doppler mode to investigate RV imaging in a

ground-based field test. Two other lasers are

planned. One will have a very short (nanosecond), high peak power pulse to yield good

range resolution. The other will use a lower

peak power, frequency-chirped pulse. To recover good range resolution, this chirped pulse

is compressed electronically in a data processor. This same pulse compression technique

has been used successfully to reduce the peak

power required in more conventional microwave radars.

‘

z

SDIO’s phaseone Strategic Defense System pkms one or

more optical sensors for cueing ERIS interceptors. However,

Lockheed-the ERIS developer–and others have proposed an

“early deployment” version of ERIS that would utilize existing radars. The computing capabilities of these radars wouid

have to be improved to handle hundreds of targets. The systems would be susceptible to electromagnetic pulse, microwave

jamming, and blast damage in the event of nuclear war. At this

time, phased-array radars are the only sensors available for early

deployment of ERIS-like BMD systems.
Work is also proceeding on diode-pumped

glass lasers, excimer lasers, and bistatic CO2

ladars. Glass lasers are typically pumped with

flash lamps, resulting in very low efficiency

(typically less than 0.2 percent), since the spectrum of the flash lamp does not match the absorption bands of the Nd:glass material. By

pumping the Nd:glass laser with an array of

incoherent laser diodes, efficiency can be increased significantly and the thermal distortion which normally limits these lasers to very

low repetition rates can be controlled.

Excimer lasers have the advantage of generating UV radiation, which demands the

smallest mirrors for a given resolution.

Key Issues for Active Sensors.—Current SDI

phase-two concepts call for ground-based radars for directing late mid-course and terminal defense. Space-based ladars are suggested

for boost-phase ranging, to observe PBV deployment, and for determining accurate target position during mid-course discrimination.

Ladars might also be used for air-borne ranging to assist terminal defense. Issues for these

active sensors include the following.

Ground-based Radar.-Ground-based radars

would have to be large, phased-array devices

to focus adequate energy on many targets. Two

key issues would be survivability and data

processing. Surge fuses at each radiating diode in the array could probably protect large

antennas from nuclear burst-generated electromagnetic pulse (EMP). Shielding the structure and building could protect interior electronics. Most EMP energy would be below 150

MHz, so radar radio frequency (RF) circuits

at 10 GHz could be safe.

However, these antennas would be susceptible to in-band radiation from dedicated jammers. It might be a challenge to design effective electronic counter-countermeasures to

protect these large and critical assets from electronic jamming by Soviet satellites. Some system architects have suggested that these radars be mobile, possibly on railroad cars.

Mobility might reduce susceptibility to

jamming.

Data processing might also be challenging.

Consider an X-band (3-cm wavelength) radar.

Its data processor might have to handle 5 million bits per second of incoming data for each

of 5,000 antenna dipoles, or a total of 25 billion bits per second for the entire radar.43 These

data must be stored and processed to determine the direction to each target (by phasing

the receiving array) and a Fast Fourier Transform (FFT) operation would have to be performed on each range bin to measure doppler

frequency shift over many pulses.

Doppler imaging radars might be fooled if

RVs (and decoys) were covered with “fronds,”

—strips painted with irregular patterns of volatile material. Attached at various places on an

object, these strips would move about at random in space as the volatile material evaporated. This motion would give different parts

of the target different doppler velocities independent of their positions on the RV or decoy

cone. Such extraneous frequency shifts might

confuse the radar processor, obscuring the image of the RV body.

Ladar Active Discrimination. –Significant

advances would be required in ladar technology before it could be utilized to observe PBV

deployment of RVs and decoys. Key issues

would be resolution, beam steering, and data

processing to handle the expected traffic.

Direct angle/angle ladar imaging of PBVs

would take very large mirrors.44 The alternative would be doppler processing to improve

cross-track resolution. While microwave syn4~his data rate assumes that radar bandwidth is 1 GHz to

yield a 15-cm range resolution. The radar tracks each target

to within 100-m accuracy before hand-over to an image mode

Processor, which maintains a shiingrange gati 100 m wide about

each high-speed target. The radar pulse repetition rate is set

by the highest expected doppler frequency shift produced by

RV rotation. For clear images of a 20-cm radius RV rotating

at 3 hertz, the pulse rewtition frequency (PRF) must be 500

hertz or higher. (This imaging doppler radar would be highly

ambiguous with respect to RV velocity, which would require

MHz type PRFs to measure actual velocity.)

44T0 image a 30-cm diameter RV, a ladar designer would like

10 resolution elements across the object to resolve shape or details, or 3 cm resolution. Thus, an impractically large 60-m mirror would be required for 3 cm resolution at 3,000 km range

with a visible laser.
97

thetic aperture radars have been successfully

operated for over 20 years, this process has

not been extended to optical wavelengths.

Building stable but powerful space-based

lasers with the coherence necessary for doppler processing would be a major challenge.

Interactive Sensors

The consensus in the SDI technical community is that passive and active sensors may not

be adequate to discriminate between RVs and

decoys in the future. The Soviet Union probably has the necessary technology to develop

decoys and real RVs with nearly the same infrared and radar signatures. Decoys would not

be extraordinarily difficult to fabricate and disperse in space, and they would weigh only a

small fraction of an RV. There is a serious question whether, once dispersed, they could be distinguished from real RVs by any passive or

active sensor. If not, the offense could overwhelm a space-based or ground-based midcourse defense system with literally hundreds

of thousands of false targets.

Mid-course decoy discrimination would become crucial if the Soviets could:

●

●

deny a phase-one boost-phase defense

through countermeasures such as moderately fast-bum (e.g., 120- second) boosters,

and

deny significant post-boost kills by moving to faster PBV deployment times or to

single warhead missiles.

If an initial U.S. deployment of kinetic

energy weapons could no longer destroy many

ICBMs in the boost or post-boost phase, and

if directed-energy weapons were not yet available, then mid-course discrimination would become indispensable to a viable BMD system.

There would be two possibilities for effective mid-course discrimination under these circumstances: ladar discrimination during postboost decoy dispersal, or interactive discrimination after the RVs and decoys were released.

As discussed in the preceding section, ladar

detection during decoy deployment would be

very challenging. Moreover, simple measurement of RV and PBV recoil velocities might

be thwarted completely if the Soviets could disperse decoys and RVs simultaneously in pairs.

Even fine doppler imaging would be foiled if

the Soviet PBV could obscure the deployment

operation with a shroud. This would leave interactive discrimination as the main approach

to keeping BMD viable in the long term.

How Interactive Discriminators Would Work.

—In interactive discrimination, a sensor system would perturb each target and then measure its reaction to determine if it were a decoy

or an RV. For example, a dust cloud of sufficient density and uniformity could be placed

in front of a group of objects. The resulting

collisions would slow down light decoys more

than heavy RVs. A ladar would monitor the

change of velocity of all objects, thereby identifying real RVs.

Two general classes of discriminators have

been proposed: kinetic energy and directed

energy perturbers.

Kinetic Energy Discriminators. —Two methods have been proposed to project particles in

front of an oncoming cloud of decoys and RVs:

rocket-born particles and nuclear-explosionprojected particles. A rocket-borne cloud would

be limited to late mid-course, unless the rockets

were fired from submarines or based in Canada or the Arctic. Presumably one rocket

would be necessary for the cylindrical cluster

(or “threat tube”) of RVs and decoys emanating from each PBV. To slow down decoys measurably, a rocket would have to carry enough

mass to cover the full lateral extent of the

threat tube with a sufficiently dense cloud. A

ladar would have to measure velocity changes

in the 10-cm/sec to l-m/see range.

Directed-energy Discriminators. -Several

forms of directed energy have been proposed

for interactive discrimination. They would all

have the advantage of long range, extending

the discrimination capability back to the beginning of the mid-course if not to the postboost phase.

The laser is the best developed directedenergy perturber currently available, although

further development would be needed to produce lasers with the brightness required for
98

interactive discrimination. Lasers could heat

unknown targets (called “thermal tagging”).

Alternatively, a short pulse of laser light could

change the velocities of targets (called “impulse tagging”).

In thermal tagging, a laser of the appropriate wavelength would heat a light-weight decoy more than an RV—assuming they both absorbed laser energy and radiated IR (thermal)

energy to the same degree. A separate IR sensor, possibly mounted on SSTS satellites,

would then detect the warmer decoy.

Pulsed lasers could shock the unidentified

objects. Energy would be deposited in microseconds instead of the milliseconds taken by

thermal tagging. A high-power pulse would

boil away material perpendicular to the surface of the target. The reaction of ablation products would cause the target to change velocity. A heavier RV would recoil less than a

decoy, providing amass-dependent indicator.

A separate ladar would monitor the change of

each object’s velocity.

The SDIO has chosen the neutral particle

beam (NPB) as the most promising interactive

discrimination perturbation source. The particle beam source is derived from well-established particle accelerators used for several decades in physics research experiments around

the world. A neutral particle beam could be

composed of hydrogen atoms,45 accelerated to

velocities about half that of the speed of light.

Since the particle beam would be relatively

broad, on the order of 2 microradian beam

width, it would not require the pointing accuracy of 50-nanoradian-wide laser beams.

These energetic particles would be deposited

several cm deep inside an RV.46 As they were

4SAn NpB could dso utilize deuterium or tritium, the heavier

isotopes of hydrogen. These heavier isotopes would experience

less divergence in the beam neutralization process after acceleration. Tritium, the hydrogen isotope with two neutrons, must

be produced in a nuclear reactor and is radioactive with a halflife of 12.3 years. Deuterium, the non-radioactive hydrogen isotope with one neutron, would most likely be used.

Another approach calls for cesium instead of hydrogen atoms

in a “momentum rich beam. ” A heavy cesium beam would impart a velocity change to the target, so it is more analogous

to a laser impulse tagger than to a hydrogen NPB.

@The electron on each hydrogen atom would be stripped off,

leaving the proton which penetrates into the target.

absorbed, these particles would produce

gamma rays and neutrons. Neutron or gammaray detectors on many satellites-located

closer to the targets than the accelerator—

might monitor the emissions coming from a

massive RV. Light weight decoys, in contrast,

would not emit much radiation.

High energy particles must be electrically

neutral to propagate through the Earth’s variable magnetic field (charged particles would

bend in unpredictable paths.) But a particle

must be charged to be accelerated. Therefore

the NPB would first accelerate negatively

charged hydrogen ions. After acceleration to

a few hundred MeV (million electron volts)

energy, this beam would be aimed toward the

target by magnetic steering coils. Once steered,

the charged beam would be neutralized by

stripping off the extra electron from each particle. Thin foils or gas cells are currently used

to neutralize beams in laboratory experiments.

A relativistic (i.e., near-speed-of-light) electron beam could also be used as a discriminator. The detector in this scheme would monitor x-rays from the more massive RV. Such

a system might be ground-based, popping up

on a rocket to monitor the mid-course phase.

The main advantage would be the avoidance

of space-based assets for interactive discrimination. However, an e-beam discriminator

would need some air to form a laser-initiated

channel, so it could only operate at altitudes

between 80 to 600 km.

Current Status of Interactive Sensors. -Interactive sensors have not yet been built for any

military mission. All the concepts described

above have been invented to solve the severe

discrimination problem unique to mid-course

ballistic missile defense.

Key Issues for Interactive Discrimination.—

The overriding issue for interactive discrimination is effectiveness in the face of evolving

Soviet countermeasures. There are some common issues for any discriminator and some issues unique to each approach.

Laser Radar. –Any discriminator would require a high resolution laser radar to accurately

locate and identify each object in space. One
99

Table 4-3.—Key Issues for Interactive Sensors

For all discriminators:

●

●

For

●

●

●

●

●

●

●

●

For

●

●

For

●

●

Laser radar required for accurate target location:

(corner cube reflector is inexpensive countermeasure.)

Rapid retargeting: 3-50 targets/second

NPB accelerator:

Voltage and duty cycle must be increased without

increasing beam emittance

Beam expansion

Beam sensing must be developed

Beam pointing system must be developed

Beam propagation in space

Space charge accumulation

Accelerator arcing in space

Weight

NPB neutron detectors:

RV detection with nuclear precursor background

Missed target indicator

laser thermal tagger:

Moderate to high power pulsed lasers

Thermal shroud on RV

For laser impulse tagger:

● Needs Iadar imager to tell orientation

● High to very high average power, microsecond-long

pulsed lasers

. Thruster-compensated RVs

For dust cloud tagger:

. Dispersal of dust cloud

SOURCE Office of Technology Assessment, 1988,

possible countermeasure to ladar would bean

inexpensive corner-cube reflector on each RV

and decoy. This corner cube would essentially

swamp the ladar receiver: the beam would be

returned on itself and the ladar would be unable to measure target characteristics. A counter-countermeasure would be a bistatic ladar

with a laser transmitter on one platform and

a light detector on a separate satellite not far

away. Reflected energy from a corner cube

would travel harmlessly back to the transmitter; thus failing to blind the receiver. Bistatic

operation would be feasible, but it would complicate system design, construction, and

operation.

Beam Steering. —A directed-energy interactive discriminator would have to steer its beam

rapidly from one object to the next. Beam

steering requirements are set by the number

of expected targets and the number of directedenergy satellites within range of those targets.

Typical estimates are that hundreds of thousands of RVs and decoys might survive the

boost phase defense.47 Assuming that midcourse discrimination of sophisticated decoys

must be completed in 15 minutes, then each

platform would have to interrogate 3 to 50 targets per second. The directed-energy source

would have to be steered accurately from one

target to the next in less than 20 to 300 milliseconds. This would be a formidable challenge.

NPB Accelerator. –Neutral particle beam

accelerator development faces many key hurdles. Beam energy must be increased by a factor of 20, which should not be difficult. Duty

cycle and beam diameter must be increased by

a factor of 100 without degrading beam quality or emittance—a more challenging task. An

accelerator would have to operate in space

without electrical breakdown or arcing that

would short out its electrical system. Communications and electronic controls would have

to operate even with electrical charge buildup in space. An NPB would have to propagate

over long distances in space with little divergence. To point accurately at targets, it would

have to be effectively boresighted to an optical system.

These same issues would have to be resolved

for an NPB weapon accelerator. A weapongrade NPB would probably dwell longer on

each target to assure destruction of at least

the internal electronics, but might otherwise

be very similar to one designed for interactive

discrimination. A more detailed discussion of

NPB accelerator issues appears in the DEW

section of chapter 5.

Neutron Detection. –Calculations indicate

that large neutron detectors placed on hundreds of separate satellites near the targets

could detect the neutron flux from RVs. The

offense might intentionally detonate nuclear

weapons in space before an attack to saturate

these neutron detectors. With sufficiently high

particle-beam energy (on the order of 200 MeV),

JTAn ~~ractive discriminator wouId not have to interrogate

all objects in space. Unsophisticated decoys, discarded booster

stages and other debris could probably be identified by passive

or active sensors. With adequate battle management to keep

track of extraneous objects, the process of “bulk filtering” would

eliminate these objects from the interactive discriminator’s target list.
100

the energy of some neutrons ejected from an

RV would be higher than that expected of neutrons emanating from nuclear detonations.

Therefore an energy threshold circuit would

eliminate most of the signal from the latter

source, allowing identification of the neutrons

from RVs.

Another issue is how to confirm that targets

had been hit by an NPB, since the neutron detectors would receive no signal from decoys.

How would a system distinguish between decoys and RVs which were missed by the beam?

One possibility, being tested in the laboratory,

would be to monitor each object with a UV sensor on the assumption that the outer surface

of the RVs (and the decoys) would emit UV

light when struck by the particle beam. This

UV sensor simply would confirm that the particle beam had hit a target.

If based on current technology, neutrondetector platform weights would be excessive.

Each platform would weigh up to 30 tonnes.

System designers hope that lighter detector

elements and power supplies can reduce this

weight to 5 tonnes per platform by the mid1990s. If this goal were achieved, then the several hundred detector satellites could be orbited with about 100 launches of the proposed

Advanced Launch System.

Laser Thermal Tagger.–Very high power

lasers would be required to tag space targets

for an interactive discriminator. A laser thermal tagger, like all interactive sensors, would

require a separate laser radar to locate targets

precisely. For example, cold RVs (and decoys)

would have to be tracked by long-wavelength

LWIR passive sensors. These sensors could

only determine a target’s position to within

18 m, assuming a 2-m sensor mirror at 3,000

km.48 But the interrogating laser beam might

have a spot size of only 1 or 2 m. A more accurate laser radar would be required to guide

an HF laser beam to the target.

46A single target could be located to within less than the 18-m

LWIR resolution element by a process called “beam-splitting”:

the target is assumed to be in the center of the IR signal wave

form. If there were two targets or a target and a decoy within

the 18-m resolution element, however, then the sensor would

falsely indicate one target located between the two objects.

Detecting small temperature rises on several hundred thousand objects would also

stress LWIR sensor technology. Monitoring

closely spaced targets would demand large

LWIR mirrors. For example, to distinguish objects spaced 10 m apart, a sensor 3,000 km

away would need a 4-m mirror. Steering this

large mirror to, say, 15 targets per second

would be another major challenge.

Decoys might be modified to respond to thermal tagging as an RV would. Due to their lower

mass, decoy surfaces should became hotter

than RV surfaces after laser illumination. However, the outer layer of the decoys could in principle be built to absorb less laser light or to

emit more IR heat. These decoys would then

reach the same temperature as an RV after exposure to laser light. Or, an RV could simply

be covered by an insulating blanket that would

decouple the exterior thermal response from

the internal RV mass. It appears that laser

thermal tagging would have limited usefulness

against a committed adversary.

Laser-impulse Discriminator.–The energy

density required for laser impulse discrimination would be in the range of 7 to 30 times more

than for thermal tagging. In addition, the laser pulses would have to be very short, on the

order of microseconds instead of milliseconds,

which makes the peak laser power extraordinarily high. This high peak power would be

difficult to generate and handle, since mirrors

and other optical components would be susceptible to damage by the intense pulses. While

less powerful than proposed laser weapons,

lasers for impulse discrimination would still

be a major development.49

Laser impulse discrimination might be countered by equipping RVs or decoys to react deceptively. Small thrusters on RVs might cause

them to move as a decoy would under a laser

impulse. Alternatively, thrusters on relatively

sophisticated decoys might counteract the laser impulse.

4YI’he primary measure of a laser’s effectiveness as a weapon

is beam “brightness,” the average power radiated into a given

solid angle. An HF laser impulse tagger would be brighter than

any laser built to date, but still a factor of 2 to 200 less bright

than that needed for BMD against a responsive Soviet threat.
101

All interactive discriminators would probably require an imaging ladar to provide adequate resolution both to hit targets with a

probe beam and to measure target response

accurately. A laser impulse discriminator

would bear the additional burden of determining target (and particularly decoy) orientation.

The orientation of a conical decoy, for example, could affect its reactive motion in response

to the laser pulse.

Dust-cloud Discriminator. –The key issue

for a dust cloud discriminator is how to position the cloud accurately in front of the oncoming

1.

2.

3.

If the particles were dispersed too widely, the

required amount would become excessive. If

clustered too closely, they could miss some decoys. As with any discriminator, a precision

ladar would be required to measure velocity

changes accurately.

Laser impulse discrimination might be countered by equipping RVs or decoys to react deceptively. Small thrusters on RVs might cause

them to move as a decoy would under a laser

impulse; alternatively, thrusters on relatively

sophisticated decoys might counteract the

impulse.

RV-decoy constellation at the proper time.

SENSOR TECHNOLOGY CONCLUSIONS

Phase 1

A boost surveillance and tracking satellite

(.BSTS) could most probably be developed by

the mid-1990s. Short-wave and middle-wave

infrared (S/MWIR) sensors, could provide

early warning and coarse booster track

data sufficient to direct SBI launches.50

Space surveillance and tracking system

(SSTS) satellites would not be available for

tracking individual RVs and decoys before

the late 1990s. The ability to discriminate

possible decoys in this time frame is in

question. Smaller but similar sensors for

a phase-one system might be placed on individual SBI platforms or on ground-based,

pop-up probes.

An airborne optical system could probably

be available by the mid-1990s to detect and

track RVs and decoys with IR sensors (although not to discriminate against a replica

decoy above the atmosphere). However, its

utility may be limited in performance and

mission:

● Performance may be limited by the

vulnerability and operating cost of its

aircraft platform, and IR sensors

600ne ~ncert~nty ig the protection of the BSTS sensors from

future airborne or spaceborne laser jarnmers which could permanently darnage IR detector elements during peacetime.

4.

5.

●

might be confused during battle by

IR-scattering ice crystals formed at 60

to 80 km altitude by debris reentering the atmosphere.

The relatively short range of airborne

IR sensors would limit the AOS mission to supplying data on approaching objects for endo-atmospheric interceptor radars, and possibly for

exe-atmospheric interceptors a short

while before RV reentry. Airborne IR

sensors, unless very forward-based,

could utilize only a small portion of the

time available in mid-course for discrimination and therefore could not

take full advantage of the fly-outrange

of ground-based exoatmospheric interceptors.

In any case, an Airborne Optical

System is not now included in SDIO

phase-one deployment plans.

Effective discrimination against more sophisticated decoys and disguised RVs in

space is unlikely before the year 2000, if at all.

Phase 2

By the late 1990s at the earliest, a space surveillance and tracking system (SSTS) might

furnish post-boost vehicle (PBV) and reen-

.
102

6.

7.

try vehicle (RV) track data with long-wave

infrared (LWIR) above-the-horizon (ATH)

sensors suitable for directing SBI launches

in the mid-course. New methods would be

needed for the manufacture of large quantities of radiation-hardened focal plane arrays. Another issue is the operation of

LWIR sensors in the presence of precursor nuclear explosions (including those

heaving atmosphere into the ATH field of

view) or other intentionally dispersed

chemical aerosols. Effective mid-course

SBI capability is unlikely before the late

1990s to early 2000s.

There are too many uncertainties in projecting sensor capabilities and the level of Soviet countermeasures to specify a discrimination capability for SSTS. It appears that

Soviet countermeasures (penetration aids

and decoys) could keep ahead of passive

IR discrimination techniques:

● Passive IR discrimination could be

available by the mid-1990s, but probably would have marginal utility

against determined Soviet countermeasures.

● Active laser radar (ladar) imaging of

PBV deployment offers some promise

of decoy discrimination, provided that

the Soviets did not mask dispersal of

decoys. Space-borne imaging ladars

probably would not be available until

the late 1990s at the earliest.

● Laser thermal tagging of RVs is unlikely to be practical given the need

for complex, agile steering systems

and given likely countermeasures such

as thermal insulation of RVs and

decoys.

● Laser impulse tagging is even less

likely to succeed in this phase because

high-power pulsed lasers would be required.

Ground-based radar (GBR) might be available by the late 1990s to direct interceptors

to recentering warheads. There maybe some

8.

9.

10.

questions about its resistance to RF jammers. Signal processors may have difficulty handling large numbers of targets in

real-time.

Phase 3

Accurate IR sensors, UV ladar, or visible

ladar would have to reside on each DEW

platform.

Interactive discrimination with neutral particle beams (NPB) appears the most likely

candidate to reliably distinguish decoys from

RVs, since the particles would penetrate targets, making shielding very difficult. Before

one could judge the efficacy of a total NPB

discrimination system, major engineering

developments would be required in: weight

reduction, space transportation, neutral

particle beam control and steering,5l automated accelerator operation in space, and

multi-megawatt space power.

It is unlikely that a decision on the technical feasibility of NPB discrimination

could be made before another decade of laboratory development and major space experiments. Given the magnitude of an

NPB/detector satellite constellation, an effective dismimination system against sophisticated decoys and disguised RVs would not

likely be fully deployed and available for

BMD use until the 2010 to 2015 period at the

earliest.

Nuclear bomb-projected particles might also

form the basis of an effective interactive discriminator, if reliable spacebased ladar systems were also developed and deployed to

measure target velocity changes. There are

too many uncertainties to project if or when

this approach might succeed.

“Since the particle beams are invisible, novel approaches would

be required to sense the direction of the beam so that it could

be steered toward the target.
Chapter 5

Ballistic Missile Defense

Technology: Weapons, Power,

Communications, and

Space Transportation
CONTENTS

Page

Introduction . ......................10 5

Weapons . ..................,......10 5

Kinetic-Energy Weapons (KEW) ....106

Directed-Energy Weapons . .........123

Power and Power Conditioning . ......142

Space Power Requirements . ........142

Space Power Generation Technology .143

Power Conditioning . .. ..........145

Communication Technology . .........146

60-GHz Communication Links .. ....147

Laser Communication Links . .......148

Space Transportation . ..............148

Space Transportation

Requirements. . . ................149

Space Transportation Alternatives ...149

Space Transportation Cost

Reduction . . . . . . ................153

Conclusions . . . . ....................153

Weapon Technology Conclusions ....153

Space Power Conclusions. . .........155

Space Communications Conclusion. ..l55

Space Transportation Conclusions ...156

Figures

Figure No. Page

5-1.

5-2a.

5-2b.

5-2c.

5-2d.

5-3a.

5-3b.

5-4.

5-5.

5-6a,

Orientation of SDI to RV .. ....111

Space-Based Interceptor Mass v.

Velocity . ....................11 4

Number of Satellites v. SBI

Velocity . ....................11 4

Number of Space-Based

Interceptors v. Velocity.. .. ....114

Constellation Mass v. SBI

Velocity . ....................11 4

Number of Projectiles v. SBI

Velocity . ....................11 5

SBI Constellation Mass v. SBI

Velocity . ..............4.....11 5

Total SBI Constellation Mass in

Orbit v. Booster Bum Time ....116

Boost and Post-Boost Kill

Effectiveness (l,400 ICBMs v.

“Realistic’’ SBIs) . ............116

Boost and Post-Boost Kill

Effectiveness (500 single-RV

ICBMs at three sites)... . ......117

5-6b.

5-7.

5-8.

5-9.

5-10.

5-11.

5-12.

5-13a.

5-13b.

Table

5-1.

5-2.

5-3.

5-4.

5-5.

5-6.

5-7.

5-8.

5-9.

5-10.

Boost and Post-Boost Kill

Effectiveness (500 single-RV

ICBMs atone site) . ...........117

Boost and Post-Boost Kill

Effectiveness (200 “mediumbum-booster” ICBMs at one

site). . . . . . . . . . . . .............118

Schematic of an Electromagnetic

Launcher (EML) or “Railgun” ..119

Lightweight Homing Projectile. .l2l

Illustration of the Relationships

Between Laser Parameters and

Power Density Projected on a

Target . . . . . . . . . . .............125

FEL Waveforms . .............125

Schematic of a Neutral Particle

Beam Weapon.. . .............129

Annual Space Launch Capacity .152

Space Transportation . .........152

Tables

No. Page

Key Issues for Electromagnetic

Launchers (EML) . .............120

Characteristics of a Ground-Based

FEL Weapons System . .........124

Characteristics of an HF Laser

Weapons System. . .............127

Characteristics of Directed Energy

Weapons Against a Fully

Responsive Soviet Threat . ......130

Possible Beam Steering and

Retargeting Requirements for

Boost-Phase Engagement . ......134

Key Issues for Free Electron

Lasers (FEL) . .................136

Key Issues for the HF Chemical

Laser . . . . . . . . . . ...............138

Neutral Particle Beam Issues ....139

Estimated Power Requirements

for Space Assets . ..............142

Current U.S. Space Launch

Inventory . . . . . . . ..............150
Ballistic

Weapons,

Chapter 5

Missile Defense Technology:

Power, Communications, and

Space Transportation

INTRODUCTION

This chapter reviews weapon technologies weapons, power systems, and communication

relevant to ballistic missile defense (BMD). It systems of most interest for the Strategic Deemphasizes the chemically propelled hit-to-kill fense Initiative (SD I). Finally, it considers the

weapons most likely to form the basis of any new space transportation system essential for

future U.S BMD deployment in this century. a space-based defense.

The chapter also covers the directed-energy

WEAPONS

A weapon system must transfer a lethal dose

of energy from weapon to a target. All existing weapons use some combination of kinetic

energy (the energy of motion of a bullet, for

example), chemical energy, or nuclear energy

to disable the target. The SDI research program is exploring two major new types of

weapon systems: directed-energy weapons and

ultra-high accuracy and high velocity hit-tokill weapons. Not only have these weapons

never been built before, but no weapon of any

type has been based in space. Operating many

hundreds or thousands of autonomous weapons platforms in space would itself be a major

technical challenge.

Directed-energy weapons (DEW) would kill

their prey without a projectile. Energy would

travel through space via a laser beam or a

stream of atomic or sub-atomic particles. Speed

is the main virtue. A laser could attack an object 1,000 km away in 3 thousandths of a second, while a high-speed rifle-bullet, for example, would have to be fired 16 minutes before

impact with such a distant target. Clearly,

Note: Complete defi”m”tions of acronyms and im”tial.z”sms

are h“sted in Appen&”x B of this report.

DEW, if they reach the necessary power levels,

would revolutionize ballistic missile defense.

DEWS offer the ultimate in delivery speed.

But they are not likely to have sufficient deployed power in this century to destroy ballistic missiles, and they certainly could not kill

the more durable reentry vehicles (RVs). In

hopes of designing a system deployable before

the year 2000, the SDI research program has

emphasized increased speed and accuracy for

the more conventional kinetic-energy weapons

(KEW), such as chemically propelled rockets.

With speeds in the 4 to 7 km/s range, and with

terminal or homing guidance to collide directly

with the target, these KEW could kill a significant number of today’s ballistic missiles.

With sufficient accuracy, they would not require chemical or nuclear explosives.

Although DEWS will not be available for

highly effective ballistic missile defense during this century, they could play a significant

role in an early 1990s decision on whether to

deploy any ballistic missile defense system.

That is, the deployment decision could hinge

on our ability to persuade the Soviets (and ourselves) that defenses would remain viable for

the foreseeable future. Kinetic-energy weapons

105
106

work initially against the 1990s Soviet missile

threat. But Soviet responsive countermeasures

might soon render those weapons ineffective.

Thus, a long-term commitment to a ballistic

missile defense system would imply strong confidence that new developments, such as evolving DEW or evolving discrimination capability, could overcome and keep ahead of any

reasonable Soviet response.

Strategic Defense Initiative Organization

(SDIO) officials argue that perceived future capabilities of DEW might deter the Soviet

Union from embarking on a costly defense

countermeasures building program; instead,

the prospect of offensive capabilities might persuade them to join with the United States in

reducing offensive ballistic missiles and moving from an offense-dominated to a defensedominated regime. To foster this dramatic shift

in strategic thinking, the evolving defensive

system would have to appear less costly and

more effective than offensive countermeasures.

Today, the immaturity of DEW technology

makes any current judgments of its costeffectiveness extremely uncertain. It appears

that many years of research and development

would be necessary before anyone could state

with reasonable confidence whether effective

DEW systems could be deployed at lower cost

than responsive countermeasures. Given the

current state of the art in DEW systems, a wellinformed decision in the mid-1990s to build and

deploy highly effective DEW weapons appears

unlikely.1

Kinetic-Energy Weapons (KEW)

Today’s chemically propelled rockets and

sensors could not intercept intercontinental

ballistic missiles (ICBMs) or reentry vehicles

.-—— —

‘The Study Group of the American Physical Society concluded

in their analysis of DEW that “even in the best of circumstances,

a decade or more of intensive research would be required to provide the technical knowledge needed for an informed decision

about the potential effectiveness and survivability of DEW systems, In addition, the important issues of overall system integration and effectiveness depend critically upon information

that, to our knowledge, does not yet exist.” See American Physical Society, Science and Technology of Directed Energy Weapons: Report of the American Physical Society Study Group,

April, 1987, p. 2.

(RVs) in space. No currently deployable projectile system has the accuracy or speed to consistently intercept an RV traveling at 7 km/s

at ranges of hundreds or thousands of kilometers. The SAFEGUARD anti-ballistic missile

(ABM) system built near Grand Forks, North

Dakota in the early 1970s, and the existing Soviet Galosh ABM system around Moscow both

would compensate for the poor accuracy of

their radar guidance systems by exploding nuclear warheads. The radiation from that explosion would increase the lethal radius so that

the interceptors, despite their poor accuracy,

could disable incoming warheads.

The goal of the SDI, however, is primarily

to investigate technology for a non-nuclear defense. This would dictate the development of

“smart” projectiles that could “see” their targets or receive external guidance signals,

changing course during flight to collide with

the targets.

The following sections discuss proposed

KEW systems, KEW technologies, the current

status of technology, and key issues.

KEW Systems

Four different KEW systems were analyzed

by SDI system architects, including spacebased interceptors (SBIs, formerly called

space-based kinetic kill vehicles or SBKKVs),

and three ground-based systems. All four systems would rely on chemically propelled

rockets.

Space-Based Interceptors (SBIs).–Each system architect proposed-and the SDIO “phase

one’ proposal includes—deploying some type

of space-based projectile. These projectiles

would ride on pre-positioned platforms in lowEarth orbits, low enough to reach existing

ICBM boosters before their engines would

burn out, but high enough to improve the likelihood of surviving and to avoid atmospheric

drag over a nominal seven-year satellite life.

The range of characteristics for proposed SBI

systems is summarized in the classified version of this report.

It would take a few thousand carrier satellites in nearly polar orbits at several hundred
107

km altitude to attack effectively a high percentage of the mid-1990s Soviet ICBM threat.

There was a wide range in the number of interceptor rockets proposed by system architects, depending on the degree of redundancy

deemed necessary for functional survivability,

on the number of interceptors assigned to

shoot down Soviet direct-ascent anti-satellite

weapons (ASATs), and on the leakage rates accepted for the boost-phase defense.

In late 1986, the SDIO and its contractors

began to examine options for 1990s deployment which would include constellations of

only a few hundred carrier vehicles (CVs) and

a few thousand SBIs. This evolved into the

phase-one design which, if deployed in the mid

to late 1990s, could only attack a modest fraction of the existing Soviet ICBMs in their

boost and post-boost phases.

Exe-atmospheric Reentry Interceptor System

(ERIS).–The ERIS would be a ground-based

rocket with the range to attack RVs in the late

midcourse phase. Existing, but upgraded, radars such as BMEWS, PAVE PAWS, and the

PAR radar north of Grand Forks, North Dakota might supply initial track coordinates to

ERIS interceptors.2 (These radars might be the

only sensors available for near-term deployments.) Alternatively, new radars or optical

sensors would furnish the track data. Upgraded radars would have little discrimination

capability (unless the Soviets were to refrain

from using penetration aids); moreover, a single high altitude nuclear explosion could degrade or destroy them.

Optical sensors might reside on a fleet of

space surveillance and tracking system (SSTS)

satellites or on ground-based, pop-up probes

based at higher latitudes. Such sensors might

supply early enough infrared (IR) track data

..—‘The range of planned ground-based radars such as the Terminal Imaging Radar (TIR), which could discriminate RVS from

decoys, might be too short to aid ERIS long-range interceptors; the TIR was planned for the lower HEDI endoatmospheric

system. A longer-range Ground-based Radar (GBR) system has

also been proposed. This system may be capable of supporting

ERIS interceptors.

Photo Credit: Lockheed Missiles and Space Company

How ERIS would work.—The ERIS vehicle would be launched from the ground and its sensors would acquire and track

a target at long range, ERIS would then maneuver to intercept the target’s path, demolishing it on impact.
108

Photo Credit: Lockheed Missiles and Space Company

ERIS kill vehicle concept.—The Integrated Avionics

Package (IAP) computer (top left) receives interceptor

position data from the on-board Inertial Measurement

Unit and target position data from the seeker, or

infrared sensor. The seeker acquires and tracks the

incoming warhead. The IAP sends guidance commands to the two transverse and two lateral thrusters,

which maneuver the vehicle to the impact point. Helium is used to pressurize the fuel tanks and also as a

propellant for the attitude control system at the aft

bulkhead. The lethality enhancement device would

deploy just before impact to provide a larger hit area.

to take full advantage of the ERIS fly-out

range. 3 If deployed, an airborne optical system

(AOS) could give some track data late in midcourse. None of these sensors has been built,

although the AirborneOptical Adjunct (AOA),

a potential precursor to the AOS airborne system, is under construction and will be test

flown in the late 1980s.

Anon-board IR homing sensor would guide

the interceptor to a collision with the RV in

the last few seconds of flight. This homing sensor would derive from the Homing Overlay Experiment (HOE) sensor, which successfully intercepted a simulated Soviet RV over the

Pacific on the fourth attempt, in 1984.

No major improvements in rocket technology would be necessary to deploy an ERISlike system, but cost would be an important

factor. The Army’s Strategic Defense Command proposes to reduce the size of the launch

vehicle in steps. The Army has proposed—

Whe ERIS, as presently designed, requires a relatively high

target position accuracy at hand-off from the sensor. The BSTS

would not be adequate for this.

Photo Credit: Lockheed Missiles and Space Company

ERIS Functional Test Validation (FTV) v. baseline ERIS

concept.–Sizes of the FTV vehicle and baseline ERIS

concepts are compared to a 6-foot-tall man. ERIS is

designed as a ground-launched interceptor that would

destroy a ballistic missile warhead in space. The FTV

vehicle is 33 feet tall, large enough to carry both an

observational payload to observe the impact with the

warhead and the telemetry to relay information to the

ground during the flight tests. The baseline interceptor

concept is less than 14 feet tall, more compact because

it will not require all the sensors and redundancies

that are demanded by flight tests.

partly to reduce costs–to-test this system with

a Functional Technical Validation (FTV) rocket

in 1990-91. This missile would have approximately twice the height, 10 times the weight,

and twice the burn time of the planned ERIS

rocket. The planned ERIS rocket system has

a target cost of $1 million to $2 million per intercept in large quantities. Research is proceeding with a view to possible deployment by the

mid-1990s.

Much development would be necessary to

upgrade the experimental HOE kinetic kill vehicle technology for an operational ERIS interceptor. The IR sensors are being radiationhardened. Since the operational sensor could

not be maintained at the cryogenically low temperatures required for the HOE experiment,

higher operating-temperature sensors are being developed, with cool-down to occur after

alert or during rocket flight.

High Endo-atmospheric Defense Interceptor

(HEDI).–The HEDI system would attack RVs

that survived earlier defensive layers of

ground-based, high-velocity interceptor
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rockets. HEDI would take advantage of the

fact that the atmosphere would slow down

light-weight decoys more than the heavier

RVs. Since it would operate in the atmosphere,

HEDI might attack depressed trajectory submarine-launched ballistic missile (SLBM) warheads that would under-fly boost and midcourse defensive layers—provided it received

adequate warning and sensor data.

According to one plan, an AOS would track

the RVs initially, after warning from the boostphase surveillance and tracking system (BSTS)

and possible designation by SSTS (if available).

The AOS would hand target track information

off to the ground-based terminal imaging radar (TIR). The TIR would discriminate RVs

from decoys both on shape (via doppler imaging) and on their lower deceleration (compared

to decoys) upon entering the atmosphere.

Interceptors would attack the RVs at altitudes

between 12 and 45 km. The HEDI system thus

would combine passive optics (IR signature),

atmospheric deceleration, and active radar

(shape) to distinguish RVs from decoys.

The penalty for waiting to accumulate these

data on target characteristics would be the

need for a large, high-acceleration missile. The

HEDI would have to wait long enough to provide good atmospheric discrimination, but not

so long that a salvage-fused RV would detonate a nuclear explosion close to the ground.

To accelerate rapidly, the HEDI 2-stage missile must weigh about five to six times more

than the ERIS missile.

The key technology challenge for the HEDI

system would be its IR homing sensor. This

non-nuclear, hit-to-kill vehicle would have to

view the RV for the last few seconds of flight

to steer a collision course.4 But very high acceleration up through the atmosphere would

severely heat the sensor window. This heated

window would then radiate energy back to the

IR sensor, obscuring the RV target. In addition, atmospheric turbulence in front of the

window could further distort or deflect the RV

——4The HEDI interceptor would probably include an explosively

driven “lethality enhancer. ”

image. No sensor has been built before to operate in this environment.

The proposed solution is to use a sapphire

window bathed with a stream of cold nitrogen

gas. A shroud would protect the window until

the last few seconds before impact. Since reentry would heat the RV to temperatures above

that of the cooled window, detection would be

possible. Recent testing gives grounds for optimism in this area.

Fabrication of the sapphire windows (currently 12 by 33 cm) would be a major effort

for the optics industry. These windows must

be cut from crystal boules, which take many

weeks to grow. At current production rates,

it would take 20 years to make 1,000 windows.

Plans are to increase the manufacturing capability significantly.

The HEDI sensor suite also uses a Nd:YAG5

laser for range finding. Building a laser ranger

to withstand the high acceleration could be

challenging.

As with ERIS, plans call for testing a HEDI

Functional Technical Validation missile, which

is 2 to 3 times larger than the proposed operational vehicle. The proposed specifications of

HEDI are found in the classified version of this

report.

Flexible Light-Weight Agile Experiment

(FLAGE).–The weapon system expected to

evolve from FLAGE research would be the last

line of defense, intercepting any RVs which

leaked through all the other layers. Its primary

mission would be the defense of military targets against short range missiles in a theater

war such as in Europe or the Middle East. The

FLAGE type of missile would intercept RVs

at altitudes up to 15 km. The homing sensor

for FLAGE would use an active radar instead

of the passive IR sensor proposed for on all

other KEW homing projectiles.

5

“Nd:YAG” is the designation for a common laser used in

research and for military laser range-finders. The “Nd” represents neodymium, the rare element that creates the lasing action, and “YAG” stands for yttrium-alumin urn-garnet, the glasslike host material that carries the neodymium atoms.
110

The FLAGE system was flown six times at

the White Sands Missile Range. On June 27,

1986, the FLAGE missile successfully collided

with an RV-shaped target drone which was

flown into a heavily instrumented flight space.

The collision was very close to the planned impact point. Another FLAGE interceptor collided with a Lance missile on May 21, 1987.

The FLAGE program ended in mid-1987

with the Lance intercept. A more ambitious

Extended Range Intercept Technology (ERINT) program succeeds it. The ERINT interceptors will have longer range and “a lethality enhancer. ” FLAGE was a fire-and-forget

missile; no information was transmitted from

any external sensor to the missile once it was

fired. The ERINT missiles are to receive midcourse guidance from ground-based radars. Six

test launches are planned at the White Sands

Missile Range.

KEW Technology

Three types of KEW propulsion have been

proposed for SDI: conventional projectiles

powered by chemical energy, faster but less

well-developed electromagnetic or “railgun”

technology, and nuclear-pumped pellets. All

system architects nominated the more mature

chemically propelled rockets for near-term

BMD deployments.

How Chemical Energy KEWs Work.–There

are three different modes of operation proposed

for chemically propelled KEWs:

● space-based rockets attacking boosters,

post-boost vehicles (PBVs), RVs, and

direct-ascent ASATs;

● ground-based rockets attacking RVs in

late mid-course outside the atmosphere,

and

● ground-based rockets attacking RVs inside the atmosphere.

Two or more rocket stages would accelerate

the projectile toward the target. The projectile would be the heart of each system and

would entail the most development.

The smart projectile for the space-based mission would need some remarkable features. It

would be fired at a point in space up to hundreds of seconds before the actual interception. 6 After separation from the last rocket

stage, the projectile would have to establish

the correct attitude in space to “see” the target: in general the line-of-sight to the target

would not correspond with the projectile flight

path. If it had a boresighted sensor that stared

straight ahead, then the projectile would have

to fly in an attitude at an angle to its flight

path to view the target (see figure 5-1).7

The projectile would have to receive and execute steering instructions via a secure communications channel from the battle manager.

Usually just a few seconds before impact, the

projectile would need to acquire the target–

either a bright, burning booster or a much dimmer PBV—with an on-board sensor. It would

then make final path corrections to effect a collision. Fractions of a second before impact, it

might deploy a “lethality enhancement device’

–like the spider-web structure used in the

Army’s Homing Overlay Experiment (HOE)–

to increase the size of the projectile and therefore its chance of hitting the target.

The SBI projectile must have these components:

an inertial guidance system,

a secure communications system,

a divert propulsion system,

an attitude control system,

a sensor for terminal homing (including

vibration isolation),

a lethality enhancement device (optional?),

and

a computer able to translate signals from

the sensor into firing commands to the divert propulsion system in fractions of a

second.

The on-board sensors envisaged by most system architects for more advanced “phase-two”

6A computer in the battle management system would estimate the actual interception aim-point in space by projecting

the motion or track of the target using the sensor track files.

7For non-accelerating targets, this look angle would not

change, even though the target and the projectile were traveling at different velocities. In this “proportional navigation”

mode, the projectile orientation would be fixed once the sensor

was aimed at the target.
111

Figure 5.1 .—Orientation of SDI to RV

(SBI)

Orientation of the space-based interceptor (SBI) to the reentry vehicle (RV) during the homing phase of the flight. (This

drawing shows a sensor bore-sighted with the axis of the SBI,

which is common for guided missiles operating in the atmosphere. For space-based interceptors, the sensor could just

as well look out the side of the cylindrical projectile.) The

SBI sensor would have to be aimed at the RV so that its lineof-sight would not be parallel to the SBI flight path (except

for a head-on collision.) For a non-accelerating RV, the angle from the sensor line-of-sight to the SBI flight path would

be fixed throughout the flight. Since targets such as ICBM

boosters and post-boost vehicles do change acceleration during flight, then this look angle and hence the orientation of

the SBI would have to be changed during the SBI flight.

SOURCE Office of Technology Assessment, 1988,

space-based interceptors may be particularly

challenging because they would perform several functions. They would track not only the

ICBM during the boost phase, but also the

PBV, RVs, and direct-ascent ASAT weapons

sent up to destroy the BMD platforms. Each

SBI would, ideally, kill all four types of targets.

In the boost phase, a short-wave infrared

(SWIR) or medium-wave infrared (MWIR) sensor with existing or reasonably extended technology could track a hot missile plume. An SBI

would still have to hit the relatively cool missile body rather than the hot exhaust plume.

Three approaches have been suggested for detecting the cooler missile body: computer algorithm, separate long-wave infrared (LWIR)

sensor, or laser designation.

A computer algorithm would steer the SBI

ahead of the plume centroid by a prescribed

distance that would depend on the look angle

of the SBI relative to the booster and on the

booster type. Predicting the separation between the plume centroid and the booster body

under all conditions might be difficult or even

impractical if that separation varied from one

booster to the next.

A separate LWIR sensor channel might acquire and track the cold booster body.8 One

designer proposed a single detector array, sensitive across the IR band, in combination with

a spectral filter. This filter would move mechanically to convert the sensor from MWIR

to LWIR capability at the appropriate time.

Finally, in some designs a separate laser on

the weapon platform or on an SSTS sensor

would illuminate the booster. In this case a

narrow-band filter on the interceptor’s sensor

would reject plume radiation, allowing the SBI

to home in on laser light reflected from the

booster body.

In the post-boost and mid-course phases of

the attack, the SBI would have to track hot

or warm PBVs and cold RVs. Therefore either

SBIs would need to have much more sophisticated LWIR sensors, or they would need something like laser designators to enhance the target signature. This laser illumination need not

be continuous, except possibly during the last

few seconds before impact. But intermittent

illumination would place another burden on the

battle manager: it would have to keep track

of all SBIs in flight and all SBI targets, then

instruct the laser designator at the right time

to illuminate the right target.

An SBI lethality enhancer might, for example, consist of a spring-loaded web which ex-

‘There is also a possibility that an SWIR or MWI R sensor

could acquire a cold booster body. At 4.3 p, for exaznple, the

atmosphere is opaque due to the C02 absorption, and the upper

atmosphere at a temperature of 2200 K would be colder than

a booster tank at 3000 K. As an SBI approached a booster, the

latter would appear to a 4.3 ~m sensor as a large, warm target

against the background of the cool upper atmosphere,
112

panded to a few meters in diameter or an explosively propelled load of pellets driven

radially outward. Weight would limit the practical diameter of expansion. System designers

would have to trade off the costs of increased

homing accuracy with the weight penalty of

increased lethality diameter.

Ground-based KEW capabilities would resemble those of space-based interceptors. Exoatmospheric projectiles that intercept the RVs

outside the Earth’s atmosphere would use

LWIR homing sensors to track cold RVs, or

they would employ other optical sensors to

track laser-illuminated targets. These interceptors would be command-guided to the vicinity of the collision by some combination of

ground-based radars, airborne LWIR sensors

(AOS) or space-borne LWIR sensors (SSTS,

BSTS, or rocket-borne probes). Long-wave infrared homing sensors in the projectile would

have to be protected during launch through

the atmosphere to prevent damage or overheating.

Current Status of Chemically Propelled

Rockets.–No interceptor rockets with BMDlevel performance have ever been fired from

space-based platforms. Operational IR heatseeking interceptor missiles such as the airto-air Sidewinder and the air-to-ground Maverick are fired from aircraft, but both the range

and the final velocity of this class of missiles

are well below BMD levels.

The SDIO’s Delta 180 flight test included

the collision of two stages from a Delta rocket

after the primary task of collecting missile

plume data was completed. However, these

two stages were not interceptor rockets, were

not fired from an orbiting platform, did not

have the range nor velocity necessary for

BMD, and were highly cooperative, with the

target vehicle orienting a four-foot reflector

toward the homing vehicle to enhance the signal for the radar homing system. Note that this

test used radar homing, whereas all SBI designs call for IR homing or laser-designator

homing. This experiment did test the tracking algorithms for an accelerating target, although the target acceleration for this nearly

head-on collision was not as stressing as it

would be for expected BMD/SBI flight trajectories.9

Engineers have achieved very good progress

in reducing the size and weight of components

for the proposed space-based interceptors.

They have developed individual ring laser gyroscopes weighing only 85 g as part of an inertial measuring unit. They have reduced the

weight of divert propulsion engines about 9

kg to 1.3 kg. Gas pressure regulators to control these motors have been reduced from 1.4

kg to .09 kg each. The smaller attitude control engines and valves have been reduced from

800 g each to 100 g each. Progress has also

been made on all other components of a SBI

system, although these components have not

as yet been integrated into a working prototype SBI system.

Ground-based interceptor rockets are one of

the best developed BMD technologies. The

Spartan and Sprint interceptor missiles were

operational for a few months in the mid 1970s.

Indeed parts of these missiles have been recommissioned for upcoming tests of SDI groundbased weapons such as the endo-atmospheric

HEDI. The production costs for these missiles

would have to be reduced substantially to

make their use in large strategic defense systems affordable, but no major improvements

in rocket technology are needed for groundbased interceptors, other than a 30 percent improvement in speed for the HEDI missile. As

discussed in chapter 4, however, major sensor

development would be necessary for these interceptors.

Key Issues for Chemical Rockets.–Chemical

rocket development faces four key issues, all

related to space-based deployment and all derived from the requirement to design and make

very fast SBIs.

Constellation Mass. —The overriding issue

for SBIs is mass. The SBIs must be so fast

‘Previous tests of IR guided projectiles such as the Homing

Overlay Experiment against a simulated RV and the F-16

launched ASAT test against a satellite, shot down nonaccelerating targets.
113

that a reasonably small number of battle stations could cover the entire Earth. But, for a

given payload, faster rockets consume much

more fuel-the fuel mass increases roughly exponentially with the desired velocity. The

designer must compromise between many battle stations with light rockets or fewer battle

stations with heavier rockets.10

These trade-offs are illustrated in figure 5-

2, which assumes a boost-phase-only defense

with three hypothetical rocket designs: a stateof-the-art rocket based on current technology;

a “realistic” design based on improvements

in rocket technology that seem plausible by

the mid-1990s; and an “optimistic” design that

assumes major improvements in all areas of

rocket development. The key parameters assumed for SBI rocket technology appear in the

classified version of this report. In all cases

analyzed, OTA assumed the rockets to be

“ideal”: the mass ratio of each stage is the

same, which produces the lightest possible

rocket. 11 The first chart in figure 5-2a shows

that rocket mass increases exponentially with

increasing velocity, limiting practical SBI velocities to the 5 to 8 km/s range for rockets

weighing on the order of 100 kg or less.

For analytic purposes, OTA has considered

constellations of SBIs that would be necessary

to intercept virtually 100 percent of postulated

numbers of ICBMs. It should be noted that

since the system architecture analyses of 1986,

SDIO has not seriously considered deploying

SBIs that would attempt to intercept anywhere near 100 percent of Soviet ICBMs and

PBVS.

12 This OTA analysis is intended only

‘“Projectile mass might not be as critical for ground-based

as for spacebased KEW projectiles, since there would be no

space transportation cost. However, the projectile mass should

still be minimized to reduce the over-all rocket size and cost,

and to permit higher accelerations and final velocities.

llThe ma99 fraction for a rocket stage is defined = the ratio

of the propellant mass to the total stage mass (propellant plus

rocket structure). The mass fraction does not include the payload mass. For the calculations reported here, an ideal rocket

is assumed: it has equal mass ratios for each stage, where mass

ratio is defined as the initial stage weight divided by the stage

weight after burn-out (both including the payload; it can be

shown that the rocket mass is minimized for a given burnout

velocity if each stage has the same mass ratio.)

“AS indicated in chapters 1,2, and 3, SDIO argues that the

deterrent utility of defenses far more modest than those needed

for “assured survival” would make them worthwhile.

to give a feel for the parameters and trade-offs

involved in a system with SBIs.

Deployment of a system of “state-of-the-art”

SBIs intended to provide 100 percent coverage of Soviet ICBMs would entail 11.7 million

kg of CVs; waiting for the development of the

“realistic” SBI would reduce the mass to orbit by a factor of two.

Figure 5-2b shows the number of SBI carrier platforms and figure 5-2c shows the number of SBIs for a 100 percent-boost-phase defense as a function of SBI velocity. The last

chart (figure 5-2d) shows the total constellation mass as a function of velocity. The number of CVs was calculated initially to optimize

coverage of existing Soviet missile fields: the

orbits of the CVs were inclined so that the CVs

passed to the north of the missile fields by a

distance equal to the SBI fly-out range.13 Each

CV therefore stayed within range of the ICBM

fields for a maximum period during each orbit.

The “optimal” number of CVs resulting from

this calculation was so low as to endanger system survivability (see ch. 11), calling for up to

100 SBIs per carrier to cover the existing Soviet ICBM threat: such concentrations would

provide lucrative targets for the offense’s

ASATs. To increase survivability, the number of CVs was therefore increased by a factor

of 3 for the data in figure 5-2. Some polar orbits were added to cover the SLBM threat from

northern waters.

The number of SBIs was calculated initially

to provide one SBI within range of each of

1,400 Soviet ICBMs sometime during the

boost phase. The booster burn time was taken

as similar to that of existing Soviet missiles,

with a reasonable interval allotted for cloudbreak, initial acquisition, tracking, and weapons launch.

One SBI per booster would not do for a robust (approaching 100 percent coverage) boostphase defense. A substantial number of SBIs

*The locations of Soviet missile fields are estimated from maps

appearing in U.S. Department of Defense, Sow”et Mih”tary Power,

1987 (Washington, D. C.: Department of Defense, 1987), p. 23.

See adaptation of this map in chapter 2 of this OTA report.
114

Figure 5-2a. -Space-based Interceptor Mass v. Velocity

Velocity (km/see)

The SBI mass versus SBI velocity. These data assume 100°/0

coverage of the current Soviet threat of 1,400 ICBMs. It should

be noted that the SDIO currently proposes a substantially

lower level of coverage for SBIs. Therefore, the absolute numbers in the OTA calculations are not congruent with SDIO

plans. Rather, the graphs provided here are intended to show

the relationships among the various factors considered. It

should also be noted that numerous assumptions underlying

the OTA analyses are unstated in this unclassified report, but ,

are available- in the classified version.

SOURCE: Office of Technology Assessment, 1988.

Figure 5-2b.-Number of Satellites v. SBI Velocity

1

I I I I

4 6 10 12

Velocity~km/see)

The number of SBI carrier satellites v. SBI velocity.

SOURCE: Office of Technology Assessment, 1988

Figure 5-2c. -Number of Space-Based Interceptors

v. Velocity (Inclined orbits + SLBM polar orbits)

40

30 —

20 —

4 6 10 12

Velocity ~km/see)

The number of space-based interceptors (SBIs) required to

provide one SBI within range of each of 1,400 existing Soviet

ICBMs before booster burnout. ‘

SOURCE: Office of Technology Assessment, 1988.

Figure 5-2d. -Constellation Mass v. SBI Velocity

4 6 8 10 12

Velocity (km/see)

The total constellation mass in orbit (SBIs and carrier vehicles, excluding sensor satellites) v. SBI velocity. The minimum constellation mass for the “realistic” SBI to be in position to attack all Soviet boosters would be about 5.3 million

kg. Faster SBIs would permit fewer carrier vehicles and fewer

SBIs, but the extra propellant on faster SBIs would result in

a heavier constellation. For reference, the Space Shuttle can

lift about 14,000 kg into polar orbit, a 5.3 million kg constellation would require about 380 Shuttle launches, or about 130

launches of the proposed “Advanced Launch System” (ALS),

assuming it could lift 40,000 kg into near-polar orbit at suitable altitudes.

SOURCE: Office of Technology Assessment, 1988.
115

would fail over the years just due to electronic

and other component failures. The number of

SBIs in figure 5-2 was increased by a plausible factor to account for this natural peacetime

attrition. In addition, during battle, some SBIs

would miss their targets, and presumably Soviet defense suppression attacks would eliminate other CVs and draw off other SBIs for

self-defense.

Given the above assumptions, figure 5-2 represents the SBI constellation for nearly 100

percent coverage of the existing Soviet ICBM

fleet in the boost phase, with modest survivability initially provided by substantial SBI

redundancy, degrading to no redundant SBIs

as “natural” attrition set in.

Note that for each type of rocket there is an

optimum velocity that minimizes the total

mass that would have to be launched into

space; lower velocity increases the number of

satellites and SBIs, while higher velocity increases the fuel mass. In OTA’s analysis, the

minimum mass which would have to be launched

into orbit for the “realistic” rocket is 5.3 million kg (or 11.7 million lb); the mass for a constellation of “optimistic” SBIs would be 3.4

million kg.

The data for figure 5-2 all assume booster

burn times similar to those of current Soviet

liquid-fueled boosters. Faster-burning rockets

would reduce the effective range of SBIs and

would therefore increase the needed number

of carrier satellites. The same SBI parameters

are shown in figures 5-3a and b with an assumption of ICBM booster burn time toward the

low end of current times. The minimum constellation mass has increased to 29 million and

16 million kg, respectively, for the “realistic”

and “optimistic” rocket designs.

Several studies of “fast-burn boosters” concluded that reducing burn-time would impose

a mass penalty, so the Soviets would have to

off-load RVs (or decoys) to reduce burn time

significantly. But these same studies showed

that there is no significant mass penalty for

burn times as low as 120 s. About 10-20 percent of the payload would have to be off-loaded

for burn times in the 70 to 90 s range.

Figure 5-3a. -Number of Projectiles v. SBI Velocity

(160 second burn-time)

0

4 6 8 10 12

Velocity (km/see)

The number of space-based interceptors v. SBI velocity for

reduced booster burntime (within currently applied technology).

SOURCE: Office of Technology Assessment, 1988.

Figure 5-3b. -SBI Constellation Mass v. SBI Velocity

(160 second burn-time)

10

0

4 6 8 10 12

Velocity (km/see)

The total constellation mass (carrier vehicles and SBIs) versus

SBI velocity for reduced booster burn times, assuming one

SBI within range of each of 1,400 boosters before burnout.

SOURCE: Office of Technology Assessment, 1988

If the Soviet Union could reduce the burn

time of its missiles below that of any currently

deployed ICBMs, then the total SBI constellation mass necessary for boost-phase intercept would increase dramatically. The minimum constellation mass to place one SBI

within range of each ICBM during its boost

phase is shown in figure 5-4 as a function of
116

Figure 5-4.-Total SBI Constellation Mass in Orbit

v. Booster Burn Time

Realistic

rocket -

120 160 200 240 280 300

Booster bum time (seconds)

The effect of Soviet booster burn time on SBI constellation

mass. If we consider 40 million kg as a maximum conceivable upper bound on constellation mass (corresponding to

2,800 Shuttle flights or 1,000 launches of the proposed ALS

system), then booster times of 120 to 150 seconds would severely degrade a 100%-boost-phase defense with chemically

propelled rockets. The ability of smaller constellations of SBIs

to achieve lesser goals would be analogously degraded by

the faster burn times.

All assumptions are the same as for the previous figures,

except for the burn-out altitude, which varies with burn-time.

SOURCE: Office of Technology Assessment, 1988.

booster burn time for the three canonical rocket

designs.

The masses described above for a boostphase-only defense are clearly excessive, particularly for a responsive Soviet threat. Adding other defensive layers would reduce the

burden on boost-phase defense. The next layer

of defense would attack PBVs, preferably early

in their flight before they could unload any

RVs.

A PBV or “bus” carrying up to 10 or more

RVs would be more difficult to track and hit

than a missile. A PBV has propulsion engines

that emit some IR energy, but this energy will

be about 1,000 times weaker than that from

a rocket plume.14 A PBV is also smaller and

less fragile than a booster tank. In short, a PBV

is harder to detect and hit with an SBI. However, a PBV is still bigger and brighter than

Whe first stage of an ICBM might radiate 1 million W/sr,

the second stage 100,000 W/sr, while a PBV may emit only 100

W/sr. On the other hand, the RV radiates only 5 W/sr, so the

PBV is a better target than an RV.

an RV; sensors might acquire the PBV if its

initial trajectory (before its first maneuver) can

be estimated by projecting the booster track.

The effectiveness of a combined boost and

post-boost defense in terms of the percentage

of RVs killed is estimated in figure 5-5 for the

“realistic” SBI rocket. The calculation assumes that 1,400 missiles resembling today’s

large, heavy ICBMs are spread over the existing Soviet missile fields.

The net effect of attacking PBVs is to reduce the number of SBIs needed to kill a given

number of RVs. For example, to destroy 85

percent of the Soviet RVs carried by ICBMs,

a boost-only defense system would require

about 26,000 SBIs in orbit. Adding PBV interceptions reduces the number of SBIs needed

to about 17,000.

A defensive system must meet the expected

Soviet threat at the actual time of deployment,

not today’s threat. For example, the Soviet

Union has already tested the mobile, solidfueled SS-24 missile, which can carry 10 warFigure 5-5.—Boost and Post-Boost Kill Effectiveness

(1,400 ICBMs v. “Realistic" SBIs)

100

0

o 40

Number of space interceptors (SBIs)

(thousands)

Percentage of reentry vehicles (RVs) killed as a function of

the number of space-based interceptors (SBIs) deployed in

space. This calculation assumes a threat of 1,400 ICBMs

spread over the Soviet missile fields. The SBIs have a plausible single-shot probability of killing a booster and a slightly

smaller chance of killing a PBV; a substantial fraction of the

SBIs are used for self-defense (or are not functional at the

time of attack).

SOURCE: Office of Technology Assessment, 1988,
117

heads. There is no reason to doubt that the

Soviets could deploy this kind of missile in

quantity by the mid-1990s. Such a fleet would

particularly stress a space-based defense if deployed at one or a few sites, since more SBIs

would be needed in the area of deployment concentration.

The effects on the combined boost and postboost defense of clustering 500 shorter-burntime, multiple-warhead missiles at three existing SS-18 sites are shown in figure 5-6a. It

would take about 23,000 SBIs to stop 85 percent of these 5,000 warheads. If the assumed

500 ICBMs were concentrated at one site (but

still with 10 km separation to prevent “pindown” by nuclear bursts), then 30,000 SBIs

would be needed (see fig. 5-6b).l5

Finally, the Soviets might deploy 200 (or

more) current-technology, single-warhead missiles atone site, as shown in figure 5-7. In this

case, no reasonable number of SBIs could intercept 85 percent of these 200 extra warheads

(50,000 SBIs in orbit would kill 70 percent).

Twice as many RVs are destroyed in the postboost period as the boost-phase. Once this concentrated deployment was in place, the defense

would have to add about 185 extra SBIs and

their associated CVs to achieve a 50 percent

probability of destroying each new ICBM deployed.

SBI Projectile Mass.—The constellation

masses shown above assume that the mass of

the smart SBI projectile (including lateral divert propulsion, fuel, guidance, sensor, communications, and any lethality enhancer) can

be reduced to optimistic levels. Current technology for the various components would result in an SBI with a relatively high mass. Thus

mass reduction is essential to achieve the results outlined above; total constellation mass

would scale almost directly with the achievable SBI mass.

“Concentrating 500 missiles atone site would have disadvantages for an offensive attack: timing would be complicated to

achieve simultaneous attacks on widely separated U.S. targets,

and Soviet planners may be reluctant to place so many of their

offensive forces in one area, even if the missiles are separated

enough to prevent one U.S. nuclear explosion from destroying

more than one Soviet missile.

Figure 5-6a.— Boost and Post-Boost Kill Effectiveness

(500 single.RV ICBMs at three sites)

I

(“Realistic” SBIs)

1

- o 10 20 30 40

Number of space-based interceptors (SBIs)

(thousands)

The percentage of RVs from modestly short-burn ICBMs killed

as a function of the number of SBIs deployed in space. This

curve corresponds to 500 such ICBMs deployed at 3 existing SS-18 sites. All SBI parameters are the same as in previous figures.

SOURCE: Office of Technology Assessment, 1988

Figure 5-6b.—Boost and Post-Boost Kill Effectiveness

(500 singIe.RIV ICBMs at one site)

- 0 10 20 30 40

Number of space-based interceptors (SBIs)

(thousands)

This curve assumes that all 500 shorter-burn ICBMs are deployed at one site (but still with 10 km separation to prevent

pin-down). All other parameters are the same as figure 5-6a.

SOURCE: Office of Technology Assessment, 1988,

Rocket Specific Impulse. —Similarly, the specific impulse of the rocket propellant would

have to be improved from current levels. The

specific impulse, expressed in seconds, measures the ability of a rocket propellant to change

mass into thrust. It is defined as the ratio of

thrust (lb) divided by fuel flow rate (lb/s).
118

Figure 5-7. - Boost and Peat-Boost Kill Effectiveness

(200 “medium-burn-booster” ICBMs at one site)

100

90 —

(“Realistic” SBIs)

I

0

0 10 20 30 40

Number of space-based interceptors (SBIs)

(thousands)

Percentage of single-warhead ICBM RVs killed as a function

of number of SBIs in space. The 200 single-warhead ICBMs

are deployed at one site with 10 km separation. All SBI parameters are as in previous figures.

SOURCE: Office of Technology Assessment, 1988.

The specific impulse of current propellants

varies from 240 to 270 s for solid fuel and up

to 390s at sea level for liquid oxygen and liquid hydrogen fuel. Assuming that BMD weapons would utilize solid fuels for stability and

reliability, then the specific impulse for current technology would be limited to the 270-s

range. 16 One common solid propellant, hydroxyl-terminated polybutadiene (HTPB)

loaded with aluminum, has an impulse in the

260-to-265-s range. This can be increased to

280s by substituting beryllium for the aluminum. Manufacturers of solid propellant say

that further improvements are possible.

Rocket Mass Fraction. —Finally, the mass

fraction-the ratio of the fuel mass to the stage

total mass (fuel plus structure but excluding

payload) –would have to be raised to meet SDI

objectives. Large mass fractions can be

achieved for very big rockets having 95 percent of their mass in fuel. It would be more

difficult to reduce the percentage mass of structure and propulsion motor components for very

small SBI rockets.

—

IThe SBI divert propulsion system in the final projectile stage

would probably use liquid fuel, and some have suggested that

the second stage also use liquid fuel.

The mass fraction can be increased by reducing the mass of the rocket shell. New lighteight, strong materials such as carbon graphite fiber reinforced composite materials or

judicious use of titanium (for strength) and aluminum (for minimum mass) may permit increased mass fractions for future rockets.

How Electromagnetic Launchers (EML) Work.

–Electromagnetic launchers or “railguns” use

electromagnetic forces instead of direct chemical energy to accelerate projectiles along a pair

of rails to very high velocities. The goal is to

reach higher projectile velocities than practical rockets can. This would extend the range

of KEW, expanding their ability to attack

faster-burn boosters before burn-out. Whereas

advanced chemically propelled rockets of reasonable mass (say, less than 300 kg) could

accelerate projectiles to at most 9 to 10 km/s,

future EML launchers might accelerate small

projectiles (1 to 2 kg) up to 15 to 25 km/s. SDIO

has set a goal of reaching about 15 km/s.

In principle, chemical rockets could reach

these velocities simply by adding more propellant. The efficiency of converting fuel energy

into kinetic energy of the moving projectile decreases with increasing velocity, however: the

rocket must accelerate extra fuel mass that is

later burned. A projectile on an ideal, staged

rocket could be accelerated to 15 km/s, but only

17 percent of the fuel energy would be converted into kinetic energy of the projectile,

down from 26 percent efficiency for a 12 km/s

projectile. Since a railgun accelerates only the

projectile, it could theoretically have higher

energy efficiency, which would translate into

less mass needed in orbit.

In practice, however, a railgun system would

not likely weigh less than its chemical rocket

counterpart at velocities below about 12 km/s,

since railgun system efficiency would probably

be on the order of 25 percent at this velocity .17

17ms assumes 50 ~rmnt efficiency for converting fuel (thermal) energy into electricity, 90 percent efficiency in the pulse

forming network, and 55 percent rail efficiency in converting

electrical pulses into projectile kinetic energy. The SDIO has

a goal of reaching 40 percent overall EML system efficiency,

but this would require the development of very high temperature (2,000 to 2,500° K) nuclear reactor driven turbines. The

total system mass might still exceed that of a comparable chemical rocket system.
119

Therefore a railgun system would have to carry

as much or more fuel than its chemical rocket

equivalent-in addition to a massive rocketengine generator system, an electrical pulseforming network to produce the proper electrical current pulses, and the rail itself.

The conventional “railgun” (see figure 5-8)

contains a moving projectile constrained by

two conducting but electrically insulated rails.

A large energy source drives electrical current

down one rail, through the back end of the moving projectile, and back through the other rail.

This closed circuit of current forms a strong

magnetic field, and this field reacts with the

current flowing through the projectile to produce a constant outward force. The projectile

therefore experiences constant acceleration as

it passes down the rail.

The final velocity of the projectile is proportional to the current in the rail and the square

root of the rail length; it is inversely proporFigure 5-8.-Schematic of an Electromagnetic

Launcher (EML) or “Railgun”

Schematic of an electromagnetic launcher (EML) or “Railgun. ” In operation, a strong pulse of electrical current forms

a circuit with the conducting rails and the projectile. This current loop generates a magnetic field. The interaction of this

field with the current passing through the moving projectile

produces a constant outward force on the projectile, accelerating it to high velocities.

SOURCE’ Office of Technology Assessment, 1988.

tional to the square root of the projectile mass.

High velocity calls for very high currents (millions of amperes), long rails (hundreds of m),

and very light projectiles (1 to 2 kg).

For the BMD mission, the projectile must

be “smart”. That is, it must have all of the components of the chemically propelled SBIs: a

sensor, inertial guidance, communications, divert propulsion, a computer, and possibly a

lethality enhancement device. The EML projectile must be lighter, and it must withstand

accelerations hundreds of thousands times

greater than gravity, compared to 10 to 20

“g’s” for chemically propelled SBIs.

Researchers at Sandia National Laboratory

have proposed another type of EML launcher

which would employ a series of coils to propel

the projectile. Their “reconnection gun” would

avoid passing a large current through the projectile, eliminating the “arcs and sparks” of

the conventional railgun. The term “reconnection” derives from the action of the moving

projectile: it interrupts the magnetic fields of

adjacent coils, and then these fields “reconnect” behind the projectile, accelerating it in

the process.

Current Status of EMLs.—Several commercial and government laboratories have built

and tested experimental railguns over the last

few decades. These railguns have fired very

small plastic projectiles weighing from 1 to

2,500 g, accelerating them to speeds from 2

to 11 km/s. In general, only the very light

projectiles reached the 10 km/s speeds.

One “figure of merit,” or index, for railgun

performance is the kinetic energy supplied to

the projectile. For BMD applications, SDIO

originally set a goal of a 4 kg projectile accelerated to 25 km/s, which would have acquired

1,250 MJ of energy. SDIO officials now state

that their goal is a 1 kg projectile at 15 km/s,

which would acquire 113 MJ of kinetic energy.

The highest kinetic energy achieved to date

was 2.8 MJ (317 g accelerated to 4.2 km/s), or

about 50 to 400 times less than BMD levels.

Finally, there have been no experiments with

actual “smart” projectiles. All projectiles have

been inert plastic solids. Some (non-operating)
120

Photo credit: Contractor photo released by the U.S. Department of Defense

Electromagnetic launcher.—This experimental electromagnetic launcher at Maxwell Laboratories, Inc., San

Diego, CA, became operational late in 1985.

electronic components, including focal plane

arrays, have been carried on these plastic

bullets to check for mechanical damage. Results have been encouraging.

Key Issues for EMLs.—Much more research

must precede an estimate of the potential of

EML technology for any BMD application.

The key issues are summarized in table 5-1.

There is uncertainty at this time whether all

these issues can be favorably resolved.

Table 5-1 .—Key Issues for Electromagnetic

Launchers (EML)

Low-mass (2 kg or less), high acceleration (several

hundred thousand g) projectile development.

High repetition rate rails (several shots per second for

hundreds of seconds).

High repetition rate switches with high current (several

million A versus 750,000 A)

Pulse power conditioners (500 MJ, 5 to 20 ms pulses

versus 10 MJ, 100 ms pulses)

Efficiency

Mass

Heat dissipation

SOURCE: Office of Technology Assessment, 1988.

EML Projectile. –If based on current technology for sensors, inertial guidance, communications, and divert propulsion systems, the

lightest “smart” projectile would weigh over

10 kg. The total mass must shrink by at least

a factor of 5, and the projectile must withstand

over 100,000 g’s of acceleration. If the projectile could only tolerate 100,000 g’s, then the

railgun would have to be 112 m long to impart

a 15 km/s velocity to the projectile. Higher acceleration tolerance would allow shorter railguns. (200,000 g’s would allow a 56-m long gun,

etc.)

The SDIO has consolidated the development

of light-weight projectiles for all kinetic energy

programs into the “Light-weight Exo-atmospheric Projectile” (LEAP) program. Although

researchers first saw a need for light-weight

projectiles for railguns, the primary initial

users of LEAP technology are to be the chemical rocket KEW programs (SBI, ERIS,

HEDI). The phase-one LEAP projectile would

weigh about 5 kg according to current designs

(see figure 5-9), if all component developments

met their goals. This projectile would weigh

too much for any railgun, and it will therefore

not be tested at high acceleration. This technology might evolve into a 2-kg projectile by

the early 1990s. In any case, there are no plans

now to build a gun big enough to test even the

phase-two 2 kilogram projectile.

High Repetition Rate.–A railgun would

have to fire frequently during an attack, engaging several targets per second. The penalty

for low repetition rates would be additional railguns in the space-based constellation to cover
121

Figure 5-9.— Lightweight Homing Projectile

LEAP PHASE 1

WEIGHT S’

LEAP PHASE 1

PAYLOAD (1 600)

SEEKER 295.0

IMU 285.0

INTEGRATED PROCESSOR 400.0

COMMAND RECEIVER 100.0

POWER SYSTEM 325.0

STRUCTURE & MISC. 195.0

PROPULSION (3,059.0

VALVES & NOZZLES 570.0

CASE & INERTS 551.0

PROPELLANT 1818.0

VALVE DRIVERS 120.0

PROJECTILE TOTAL 4659.(

LEAP PHASE 2

STATEMENTS

LEAP PHASE 2

PAYLOAD (670)

SEEKER 150.0

IMU 70.0

INTEGRATED PROCESSOR 150.0

COMMAND RECEIVER 25.0

POWER SYSTEM 175.0

STRUCTURE & MISC. 100.0

PROPULSION 0 (1325. 0

VALVES & NOZZLES 375.0

CASE & INERTS 215.0

PROPELLANT 690.0

VALVE DRIVERS 45.0

PROJECTILE TOTAL 1995,0

Illustration of planned projectiles for the Lightweight Exoatmospheric Projectile (LEAP) program. This program is developing

projectile technology for both the rocket propelled and electromagnetic launcher (railgun) programs. However, the phase 1

projectile at 5 kg and even the more conceptual phase 2 projectile with a mass projection of 2 kg are too heavy for any existing

or planned railguns. There are no plans to test either projectile at the 100,000s of g acceleration necessary for rail gun operation. These projectiles will benefit the SBI, ERIS, and HEDI programs.

SOURCE: Office of Technology Assessment, 1988.

the threat. Most railguns to date have been

fired just once: the rails eroded and had to be

replaced after one projectile. Newer systems

can fire ten shots per day, and at least one experiment has fired a burst of pellets at a rate

of 10/s. Researchers at the University of Texas

plan to fire a burst of ten projectiles in 1/6 of

a second, or a rate of 60/s.

Key issues for high repetition-rate guns are

rail erosion,18 heat management, and high repetition-rate switches to handle the millionampere current levels several times per second.

Conventional high repetition-rate switches can

‘*New rail designs have shown promise of minimum erosion

in laboratory tests; it remains to be proven that rails would survive at weapons-level speeds and repetition rates.
122

handle up to 500 A today, although one special variable resistance switch tested by the

Army carried 750,000 A. An Air Force test successfully switched 800,000 A, limited only by

the power supply used. EML systems would

have to switch 1 to 5 million A.

EML Power. –An EML would consume high

average electrical power and very high peak

power during each projectile shot. Consider

first the average power requirements: a 1 kg

projectile would acquire 112 MJ of kinetic

energy if accelerated to 15 km/s. Assuming 40

percent efficiency and 5 shots per second, then

the EML electrical system would have to deliver 280 MJ of energy per shot or 1.4 GW of

average power during an attack which might

last for several hundred seconds. For comparison, a modern nuclear fission power plant delivers 1 to 2 GW of continuous power.

The SP-100 nuclear power system being discussed for possible space application would

produce only 100 to 300 kW of power. The only

apparent near-term potential solution to providing 2.5 GW of power for hundreds of seconds would be to use something like the Space

Shuttle Main Engine (SSME) coupled to a turbogenerator. Assuming 50 percent electrical

conversion efficiency, then one could convert

the SSME 10 GW of flow energy into 5 GW

of average electrical power while the engines

were burning.

High average power would not suffice. The

electrical energy would have to be further concentrated in time to supply very short bursts

of current to the railgun. For example, a 112-

m long railgun with 100,000-g acceleration

would propel a projectile down its length in

about 15 milliseconds (ins) to a final velocity

of 15 km/s. The peak power during the shot

would be 50 GW.l9 And the EML system designer would like to shorten the 112-m railgun

length and increase acceleration, which would

mean further shortening the pulse length and

increasing peak power.

Several techniques are under consideration

to convert the average power from something

like the SSME into short pulses. One laboratory approach is the homopolar generator: this

device stores current in a rotating machine

much like an electrical generator and then

switches it out in one large pulse. Existing

homopolar generators can supply up to 10 MJ

in about 100 ms; therefore, energy storage capacity must increase by a factor of 50 and the

pulse length shorten by a factor of 5 to 20.

EML Mass to Orbit.–The mass of an EML

system based on today’s technology would be

excessive. A homopolar generator to supply

280 MJ per pulse would weigh 70 tonnes

alone. 20 The rails would have to be long to limit

acceleration on sensitive “smart projectiles,

which would have to be very strong (massive)

to resist the outward forces from the high rail

currents. The platform would have to include

an SSME-type burst power generator, a thermal management system to dispose of the

energy deposited in the rails, divert propulsion

to steer the railgun toward each target, and

the usual satellite communications and control functions.

Given the early stage of EML research, estimates of total platform mass could be in error

by a factor of 10. At this time, a total mass

of about 100 tonnes would seem likely, meaning that each EML would have to be launched

in several parts, even if the United States developed an Advanced Launch System (ALS)

that could carry about 40 tonnes maximum per

flight to high inclination orbits. It is conceivable that, in the farther term, superconductive

electrical circuits could significantly reduce the

mass of an EML. Lighter compulsators (see

below) might also reduce EML mass.

Nuclear-Driven Particles.–A nuclear explosion is a potent source of peak power and

energy. If even a small fraction of the energy

in a nuclear explosion could be converted into

kinetic energy of moving particles, then an extremely powerful nuclear shotgun could be imlgFor a fi~e of referen~, consider that the total power av~-

able from the U.S. power grid is several hundred gigawatts.

20Assuming today’s energy density for homopolar generators

of 4 kJ/kg.
123

agined. These particles could be used for interactive discrimination as described above,

since the particles would slow down light decoys more than heavy RVs With more power,

nuclear-driven particles could conceivably destroy targets. This concept is discussed in more

detail in the classified version of this report.

Directed-Energy Weapons

Directed-energy weapons (DEW) offer the

promise of nearly instantaneous destruction

of targets hundreds or thousands of km away.

While a KEW system would have to predict

target positions several minutes in the future

and wait for a high speed projectile to reach

the intended target, the DEW could—in principle—fire, observe a kill, and even order a repeat attack in less than a second.

DEW Systems

Although no DEW are planned for phaseone BMD deployment, both ground-based and

space-based DEW systems are possible in the

next century .21 Candidate DEW systems

include:

●

●

●

●

●

free electron lasers (FEL) (ground-based

or space-based),

chemical lasers (space-based),

excimer lasers (ground-based),

x-ray laser (pop-up or space-based), or

neutral particle beam (space-based).

The FEL is the primary SDIO candidate for

ground-based deployment (with the excimer

laser as a back-up). The hydrogen-fluoride (HF)

laser and the neutral particle beam weapon are

the primary candidates for space-ased DEWS,

although a space-based FEL or other chemical laser concepts might also be possible.

Ground-Based Free Electron Laser (GBFEL).

–A GBFEL system would include several

ground-based lasers, “rubber mirror” beam directors to correct for atmospheric distortions

and to direct the beams to several relay mirrors in high-Earth orbit, and tens to hundreds

ZISDICI a9Wrt9 that some versions of DE W could be deployed

late in this century. It is examining designs for “entry level”

systems with limited capabilities.

of “battle-mirrors’ in lower Earth orbit to focus the beams on target. It would take several laser sites to assure clear weather at one

site all the time. Several lasers per site would

provide enough beams for the battle. Ideally

these lasers should beat high altitudes to avoid

most of the weather and atmospheric turbulence. But the FEL, as currently envisioned,

requires very long ground path lengths for

beam expansion and large quantities of power.

The logical location for relay mirrors would

be geosynchronous orbit, so that the groundbased beam director would have a relatively

fixed aim point. The effects of thermal

blooming 22 may best be avoided, however, by

placing the relay mirrors in lower orbit: the motion of the laser beam through the upper atmosphere as it follows the moving relay mirror

would spread the thermal energy over a large

area.23

Adaptive optics would correct for atmospheric turbulence. The optical system would

sense turbulence in real time and continuously

change the shape of the beam-director mirror

to cancel wave-front errors introduced by the

air. A beacon would be placed just far enough

in front of the relay satellite that the satellite

would move to the position occupied by the

beacon in the time it took for light to travel

to the ground and back. A sensor on the ground

would detect the distortions in the test beam

of light from the beacon, then feed the results

to the “rubber mirror” actuators. With its

wave front so adjusted, the laser beam would

pass through the air relatively undistorted.

—

‘

zThermal blooming occurs when a high-power laser beam

passes through the atmosphere, heating the air which disturbs

the transmission of subsequent beam energy. See the section

below on key DEW issues for details.

23 For exmple, a 1O-m dimekr laser beam which tracked a

relay mirror at 1,000 km altitude would pass through a clean,

unheated patch of air at 10 km altitude after 140 ms. If thermal

blooming resulted from relatively long-term heating over a few

seconds, then scanning across the sky could ameliorate its effects. While beam energy at altitudes below 10 km would take

longer than 140 ms to move to unheated patches of the atmosphere, lower altitude blooming could be more readily corrected

by the atmospheric turbulence compensation systems proposed

for ground based lasers: atmospheric compensation works best

for “thin lens” aberrations close to the laser beam adaptive mirror on the ground.
124

This concept is discussed further below, under the heading of “Key DEW Issues.”

Table 5-2 compares the characteristics of current research FELs with those needed for

BMD operations, as derived horn elementary

considerations in the American Physical Society study .24 The key figure of merit is beam

brightness, defined as the average laser output power (watts) divided by the square of the

beam’s angular divergence. Brightness is a

measure of the ability of the laser beam to concentrate energy on the target (see figure 5-10).

Another important figure of merit is the retarget time—the time needed to switch from one

target to another.

~iAmerican Physical Society, op. cit., footnote 1, chapters 3

and 5.

Existing FELs operate in a pulsed mode: the

energy is bunched into very short segments,

as illustrated in figure 5-11 for the radio frequency linear accelerator (RF linac) and for the

induction linear accelerator, two types of accelerators proposed for the FEL. The power

at the peak of each pulse is much higher than

the average power. In the proposed induction

linac FEL, peak power might exceed average

power by 60,000 times. But it is the average

power that primarily determines weapons effestiveness. 26

The RF linac experiments to date have

produced 10 MW of peak power at 10 μm wave

“Short pulses of energy may foster coupling of energy into

a target, however, so the average power required from a pulsed

laser could, in principle, be less than the average power of a

continuous wave (CW) laser. This will be the subject of further

SDI research.

Table 5-2.—Characteristics of a Ground-Based FEL Weapons System

Operational requirements against

Current status a fully responsive Soviet threata

Free Electron Laser. . . . . . . . . . . . . . . . . . . . . . . RF Induction

Number of laser sites . . . . . . . . . . . . . . . . . . . — 5-8

Wavelength μm). . . . . . . . . . . . . . . . . . . . . . . . 9-35 8,800 .8 to 1.3

Average power (MW) . . . . . . . . . . . . . . . . . . . . .006 .000014 100 to 1,000

Peak power (MW). . . . . . . . . . . . . . . . . . . . . . . 10 1,000 b

Beam diameter (m) . . . . . . . . . . . . . . . . . . . . . (4)C

(4) 10 to 30

Brightness (W/sr) . . . . . . . . . . . . . . . . . . . . . . . 3.6x 1014 1 x 106

several x1022

Peak brightness (W/sr) . . . . . . . . . . . . . . . . . . 6.3x 1017 4.9 x 1013 d

Beam director:

Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) 10’

Number of actuators. . . . . . . . . . . . . . . . . . . . (103

) 103 to 104

Frequency response (Hz) . . . . . . . . . . . . . . . . 103hundreds

Relay mirrors:

Number of mirrors. . . . . . . . . . . . . . . . . . . . . . — 3-5?

Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . – 10 or more

Altitude (km) . . . . . . . . . . . . . . . . . . . . . . . . . . . – tens of thousands

Steering rate (retargets/s). . . . . . . . . . . . . . . . – 4-1o

BattIe-mirrors:

Number of mirrors: . . . . . . . . . . . . . . . . . . . . . 30-150

Diameter (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . (4) 10

Altitude (km) . . . . . . . . . . . . . . . . . . . . . . . . . . . – 1,000-4,000

Steering rate (retargets/s). . . . . . . . . . . . . . . . – 2-5

operational requirements are taken from American Physical Society, Science and Technology of Dkected-Energy Weapons: Report to the American Physical Society

of the Study Group, April 19S7. SDIO dlsagreea with some of the numbers, but their disagreements are classified and may be found in the classified version of this

report. Further, SDIO has identified BMD missions other than dealing with a fully responsive Soviet threat. An “entry-level” syatem (with a brightness on the order

of 10*0), might be developed earlier than the one with the above characteristics and would have less stressing requirements.

bSagment5 of ~ S.meter ~tive minor have baen built, and a A-meter, T.segmmt mirror is under construction. Parentheses in this table indicate that the mirror t=hnOlOgy exists, but the mirrors have not yet been integrated with the laser. CA weapons Systam would require the average ~wer levels listed above. The FEL is a pulsed laser—the powar of each pulsa iS much higher than the average power

when the pulses are both on and off. Depending on how targets and pulses Intaract, these short pulses might be lethal even with lower average power.

dpeak brightness, like peak power, is not the relevant measure Of weapons lethatity.

eThe American physl~ai s~lety, op. cit., footnote a, estimated that brightnesses on the order of 10” Wlsr might be necessary Io counter a responsive threat. A lo-meter

diameter mirror would be requirad for the lower power (1 OOMW) FEL module to raach 10’* WLsr brightness. The more probable approach would be to combine the

beams from ten lo-meter mirrors in a coherent array.

SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 19S7 and 19SS.
125

Figure 5-10. -Illustration of the Relationships

Between Laser Parameters and Power Density

Projected on a Target

Laser

D

Laser output power = P (watts)

Beam diameter = D (meters)

For a diffraction-limited beam,

wavelength)

Brightness = B

Power density on target = I (watts/square cm)

R

2

Illustration of the relationships between laser parameters and

power density projected on a target. The key figure of merit

for any laser is its brightness. Brightness measures the ability

of the laser to concentrate power on a distant target. High

brightness requires high laser power and low angular divergence. Low angular divergence in turn requires short wavelength and a large beam diameter. The power density on target is equal to the laser brightness divided by the square of

the distance to that target.

SOURCE: Office of Technology Assessment, 1988.

length, but only 6 kW of average power–which

would translate into a brightness 100,000,000

times less than the level needed for a BMD

weapon against hardened Soviet boosters.26

This 6-kW average power was averaged over

a 100-microsecond long “macropulse” in a

given second.

ZeThi9 brightne99 calculation assumes that the beam would

be expanded to fill a stateof-theart 4-m diameter mirror and

was diffraction-limited.

Figure 5-11 .—FEL Waveforms

Radio frequency LINAC FEL waveform

Duty cycle: Much lower than radio frequency LINAC

Existing laser waveforms from the radio frequency linear accelerator (RF Iinac) free electron laser (FEL) and the induction linear accelerator FEL. The laser light is emitted in very

short pulses. The peak power during these short pulses would

have to be extremely high to transmit high average power to

the targets. This peak intensity, particularly for the induction

Iinac FEL, would stress mirror coatings and could induce

other nonlinear losses such as Raman scattering in the

atmosphere. Therefore, a weapon-grade induction-linac FEL

would have to have higher repetition rates, perhaps on the

order of 10 kilohertz.

SOURCE: Office of Technology Assessment, 1988.

It should be noted, however, that these experiments were not designed for maximum

average power. Low repetition rates were used

primariliy for economic reasons. SDIO scientists say that scaling up the number of macropulses from 1/s to 5,000/s is not a serious problem. If correct, this would mean that 30-MW

average power could be produced with technology not radically different from today ’s. In

addition, aground-based weapon would use a

wavelength an order of magnitude smaller. The

brightness scales as the inverse of the wavelength squared. For a given mirror diameter,

then, if a similar power output could be

produced at a smaller wavelength, and the high

repetition rate were achieved, the brightness

would only need to be increased by a factor

of about 200 for 30 MW at 1 μm. Accomplish-
126

Photo credit: U.S. Department of Defense

Model of ALPHA experimental chemical laser.—This

experimental chemical laser and its large vacuum

chamber have been constructed by TRW at a test site

near San Juan Capistrano, CA. The cylindrical configuration of the laser design may be most

suitable for basing in space.

ing both modifications would entail significant

development work.

Space-based Chemical Lasers.—Placing highpower lasers directly on satellites would eliminate the needs for atmospheric compensation,

redundant lasers to avoid inclement weather,

and relay mirrors in high orbits; it would also

reduce beam brightness requirements by a factor of 4 to 10 (depending on the wavelength

and atmospheric factors) since the atmosphere

would not attenuate the beam.27 These advantages are offset by the engineering challenge

of operating many tens or hundreds of lasers

autonomously in space and by the possible

higher vulnerability of lasers relative to battlemirrors.

270ne defense contractor estimated that a space-based chemical laser system, including space transportation, would cost

about 10 times less than the proposed ground-based free electron laser weapon system.

The laser should operate at short wavelength

(to keep the mirror sizes small) and should be

energy efficient (to reduce the weight of fuel

needed in orbit). Although its wavelength band

(near 2.8 μm) is rather long, the hydrogen fluoride (HF) laser is the most mature and most

efficient laser available today. Table 5-3 compares the characteristics of a potential high

performance HF laser BMD system with the

current mid-infrared chemical laser (MIRACL)

(using deuterium fluoride, or DF) operating at

the White Sands Missile Range in New Mexico. 28

DEW Technology

How DEW Work.—Directed energy weapons

would change stationary, stored energy from

a primary fuel source into a traveling beam of

energy that could be directed and focused on

a target. Several stages of energy conversion

may be necessary. The challenge is to build an

affordable, survivable, and reliable machine

that can generate the necessary beam of

energy. Lasers can be driven by electrical

energy, chemical energy, or nuclear energy.

Free Electron Lasers.—Through 1987, the

SDIO chose the FEL research program to receive the most DEW emphasis (recently, SDIO

has returned to favoring research in spacebased chemical lasers). The FEL uses a relativistic29 electron beam from an accelerator to

amplify a light beam in a vacuum. The key

advantage of the FEL is the lack of a physical

gain medium: all other lasers amplify light in

a solid, liquid, or gas. This gain medium must

be stimulated with energy to produce an excited population inversion of atoms or molecules. The fundamental limitation with these

lasers is the need to remove waste heat before

it affects the optical transparency of the

medium. The FEL achieves its gain while pass28The SDIO is also considering lower-performance, “entry

level, ” space-based chemical lasers for more limited BMD

missions.

29A beam of particles is deemed “relativistic” when it is accelerated to speeds comparable to a fraction of the speed of light

and acquires so much energy that its mass begins to increase

measurably relative to its rest mass.
127

Table 5-3.—Characteristics of an HF Laser Weapons System

Estimated operational

Current status requirements a

Number of laser satellites . . . . . . . . . — 50-150

Altitude (km) . . . . . . . . . . . . . . . . . . . . . 800-4,000

Beam diameter (m) . . . . . . . . . . . . . . . 1.5b10

Power (MW) . . . . . . . . . . . . . . . . . . . . greater than 1 hundreds (single beam)

Brightness (W/sr) . . . . . . . . . . . . . . . . . several x 1017C several x 1021

Phased array alternative:

Number of beams . . . . . . . . . . . . . . 7

Beam diameter (m) . . . . . . . . . . . . . 10

Total power (MW): . . . . . . . . . . . . . . 100 (14 MW per beam)

aThese numbers derived from first principles, and from American Physical Society, SCienCe and Technology of Directed-Energy

Weapons. Report of ttre American Physical Society Study Group, April 1987, which contains estimates of booster hardness

for a fully responsive threat The SDIO neither confirms nor denies these estimates. Current SDIO estimates may be found

In the classified version of this report, In addition, SDIO has identified earlier entry-level systems with less stressing mls.

sions and less stressing requirements with brightnesses on the order of 1010 WLsr.

bThe LAMP mirror, not yet integrated with a high-power laser, has a diameter of 4 m. CAS~Unling perfect beam quality for a multi-megawatt system with the characteristics of the MIRACL laser

SOURCE Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organ izat!on, 1987 and 19fM3

ing through an electron beam plasma, so much

of the “waste heat” exits the active region

along with the electron beam at nearly the

speed of light.

Two types of electron beam accelerator are

currently under investigation in the SDIO program: the radio frequency linear accelerator

(RF linac) and the induction linac.30

In the RF linac, electrical energy from the

primary source is fed to radio-frequency generators that produce an RF field inside the accelerator cavity. This field in turn accelerates

low energy electrons emitted by a special

source in the front end of the accelerator. The

accelerator raises this electron beam to higher

and higher energy levels (and hence higher velocity) and they eventually reach speeds approaching that of light. Simultaneously, the

electrons bunch into small packets in space,

corresponding to the peaks of the RF wave.

This relativistic beam of electron packets is

inserted into an optical cavity. There the beam

passes through aperiodic magnetic field (called

a “wiggler” magnet) that causes the electrons

‘“Other types of accelerators are possible for a free electron

laser, such as the electrostatic accelerator FEL under investigation at the University of California at Santa Barbara, but

the RF linac and induction linac have been singled out as the

primary candidates for initial SDI experiments.

to oscillate in space perpendicular to the beam

axis. As a result of this transverse motion,

weak light waves called synchrotrons radiation

are generated. Some of this light travels along

with the electron packets through the wiggler

magnets. Under carefully controlled conditions, the electron beam gives up some of its

energy to the light beam. The light beam is

then reflected by mirrors at the end of the optical cavity and returns to the wiggler magnet synchronously with the next batch of electrons. The light beam picks up more energy

from each pass, and eventually reaches high

power levels. This type of FEL is an optical

“oscillator”: it produces its own coherent light

beam starting from the spontaneous emission

from the synchrotrons radiation.

As more energy is extracted from the electron beam, the electrons slow down. These

slower electrons are then no longer synchronized with the light wave and the periodic

magnet, so the optical gain (amplification)

saturates. To increase extraction efficiency, the

wiggler magnet is “tapered”: the spacing of

the magnets or the magnetic field strength is

varied so that the electrons continue in phase

with the light wave and continue to amplify

the beam as energy is extracted.

For high-power weapon applications, the

power from an oscillator might be too weak:
128

the limit for an RF linac FEL oscillator is near

20 MW. In this case additional single-pass amplifiers can boost the beam energy. This system is called a master oscillator power amplifier (MOPA) laser.

In the second type of FEL, the induction

linac, large electrical coils accelerate narrow

pulses of electrons. The high energy electrons

interact with an optical beam as in the RF linac

FEL, but the optical beam as currently planned

would be too intense to reflect off mirrors and

recirculate to pickup energy in multiple passes

as in the RF oscillator. Rather, all of the energy

transfer from the electron beam to the optical

beam would occur on a single pass. This would

entail very high gain, which demands very high

density electron beams and very intense laser

light coming into the amplifier. The induction

linac FEL therefore depends on an auxiliary

laser to initiate the optical gain process; this

limits the tunability of the induction linac FEL

to the wavelengths of existing conventional

lasers of moderately high power.

The process of converting electron energy

into light energy can theoretically approach

100 percent efficiency, although it may take

very expensive, heavy, and fragile equipment.3l

Nevertheless, the FEL could achieve very high

power levels, and, unlike other lasers, the RF

linac FEL can be tuned to different wave

lengths by changing the physical spacing or

field strength of the wiggler magnets or the

energy of the electron beam.92 Tunability is

desirable for ground-based lasers, which must

avoid atmospheric absorption bands (wavelengths of light absorbed by the air) if they are

to reach into space.

Chemical (EIF) Lasers.–The HF laser derives its primary energy from a chemical reaction: deuterium and nitrogen trifluoride

~lTot~ sy9~m efficienW would probably be abOUt 20 Per@nt25 percent at best, assuming areasonably optimistic 50 percent60 percent efficiency to convert chemical to electrical energy

using a rocket-driven turbine, and 40 percent efficiency to generate RF power.

‘The wavelength of the FEL is proportional to the wiggler

magnet spacing and inversely proportional to the square of the

electron beam energy. Higher beam energies are necessary for

the short wavelengths needed for BMD.

gases react in a device resembling a rocket engine. Hydrogen gas mixes with the combustion products. Chemical energy raises the resulting HF molecules to an excited state, from ,

which they relax later by each emitting a photon of light energy in one of several wavelength

lines near 2.8 μm in the MWIR. A pair of opposing mirrors causes an intense beam of IR

energy to build up as each pass through the

excited HF gas causes more photons to radiate instep with the previously generated light

wave.33 Some additional electrical energy runs

pumps and control circuits.

Excimer Laser.—In an excimer34 laser, electrical energy, usually in the form of an electron beam, excites a rare gas halide35 such as

krypton fluoride or xenon chloride.36 These

gases then emit in the ultraviolet (UV) region

of the spectrum, with wavelengths in the range

from.2 to .36pm. This very short wavelength

permits smaller optical elements for a given

brightness. However, the optical finish on

those UV optics would have to be of proportionately higher quality.

Ultraviolet light is also desirable for space

applications, since its high energy generally

causes more damage to the surfaces of targets

than does that of longer-wavelength visible or

IR light. One drawback is that internal mirrors resistant to UV radiation damage are more

difficult to make. Another is that UV cannot

readily penetrate the atmosphere. These obstacles, combined with their relative immaturity and low efficiency, have relegated high

power excimers to a back-up role to the FEL

for the ground-based BMD laser.

‘This process of repeat43d radiation in step is called “stimulated emission”: the traveling wave of light stimulates the excited molecule to radiate with the same phase and direction as

the stimulating energy. The resulting beam of light is “coherent”: it can be focused to a very small spot. The term “laser”

is derived from the phrase “Light Amplification by Stimtiatsd

Electromagnetic Radiation.” ~iEXCimer is sho~ for “exci~ state dimer”; the excitation

of these rare gas halides produces molecules that only exist in

the excited state, unlike other lasing media which decay to a

ground state after emitting a photon of light.

S5A “h&de” is a compound of two elements, one of which iS

a halogen: fluorine, chlorine, iodine, or bromine.

“Krypton fluoride produces a wavelength too short to penetrate the atmosphere; for ground-based applications, xenon chloride would be of interest.
Passing a laser beam through a Raman gas

cell can improve its quality. This cell, typically

filled with hydrogen gas, can simultaneously

shift the laser frequency to longer wavelengths

(for better atmospheric propagation), combine

several beams, lengthen the pulse (to avoid

high peak power), and smooth out spatial variations in the incoming beams. A low-power,

high quality “seed” beam is injected into the

Raman cell at the desired frequency. One or

more pump beams from excimer lasers supply

most of the power. In the gas cell, Raman scattering transfers energy from the pump beams

to the seed. This process has been demonstrated in the laboratory with efficiencies up

to 80 percent.

X-ray Laser.—A nuclear explosion generates

the beam of an x-ray laser weapon. Since this

type of laser self-destructs, it would have to

generate multiple beams to destroy multiple

targets at once. It has been proposed that xray lasers would be based in the “pop-up”

mode; their launch rockets would wait near the

Soviet land mass and fire only after a full-scale

ICBM launch had been detected. Since the xrays could not penetrate deeply into the atmosphere unless self-focused, the earliest application for the x-ray laser would likely be as an

ASAT weapon.

Neutral Particle Beam (NPB) Weapon.—The

NPB weapon, like a free electron laser, would

use a particle accelerator (see figure 5-12). This

accelerator, similar to those employed in high

energy physics experiments, would move

charged hydrogen (or deuterium or tritium)

ions to high velocities. Magnetic steering coils

would aim the beam of ions toward a target.

As the beam left the device, a screen would

strip the extra electrons off the ions, resulting in a neutral or uncharged beam of atoms.37

Unlike laser beams, which deposit their

energy on the surface of the target, a neutral

particle beam would penetrate most targets,

causing internal damage. For example, a 100-

MeV particle beam would penetrate up to 4

37A charged beam could not be aimed reliably, since it would

be deflected by the Earth’s erratic magnetic field, so the beam

must be uncharged or neutral.

129

Figure 5-12.— Schematic of a Neutral

Particle Beam Weapon

LH2/Lox- Liquid oxygen

turbine

Turbogenerator Liquid hydrogen

I

Beam expander

magnets

Schematic of a neutral particle beam weapon. Primary power

might be generated by firing a rocket engine, similar to the

Shuttle main engine, coupled to an electrical generator. Alternately, the hydrogen and oxygen could be combined in a

fuel cell to produce electricity. The resulting electrical current would drive the accelerator that would produce a beam

of negatively charged hydrogen ions. This negatively charged

beam would be expanded and directed toward the target by

magnets. Just before leaving the device, the extra electron

on each hydrogen ion would be stripped off, leaving a neutral particle beam that could travel unperturbed through the

earth’s erratic magnetic field.

SOURCE: Office of Technology Assessment, 1988

cm into solid aluminum and a 200-MeV beam

would deposit energy 13 cm deep.38 These

penetrating particles could damage sensitive

circuits, trigger the chemical high explosives

in nuclear warheads, and-at high enough incident energy levels—melt metal–components.

Shielding against neutral particle beams would

be difficult; imposing a large weight penalty.

As mentioned in chapter 4, the NPB may

be usable first as an interactive discriminator.

The beam of energetic hydrogen atoms would

dislodge neutrons from massive RVs (the disY3ee W. Barkas and M. Berger, Tables ofEnergy Losses and

Ranges of Heavy Charged Particles, (Washington, DC: NASA,

1964).
130

criminator NPB would presumably dwell on

each RV and decoy for too short a time to damage the RV). Separate satellites with neutron

detectors would determine which targets were

RVs and which were light-weight decoys. The

NPB technology development would be the

same for the weapon and the interactive discrimination programs, giving it multi-mission

capability.

Current Status of DEW.–Directed energy

weapons are at various stages of development

as discussed below, but none could be considered ready for full-scale engineering development or deployment in the next decade.39

The characteristics of three potential DEW

systems are summarized in table 5-4. A key

figure of merit is the brightness of the beam.

Precisely what brightness would destroy different targets is still under investigation: the SD I

research program is measuring target lethality for different wavelengths and for different

classes of targets. The brightness levels of table 5-4 are derived from physical first principles and assume that the Soviets could convert their missiles to hardened, solid-fueled

boosters by the time DEWS could be deployed. 40

WSDIO has recently been considering “entry-level” options

that it currently considers feasible for phase-two deployment.

40SD10 is considering “entry level” DEWS that would have

much lower brightness and might be effective against today’s

more vulnerable boosters. A synergistic mix of KEW-DEW

boost-phase intercept capability and DEW discrimination is being considered by SDIO as possible parts of a phastwo system.

The Accelerator Test Stand (ATS) neutral

particle beam experimental accelerator at Los

Alamos National Laboratory is the weapon

candidate closest to lethal operating conditions: its brightness would need to rise by

about a factor of 10,000 to assure destruction

of electronics inside an RV at typical battle

ranges (thousands of km). However, in this kill

mode, it maybe hard to determine whether the

electronics actually had been destroyed.

Another factor of 10 to 100 might be needed

to produce visible structural damage.

The MIRACL DF chemical laser operating

at White Sands has greater than 1 megawatt

output power, but its relatively long wavelength, the challenge of unattended space operation, and the uncertainty of scaling this laser to the power levels necessary for ballistic

missile defense would make a deployment decision now premature. The brightness of an HF

or DF laser would have to be increased by a

factor of 10,000 to 100,000 over current levels

to be useful against responsively hardened Soviet boosters. However, an “entry-level” system that might be useful against current boosters would entail an increase in brightness of

only several hundred to several thousand

times.

To test some aspects of a space-based HF

laser, TRW is installing its “Alpha” laser in

a large space-simulation chamber near San

Juan Capistrano, California. The Alpha laser

uses a cylindrical geometry (MIRACL uses linTable 5.4.—Characteristics of Directed Energy Weapons Against a Fully Responsive Soviet Threata

FEL—ground-based HF—space-based NPB—space-based

Primary energy source. . . . . . . . Electric Chemical Electric

Wavelength or energy . .......0.8-1.3 μm 2.7 μm 100-400 MeV

Required brightness (W/sr) . . . .Several X 1022 Several x 1021 Several x 1019 (for

electronics kill)

Current brightness (W/sr) . . . . .Several x 1014 Several x 1017b (potential

(considering for about 1010 1015-1016 (considering

unintegrated if unintegrated unintegrated

components) components considered) components)

Minimum penetration

altitude (km) . . . . . . . . . . . . . . . About 30 About 30 130-170

aTh~ “u~ber~ in this table are ~btai”~d from the American physical Society, science and Technology of Difecfed-EnergY Weapons.’ f?8pOff Of the American ~hySkd

Society Study Group, April 1987, and apply to an advanced BMD system against a responsive threat. The estimates are neither confkmed nor denied by SDIO. SDIO

has identified other BMD missions for which lower “entry-level” systems with lower specifications (on the order of 1010 Wlsr) would be adequate.

bAss uming perfect beam quality for a system with the characteristics of the MIRACL laSer.

SOURCE: Office of Technology Assessment, American Physical Society Study Group, and Strategic Defense Initiative Organization, 1987 and 1988.
131

ear flow) with the supersonic gas flowing outward from a central 1.1-m diameter cylinder

formed by stacking rings of carefully machined

nozzles. The laser beam will take the form of

an annulus passing just outside the radially

directed nozzles. A complex aspheric mirror

system will keep the laser beam within this narrow ring. The goal of this program is to demonstrate multi-megawatt, near-diffraction

limited operation in 1988.

The brightness of a 4-m diameter (the size

of the Large Aperture Mirror Program mirror),

perfect, diffraction-limited beam41 from, for example, a 1-MW laser, would be over 1018 watts/

steradian (W/sr). The Alpha laser was designed

to be scaled to significantly higher levels by

stacking additional amplifier segments. It

would take a coherent combination of many

such lasers to make a weapon able to engage

a fully responsive missile threat.

Chemical lasers to meet a responsive Soviet

missile threat would need brightnesses of 1021-

1022 W/sr. The level needed would depend on

the target dwell and retarget times. These

times, in turn, depend on the laser constellation size and geometry, booster burn time and

hardness, and number of targets which must

be illuminated per unit time. If the Soviets were

to increase the number of ICBMs in a particular launch area or decrease booster burn times,

then the laser brightness needed would increase.

The brightness of aground-based FEL would

have to increase by a factor of 4 to 10 to account for energy losses as the beam passed

through the atmosphere and travelled to and

from relay mirrors in space. Several free electron lasers have been built. None has operated

within a factor of 100 million (108

) of the lethal

brightness levels needed for a fully-responsive

BMD system. Part of the reason is the low

repetition rate of the pulses in experimental

machines. For example, one experiment ran

with the accelerator operated at a rate of one

electron beam pulse every two seconds. Future

accelerators will probably increase this rate to

“See American Physical Society, op. cit., footnote 1, p. 179.

thousands of pulses per second. This will increase average brightnesses accordingly, although, as previously discussed, several more

factors of 10 improvement would be needed.

Lawrence Livermore National Laboratory is

conducting experiments with an FEL based

on an induction linear accelerator (linac). Boeing Aerospace is constructing an RF linac

FEL, based on technology developed by Boeing and Los Alamos National Laboratory.

Initial experiments on the Livermore FEL

in 1985 produced microwave beams at 8.6 mm

wavelength with peak powers of 100 MW.

More recently, the peak power risen to 1.8 GW

(1.8x109W),42 although this intensity lasts for

only 15 nanoseconds (15x10-9

s); the average

power at the repetition rates of one shot every

2 seconds was only 14 W. Scaling to shorter

wavelengths demands higher quality and very

high-energy electron beams. Livermore Laboratory achieved FEL lasing at 10 μm in the

far IR with its “Paladin” laser experiment in

late 1986. Boeing and a TRW/Stanford University collaboration have operated 0.5 μm visible lasers, but at low average power levels.

The Boeing RF linac FEL has the advantage

of multiple optical passes through the wiggler

of the optical oscillator. This means that high

gain is not necessary, as it is with single-pass

induction linacs.43 The RF linac also has more

tolerance of variations in electron beam quality or emittance. The emittance of the RF linac

electron beam could grow (i.e., deteriorate) by

almost a factor of 10 without deleterious effects. In contrast, the induction linac electron

beam cannot increase in emittance by more

than a factor of two without degrading optical beam brightness. 44 However, there has been

more uncertainty as to whether RF linacs could

be scaled to the high current levels needed for

BMD. Induction linacs, on the other hand,

4ZAn&ew M. Sessler and Douglas Vaughan, ‘‘Free-Electron

Lasers, ” Amen”can Scientist, vol. 75, January-February, 1987,

p. 34.

4$The RF Iinac fight require single-pass amplifiers in addition to their multi-pass oscillators (MOPA configuration) to

achieve weapons-class power levels.

44 Private communication, John M.J. Madey, 1987.
132

have inherently high-current capability. Recently, the two FEL concepts have appeared

on the whole to compete closely with one

another.

Excimer lasers have been utilized for lower

power research and some commercial applications. The UV energy from an excimer laser

is generally more damaging than visible or IR

energy. However, UV light can also damage

mirrors and other optical components within

the laser system, making high-power operation

much more difficult. Scaling to higher power

is possible, but SDIO has judged the excimer

program less likely to succeed, and has cut it

back. The Air Force ASAT program is funding continued excimer laser research jointly

with SDIO.

Los Alamos National Laboratory researchers have conducted NPB-related experiments

on their ATS. They have produced a current

level of 0.1 A at 5 MeV. Rocket-borne tests

of parts of a NPB system were planned for the

late 1980s. The SDIO had planned a series of

full space tests to begin in the early 1990s, including a NPB accelerator with a target satellite and a neutron detector satellite as part of

the interactive discrimination experimental

program. Recently, scheduling of these tests

has been delayed due to funding constraints.

Key DEW Issues.–With such a wide gap between operational requirements and the current status of DEW, many key technical issues remain. DEW research over the next 10

to 20 years could resolve some issues judged

crucial today, but could also uncover other, unforeseen, roadblocks. Some of the current issues of concern (large mirrors, pointing and

tracking, and lethality measurements) are

generic to all laser systems, while others are

specific to particular weapon systems.

Large Mirrors. –All laser systems (except

the x-ray laser) need very large mirrors to focus the beam to a small spot at the target.45

‘

bSpot size k inversely proportional to mirror diameter. Laser brightness, the primary indicator of weapon lethality, increases as the square of mirror diameter. Thus doubling the

mirror size from 2 meters to 4 meters would increass laser brightness by a factor of 4.

This is true for both ground-based lasers with

multiple relay mirrors in space and for spacebased lasers with the mirror adjacent to the

laser. In either case, the size of the last mirror

(closest to the target) and its distance from the

target determine the size of the laser spot focused on that target. To achieve the brightness levels of 1021 to 1022 W/sr for BMD against

a fully responsive threat, laser mirrors would

have to be at least 4 m (assuming mirrors were

ganged into coherent arrays), and preferably

10 to 20 m, in diameter.

The largest monolithic telescope mirrors

today are about 5 m in diameter (Mt. Palomar),

and the largest mirror built for space application is the Hubble Space Telescope at 2.4 m.

The Hubble or Palomar mirror technologies

would not simply be scaled up for SD I applications. The current trend both in astronomy and

in military applications is to divide large mirrors into smaller segments. Electro-mechanical actuators within the mirror segments adjust their optical surfaces so that they behave

as a single large mirror.

Even for these segments, direct scaling of

old mirror manufacturing techniques using

large blocks of glass for the substrate is not

appropriate: these mirrors must weigh very little. They must be polished to their prescribed

surface figure within a small fraction of the

wavelengths they are designed to reflect.

Brightness and precision make opposite demands: usually, a thick and relatively heavy

substrate is necessary to keep good surface figure. SDIO has developed new technologies to

reduce substrate weight substantially.

Two segments of a 3-segment, 3-m mirror

(HALO) have been built. The 7-segment, 4-m

mirror (LAMP) is now assembled and currently

being tested. One segment of a l0-m mirror

is to be built by 1991, but there are no current

plans to assemble a complete 10-m mirror. Recently, the SDIO has begun tests of the lightweight LAMP mirror, designed for spacebased lasers.

Durable, high-reflectivity mirror coatings are

essential to prevent high laser power from

damaging the mirrors. The largest mirror that
133

has been coated with a multi-layer dielectric

coating to withstand high energy-density

levels is 1.8 m in diameter. Multi-layer dielectric coatings are generally optimized to produce maximum reflectivity at the operating

wavelength. Their reflectivity at other wavelengths is low (and transmission is high), meaning that off-wavelength radiation from another

(enemy) laser could penetrate and damage

them. 46 These coatings may also be susceptible to high-energy particle damage in space,

either natural or man-made.

Finally, the optical industry must develop

manufacturing techniques, infrastructure, and

equipment to supply the hundreds of large mirrors for BMD DEW deployment. The SDI research program has targeted mirror fabrication as a key issue, and progress has been good

in the last few years. Techniques have been

developed to fabricate light-weight, segmented

mirrors with hollow-cored substrates and actuators to move each segment to correct for

surface figure errors.

These active mirrors could correct both for

large-scale manufacturing errors and for operational changes such as distortions due to thermal warping. They could even correct for broad

phase errors in the laser beam. The price would

be added complexity. A complex electro-mechanical-optical servo system would replace a simpler static mirror. And, to make the necessary

corrections, another complex wave-front detection system would measure the phase distortions of the laser beam in real time.

With reliable active mirrors, it might be possible to coherently combine the output energy

from two or more lasers. The brightness of “N”

lasers could theoretically be increased to “N2

”

times that of a single laser with this coherent

addition. (See section below on chemical lasers

for more details.)

‘

aDielectri

c coatings are nominally transmissive off the main

wavelength band, but there are always defects and absorbing

centers that absorb energy passing through, often causing damage and blow-off. At best the transmitted energy would be deposited in the substrate, which would then have to be designed

to handle the high power density of offensive lasers.

Pointing, Tracking and Retargeting Issues.

—A DE W beam must rapidly switch from one

target to the next during a battle. Assuming

that each DEW battle-station within range of

Soviet ICBMs would have to engage 2 ICBMs

per second,47 then the beam would have to slew

between targets in 0.3 s to allow 0.2 s of actual laser dwell time. In addition, the mirror

would have to move constantly to keep the

beam on the target: the target would move 1.4

km during 0.2s exposure, and the beam would

have to stay within a 20- to 30-cm diameter

spot on the moving target.

Large 10- to 30-meter mirrors could move

continuously to track the general motion of a

threat cloud, but jumping several degrees to

aim at a new target in 0.3 s would be rather

difficult. 48 One solution would be to steer a

smaller, lighter-weight secondary mirror in the

optics train, leaving the big primary mirror stationary. This approach would yield only limited

motion, since the beam would eventually walk

off the primary mirror; in addition, the smaller

secondary mirror would be exposed to a higher

laser intensity, making thermal damage more

difficult to avoid.

Alternatively, small-angle adjustments

could be made with the individual mirror segments that would constitute the primary mirrors. These mirror segments would probably

have mechanical actuators to correct for gross

beam distortions and thermal gradient-induced

mirror warpage. Again, moving individual mirror segments would produce only limited angular motion of the total beam.49

4Tof a laser battle-station fleet of 120, perhaps 10 to 12 would

be within range of the missile fields. Assuming that average

Soviet booster bum times were in the range of 130 seconds by

the time a DEW system could be deployed, and allowing 30

seconds for cloud break and initial track determination, then

each DEW platform in the battle space would have to engage

an average of about 130 ICBMS in 100 seconds. This required

targeting rate of about 1.3 per second could be increased by

factors of 2 to 5 or more if the Soviet Union decided to deploy

more ICBMS, and if they concentrated those extra ICBMS at

one or a few sites. For this discussion, a figure of 2 per second

is taken.

4aSlewing requirements cm be minimized by using appropriate algorithms.

4BAl~rnate Concepts are being kveStigated to wow retmgeting at large angles with steerable secondary mirrors.
134

The mirror servo system would have to accomplish these rapid steering motions without introducing excessive vibration or jitter

to the beam. To appreciate the magnitude of

the steering problem, consider that a vibration

that displaced one edge of a 10-m mirror by

1 micrometer (1 μm—40 millionths of an inch

or twice the wavelength of visible light) would

cause the laser to move one full spot diameter

on the target.50 This small vibration would cut

the effective laser brightness in half. Allowable jitter is therefore in the 20 nanoradian, or

one part in 50 million, range. Since any servo

system would undoubtedly exceed these jitter limits immediately after switching to a new

target, there would be a resettling time before

effective target heating could begin. This

resettling time would further decrease the allowable beam steering time, say from 0.3 s to

0.2 or 0.1 s.

Non-inertial methods of steering laser beams

are under investigation. For example, a beam

of light passing through a liquid bath which

contains a periodic acoustical wave is diffracted at an angle determined by the acoustical frequency. By electronically changing the

acoustical frequency in the fluid, the laser beam

could be scanned in one direction without any

moving parts. Two such acousto-optic modulators in series could produce a full twodimensional scanning capability.

Alternatively, the laser beam could be

reflected off an optical grating that diffracted

it at an angle that depended on its wavelength.

If the laser wavelength could be changed with

time, then the beam could be scanned in one

direction. Most of these non-inertial scanning

techniques could not operate at weapons level

laser power without damage. Others place constraints on the laser, such as limiting tunability.

Approximate beam steering and retargeting

levels are summarized in table 5-5. These parameters would vary with specific weapons design, system architecture, and assumed

threats. In general, demands on beam steer-

‘Whis assumes a 20-cm spot diameter on a target 1,000 km

away, or an angular motion of 200 nanoradians.

Table 5-5.—Possible Beam Steering and Retargeting

Requirements for Boost-Phase Engagement

Retargeting rate . . . . . . . . . . . . . . . . . . . . 2 targets/second

Retarget time. . . . . . . . . . . . . . . . . . . . . ..0.1 to 0.2 seconds

Jitter resettling time. . ...............0.1 to 0.2 seconds

Average laser dwell time . . . . . . . . . . . . . 0.2 seconds

Laser angular beamwidtha

. . . . . . . . . . . . 120 nanoradians

Allowable beam jitter . . . . . . . . . . . . . . . . 20 nanoradians

aThe diffraction. [imited beam spread for a 1 ~m laser with a 10 m diameter mirror is 120 nr,

SOURCE: Office of Technology Assessment, 1988.

ing speed and precision would increase if the

DEW range were extended (by deploying fewer

than the 120 battle stations assumed here, for

example), or if the Soviets increased the offensive threat above 1,400 ICBMs with average

burn time of 130 s assumed above.

Beam steering and retargeting needs for

post-boost and midcourse battle phases could

be more stressing if boost-phase leakage were

high and discrimination were not reasonably

effective. In general there would be more time

for midcourse kills, and more DEW platforms

would engage targets, but the hard-shelled

RVs would withstand much more laser irradiation and hence impose longer dwell times.

Lasers do not appear likely candidates for midcourse interception of RVs.

A neutral particle beam weapon (NPB) would

not have to dwell longer on RVs than on boosters or PBVs, since energetic particles would

penetrate the RV. Without midcourse discrimination, the NPB system might have to kill

from 50,000 to 1,000,000 objects surviving the

boost phase, and a weapon platform would

have to kill an average of 3 to 50 targets per

second. At the other extreme, with effective

discrimination, each NPB platform in the battle might have to engage only one RV or heavy

decoy every 20 s.51

Atmospheric Turbulence and Compensation

for Thermal Blooming. -One current DEW

candidate is the ground-based free electron la61 A55me–6,()()() RVS, 6,()()0 heavy decoys, and 10 Percent 1e*-

age from the boost phase defense. If the discrimination system

reliably eliminated all light decoys and debris, then, with 30

of the 120 DEW platforms in the midcourse battle, each platform would engage, on the average, one target every 22.5

seconds.
135

ser. The beam from this laser would be directed

to a mirror in space that would reflect the beam

to “fighting mirrors” closer to the targets. The

laser beam would be distorted in passing

through the atmosphere, for the same reason

that stars “twinkle.” If not corrected, atmospheric distortion would scramble the beam,

making it impossible to focus with sufficient

intensity to destroy ICBM boosters.

Techniques have been developed to measure

this distortion of the optical wave front and

to modify the phase of low power laser beams

to nearly cancel the effects of the turbulent

atmosphere. To correct distortion, the mirror

is manufactured with a flexible outer skin or

with separate mirror segments. Mechanical actuators behind the mirror surface move it to

produce phase distortions that complement

phase errors introduced by the atmosphere.

This “rubber mirror” must continuously adjust to cancel the effects of atmospheric turbulence, which varies with time at frequencies

up to at least 140 hertz (cycles per second).

To measure atmospheric distortion, a test

beam of light must be transmitted through the

same patch of atmosphere as the high power

laser beam. For the BMD application, this test

beam would be projected from a point near the

relay mirror in space, or a reflector near that

relay mirror would return a test beam from the

ground to the wave-front sensing system. Signals derived from the wave-front sensor computer in response to the test beam would drive

the mirror actuators to correct the high-power

laser beam.

The wave-front sensor must generate a coherent reference beam to compare with the distorted beam, as in an interferometer. One technique, called shearing interferometry, causes

two slightly displaced versions of the incoming distorted image to interfere. A computer

then deduces the character of the distorted

wave front by interpreting the resulting interference fringes,

Another wave-ront sensor system under investigation filters part of the incoming reference beam to produce a smooth, undistorted

wave front. This clean wave front can then be

combined with the distorted wave front, producing interference fringes that more clearly

represent the atmospheric distortion. Unfortunately the energy levels in the filtered wave

front are too low, so an operational system

might need image intensifiers.

Atmospheric compensation of low power

beams has been demonstrated in the 1aboratory and in tests during late 1985 at the Air

Force Maui Optical Station (AMOS) in Hawaii.

In this test, an argon laser beam was transmitted through the atmosphere to a sounding

rocket in flight. A reflector on the sounding

rocket returned the test signal. Wave-front errors generated on Maui drove a “rubber mirror” to compensate for the turbulence experienced by a second Argon laser beam aimed at

the rocket. A set of detectors spaced along the

sounding rocket showed that this laser beam

was corrected to within a factor of two of the

diffraction limit.

Successful atmospheric compensation will

entail resolution of two key issues: thermal

blooming and fabrication of large, multielement mirrors. As a high-power laser beam

heats the air in its path, it will create additional

turbulence, or “thermal blooming,” which will

distort the beam. At some level, this type of

distributed distortion cannot be corrected. For

example, if thermal blooming causes the laser

beam to diverge at a large distance from the

last mirror, then the test beam returning from

the relay satellite would also spread over a

large area and would not all be collected by

the wave-front sensor. Under these conditions,

complete compensation would not be possible.

Laboratory tests of thermal blooming were

planned at MIT’s Lincoln Labs and fieldtesting was planned for early 1989 using the

high-power MIRACL laser at the White Sands

Missile Range. The latter series of tests is on

hold due to lack of funding.

The mirror for a BMD FEL would need 1,000

to 10,000 actuators for effective atmospheric

compensation. 52 Experiments to date have

used cooled mirrors with a relatively small

52American Physical Society, op. cit., footnote 1, p. 190.
136

number of elements, and Itek is currently

building a large uncooled mirror with many

more.

Nonlinear optical techniques may offer an

alternative to the active-mirror correction of

atmospheric turbulence. Laboratory experiments at low power have already demonstrated

beam cleanup by stimulated Brillouin scattering, for example. In this technique, a beam of

light with a wave front distorted by the atmosphere enters a gas cell. The beam passes partially through this gas and is reflected back

with complementary phase distortion. This

complementary or “conjugate” phase exactly

cancels the phase distortions introduced by the

atmosphere. The key is to amplify the phase

conjugate beam without introducing additional phase errors. If perfected, this approach

would eliminate moving mirror elements.

Target Lethality.— One term in the DEW effectiveness equation is the susceptibility of current and future targets to laser and neutral particle beams. Current U.S. missile bodies have

been subjected to HF laser beams in groundbased tests, and various materials are being

tested for durability under exposure to highpower laser light.53 Laser damage varies with

spot size, wavelength, pulse length, polarization, angle of incidence, and a large range of

target surface parameters, making lethality

test programs complex.54 FEL beams with a

series of very short but intense pulses may produce an entirely different effect than continuous HF chemical laser beams.

Measuring the lethality of low-power neutral particle beam weapons intended to disrupt

electronics could be more complicated. Damage thresholds would depend on the electronics

package construction. However, current plans

call for particle beam energy density which

would destroy virtually any electronic sys691~ ~~~ highly publicized test at the White S~ds Missile

Range, a strapped-down Titan missile casing, pressurized with

nitrogen to 60 pounds per square inch pressure to simulate flight

conditions, blew apart after exposure to the megawatt-class

MIRACL laser.

“Computer models have been developed to help predict target lethality, and these models will be refined and correlated

with ongoing lethality measurements.

tem.55 The kill assessment issue for NPB weapons would then become one of hit assessment:

the system would have to verify that the particle beam hit the target.

FEL.-The two types of FEL systems (induction linac and RF linac) face different sets

of key issues (table 5-6). The induction linac

FEL has the potential of very high power, but

all of the laser gain must occur on one pass

through the amplifier as currently designed.

(Almost all other lasers achieve their amplification bypassing the beam back and forth between two mirrors, adding up incremental

energy on each pass.)

To achieve BMD-relevant power levels on

one pass, the FEL beam diameter must be very

small, on the order of a millimeter (mm). Furthermore, the beam must be amplified over a

very long path, on the order of 100 m. But a

millimeter-diameter beam would naturally expand by diffraction over this long path length,56

so the induction linac must utilize the electron

beam to guide and constrain the light beam

while it is in the wiggler magnet amplifier,

much like a fiber optic cable. This optical guiding by an electron beam has been demon-

‘What is, the NPB would be designed to deliver 50 J/gin at

the target, whereas 10 J/gin destroys most electronics (see American Physical Society, op. cit., footnoti 1, p. 306. This would

assure electronics kill unless massive shielding were placed

around key components.

66A l.mkm of unconstrained l-pm light would expand

to 120 mm after traveling 100 m.

Table 5.6.–Key Issues for Free Electron Lasers (FEL)

For induction linear accelerator driven FELs:

—Electron beam guiding of the optical beam

—Generation of stable, high current, Iow-emittance

e-beams

—Scaling to short wavelengths near 1 μm

—Raman scattering losses in the atmosphere

For radio frequency accelerator-driven FELs:

—Scaling to 100 MW power levels

—Efficiency

—Mirror damage due to high intercavity power

—Cavity alignment

For any FEL:

—Long cavity or wiggler path lengths

—Sideband instabilities (harmonic generation)

—Synchrotron/betatron instabilities (lower efficiency)

SOURCE: Office of Technology Assessment, 1988,
strated, but not under weapon-like FEL conditions.

Two other disadvantages derive from this

narrow, intense beam of light produced by the

induction linac FEL. First, the beam is so intense that it would damage any realizable mirror surface. The current plan is to allow the

beam to expand by diffraction after leaving

the FEL, traveling up to several km in an

evacuated tunnel before striking the director

mirror which would send the beam to the relay mirror in space.

A second disadvantage of such intense

pulses of light is that they would react with

the nitrogen in the atmosphere by a process

called “stimulated Raman scattering. ” Above

a threshold power density, the light would be

converted to a different frequency which

spreads out of the beam, missing the intended

target. 57 Again, this effect could be ameliorated

by enclosing the beam in an evacuated tube,

allowing it to expand until the power density

were low enough for transmission through the

atmosphere to the space relay. On the return

path to the target, however, the beam would

have to be focused down to damage the target.

The experimental induction linac at Livermore currently uses a (conventional, non-FEL)

laser-initiated channel to guide the electron

beam before it is accelerated. This beam has

drifted several millimeters laterally during the

FEL pulse in initial experiments, severely

limiting FEL lasing performance because the

electron beam does not remain collinear with

the FEL laser beam.

The RF linac FEL, as currently configured,

has shorter pulse lengths (20 picosecond v.

15 nanoseconds for the induction linac FEL)

but much higher pulse repetition rates (125

MHz v. 0.5 MHz58), giving it higher duty cyS~The R~~t~eshold for stimulated gain in nitrogen gas

at one P light is about 1.8 MW/cm2. Above this power density, the atmosphere becomes a singk+pass nitrogen laser: much

of the beam energy is converted to different (Stokes and antiStokes) wavelengths which diverge and cannot be focused on

the target.

‘aThe induction linac at Livermore could be operated up to

1 kHz for up to 10 pulses. An o~rational linac FEL would have

a repetition rate as high as tens of kilohertz.

137

cle and lower peak intensity for a given output average power level. It is uncertain

whether the RF linac can be scaled up to produce power levels which seem probable for the

induction linac. By adding a set of power amplifiers in series, it might be possible to reach

the power needed for a lethal laser weapon with

an RF linac FEL.

The RF linac generates very high power

levels inside the optical cavity. Mirror damage is therefore an issue, as is the problem of

extracting energy out of the cavity at these

high power levels. Cavity alignment is also critical: the mirrors must be automatically aligned

to maintain path-lengths within micrometers

over many tens of meters during high-power

operation.

The RF linac currently has low efficiency.

In 1986, Los Alamos National Laboratory and

a TRW-Stanford team demonstrated an energy

recovery technique whereby much of the unused energy in an electron beam was recovered

after the beam passed through a wiggler-amplifier. In principle, this energy could be coupled

back to the RF generator to improve efficiency

in an operational system. At the higher optical energy levels envisaged for the amplifiers,

the RF linac amplifier should achieve 20 percent to 25 percent conversion efficiency, making energy recovery less advantageous.

An FEL would tend to be fragile. Accelerators are notorious for demanding careful alignment and control, taking hours of manual

alignment before operation. Major engineering developments in automatic sensing and

control would be necessary before an FEL

could become an operational weapon. Los

Alamos is working to automate its ATS particle beam accelerator; FEL systems would

have to incorporate similar automation, with

the added complexity of optical, as well as accelerator, alignment.

An FEL may suffer from electron beam (ebeam) instabilities. For example, unwanted longitudinal e-beam excursions could create “sideband instabilities,” in which part of the optical

energy would be diverted to sideband frequencies. Laser light at these extraneous frequen-
138

cies could damage optical components designed to handle high power only at the main

lasing frequency. Such sideband frequencies

have been observed in FEL experiments. Lateral motion of the e-beam, called “synchrotron/betatron instabilities” could reduce FEL

efficiency, although calculations indicate that

this should not be a problem.

Chemical Laser Issues.—The chemical HF

laser has some disadvantages relative to the

FEL. Its longer wavelength (2.8-pm range)

would demand larger mirrors to focus the beam

on target. In general, targets would reflect a

higher percentage of IR light than visible or,

particularly, UV light. Hence, for a given mirror size, an HF laser would have to generate

7 to 10 times more power than an FEL laser

operating at one μm, or 80 to 200 times more

power than a UV laser, to produce the same

power density at the target.

Chemical laser experts do not believe that

an individual HF laser could be built at reasonable cost to reach the 1021 to 1022 W/sr

brightness levels needed for BMD against a

responsive threat, since the optical gain volume is limited in one dimension by gas flow

kinetics, and by optical homogeneity in the

other directions. However, by combining the

outputs from many HF lasers, it might be possible to produce BMD-capable HF arrays (table 5-7).

These beams must be added coherently: the

output from each laser must have the same freTable 5-7.—Key Issues for the HF Chemical Laser

Coherent beam combination: (many HF laser beams would

have to be combined to achieve necessary power levels)

Required beam brightness

against a responsive threat . . . several x 1021 W/sr a

Reasonable HF Laser brightness

for a single large unit (10 MW

power and 10-m mirror). . . . . . . 8.6 x l o”

Coherent Array of seven 10

MW/10-m HF lasers . . . . . . . . . . 4.2x1021

~he American Physical Society, Science and Technology of Directed-Energy

Weapons: Repori of the Arnedcan Physical Society Study Group, April 1967,

p. 55, estimated hardnees for a reaporwive threat to be well in excese of 10

kJ/cm2. Given a range of 2,000 km and a dwell time of 0.2s, the denoted brightness is appropriate.

SOURCE: Office of Technology Asae~wnent, 1966.

quency and the same phase.59 Controlling the

phase of a laser beam is conceptually easy, but

difficult in practice–particularly at high power

and over very large apertures. Since an uncontrolled HF laser generates several different frequencies in the 2.6 to 2.9 μm band, the laser

array would have to operate on one spectral

line, or one consistent group of lines.

Three coherent coupling techniques have

been demonstrated in the laboratory:

1. Coupled Resonators—the optical cavities

2

3

of several lasers are optically coupled, so

they all oscillate in phase;

Injection Locked Oscillators-one low

power oscillator output light beam is injected into the optical cavity of each laser;

Master Oscillator/Power Amplifier (MOPA)

—each laser is a singh-pass power amplifier fed by the same master oscillator in

parallel.

In one experiment, 6 CO, lasers were joined

in the coupled resonator mode. With incoherent addition, the output would have been 6

times brighter than that of a single laser; with

perfect coupling, the output would have been

36 times brighter. The experiment actually

produced 23.4 times greater brightness. Experiments are under way to couple two l-kW,

HF/DF lasers (with the coupled resonator approach) and to demonstrate MOPA operation

of two HF laser amplifiers.60

Neutral Particle Beam.—Although accelerator technology is well established for groundbased physics experiments, much research, development, and testing are prerequisite to a

judgment of the efficacy of a space-based particle beam weapon system. Key issues are presented in table 5-8.

~If ~d~ ~heren~y, & &am brightness of “N” ISSSrS wollld

be “Nz” times the brightness of one laser. If the “N” lasers

were not coherent, then the brightness of the combination would

be the sum or “N” times the brightness of one laser.

‘Actually, the MOPA experiment will utilize one amplifier

with three separate optical cavities: one for the master oscillator and two for the amplifiers. (Source: SDI Laser Technology

Office, Air Force Weapons Laboratory, unclassified briefing to

OTA on Oct. 7, 1986.)
139

Photo Credit: U.S. Department of Defense,

Strategic Defense Initiative Organization

Artist’s conception of a phased array of lasers.—Since

it may be impractical to build a single module spacebased chemical laser of a size useful for ballistic

missile defense, scientists and engineers are exploring

the possibility of using several smaller laser modules

that would be phase-locked to provide a single

coherent beam. This technique could increase the attainable power density on a target by a factor

of N* (instead of N for incoherent addition),

where N is the number of modules.

Table 5-8.—Neutral Particle Beam Issues

● Major issues:

—Beam divergence: 50 times improvement required

–Weight reduction (50 to 100 tonnes projected)

—Kill assessment (or hit assessment)

. Other issues:

—Beam sensing and pointing

—Duty factor: 100 times improvement required

—Ion beam neutralization (50°/0 efficient)

—Space charge accumulation

—ASAT potential

SOURCE Office of Technology Assessment, 1988

The NPB ATS now at Los Alamos generates the necessary current (100 mA) for a NPB

weapon, but at 20 to 40 times lower voltage,

about 100 times lower duty cycle, and with

about 50 times more beam divergence than

would be needed for a space-based weapon. A

continuous ion source with the necessary current levels has been operated at the Culham

Laboratory in the United Kingdom with 30-s

pulses, but not as yet coupled to an accelerator.

Researchers have planned a series of groundbased and space-based experiments to develop

beams meeting NPB weapons specifications.

It is possible that these experiments would encounter unknown phenomena such as beam instabilities or unexpected sources of increased

beam divergence, but there are no known physics limitations that would preclude weapons

applications.

High energy density at the accelerator would

not be sufficient for a weapon. The beam would

have to be parallel (or well-collimated, or have

“low emittance” in accelerator parlance), to

minimize beam spreading and maximize energy transmitted to the target. In general,

higher energy beams have lower emittance, but

some of the techniques used to increase beam

current might increase emittance, possibly to

the point where increased current would decrease energy coupled to the distant target.

With high emittance, the NPB would be a

short-range weapon, and more NPB weapons

would be necessary to cover the battle space.

The divergence of existing, centimeter-diameter particle beams is on the order of tens of

microradians; this divergence would have to

be reduced by expanding the particle beam diameter up to the meter range.6l This large beam

would have to be steered toward the target

with meter-size magnets. Full-scale magnetic

optics have not been built or tested. However,

one-third scale optics have been built by Los

Alamos National Laboratory and successfully

tested at Argonne National Laboratory on a

50 MeV beam line.

The weight of the NPB system would have

to be reduced substantially for space-based

operation. The RF power supply alone for a

81 In theory, beam divergence decreases as the beam size is

increased. In practice, the magnets needed to increase the beam

diameter might add irregularities in transverse ion motion, which

could contribute to increased beam divergence; not all of the

theoretical gain in beam divergence would be achieved.
140

weapon-class NPB would weigh 160,000 kg

(160 tonnes) if based on existing RF radar technology.62 Using solid-state transistors and reducing the weight of other components might

reduce RF weight about 22 tonnes.63 One study

concluded that a total NPB platform weight

of 100 tonnes is “probably achievable. ”64 Los

Alamos scientists have estimated that the

NPB platform weight for an “entry level,” 100-

MeV, NPB system could be 50 tonnes. Some

day, if high-temperature, high-current superconductors became available, NPB weights

might be reduced substantially.

Thermal management on a NPB satellite

would be challenging. A NPB weapon might

produce 40 MW of waste heat.65 One proposal

is to use liquid hydrogen to dispose of this heat.

About 44 tonnes of hydrogen could cool the

NPB for 500 s.66 The expulsion of hydrogen

gas would have to be controlled, since even a

minute quantity of gas diffused in front of the

weapon could ionize the beam, which would

then be diverted by the Earth’s magnetic field.

Since the hydrogen gas would presumably

have to be exhausted out opposing sides of the

spacecraft to avoid net thrust, it might be difficult to keep minute quantities of gas out of

the beam.

A state-of-the-art ion accelerator (the

Ramped Gradient Drift Tube Linac) can raise

beam energy about 4 MeV per meter of acceler-

— -.—. . . . .

‘The vacuum-tube (klystron) RF power supply for the PAVE

PAWS radar system weighs approximately 2 g/W of power. A

NPB weapon wo~d emit an average power of 20 MW (2x107

watts), assuming 200 MeV beams at a current of 0.1 A. Assuming an overall efficiency of 25 percent (50 percent accelerator

efficiency and 50 percent beam neutralization efficiency), the

power supply would have to generate 80 MW average power,

and would weigh 160 tonnes.

‘This assumes that the RF power is generatid with l-kW,

commercial quality power transistors (80,000 transistors would

be required for the hypothetical 80 MW supply). These transistors can only be operated at 1 percent duty factor. New cooling

technology wo~d have to be developed to operati at the 100

percent duty factor required for a NPB weapon. (See American

Physical Society, op. cit., footnote 1, pp. 149 and 361.) The overall efficiency of these power supplies would be 40 percent.

“Ibid., p. 152.

66Assuming azoo.d,loo.Itiev”beam, 50 percent neutr~ization efficiency, and 50 percent power generation efficiency.

66Assuming heat of vaporization OIlly (450 J/g)* ~d no ‘temperature rise in the hydrogen. If the gas temperature were allowed to rise by 100° K, then the hydrogen mass could be reduced to about 14 tonnes.

ator length. At this gradient rate, a 200-MeV

beam would have to be over 50 m long. This

accelerator could be folded, but extra bending

magnets would increase weight and could reduce beam quality. The gradient could be increased, but if the ion beam energy were increased in a shorter length, then there would

be more heating in the accelerator walls. This

implies another system trade-off: reducing

length in an attempt to cut weight might eventually reduce efficiency, which would dictate

heavier RF power elements and more coolant.

Again, future superconductors might ameliorate this problem.

The beam would have to be steered to intercept the target. A NPB would have two advantages over laser beams: the convenience of

electronic steering and a lesser need for steering accuracy. Magnetic coils could steer negatively charged hydrogen ions before the extra

electrons were stripped off. However, the angular motion of electronic steering would be

limited: the entire accelerator would have to

maneuver mechanically to aim the beam in the

general direction of the target cluster. Like laser weapons, a NPB must have an agile optical sensor system to track targets. However,

the divergence of the NPB is larger than most

laser beams (microradians versus 20 to 50

nanoradians), so the beam steering need not

be as precise.

On the other hand, a hydrogen beam could

not be observed directly. The particle beam

direction is detected in the laboratory by placing two wires in the beam. The first wire casts

a shadow on the second wire placed downstream. By measuring the current induced in

this downstream wire as the upstream wire is

moved, the beam direction can be estimated

to something like 6 microradian accuracy.

New techniques would be needed to sense

the beam direction automatically with sufficient accuracy. One approach utilizes the fact

that about 7 percent of the hydrogen atoms

passing through a beam neutralization foil

emerge in a “metastable” excited state: the

electrons of these atoms acquire and maintain

extra energy. Passing a laser through the beam

can make these excited atoms emit light. The
141

magnitude of this fluorescence depends on the

angle between the particle beam and the laser

beam. Thus the NPB direction can be deduced

and the beam boresighted to an appropriate

optical tracking system. Laboratory tests have

demonstrated 250 microradian accuracy, compared to the l-microradian accuracy necessary.67 More recent tests at Argonne at 50 MeV

have yielded better results.

The current technique to neutralize the

hydrogen ions is to pass them through a thin

foil or a gas cell. This process strips off, at

most, 50 percent of the electrons, cutting the

efficiency of the system in half and thus increasing its weight. A gas cell is not practical

for space applications. A stripping foil must

be extraordinarily thin (about .03 to.1 μm, or

ten times less than the wavelength of visible

light). In the proposed NPB weapon, a thin foil

1 m in diameter would have to cover the output beam. Clearly such a foil could not be selfsupporting, but Los Alamos scientists have

tested foils up to 25 cm in diameter that are

supported on a fine wire grid. This grid obscures about 10 percent of the beam, but has

survived initial tests in beams with average

power close to operational levels.

Another beam neutralization concept is to

use a powerful laser to remove the electrons-a

technique that some assert may yield 90 percent efficiency. However, the laser stripping

process would call for a 25 MW Nd:YAG laser

(near weapon-level power itself), and it would

eliminate the excited state hydrogen atoms

needed for the laser beam sensing technique.68

Charged hydrogen ions that escaped neutralization might play havoc with an NPB satellite. The accumulation of charge might severely degrade weapon system performance in

unforeseen ways, although NPB scientists are

confident that this would not bean issue.69 The

Beam Experiment Aboard Rocket (BEAR) experiment with an ion source and the planned

67See American Physical Society, op. cit., footnote 1, p, 172.

‘

8 See American Physical Society, op. cit., footnote 1, p. 148.

agone ~ugge9ted that the neutralizing foil be thicker so that

two electrons are stripped from some hydrogen ions, forming

positive hydrogen ions (protons) to help neutralize the charge

in the vicinity of the spacecraft.

Integrated Space Experiment (ISE) should answer any remaaining doubts about space-charge

accumulation.

Arcing or electrical breakdown that could

short out highly charged components may also

be a problem in space. Dust or metal particles

generated in ground-based accelerators fall

harmlessly to the ground. In space, floating

particles could cause arcing by forming a conducting path between charged components.

Existing accelerators demand many hours

of careful manual alignment before an experiment. Neutral particle beam weapons would

have to operate automatically in space. Current plans call for the ATS accelerator at Los

Alamos to be automated soon.

Kill assessment might be difficult for weak

particle beam weapons. Damage deep inside

the target might completely negate its function with no visible sign. The choices would

be either to forgo kill confirmation or to increase NPB energy levels until observable

damage were caused, possibly the triggering

of the high-energy explosive on the RV. The

current plan is to forgo kill confirmation per

se, but to increase the NPB power level to assure electronic destruction. Sensors would determine that the particle beam had hit each

target. Experiments are planned to assess

whether UV light emissions would indicate

that a particle beam had struck the surface of

a target.

The planned (and now indefinitely postponed) ISE illustrates a point made in chapter 11 of this report: many BMD weapons

would have ASAT capabilities long before they

could destroy ballistic missiles or RVs. The

ISE accelerator, if successful, would have

ASAT lethality at close range, although for

a limited duty cycle. Beam divergence might

limit range, but it could probably destroy the

electronics in existing satellites within 500 to

1,000 km.70 Even though not aiming a beam

‘This experiment could have nearly BMD-level lethality, possibly raising issues with respect to the ABM Treaty. However,

it would not have the necessary beam sensing and pointing or

the computer software and hardware for a BMD weapon; SDIO

considers the experiment to be treaty compliant.
142

at other than a target satellite, this experiment

conceivably might disrupt nearby satellite electronics. Although this may not be serious (calculations indicate that it should not), there is

enough uncertainty to cause ISE planners to

ask whether they should wait until the Space

Shuttle had landed before turning on the ISE.

X-ray Laser.—The nuclear bomb-driven xray laser is the least mature DE W technology.

To date this program has consisted of theoretical and design work at Livermore National

Laboratory and several feasibility demonstration experiments at the underground Nevada

nuclear weapons test site. Actual x-ray generation technology may or may not reach suitable levels in the years ahead; currently the

methods to convert this technology into a viable weapons system remain paper concepts.

POWER AND POWER CONDITIONING

The average electrical power consumed by

some proposed BMD spacecraft during battle might be factors up to 100,000 over current satellite power levels. Most existing satellites are powered by large solar arrays that

would be vulnerable to defense suppression attack. To provide sufficient survivable power

for space applications, most BMD satellites

would require either nuclear reactors, rocket

engines coupled to electrical generators, or advanced fuel cells.

In addition to high average power, some proposed weapon satellites would demand high

peak power: energy from the prime source, either a nuclear reactor or a rocket-driven turboalternator, would have to be stored and compressed into a train of very high current pulses.

For example, a railgun might expend 500 MJ

of energy in a 5-millisecond (ins) pulse, or 100

GW of peak power. This is about 1,000 times

more than current pulse power supplies can

deliver.

The following sections outline satellite power

demands and the technologies that might

satisfy them. While space systems would call

for the primary advances, ground-based FELs

would also depend on advances in pulsed-power

supply technology. Some of the technology developed for space-borne neutral particle beam

systems, such as RF power sources, might be

applicable to FELs.

Space Power Requirements

Estimates of power needs of space-based

BMD systems are summarized in table 5-9.

Since most of these systems have not been designed, these estimates could change significantly: the table only indicates a possible range

of power levels. Power is estimated for three

modes of operation: base-level for general satellite housekeeping and continuous surveillance operations lasting many years; alert-level

in response to a crisis, possibly leading to war;

Table 5-9.—Estimated Power Requirements for Space Assets

(average power in kilowatts)

Mode of operation Base Alert Burst (battle)

BSTS . . . . . . . . . . . . . . . . . . . . . . 4-1o 4-1o 4-1o

SSTS (IR) . . . . . . . . . . . . . . . . . . 5-15 5-15 15-50

Ladar. . . . . . . . . . . . . . . . . . . . 15-20 15-20 50-100

Ladar imager . . . . . . . . . . . . . 15-20 15-20 100-500

Laser illumination . . . . . . . . . 5-1o 5-1o 50-100

Doppler Iadar. . . . . . . . . . . . . 15-20 15-20 300-600

SBI carrier . . . . . . . . . . . . . . . . . 2-30 4-50 1o-1oo

Chemical laser . . . . . . . . . . . . . 50-100 100-150 100-200

Fighting mirror . . . . . . . . . . . . . 10-50 10-50 20-100

NPB/SBFEL . . . . . . . . . . . . . . . . 20-120 1,000-10,000 100,000-500,000

EML (railgun) . . . . . . . . . . . . . . . 20-120 1,000-10,000 200,000-5,000,000

SOURCE: Space Defense Initiative Organization, 1988.
I143

and burst-mode for actual battle, which may

last hundreds of seconds.

In addition to average-power and survivability perquisites, a space-based power system

would have to be designed to avoid deleterious effects of:

● thrust from power-generating rockets upsetting aiming,

● torque due to rotating components,

● rocket effluent disrupting optics and beam

propagation,

● vibration on sensors and beam steering,

● thermal gradients, and

● radiation from nuclear reactors.

Power systems would also have to operate

reliably for long periods unattended in space.

Space Power Generation Technology

There are three generic sources of electrical

power in space: solar energy, chemical energy,

and nuclear energy.

Solar Energy

Solar panels have supplied power for most

satellites. The sun produces about 1.3 kW of

power on every square meter of solar array surface. An array of crystalline silicon cells converts the sun’s energy into direct electrical current through the photovoltaic effect, with an

efficiency of about 10 percent. Thus a l-m2

panel of cells would produce about 130 watts

of electricity, assuming that the panel were oriented perpendicular to the sun’s rays. A 20-

kW array, typical for a BMD sensor, would

then have about 150 m2–roughly, a 12-m by

12-m array. The Skylab solar array, the largest

operated to date, produced about 8 kW. NASA

has built, but not yet flown in space, a 25-kW

experimental solar array designed to supply

space station power.

The major disadvantage of solar arrays is

that their large size makes them vulnerable to

attack. Crystalline silicon photovoltaic cells are

also vulnerable to natural and man-made radiation. One approach to reduce both vulnerabilities to some degree would be to concentrate

the sun’s rays with a focusing optical collector. The collector would still be vulnerable, but

if the system efficiency could be improved, then

the area of the collector would be smaller than

equivalent ordinary solar cell arrays.

There are two other ways to convert the

energy from solar collectors into electricity.

One is to use solar thermal energy to drive a

conventional thermodynamic heat engine. The

other is to focus sunlight on more radiationresistant and higher-efficiency photovoltaic

cells such as gallium arsenide. Depending on

the temperature of the working fluid in a thermodynamic heat engine cycle, efficiencies of

20 percent to 30 percent might be achieved.

Gallum arsenide cells have shown up to 24 percent efficiency in the laboratory, so 20 percent

efficiency in space may be reasonable. Thus,

either technology could cut the required collector area in half compared to conventional

solar cells, or 75 m2 per 20-kW output. Neither

approach has been tested in space, but NASA

is pursuing both for future space applications.

Nuclear Energy

Nuclear energy has also been used in space.

There are two types of nuclear energy sources:

radioactive isotope generators that convert

heat from radioactive decay to electricity, and

nuclear fission reactors. Both have flown in

space, but the radioactive isotope generator

is more common.

Both radioactive decay and a controlled fission reaction produce heat as the intermediate energy form. This heat can be converted

into electricity by static or dynamic means.

A static power source produces electricity

directly from heat without any moving parts,

using either thermoelectric or thermionic converters. These converters generate direct current between two terminals as long as heat is

supplied to the device. The efficiency and total practical power levels are low, but for applications of less than 500 W, the advantage

of no moving parts makes a radioisotope thermoelectric generator (RTG) a primary candidate for small spacecraft.

To produce more than 500 W, a radioisotope

source could be coupled to a dynamic heat en-
144

gine. One dynamic isotope power system

(DIPS) with 2 to 5 kW output has been groundtested. This system weighs 215 kg. However,

the U.S. production capacity for radioactive

isotopes would limit the number of satellites

that could be powered by DIPS.

BMD satellites needing more than 5 to 10

kW of power might carry a more powerful nuclear fission reactor. Static thermoelectric converters would still convert the heat to electricity. This is the approach proposed for the

SP-100 space power program, the goal of which

is to develop elements of a system to provide

power over the range of 10 to 1,000 kW. The

Departments of Defense and Energy and

NASA are producing a reference design incorporating these elements to produce a 100-kW

test reactor.

71 This is the major focus for the

next generation of space power systems.

The SP-100 reactor, as currently designed,

would use 360 kg of highly enriched uranium

nitride fuel with liquid lithium cooling operating at 1,3500 K. This heat would be conducted

to 200,000 to 300,000 individual thermoelectric elements which would produce 100 kW of

electricity. The overall efficiency of the system would be about 4 percent, which would

entail the disposal of 2.4 MW of waste heat.

Large fins heated to 8000 K would radiate this

heat into space.

The SP-100 program faces numerous challenges. In addition to being the hottest running reactor ever built, the SP-100 would be

The estimated mass of the SP-100 is 3,000

kg, or a specific mass of 30 kg/kW. Original

plans called for building a ground-test prototype SP-100 based on the 100-kW design by

1991, with a flight test several years later. Subsequently a 300-kW design was considered

which would have pushed initial hardware

toward 1993, but current schedules are fluid

due to uncertain funding.

the first space system to: To produce power levels in excess of a few

●

●

●

ž

●

●

use uranium nitride fuel,

be cooled by liquid lithium,

use strong refractory metals to contain the

primary coolant,

have to start up with its coolant frozen,

have two independent control mechanisms

(for safety), and

use electronic semiconductors under such

intense heat and radiation stress.72

710riginal SDIO plans called for designing a 300 KW system,

but as of this writing the goal has been reduced to 100 KW.

‘*See Eliot Marshall, “DOE’s Way-out Reactors,” Science,

231:1359, March 21, 1986.

hundred kW, one would have to take the next

step in the evolution of space nuclear power

systems: a nuclear reactor coupled through a

dynamic heat engine to an electrical generator. In principle, large reactors in space could

generate hundreds of MW, satisfying the most

stressing BMD average power demands.

A “multimegawatt,” or MMW, project has

begun to study some of the fundamental issues raised by large reactors, including daunting engineering challenges such as high temperature waste heat disposal in space, safety

in launch, operation, and decommissioning.

These large nuclear systems might have to be
745

operated “open-cycle,” requiring much “fuel”

in the form of cooling gas to dispose of excess

heat. At this writing the MMW project is in

the conceptual phase with no well-defined research program. Multi-megawatt nuclear reactors in space would have to be considered

a 20-to-30 year project.

In summary, space nuclear power systems

would require extensive development to

achieve reliable space operation at the 100-300

kW level by the mid-to-late 1990s. Given current engineering and budget uncertainties, development of megawatt-class nuclear power

systems for space cannot be projected until

well into the 21st century.

Chemical Energy

Satellites frequently employ chemical energy

in the form of batteries, fuel cells, and turbogenerators. Batteries would be too heavy for

most BMD applications, except possibly for

pop-up systems with very short engagement

times. Fuel cells, which derive their power by

combining, e.g., hydrogen and oxygen, are under active consideration for driving the accelerators of NPB weapons.

For the short bursts of MMW power needed

by some BMD weapons, an electrical generator driven by a rocket engine (e.g., burning liquid hydrogen and liquid oxygen) might be the

only available technology in the foreseeable future. The Space Shuttle main engine (SSME)

develops about 10 GW of flow power, which

could generate 5 GW of electrical energy if it

could be coupled to a turboalternator. Alternatively, rocket exhaust could, in principle, be

converted to electricity by magnetohydrodynamics (MHD).

The engineering challenges of using rocket

engines to produce electrical power on board

a BMD satellite are posed not only by the

generator itself, but by its effects on other components such as sensors, electronics, and weapons. Two counter-rotating and counter-thrusting rockets would probably be essential to

cancel torque and thrust. Even then, sensors

and weapons-aiming devices would have to be

isolated from vibration. Similarly, the effluent

from the rocket engines must not interfere with

sensors or weapon beam propagation, and electrical noise must not interfere with communication or data processing electronics.

It might be necessary to place rocket engines

and power generators on separate platforms

hundreds or a few thousands of meters away

to achieve the necessary isolation, transmitting power by cable or microwaves. This

method, however, would raise vulnerability issues, presenting to the adversary an additional

target and a vulnerable umbilical cord.

Power Conditioning

Power conditioning is matching the electrical characteristics of a power source with those

required by the load. A generator might produce a continuous flow of electrical current,

but a load, such as railgun firing, would require

a series of very high-current pulses. Power conditioning equipment would convert the continuous flow into pulses.

In some cases the projected power conditioning device requirements exceed existing capabilities by two or three orders of magnitude,

even for ground-based experiments. In many

areas, no space-qualified hardware exists at

any power level. Pulsed power technology development efforts are underway in capacitive

and inductive energy storage, closing and opening switches, transformers, RF sources, ACDC converters, and ultra high-voltage techniques and components.

Particle accelerators that drive the FEL and

the NPB use RF power. Railgun requirements

would present the greatest challenge: very

short (millisecond) pulses of current several

times a second. Many electrical components

would have to be developed to produce the

proper current pulses for a railgun.

A homopolar generator combined with an inductor and opening switch is now the primary

candidate for the generation of very short

pulses. A homopolar generator is a rotating

machine that stores kinetic energy in a rotating armature. At the time of railgun firing,

brushes would fall unto the armature, extract-
146

ing much of its energy in a fraction of a second. This would result in a sudden jerk in the

torque of the generator, which would disturb

a spacecraft unless compensated by a balanced

homopolar generator rotating in the opposite

direction.

The brushes would also wear out, which

raises questions about the durability of a railgun with high repetition rates. Very fast

switches would be essential. These switches

would have to be light enough to move rapidly, but heavy enough to handle the extraordinarily high currents.

Researchers at the University of Texas have

investigated one advanced modification to the

homopolar generator. They have replaced

brushes and switches with inductive switches

in a “compulsator,” a generator which produces a string of pulses. By replacing noncurrent carrying iron with graphite-epoxy composites, these compulsators could be much

lighter than the homopolar generators.

While space applications drive power development requirements, emerging ground-based

defensive systems would also stress existing

power sources. Ground-based BMD elements

might require diesel and turbine driven electric generators and MHD generators for mobile applications. A fixed-site system such as

the FEL might draw on the commercial utility grid, dedicated power plants, or superconducting magnetic energy storage (SMES). The

electrical utility grid could meet peacetime

housekeeping power needs and could keep a

storage system charged, but, due to its extreme

vulnerability to precursor attack, could not be

relied on to supply power during a battle.

Therefore, a site-secure MMW power system

would probably be necessary.

Superconducting magnetic energy storage

is a prime candidate for ground-based energy

storage; an SMES system would be a large,

underground superconducting coil with continuous current flow. The science of SMES is

well established, but engineering development

remains.

Recent discoveries of high-temperature superconductors could have an impact on future

power supplies and pulse conditioning systems. Given the likely initial cost of manufacturing exotic superconducting materials and

the probable limits on total current, their first

applications will probably be in smaller devices

such as electronics, computers, and sensor systems. But if:

●

●

scientists could synthesize high temperature superconducting materials able to

carry very large currents; and

engineers could develop techniques to

manufacture those materials on a large

scale suitable for large magnetic coils, RF

power generators, accelerator cavity walls,

the rails of electromagnetic launchers, etc.;

then superconductors could substantially reduce the power demand. Efficiency of the

power source and power conditioning networks

could also be improved. High temperature superconductors would be particularly attractive

in space, where relatively cold temperatures

can be maintained by radiation cooling.

COMMUNICATION TECHNOLOGY

Communication would be the nervous sys- dreds to several thousands of weapons plattem of any BMD system. A phase-one defense forms in low-Earth orbits. “

would include hundreds of space-based components, separated by thousands of kilometers, Three fundamental communication paths

for boost and post-boost interception. A would link these space assets: ground to space,

second-phase BMD system would include space to space, and space to ground. Ground

many tens of sensors in high orbits and hun- command centers would at least initiate the
147

battle; they would also receive updates on

equipment status and sensor data in peacetime

and as the battle developed.

The attributes of an effective communication system would include:

● adequate bandwidth and range,

● reliability,

● tolerance of component damage,

● security from interception or take-over,

● tolerance of nuclear effects, and

● jam- or spoof-resistance.

The bandwidths, or frequency space available, from links in the millimeter-wave bands

would be adequate for most near-term BMD

functions. The most demanding element would

be the boost surveillance and tracking system

(BSTS) satellite, with perhaps a l-million-bitper-second data rate. Second-phase elements

such as a space surveillance and tracking system (SSTS) sensor satellite might operate at

much higher rates, up to 20 million bits per

second, while battle management might take

50 or more million bits per second of information flow. Various additional data for synchronization signals would have to be communicated. Transmission bandwidths might have

to be very large-perhaps 1-10 gigahertz (GHz)

–to reduce the chances of jamming.

The communication system must be durable and survivable even if some nodes fail due

to natural or enemy action. Redundant links

in a coupled network might assure that messages and data got through even if some satellites were destroyed. Tying together a vast

BMD space network would be challenging,

especially given that the satellites in low-Earth

orbit would constantly change relative positions.

One key issue for BMD communications is

jamming by a determined adversary. Successfully disrupting communications would completely negate a BMD system that relied on

sensors and command and control nodes separated from weapons platforms by tens of thousands of km. Jammers could be developed, deployed, and even operated in peacetime with

little risk of stimulating hostile counteraction.

Space-to-ground communication links would

be particularly vulnerable. Ground-based, shipbased or airborne high-power jammers might

block the flow of information to satellites. In

wartime, nuclear explosions could disrupt the

propagation of RF waves. Ground-based

receivers would also be susceptible to direct

attack. Even space-based communications

would be susceptible to jamming.

Recently there have been two primary SDI

candidates for BMD communication links in

space: laser links and 60-GHz links. A 60-GHz

system once seemed to promise a more jamresistant channel for space-to-space communications, since the atmosphere would absorb

enough 60-GHz energy to reduce the threat of

ground-based jammers. Recent analyses, however, indicate that space and air-based jammers may limit the effectiveness of 60-GHz

links.

60-GHz Communication Links

The operating frequencies of space communication systems have been steadily increasing. For example, the Milstar communications

satellite will use the extremely high frequency

(EHF) band with a 44-GHz ground-to-space uplink and a 20-GHz downlink. These high frequencies allow very wide bandwidth (1 GHz

in the case of Milstar) for high data transmission rates, but also for more secure communications through wide-band-modulation and

frequency-hopping anti-jamming techniques.

For space-to-space links, BMD designers are

considering even higher frequencies-around

60 GHz. This band includes many oxygen absorption lines. It would be very difficult for

ground-based jammers to interfere with 60-

GHz communications between, for example,

a BSTS early warning satellite and SBI CVs:

oxygen in the atmosphere would absorb the

jamming energy.

Pre-positioned jammer satellites, or possibly rocket-borne jammers launched with an attack, might still interfere with 60-GHz channels. The main beam of radiation from a

60-GHz transmitter is relatively narrow, mak-
148

ing it difficult for an adversary to blind the

system from the main lobe: the enemy jammer transmitter would have to be located very

close to the BMD satellite broadcasting its

message. But a 60-GHz receiver would also

pick up some energy from the “sidelobes” and

even some from the opposite side of the receiver

(the “backlobes”). While a receiver may be

10,000 to 100,000 times less sensitive to energy

from these sidelobes than from the main lobe,

it must be extremely sensitive to pickup signals from a low-Earth orbit satellite tens of

thousands of km away.

At high-Earth orbit, a near-by jammer with

only a few hundred watts of power could overwhelm a much more powerful 60-GHz system

on a sensor satellite. This neighbor might

masquerade as an ordinary communications

satellite in peacetime. In wartime, it could aim

its antenna at the BSTS and jam the channel.

The countermeasure would be to station the

BSTS out of standard communications satellite orbits.

Laser Communication Links

The low-power diode laser offers the possibility of extremely wideband, highly directional, and, therefore, very jam-proof communications. The MIT Lincoln Laboratory has

designed a 220 megabit-per-second (Mbs) communication link that would need just 30 milliwatts of laser power from a gallium aluminum arsenide (GaAIAs) light emitting diode

(LED) to reach across the diameter of the geosynchronous orbit (about 84,000 km). The receiver, using heterodyne detection,73 could pull

in a signal of just 10 picowatts (10-11 W)

power. A 20-cm mirror on the transmitter

would direct the laser beam to an intended receiver.

The high directionality of narrow laser

beams also complicates operation. A wideangle antenna could flood the receiver area with

signal, even sending the same message to many

receivers in the area at one time. A narrow laser beam must be carefully aimed at each satellite. This would require mechanical mirrors

or other beam-steering optics, as well as software to keep track of all friendly satellites and

to guide the optical beam to the right satellite. The lifetime of a laser source and an agile

optical system may be relatively short for the

first few generations of laser communication

systems.

A laser communication system, as presently

designed, would require up to eight minutes

to establish a heterodyne link between a transmitter and a receiver. Plans call for reducing

this acquisition time to one minute. With a

very narrow laser beam, even minute motions

of the transmitter platform could cause a

momentary loss of coupling, forcing a delay

to reacquire the signal.

While laser links might provide jam-proof

communications between space-based assets

of a BMD system, laser communications to the

ground would have to overcome weather limitations. One approach would use multiple

receivers dispersed to assure one or more clear

weather sites at all times. Alternatively, one

could envisage an airborne relay station, particularly in time of crisis.

“The common “heterodyne” radio receiver includes a local

oscillator which generates a frequency that is combined or

“mixed” with the incoming radio signaL This process of “mixing” the local oscillator signal with the received signal improves

the ability to detect a weak signal buried in noise, and reduces

interference. A laser heterodyne receiver would include its own

laser source, which would be “mixed” at the surface of a light

detector with the weak light signal from a distant laser transmitter.

SPACE TRANSPORTATION

Reasonable extensions of current U.S. space launch capabilities would be necessary to lift

transportation capability might launch the several hundred to over one thousand carrier

tens of sensor satellites envisaged by some vehicles and their cargoes of thousands to tens

BMD architectures, but entirely new space of thousands of kinetic kill missiles into space
149

in a reasonable period of time. Therefore space

launch capability would have to evolve along

with phase-one and phase-two weapon systems

to assure the United States—and to persuade

the Soviet Union-that a defense-dominated

world would be feasible and enduring.

Space Transportation Requirements

Space-based interceptors and their carrier

satellites would dominate initial space deployment weights. Assuming that a phase-one deployment would include a few hundred CV\sS

and a few thousand SBIs based on the "stateof-the-art” rockets described above, then total launch weight requirements might be in the

range of 1 million to 2 million kg.

The range of weights estimated by SD I system architects for a more advanced phase varied from 7.2 to 18.6 million kg. The large range

of weight estimates reflects differences in architectures, and particularly differences in survivability measures. Several contractors indicated that survivability measures—such as

shielding, decoys, proliferation, and fuel for

maneuvering-would increase weight by a factor of about three. One could infer that the

heavier designs might be more survivable.

Additional space transportation would be required over time for servicing, refueling, or

replacement of failed components. One unresolved issue is how best to maintain this fleet

of orbiting battle stations: by originally including redundant components such as interceptor missiles on each satellite, by complete

replacement of defective satellites, by on-orbit

servicing, or by some combination of the above.

One contractor estimated, for example, that

it would take 35 interceptor missiles on each

battle station to assure 20 live missiles after

10 years, with the attrition due entirely to natural component failures.

Soviet countermeasures might drive up

weight requirements substantially in later

years. Increased Soviet ICBM deployments

might be countered with more SB I platforms.

Defense suppression threats such as directascent ASATs might be countered in part by

proliferation of SBI battle stations or by other

heavy countermeasures. Advanced decoys dispersed during the post-boost phase of missile

flight might require some type of interactive

discrimination system in space. Reduced Soviet booster burn times would eventually impel a shift to DE W. Deploying these countermeasures would necessitate additional space

transportation capability. Directed-energy

weapon components in particular would probably be very heavy. The range of SDI system

architects’ estimates for some far-term systems was from 40 million to 80 million kg.

Space Transportation Alternatives

There seem to be two fundamental options

for lifting the postulated BMD hardware into

space: use derivatives of existing space transportation systems; or design, test, and build

anew generation space transportation system.

The first option might be very costly; the second might postpone substantial space-based

BMD deployment into the 21st century.

Some BMD advocates outside the SDIO

have suggested that existing United States

space launch systems might be adequate for

an initial spacebased BMD deployment in the

early 1990s. But the existing United States

space launch capability is limited in vehicle inventory, payload capacity per launch, cost,

launch rate, and launching facilities. As shown

in table 5-10, today’s total inventory of U.S.

rockets could lift about 0.27 million kg into

low-Earth orbit (180 km) at the inclination angle of the launch site (28.50 for the Kennedy

Space Center in Florida) .74

The bulk of early SBI deployments would

have to be launched into near-polar orbits from

Vandenberg AFB, which would now only be

possible for the 6 remaining Titan 34D vehicles with a combined lift capacity of 75,000 kg.

“Missile launch capacity is usually specified in terms of the

payload which can be lifted into direct East-West flight at an

altitude of 180 km, which produces an orbit inclined at the latitude of the launch point. Extra propellant is required to lift the

payload to higher inclinations or to higher altitudes. Proposed

BMD weapons systems would require higher inclinations (700

to850, and higher altitudes (600 to 1,000 km), which translates

into lower payload capacity.
150

This would correspond to about 6 percent of

the initial phase-one BMD space deployment

requirements. Some have suggested refurbishing Titan-IIs, which have been retired from the

ICBM fleet. If all 69 Titan-IIs were refurbished, then the United States could lift

another 130,000 kg into polar orbit, or another

11 percent of the near-term BMD needs.

The rate of missile launch might also be

limited by the existing space transportation

infrastructure. Launching one Shuttle now

takes a minimum of 580 hours at the Kennedy

Space Center (and might take about 800 hours

at Vandenberg AFB75), limiting potential

launches to one per month or less from each

complex. After the Shuttle accident, NASA

estimated that 12 to 16 flights per year would

be reasonable. Clearly 16 launches per year

would not be sufficient for BMD deployment.76

Several aerospace companies have proposed

building launch vehicles with increased lift capacity to meet SDI, DoD, and civilian space

transportation demands. Many of these vehicles would be derived from various Shuttle or

75Completion of the Vandenberg Shuttle launch site SLC-6

has been postponed until 1992.

‘“Assuming 16 Shuttle launches per year with 9,000 kg payload to low polar orbit, it would take between 8 to 12 years to

deploy a phase-one BMD system and 48 to 125 years to deploy

a phase-two system weighing 7 to 18 million kilograms.

Titan predecessors, such as the Titan-4, included in table 5-10. Twenty-three Titan-4s will

be built by 1988, but these have only marginally increased lift capacity. A major increase

in lift capacity to the 40,000 to 50,000 kg range

would be required for an effective space-based

BMD system. Even for a phase-one system,

far more would be needed by the mid-1990s.

Both SDIO and Air Force officials have called

for anew space transportation system that is

not a derivative of existing technology.

Four aerospace companies analyzed various

space transportation options under joint Air

Force/NASA/SDIO direction. The Space Transportation Architecture Study (STAS) compared manned v. unmanned vehicles, horizontal v. vertical takeoff, single v. 2-stage rockets,

and various combinations of reusable v. expendable components. 77 The &r Force, after

reviewing the initial STAS work, appears to

be leaning toward a decision that the BMD deployment should use an unmanned, expendable, 2-stage heavy-lift launch vehicle (now

called the ALS or advanced launch system) .78

TTThe spice Trmsportation Architecture Study (S’I’AS) was

a joint Air Force/NASA/SDIO study on future space transportation systems. The Air Force Systems Division contracted with

Rockwell and Boeing, while NASA employed General Dynamics

and Martin Marietta to analyze U.S. civilian and military space

requirements and possible alternatives to satisfy them.

7sThe name HLLV (heavy lift launch vehicle) was changed

to ALS in April 1987.

Table 5-10.—Current U.S. Space Launch Inventorya

Payload per vehicle (thousands of kg)

Inventory LEO Polar

quantity (180 km) (180 km) Geo

Shuttle . . . . . . . . . . . . . . . 3 25 15 Centaur-G:4.5

IUS: 2.3

Titan 34D . . . . . . . . . . . . . 15,3 12.5 IUS: 1.8

Titan-4 b 

. . . . . . . . . . . . . . (23) 17.7 14.5 Centaur-G:4.6

IUS: 2.4

Titan 11-SLV . . . . . . . . . . . (13)C

3.6 1.9

Delta. . . . . . . . . . . . . . . . . 2.9

Delta (MLV) . . . . . . . . . . . (7) 4 1.5

Atlas. . . . . . . . . . . . . . . . . 13 6

scout . . . . . . . . . . . . . . . . 21 .26

(ALS) d . . . . . . . . . . . . . . . . ? (50-70) (40-55)

aparentheses indicate future SyStemS.

bThe Titan.4or the complementary Expendable Launch Vehicle (CELV) iS the latest in the line Of Titan miSSile configurations;

23 have been ordered.

CThe Titan 11.sLvs are Ming refurbished from the ICBM inventory. The first Titan-n may be available by 1989. An additional

56 Titan II could be refurbished from the retired ICBM fleet.

dThe Advanced Launch System is proposed to deploy the bulk of the BMCI SpaCe CO171pOIlWlk.

SOURCE: Office of Technology Assessment, 1988.
151

An interim STAS study suggested that such

a vehicle would have to evolve to a partially

reusable system to meet SDIO cost reduction

goals. The STAS contractors projected that

development of a heavy-lift unmanned vehicle would require about 12 years, although at

least one aerospace company estimates that

an ALS could be developed in 6 years. If the

original 12-year estimate is correct, significant

space deployment of a BMD system could not

begin until the turn of the century even if the

weapon systems were ready earlier. If the 6-

year estimate were correct, then initial deployment could begin by 1994.

To deploy space-based assets earlier, SDIO

has suggested a two-tier level program: build

part of an ALS by the mid-1990s, but design

this system to evolve into the long-range system by the year 2000. The initial system would

include some of the advanced features of the

heavy-lift launch vehicle concepts outlined by

STAS, but would not have a fly-back booster

and would not meet the SDIO cost goals of

$300 to $600 per kilogram. The interim goal

would be to reduce the current costs of $3,000-

$6,000 per kilogram to $1,000-$2,000. Building a space transportation system while trying to meet these two goals simultaneously

could be risky. Compromises might be required

either to meet the early deployment date or

to meet the long-term cost and launch rate

goals.

The estimated launch rate for a fully developed ALS vehicle is about once per month per

launch complex.79 Assuming a 40,000-kg payload to useful BMD orbits, then between 30

and 45 successful flights would be required for

a phase-one BMD deployment and from 180

to 460 flights for a much larger second phase.

Allocating 5 years to deploy the latter system,

the United States would need to build three

to eight new launch facilities.80

—. -—‘YFhe current maximum launch rate for Titans is three per

year from each pad, which might be increased to five per year.

Further increases are unlikely because the Titans are assembled on-site. This is one of the reasons an entirely new space

launch system would be needed to meet the SDI launch rates.

*’The United States now has four launch pads for Titan-class

boosters, two on the east coast and two on the west coast. One

west coast pad is being modified to handle the CELV. Since

Photo credit: U.S. Department of Defense,

Strategic Defense Initiative Organization

Advanced launch system (ALS).—Large-scale deployment of space-based interceptors (SBI) or other

weapons in space will require a dramatic expansion

of US. space-launch capabilities. Various proposals,

including a Shuttle-derived, unmanned launch vehicle

such as this have been under consideration by

the Air Force, NASA, and the SDIO.

Figure 5-13 presents one very optimistic sce

nario which might lead to space launch facilities adequate for proposed second-phase BMD

SBIs would have to be launched from Vandenberg to reach nearpolar orbits, all early deployments would have to be from one

pad. The estimated time to build a new launch pad complex

is 7 to 10 years,
152

Figure 5-13a. —Annual Space Launch Capacity

(near polar orbits at 800 km)

1985 1990 1995 2000 2010

Year

This is one possible scenario to achieve the 2 million kg per

year space launch capability into near-polar orbits required

for an intermediate ballistic missile defense system. This system could conceivably reach this goal by the year 2003, assuming that three new launch pads were built at Vandenberg

AFB, and the proposed Heavy Lift Launch Vehicle (HLLV)/Advanced Launch System (ALS) could be developed, flight

tested, and ready for initial service with 30,000 kg lift capacity by 1994. This would be 5 years ahead of the schedule initially suggested by the Space Transportation Architecture

Study (STAS). The HLLV is further assumed to evolve into

a 44,000 kg capability by the year 2000, without any engineering delays. The SDIO launch goals as of early 1987 are shown

for comparison.

SOURCE: Office of Technology Assessment, 1988.

systems by 2000-2005. This scenario assumes

that the SDIO two-level space transportation

development approach would be successful: an

interim ALS vehicle, with a 30,000 kg capability to near-polar orbits, would be available

by 1994; a more advanced ALS would come

online in 2000 with 44,000 kg capacity. Three

new launch pads would be built (although there

is no room for three new pads at Vandenberg

AFB, the only existing site in the contiguous

United States with near-polar orbit capability).

Assuming approval to proceed with the new

launch system in 1988, the first flights of the

new ALS would begin in 1994, using the refurbished SLC-6 launchpad at Vandenberg, built

originally for the Space Shuttle. The three new

pads would become operational in 1997,1998,

and 1999. Flights would be phased in at each

site, increasing up to 12 flights per year per

pad. With these assumptions, the SDIO goal

Table 5-il.—Space Transportation

Vehicle capacity to 600 km, high inclination:

(thousands of kilograms)

CELV (Titan-4) 115

Titan 34D

Earty HLLV 30 (1995-2000)

Final HLLV 44 (2000+)

Total

Number of launches per year annual

launch

Launch pads: 4-East: SLC-6

(34D/CEL) (HLLV) (HLLV) (HLLV) (HLLV)

Capacity

(M kg)

Year

1965 3 003

1986 3 003

1987 3 003

1966 3 003

1989 4 005

1990 4 005

1991 5 0.06

1992 5 0.06

1993 6 007

1994 6 1 010

1995 7 2 014

1996 7 6 026

1977 6 6 1 036

1998 6 10 2 2 051

1999 6 12 4 4 2 075

2000 6 12 6 6 4 132

2001 6 12 8 8 6 1.59

6 12 10 10 6 165

2000 6 12 12 12 10 212

2004 6 12 12 12 12 220

6 12 12 12 12 2.20

2006 8 12 12 12 12 220

2007 12 13 12 12 225

2008 8 12 13 13 12 229

2009 6 12 14 13 13 236

2010 6 12 14 14 13 242

Tabular data for figure 3-13a.

SOURCE: Office of Technology Assessment, 1988.

of 2 to 2.5 million kg per year could be achieved

by 2003.

If the United States were to operate 10

launch facilities, each with one ALS launch per

month, then it would take about 10 years to

orbit the 50 million kg estimated for a far-term,

third-phase system.81 If political or strategic

considerations (such as transition stability)

would not allow as long as 10 years to deploy,

then the United States would have perhaps

three choices:

1. develop another new vehicle with lift capacity above 50,000 kg to 800-km, high

inclination orbits;

2. build and operate more than ten ALS

launch facilities simultaneously; or

ElThe 50 fion kg assumes the low end of the 40 to 80 ~-

iion kg estimated above for phase three with spacebased lasers.

A successful ground-based laser system could reduce this estimate by about 15 million kg, or 25 to 65 million kg for a total

phase-three constellation.
153

3. improve launch operations to reduce turnaround time below 30 days per pad.

The country would have to expand booster

manufacturing capacity to meet this demand

for up to 120 launches per year. Historically,

Titan production lines completed up to 20 missiles per year, and Martin Marietta has estimated that it could easily produce 14 of the

Titan class per year with existing facilities.82

Space Transportation Cost Reduction

Identifying 42 technologies related to space

transportation, the STAS listed several where

research might lead to reduced operating costs

(it emphasized the first three as offering especially high leverage for cost reduction):

●

●

●

●

●

●

●

●

●

lightweight materials,

expert systems and automated programming to cut software costs,

better organization,

reducing dry weights substantially,

better ground facilities,

higher performance engines,

fault-tolerant avionics,

reusability of major components, and

better mating of spacecraft to launch vehicle for reduced ground costs.

— ..——— sZThiS would inClu& 5 CELVS, 6 Titan 11s, and 3 Titan 34Ds.

The operating (as opposed to life-cycle) costs

of space transportation are currently estimated

at $3,300 to $6,000 per kilogram of payload

to low-Earth orbit, and $22,000 to $60,000 per

kilogram to geosynchronous orbit. At that

rate, it would cost $24 billion to $200 billion

to launch a phase-two BMD system, and $140

billion to $450 billion for a responsive phasethree deployment, based on the constellation

weights estimated by various SDIO system

architects. The SDIO has set a goal of reducing launch operating costs by a factor of 10.

Operating costs are estimated at about one

third of the total life-cycle costs of a space

transportation system. Based on current operating costs, total life-cycle costs for transporting a phase-two BMD system into space might

be $72 billion to $600 billion; for phase three,

the costs might range from $420 billion to

$1.35 trillion. Reaching the goal of reducing

operating costs by a factor of 10 would reduce

life-cycle costs for space transportation by only

30 percent. Assuming that this percentage

would be valid for a new space transportation

system, and assuming a 10 to 1 reduction in

operating costs only, then the total life-cycle

costs for space transportation might be $50

billion to $420 billion for a phase-two deployment and $290 billion to $900 billion for a

phase-three deployment. Clearly the other kinds

of costs for space transportation would have to

be reduced along with the operating costs.

CONCLUSIONS

Weapon Technology Conclusions

Phase One

Kinetic Energy Weapons.—KEWs (or else the

kinds of nuclear-armed missiles developed for

BMD in the 1960s) would most likely be the only

BMD weapons available for deployment in this

century and possibly the first decade of the 21st

century. Several varieties of non-nuclear, hitto-kill KEW form the backbone of most nearand intermediate-term SDI architecture proposals. Considering the steady evolution of

rockets and “smart weapon” homing sensors

used in previous military systems, it seems

likely that these KEWs could have a high probability of being able to destroy individual targets typical of the current Soviet ICBM force

by the early to mid-1990s. The key unresolved

issue is whether a robust, survivable, integrated system could be designed, built,

tested, and deployed to intercept—in the face

of likely countermeasures—a sizeable fraction

of evolving Soviet nuclear weapons.

Space-Based Interceptors.—SBIs deployed in

the mid to late 1990s could probably destroy

75-9220 - 88 - 6
154

some Soviet ICBMs in their boost phase. The

key issue is whether the weight of the SBI

projectiles could be reduced before Soviet

booster burn times could be shortened, given

that existing SS-24 and SS-25 boosters would

already stress projected SBI constellations.

The probability of post-boost vehicle (PBV) kills

is lower due to the smaller PBV size and IR signal, but SBIs might still achieve some success

against current PBVs by the mid to late 1990s.

Exe-atmospheric Reentry Interceptor System.—The ERIS, which has evolved from previous missiles, could probably be built by the

early to mid-1990s to attack objects in late midcourse. The key unknown is the method of

tracking and discriminating RVs from decoys.

Existing radar sensors are highly vulnerable,

the SSTS space-based IR sensor probably

would not be available until the late 1990s to

early 2000s, and the AOS airborne sensor

would have limited endurance and range. This

would leave either new radars or some type of

pop-up, rocket-borne IR probe, which have

apparently received little development effort

until recently. Given the uncertainty in sensors suitable for the ERIS system, its role

would probably be confined to very late midcourse interceptions and it might have limited

BMD effectiveness until the late 1990s.

Phase Two

High Endo-atmospheric Defense Interceptor.

—The HEDI could probably be brought to operational status as soon as the mid-1990s. To overcome the unique HEDI window heating problem, the HEDI on-board homing IR sensor

needs more development than its ERIS cousin.

But the HEDI system does not depend on longrange sensors to achieve its mission within the

atmosphere. The HEDI could probably provide some local area defense of hardened targets by the mid-1990s against non-MaRVed

RVS.

83 HEDI performance against MaRVed

RVs appears questionable.

‘

S

’’ MaRV” refers to maneuvering reentry vehicles, or RVs

which can change their course after reentering the atmosphere

to improve accuracy or to avoid defensive interceptors.

SBIs against Reentry Vehicles.-The probability that SBIs would kill RVs in the mid-course

is low until the next century, given the difficulty

in detecting and tracking many small, cool RVs

in the presence of decoys, and given uncertainties in the SSTS sensor and battle management

programs.

Phase Three

Directed-Energy Weapons. -It is unlikely that

any DEW system could be highly effective before 2010 to 2015 at the earliest. No directed

energy weapon is within a factor of 10,000 of

the brightness necessary to destroy responsively designed Soviet nuclear weapons. (OTA

has not had the opportunity to review recent

SDIO suggestions for “entry level” DEWS of

more modest capability. SDIO contends that

effective space-based lasers of one to two

orders of magnitude less than that needed for

a responsive threat could be developed much

sooner.) At least another decade of research

would likely be needed to support a decision

whether any DEW could form the basis for an

affordable and highly effective ballistic missile defense. Further, it is likely to take at least

another decade to manufacture, test, and

launch the large number of satellite battle stations necessary for highly effective BMD.

Thus, barring dramatic changes in weapon and

space launch development and procurement

practices, a highly effective DEW system is

unlikely before 2010 to 2015 at the earliest.

Neutral Particle Beam.-The NPB, under development initially as an interactive discriminator, is the most promising mid-course DEW.84

Shielding RVs against penetrating particle

beams, as opposed to lasers, appears prohibitive for energies above 200-MeV. Although laboratory neutral particle beams are still about

10,000 times less bright than that needed for

sure electronics kills of RVs in space, the necessary scaling in power and reduction in beam

divergence appears feasible, if challenging.

8iThe NPB would have virtually no boost phase capability

against advanced “responsive” boosters since particle beams

cannot penetrate below about 150 kilometers altitude.
155

However, as discussed in chapter 4 under the

topic of NPB interactive discrimination, it is

unlikely that engineering issues could be resolved before the late 1990s, which would most

likely postpone deployment and effective system operation to at least 2010-2015.

Free Electron Laser.—The free electron laser

(FEL) is one of the more promising BMD DEW

weapon candidates. The FEL is in the research

phase, with several outstanding physics issues

and many engineering issues to be resolved.

Even if powerful lasers could be built, the high

power optics to rapidly and accurately steer

laser beams from one target to the next could

limit system performance. Although the basic

system concept for an FEL weapon is well developed, it is too early to predict BMD performance with any certainty.

Chemical Laser.—There are too many uncertainties to project BMD performance for the

chemically pumped hydrogen fluoride (HF) laser. The HF laser has been demonstrated at

relatively high power levels on the ground, although still 100,000 times less bright than that

needed for BMD against a responsive threat.

Scaling to weapons-level brightness would require coherent combination of large laser

beams, which remains a fundamental issue.

This, coupled with the relatively long wavelength (2.8 micron region), make the HF laser

less attractive for advanced BMD than the

FEL.

Electromagnetic Launcher.—There are too

many uncertainties in the EML or railgun program to project any significant BMD capabilities at this time.

Space Power Conclusions

Phase One

Power Requirements.—Nuclear power would

be required for most BMD spacecraft, both to

provide the necessary power levels for stationkeeping, and to avoid the vulnerability of large

solar panels or solar collectors.

Dynamic Isotope Power System.–The DIPS,

which has been ground-tested in the 2 to 5 kW

range, should be adequate and available by the

mid to late 1990s, in time for early BSTS-type

sensors.

Phase Two

Nuclear Reactors.—Adequate space power

may not be available for SSTS or weapon platforms with ladars before the year 2000. For

BMD satellites that require much more than

10 kW of power the SP-100 nuclear reactor/

thermoelectric technology would have to be developed. This is a high-risk technology, with

space-qualified hardware not expected before

the late 1990s to early 2000s.

Phase Three

Chemical Power.—Chemically driven energy

sources (liquid oxygen and liquid hydrogen driving turbogenerators or fuel cells) could probably be available for burst powers of MW up to

GW to drive weapons for hundreds of seconds

by 2000-2005.

Power for Electromagnetic Launchers. -Highcurrent pulse generators for electromagnetic

launchers (EML) would require extensive development and engineering, and would most

likely delay any EML deployments well into the

21st century.

High-Temperature Superconductors.–Research on high-temperature superconductors

suggests exciting possibilities in terms of reducing the space power requirements and improving power generation and conditioning efficiencies. At this stage of laboratory discovery,

however, it is too early to predict whether or

when practical, high current superconductors

could affect BMD systems.

Space Communications Conclusion

Laser communications may be needed for

space-to-space and ground-to-space links to

overcome the vulnerability of 60-GHz links to

jamming from nearby satellites. Wide-band

laser communications should be feasible by the

mid-1990s, but the engineering for an agile

beam steering system would be challenging.
156

Space Transportation Conclusions

Phase One

Mid-1990s Deployments.—Extrapolating

reasonable extensions of existing space transportation facilities suggests that a limitedeffectiveness, phase-one BMD system begun

in the mid-1990s could not be fully deployed

in fewer than 8 years.85 Assuming that the

hardware could be built to start deployment

in 1994, the system would not be fully deployed

until 2002. A more ambitious launcher-development program and a high degree of success

in bringing payload weights down might

shorten that period.

Phase Two

New Space Transportation System.—A fully

new space transportation system would be required to lift the space assets of a “phase-two”

BMD system. This system would have to include a vehicle with heavier lift capability

(40,000 to 50,000 kg v. 5,000 kg for the Tita.n4), faster launch rates (12 per year v. 3 per year

per pad), and more launch pads (4 v. 1).

‘J6’l”& ~Sumes that two launch pads at Vandenberg AFB,

4-East and the SLC-6 pad intended for the Shuttle, are modified to handle the new Titam4 complementary expendable launch

vehicle (CELV), and the launch rates are increased from three

Titans per year per pad up to six per year.

Optimistic Assumptions.—Even under very

optimistic assumptions,86 the new space transportation system would be unlikely to reach the

necessary annual lift requirements for a largescale, second-phase BMD until 2000-2005, with

full phase-two deployment completed in the 2008-

2014 period.

Phase Three

Ultimate DEW Systems.–It might take 20 to

35 years of continuous launches to fully deploy

far-term, phase-three BMD space assets designed

to counter with very high effectiveness an advanced, “responsive,” Soviet missile threat. This

estimate assumes deployment of the proposed

ALS space transportation system and the kind

of advanced space-based laser constellation

suggested by SD I system architects. A set of

ground-based laser installations could reduce

the space launch deployment time estimate to

12-25 years.

~his assumes that the SDIO bifurcated goal is met: a revolutionary space transportation system with 10 times lower cost

is developed in 12 years, while a near-term component of that

system yields a working vehicle of reduced capability by 1994.
Chapter 6

System Development,

Deployment, and Support
CONTENTS

Introduction. . . . ....... . . . . . . ....... . . . . . . . . .

Architecture Definition . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Fabrication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Deployment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Operation and Maintenance . . . . . . . . . . . . . . . . . . . . . . . .

Example BMD Sub-System: SSTS. . . . . . . . . . . . . . . . . .

Key System Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Automatic Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Scheduling and Deployment: An Illustrative Scenario

System Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Scheduling and Deployment . . . . . . . . . . . . . . . . . . . . . .

Box

6-A. SDIO

Figure No.

Demonstration

Box

Experiments . . . . . . . . . . . . .

Figures

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Page

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6-1. Number of Space-Based Interceptors Launched Into Space . ......171

6-2. Number of Warheads Leaking Through Boost and

Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-3. Number of Warheads Leaking Through Boost and

Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6-4a. Number of Warheads Leaking Through Boost and

6-4b. Number of Warheads Leaking Through Boost and

Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tables

Table No,

Post-Boost

..172

Post-Boost ””””

..173

Post-Boost ..174

Post-Boost

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Page

6-1. Examples of Current v. Required Manufacturing Capacity for

Proposed BMD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .163

6-2. SSTS Sub-Systerms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............167

6-3. Three-Color LWIR Sensor Components . . . . . . ........ . ..........167

6-4. Assumed Distribution of Soviet ICBMs for 1990-2010 . ............170
Chapter 6

System Development,

Deployment, and Support

INTRODUCTION

The preceding chapters review the status of

key ballistic missile defense (BMD) technologies, describing the progress made and the

additional advances still needed to meet various BMD goals. These technologies would

have to work together in an integrated system.

The United States would have to develop the

infrastructure to fabricate, test, deploy, operate, and maintain that system, and modify it

in response to Soviet countermeasures. In the

case of space-based elements, now considered

essential for a highly effective defense, the

United States would have to design, test, and

build anew space transportation system. Anything but the fastest development of this transportation system could delay all but the most

modest space-based BMD deployment to well

into the 21st century.

This chapter explores the steps involved in

moving from the current research and development phase to operational status. These

steps include:

● architecture definition,

● system development,

● system testing,

● fabrication,

● deployment, and

● operation and maintenance.

Given the complexity of a global BMD system and the immaturity of many technologies,

this chapter can only outline and give some

indication of the multitude of challenges that

would face engineers and manufacturers if a

decision were made to proceed to full-scale engineering development (FSED) and then to deployment. From the beginning, the development and deployment of dependable computer

software would be a key issue; the subject of

software is deferred until chapter 9.

ARCHITECTURE DEFINITION

The first step toward deployment would be

to complete the detailed system design or architecture. As noted in chapter 3, five defense

contractors have competed with different

BMD system designs. The Strategic Defense

Initiative Organization (SDIO) has conducted

additional analyses outside the main architecture contracting framework. A single system

architect is to be chosen in 1988. This architect is to define the actual BMD system in detail, providing information for a decision on

whether to proceed to the next step: full-scale

engineering development. The SDIO has proNote: Complete definitions of acronyms and initialisms

are listed in Append-x B of this report.

posed an early 1990s decision on FSED but

its schedules are slipping as a result of funding levels that are below its earlier expectations.

In the meantime, common elements in the

existing architecture studies can be used to

guide the research program.’ All of the spaceand ground-based architecture designs included space-based infrared (IR) sensors and

space-based interceptors (SBIs). All assumed

1Each architect defined three architectures: a combination

space and ground-based system, a ground-only system, and a

theater defense system. In addition, most architects have considered various time-phased options. For this discussion we are

considering primarily the combined space- and ground-based

architectures.

159
160

some type of ground-based exe-atmospheric The “concept validation” program approved

reentry interceptor system (E RI S). All saw a by the Secretary of Defense in September 1987,

critical need for midcourse interactive discrimi- included work on SBIs, ERIS, and associated

nation, although this task might be too diffi- sensors and battle management technology.

cult for a near-term, phase-one deployment.

SYSTEM DEVELOPMENT

The system engineer must combine various

components and sub-systems defined by the

architecture into a working system. A typical

BMD system as envisioned by system architecture contractors for intermediate-term

(“phase-two”) deployment might have included

30,000 major sub-systems of nine different

types (for suggested major components of a

phase-two system, see table 1-2 inch. 1). The

sub-systems would be tied together by a communications network. These sub-systems

would have to work together under the direction of battle management computers.2

For each of these components, the system

engineer would have to consider the following

issues:

● Mass is particularly critical for SBIs: they

would have to be light to reduce space

transportation costs and to achieve the

necessary velocity during battle.

● Total volume may be limited by the space

transportation system. All space sub-systems would have to conform to the launch

vehicle internal dimensions, preferably

with minimum wasted payload space.

● For early deployment (late 1990s), the

choices for space base-load power would

be limited to solar (which is vulnerable),

or nuclear, which would have to be developed and space-qualified in the power

ranges needed for BMD. Far-term directed-energy weapons could be driven by

liquid oxygerd/liquid hydrogen turbogenerators or fuel cells for a few hundred seconds. The weight of power supplies might

dominate future systems.

‘As discussed inch 7, this battle management function would

likely be distributed among many computers on different satellites for survivability.

●

●

●

●

●

•

Heat rejected by the various devices

would have to be minimized and properly

managed, since cooling systems take up

weight and power.

Almost all sub-systems would have to be

produced in large quantities compared to

previous space systems. These components would have to be capable of mass

production, as compared to the one-of-akind laboratory fabrication used in many

of the SDIO technology demonstration

projects. The United States has never

mass-produced any satellites.

All components would have to withstand

severe radiation environments, including

nearby nuclear explosions. This would be

particularly stressing on electronic components such as IR detectors. The detectors and most electronics used for demonstration experiments would not be

suitable for BMD deployment.

These systems would have to endure and

operate on call after sitting dormant (except possibly for periodic tests) for years.

The current goal is at least 5-year life for

first-phase deployment, with 7 years desirable. Limited lifetimes would further burden the space transportation system with

replacement or repair missions.

Many systems might have to operate

within seconds or minutes after warning,

although there might be an alert status

lasting for days or weeks. Trade-offs between long alert times and fuel consumption might be necessary.

All space-based systems would have to

operate automatically, compared to the

careful “hand tweaking” common in experiments. In particular, there would be

little or no opportunity for the routine
161

maintenance common to all terrestrial military systems.

● Various sub-systems and components

would have to work together. For example, radiation from a nuclear power supply must not degrade the operation of sensitive IR sensors or electronics. Similarly,

fumes from a propulsion system must not

fog the optics of critical sensors, and vibration from power sources must not degrade

weapons pointing accuracy.

● If components are prone to failure, they

should be easily replaceable or adjustable.

For space-based systems, a key issue

would be whether to replace entire satellites when they failed, or to attempt periodic manual or robotic repair.

● All systems and components should survive both natural and man-made environments. Survivability measures such as

decoys, redundancy, shielding, maneuverSYSTEM

Testing of both hardware and software is essential to any engineering project. Components

are tested and modified to overcome deficiencies. Sub-systems are tested and modified. Finally, prototypes of the complete system are

built and tested under full operating conditions

whenever possible. These system tests invariably reveal faults in the original design, faults

which must be corrected before production

begins.

A ballistic missile defense system could not

be tested in a full battle condition. Instead,

the systems engineer would have to rely on

some combination of computer simulations and

operation under simulated conditions. The

anti-ballistic missile (ABM) treaty prohibits

space-based tests in an ABM mode which

would be necessary to establish even minimal

confidence in SBIs.

In place of complete system testing, the

SDIO is developing the National Test Bed.

This test bed (see ch. 8) is to tie together many

●

●

ability, electronic jamming, and shootback would add mass to space-based components. One system architect estimated

that survivability measures would account for 70 percent of on-orbit mass for

SBI systems.

The communications channels would have

to be secure against interception, manipulation, and jamming.

The systems should be safe in manufacture, assembly, transport, and operation.

SDIO is funding research in all of these

areas. Optimists believe these characteristics

may be achievable; pessimists question whether the break necessary from past practice and

experience is possible; others say it is too early

in the research program to judge whether the

United States could achieve all of these attributes in a working system.

TESTING

communication nodes and computers via satellite, simulating some of the complexity of

BMD. Some types of hardware (such as sensors) would also be coupled into this test system as they became available, “talking” to the

computers as they would in a real battle. The

cost of simulation will be high, but this is the

only way to give leaders some degree of confidence in system operation. One of the key judgments the President and Congress will have

to make about the SDI program will be the

level of confidence to be placed in a global system that has never been tested in a full operational mode.

Testing so far under the SDIO program has

been limited to the component or sub-system

level, usually under simplified or artificial conditions. These experiments have yielded valuable information necessary for the ongoing research and development effort; the United

States should not, however, confuse a demonstration test with operational readiness (see

box 6-A).
162

Box 6-A.—SDIO Demonstration Experimental

Homing Overlay Experiment: The HOE demonstrated on the fourth test (June 10, 1984) that

an experimental IR homing vehicle can acquire and collide with a simulated reentry vehicle in flight.

The RV was launched aboard a test ICBM from Vandenberg AFB in California. After detection

by radars on Kwajalein, a rocket carrying the experimental ground-launched interceptor was fired

from a nearby island toward the oncoming RV. The IR sensor on the interceptor then acquired the

RV and guided the interceptor to a direct hit high above the Pacific.

While this was an encouraging and successful experiment, it does not mean that the United

States could deploy operational exoatmospheric interceptors tomorrow. The HOE experiment used

parts of an existing missile, too large and expensive for an affordable BMD system. The IR sensor

was cooled for many hours prior to the test; an operational system could not be maintained at such

cold temperatures. The detectors were not hardened against nuclear radiation; new types of detectors would be required for the operational system. The simulated RV fired from Vandenberg AFB

in California radiated about 10 times more IR energy than that expected from today’s Soviet RV,

and future RVs could have even lower IR signatures with thermal shrouds. There was only one

RV, and the experimenters knew when and where it would be fired; the real issue for exoatmospheric

interception is decoy discrimination-separating one RV out of a cloud of hundreds or thousands

of other objects, including tethered balloons. Opinions differ on how difficult this would be.

Delta 180: The Delta 180 mission (Sept. 5, 1986) launched a Delta missile into space; the two

upper stages of this missile were both placed in orbit. Each contained sensors later used to measure

radiation from the other and from another missile launched from White Sands, New Mexico during

one orbit. One stage also contained a radar sensor used to guide the two stages into a collision course

at the end of the experiment.

The Delta 180 was a very successful measurement program, providing useful information about

radiation from rocket exhaust plumes, both at close range in space and from the ground-launched

Aries rocket. Some radiation patterns confirmed expectations, but there were some surprises which

could improve our ability to detect and track future missile plumes. Tracking algorithms were also

tested in the final interception with the target stage accelerating, which is more difficult than for

targets with constant velocity. The entire Delta 180 mission took only 18 months from start to

finish, requiring extraordinary management and dedicated performance by defense contractors.

However, this measurements program should not be confused with a demonstration of the near

operational readiness of space-based interceptors. This interception had little resemblance to the

BMD problem–and could not have without violating the ABM treaty. The relative velocities and

ranges of the two stages were far less than those required for BMD. The target stage had a large

radar reflector (over 1 square meter). The size and mass of the interceptor stage (over 2,000 kg compared to a goal of less than 200 kg for SBIs) would eliminate any possibility of achieving the velocities required of a SBI to kill an ICBM. All planned SBIs discussed to date would require an IR

sensor for final homing, while Delta 180 used a Phoenix air-to-air missile radar. Finally, the near

head-on aspect of the final kill would not be typical for a BMD mission, and did not stress the divert

capability of the interceptor.

FLAGE: Six of nine planned tests of the “flexible, light-weight agile guided experiment’ ’( FLAGE)

short-range terminal interceptor missile have been completed. On the second test, the radar-guided

homing interceptor passed very close to the target, again indicating that hit-to-kill interceptors

are feasible under appropriate conditions.

In the FLAGE tests, the target vehicle was flown into a highly instrumented volume of air

above the White Sands Missile Range. Although artificial, this controlled environment is appropriate for an experiment, which should collect as much data as possible. The successful interception

‘These comments on the SDI validation experiments should not be construed as criticism of SDIO management. These are all sound

experiments properly designed to collect bits of information necessary on the path to developing a working system. At this time we have

no major element of a non-nuclear ballistic missile defense system which has been tested in a system mode with equipment suitable for actual

operation.
163

does not imply that the United States could build a FLAGE interceptor system today that would

be effective against uncooperative targets in all types of weather. A FLAGE-derived interceptor

would not be suitable for defending soft targets such as cities.

MIRACL Laser Test: The MIRACL DF laser at White Sands was aimed at a strapped-down

Titan rocket casing. The booster casing was stressed with high pressure nitrogen to simulate the

stresses expected in flight. The laser beam heated the skin of the tank, which then exploded in a

few seconds as the shell weakened.

This experiment essentially tested target lethality: how much IR energy is required to weaken

a Titan tank until it ruptures? The laser beam was about 100,000 times less bright than one required

to destroy a responsive Soviet booster from a distance of 1,000 km or more. It was not a test of

a directed-energy weapon system. The key issues for any DEW are target acquisition and tracking,

beam pointing over very large distances, and particularly the questions of retargeting and beam

jitter: could one keep the laser beam focused on one spot on the booster body while the booster

and the DEW platform travel through space at many kilometers per second? Other more complex

experiments would be required to answer these crucial questions. Real confidence in any DEW would

require space-based testing under dynamic conditions.

FABRICATION

Once a system had been developed and

tested to the degree possible, it would have to

be manufactured. The manufacturing tools and

facilities to fabricate much of the specialized

equipment needed for BMD are not yet available. In some cases, expansion or modification

of existing manufacturing facilities might be

adequate. In other cases, entirely new manufacturing techniques would have to be developed and skilled workers trained. The SDI

research program is addressing some key manufacturing issues, such as mirror and focal

plane array (FPA) fabrication techniques.

Some of the key manufacturing challenges

are summarized in table 6-1, along with an estimated comparison of current manufacturing

capacity with second-phase BMD needs. These

comparisons are not always valid, however.

For example, current (FPA) manufacturing capacity is for non-radiation hardened arrays

with less than 180 detector elements. Ballistic missile defense sensors must survive in a

radiation environment, so new types of detectors are being developed, along with all new

manufacturing techniques.

The items in table 6-1 represent only phasetwo BMD deployments, excluding items such

as interactive discrimination apparatus and

Table 6-1.— Examples of Current v. Required

Manufacturing Capacity for Proposed BMD Systems

Required

Current capacity for

capacity Phase-n BMD

Large area mirrors (square

meters per year). . . . . . . . 1-2 100-2,000

Focal plane arrays (number

of elements made per

year) . . . . . . . . . . . . . . . . . . 106

107-10’

Sapphire windows (for

HEDI; number per year) . 50 600-1,000

Precision guided missiles

(per year) . . . . . . . . . . . . . . loos 1,000-5,000

Satellites (per year) . . . . . . . 10s 300-500

Space-launch rockets . . . . . 10s loos

SOURCE: Office of Technology Assessment, 1988,

directed-energy weapons (DEW).3 Building

hundreds of space-qualified neutral particle

beam accelerators or high power lasers with

their rapid pointing and retargeting mechanisms would certainly stress manufacturing

capability.

Any manufacturing process must minimize

cost and delivery time while maintaining high

quality. These three virtues have added significance for BMD.

‘Note, however, that recently the SDIO has suggested the

possibility of including such elements in phase two.
164

Photo credit: U.S. Department of Defense

Delta 180 payload—The payload of the Delta 180

experiment, atop a Delta booster, is shown during

shroud installation on Pad 17 at Cape Canaveral.

Multiple boxes carrying optical sensors are mounted

on the side of the rocket’s second-stage truss at

bottom. The mast on top of the third stage is a Phoenix

missile sensor, which helped guide an intercept

between the two vehicles to obtain rocket motor plume

data at short distances.

.

,

Photo credit: U.S. Department of Defense

Lethal test of high-velocity projectiles.—Electromagnetic launchers might hurl small homing projectiles

at distant missile stages or warheads. In this test of

the effects of high-velocity impact, a small (unguided)

plastic projectile hit a cast aluminum block at 7 km/s.

This was a test of lethality, not of a weapon:

the projectile was not launched from an

electromagnetic launcher.

Photo credit: U.S. Department of Defense

Laser lethality test.— In September 1986 this test at

White Sands Missile Range, N. M., investigated the

possible effects of a laser beam on a rocket booster.

The test vehicle was the second stage of a Titan I

booster missile body. External loads were applied to

the booster to simulate flight conditions typical of

current operational Soviet missile systems. The test

vehicle contained no liquid propellant or explosives.

It was irradiated with a high-energy laser beam for

several seconds before being destroyed. The laser

used, the Mid-Infrared Advanced Chemical Laser,

generates a beam energy greater than 1 MW/sr. It is

a test laser, not developed for deployment in space.
165

cost

The projected costs for a BMD system will

strongly affect a national decision on whether

to proceed with production or deployment. In

addition to total costs, the incremental costs

of BMD would have to be less (some think substantially less) than the perceived incremental

cost of Soviet countermeasures. Thus the unit

costs of a deployed SBI might have to be less

than l/370th to l/12th the cost of a Soviet

booster.’ On the other hand, the leverage provided by a successful “adaptive preferential

defense” might improve this cost-exchange ratio (see ch. 1).

The allowable costs for aground-based exoatmospheric interceptor would depend on the

system architecture. With low leakage from

the boost phase and good discrimination, each

interceptor would have to engage only a small

percentage of the attacking Soviet reentry vehicles (RVs), and the interceptor could be relatively expensive. If discrimination were poor,

which might be the case in a phase-one deployment, then the interceptor might be competing with cheap decoys. The defense would not

be cost-effective at the margin if every exoatmospheric interceptor had to costless than

10 light-weight decoys, or even less than 10

heavy decoys.5

Time

The time to manufacture components for

BMD might be crucial in several respects.

‘The 12-tOl cost ratio assumes that 8 percent of the SBIS

would be within range of the Soviet missile fields and that one

SBI is fired at each booster or PBV. Them are no extra SBIS

for redundancy or shoot-back against Soviet ASATS. In this

case the United States would have to add about 12 SBIS (and

another carrier satellite) for each new Soviet booster. The 370-

t~l cost ratio comes from a concentrated basing of new Soviet

boosters in a relatively small area say 150 km by 150 km. In

this case the United States would have to deploy 370 extra SBIS

and their associated satellites for each new Soviet booster to

achieve an 85 percent probability of destroying that extra RV.

‘If the boost phase defense let through 10 percent of the

boosters, and each booster carried 10 warheads, 10 heavy decoys, and 100 light decoys, then the exe-atmospheric interceptor system would have to engage one warhead, one heavy decoy, and 10 light decoys for each booster launched With perfect

discrirnin ation one deployed interceptor would have to cost less

than one loaded booster. Without any discrimination, one interceptor would have to cost l/12th of the booster.

Ideally the system should be deployed quickly

to avoid transition instabilities, although system architects differ on this point. Components

could be produced and stockpiled until deployment began.6 To the degree that space transportation would pace deployment, production

times would not be critical.

But the United States would also be locked

in a race with Soviet countermeasures. If the

United States could not produce and deploy

enough SBIsbefore the Soviets had reduced

a substantial number of their booster burn

times below 140 seconds, then BMD boostphase system effectiveness would drop significantly, perhaps to zero. The SBIs might force

the Soviets to faster post-boost vehicle (PBV)

dispersals, which could reduce the number of

RVs At some point, however, there would be

no sense in deploying SBIs(and particularly

SBIs Which did not have any midcourse capability against RVs) until DEW were developed.

(See also ch. 5 and the key-issues section at

the end of this chapter for more analysis of SBI

effectiveness against boosters with moderately

fast burn times.)

On the other hand, if the United States could

produce and deploy an SBI system in a few

years, and if it could build and deploy a credible DEW system as the Soviet Union converted to faster-burning boosters and fast-dispersing PBVs, then BMD effectiveness might

continue. .

Production time involves not only the production rate, but the time to design, build, and

debug the manufacturing facilities, including

necessary training of production workers.

Since many new technologies are contemplated, there might be relatively long periods

before routine production could begin.

Quality

Quality control would be essential, particularly for space-based deployment. Repair or

even replacement of failed assets in space

‘See U.S. Congress, Office of Technology Assessment, Ballistic Missile Defense Technologies, OTA-ISC-254 (Washington, D. C.: U.S. Government Printing Office), p. 119.
166

might severely stress space transportation, just to lift the initial BMD equipment into

particularly if space launch facilities were com- place.

pletely occupied over a period of 5 to 10 years

DEPLOYMENT

Given that some boost-phase defense capability would be key to a highly effective BMD

system, and given that the United States currently has very little space launch capability,

deployment of space-based assets would most

likely limit the operational starting date for

BMD. As shown in the space transportation

section of chapter 5, the United States would

have to build a new space launch system to

lift into orbit the necessary number of SBIs

and their supporting satellites. The timing of

the development and availability of a new

space launch system is unclear, but it is doubtful that it would be possible to launch significant numbers of SBIs before the mid to late

1990s.

Several years of continuous space launch

activities from several launch pads would then

be necessary to deploy enough SBIs to provide

one shot against each missile or PBV in today’s

fleet of Soviet intercontinental ballistic missiles (ICBMs). The SDIO, however, does not

propose deploying that many SBIs in a firstphase system. It argues that lesser capabilities

would still have worthwhile deterrent value.

(See section below on scheduling and deployment issues for discussion of the effect of deployment rates on SBI system effectiveness.)

OPERATION AND MAINTENANCE

Once deployed, the BMD system would have

to be kept in operating order. Ground-based

elements such as ERIS could be periodically

tested, disassembled, and repaired as needed.

For space-based assets, both testing and repair would be difficult unless built into the initial design. Methods would be needed to determine if the sensor or the guidance system

on a dormant SBI would operate in a war. Computer systems would have to be exercised to

make sure radiation in space had not altered

a key software bit that might subsequently inhibit successful operation. The status of dormant space assets would have to be monitored

carefully and frequently.

Once defective space systems were diagnosed, they would have to be replaced or

repaired. The system architecture would have

to incorporate some combination of redundancy or on-orbit repair or replacement to

maintain the total system. The space transportation system would have to be sized to handle this load.

Space-based assets might also need to be

modified in response to Soviet countermeasures. SBI sensors initially designed for tracking only booster plumes with short or mediumwave IR sensors might become worthless

against faster-burning boosters. Should a

second-phase system add LWIR sensors to

previously deployed SBIs to give them midcourse kill capability? Trade-off studies would

determine whether it would be more cost-effective to replace components on obsolete satellites or simply to add entirely new satellites.
167

EXAMPLE BMD SUB-SYSTEM: SSTS

To appreciate some of the complexity of a

BMD system, consider just one of the systems

in table 1-2: a moderately sophisticated Space

Surveillance and Tracking System (SSTS). The

potential sub-systems of an SSTS are shown

in table 6-2. Almost every subsystem on this

list would require development to meet the

probable BMD specifications.

At the next level down, just one sub-system

from the SSTS, a three-color LWIR sensor,

Table 6-2.—SSTS Subsystems

Development

required

Propulsion (for station-keeping) . . . . . . . . . . .

Communications (space-to-space) . . . . . . . . .

Communications (space-to-ground) . . . . . . . .

Power source . . . . . . . . . . . . . . . . . . . . . . . . . . .

Three-color LWIR sensor(s) . . . . . . . . . . . . . . .

SWIR/MWIR sensor(s). . . . . . . . . . . . . . . . . . . .

Laser ranger/designator . . . . . . . . . . . . . . . . . .

Star tracker(s). , . . . . . . . . . . . . . . . . . . . . . . . . .

Computer and memory. . . . . . . . . . . . . . . . . . .

Waste heat rejection. . . . . . . . . . . . . . . . . . . . .

Support structure . . . . . . . . . . . . . . . . . . . . . . .

Low

High

High

High

High

Medium

High

Medium

Medium

Medium

Low

would include the components listed in table

6-3. Again, most of these components must be

developed to meet BMD specifications. An

analysis of the other SSTS sub-systems and

the other major sub-systems in the three

phases of SDI would reveal literally hundreds

of sizeable development programs which would

have to come together to form the complete

system.

Table 6.3.—Three-Color LWIR Sensor Components

Development

required

Primary mirror . . . . . . . . . . . . . . . . . . . . . . . . . .

Secondary mirror. . . . . . . . . . . . . . . . . . . . . . . .

Cryo-cooler. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Three-color focal plane array (FPA) . . . . . . . .

Signal processor . . . . . . . . . . . . . . . . . . . . . . . .

Three-axis gimbal . . . . . . . . . . . . . . . . . . . . . . .

Servo control system . . . . . . . . . . . . . . . . . . . .

Thermal control system . . . . . . . . . . . . . . . . . .

Sun shield . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Support structures . . . . . . . . . . . . . . . . . . . . . .

High

High

Medium

High

High

Medium

Medium

Low

Low

Low

SOURCE: Office of Technology Assessment, 1988 SOURCE: Office of Technology Assessment, 1988.

KEY SYSTEM ISSUES

Building and deploying a system on the scale

of proposed BMD architectures would stress

the U.S. engineering and manufacturing infrastructure on many fronts. However, three critical systems issues are unique to ballistic missile defense with space-based components: the

lack of realistic system testing, the necessity

for automated, computer-controlled operation,

and the difficulties of scheduling and space deployment.

System Testing

The inability to test fully a global BMD system (both hardware and software) would cast

doubt on its operational effectiveness. The

administration and Congress will have to decide on the deployment of a system whose performance would have to be predicted largely

by computer simulations. The National Test

Bed and future component tests would improve the verisimilitude of those simulations,

but they could not encompass all of the complexity of the real world. Some issues such as

sensor operation against a nuclear explosion

background in space could not be tested even

at the component level without abandoning the

Limited Test Ban Treaty. Except in computer

simulations, the system could not be tested,

short of war, with even 10 percent of the possible wartime threat.
168

It is true that all military systems are subject to uncertainty when they first go into battle. A fighter aircraft, despite the best flight

test program, can never be tested with all the

variables that will arise in a real battle. An

aircraft-carrier battle group could never anticipate all possible situations in some future battle with a capable adversary, and might be susceptible to unforeseen vulnerabilities. The U.S.

carrier battle groups have never fought against

an enemy with modem “smart weapons. ”

There is uncertainty in the performance predictions of these conventional military systems.

A global BMD system would have even more

complex, untestable sub-system interactions.

Even full interception tests, using SBIs fired

against ICBMs launched from Vandenberg

AFB, could involve at best a salvo launch of

a few missiles. This would not substitute for

the launching of a thousand missiles by the

Soviet Union at a time of their choosing,

preceded by anti-satellite weapon (ASAT) attacks and nuclear precursor explosions. Individual components such as sensors, data processors, and communication equipment could

be tested by themselves to full operational capacity in the laboratory or in simulated space

chambers, and some effects of nuclear explosions could be tested at the Nevada test site.

In any case, the complete BMD system could

not be tested as an integrated unit against a

real threat. Neither, on the other hand, could

the Soviet offensive ballistic missile force.

Automatic Operation

Automation has made dramatic changes in

factories and some military weapons systems.

Robotics is firmly established in many manufacturing situations, and will grow in the future. However, space-based BMD systems

would cross into new engineering domains of

automatic operation on several counts:

● continuous unattended stand-by operation for years,

a continuously changing constellation of

components which would have to operate

together as a unit, and

operation under adverse conditions against

an opponent determined to defeat the

system.

None of these limitations is encountered in

automated factories.

Automatic fire control systems are common

in today’s weapons. Human intervention is always possible, however, to repair and maintain the system. The United States has never

operated a weapon system in space. Both the

United States and the Soviet Union have operated sensor systems ins ace for surveillance

and early warning of ballistic missile attack.

The challenge would be to integrate more sophisticated early warning satellites with actual weapon platforms thousands of kilometers away.

Sensor satellites currently in orbit operate

autonomously, with directions from a few

ground-based mission control nodes. Once the

battle began, BMD systems might require the

autonomous operation of 30 to 40 sensors

working in conjunction with hundreds or thousands of SBI carrier satellites. Sensors and carrier satellites would be moving in different orbits, so that the particular weapons platforms

and sensors making up a “battle group” (in

one possible battle management architecture)

would be constantly changing with time. (See

ch. 7 on wartime operation.) These battle

groups would have to be connected by secure

communication links. Higher system effectiveness would entail tighter coordination.

Automatic operation would be further challenged by Soviet defense suppression tactics.

The system would ideally adapt to lost or noisy

communication links and continue to manage

the battle on the basis of degraded information. (See ch. 9 for a fuller discussion of BMD

software dependability.)
169

Scheduling and Deployment:

An Illustrative Scenario

If an administration and Congress were to

decide that our national security would be improved by deploying some type of BMD, a major issue would be when to begin deployment.

Early deployment (e.g., 1995-2000) of a phaseone system would risk “locking in” immature

BMD technology that might be less effective

against the projected threat. Waiting for more

advanced technology would give the Soviet

Union more time to prepare countermeasures,

increasing the risk that the defense effectiveness would remain low. Early deployment

would strain space transportation facilities,

and the long deployment time would preclude

a fast transition from offense- to defensedominated status. But a decision to wait for

later deployment could, some fear, indefinitely

postpone any deployment at all.

Ballistic missile defense system effectiveness would depend not only on the U.S. deployment schedule, but also on the timing of Soviet countermeasures. The longer it took to

deploy a defense, the more time the Soviets

would have to respond by improving their

offensive forces. To illustrate the interplay between defensive and offensive deployments

over time, OTA constructed a plausible scenario for the 1994-2010 period, then estimated

the effectiveness of an SBI system as a function of time. For the defense, we assume that:

● SBI deployment would be limited only by

the capacity of future United States space

transportation systems. That is, the

United States could produce and operate

in space as many SBIs as it could launch.

Note that it is emphatically not the SDIO

proposal to deploy this many SBIs.

• The SDIO two-track space transportation

scenario succeeds in building a heavy lift

expendable launch vehicle by 1994 with

30,000 kg lift to near polar orbits, and this

same technology simultaneously evolves

into an economical, partially reusable vehicle with 44,000 kg capacity by the year

2000.

Three new launch pad complexes would

be built at Vandenberg AFB and launch

rates would be increased from 3 per year

per pad up to 12 per pad per year, bringing the total lift capacity to near polar orbits to 2.2 million kg per year by 2004.

Three different classes of SBIs might be

available with varying masses and velocities: a “state-of-the-art” a “realistic,” and

an “optimistic” interceptor. (Specification

of the characteristics of each are in the

classified version of this report.)

The SBIs would be replaced at the end of

a useful life of 5 to 1O-years, which limits

the number of SBIs in orbit unless the

space transportation system capacity continues to grow with time.

For the Soviet offensive response, OTA

assumed:

●

●

●

●

a gradual decrease in the burn-time of Soviet ICBM boosters and in the RV and decoy dispersal time of its PBVs through

the introduction of one new class of 10-

warhead missiles every five years;

that these new missiles would be clustered

at three existing SS-18 missile sites, which

would cover an area of 500,000 square km;

retirement of old Soviet missiles as the

latest models were introduced, keeping the

total RV count at 10,000 (case 1), or an

increase of their ICBM’s by 100 per year

after the year 2000 (case 2);

no other Soviet countermeasures, except

a significant Soviet A SAT capability, imlied by our resewing a substantial fraction of U.S. SBIs for self-defense or to account for inoperable SBIs that fail over

time.

While these assumptions are technically

plausible, they are not based on any Department of Defense or intelligence community

estimates of what the Soviets could or would

do.
170

Table 6-4.—Assumed Distribution of

Soviet ICBMs for 1990.2010

All ICBMs are assumed to carry 10 warheads. Please note

that the mix of forces here reflects neither Department of

Defense nor intelligence community estimates of what the

Soviets actually may do. Instead, this table merely lays out

a purely hypothetical sequence of a phasing-in of fasterburning ICBMs at 5-year internals beginning in 1990, Older

missiles are retired as new ones are deployed, keeping the

total RV count fixed at a hypothetical number of 10,000. The

slow-burn boosters are distributed over existing Soviet missile fields, while the other four classes are assumed concentrated at three existing sites.

Number of ICBMs

ICBM type SBB MBB-1 MBB-2 FBB-1 FBB-2

Year:

1991 . . . . . .

1992 . . . . . .

1993 . . . . . .

1994 . . . . . .

1995 . . . . . .

1996 . . . . . .

1997 . . . . . .

1998 . . . . . .

1999 . . . . . .

2000 . . . . . .

2001 . . . . . .

2002 . . . . . .

2003 . . . . . .

2004 . . . . . .

2005 . . . . . .

2006 . . . . . .

2007 . . . . . .

2008 . . . . . .

2009 . . . . . .

2010 . . . . . .

500

500

500

400

300

200

100 —

—

—

—

—

—

—

—

—

—

—

500 – – –

500 – – –

500 – – –

500 – – –

500 – – –

500 100 — —

500 200 – –

500 300 – –

500 400 – –

500 500 – –

400 500 100 –

300 500 200 –

200 500 300 –

100 500 400 –

— 500 500 –

— 400 500 100

— 300 500 200

— 200 500 300

100 400

— — 500 500

Legend:

SBB: Slow-Burn Booster

MBB-1: Medium-Burn Booster—First Generation

MBB-2: Medium-Burn Booster—Second Generation

FBB-1: Fast-Burn Booster-First Generation

FBB-2: Fast-Burn Booster-Second Generation

SOURCE: Office of Technology Assessment, 1988

Space Transportation Limits on Deployment

As indicated in chapter 5, a new space transportation system would be needed to launch

the space-based assets of a highly effective

BMD system. Even a more modest system,

such as that proposed by SDIO for the first

phase, would call for considerable new space

transportation capacity. The SDIO has identified two potentially conflicting space transportation goals: reducing launch costs by a factor of 10 and beginning some launches in the

mid-1990s. Derivatives of existing Shuttle/

Titan launch systems are not likely to lead to

major cost reductions; an entirely new system

would be needed. But a revolutionary new

space transportation system would not likely

be ready before the year 2000.

To achieve both the cost and schedule goals,

SDIO has proposed a dual-track formula: a new

space transportation system would be developed with a goal of a tenfold cost reduction

by 2000 or so, but parts of this new system

would be available by the mid-1990s for early

deployments, probably with reduced lift capacity and higher cost. This approach might create design compromises. Either cost reductions

might have to be postponed to meet the schedule, or the schedule might have to be slipped

to meet the eventual cost goals: a space transportation system designed to meet just one of

these goals might look quite different from

the hybrid. In this scenario, however, we assume that both goals could be achieved simultaneously.

The United States now has one pad capable

of launching more than 10,000 kg to the high

inclination orbits and altitudes of several hundreds of kilometers to be occupied by the SBI

constellation. 7 The Shuttle pad at Vandenberg

Air Force Base could be modified by 1992 to

launch the Titan-4 (CELV) vehicle with a capacity of about 14,500 kg to SBI orbits. In the

past, building new launch pads has taken from

7 to 10 years and there is some question

whether there is adequate space at Vandenberg to add even a few more pads and their

necessary assembly facilities. (The Air Force

has been examining the possibility of launching rockets from an off-shore oil rig.) Survivability of launch facilities would also be questionable if all U.S. polar-orbit pads were located

at one coastal site. In this scenario, we assume

that these difficulties are overcome.

Launch rates have been in the range of three

to five per year from one pad. This rate is

‘The 4-East pad at Vandenberg Air Force Base in California

is equipped to launch the Titan 34D and Titan-4 (CELV) vehicles mto polar or high inclination orbits. The 4-West pad at Vandenberg can handle tlw Titan-2 vehicle, which has less than 2,000

kg capacity. Two pads at Kennedy Space Center (#40 and #41)

can launch Titan 34Ds and Titan-4s, but not into near-polar

orbits. There are no Delta launch facilities at Vandenberg.
171

limited by the necessity to assemble the launch

vehicles at the site. Studies are underway to

determine if these launch rates could be increased to 6 per year, with some experts suggesting that rates up to 8 per year might be feasible in the future for the Titan class vehicles,

and 12 per year per pad for the new vehicle.

SBI Characteristics

OTA analyzed three classes of SBIs, corresponding to assumed improvements in SBI

technology as discussed in chapter 5. The

“state-of-the-art” rocket would probably be the

best technology available for a first-phase deployment in the mid-1990s. For the most part,

this SBI would use components that have been

demonstrated in the laboratory (as of 1988),

but not as yet assembled into a working system. The “realistic” SBI represents a plausible level of technology after more component

research and development, and might be available by the mid to late 1990s; the overall rocket

mass assumption of well under 100 kg would

be particularly challenging. The “optimistic”

SBI assumes improvements in all areas of development, and would be much less likely, but

possible. Other assumptions about SBI redundancy factors and kill probabilities are the

same as those applied earlier in chapter 5 of

this report.

Given the optimistic space launch projections from chapter 5 and the different assumptions for SBI masses, one can estimate the total number of SBIs that might be placed in

orbit as a function of time, as shown in figure

6-1. The lifetimes of SBIs in space would be

critical, since defunct interceptors would have

to be replaced, taking space transportation capacity away from the tasks of increasing SBI

deployments or other BMD assets. It might

turn out, however, that on-orbit repair could

reduce the numbers of spares and replacements

needed. As shown in figure 6-1, the number

of state-of-the-art rockets would reach a plateau by about 2006 if better SBIs could not

be developed: a space transportation system

sized to put the original constellation in place

would operate full-time just to replace these

.

20

0

I

1990 1995 2005 2010

Year

Maximum number of SBIs that could be launched into orbit

based on the assumed space transportation revolution described in chapter 5. This chart assumes that all space launch

capability is devoted to SBIs and their associated carrier vehicles. The net mass per SBI, including the pro-rated share

of the carrier vehicle mass, would be 334, 179, and 129 kilograms for “state-of-the-art,“ “realistic,” and “optimistic” SBIs,

with life-times of 5, 7, and 10 years.

SOURCE: Office of Technology Assessment, 1988.

SBIs and maintain the constellation in a

steady-state constellation.

For the lighter and faster “optimistic” SBIs,

the assumed transportation system could lift

up to 160,000 SBIs into orbit by the year 2010.

This assumes that no other space assets would

be launched into near polar orbits during the

entire 1994-2010 period. Thus any later deployments of interactive discrimination systems

or directed-energy weapons would reduce the

possible number of SBIs in orbit. In any case,

it is obvious that the United States would not

try to manufacture, lift into space, and manage a constellation of 160,000 SBIs.

The “optimistic” SBI effectiveness curves

which follow are therefore unrealistic; they are

shown only to indicate upper bounds on SBI

boost and post-boost effectiveness. They suggest that while SBIs might be considered for

a system intended to enhance deterrence, they

would not, by themselves, be suitable for a system intended to assure very RV low leakage

rates. They also suggest that, barring substan-
172

tial offensive force reductions, the initial effectiveness of an SBI system might be eroded

by appropriate countermeasures. In that case,

directed-energy weapons might have to be

brought on line just to maintain previous defense capability.

Assumed Soviet Offensive Countermeasures

As the U.S. space transportation system

(and hence the number of possible SBIs in

space) grew, Soviet ICBM and submarinelaunched ballistic missile SLBM forces would

most likely also change with time. One central

question for evaluating BMD effectiveness is

whether reasonable Soviet countermeasures

could keep ahead of possible U.S. BMD deployments. Here, OTA analyzed the effects of just

three Soviet countermeasures: reduced booster

burn and PBV dispersal times and clustering

of new missiles at three existing missile sites.

These analyses assumed that the Soviet Union

reduced its booster burn and PBV times gradually over the next two decades, introducing

anew class of weapon each 5 years with moderately improved performance. Three cases were

assumed: optimistic (relatively long booster

bum times), base case, and pessimistic threats.

Even the “pessimistic” threat case assumes

a 90-second bum-time by 2006, still more than

the 60- to 80-second burn-times deemed feasible for the next century by some rocket experts. Thus these threat assumptions are all

conservative compared to what may be technically feasible.

SBI Boost and Post-Boost Effectiveness

We next calculated the maximum possible

number of RVs that could be destroyed each

year by SBIs in either the boost or the postboost phase, simply by calculating how many

SBIs would be within range of the booster or

the post-boost vehicle at the time each RV was

deployed.

We assumed uniform, serial RV deployment

over the PBV dispersal time. Each SBI attacked the booster first if it was within range,

then the PBV at the earliest possible time. Two

shots were taken if more than one SBI could

reach a booster or PBV. Perfect battle management was assumed: the battle manager

knew exactly where all boosters and SBIs

would be at burnout, and assigned SBIs to

their highest value targets without error. These

calculations assumed that a substantial fraction of SBIs are used for self-defense (or are

inoperable)—an on orbit repair system, however, might reduce the extra numbers needed.

Other assumptions were that each SBI had a

reasonably high single-shot kill probability

against the boosters and and a slightly smaller

one against the PBV.

The resulting system effectiveness (the number of RVs leaking through the boost and postboost SBI defense) is plotted as a function of

time in figure 6-2 for the three canonical SBIs

Figure 6-2.-Number of Warheads Leaking Through

Boost and Peat-Booat Defenses

Year

BMD system effectiveness in terms of the number of RVs

(out of a hypothetical attack of 1,000 missiles with 10,000 RVs)

which would leak through a boost and post-boost defense,

limited only by the ability of the U.S. space transportation

system to lift space-based interceptors (SBIs) into orbit (figure 6-1 indicates the number of SBIs available each year for

each type of SBI). The SBIs have a reasonable probability y of

destroying a booster and a slightly smaller probability y of killing a PBV; a substantial percentage of the SBIs are used for

self-defense or are otherwise inoperative. The Soviet threat

has a constant 10,000 warhead level, but with decreases in

booster burn-times and PBV dispersal times as described in

the text.

SOURCE: Office of Technology Assessment, 1988,
173

of chapter 5, assuming OTA’s hypothetical Soviet threat. In the very near term, the United

States could only deploy the “state-of-the-art”

SBI. According to these simplified calculations, this type of defense could at best destroy

2,500 RVs out of the OTA-postulated 1,000-

missile, 10,000-RV threat by 1998 when the

United States would have orbited 4,100 SBIs;

7,500 RVs (and their associated decoys) would

pass through to the later defensive layers.

Performance would degrade over time with

quicker dispersal of Soviet RVs. If the United

States could develop the lighter and faster “optimistic” SBIs, then the defense could reach

50 percent effectiveness by 2001, but this

would imply the deployment of 40,000 SBIs

by then. Furthermore, to maintain this approximate level of effectiveness with 5,000 warheads leaking through to the midcourse, the

United States would have to continue deploying these SBIs, reaching levels of 160,000 SBIs

by 2010. Even then, the Soviet penetration to

the midcourse would have increased slightly

to 6,000 warheads.

The most likely “realistic” SBI would result

in a minimum leakage of 6,000 warheads to

midcourse. To come close to maintaining this

leakage, the United States would have to continue devoting all space launch capability to

the SBI system; by 2010 there would be 90,000

SBIs in orbit and 8,000 warheads would survive to midcourse. Again, such figures illustrate that SBIs should not be expected to stop

high percentages of Soviet missiles in a massive attack. Nor is it reasonable to expect them

to sustain initial boost- and post-boost phase

capabilities against a “responsive” Soviet missile threat of the future. The SDIO does not

support either expectation.

The sensitivity of SBI effectiveness to the

Soviet threat is shown in figure 6-3, assuming

the “realistic” SBI rocket parameters in all

cases. With the “optimistic threat” scenario,

the SBI BMD system could achieve 50 percent effectiveness by 2005, assuming that the

United States had deployed 70,000 SBIs.

Again, this constellation would have to be increased to 90,000 by the year 2010, and even

then the Soviet RVs leaking through could

Figure &3. -Number of Warheads Leaking Through

Boost and Post-Boost Defenses

10,

A

4 Optimistic threat J

I

3

l - - I

2 —

(Realistic SBl)

1 —

I I I

1990 1995 2005 2010

Year

Boost and post-boost system effectiveness as a function of

time for three different Soviet threat models described above.

“Realistic” SBIs were used in all cases.

SOURCE: Office of Technology Assessment, 1988.

number 6,000 warheads and be increasing.

These numbers suggest that directed-energy

weapons would be needed, sooner or later, to

achieve and sustain high kill levels against advanced Soviet boosters and PBVs.

The previous two figures assume that the

Soviets retire old missiles as new ones are deployed, keeping the total at 10,000 warheads

available. In the absence of arms control treaties, they could keep old missiles in place, and

continue to add faster-burning boosters. The

BMD effectiveness for this situation is shown

in figure 6-4, assuming that all initial mediumburn boosters are retained, and that 100 of the

faster-burning boosters (FBB) are added each

year after 2000. Under the most optimistic (for

the defense) conditions, the Soviets could maintain 6,000 warheads surviving into mid-course.

Assuming penetration aids to be available

by the 2000-2005 time period, these 6,000 warheads and their associated decoys would make

passive midcourse discrimination and RV kills

very difficult. The leakage against SBIs in all

cases would increase with time, most likely

reaching the 10,000 warhead level by 2010, despite the presence of up to 160,000 SBIs in

space.

8

8For analytic purposes, we have ignored the questions of maintenance and battle management of so many interceptors.
174

~ Optimistic SBI 2 —

o~ I I I

1990 1995 2000 2010

Year

Boost and post-boost effectiveness limited only by space

t ransportat ion capability, assuming that the Soviet threat increases in quality (shorter deployment times) and quantity

(after 2000). Effectiveness shown for three different types of

space-based interceptors against the “base case” Soviet

threat.

SOURCE: Office of Technology Assessment, 1988,

1.

2.

3.

Figure 6-4b. -Number of Warheads Leaking Through

Boost and Post-Boost Defenses - —

01 I I I

1990 1995 2005 2010

Year

Boost and post-boost effectiveness against three different

Soviet threats, all assuming “realistic” space-based interceptors.

SOURCE: Office of Technology Assessment, 1988.

SYSTEM CONCLUSIONS

Testing

If the United States abandoned or achieved

modification of the ABM Treaty, it could

test a limited constellation of SBIs against

a few ICBM’s launched from Vandenberg

AFB. But this would not replicate the conditions of a massive, surprise launch of

hundreds or thousands of ICBMs, ASATs,

and nuclear precursors from the Soviet

Union.

A BMD system could not be tested against

the real threat of up to thousands of ICBMs

combined with defense suppression and nuclear precursors. However, neither could

such a coordinated offensive attack be

fully tested.

Key elements, such as IR sensors, could not

be realistically tested against a background

disrupted by nuclear explosions without

abrogating the Limited Test Ban Treaty.

Automation

4. No technical barriers appear to preclude

automatic operation of a space-based BMD

system, but the task of operating an automatic, constantly changing constellation of

sensors and weapons platforms in the face

of defense suppression tactics would be a

major challenge with little or no analogous

experience from any other automated

systems.

Scheduling and Deployment

Phase One

5. A near-term deployment (1995-2000) of state

of-the-art SBIs might stop up to 2,500 of

an assumed constant 10,000 Soviet warhead

threat in the boost and post-boost phases—

if the United States devoted all of its space

launch capability to lifting SBIs into orbit.
175

This assumes that the burn times and

post-boost vehicle dispersal times of future Soviet ICBM’s decrease over time in

a reasonable manner. Of course, fewer

SBIs could kill similar percentages of

boosters if a smaller attack were assumed.

The SDIO argues that defenses that are

far from perfect still offer significant enhancement of deterrence (see chs. 1,2, and

SBIs and into midcourse after 2005, reaching the pre-BMD levels of 10,000 leaking

warheads by 2010. Therefore, SBIs should

not be expected to achieve the strategic

goal of “assured survival” against nuclear

attack by a Soviet missile force unconstrained by arms reductions and limitations.

Phase 3

3 ).

Phase 2 8.

6. An intermediate-term or “phase-two” deployment of more advanced SBIs might kill

up to 5,000 of the hypothesized fixed number of 10,000 Soviet RVs in the boost and

post-boost phases, but only by orbiting from

90,000 to 160,000 SBIs. Therefore, the

United States would be unlikely to rely

on SBIs for continued boost-phase interception of advanced Soviet missiles. 9.

7. Given the assumptions of OTA analyses,

under the most optimistic conditions the Soviet Union could maintain an RV leakage

into midcourse at or above the 6,000 warhead level by increasing the number of

ICBMs deployed by 100 per year after the

year 2000. Under any of the assumed conditions, the Soviet Union could increase

the rate of warhead penetration against

A highly effective BMD system would require either very effective midcourse discrimination or a very effective directedenergy weapon (DEW) system, and preferably both, since an SBI system, as limited

by the most optimistic space transportation system, could never assure that fewer

than 5,000 Soviet warheads and their associated decoys would leak through to the

midcourse,

As concluded in chapter 5, it is unlikely

that the United States could determine

the feasibility of DEW systems by the late

1990s, and deployment probably could not

begin until 2005-2010 at the earliest. It

therefore appears likely that the Soviet

Union, unless constrained by offensive arms

control agreements, would be able to maintain leakage rates of a few thousand nuclear

warheads until at least the period 2005-2010.
Chapter 7

System Integration

and Battle Management
CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Battle Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conduct of the Battle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Battle Management Functions . . . . . . . . . . . . . . . . . . . . .

Interactions Among the Phases . . . . . . . . . . . . . . . . . . . .

System Performance and Interaction . . . . . . . . . . . . . . . .

Opportunities for Human Intervention . . . . . . . . . . . . . .

Supporting Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Recovery From Damage and Failures . . . . . . . . . . . . . . . .

Complexity of Battle Management . . . . . . . . . . . . . . . . . . . .

Battle Management Architecture . . . . . . . . . . . . . . . . . . . . .

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Box

7-A.

7-B.

7-c.

Boxes

Algorithms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Centralization, Distribution, and Coupling . . . . . . . . .

Hierarchies and System Design .. ... . . . . . . . . .

Figure No.

7-1. Analytic Chart

BMD System .

Missile

Table No.

7-1. Ballistic

of Battle

. . . . . . . .

Figure

Management Functions in a

. . . . . . . . . . . . . . . . . .

Table

Battle Management

. . . . . . .

.... . . . . . . . . . . . . 179

. . . . . . . . . . . . . . . . . . 179

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Page

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. . . . . . . . . . . . . . .. 192

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Phase-One

. . . . . . . . . . . . . . . .

Functions . . . . . . . . . . . . . .

Page

.196

Page

.181
Chapter 7

System Integration and Battle Management

INTRODUCTION

Chapter 6 discusses developing, deploying,

and maintaining a ballistic missile defense

(BMD) system. Once deployed, BMD components would have to work together to form a

fighting system. Maintaining such integration

would require regular, routine support. This

chapter looks at integrated operation of the

system. Although some system capabilities

could be used during peacetime, e.g., for surveillance, fully integrated use would only be

required during battle.1 Accordingly, most of

this chapter is concerned with battle management, i.e., how the system would be managed

to fight effectively.

‘Peacetime simulations of battles would also require considerable integration, but would probably omit operations such

as use of interactive discriminators and firing and controlling

weapons.

A major assumption in the discussion that

follows is that the system is sufficiently wellintegrated during peacetime that it can be

moved promptly to a full fighting status. As

examples, the communications network that

permits battle managers to exchange information would have to be working and the battle

managers would need timely data on the number, kinds, and locations of resources available

to them. Peacetime activities needed to keep

the system integrated, such as sending updates

of resource-availability data and new versions

of software to battle managers, would have to

be performed routinely. Operational readiness,

testing and evaluation, and repair or replacement off ailing system components would also

have to be routine.

BATTLE MANAGEMENT

The battle management portion of a BMD

system would combine the data provided by

sensors and the capabilities offered by weapons into a defensive system. The battle management computers would provide computational and decisionmaking capability. The

battle management software would be the glue

bonding the components together into a fighting system. Battle management includes strategy, tactics, resource allocation algorithms,

and status reporting.

Battle management computing may be distributed among many different platforms or

consolidated on just a few. In either case, the

battle management functions would remain

the same, although the capabilities needed in

supporting functions, such as communications,

Note: Complete definitions of acronyms and intialisms

are listed in Appendix B of this report.

might vary. This chapter describes the conduct

of a battle from the battle management viewpoint, from alert through the actual battle sequence. The scenarios only meant to be illustrative, not comprehensive. Its purpose is to

convey a sense of the complexity of the battle

management task, not to provide an actual battle management system design.

In peacetime, the system might be in a quiescent mode, conserving fuel and other resources,

with some components shielded from space.

As the probability of a battle increased, the

system might move through a series of alert

levels, during which sensors such as the Boost

Surveillance and ‘hacking System (BSTS) and

Space Surveillance and Tracking System

(SSTS) would be fully opened to space and

weapons would be prepared for battle, including warm-up and status checks. At the highest alert level, the system would be fully prepared to fight a battle.

179
180

The battle may be partitioned into different

phases, each distinguished by a different set

of offensive actions. The phases are boost, postboost, mid-course, and terminal. For an individual reentry vehicle (RV) or decoy, the phases

occur in the sequence given. Different RVs and

decoys may be in different phases concurrently, requiring the defense to fight in different

phases at the same time. In addition, the BMD

system might have to defend itself against defense suppression attacks during any phase.

For a description of the phases of ballistic missile flight, see chapter 3, table 3-6.

CONDUCT OF THE BATTLE

Our battle scenario assumes, for simplicity,

a system with a second-phase architecture as

described in chapter 3. We assume that it is

in an alert stage from which it could be moved

directly to fully automated battle management.2 The battle would commence with the

launch of Soviet intercontinental ballistic missiles (ICBMs). We assume a mix of ICBMs,

some of which would burnout above the atmosphere, and some of which would bum out in

the atmosphere. The ICBMs release post-boost

vehicles (PBVs) carrying both RVs and decoys.

The PBVs would maneuver to dispense their

payloads, inserting each RV and decoy into a

preplanned trajectory. RVs and decoys would

then coast until they started to reenter the

atmosphere. RVs would continue on to their

targets, accompanied partway by those decoys

designed to simulate reentry.

Besides launching ICBMs, the Soviets might

employ a variety of defense-suppression measures. For example, they might launch directascent anti-satellite weapons (ASATs) at BMD

system satellites. Such a weapon might carry

one or more warheads and decoys. The defense

suppression attack might begin before an

ICBM launch.

The following sections describe briefly the

functions that a BMD system would have to

perform during the battle. Requirements for

recovering from damage and failures occurring

‘Although the assumption that there would be sufficient prior

warning to an attack that a BMD system could be moved to

an alert stage makes the scenario easier to describe, the system’s designers could not depend on such an assumption to be

true. There would have to be some provision to go from peace

time to battle in seconds in the event that no warning is received or that such warnings are ignored.

during battle are given simplified treatment

later. Table 7-1 gives a more detailed description, with examples, of the defensive functions,

organized by function and by missile flight

phase.

Battle Management Functions

In all phases of a battle the defensive system would have to track targets, assign weapons to destroy targets, aim, fire, and control

those weapons, and assess the damage they

do. It would also continually report on system

status to human commanders, transmit information among computer battle managers

within a battle phase, and from the battle

managers in one phase (e.g., boost) to the battle managers in another phase (e.g., terminal).

Additional functions, unique to each phase, are

described in the following sections.

Each of these functions would involve making many decisions in short spaces of time

using data obtained from a variety of sources.

For example, aiming a weapon at a target

might be based on tracking data obtained from

a BSTS combined with data from a laser range

finder located on satellite battle station, and

would require the prediction of an intercept

point for the target and weapon.

Boost Phase

The task of detecting booster launches would

be unique to the boost phase as would be predicting the approximate trajectories of PBVs

from those boosters penetrating the boostphase defense. Trajectory prediction would

needed so that space-based interceptors (SBIs)
181

Table 7-1 .—Ballistic Missile Defense Battle Management Functions

The components of any ballistic missile defense (BMD) system would have to be tied together in a battle management

network. The table below lists the kinds of functions such a battle management system would have to perform, assuming

a second- phase architecture of the type shown in Table 3-5. Computers would have to perform most of the functions. The

BMD system architecture would specify locations and interrelationships among the computers. The system might be more

or less centralized, more or less hierarchical. The elements of the system need to be tied together in a communications network. Chapter 8 of this report further discusses battle management communications and computation requirements.

Because different system components often perform the same functions in different ways, the table gives hypothetical

examples of how the functions are accomplished in different battle phases. The “hypothetic/ examples’ are just that: this

table does not purport to outline a complete BMD architecture.

The table is organized into 6 sections. The first 5 sections correspond to the boost, post-boost, mid-course, terminal,

and self-defense parts of a BMD battle. The sixth outlines a battle of BMD system self-defense against anti-satellite weapons.

The functions and their descriptions are the same for each section of the table; what varies is the way the functions are accomplished. Thus, to find out how objects might be tracked as part of the acquisition and discrimination function during the

terminal phase, one reads the hypothetical example in the section of the table devoted to the terminal phase.

Boost Phase

Function Description Hypothetical Boost Phase Example

Acquire and discriminate objects Sense objects of interest

Distinguish between targets to attack and

decoys or debris

Track objects

Associate and Correlate objects sensed by

different means or from different platforms

Assess Situation

Decide Course of Action

Estimate whether enemy is attacking, and if so

with how many of what kinds of weapons with

what battle tactics

Assess which of own BMD forces are available

for battle

Authorize firing when ready, based on direction

from higher authority if available, or as preauthorized if not

Determine strategy and battle plan

Select Targets & Direct Weapons Choose which targets to strike

Employ Weapons

Assign weapons

Prepare engagement instructions

Assess kill: decide which targets have been

destroyed

Control weapon

Enable weapon

Prepare weapon

Short and mid-wavelength infrared telescopes

on BSTS detect hot exhaust plumes from

launch of boosters

BSTS starts track files to distinguish moving

ICBMs from stationary background and cloud

clutter

SSTS sensors start to observe and record paths

of identified objects

Battle management computers compare

information gathered on two separate SSTS

platforms and give same identification number

to the same observed objects

BSTS detects ICBM launches, notes numbers

and locations of launch sites, and determines

types of missiles

Battle management computers determine which

space based space-based interceptor (SBI)

carrier vehicles (CVs) are in range of launched

ICBM boosters, and will be in range of PostBoost Vehicles (PBVs) when they are released

from ICBMs.

If 3 or more ICBMs are launched within 1

minute, space battle management computers

are programmed to command launch of spacebased SBIs when the 4th ICBM is detected

Determine plan for which kinds of ICBMs from

which locations and which PBVs to attack

first based on trajectories. CV positions.

predicted RV impact points, and predicted

times of PBV separation from missile

Select the booster or PBV whose trajectory will

place it closest to the fly-out range of a

particular CV

Battle management computer decides that SBIs

no. 7888 and 7930 should attack target

booster no. 754, and commands CVs to flightcheck SBIs.

Battle management computers send flight plans

and target track information to CVs.

Remove a booster from the active target list

Feed target information to SBI guidance

package

Conduct flight check of SBI
182

Table 7-1 .—Ballistic Missile Defense Battle Management Functions—continued

Launch weapon Open launch tube, eject and orient SBI, ignite

SBI rocket motor

Fly-out and kill Battle manager transmits guidance update to

SBI based on SSTS tracking data; SBI homes

in on target booster or PBV

Post-Boost Phase

Function Description Hypothetical Post Boost Phase Example

Acquire and discriminate objects Sense objects of interest

Assess Situation

Decide Course of Action

Distinguish between targets to attack and

decoys or debris

Track objects

Associate and correlate objects sensed by

different means or from different platforms

Estimate whether enemy is attacking, and if so

with how many of what kinds of weapons with

what battle tactics

Assess which of own BMD forces are available

for battle

Authorize firing when ready

Determine Strategy and Battle Plan

Select Targets & Direct Weapons Choose which targets to strike

Assign weapons

Employ Weapons

Prepare engagement instructions

Assess kill: decide which targets have been

destroyed

Control weapon

Enable weapon

Prepare weapon

Launch weapon

Fly-out and kill

Mid-Course Phase

Infrared telescope on SSTS detects PBV after it

separates from missile, and RVs and decoys

after separation from PBV

From differences in IR signatures and other

data, such as PBV recoils, sensor systems on

SSTS distinguishes among PBVs, expended

boosters, RVs, and decoys

SSTS sensors continue to observe and record

paths of identified objects

Computers on two separate SSTS platforms

compare information gathered by each and

give same identification number to the same

observed objects

Battle management computers determine

whichs CVs are in range of targetable PBVs

and RVs

Battle management computers determine plan

for attacking targetable PBVs that have

survived earlier SBI intercepts and when to

start attacking RVs that have been deployed

from PBVs

Battle management computers target the PBVs

that will first be in range of SBIs

Battle management computer decides that

space-based SBI no. 12,543 should attack PBV

no. 328 and commands CVs to flight check

SBIs

Battle management computers send flight plans

and target track information to CVs

Remove a PBV from the active target list

Feed target information to SBI guidance

package

Conduct flight check of SBI

Open launch tube, eject and orient SBI, and fire

SBI rocket motor

Battle manager transmits guidance update to

SBI; SBI homes in on target PBV or RV

Function Description Hypothetical Mid-course Example

Acquire and discriminate objects Sense objects of interest Infrared telescope on SSTS detects RVs and

decoys

Distinguish between targets to attack and From differences in motion after passage

decoys or debris through dust cloud, laser range-finding radar

on SSTS identifies target RVs v. decoys

Track objects SSTS sensors continue to observe and record

paths of identified objects
183

Table 7-1 .—Ballistic Missile Defense Battle Management Functions—continued

Assess Situation

Associate and correlate objects sensed by

different means or from different platforms

Estimate whether enemy is attacking, and if so

with how many of what kinds of weapons with

what battle tactics

Assess which of own BMD forces are available

for battle

Decide Course of Action Authorize firing when ready

Determine Strategy and Battle Plan

Select Targets & Direct Weapons Choose which targets to strike

Assign weapons

Employ Weapons

Prepare engagement instructions

Assess kill: Decide which targets have been

destroyed

Control weapon

Enable weapon

Prepare weapon

Launch weapon

Fly-out and kill

Terminal Phase

Computers on two separate SSTS platforms

compare information gathered by each and

give same identification number to the same

observed objects

Computers determine which ERIS interceptors

are in range of RVs

Determine plan for which RVs to attack first

Select the RVs within shortest flight time of a

particular ERIS site

Battle management computer decides that ERIS

interceptor no. 3001 should attack target RV

no. 10,005 and commands fire control

computer to flight check the interceptor

Battle management computer sends flight plan

and target track information to ERIS fire

control computer

Remove an RV in mid-course from the active

target list

Feed target information to ERIS guidance

package

Turn on ERIS warhead sensor

Cool down ERIS homing sensor

Fire ERIS rocket motor

Battle manager transmits guidance updates to

ERIS; ERIS homes in on target RV

Function Description Hypothetical Terminal Phase Example

Acquire and discriminate objects Sense objects of interest Infrared telescope on AOS detects RVs and

decoys based on data received from SSTS;

Assess Situation

Decide Course of Action

Select Targets & Direct Weapons

Distinguish between targets to attack and

decoys or debris

Track objects

Associate and correlate objects sensed by

different means or from different platforms

Estimate whether enemy is attacking, and if so

with how many of what kinds of weapons with

what battle tactics

Assess which of own BMD forces are available

for battle

Authorize firing when ready

Determine Strategy and Battle Plan

Choose which targets to strike

Assign weapons

Prepare engagement instructions

Assess kill: Decide which targets have been

destroyed

AOS ‘passes data to TIR

From differences in motion after passage

through the upper atmosphere, ground-based

radar identifies target RVs v. decoys

Ground-based radars continue to observe and

record paths of identified objects

Ground-based battle management computer

compares track information handed-off by

space-based battle management computer

with ground-based radar data and gives same

identification number to the same observed

objects

Computers determine which HEDIs are in range

of incoming RVs

Choose plan for which RVs to attack first

Select the RVs nearest to a target and that can

be reached by a HEDI

Decide that HEDI no. 1897 should attack target

RV no. 257

Ready flight plan and target tracking

information for HEDI

Remove an RV in terminal from the active target

list
184

Employ Weapons Control weapon Feed target information to HEDI guidance

package

Enable weapon Turn on HEDI warhead sensor

Prepare weapon Cool down HEDI homing sensor

Launch weapon Fire HEDI rocket motor

Fly-out and kill Battle manager transmits guidance update to

HEDI; HEDI homes in on target RV

BMD System Self-Defense

Function Description Hypothetical SeIf-Defense Example

Acquire and discriminate objects

Assess Situation

Decide Course of Action

Select Targets & Direct Weapons

Employ Weapons

Sense objects of interest

Distinguish between targets to attack and

decoys or debris

Track objects

Associate and correlate objects sensed by

different means or from different platforms

Estimate whether enemy is attacking, and if so

with how many of what kinds of weapons with

what battle tactics

Assess which of own BMD forces are available

for battle

Authorize firing when ready

Determine Strategy and Battle Plan

Choose which targets to strike

Assign weapons

Prepare engagement instructions

Assess kill: decide

destroyed

Control weapon

Enable weapon

Prepare weapon

Launch weapon

Fly-out and kill

which targets have been

Infrared telescope on SSTS detects direct -

ascent ASAT

SSTS sensors continue to observe and record

paths of identified objects

Computers on two separate SSTS platforms

compare information gathered by each and

give same identification number to the same

observed objects

Computers determine target of ASAT and which

CVs may be used to defend against

approaching ASAT

Choose plan for which ASATs to attack first

Select the ASAT nearest to a particular CV

Battle management computer decides that SBI

no. 1024 should attack target ASAT no. 128,

and commands CVs to flight check SBIs

Battle management computers send flight plans

and target-track information to CVs -

Remove an ASAT from the active target list

Feed target information to SBI guidance

package

Conduct flight check of SBI

Open launch tube, eject and orient SBI, and fire

SBI rocket motor

Battle manager transmits guidance update to

SBI; SBI homes in on target ASAT

NOTE: The first two columns of this table draw heavily from work of Albert W. Small and P. Kathleen Groveston, Strategic Defense Battle Operations Framework, (Bedford,

MA: The MITRE Corp., July 1985). The hypothetical examples are supplied by OTA.

could be launched in time to intercept the

PBVs before they dispensed their payloads.

Post-Boost Phase

Tasks unique to the post-boost phase would

be noting the separation of PBV from missile

and observing the PBV as it dispensed its payload. To have a chance to destroy most PBVs,

the interceptors would have to have been

launched during boost phase, perhaps before

PBV separation. To intercept the PBVs, the

system would have to guide the SBIs launched

earlier. For PBVs that survived to dispense

their payloads, the system might start discriminating between RVs and decoys, perhaps

by trying to observe differences in PBV recoil

during the dispensing process.

Mid-Course Phase

The primary problem for the defensive system during mid-course would be to discriminate real warheads from decoys. The number
185

of decoys maybe in the hundreds of thousands,

or even greater. Decoys and warheads may appear very similar to optical, infrared, radar,

and other sensors, both passive and active.3

Terminal Phase

During terminal phase, the defensive system

would have to discriminate RVs from decoys

using data handed off from earlier phases and

using the atmosphere and radar signatures as

discriminators.

Table 7-1 shows the different functions a

BMD system would have to perform during

battle, and how different components would

participate in different phases of the battle.

The table assumes a second-phase architecture,

such as described in table 3-6. It also shows

the functions that would serve to defend the

system against defense suppression threats.

Interactions Among the Phases

A BMD system would not be a single, monolithic entity. Instead, it would comprise many

different elements, some of which would participate in only one or two phases of the battle. In most system architectures, battle management would be conducted by different

battle managers during the different phases.

Furthermore, some battle managers might be

fighting one phase of the battle while others

are fighting a different phase. Boost, postboost, and mid-course managers would be located in space, while terminal phase managers

would likely be on the ground. For the system

to function most effectively, information, such

as tracks and status of RVs and decoys, would

have to be communicated from battle managers in earlier phases to battle managers in later

phases.

Interactions for Tracking Purposes

Establishing, distributing, and correlating

track information is a good example of a problem involving interaction among different system elements and cooperation among battle

‘Chapters 4 and 10 discuss the issues of discrimination during mid-course in more detail.

managers. Detecting, identifying, and noting

the current position of a target would not be

sufficient for guiding a BMD weapon system.

The target would move between the time that

the sensor records its position and the time

the weapon is fired. An SBI traveling at, say,

8 km/see, would take 250 seconds to reach an

RV target if the SBI were fired at a range of

2,000 km from the impact point. During those

250 seconds, the RV would move 1750 km. Just

as the hunter must lead the duck in flight as

he fires his shotgun, the BMD system would

have to aim its SBI well ahead of the speeding

RV.4

The BMD system would therefore have to

keep track of each target’s motion and predict

where the target would be at later times. The

sensors would have to generate a‘ ‘track file,

i.e., a history of each target’s motion through

space. Given the target’s past history in terms

of position, velocity, and acceleration in three

dimensions, a computer could then predict its

future position. This prediction could then be

used to aim and fire the weapon system. After

the SBI was fired, the sensors would have to

continue to track the target (and possibly the

SBI), the track files would have to be updated,

and mid-course guidance corrections sent to

keep the SBI on a collision course. Mid-course

corrections would be mandatory if the target

acceleration changed after the SBI was fired,

as would occur with multi-stage missiles.

For directed-energy weapons or interactive

sensor systems, the delay from the time that

energy is emitted by the target until it reaches

the sensor, and then from the firing of the

weapon until the arrival of the kill energy traveling at the speed of light, would be very short,

but not zero. At 2,000 km range, for example,

13 milliseconds would elapse from the last sensor reading until the time a laser beam could

reach the target. The RV would move about

91 meters in this time, so some predictive ca4However, the RV would be moving on a ballistic or free-fall

flight with no external acceleration other than the force of gravity. Therefore, predicting its future path or trajectory would

be possible provided that the sensor generated two or more accurate three dimensional target positions. Predicting the future

path of an accelerating, multi-stage missile would be much more

difficult. RVs that could maneuver would worsen the difficulties.

75-922 0 - 88 - 7
186

pability would be required.’ In addition, the

observable characteristics of the target would

change drastically during the tracking operation. The sensor systems would begin tracking the hot booster plume. For boost phase

kills, the sensors would have to acquire and

track the cold booster body, or rely on calculations of the missile body position relative

to the booster plume for all booster/sensor

orientations. After the last booster stage had

burned out, the battle management computers

would have to continue the track file on the

surviving post-boost vehicles (PBV), even

though tracking data might be derived from

other types of sensors. Finally, as individual

RV’s and decoys were deployed, the track files

would have to proliferate, taking the last PBV

track projection as the recently deployed RV

track file, until it could be verified and updated

by long wavelength infrared sensors.

The data handling problem would be compounded by RV’s that survived the boost and

post-boost defensive attacks. The surviving

RV’s would usually pass out of the field of view

of the initial sensor. The track file obtained

from one sensor’s data would then have to be

correlated with the data from another sensor.6

Track data would be passed to the appropriate weapons platform at each stage of the battle. Eventually the track files of surviving targets should be passed on to the ground-based

terminal defensive systems to aid in the final

kill attempts. Information on decoys and other

rocket or killed target debris should also be

‘In general it would take additional time for the sensor signal

processor to analyze the sensor data and for the track file to

be updated after the last signal was received; the actual elapsed

time between observation and the order to fire the weapon might

be 5 to 10 seconds, so the target might move as much as 70

km even for a directed-energy weapon.

6

If battle managers, sensors, and weapons were organized into

autonomous battle groups, then each battle manager would have

to hand off and receive track data as targets passed through

the field of view of sensors in its group, or it would have to perform all of its own target acquisition and discrimination. If there

were a single battle manager to handle all tracking and correlation, it would have to maintain track files on all targets. Such

an organization would tie system survivability to survivability of the central battle manager. Finally, if battle groups were

to use fixed battle management platforms, but different sensors and weapons as the battle continued, then the battle

managers would have to correlate tracks from different sensors

as the sensors moved in and out of its group.

transmitted, to avoid attacking too many false

targets.

System Performance and Interaction

The ability to correlate data well from different sensors (required to get accurate threedimensional track histories) could have a

strong effect on system performance, as could

the ability to correlate track files exchanged

among battle managers both within and between phases. Poor performance in the early

phases would mean many RVs leaking into

later phases, with possible overload of resources assigned to mid-course and terminal

phases.

Distinctions Among the Phases

In all phases the defensive system would

have to perform many similar functions, such

as tracking; weapons assignment; aiming, firing and controlling weapons; and reporting status. Sensors and weapons would vary considerably from one phase to another, however. For

example, boost, post-boost, and mid-course

tracking would be done primarily by spacebased infrared sensors such as those incorporated in the BSTS and SSTS systems. Terminal phase tracking would be done by a combination of airborne infrared sensors and

ground-based radars. The software and hardware used to perform the sensing, discrimination, and tracking functions indifferent phases

would likely be quite different; aiming, firing,

and controlling weapons might be similar.

Some phases would require unique functions.

A good example is interactive discrimination

of RVs from decoys for mid-course defense.

Candidates for such discrimination, such as

neutral particle beam systems, would likely be

controlled by unique computer software and

hardware adapted to the task.

Reconfiguration

In addition to fighting the battle, the system would also have to be able to reconfigure

itself during and after the battle, to compensate for damage done to it, in preparation for

further or continued engagements. In the post-
battle case, this might be done with human

assistance.

Opportunities for Human Intervention

Tracking and discriminating objects, aiming and firing weapons, and managing the battle would require computers. During peacetime, humans could monitor surveillance data

after it had been processed and displayed in

a form suitable for human interpretation.

Human decisionmakers could deduce from the

events monitored, among other things, whether and when the defensive system should be

placed in alert status, ready to cope with a battle. Once the battle started, however, the response time and data processing requirements

would severely limit the opportunities for human intervention. There are four possible human intervention points under consideration

in currently suggested BMD architectures:

●

●

●

●

the decision to move the system from

peacetime status to alert status,

the decision to release weapons,

the decision to switch to a back-up for one

or more of the algorithms (see box 7-A)

used by the battle management computers, and

selection from a pre-specified set of tactics for terminal phase, made as a result

of observations of earlier phases of the

battle.

Transition to Alert Status

Humans could decide to move from one level

of alert to another in hours or minutes, as compared to fractions of seconds for computers
188

Box 7-A.—Algorithms

Methods for solving problems by the use of computers are often expressed as algorithms. As

described by John Shore,

An algorithm is a precise description of a method for solving a particular problem using operations

or actions from a well-understood repertoire.1

More technical definitions often require that the description contain a finite number of steps,

each of which can be performed in a finite amount of time, and that there be specific inputs and

outputs. As explained by computer scientist Donald Knuth,

The modern meaning of algorithm is quite similar to that of recipe, process, method, technique, procedure, routine, ...2

Carrying out the steps of an algorithm is known as “executing it.” If one thinks of a recipe

for baking a cake as an algorithm, then executing the algorithm consists of following the recipe

to produce the cake. The following is a simplified example of an algorithm that might be used in

the early stages of the design of a BMD system. The purpose of the algorithm is to detect the launch

of boosters. We assume that the system uses a sensor on a satellite that can scan the Soviet Union.

The sensor is composed of a number of different elements, each of which is sensitive to the radiation

emitted by a booster. The Soviet Union is divided into regions, and each detector element periodically scans sequentially across a number of regions.

1.

2.

3.

4.

5.

6.

For each detector element, record all detected radiation sources greater than the threshold

for a booster as the detector scans across the Soviet Union. Record the time of occurrence

of each detection as well as the intensity of the source.

For each source recorded, identify its region of origin.

Compare the occurrences of sources in the current scan with occurrences from the previous

two scans. Count all events consisting of occurrences of sources in the same region for three

consecutive scans. Flag each such event as a launch.

If data have been saved from more than 2 consecutive scans, discard the data from the oldest

scan and save the data from the current scan.

If no launches were observed, go back to step 1, otherwise continue with step 4.

If launches were observed, notify the system operator.

While this description is simplified, e.g., omitting consideration of booster movement across

regions, it is an algorithm because the operations needed to perform each step could be completely

specified; furthermore, it could be implemented as a computer program.

Although the number of steps used in describing an algorithm must be finite, the definition

does not require that the algorithm terminate when executed. Many algorithms are designed to

be non-terminating, such as the following simplified description of how a radar processing system

might operate:

1. Send out radar pulse.

2. Wait a pre-calculated interval for a return pulse.

3. If there was a return pulse calculate the distance to the object.

4. Go back to step 1.

Despite not terminating, this algorithm still produces useful results. Some algorithms terminate

under certain conditions, but do not terminate and produce no results under other conditions. Conditions under which algorithms do not produce the desired results are known as error or exception

conditions and the occurrence of such conditions as undesired events. For the following simplified

algorithm, which tracks a target based on radar returns, the failure of the radar pointing mechanism

is an undesired event that causes the algorithm to continue endlessly, producing no useful result.

1. Retrieve the last known target location, velocity, and acceleration.

2. Calculate the estimated current target location.

‘John Shore, The Sachertorte Algorithm and Other Antidotes to Computer Ati”ety (New York, NY: Viking Press, 1985), p. 131.

‘Donald E. Knuth, The Art of Computer Programrm”ng, Vol. 1: Fundamental Algorithms (Reading, MA: Addison Wesley, 1974), p. 4.
189

3.

4.

5.

A

Point the radar at the estimated current target location and attempt object detection.

If the radar locates no object at the estimated range to the target, revise the estimated target

location and return to step 3. Otherwise, continue with step 5.

If the radar locates an object at the estimated range to the target, update the target location,

velocity, and acceleration.

real version of such a tracking algorithm would have to take into account the possibility that

the radar might fail in any of several different ways, or that earlier estimates of target position

might be grossly wrong.

I

during battle. The humans’ decisions could be

based on data gathered from sources both

within and outside the BMD system. The computerized battle-time decisions would be based

on data acquired by the system’s sensors.

Weapons Release

Once a human had permitted the transition

to the highest level of alert, the system would

function automatically, responding to threats

as it perceived them.7 It would be possible to

build human intervention points for the release

of weapons into the battle management process. In the first-phase and second-phase systems described in chapter 3, the first weapons

to be released would be the SBIs. The period

from the time that a missile launch was first

perceived by BMD sensors until SBIs would

have to be launched to intercept a missile still

in its boost phase would be quite short—a few

minutes at most. Accordingly, if humans were

to control the release of weapons, they would

have to monitor the defense system’s operation continuously once it had moved to the

highest alert status.

Since it may be necessary to release hundreds or thousands of SBIs within minutes,

a human operator would not be able to authorize release of individual SBIs. Because of the

rapid reaction times needed, continual human

intervention in the weapons release process

would likely degrade system effectiveness unacceptably. It might be feasible to intervene

when previously unused weapon systems were

‘The AEGIS ship defense system, often cited as performing

many of the same functions as a BMD system, reacts completely

automatically to incoming threats when in the highest level of

alert mode. For some threats, AEGIS must react within 15 seconds from the time a threat is detected.

brought into the battle. As an example, if

neutral particle beam (NPB) weapons had not

been used before enemy missiles reached midcourse, then a human might be called onto authorize their use during the mid-course part

of the battle. Even such occasional intervention might degrade performance somewhat.

Switching to Back-ups

During the course of the battle it might be

possible for a human observer to determine

that a BMD system was malfunctioning. For

a human to notice, the malfunction would probably have to be gross, such as a failure to fire

interceptors or firing interceptors in obviously

wrong directions. If the problem lay in the algorithms used by the battle management computers, and if the system were designed in such

away that back-up algorithms were available

to the computers, then the human might command the battle management computers to

switch to a particular back-up algorithm.8

Human intervention of this type is rarely

used in existing systems because the human

cannot interpret the situation correctly in the

available time, and because it is difficult to design the software to switch algorithms successfully in mid-computation. Inmost systems, the

gain is not worth the added software complexity. The potential gain for BMD from such intervention would be that in the cases where

‘Switching to a back-up algorithm should not be confused with

situations where a computer uses built-in diagnostics to determine the occurrence of a hardware malfunction and then automatically switches operation to a redundant component. Such

diagnostics and hardware redundancy for automatic switching

are now used in some critical applications, such as airline transaction systems, telephone switching systems, and battle management systems such as AEGIS.
190

the system had been badly spoofed by the

enemy, and the human operator quickly recognized the symptoms, cause, and needed corrective action, recovery might be possible in

time to continue the battle. The risk would be

that the operator would misjudge the situation, or that the complications involved in provialing the appropriate interface to the operator, both in additional software, hardware, and

communications capability, would make the

system less reliable.

Selection of Tactics

Because the boost, post-boost, and midcourse phases of a BMD battle would last 20-

30 minutes, a human commander might be able

to evaluate the results of those phases in time

to affect the tactics used during the terminal

phase. To do so, he would have to be presented

with status reports during the battle. Based

on his analysis of the battle, and on choices

of previously-determined tactics presented to

him by an automated battle manager, he could

choose the terminal phase tactics to be used.

Again, because of the time-scales and data

volumes involved, he would probably not be

able to alter his choice once the terminal phase

began.

Increasing degrees of human intervention

would require increasing complexity in the interface between humans and the battle management system. A sophisticated interface between human and computer would be needed,

allowing the human to observe status and issue commands, and, when appropriate, receive

acknowledgements. Such an interface would

add complexity to the software. Furthermore,

SUPPORTING

Table 7-1 shows the primary battle management functions, but does not include several

supporting functions. Most important of these

are communications and recovery from damage and from failures. Both of these functions

are needed in all phases, with communications

playing its traditionally crucial role in battle

management and with recovery invoked as

the human operator(s) would probably have to

have authority to release weapons, as there

might not be time to consult with higher authorities.

Common to all BMD system designs that

require human intervention at any stage is the

need to provide secure, rapid communications

between the human and the battle management computers. If part of the system were

in space, then most likely there would be a need

for space-to-ground communications.9

For all of the preceding reasons, it seems

likely that a BMD system would operate

almost completely automatically once moved

to an alert status in preparation for battle.

The preceding analysis illustrates the difficult trade-offs involved in designing a battle

management system. In considering the interface between humans and the system, the designer must trade off communications security against the need for human intervention

against system structure against complication

of the computing tasks. He must also balance

system performance against all other considerations, deciding whether the system could perform better and more dependably with the aid

of a human than without, and whether any extra complication in the human-computer interfaces would be worth whatever capability and

trustworthiness might be added by the human.

‘Even if a human operator were space-based, he might need

to communicate with higher authority on the ground. Such communications would probably not require as rapid data communication rates as battle-management-to-operator communications.

FUNCTIONS

needed. Both communications and recovery

procedures would be completely automated

during a battle. Because of the short decision

times involved during boost, post-boost, and

terminal phases, recovery would have to be extremely rapid; delays would result in RVs moving on to the next phase or reaching their targets. Long delays in recovery could also reduce
191

the opportunities for a battle manager fighting in one phase to pass information along to

battle managers fighting in the next phase.

Communications

Automated communications links between

sensors and battle management computers, between different battle management computers,

between battle management computers and

weapons, and between battle management

computers and humans, would all be needed

for effective battle management. Data would

be continually transmitted over a battle management communications network during all

battle phases.

Recovery From Damage and Failures

Present in all phases would be the need to

recognize and recover from system failures and

from damage to system resources. Individual

system components would have to monitor and

report their own status continually. They

would have to try to recover from local failures,

whether internally generated—perhaps by a

software error-or externally generated—perhaps by a detonation of a nearby nuclear antisatellite weapon causing radiation damage in

a computer chip.

Some instances of system damage and failure, such as destruction of several adjacent battle management platforms or communications

controllers, would require recovery based on

“global information,“ i.e., information about

the status of the entire BMD system and the

entire battle. Examples are knowing how to

reroute communications around damaged

nodes in the communications network, or

knowing which battle management computers

were in position, both physically and in terms

of resources available, to take over the functions of a disabled battle manager.

COMPLEXITY OF BATTLE MANAGEMENT

Conduct of a successful BMD battle would

be similar to the conduct of a large conventional battle in that it would require the orchestration of many different kinds of components under precise timing constraints. The

problem may be ameliorated somewhat by preplanning some of the orchestration. The difference is that in a BMD battle the time constraints would be tighter, the battle space

would be larger, the fighting would largely be

automated, the components would be previously untested in battle, and there would be

little chance to employ human ingenuity to

counter unanticipated threats or strategies.

The only kind of BMD system for which the

U.S. has battle management software development experience and an understanding of

the attendant problems is a terminal defense

system, such as SAFEGUARD. Some consider even this experience as suspect, since

SAFEGUARD and other systems like it were

never used in a real battle. Adding a boostphase defense would add complexity to the system and require the inclusion of technologies

hitherto untried in battle. It would also be the

first time that software was used to control

highly automated space-based weapons.

Adding amid-course defense would probably

increase the software complexity past that of

any existing systems. The burden of effectively

integrating information from different sensors,

controlling different weapons, coordinating interactive discrimination to distinguish among

hundreds of thousands of potential targets,

and selecting effective strategy and tactics—

all while trying to defend against active countermeasures-would fall on the software. (Software issues are discussed in detail in ch. 9.)

Approaches to reducing complexity center

on “divide and conquer” strategies applied to

architecture definition, aided by simulations

of the effectiveness of different battle management architectures. Those who favor such approaches believe that the system could be

designed and built in small, relatively inde-
192

pendent pieces that could each be adequately

tested, and that could be jointly subjected to

peacetime tests. As an example, each interceptor carrier vehicle might contain a battle manager, designed to fight independently of other

battle managers. They argue that the system

could be easily expanded by adding more

pieces, e.g., more CVs each with its own battle manager. Since the pieces would tend to

be independent, the reliability of the system

would be more strongly related to the reliability of an individual piece, rather than to the

joint reliability of all pieces, i.e., knowing that

individual pieces were reliable would suggest

that the whole system was reliable. Also, the

failure of a single piece might not be as catastrophic as in an architecture where the pieces

were highly interdependent.

Those who doubt the effectiveness of such

a strategy in the face of the complexity induced

by BMD requirements argue that making the

pieces independent would require making them

very complex. They further note that historically no approach has led to the development

of a weapon system whose software worked

correctly the first time it was used in battle.

The greater complexity of BMD software over

existing weapon systems leads them to believe

that a BMD system would have little chance

of doing so.

BATTLE MANAGEMENT ARCHITECTURE

A battle management architecture is a specification of the battle management functions

to be performed by different system components and the relationships among those components. Components may be software, such

as a set of computer programs that would allocate weapons to targets, or hardware, such

as the computer(s) used to execute those programs. (See also ch. 3.)

A significant architectural tradeoff concerns

the degree of coupling among battle managers

(see box 7-B). Some proposed architectures use

a very loosely coupled system with little communication among battle managers, similar to

the “almost perfect” architecture described in

the Fletcher Report.10 Such architectures tend

to locate battle managers on board carrier vehicles. Others use a more tightly coupled system with track and other data exchanged

among battle managers for coordination purposes. They often locate battle managers on

separate platforms.

IOJ~es C. Fle~her, Study Chairman tmd B. IMcMill~,p~el

Chairman, Report of the Study on Mimi.mitingtbe Threat Posed

by Nuclear Balh”stic Missiles: Volume V, Battle Management,

Commum”cations, and Data Processing, (Washington, DC: De

partment of Defense, Defensive Technologies Study Team, Oct.

1983), p. 19.

Box 7-B.—Centralization, Distribution,

and Coupling

A centralized system concentrates computing resources in one location and may consist

of several processors that share the same

memory and are housed together physically.

Such a system is known as a multiprocessor.

The processors are able to communicate with

each other at very high data rates, and are

said to be ightly coupled. As the processors

are physically moved apart, acquire their

own, separate memories, and as the data communication rates among them decrease, they

acquire the characteristics of a distributed

system, also called decentralized, and are said

to be loosely coupled.

An important factor in understanding the

degree of coupling is the criterion used to partition the battle space into segments so that

each battle management computer has responsibility for a segment. Indeed, criteria for

segmentation are one way of distinguishing

among architectures. Segments might be geographically determined and of fixed location,

or might be determined by the clustering of

targets as they move through space, or might

be determined by the location of battle
193

managers, CVs sensors, and other system resources during a battle.

Although the Eastport Group” recommended

that BMD battle managers be hierarchically

structured, the Fletcher Report12 suggested

that a logical battle management structure

that was almost perfect would not be hierarchical, but would consist of a single battle manager replicated a number of different times,

with each copy physically located on a different platform. The Fletcher report also noted

that such an architecture might be very costly,

that there might be equally effective and

cheaper alternatives, and that it was important to look at technical issues that distinguished among those alternatives. An example given was the effectiveness of algorithms

that allocated weapons to targets based only

—...—-

1lEastport Study Group Report, “Report to the Director, Strategic Defense Initiative Organization, 1985. ”

on local data. As yet, few detailed studies of such

technical issues appear to have been made.

The Eastport Group recommended the development of a decentralized, hierarchical battle management architecture. ’3 Architectures

currently under consideration for BMD systems are consistent with that recommendation.

In a typical such architecture, each battle manager would report as necessary to battle

managers at higher levels, and would receive

commands from them. There might be 3 layers

of battle managers; the lowest layer would be

local battle managers, which perform the fighting functions. The next layer would be regional

—.—— 13The EastPort Study Group is the name used to refer to the

SDIO Panel on Computing in Support of Battle Management.

It was appointed “to devise an appropriate computational/communication response to the SDI battle management computing problem and make recommendations for a research and technology development program to implement the response. ” Its

‘

zFletcher Report, op. cit~ footnote 10, pp. 9-21. report was issued in-De;ember 1985.

Box 7-C.—Hierarchies and System Design

Designers of systems find it useful to impose a structure on the design. For complex systems,

several different structures may be used, each allowing the designer to concentrate on a different

concern. In systems where many components are involved, such as complex software systems, large

communications systems, and complex weapons systems, the structures used are often hierarchical. Each hierarchy may be specified by identifying the participating components and a relationship

among them. The military command structure is an example of a hierarchy. The components are

individuals of different rank, and the relationship is “obeys the commands of, ” e.g., a lieutenant

obeys the commands of a captain.

Many proposed SDI battle management architectures use some variation of the relationship

“resource contentions are resolved by. ” Thus, local battle managers’ resource contentions are resolved by regional battle managers. However, another important battle management hierarchy is

“communicates track data to. ” This latter hierarchy is important in determining communications

needs for the BMD system, and is sometimes confused with the former.

A tree is a special form of hierarchy in which a component at one level is only related to one

component at the next higher level. The military chain of command is an example. A lieutenant

is only commanded by one captain, although a captain may have several lieutenants under his command. The Eastport Group considered battle management architectures that were structured as

trees to be the most promising for SDI.’

Structures may describe relationships among entities in a design, independent of physical relationships among system components. Such structures are sometimes known as logical structures.2

‘The Eastport Study Group is the name used to refer to the SDIO Panel on Computing in Support of Battle Management. It was appointed “to devise an appropriate computationallcommunication response to the SDI battle management computing problem and make recommendations for a research and technology development program to implement the response. ” Its report was issued in December 1985.

‘!%, for example, Report of the Study on Eliminating the Threat Posed by Nucfear BaJlistic Missdes, James C. Fletcher, Study Chairman,

Volume V, Battle Management, Communications, and Data Processing, (Washington, DC: Department of Defense, Defensive Technologies

Study Team, October 1983), p. 18.
194

battle managers, which would resolve contentions for resources among local battle managers, as battle managers, sensors, and weapons

moved, and which would assign responsibility for targets passing between battle spaces.

At the top would be a global battle manager

that would establish strategy for the regional

and local battle managers and that which

would provide the interface between humans

and the system. The battle space would be partitioned into segments such that each battle

manager in the lowest layer of the hierarchy

had responsibility for a segment. As battle

managers and targets moved through the battle space, information concerning them, such

as type of target, location of target, and trajectory of target, would have to be moved from

one computer to another.

Some recent proposals have suggested fewer

layers in the battle management hierarchy. In

such architectures, the hierarchy of automated

battle managers is flat, i.e., there are no regional battle managers, and the top layer is

a human commander. Such organizations have

been designed so that battle managers may

act almost independently of each other.

The volume of data to be communicated

among the battle management computers

would depend on the degree of coordination

among the battle managers required by the

battle management architecture. (See chapter

8 for estimates of communication requirements.) The determining factor is the amount

of target tracking information that would have

to be exchanged. Since there might be hundreds of thousands of objects to be tracked during mid-course, architectures that required

tracks of all objects to be exchanged among

battle managers would place a heavier load on

communications than those that required no

object tracks to be exchanged. The price paid

for exchanging less information, however,

would be the traditional one: the ability to coordinate the actions of different battle

managers would be hampered and the overall

efficiency of battle management might be decreased.

The efficiency-volume trade-off may be seen

by considering the transition from one phase

of the battle to the next. As an example, the

terminal-phase battle managers would have

the best chance to destroy targets if they received target-tracking information from the

mid-course battle managers. Without such information they would have to acquire, track,

and discriminate among targets before pointing and firing weapons. With such information

they would only have to continue tracking targets and point and fire weapons. In such a situation, one might suggest combining the midcourse and terminal-phase battle managers

into one set of programs on one computer as

opposed to a more distributed system withinformation transmitted among battle managers. Unfortunately, this organization would

probably complicate the battle management

task, since there would be somewhat different

functions to be performed in the different

phases, and since the way functions would be

implemented in different phases would be

different. 14

The Eastport Group believed that a hierarchical battle management organization would

simplify the computing job of each battle manager and would allow the battle managers to

act without frequent interchange of information.15 The concerns of each battle manager

could then be simplified more than in a nonhierarchical organization, battle managers

could be added to the system as needed, and

the system would still survive if a few lower

level battle managers were lost.

14Since different weapons would be used in the terminal phase

as compared to the mid-course, pointing and controlling the

weapons would be done differently. Similarly, different sensors

may be used to discriminate between targets and decoys, requiring the allocation of resources with different characteristics and therefore a different resource allocation algorithm.

“Some earlier proposed architectures required the battle

managers to be tightly coupled, exchanging considerable information with each other frequently. The Eastport Study Group

rejected such an architecture because of the computing and communications burden it would place on the battle managers, and

because of the complexity it would induce in the battle management software.
195

Decentralizing battle management means

that the battle management task would be

physically distributed among different computers. Decentralization would permit other

battle managers to continue fighting even if

a local battle manager were disabled. However,

if the degree of coupling were high, the loss

of data from the disabled battle manager might

result in reduced effectiveness of the others.

Without a specific design and a way of effectively testing architectures, it is difficult to

verify claims about their merits and deficiencies. Such tests would have to be based on

simulations and on whatever peacetime tests

could be conducted.16 However, the apparent

disadvantages of a decentralized, hierarchical

system would be:

● contentions for resources would have to

be resolved at upper levels of the hierarchy, possibly adding complexity to the

computational problem as a whole,

the actions of battle managers would be

based mainly on local data, perhaps resulting in inefficiencies, e.g., adjacent battle

managers might both shoot at some of the

same targets, thereby wasting shots, un-

—.—‘The— proposed National Test Bed, might provide some of the

simulation capabilities needed for architecture evaluation.

less sophisticated battle management algorithms to compensate for the information loss could be developed,

. if strategic and tactical decisionmaking

were concentrated at one level in the hierarchy, disabling some or all of that level

could greatly reduce system performance.

Such damage would be easier to accomplish if there were relatively few battle

managers at that level, as might be true

at the higher levels of the hierarchy.

The Eastport group believed that the advantages of a hierarchical, decentralized system

far outweighed the disadvantages. Evaluation

of advantages and disadvantages must await

a design specific enough to be tested, and an

effective test method.

No matter the choice of structure for battle

management, some technology would be strained

and software dependability would be a key issue. Centralization would appear particularly

to stress computational performance and survivability. Decentralization would appear to

require more sophisticated software at the local battle manager level and would increase

the weapons supply needed. All architectures

would require secure communication, whether

to exchange track data, or to receive sensor

data, or to communicate with the ground.

CONCLUSIONS

Ballistic missile defense battle management

would be an extremely complex process. The

number of objects, volume of space, and speed

at which decisions would have to be made during a battle preclude most human participation.

Aside from authorizing the system to move

to alert status, prepared to fight automatically,

at best the human’s role would be to authorize the initial release of weapons and to change

to back-up, previously-prepared, strategy or

tactics. Decisions about which weapons to use,

when to use them, and against which targets

to use them would all be automated. Inclusion

of human intervention points would likely add

complexity to an already complex system and

to compromise system performance in some

situations. On the other hand, if an attacker

had successfully foiled the primary defensive

strategy, human intervention might allow recovery from defeat.

Battle management architectures as yet proposed are not specific enough for their claimed

advantages and disadvantages to be effectively evaluated. Such evaluation must await

both better architecture specifications and the

development of an effective evaluation technique, perhaps based on simulation.
196

w
Chapter 8

Computing Technology
CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ....199

The Need for Automation . . . . . . . . . . . . . . . . . . . . . . . . . . .................199

Integrating Sensors, Weapons, and Computers. . . . . . . . . . . . . . . . . . . . . . ....199

Communications and Computer Networks. . ............................201

Communications Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... ... ... ..202

Simulations and the National Test Bed. . ........................,.......205

Simulations and Systems Development . ...............................205

Current Battle Simulation Technology . . . . . . . ......................,...206

The National Test Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............207

Computers in Support of BMD System Development . ...................208

Computing Technology Trade-offs. . . . . . . . .............................208

Processor Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................210

Capabilities of Existing Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...210

New Computer Architectures. . . . . . . . . . . . . . . . . . . . . . . . . ................213

Space Qualification and Radiation Hardening . ..........................214

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......................,,.215

Reliable, Secure Communications . . . . . . . . . . . . . . . . . . . . . ................215

Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......216

Technology and Architectural Trade-offs . ..............................216

Computational Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........216

Boxes

Box Page

8-A. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .....................,206

8-B. MIPS, MOPS, and MEGAFLOPS . . . . . . . . . . . . . . . . . . . . . . . . . . .......,213

Tables

Table No. Page

8-l. Computers in a Ballistic Missile Defense System . . . . . . . . . . . . . . . . . . . . . .200

8-2. Computing Performance Requirements.. . ............................211
Chapter 8

Computing Technology

INTRODUCTION

This chapter discusses the demands that ballistic missile defense (BMD) systems would

place on computing technology, and the tradeoffs that would have to be considered in satisfying those demands. Initial sections discuss

why BMD would need computers and how it

would use them for battle management, weapons control, sensor data processing, communications, and simulation. Later sections describe the technology used to build computers

and the requirements that the Strategic Defense Initiative (SDI) imposes on that technology. The chapter concludes with key issues

posed by SDI computing needs. Any description of computing technology must be accompanied by a discussion of software and software technology issues; these can be found in

chapter 9 and appendix A.

The Need for Automation

The rapid response times and volume of data

to be processed would require the use of computers in every major BMD component and

during every phase of battle. Humans could

not make decisions fast enough to direct the

battle. The launch of thousands of intercontinental ballistic missiles (ICBMs), some employing fast-burn rocket boosters, might permit less than 60 seconds to detect, track, aim,

and fire weapons at the first boosters to clear

the atmosphere. During mid-course it might

be necessary to account for a million objects

and to discriminate among hundreds of thousands of decoys and thousands of reentry vehicles (RVs). In the terminal stage, RVs in the

atmosphere would have to be quickly located,

tracked, and destroyed by interceptor missiles.

Mutual occupation of space by two defensive systems of comparable capabilities might

require considerably faster reaction times than

Note: Complete definitions of acronyms and intialisms

are listed in Appendix B of this report.

those needed to meet a ballistic missile attack

alone. Countering an attack by space-borne

directed-energy weapons would require response times of seconds or less to avoid the

loss of critical defensive capabilities. The critical part of such a battle could well be over before humans realized that it was taking place.

Although automated decision-making is a focus of concern for the use of computers in

BMD, computers would also serve many other

purposes. Table 8-1 shows many of the places

where computers would be used.

Integrating Sensors, Weapons,

and Computers

An automated BMD system would require

some degree of coordination among different

computers, but there would be many places

where computers would act independently of

each other. Table 8-1 shows many such cases,

e.g., computers incorporated into sensor systems, such as radars, to perform signal processing on data perceived by the sensor. In each

case the computer may be specially designed

for its job and is physically a part of the system of which it is a component.

As an example, an imaging radar would build

up an image of an object such as an RV by

analyzing the returns from the object over a

period of seconds. The radar would process

each return individually and store the results.

With sufficient individual returns, the radar

could analyze them to form an image of the

object. A single computer incorporated into

the radar would perform the processing, storage, and analysis. From the viewpoint of an

external observer, such as a battle management computer residing on a different platform, the radar is a black box that produces

an image of an RV. The external observer need

know nothing about the computer inside the

199
200

Table 8-1 .—Computers in a Ballistic Missile Defense System

Component Purpose of Computers

First. and Second. Phase Systems:

Battle Management Computersa

Boost Phase Surveillance and

Tracking Satellite (BSTS)

Space Surveillance and Tracking

Satellite (SSTS)

Laser Thermal Tagger

Carrier Vehicle (CV) for Spacebased Interceptors (SBI)

Space-based Interceptor (SBI)

Airborne Optical System (AOS)

Exe-atmospheric Interceptor

System (ERIS)

Ground-based Terminal Imaging

Radar

High Endo-atmospheric

Interceptors (HEDI)

Third Phase, Add:

Ground-based Laser, Space-based

Mirrors

Space-based Neutral Particle

Beam (NPB)

Radiation Detector Satellites

Coordinate track data (e.g, maintain a track data base and correlate data from multiple sensors);

maintain status of and control defense assets;

select strategy;

select targets;

command firing of weapons; assess situation.

Process signals to transform IR sensor data into digital data representing potential booster tracks;

process images to recognize missile launches and to produce crudely-resolved booster tracks;

communicate with battle management computers;

maintain satellite platform: guidance, station keeping, defensive maneuvering;

housekeeping.

Process signals to transform IR, laser range-finder, and radar sensor data into digital data

representing potential tracks;

process images and data for fine-tracking of launched boosters, post-boost vehicles, RVs and

decoys and to discriminate RVs from decoys;

point sensors;

communicate with other elements of the BMD system;

guidance, station keeping, defensive maneuvering,

housekeeping (maintain mechanical and electronic systems).

Point the laser beam;

communicate with other elements of the BMD system;

maintain satellite platform: guidance, station keeping, defensive maneuvering;

housekeeping.

Monitor status of SBIs;

control launching of SBIs;

communicate with battle manager;

maintain satellite platform: guidance, station keeping, defensive maneuvering;

housekeeping.

Guide flight based on commands received from battle manager;

Track target and guide missile home to target;

communicate with battle manager;

housekeeping.

Process signals to transform IR data into digital data representing potential tracks;

process images and data for fine-tracking of post-boost vehicles, RVs and decoys and, if possible,

discriminating RVs from decoys;

point sensors;

communicate with other elements of the BMD system;

control of airborne platform;

housekeeping.

Guide flight;

process signals and images from on-board sensor for terminal guidance and target tracking;

communicate with battle manager and SSTS, AOS, and probe sensors;

housekeeping.

Process signals and images to convert radar returns to target tracks;

process images and data to discriminate between decoys and RVs;

control radar beam;

communicate with battle managers and other elements of BMD system;

housekeeping.

Guide missile flight based on commands received from battle manager;

process signals and images from on-board sensor for terminal guidance and target tracking;

communicating with battle manager;

housekeeping.

Manage laser beam generation;

Control corrections to beam and mirrors for atmospheric turbulence;

steer mirrors;

communicate with battle manager;

housekeeping.

Manage particle beam generation;

steer the accelerator;

track potential targets;

communicate with battle manager and neutron detector;

maintain satellite platform: guidance, station keeping, defensive maneuvering,

housekeeping.

Discriminate between targets and decoys based on sensor inputs;

communicate with battle manager and/or SSTS;

maintain satellite platform: guidance, station keeping, defensive maneuvering;

housekeeping. 

aMaY be carried on sensor platforms, weapon platforms, or separate platforms; ground-based units maY be mobile.

SOURCE: Office of Technology Assessment, 1988,
201

radar, or how it operates, but only the form

and content of its output.

Customizing the Computer for the Application

The above “black box” design strategy is

based on sound engineering principles and

tends to simplify the battle management architecture, but it still involves some difficult

trade-offs. One such trade-off is that between

developing special-purpose computers for different sensors and weapons versus utilizing

commercially available hardware. Utilizing

commercially available computers may simplify the job of software development. There

would be people available who have experience

with existing hardware. In addition, support

tools for software development on available

computers already exist. As a result, software

developed for commercially available computers would probably be more reliable, more

efficient, and less expensive than software developed for new computers built specifically

for BMD. Furthermore, software development

would not have to wait for development of the

hardware, reducing the risk of not meeting

schedules.

On the other hand, hardware specially built

for BMD is likely to be more efficient and better suited to the job, possibly offsetting efficiency losses in software. Moreover, maintainability, reliability of the hardware, and life-cycle

cost would have to be taken into account. Software experts at OTA’s SDI Software Workshop suggested that hardware customization

v. software reliability and cost was an important trade-off that should be resolved in favor

of simplifying software development. However, some SDI computing might require the

use of novel hardware designs, even though

this might require designing new and complex

software from scratch.

Communications and Computer

Networks

Battle management requires communications among the battle managers, sensors, and

weapons forming a BMD system and between

the battle managers and the human operators

of the system. Space-to-space, space-to-ground,

and ground-to-space communications would be

required. As in traditional battle management,

information must be sent in useful form, on

time, and securely to the place where it is

needed. Also as in traditional battle management, information transmitted among battle

managers concerns the location of targets and

weapons, the status of resources, and decisions

that have been made. Distinct from traditional

battle management, information transmitted

in a BMD system would all be digitally encoded and the transmissions controlled by computers. As noted in chapter 7, the rate and volume of data to be transmitted depend on the

battle management architecture.

Estimates of Communications Requirements

The Fletcher Report estimated that the peak

data rate needed by any communications channel in a BMD system would be about 107 bits

per second (bps).’ This estimate assumed that

an entire track file would have to be transmitted, that the file would contain 30,000

tracks, and that each track could be represented in 200 bits. Except for the number of

tracks in a track file, the estimate is based on

conservative assumptions. Furthermore, it

scales linearly with the number of tracks, i.e.,

a track file containing 300,000 tracks would

require a peak rate of about 108 bps.

In more recent work, analysts have made

more specific assumptions about architectures

and have been able to produce more refined,

but still rough, estimates. For example, one

study of boost-phase communications uses a

highly distributed architecture consisting of

sensor satellites and satellite battle groups

composed of battle management computers,

sensors for booster tracking, and space-based

interceptor (SBI) carrier satellites. Additional

assumptions were made about numbers of targets tracked per sensor in the battle, number

‘James C. Fletcher, Study Chairman and B. McMi&n, Panel

Chairman, Report of the Study on E!inn”nating the Threat Posed

by Nuclear Ballistic Missiles: Volume V, BattJe Management,

Communications, and Data Processing, (Washington, DC: Department of Defense, Defensive Technologies Study Team, Oct.

1983), p. 19.
202

of bits per target track, non-uniform message

traffic density, number of relays per message,

varying message types (examples are track

data, status information, and engagement

data), and number of seconds per frame. The

result was a peak link data rate for boost phase

within the transmission rates of current technology.2

The Fletcher Report noted, and OTA concurs, that:

The technology exists today to transmit 107

to 108 bits/sec over data links of the length and

kind needed for a BMD system. Therefore,

even with 300,000 objects in the track file, existing communication technology could handle the expected data rates. Cost and complexity will vary with the rate designed for, but

the Panel concludes that communication rates,

per se, will not be a limiting factor in the design of a BMD system.3

Communications Networks

Regardless of volume, communications

would have to be secure and reliable. It would

have to survive attempts by an enemy to intercept, jam, or spoof communications, at best

rendering the system ineffective, at worst taking control of it for his own purposes. It would

also have to survive physical damage incurred

in a battle or defense suppression attack. Understanding the threats requires understanding how communications would function in a

BMD system.

Current communications technology, including that proposed for BMD systems, involves

establishing a network of computers, each acting as a communications node, that transmit

data to each other. One example of an existing network that is widely distributed geographically is the ARPA network, initially developed by the Defense Advanced Research

Projects Agency (DARPA) as an experimental

network. Another example is the AT&T long

distance telephone network. Both differ considerably from a space-based battle communications network, which would have:

‘Personal communication, Ira Richer, The Mitre Corp.

The Fletcher Report, op. a-t., footnote 1, p. 40.

1.

2.

3.

4.

5.

more nodes and more available direct connections between nodes;

different delays between nodes (perhaps

5 milliseconds for the example distributed

space-based network described earlier as

compared to more than 25 milliseconds for

the ARPA network);

more stringent security requirements;

a need to re-establish links every few minutes; and, probably

long repair times for individual nodes.

Nonetheless, the problems are sufficiently

similar that the terrestrial networks are useful examples. Each node in the network communicates with several other nodes. Users of

the network communicate by submitting messages to the network.4 The computers controlling and comprising the network route the message from one node to another until it reaches

its destination.

Some of the major issues that must be resolved in designing a communications network

are:

●

●

●

●

●

●

the physical arrangement of the nodes and

the interconnections among the nodes;

the unit of data transmission, which may

be a complete message or part of a

message;

the algorithm used to decide what route

through the network each unit of data

transmission will take;

the algorithm used to encode units of data

transmission so that they may be reliably transmitted;

the algorithm used by nodes for interchanging data so that the start and end

of each data transmission may be determined; and

the methods used to ensure that data communications are secure and cannot be

jammed, spoofed, or otherwise rendered

unreliable.

4

In the AT&T network, messages are sent across the network

to establish a circuit to be used for a long distance call when

a subscriber dials a long distance number. Generally, once a

circuit is established, it is dedicated to a call, and communications on it may be sent in non-message form as analogue signals or may be encoded digitally into messages.
204

Message Transmission

Information to be sent over a digital communications network, such as used in BMD

systems, is organized into messages. In some

networks, known as “packet-switched” networks, for transmission purposes the messages

are organized into blocks of data “packets.”7

In a packet-switched network the user submits

his message to the network unaware of how

the message will be organized for transmission.

The software that controls the network must

incorporate a method for extracting messages

from packets when the packets reach their destinations.

Security of Communications

Secure network communications require that

the routing algorithm be correct, that nodes

cannot be fooled into sending messages to the

wrong recipient, and that the physical communications links are secure from unauthorized

interceptions. Since a network by its nature

involves access to many computer systems, it

affords potential saboteurs a chance to access

many different computers. Both the ARPA

network and the AT&T telephone network

have been fooled on many occasions into permitting unauthorized access to the network

and, in the case of the ARPA network, to computers on the network. The managers of both

networks continually try to improve their protection against such access, but no workable

foolproof protection techniques have been

found. 8 As noted by Lawrence Castro, Chief

of the Office of Research and Development at

the National Computer Security Center,

(continued from previous page)

L?7EE Trans. on Corrmunications, vol. COM-26, No 12, December 1978.

The reader should keep in mind that the ARPANET was designed as an experimental network, and not as a high reliability network intended for commercial use.

‘Depending on the situation, several messages maybe combined into one packet, or one message maybe split across several packets. In either case, the benefit of packet switching is

that network resources may be shared, leading to more efficient

routing of messages and more efficient use of the network, The

disadvantage is that the job of the routing algorithm may be

complicated, and routing may become more difficult to debug.

“Access to a network is frequently separated from access to

the computers using the network. Entrance to the ARPA network is through computers dedicated to that job, known as terCurrent computer networking technology

has concentrated on providing services in a benign environment, and the security threats to

these networks have been largely ignored.

While literature abounds with examples of

hackers wreaking havoc through access to

public networks and the computers connected

to them, hackers have exploited only a fraction of the vulnerabilities that exist. Techniques need to be developed that will prevent

both passive exploitation (eavesdropping) and

active exploitation (alteration of messages or

message routing.)9

Gaining unauthorized access to a BMD communications network would at least require

communications technology as sophisticated

as that used in the design and implementation

of the network. Furthermore, an enemy would

have to penetrate the security of the data links,

which would likely be encrypted. Since network

communications would be used for coordination among battle managers, and would probably involve transmission of target and health

data,"10 the worst result of compromise of the

network would be that the enemy could control the system for his own uses. Disruption

of communications could result in disuse or

misdirection of weapons and sensors, causing

the BMD system to fail completely in its mission. To achieve such disruption, it would not

be necessary for a saboteur to gain control of

a battle management computer, but only to

feed it false data. Less subtle ways to achieve

the same means might be to destroy sufficient

rninal access computers (TACS). Until recently, such access was

available to anyone who had the telephone number of a TAC.

Several s~called hackers have made use of TAC facilities to

gain entrance to Department of Defense computer systems connected to the ARPA network, and they have been successfully

prosecuted for doing so, Partly as a result of such unauthorized

use of TACS, password protection has been added to TAC access procedures. The telephone companies wage constant war

against people who attempt to use their long distance networks

without paying.

‘Lawrence Castro, “The National Computer Security Center’s

R&D Program,” Journal of Electronic Defense, vol. 10, No. 1,

January 1987.

‘The health of a resource, such as a sensor satellite or weapon

satellite, is how well the resource is able to perform its mission

and what reserves are available to it. Example measures of satellite health are battery power and efficiency of solar cells. For

a BMD satellite, such as a carrier vehicle for SBIS, additional

data specific to the function of the satellite, such as number

of SBIS remaining, would be included.
203

Since most of these issues are resolved in

software, the solutions chosen have a strong

effect on the complexity of the software and

the reliability of the battle management system as a whole. The more critical of these issues are discussed in the following sections.

In almost every case, the trade-off is that adding sophistication to the algorithm(s) used to

solve the problem results in software that is more

complicated and more difficult to debug.

Network Topology

The arrangement of interconnections among

nodes is known as the “network topology.” In

attempts to improve the efficiency and reliability of networks, numerous topologies have

been tried. As an example, until recently the

AT&T long distance telephone switching system used a hierarchical topology to establish

a circuit to be used for a long distance call.5

Nodes were organized into levels. Messages

requesting the circuit were sent from a lower

level to a higher level, then across the higher

level and back down to a lower level. If all messages must pass through one or two nodes,

then under heavy loads those nodes may form

bottlenecks that decrease network performance. If the nodes break down under the traffic load, the network cannot not function at

all. As a result, most networks employ algorithms that decide what route each message

will take through the network. The route may

vary according to the prevailing load conditions and the health of the nodes in the

network.

Routing in Networks

In geographically distributed networks with

many nodes, the routing algorithm is a sophisticated computer program. Frequently, network performance degrades as a result of incorrect assumptions or errors in the design and

implementation of the routing algorithm. Find5T0 help alleviate bottlenecks in the system, AT&T is now

moving toward anon-hierarchical system where nodes can communicate directly with each other rather than going through

a hierarchy. Note that decisions to change the structure of the

long distance system are made as the result of observing its

behavior over extensive periods of use by millions of subscribers.

ing and correcting the reason for degraded performance requires knowledge of the network

status, including traffic loads at nodes and

health of nodes. Since traffic load in particular varies second-by-second, debugging network routing software is a difficult and timeconsuming job.

One can only have confidence in relatively bugfree operation by permitting the network to function under operational conditions long enough

to observe its performance under varying loads.

Stress situations, e.g. especially heavy traffic

conditions, tend to cause problems. In operation

such conditions are relatively infrequent; they

are also hard to reproduce for debugging purposes. Nevertheless, for a dedicated network

such as a BMD communications system, it

may be easier to simulate heavily loaded conditions than for a commercial network.

Either software failures, such as an error in

a routing algorithm, or hardware failures may

cause catastrophic network failure. In December, 1986, the east coast portion of the ARPA

network was disconnected from the rest of the

network because a transmission cable was accidentally cut. Although the ARPA network

had evolved over more than 15 years, an opportunity for a single-point catastrophic failure remained in the design.

Sometimes the interaction of a hardware failure and the characteristics of a particular routing algorithm CM cause failure. In 1971 normal operations of the ARPA network came to

a halt because a single node in the network

transmitted faulty routing information to

other nodes. Transmission of the faulty data

was the result of a computer memory failure

in the bad node. Based on the erroneous data,

the routing algorithm used by all nodes caused

all messages to be routed through the faulty

node. The routing algorithm was later revised

to prevent the situation from recurring, i.e.,

the software was rewritten to compensate for

certain kinds of hardware failures.6

‘For a more complete description of this problem, see J.

McQuillan, G. Falk, and I. Richer, “A Review of the Development and Performance of the ARPANET Routing Algorithm, ”

(continued on next page)
205

communications nodes that routing algorithms

become overstressed and fail, or to destroy

sufficient nodes that battle managers can no

longer communicate with each other. The

former attack requires that the enemy have

some knowledge of the routing algorithms

used; the latter may require considerable expenditure of physical resources such as antisatellite missiles.

Even passive observation of a BMD communications network could reveal enough

about the battle management and communications algorithms used by the network to permit an enemy to devise means of circumventing those algorithms and thereby rendering the

defense partially or totally ineffective. To prevent an enemy gaining such knowledge by observation of communications, encryption of

communication links and techniques for disguising potentially revealing changes in message traffic would have to be incorporated a

network design.

Although encryption and other technology

could make passive exploitation quite difficult,

a saboteur could perhaps gain access to the

communications software and hardware. Analysis of the sabotage questions, however, beyond the scope of this study.

Achieving secure, reliable, adequate communications requires the conjunction of at least

two technologies. The technology for physical

communications, such as laser communications, needs to provide a medium that is difficult to interceptor j am and that can meet the

required transmission bandwidth. The network

technology must provide adequate, secure

service for routing messages to their destinations.

SIMULATIONS AND THE NATIONAL TEST BED

Preceding sections have discussed the role

of computers during battle. Computing technology would also play a key role in prepara- ●

tion for battle and in maintaining battlereadiness. Computer simulations (box 8-A)

would be needed:

●

●

●

●

to anticipate threats against the system,

to model different ways in which the system might work,

to provide a realistic environment in which

system components may be tested during

their development, and ●

to test the functioning of the system as

a whole, both before and after deployment.

Simulations and Systems Development

Simulators are useful during all stages of the

development of complicated systems.

● During the early stages of the development of a system, simulators may predict ●

the behavior of different system designs.

An example is simulators that predict

stresses on parts of a bridge for different

bridge designs.

During the middle stages of development,

simulators may test individual components of a system by simulating those

parts not yet built or not yet connected

together. An example is a simulator that

reproduces the behavior of the different

parts of an aircraft before the aircraft’s

systems are integrated. A radar simulator

can feed data to the radar data processor

before the radar itself has been finished.

During testing, a simulator can be used

to reproduce the environment in which the

system will operate. Avionics computers

and their software are tested before installation by connecting them to an environmental simulator that reproduces the

flight behavior of the aircraft’s systems

to which the computers will be connected

when installed in the airplane.

After deployment, simulators test the

readiness of systems by mimicking the

environment-including stress conditions
206

Box 8-A.—Simulations

A simulation is a system that mimics the behavior of another system. The difference between

the simulation and the system being mimicked (called the target), is that the simulation does not

accurately reproduce all of the behavior of the target. Behavior not accurately reproduced is either

unimportant to the users of the simulation, unknown to the builders of the simulation, or too expensive to reproduce. Many simulators operate by solving a set of mathematical equations that predict

the behavior of the target system under the desired conditions. This process is known as modelling

the behavior of the target, and such a simulator is often called a model. Others may do no more

than supply a previously determined sequence of values on demand or at fixed time intervals.

Airplane flight simulators are good examples of simulators. Flight simulators used for pilot

training reproduce flight conditions well enough to help train pilots how to fly, but not to grant

them licenses. No one would trust a pilot all of whose flight time was logged on a simulator. Flight

simulators are just not sufficiently accurate reproductions of flight conditions to ensure that the

pilot knows what it feels like to fly a real plane. However, a pilot who already has a license may

use a simulator to qualify for another aircraft in the same class as his license, e.g., a pilot qualified

for a DC-10 could qualify to fly a Boeing 747 based only on simulated flights.

Constructing an accurate simulation requires that the target behavior be well understood and

that there be some method for comparing the behavior of the simulator with the behavior of the

target. In cases where the physical target behavior is unavailable for comparison, simulator behavior may be compared to other simulators modeled on the same target, or to predictions made

by mathematical models of the target. (In cases where the simulator itself is a model, a different

model may be used for comparison. If a different model is unknown another simulator already known

to be reliable, or hand calculations, maybe used.) A simulator that models the trajectory of a missile

in flight can be checked against actual missiles and the equations of motion that are known to govern such trajectories. A simulator that models the behavior of the Sun can only be compared to

observed solar behavior, and may be quite inaccurate when used to predict behavior under previously unknown conditions.

–for which it is critical that the system attacking a single ship. Such simulations can

operate correctly. Such simulators are

often build into the system and contain

means of monitoring its behavior during

the simulation. The design of the SAFEGUARD anti-ballistic missile system of

the early 1970s incorporated a simulator

called the system exerciser” to permit

simulated operation of SAFEGUARD

during development and after deployment.

now be run 30 times slower than real time, i.e.,

30 seconds of processor time devoted to running the simulation corresponds to 1 second

in an actual engagement. However, the demand is now to develop simulators that can

model many missiles against many ships.

Work at the U.S. Army’s Strategic Defense

Command (USASDC) Advanced Research

Center (ARC) is representative of current BMD

simulation technology. In late 1986, ARC reCurrent Battle Simulation Technology searchers completed a set of mid-course BMD

battle simulations. The simulations empoyed

As faster, deadlier, and more expensive 6 Digital Equipment Corporation VAX 11/780

weapons, such as guided missiles, have been computers coupled by means of shared memadded to arsenals, the demand on simulation ories. Four of the computers could simulate

technology to analyze their effects has in- battle managers, one simulated surveillance

creased. For example, in the early 1970s, sin- sensors (all of the same type) and weapons

gle engagement simulations modeled such (ground-launched homing interceptors of the

events as defending against a single missile Exoatmospheric Reentry vehicle Interceptor
207

System), and one simulated 32 other engaged

platforms.”

The ARC researchers ran three battle management design cases:

1.36 battle managers that communicated

among each other, known as the distributed case;

2. a single centralized battle manager; and

3.36 autonomous battle managers that did

not communicate with each other, known

as the autonomous case.

The centralized and distributed cases assumed 236 and 237 interceptors respectively,

and the autonomous case assumed 660. The

maximum threat simulated was 1,000 objects,

which required 7 hours to run. The centralized

and distributed cases took 3 and 4 hours respectively to run against a threat of 216 RVs.

The simulation took 15 months to develop, and

included about 150,000 lines of code, much of

it in the Pascal programming language. (Code

for the battle managers was replicated for some

simulations; the replication is not included in

the 150,000 lines.)

Perhaps the largest stumbling block in running larger scale and more realistic simulations

for the ARC is the lack of computing power.

SDIO expects the EV88 experiment sequence,

running through fiscal year 1990, to conduct

larger scale simulations involving the ARC,

the Airborne Optical Adjunct, prototype

space-based BMD components, and the National Test Bed. This series of experiments will

require considerably more computing power

than is now in place at the ARC.

Simulation experts agree that computing

power is currently the major limitation in performing large scale simulations. However,

other factors complicate the situation. Where

equipment or environments are not well-understood or include many random variables, the

accuracy of simulations is difficult to verify.

This is the case, for example, in simulations

of sea conditions surrounding missile v. ship

engagements.

11Depending on the architecture being simulated, the other

platforms were either battle managers or sensors.

Some military simulation experts noted to

OTA staff that every time they performed

simulated threat assessments without prior access to the real equipment being modeled, the

behavior of the real equipment surprised them.

They strongly emphasized that it was only

when a simulation could be compared to an actual experiment that the verisimilitude of the

simulation could be checked.12 The implication

for BMD is that actual Soviet decoys and missiles would have to be examined and observed

in operation to simulate their workings accurately. Similarly, the battle environment, including nuclear effects—where appropriate—

and enemy tactics, would have to be well understood to conduct a battle simulation properly.

The National Test Bed

The SDIO is sponsoring the development of

a National Test Bed (NTB)—a network of computers and a set of simulations to execute on

those computers. A threat model is to simulate the launch of Soviet missiles and display

their trajectories after launch. Another model

would simulate a complete BMD battle to exercise a deployed BMD system.

The NTB would be utilized in all phases of

the development and deployment of a BMD

system. It should permit experimentation with

various system and battle management architectures, battle management strategies, and

implementations of architectures. It would be

the principal means of testing BMD system

components and subsystems as well as the entire BMD system, thereby providing the basis for their reliability.

Preliminary design work studies for the estimated $1 billion NTB were completed in December 1986.13 Initially, the NTB is to be a network of computers, each simulating a different

aspect of a BMD engagement. The number of

computers linked for any particular engagelzExwrienCe cit~ here is drawn from discussions with Scientists from the Naval Research Laboratory’s Tactical Electronic

Warfare Division about simulations of Naval warfare.

‘3Major James Price, SDIO’S assistant NTB director, described the NTB as a $1 billion program through 1992 in an

interview reported in Defense Electronics in February, 1987.
208

ment would vary depending on the completeness and depth of detail required. Initial capabilities would not permit simulation of a full

battle involving hundreds of thousands of

objects.

A major use of the NTB would be to conduct experiments with different BMD technologies and strategies. The currently visualized

NTB would link sensors, weapons, or battle

managers to simulations that reproduce the

data they would handle during a battle. The

object could then be tested under varying conditions. The results of such experiments would

be quite sensitive to the verisimilitude of the

simulations. Accordingly, it is important that

there be a way to verify the accuracy of the

simulations used in NTB tests and experiments.

Computers in Support of

BMD System Development

A BMD system to counter the Soviet ballistic missile threat might be the most complicated system ever built. It would involve the

use of many different technologies, the automated interplay of thousands of different computers, sensors, and weapons, and the development of more software than has been used

in any single previous project. Accordingly,

managing the development of such a system

would require considerable computer support

to track progress, to identify problems, and

to maintain the status of components under

development, in test, and deployed.

Computers would also be used to design, generate, and test system hardware and software.

Engineers and managers are likely to be geographically dispersed and would need to transfer information from one computer to another.

The interaction among people would only be

effective if there were a means for effective interaction among the computer systems that

they use. Previous sections of this chapter have

concentrated on the role of computers in the

operation and testing of a BMD system. But

it is clear that effective computing technology

would be needed not just in a strategic battle,

but long before system deployment and

throughout the lifetime of the system.

Computing Technology Trade-offs

Chapter 7 and the preceding sections have

portrayed some of the trade-offs involved in

using computers for ballistic missile defense.

The following list summarizes those trade-offs.

●

●

●

●

●

●

Processing power required v. volume of

data communications among battle managers. Sharing information among battle

managers relieves them of some of the

tasks that they might otherwise have to

perform, and decreases the processing

load on each of the battle managers, but

increases the data communications rate

requirements and also requires that communications be secure and reliable.

Performance v. volume of data communications. Sharing data among battle managers allows the system to operate more

efficiently, but, as in the previous tradeoff, greater dependence on communications requires greater communications capacity, reliability, and security.

Performance v. degree of automation. Permitting human intervention during a battle degrades performance under some conditions, but may permit recovery from

failures caused by the inability of an automated system to recover from unanticipated and undesired events.

Processing power required v. battle management organization. A distributed organization would require less processing

power from each computer but more communications than centralized battle management, which requires placing a considerable concentration of processing power

in one computer system.

Software complexity v. battle management organization. A hierarchical battle

management architecture simplifies the

software design but may leave the system

less survivable because of the possibility

of command layers being disabled. A decentralized battle management structure

would increase the complexity of the communications software and might require

more weapon resources, but might result

in a more survivable system.

Software expense and reliability v. hardware customization. Customizing hard-
209

Photo credit: Electronic Systems Division, US Air Force Systems Command

Computer simulations are to play a key role in the development and testing of SDI systems. The photographs here are

of video displays screens generated by a ballistic missile defense battle simulation program developed at the MITRE

Corporation. The circles in the scene above depict areas of coverage for a system of space-based interceptors. The scene

below indicates the tracks of ICBM boosters a few minutes after launch.
ware to perform efficiently at specific

tasks could improve hardware capabilities, but might result in longer software

development schedules and decreased

software reliability because of lack of experience with and lack of development

tools for the hardware.

These trade-offs represent important architectural issues that strongly affect the computing technology needed for BMD. For most

of them, the SDI system architects have not

yet explored the alternatives in sufficient detail to be able to quantify choices. As a result,

there are still only crude estimates of the

speeds and sizes of the computers needed, and

the rates at which data would have to be communicated among the elements of a BMD

system.

PROCESSOR TECHNOLOGY

Table 8-1 shows many of the places where

computers would be used in the fighting components of a BMD system and the jobs they

would perform. Rough estimates of the required memory capacities and speeds are in

included in the classified version of this report.14 Estimates of computer performance requirements for various BMD functions are

shown in table 8-2.

Processing requirements are highly dependent on the system and battle management

architectures, and on the threat. Without

detailed architectural specifications and a precise specification of the algorithms to be used,

estimates of speed and memory requirements

accurate to better than a factor of 10 probably

cannot be made.

Because of the variety of jobs they would

perform, BMD computers would vary considerably in speed and memory capacity. Special purpose computers would probably execute some computing tasks, such as signal

processing. General-purpose computers faster

than any now existing would probably be

needed for computationally stressful tasks

such as discrimination of RVs and decoys in

mid-course. All space-based computers would

have to be radiation-hardened beyond the

limits of existing computers.15

14 For many of the system elements shown in table 8-1, estimates for processing speed and size are not available. The most

computationally intensive tasks are probably signal processing for the IR and optical sensors incorporated into BSTS, SSTS,

and AOS, especially for the mid-term and far-term architectures.

“Radiation hardening to within an order of magnitude of SDI

requirements for some critical components of computer systems

In addition to their use in the fighting components of the system, computers would also

be used:

1. in simulators;

2. to help design, test, exercise, and train

people in the use of the system; and

3. to assist in supporting the system

throughout its lifetime.

Capabilities of Existing Computers

The processing power of a computer is determined by the operating speed of its components and the way they are interconnected (see

box 8-B). Processing and memory components

are built from semiconductor chips, whose

speed is limited by the number and arrangement of circuits that can be placed on a chip.

Developments in chip design and production

technology, including advances from large

scale integrated circuits (LSI) to very large

scale integrated circuits (VLSI), have increased

processor speeds for general purpose computers by a factor of three to four approximately

every 2 years for about the past 10 years. Much

of this progress has been the result of refinements in chip design and production. As a result, some existing supercomputers, such as

the Cray XMP series or Cray 2, may be close

to satisfying most SDI data processing needs,

except that such machines are not packaged

in a suitable form.

has been demonstrated. A complete computer system that is

space qualified and radiation-hardened to within an order of magnitude of SDI requirements for spaceborne computers has yet

to be built.
Table 8-2.—Computing Performance Requirements

— —. First Increment

SDS requirement

F O IIOW on SDS

SDS functions Present state of the art requirement Risk reduction programs

Space based —

General purpose processing hardware

● Command defense ● Throughput: 1 MIPS

● Maintain positive control (space qualified)

● Assess situation

● Select and implement

mode

• Coordinate with others

and higher authority

● Maintain readiness

● Reconfigure and

reconstitute

. Engagement

management

● Weapon guide and home

● Assess kill

● Data distribution

DEM/VAL objective

● Throughput: 500 MIPS

• Memory: 500 MBYTES

● Architecture:

heterogeneous

● Technology: VHSIC

(CMOS), GaAs, SOI

. MTBF: 10 years

● Throughput: 10-50 MIPS

● Memory: 1000 MBYTES

● Architecture:

heterogeneous

. Technology: VHSIC

(CMOS), GaAs, SOI

● MTBF: 10 years

● Throughput: 50-150

MIPS

. Memory: 1000 MBYTES

● Architecture:

heterogeneous

● Technology: VHSIC

(CMOS), @/%i, SOI

● MTBF: 20 years

● VHSIC

● DARPA strategic

computing

● SD I BM/C3

● MCC

Special purpose computing hardware

● Sense and bulk filter . Throughput: 350

● Track MFLOPS (space

● Type and d incriminate qualified)

● Data distribution

c Throughput: 2000

MFLOPS

● Memory: 500 MBYTES

● Architecture:

heterogeneous

. Technology: VHSIC

(CMOS), GaAs, SOI

● MTBF: 10 years

● Throughput: 500

MFLOPS

● Memory: 1000 MBYTES

● Architecture:

heterogeneous

. Technology: VHSIC

(CMOS), @i/k, SOI

c MTBF: 10 years

. Throughput: 1-10

MFLOPS

. Memory: 1000 MBYTES

. Architecture:

heterogeneous

. Technology: VHSIC

(CMOS), GaAs, SOI

. MTBF: 20 years

● DARPA

c SDI BM/C3

● SD I sensors

● Commercial

Common hardware characteristics

● Space qualified

● Hardness (unshielded)

Total dose: 104 rad

Upset: 1O-e/sec

Survive: 10° radlsec

Neutrons/cm*: 10’0

c Space qualified

● Hardness (unshielded)

Total dose: 3x 107 rads

Upset: 1O-’’/sec

Survive: 10” rads/sec

Neutrons/cm’: TBD

● Shielded: none

● Fault tolerant

● Secure

● Space qualified

● Hardness (unshielded)

Total dose: 3 x 107 rads

Upset: 1O-’’/sec

Survive: 10” rads/sec

Neutrons/cm z

: TBD

● Shielded: 5 x JCS

c Fault tolerant

● Secure

● Space qualified

c Hardness (unshielded)

Total dose: 3x 107 rads

Upset: 1O-’’/sec

Survive: 10” rads/sec

Neutrons/cm’: TBD

. Shielded: 10x JCS

● Fault tolerant

● Secure

● DARPA

● SDI BM/C3

● SD I sensors

Software

● All above ● FORTRAN, JOVIAL ● Size

Element: 0.5 MSLOC

Total: 1.5-3 MSLOC

● Fault-tolerant

● Secure

. Ada, COMMON LISP, C

● SAIPDL

● Size

Element: 1 MSLOC

Total: 5 MSLOC

c Fault-tolerant

● Secure

● Ada, COMMON LISP, C

● Size

Element: 2 MSLOC

Total: 5-10 MSLOC

. Fault-tolerant

● Secure

● Ada, COMMON LISP, C

● AdaJPO

● SPC

c SE I

● SDI BM/C3

● DARPA

● STARS

Ground based

Genera/ purpose processing hardware

● Command defense ● Throughput: 30-100

● Maintain positive control MIPS

● Assess situation ● Technology: bipolar LSI

● Select and implement

mode

. Coordinate with others

and higher authority

● Throughput: 500 MIPS

● Memory: 500 MBYTES

● Architecture:

heterogeneous

. Technology: VHSIC

(CMOS)

● MTBF: 1 year

● Throughput: 10-50 MIPS

● Memory: 1000 MBYTES

c Architecture:

heterogeneous

● Technology: VHSIC

(CMOS)

. MTBF: 1 year

c Throughput: 10-50 MIPS

. Memory: 1000 MBYTES

● Architecture:

heterogeneous

● Technology: VHSIC

(CMOS), GaAs, SOI

● MTBF: 1 year

● VHSIC

● DARPA strategic

computing

● SDI BM/Cs

● MCC
Table 8-2.—Computing Performance Requirements—continued

First increment Follow on SDS

SDS functions Present state of the art DEM/VAL objective SDS requirement requirement Risk reduction programs

. Maintain readiness

● Reconfigure and

reconstitute

● Engagement

management

. Weapon guide and home

. Assess kill

● Data distribution

Special purpose computing hardware

● Sense and bulk filter . Throughput: 100-1000

. Track MFLOPS vector

. Type and discriminate processing

. Data distribution

Common hardware characteristics

Software

● All above ● FORTRAN, JOVIAL

● Readiness, test, health

and status report

●

●

●

●

●

●

●

•

●

Throughput: 2000

MFLOPS

Memory: 500 MBYTES

Architecture:

heterogeneous

Technology: VHSIC

(CMOS)

MTBF: 1 year

Size

Element: 0.5 MSLOC

Total: 1.5-3 MSLOC

Fault-tolerant

Secure

Ada, COMMON LISP, C

SA/PDL

●

●

●

●

●

●

●

●

●

●

●

SOURCE: Strategic Defense Initiative Organization, U.S. Department of Defense, 1987

Throughput: 1-3 GFLOPS

Memory: 1000 MBYTES

Architecture:

heterogeneous

Technology: VHSIC

(CMOS)

MTBF: 1 year

Fault tolerant

Redundant

Performance monitor

Fault location

Hardness

3 PSl plus

associated effects

Size

Element: 1.6 MSLOC

Total: 3 MSLOC

Fault-tolerant

Secure

Ada, COMMON LISP, C

●

●

●

●

●

●

●

●

●

●

Throughput: 1-10

GFLOPS

Memory: 1000 MBYTES

Architecture:

heterogeneous

Technology: VHSIC II

(CMOS), GsAs, SOI

MTBF: 1 year

Fault tolerant

Redundant

Performance monitor

Fault location

Hardness

5 PSl plus

Associated effects

Size

Element: 2.3 MSLOC

Total: 5 MSLOC

Fault-tolerant

Secure

Ada, COMMON LISP, C

● DARPA

● SDI BM/C3

● SDI sensors

. Commercial

● DARPA

● SDI BM/C3

● SD I sensors

● Ada J PO

● S P C

. SE I

● SDI BM/C3

● DARPA

● STARS
213

Box 8-B.–MIPS, MOPS, and MEGAFLOPS

The processing power of a computer is often expressed as the rate at which it can execute instructions, measured in instructions per seconds, or ips. A computer that can execute a million instructions per second is a 1 mips machine. Although mips give a crude measure of the speed of

a computer, there is too much variability in the time it takes to execute different instructions on

the same machine and in the instructions used by different machines for mips to be a true comparative measure of processing power.

Complex instructions may take four or five times longer to execute than simple instructions

on the same machine. A complex instruction on one machine may have the same effect in two-thirds

the time as three simple instructions on a different machine. To simulate operating conditions, a

mix of different instructions are often used in measuring computer performance. Such measurements are sometimes characterized as operations per second, or ops, rather than ips. A computer

that can execute a million operations per second is called a 1 mops machine. BMD signal processing

needs have been estimated to be as much as 50 billion ops (50 gigops).

One class of instructions, known as floating point instructions, are important in numerical calculations involving numbers that vary over a wide range, but are very costly in terms of execution

time. A common option on computers is an additional processor, sometimes known as a floating

point accelerator, specialized to perform floating point operations. The speed of computers designed

to perform numerical floating point operations efficiently is usually measured in floating point operations per second, or flops. A computer that can execute a million floating point instructions per

second is a 1 megaflops machine.

To compensate for differences in instruction sets and instruction effects on different computers,

standard mixes of instructions are used to compare the performance of different computers. For

applications involving widely-ranging numerical calculations, such as track correlation, floating point

instructions are included in the mix. The variation in machine performance between machines may

be a factor of three or four, depending on the mix, the machines involved, and other factors.

For purposes of estimating processing power needs for SDI BMD, the requirements are not

yet known to better than a factor of about 10, which dominates differences in performance on different instruction mixes. Accordingly, estimates in this report will generally be given in terms of mips

or mops.

If progress can be continued at the same rate the features used to construct circuits on the

as in recent years, sufficiently powerful proc- chip. Increases in processor speeds may then

essors to meet the most stressing requirements

of SDI BMD should be available in about 10

years. An obstacle to satisfying BMD processing power requirements is that the processors with the largest requirements are those

that would have to be space-based and therefore radiation hardened. Special development

programs would be needed to produce adequate space qualification and radiation hardening for the new processors.

New Computer Architectures

Current chip production technology may

soon reach physical limitations, such as the

number of off-chip connectors and the size of

have to await new chip production technology

or new ways of building processors, e.g., optical techniques. An alternative to increasing

computer speeds without improving component speeds is to find better ways of interconnecting components, i.e. better computer architectures, and better ways of partitioning

computing tasks among computers. Computers constructed by interconnecting many small

computers in ingenious ways, such as the

Hypercube computers developed at Cal Tech

and later produced by Intel as the iPSC machine, are just now appearing on the market.16

“ C.L. Seitz, “The Cosmic Cube,” Communications of the

ACM, January 1985.
214

The iPSC is estimated to run at 100 mips and

8 mflops, but is well-suited only for scientific

computing tasks that can be organized to take

advantage of the iPSC’s architecture. Whether

or not such architectures will be useful for the

most computationally-intensive BMD tasks

will depend on what algorithms are used.

Novel computer architectures, despite their

potential processing power, have the drawback

that the software technology base needed to

capitalize on their potential must be developed.

New software is needed to run programs on

new computers, to help users decompose their

problems to utilize the machine’s potential, and

to convert existing software to execute on the

machine. As an example, to meet Department

of Defense (DoD) standards, a computer such

as the iPSC would need a compiler for AdaTM

(the DoD’s standard programming language

for weapon systems) and an operating system

compatible with AdaT M. Although advances

in computing hardware have come rapidly,

software development is notoriously slow and

costly.

Space Qualification and

Radiation Hardening

Space-qualified general purpose computers

lag ground-based computers in processing

power by a factor of 20 or more. The fastest

space-qualified-but not radiation-hardened—

processors today achieve processing rates of

about 1 mips.17 Adequate radiation hardening

of the computers imposes a more significant

penalty in cost than in processing speed. The

most promising technology for meeting both

speed and radiation hardening requirements

currently uses gallium arsenide (GaAs) rather

than silicon in the manufacture of chips. Although GaAs is more radiation resistant, high

defect densities reduce manufacturing yields,

making chip production costlier. The higher

defect densities also impose smaller chip sizes

and fewer electronic circuits per chip. The con17 The Sperry 1637 and Delco MAGIC V avionics processors

achieve a rate of about 1 mips, but neither are radiation-hardened

nor have they been used in space applications. The Rockwell

IDF 224 and Delco MAGIC 362S space-qualified processors

achieve a rate of about 600 kops for instruction mixes that do

not include floating point operations.

sequent lower overall level of integration may

require processors to have more components

and be less reliable. Researchers in chip production say that current problems with manufacturing yields and circuit densities are temporary and will be solved. As Milutinovic

states,

. . . many problems related to materials are

considered temporary in nature, and one

prediction states that the steady-state cost

will be about one order of magnitude greater

for GaAs than for silicon.18

Space-based computers must be able to withstand long-term cumulative doses of radiation

and neutron flux, short bursts of a few highlyenergetic particles (known as transient events),

and electromagnetic pulses (EMP) resulting

from nuclear detonations. Although shielding

may protect semiconductors against all three

phenomena, it incurs a corresponding weight

penalty. Gallium arsenide is a promising material for semiconductors because it is more

resistant to cumulative radiation and neutron

flux damage than silicon. Resistance of GaAs

to transient events is dependent on the particular chip design.

It may be possible to harden space-based

computers to survive the radiation of a nuclear

weapons battle environment. But it is important to consider the effects of such an environment on software as well as on hardware.

A transient radiation-caused upset might interrupt the current operation of computer hardware, leading either to a resetting of the processor or to the changing of a bit in memory

or in the internal circuitry of the processor. The

processor may continue to function, but the

state of the computation maybe altered, causing an error in software processing, i.e., a system failure.

Consider as an analogy the effects of a single digit error on the computation of an entry

for an income tax form. The error may be so

small as to be hardly noticeable, and it may

even make no difference because the tax scales

1’Veljko Milutinovic, “GaAs Microprocessor Technology, ”

Computer, October 1986, pp. 10-13.
215

are incremental, not continuous. On the other

hand, a larger error in a single digit may have

a considerable effect on the amount of tax paid.

In either case, the error may propagate

through later entries on the form until it is noticed and corrected. Unless the taxpayer

checks his entries for reasonableness, he may

not find the error. The IRS may find the error

by duplicating the taxpayer’s calculations, or

by performing consistency and reasonableness

checks.

The effects of transient events on computing accuracy are difficult to predict. Designing software to cope with such events is a formidable problem, requiring one to forecast all

possible symptoms of upsets and provide errorrecovery measures for them.19 It is also difficult to simulate the occurrence of transient

events realistically enough to test the software

design. There is little experience with softwareintensive systems operating under conditions

likely to produce transient events.

19TWhe design problem maybe simplified somewhat by grouping possible symptoms into classes so that all events in a particular class may be handled in the same way. Grouping events

into classes and devising the appropriate response for each class

is a very difficult design problem.

CONCLUSIONS

A BMD system to counter the Soviet ballistic missile threat might be the most complicated artifact ever built. It would involve the

application of many different technologies; the

automated interplay of thousands of different

computers, sensors, and weapons; and the development of more software than has been used

in any single previous project. An advanced

BMD system would require computers in every

fighting element of the system and in many

supporting roles.

The degree of automation demanded entails

not only advances in software technology (addressed in chapter 9) but also advances in secure computer networking, processing power,

and radiation hardening of electronics. The extent and importance of simulations—in developing, exercising, and otherwise maintaining

the system, as well as in training people in its

use—would require an advance in simulation

technology.

Because several difficult architectural tradeoffs have not yet been sufficiently addressed,

the scope of the advances needed cannot be

well predicted. Until an architectural description is available that clearly specifies battle

management structure and allocates battle

management functions both physically and

within that structure, better predictions will

not be possible.

Further discussion of the computing technology issues involved in producing an automated BMD system follows.

Reliable, Secure Communications

Common to all BMD systems that require

human intervention at any stage is the need

to provide secure, rapid communications between the human and the battle management

computers. If part of the system is in space,

then most likely there would be a need for

space-to-ground communications. Battle management requires communications among the

battle managers, the sensors, and the weapons forming a BMD system. The computers

forming the communications network would

digitally encode and control all the transmissions.

Achieving secure, reliable, adequate communications would call for simultaneous advances

in at least two technologies. First, hardware

technology, such as laser communications,

needs to provide a medium that is difficult to

intercept or jam and that can meet the required

transmission bandwidth. Second, network

technology must provide adequate, secure, survivable service for routing messages to their

destinations. When damaged, the network

must be able to reconfigure itself without sig-
216

nificantly disrupting communications. Such

performance would take sophisticated network

control software-probably beyond the current

state of the art. Proposed solutions to these

problems are either untried or have only been

tried in ground-based laboratory situations.

Simulations

Simulations would play a key role in all

phases of a BMD system’s life cycle. The SDIO

is building a National Test Bed (NTB) to facilitate the development and use of BMD simulation technology. A full-scale NTB should permit experimentation with different system and

battle management architectures, different

battle management strategies, and different

implementations of architectures. It would be

the principal means of testing and predicting

component, subsystem, and system reliability.

Initially, the NTB would be a link among computers, each simulating a different aspect of

a BMD engagement. The number of computers

linked for any particular engagement would

vary with the completeness and depth of detail required. Initial capabilities would not permit simulation of a full battle involving hundreds of thousands of objects. Battle

simulations on a scale needed to represent a full

battle realistically have not been previously attempted. It would be crucial, but very difficult,

to find a way of verifying the accuracy of such

simulations, when and if they are developed.

Technology and Architectural

Trade-offs

Many difficult trade-offs have yet to be adequately addressed in the design of a BMD system to meet SDI requirements. Novel design

ideas or advances in computing technology

may decrease the importance of some of these

trade-offs. However, no architecture has yet

been specified sufficiently to permit clear tradeoff studies. Issues that should be addressed

include:

● simplifying software at the cost of adding computational burden to the

hardware,

●

●

●

●

●

●

simplifying battle management software

by structuring it hierarchically at the expense of survivability,

increasing survivability by decentralizing

battle management at the expense of increasing communications complexity,

customizing hardware for specific applications at the expense of increased software development cost and decreased software reliability,

simplifying the problem of communications security at the cost of decreasing the

possibilities for human intervention during battle,

increasing the amount of human control

during battle at the expense of fighting

efficiency, and

improving fighting efficiency at the cost

of increasing the complexity and volume

of communications (and, thereby, the risk

of catastrophic communications failure).

None of these trade-offs is easy to make and

few can be quantified. Compounding the difficulty is that many of the system elements—

e.g., the Boost-phase Surveillance and Tracking System and Space Surveillance and Tracking System sensors, SBIs and associated CVs,

high-powered lasers, and neutral particle

beams-are still in the research or development

stages. Moreover, no previous system has ever

required the automated handling of many

different devices and different kinds of devices

as would an SD I missile defense. Nonetheless,

tentative conclusions on some trade-offs have

been reached. Most trade-offs could be properly explored by use of an appropriate simulation, such as might be provided by a full-scale

National Test Bed.

Computational Requirements

Processing requirements are highly dependent on the system design, the battle management architectures, and the threat. Because

detailed architectural and algorithmic specifications for an SD I BMD system are not yet

available, estimates of speed and memory requirements accurate to better than a factor of

10 probably cannot be made. However, prog-
217

ress in processing speed has been rapid historically. If it continues at the same pace, it

should yield sufficiently powerful processors

to meet SDI needs within 10 years or less. Such

processors might still have to be space qualified and radiation hardened.

An additional problem in providing radiation-hardened computing hardware is the lack

of experience in building software tolerant of

radiation-induced faults. There is little experience with complex, large-scale software systems that must operate efficiently despite the

occurrence of radiation-induced transient effects in the hardware.

75-9220- 88 - 8
Chapter 9

Software
CONTENTS

Page

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...221

The BMD Software Debate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ....221

Generic Software Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....223

The Software Crisis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..........225

The Nature of Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........225

Failures and Errors in Computer Programs. . .......................226

Tolerating Errors. . . . . . . . . . . . . . . . . . . . . . .........................226

Tolerating Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............227

Traditional Reliability Measures . . . . . . . . . . . . . . . . . . . . . .............227

Traditional Reliability Measures Applied to Software . ...............227

Software Dependability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........228

Figures of Merit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...............229

Correctness . . . . .. .. .. .. .. .. .. .. ... ... .... . ...229

Error Incidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................231

Trustworthiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................231

Fault Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .................232

Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............233

Security .. .. .. .. .. .. .. .. .. . $ . . . . . . . . . . . . .. .. .. .. .. .$ ...4....233

Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........234

Appropriate Measures of Software Dependability. . ..................234

Characteristics of Dependable Systems . .............................234

Observations of External Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..235

Observations of Internal Behavior .. .. .. .. .. .. .. .. .. .. .. ... ... ....23$

Factors Distinguishing DoD Software Development . ................239

Software Dependability and Computer Architecture . ................241

Software Dependability and System Architecture . ..................241

Software Dependability and System Dependability . .................242

Software Dependability and the SDI . .............................242

Development Approaches That Have Been Suggested. . ..............245

summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 245

Estimating Dependability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....245

Technology for Preventing Catastrophic Failure. . ...................246

Confidence Based on Peacetime Testing. . ..........................246

Establishing Goals and Requirements . .............................247

SDIO Investment in Battle Management, Computing Technology,

and Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Conclusions . . . . . . . . . . . . . . . ... ... .. 249

Tables

Table No. Page

9-1. Characteristics of Dependable Systems Applied to SDI,

SAFEGUARD, and the Telephone System . ......................243

9-2. SDIO Battle Management Investment . . . . . . .....................248

9-3. Funding for OTA-Specified problems . ...........................248
Chapter 9

Software

INTRODUCTION

The performance of a ballistic missile defense

(BMD) system would strongly depend on the

performance of its computers. Chapter 8 describes the pervasiveness of computers in the

operation of a BMD system, and as well as in

its development, testing, and maintenance.1 Sequences of instructions called software would

direct the actions of the computers, both in

peacetime and in battle. As shown in table 8-

1, software is responsible both for the actions

of individual components of the system (e.g.,

a radar), and for coordinating the actions of

the system as a whole. As coordinator, software maybe thought of as the glue that binds

the system together. As the system manager,

software assesses the situation based on data

gathered by sensors and reports from system

components, determines battle strategy and

tactics, and allocates resources to tasks (e.g.,

the weapons to be fired at targets.)

The role of software as battle manager is crucial to the success of a BMD system. If software in a particular component failed-even

if the failure occurred in all components of the

same type simultaneously-other components

of different types might compensate. But if the

battle management software failed catastrophically, there would be no way to compensate. Furthermore, the battle management

software may expected to compensate for systemic failures, both because of its role as manager and because software is perceived to be

more flexible than hardware. Consequently, the

battle management software would have to be

the most dependable kind. Thus it is the focus

of most of the SD I software debate.

The BMD Software Debate

The envisaged BMD system would be complex and large, would have to satisfy unique

requirements, and would have to work the first

time it is used in battle. Many computer scientists, and software engineers in particular,

have declared themselves unwilling to try to

build trustworthy software for such a system.

They claim that past experience combined with

the nature of software and the software development process makes the SDI task infeasible. David Parnas has summarized their major arguments.2 Other computer scientists,

however, have stated that their belief that the

software needed for a Strategic Defense Initiative (SDI) BMD could be built with today’s

software engineering technology. Frederick

Brooks, for example, has said:

I see no reason why we could not build the

kind of software system that SDI requires

with the software engineering technology

that we have today.3

Those willing to proceed believe that an

appropriate system architecture and heavy use

of simulations would make the task tractable.

Their arguments are summarized in a study

prepared for the Strategic Defense Initiative

Organization (SDIO) by a group known as The

Eastport Group.4 The critical role played by

the software in BMD makes it important to

understand both positions.

*Table 8-1 illustrates many of the ways in which computers

would be used in a deployed BMD system.

Note: Complete definitions of acronyms and initialisms

are listed in Appendix B of this report.

‘David L. Pamas, “Software Aspects of Strategic Defense

Systems, ” American Scientist, 73:432-40, September-October

1985.

3From a statement by Dr. Frederick P. Brooks at the Hearings before the Subcommittee On Strategic and Theater Nuclear Forces of the Committee On Armed Services, United States

Senate, S. Hrg. 99-933, p. 54.

4Eastport Study Group, “A Report to the Director, Strategic Defense Initiative Organization, ” 1985.

221
222

The Role of Software in BMD

Software for BMD would be expected to:

●

●

●

●

be the agent of system evolution, permitting changes in system operation through

reprogramming of existing computers;

perform the most complex tasks in the

system, such as battle management;

be responsible for recovery from failures,

whether they are hardware or software

failures; and

respond to threats, both anticipated and

unanticipated, against the system.

A BMD system would not be trustworthy

and reliable unless both hardware and software

were trustworthy and reliable. Because of rapid

progress in hardware technology in recent

years, and because of differences in their natures, hardware reliability is not as hotlydebated an issue as software reliability. As

Brooks puts it in his discussion of current software engineering technology:

Not only are there no silver bullets now in

view, the very nature of software makes it unlikely that there will be any-no inventions

that will do for software productivity, reliability, and simplicity what electronics, transistors, and huge-scale integration did for computer hardware. We cannot expect ever to see

twofold gains every two years.

First, one must observe that the anomaly

is not that software progress is so slow, but

that computer hardware progress is so fast.

No other technology since civilization began

has seen six orders of magnitude in performance-price gain in 30 years.5

Software Complexity

The software engineer called upon to produce

large, complex software systems is partly a victim of his medium. Software is inherently flexible. There are no obvious physical constraints

on its design (e.g., power, weight, or number

of parts) so software engineers undertake tasks

of complexity that no hardware engineer

would. Brooks summarizes the situation as

follows:

Software entities are more complex for their

size than perhaps any other human construct

because no two parts are alike . . . In this respect, software systems differ profoundly

from computers, buildings, or automobiles,

where repeated elements abound.

Digital computers are themselves more complex than most things people build: They have

very large numbers of states. This makes conceiving, describing, and testing them hard.

Software systems have orders-of-magnitude

more states than computers doe

Software Issues

Of course, complex systems are successfully

built and used. However, given the current

state of the art in software engineering, complex systems are not trusted to be reasonably

free of catastrophic failures before a period of

extensive use. During that period, errors causing such failures may be found and corrected.

A central issue in the debate over BMD software is whether it can be produced so that it

can be trusted to work properly the first time

it is used, despite the probable presence of errors that might cause catastrophic failures. A

critical point in the debate over this issue is

how one would judge whether or not the software was trustworthy. If evaluations of trustworthiness were to rely on the results of simulations of battles, then a second critical point

is how closely and accurately actual BMD battles could be simulated.

A second central issue in the software debate is whether a BMD system imposes unique

requirements on software. Critical points surrounding this issue are:

● whether there are existing similar systems

that could serve as models for the development of BMD software;

● whether requirements would be sufficiently well understood in advance of use

so that trustworthy software could be designed;

‘Frederick P. Brooks, Jr., “No Silver Bullet, Essence and Accidents of Software Engineering, ” IEEE Computer vol. 20, No.

4, April 1987, p. 10. 6

Ibid.
223

● whether all potential threats against a

BMD system could be anticipated, and,

if not; and

● whether the software could be designed

to handle unanticipated threats during the

course of a battle.

Adding fuel to the debate over whether software could meet BMD requirements is the slow

progress in software technology in recent years

when compared to hardware technology.

An obstacle to settling this issue is the current uncertainty over the purposes of a BMD

system. Software requirements would depend

on the threat and countermeasures to be faced,

the expected strategies of both the offense and

the defense; and the technology to be used in

the system, e.g., kinetic-energy v. directedenergy weapons. A system intended to defend

the population would have different requirements than one intended to defend only critical military targets. A system to be deployed

in phases would oblige the software developers

to know the changes in requirements and architecture to be expected between each phase before they designed the software for the initial

phase.

Among the developers of large, complex systems who attended OTA’s workshop on SDI

software, there was unanimous agreement that

software development should not be started

until there was a clear statement of the requirements of the system.7 All system requirements

would not have to be known in detail before

software development could be started. But

if the requirements for a system component

could not be written, neither could the specifications for the software that was part of that

component.

Catastrophic Failure

Both critics and supporters of the feasibility of building software to meet SDI requirements agree that large, complex software sys7Attendees at the workshop, held Jan. 8, 1987 in Washington, DC, included software developers who participated in the

development of SAFEGUARD, Site Defense, telephone switching systems, digital communications networks, Ada compilers,

and operating systems.

terns, such as an SDI BMD system would need,

would contain errors. They disagree on whether

the software could be produced so that it would

not fail catastrophically. Several different

meanings of catastrophic failure have been

used. It is sometimes related to whether or not

a BMD system would deter the Soviets from

launching ICBMs at the United States:

Ballistic missile defense must . . . be credible enough in its projected wartime performance during peacetime operations and testing

to ensure that it would never be attacked.8

It can also be taken to mean that

The system has failed catastrophically if the

U.S. bases its defense on the assumption that

the system will function effectively in battle

and then a major flaw is discovered so that

we are defenseless.9

This chapter assumes a technical definition:

a catastrophic failure is a decline in system performance to 10 percent or less of expected performance. A BMD system designed to destroy

10,000 warheads would be considered to have

failed catastrophically if it stopped only 1,000

of the 10,000. The figure 10 percent is an arbitrary one; it has been adopted as illustrative

of a worst-case failure.

Generic Software Issues

Much of the debate concerning BMD software is about software problems common to

all complex, critical software systems.10 A good

example is whether software can be designed

to recover from failures automatically. BMD

proponents argue that producing trustworthy

BMD software would not call for general solutions to such problems. They feel that the

specificity of the application permits specialcase solutions that would work well enough

for BMD. Opponents argue that BMD software would demand better solutions for such

problems as failure-recovery than any system

‘Charles A. Zraket, “Uncertainties in Building a Strategic

Defense, ” Science 235:1600-1606, March 1987.

‘David L. Parnas, personal communication, 1987.

10 AS descri~, for ex~ple, in David L. p~as, “Softw~e

Aspects of Strategic Defense Systems, ” op. cit., footnote 2.
224

previously built. They say that approaches proposed for SDI have been tried in the past and

have not been shown to be effective.

This chapter is primarily concerned with

arguments over the generic issues. First, there

are as yet no clear statements of BMD software requirements, whether for battle managementor particular BMD system components,

let alone proposed software designs or proposed solutions for BMD for any of the generic

problems. Application-specific analysis must

await those requirements, designs, and solutions.

Second, there seems to be agreement that

BMD software would be more complex than

any previously built. The first conclusion of

volume V of the Fletcher report was:

Specifying, generating, testing, and maintaining the software for a battle management

system will be a task that far exceeds in complexity and difficulty any that has yet been

accomplished in the production of civil or military software systems.11

Third, tasks for BMD software differ in important ways from the tasks performed in today’s weapons systems and command, control,

and communications systems. It is true that

many BMD software tasks would resemble

those for current systems: e.g., target tracking, weapons release and guidance, situation

assessment, and communications control in

real time. The differences from current systems

are

●

●

●

●

●

●

that a BMD system would:

permit less opportunity for human intervention,

have to handle more objects in its battle

space,

have to manage a larger battle space,

use different weapons and sensor technology,

contain vastly more elements,

have more serious consequences of failure,

“James C. Fletcher, Study Chairman and Brockway McMillan, Panel Chairman, Report of the Study on Wm.hMting the

Threat Posed by NucJear BaW”stic Missiles, Volume V: Battle

Management, Cornmuzu”cations, and Data Processing (Washington, DC: Department of Defense, Defensive Technologies

Study Team, October 1983).

• have to operate in a nuclear environment,

● be under active attack by the enemy, and

● be useless if it failed catastrophically during its first battle.

Accordingly, the debate over generic software issues is an appropriate one for BMD

software.

The purpose of this chapter is to examine

the key issues in the debate over the feasibility of meeting BMD software requirements.

This chapter:

1. discusses why there is such a debate and

2

3

4

includes a definition of key terms, such

as “catastrophic failure” and “trustworthiness”;

analyzes properties often claimed to be important for BMD software-e. g., trustworthiness, reliability, correctness, low error incidence, fault tolerance, security, and

safety, (including a discussion of the meaning of “reliability” as applied to software

and why there is no single, simple measure of software dependability);

identifies the major factors that affect

software dependability; and

characterize the demands placed on BMD

software and the BMD software development process in terms of the factors affecting dependability.

The remainder of this chapter begins with

a brief discussion of Department of Defense

(DoD) software experience, the nature of software, traditional reliability measures, and the

pitfalls inherent in applying such measures to

software. Following sections deal with properties such as trustworthiness, correctness,

fault tolerance, security, and safety, and with

the factors that lead people to have confidence

that systems have such properties. (The available technology for incorporating these properties into software is analyzed in app. A.) The

chapter then presents an analysis of Strategic Defense Initiative BMD requirements from

the viewpoint of those factors. The chapter concludes with: a discussion of why BMD software development is a difficult job—perhaps

uniquely so; why we are unlikely to have more

than a subjective judgment of how trustwor-
225

thy the software is, once produced:

mary of the key software issues.

The Software Crisis

and a sumSince the mid-1970s DoD officials have increasingly recognized the difficulties in producing command, control, and information

processing software for weapon systems.12 As

Ronald Enfield says:

In the 1970s, the world’s largest customer

for computers–the U.S. Department of Defense—changed its focus from hardware to

software as a major obstacle to progress in developing advanced weapons. Reliable software

is also a crucial component of complex systems

such as nuclear power plants, automatic tellers, and many other technologies that touch

our lives in critical ways. Yet, as the software

for these systems has grown increasingly complicated, it has become more prone to error.13

The complex of problems associated with

trying to produce software that operated properly, on time, within budget, and maintainably over its lifetime was dubbed “the software

crisis. ” DoD has found that the software crisis

is sometimes forcing the military to wait for

software to be debugged before it can use new

systems. Progress in alleviating this crisis has

been slow, and the same problems would apply to producing software for BMD. Both the

Fletcher and Eastport Group reports agreed

that software development for BMD would be

a difficult, if not the most difficult, problem

in BMD development. The Eastport Group

noted that:

Software technology is developing against

inflexible limits in the complexity and reliability that can be achieved.14

To understand why DoD and other developers of large, complex software systems have

been experiencing a software crisis, it is first

l%An emly ~~y9i9 of the problem can be found in Don~d

W. Kosy, “Air Force Command and Control Information Processing Requirements in the 1980s: Trends in Software Technology, ” Rand Report R-1012-PR, June 1974.

18Ron~d L. Enfield, “The Limits of Software Reliability, ”

Technology Rew”ew, April 1987.

“Eastport Study Group Report, op. cit., footnote 4.

necessary to understand the nature of software

and the demands made on it.

The Nature of Software

Digital computers are among our most flexible tools because the tasks they do can be

changed by changing the sequences of instructions that direct them. Such instruction sequences are called programs, or software and

are stored in the computer’s memory. Flexibility is attained by loading different programs

into the memory at different times.15 Each

make and model of computer has a unique set

of instructions in which it must be programmed, generically known as machine instructions or machine language.

To simplify their job, programmers have developed languages that are easier to use than

machine language. These languages, such as

FORTRAN, COBOL, and Ada, are known as

high level languages, and require the programmer to know less about how a particular computer works than do machine languages. The

language in which a program is written is

known as the source language for the program,

and the text of the program is called the source

program or source code.16 A program whose

source language is a high level language must

be translated into machine language before being loaded into the computer’s memory for execution. Some lines of text in a source program

may be translated into many machine instructions, some into just a few.

There are several measures of program size.

One measure is the number of lines in the text

of the source program, also known as lines of

source code (LOC), or number of machine language instructions. Size is greatly variable: a

simple program to add a list of numbers may

require 10 or fewer instructions, while a word

ITo protect them horn change, and to enhance their performance, some programs are loaded into memories that are either

unchangeable or that must be removed from the computer to

be changed. However, most of the memory in nearly all computer systems is of a type that is reloadable while the computer

is running.

leInstructions and data are encoded into a computer’s memory as numbers, and programs are sometimes known as codes.
processing program may take 10,000 LOC (10

KLOC). The Navy’s AEGIS ship combat software consists of approximately 2 million instructions.

Size alone is not a good measure of program

difficulty. Large programs can be simple, small

ones very complex. The size of a program is

influenced by the language, computer, programmer’s expertise, and other factors. A more

important question is, “How complex is the

problem to be solved by the program and the

algorithms used to solve it?’’17 Compounding

the problem is the lack of a standard method

for measuring complexity.

Failures and Errors in

Computer Programs

Since a computer can only execute the instructions that are stored in its memory, those

instructions must be adequate for all situations

that may arise during their execution.18 Incorrect performance by a computer program during its operation is known as a failure. Failures

in computer programs result from:

●

●

●

the occurrence of situations unforeseen by

the computer programmer(s) who wrote

the instructions,

a misunderstanding by the programmer(s)

of the problem to be solved (including misunderstandings among a group of programmers), or

a mistake in expressing the solution to the

problem as a computer program.

Each of these situations can cause errors in

the instructions making up computer programs, errors manifested as failures when particular inputs occur. 19 The effects of errors in

1~~ chap~r 8 for a discussion of algorithms.

18some progr~s, kIMNVII as self-mdfying progr~st add ti

or modify their own instruction sequences and then execute the

resulting instructions. Nonetheless, the response of the program

to input data is completely determined by the instructions that

are initially stored in its memory.

leErrors in programs are often called bugs, although the term

originally meant any cause of incorrect behavior. The origin of

the term is described in John Shore, The Sachertorte A&on”thm,

(New York, NY: Penguin Books, 1986).

programs range from minor inconveniences

(e.g., misspelled words in the program’s output) to catastrophic failures-e. g., the cessation of all processing by the computer, wrong

answers to problems like computing missile

tracks, or overdoses of radiation to devices controlled by the computer.20 21

Tolerating Errors

Errors in large computer programs are the

rule rather than the exception. Freedom from

errors cannot be guaranteed and is extremely

rare. Since correcting an error requires changing the list of instructions that make up the

program, the process of removing an error may,

and often does, introduce anew error. For large

software, the process of correcting errors is so

time consuming and expensive that modifications to the software are distributed only a few

times a year. As a result, lists of known errors

are often published and distributed to users.22

Where there is a high degree of human interaction with the program during its operation,

the human user can usually circumvent situations where the program is known to fail-often

by restricting the data input to the program

or by not using features of the program known

to be failure-prone.

The more critical the task(s) of the program

and the smaller the degree of human intervention in the program’s operation, the smaller

the tolerance for errors. Accordingly, large,

critical programs commonly include consistency checks whose goal is to try to detect

failures, prevent them when possible, and recover from them when not. This approach is

*°For a sample of the variety of problems involving computers

and software, see ACM SIGSOFTSoftware Engineering Notes

11(5):3-35, October 1986.

ZIThe Occumence of a failure condition is sometimes known

as anina”dent. The software may contain instructions that permit it to recover from such an incident. If the software successfully corrects the condition, it remains no more than an incident. Successful recovery from incidents requires good

understanding of their causes and corrections, and requires that

not too many occur at once.

2~Mmu~ PWS describing programs used with the UNIX operating system, developed and sold by AT&T Bell Laboratories

and a currently popular operating system, contain as standard

sections a description of the known bugs in the programs.
227

discussed in more detail in a later section on

fault tolerance.

Tolerating Change

As previously noted, change is both the

blessing and the curse of the software engineer.

Software is expected to be flexible, and his designs must accommodate change. Without its

flexibility, software would be as useful. Although software does not wear out in the sense

that hardware does, complex software systems

apparently tolerate only a certain amount of

change. The critical point occurs when changes

introduce more errors than they fix, i.e., each

change, on the average, introduces more errors

than it removes.23 It appears likely that increasing the rate of change decreases the time to

reach the critical point. Brooks devotes a chapter to a discussion of the effects of changes

in complex systems, concluding with:

Program maintenance is an entropy-increasing process, and even its most skillful execution only delays the subsidence of the system

into unfixable obsolescence.24

Although Brooks’s discussion is more than

10 years old, it is still valid. Systems that tend

to be very long-lived, e.g., 20 years old or more,

undergo complete software redevelopment

every few years. As an example, the Navy’s

Naval Tactical Data System, first built in the

early 1960s, has undergone at least five major

rewrites.

Traditional Reliability Measures

Reliability is one measure of system behavior. In engineering, reliability is often expressed as the average time between failures.

For inexpensive consumer items, such as light

bulbs, it is defined as the expected lifetime of

the item, since such items are completely

replaced when they fail. Complicated, expensive systems, such as automobiles, computer

“M. Lehman and L. Belady, “Programming System Dynamics,” ACM SIGOPS Third Symposium on Operating System Principles, October 1971.

ZiFrederick p. Brooks, Jr., The Mytlu”caJ Man-Month: Essays

on Software En~”neering, (New York, NY: Addison-Wesley,

1975).

systems, and weapon systems, are designed

to outlive any particular component by allowing repair or replacement of components when

they fail. Failure of a windshield wiper blade

only requires the quick, inexpensive replacement of the blade by another that meets the

same specifications as the failed one.

Reliability of complicated systems is traditionally measured in mean time between failure (MTBF), or an equivalent measure such as

failure rate. MTBF is measured by counting

failures during operation and then dividing by

the length of the observation period. For systems with no operational history, MTBF must

be predicted on the basis of estimates of the

MTBF of each of the system’s components.

Usually such an estimate is made using the

assumption that component failures are random, statistically independent events. Without such an assumption, the analysis is much

more difficult and often impractical for complex systems.

Reliability as measured by MTBF is useful

for systems with the following characteristics:

●

●

●

the time to repair the system is unimportant to the user, perhaps because a temporary replacement is available or the user

has no need of the system for a while; or

the time to repair the system is important,

but can be kept very short compared to

the MTBF, perhaps by keeping a stock

of replacement parts on hand; and

there are no failures so serious as to be

unacceptable, e.g., failures that could result in human deaths.

Traditional Reliability Measures

Applied to Software

The concept of MTBF has historically been

of limited use for critical software. For applications such as BMD, repair time is extremely

important. If the system, or parts of it, were

to fail, the user would have either no response

or a weakened response to an ICBM attack.

Accordingly, the concept of MTBF alone is not

sufficient to judge whether or not the system

would behave as desired. Furthermore, the
228

models often used for predicting MTBF are

based on assumptions that are invalid for software. Many models assume that component

failures are independent and that they are random, i.e., unrelated to system inputs and

states. Software components do not fail randomly: they contain errors that cause failures

in the event of particular inputs and particular states. The failure of one component often

causes others to fail because software components tend to be closely interrelated.

Replacing a software component by a copy

of itself will cause exactly the same failure under the same conditions that caused the original to fail. Remedying a failure consists of modifying a component to remove an error in its

list of instructions, not replacing a failed component with a copy. Once modified, the component can no longer be considered to be the

same as the original, and previous failure data

do not apply to it. Finally, a failure in one component is likely to lead to failures in others.

Consequently, a stock of replacement components cannot be kept on hand in hopes of reducing repair time.

Regardless of whether MTBF were used to

indicate software or hardware reliability for

a BMD system, some failures would be clearly

more disastrous than others. To be useful,

MTBF would have to be calculated for different classes of failures.

In recent work, researchers have shown that

if inputs are characterized in statistically

sound ways, it is possible in testing to determine with high confidence a meaningful MTBF

for a program.25 Nonetheless, MTBF remains

inadequate as the sole means of characterizing software dependability.

~5A~en Cumit, Michael Dyer, and Harlan D. M~s, “Certifying the Reliability of Software, ” L??EE Transactions On Software En&”neering, SE-12(1), January 1986, pp. 3-11.

SOFTWARE DEPENDABILITY

Computer scientists and software users have

devised a variety of ways to evaluate software

dependability. As in deciding which automobile to buy, the buyer’s concerns should determine which qualities are emphasized in the

evaluation. Qualities commonly considered are:

●

●

●

●

●

●

correctness-whether or not the software

satisfies its specification;

trustworthiness-probability that there

are no errors in the software that will cause

the system to fail catastrophically;

fault tolerance-either failure prevention,

i.e., capability of the software to prevent

a failure despite the occurrence of an abnormal or undesired event—or failure recovery, i.e., capability of the software to

recover from a failure when one occurs;

availability-probability that the system

will be available for use;

security-resistance of the software to unauthorized use, theft of data, and modification of programs;

error incidence—number of errors in the

software, normalized to some measure of

size; and

● safety—preservation of human life and

property under specified operating conditions.

For critical software, correctness and trustworthiness are important indicators of dependability. Fault tolerance assumes importance

when the system must continue to perform—

as in the midst of a battle—even if performance degrades. Security is important when valuable data or services maybe stolen, damaged,

or used in unauthorized ways. Safety is important in applications involving risk to human

life or property. Error incidence is important

in assessing whether or not apiece of software

should stay in use.

OTA’s characterization of BMD software debendability will include all of the above-listed

qualities because:

● national survival may depend on the

proper operation of BMD software;
229

● such software would have to be trusted

to operate well during the entire course

of a battle; and

● it would have a long lifetime.

Early versions of a BMD system may not

have goals as ambitious as later, more capable versions. Nevertheless, we still would want

to be confident that the software would operate well during the course of a battle, would

do so without undue pause for failure recovery, would be secure, and would be safe to operate. In addition, since it would surely undergo

continual modification during its lifetime, we

would need to be sure that it was being maintained without repeated introduction of new

errors.

Dependability needs to be attended to from

the beginning of software development, for it

is not easily added on later. Software designs

often must be redone after system delivery

when performance has been emphasized at the

cost of such factors as correctness, fault tolerance, or security. The cost of redoing software

may greatly exceed the original cost. Software

designed for dependability may contain mechanisms for later improving its correctness,

trustworthiness, fault tolerance, security, and

safety later. For example, fault tolerance was

strongly considered in the design of the

SAFEGUARD software. During tests of the

prototype system engineers realized that the

wrong set of faults had been accommodated.

Because the mechanism for detecting and responding to faults had been incorporated into

the design, the set of faults tolerated by the

system was changed in a matter of only a few

weeks. This change involved perhaps 10 percent of the lines of code in the operational software.

26

Figures of Merit

No single figure of merit can indicate dependability. Single figures of merit generally focus

on some single characteristic, such as the cost

to discover a password that would permit entry to a computer system. Because software

“Victor Vyssotsky, personal communication, 1987.

engineering is a young discipline, software

engineers do not yet know very well how to

evaluate software quantitatively. And because

information permitting numerical evaluation

of software is usually considered proprietary,

few data are available anyway for such analysis. Accordingly, we would not expect a useful quantitative evaluation of BMD software

dependability to be available for many years.

Therefore, only a brief analysis of each software property contributing to dependability

follows.

Trustworthiness is probably the most important quality for BMD software. The application is critical. Software engineers are unable

to produce complex software that is correct

and error-free at the current state of the art.

Although BMD software should still be as

nearly correct, highly available, error-free, secure, and safe to use as possible, we must above

all know whether or not it could be trusted.

Correctness

Software developers work from specifications, both written and verbal, that are intended to convey the desired system behavior.

The specifications are frequently developed by

people with little familiarity with software, e.g.,

a Naval officer untrained in software development who writes specifications for a ship’s

combat management system. “Correct” software exhibits exactly the behavior described

by its specifications. To convince himself and

his customer that he has done his job, the software developer must somehow demonstrate

that his software is correct.

Mathematical Correctness

Because no single technique has proved completely effective to demonstrate program correctness, software developers use a variety of

techniques try to demonstrate that their software adequately approximates its specifications. Computer scientists, in recognition of

the problems involved, have devoted considerable research to such techniques. They have

investigated formal and informal, mathematical and non-mathematical ideas. Much of the
230

research attention has been focused on developing “program verification’ ’-mathematical

techniques to verify that a computer program

is correct with respect to properties required

of it. Some progress has been made in mathematically proving that programs are correct.

It is unlikely, though, that a sudden breakthrough will occur leading to order-of-magnitude gains in productivity and greatly improved dependability. Brooks analyzed this

possibility:

Can both productivity and product reliability be radically enhanced by following the profoundly different strategy of proving designs

correct before the immense effort is poured

into implementing and testing them?

I do not believe we will find productivity

magic here. Program verification is a very

powerful concept, and it will be very important for such things as secure operating system kernels. The technology does not promise, however, to save labor. Verifications are

so much work that only a few substantial programs have ever been verified.

Program verification does not mean errorproof programs. There is no magic here, either.

Mathematical proofs can also be faulty. So

whereas verification might reduce the program-testing load, it cannot eliminate it.

More seriously, even perfect program verification can only establish that a program

meets its specification. The hardest part of the

software task is arriving at a complete and

consistent specification, and much of the essence of building a program is in fact the

debugging of the specification.”

Although mathematical techniques for demonstrating correctness are not frequently applied, other techniques-such as design reviews, code reviews, and building software in

small increments—are. The one technique always used by software developers, however,

is testing.

Testing

Program developers test a program by placing it in a simulated operating environment.28

27 Frederick p. Br~k9, Jr., “NO Silver Bullet, Essence ~d Accidents of Software Engineering, ” op. cit., footnote 5, p. 16.

~sFor presentation pUrpOSSS, the discussion Of testing here

is simplified, omitting, e.g., component testing. Appendix A

contains a more complete discussion.

The simulation supplies inputs to the program,

and the testers examine its output for failures.

29 They report any failures to the programmers, who correct the relevant errors and resubmit the program for testing. The sequence

continues until the developers agree that the

program has passed the test. The final stage

of testing developmental software for large and

critical systems, especially military software,

is acceptance testing. A previously agreedupon test is run to show that the software

meets criteria that make it acceptable to the

user.

It has been shown that testing of every possible state of the program, known as exhaustive testing, is not practical even for simple

programs. To illustrate this point, John Shore

calculated the amount of time required to test

the addition program used by 8 digit calculators to add 2 numbers. He estimated that, at

the rate of one trial per second it, would take

about 1.3 billion years to complete an exhaustive test.30

For large, complicated programs, the number of tests that can be run practically is small

compared to the number of possible tests.

Therefore, developers apply a technique called

scenario testing. They observe the program’s

behavior in an operational scenario that the

program would typically encounter. They may

establish the scenario by simulating the operational environment, such as an aircraft flight

simulator. Alternatively, they may place the

software in its actual environment under controlled conditions. For example, a test pilot

may put an aircraft with new avionics software

through a series of pre-determined neuvers.

In the former case, the simulator must first

be shown to be correct before the results can

be considered valid. If the simulator itself relies on software, showing the validity of the

simulation may be as difficult or more difficult than showing the correctness of the program to be tested.

z9G~ W9ter9 cue~ly determine the inputs to be used in

advance, often including some tests using random inputs, and

some using nonrandom, so as to get representative coverage

of the expected operational inputs.

‘“John Shore, op. cit., footnote 19, pp. 171-172.
231

For systems like aircraft, such tests are so

expensive that only relatively few scenarios

can be flown. Flight tests of the avionics software for the Navy’s A-7 aircraft, including

land- and carrier-based tests, cost approximately $300,000. Scenario tests for the SAFEGUARD system consisted of installing a test

version of the system at Kwajalein missile

range and firing one or two missiles at a time

at it.

Since exhaustive testing is not practical,

testing cannot be relied upon to show that a

computer program completely and exactly behaves according to its specifications or even

that it contains no errors. As stated by computer scientist Edsger Dijkstra:

Program testing can be used to show the

presence of bugs, but never to show their absence!31

The deficiencies of testing as a means of

showing correctness and freedom from errors

have moved software engineers to seek other

methods, such as mathematical. They have

also sought means of measuring error incidence. In addition, they are developing methods for random testing that permit statistical

inferences about failure rates.32

Error Incidence

Some assert that error incidence-measured,

for example, by the number of errors found per

thousand lines of source code-measure program correctness. Those making this assertion

assume that it is possible to count errors unambiguously and that the more errors a program has the less its behavior will conform to

its specifications. They then portray the debate over BMD software dependability as

hinging on the question of whether or not the

software would contain errors, and how many

it would contain.

Both critics and proponents of an attempt

to build SDI software agree that any such software would contain errors. As put by the Eastport Group:

Simply because of its inevitable large size,

the software capable of performing the battle

management task for strategic defense will

contain errors. All systems of useful complexity contain software errors.33

Ware Myers notes:

The whole history of large, complex software

development indicates that errors cannot be

completely eliminated.34

David Parnas asserts that:

Error statistics make excellent diversions

but they do not matter. A low error rate does

not mean that the system will be effective. All

that does matter is whether software works

acceptably when first used by the customers;

the sad answer is that, even in cases much simpler than SDI, it does not. What also matters

is whether we can find all the “serious’ errors

before we put the software into use. The sad

answer is that we cannot. What matters, too,

is whether we could ever be confident that we

had found the last serious error. Again, the

sad answer is that we cannot. Software systems become trustworthy after real use, not

before.35

Trustworthiness

Since correctness and error rates are not the

real issues in the software debate, trustworthiness has become the focus. The issue is

whether or not BMD software could be produced so that it would be trustworthy despite

the presence of errors. In common usage, reliability and trustworthiness are often considered

to be the same. In engineering usage, reliability has become associated with specific measures, such as MTBF. There have been few attempts to quantify trustworthiness, despite

~lJ, D~, E.W. Dijkstra, and C.A.R. Hoare, “Notes on Structured Programmingg,” structured Prognunmin g (London: Academic Press, 1972), p. 6.

‘zSee the discussion on the cleanroom method in appendix A

for more details.

g3Eastport Study Group Report, Op. cit., footnote 4.

“Ware Myers, “Can Software for the Strategic Defense Initiative Ever Be Error-Free?” IEEE Computer XX:61-67, November 1986.

‘

sDavid L. Parnas, “SDI Red Herrings Miss the Boat, ” (Letter to the Editor), 1lL!?lZ Computer 20(2):6-7, February 1987.
232

the desirability of trustworthy systems. One

possible reason may be that trust is determined

qualitatively as much as quantitatively: people judge by past experience and knowledge

of internal mechanisms as much as by numbers representing reliability. Another possible

reason is that most systems in critical applications are safeguarded by human operators.

Although the systems are trusted, the ultimate

trust resides in the human operator. Nuclear

power stations, subway systems, and autopilots are all examples.36

As noted in chapter 7, a BMD system would

leave little time for human intervention: trust

would have to be placed in the system, not in

the human operator. Accordingly, it is important to be able to evaluate the trustworthiness

of BMD software. One suggested definition is

that trustworthiness is the confidence one has

that the probability of a catastrophic flaw is

acceptably l0W.

37 Trustworthiness might be described by a sentence such as “The probability of an unacceptable flaw remaining after

testing is less than 1 in 1,000.”38 (This measure of trustworthiness has only recently been

suggested, and no data have yet been published to support it.) Estimating trustworthiness consists of testing the software in a randomly selected subset of the set of internal

states with a randomly selected subset of the

possible inputs. The set of possible inputs and

internal states must be known. It must be possible to recognize a catastrophic test result,

i.e., the expected operating conditions must

be well-understood. For BMD systems, this

means understanding the expected threat and

countermeasures as well as testing under conditions closely simulating a nuclear environment.

SeEven when hum~ operators are aware of a problem they

sometimes do not or cannot react quickly enough, or with the

proper procedures, to prevent disaster.

s~Da~d L. prom, “when Can We Tlllst $Oftw- SYS~mS’?”

(Kewote Address), Comput=Assuranm, Software Systems Integrity: Software Safety and Rvcess secun”ty Conference, July

1986.

~aDa~d L. Panas, person~ Communication, 1987.

Fault Tolerance

Realizing that errors in the code and unforeseen and undesired situations are inevitable, software developers try to find ways of

coping with the resulting failures. Software is

considered fault-tolerant if it can either prevent or recover from such failures, whether

they are derived from hardware or software

errors or from unanticipated input. Techniques

for fault tolerance include:

● back-up algorithms,

● voting by three or more different implementations of the same algorithm,

● error-recovery programs, and

● back-up hardware.

Program verification techniques, discussed

above, attempt to prove correctness by mathematical analysis of the code. In contrast, faulttolerant techniques attempt to cope with failures by analyzing how a program behaves during execution.

Since a BMD system would have to operate

under widely varying conditions for many

years, its software would have to incorporate

a high degree of fault tolerance. Unfortunately,

there are no accepted measures of fault tolerance, and design of fault-tolerant systems is

not well understood. As an example, space

shuttle flight software is designed in a way

thought to be highly fault-tolerant. Four identical computers, executing identical software,

vote on critical flight computations A fifth, executing a different flight program, operates in

parallel, providing a backup if the other four

fail. On an early attempted shuttle launch, this

flight system failed because the backup program could not synchronize itself with the four

primary programs. The failure, occurring just

20 minutes before the scheduled lift-off, caused

the flight to be postponed for a day. It was

a direct result of the attempt to make the software fault-tolerant.

The price for fault tolerance is generally paid

in performance and complexity. A program incorporating considerable code for the purpose

of detecting, preventing, and recovering from

failures will be larger and operate more slowly
233

than one that does not. A successful fault-tolerant design will result in a system with higher

availability than a corresponding system built

without regard for fault tolerance. Producers

of non-critical software may not care to pay

the price. Those concerned with critical systems that must operate continuously often feel

that they must.

Availability

Systems that are intended to maintain continuous operation are often evaluated by calculating their availability, i.e., the percent of

time that they are available for use. Availability is easily measured by observing, for some

interval, the amount of time the system is unavailable (the “down” time) and available (the

“up” time) and then calculating (up time)/(up

time + down time). As with other figures of

merit, availability figures are useful when the

conditions under which they were measured

are well-known. Extrapolation outside of those

conditions is risky. Since prediction of availability is equivalent to prediction of MTBF and

mean time to repair (MTTR)-measures of upand down-times, respectively—availability is

at least as difficult to predict as MTBF and

MTTR are individually.

Security

Computer users concerned with preserving

the confidentiality of data and the effectiveness of weapon systems, such as banks or the

military, consider security a necessary condition for dependability. Breaches of security

that concern such users include:

●

●

●

knowledge by an opponent of the algorithms implemented in a computer controlling a weapon system, allowing him

to devise ways of circumventing the strategy and tactics embodied in those algorithms;

access by unauthorized users to sensitive

or classified data stored in a computer;

denial of access by authorized users to

their computers, thereby denying them

the capabilities of the computer and the

data stored in it; and

● substitution of an opponent software for

operational software-(changing even a few

instructions may be potentially disastrous), allowing the opponent to divert the

computer to his own uses.

Many of the preceding concerns only apply

if a computer must use a potentially corruptible communications channel to receive data or

instructions from another computer or from

a human.39 Any BMD system would contain

such links. (The possibility that a link could

be corrupted and measures for preventing such

corruption are discussed in ch. 8.) Over these

channels one might:

●

●

●

●

load revised programs into the memories

of the BMD computers;

correct errors in existing programs;

change the strategy incorporated into existing programs; or

accommodate changes to software requirements, such as might be caused by

the introduction of new technology into

the BMD system.

In addition, any BMD architecture would

contain communications channels for the exchange of data between battle management

computers and sensors and among battle management computers.

Since the 1960s, when computers started to

be used on a large scale in weapon systems,

the DoD has expended considerable effort to

find ways of making computer systems secure.

As yet, no way has been found to meet all the

security requirements for computers used in

the design, development, and operation of

weapon systems. As Landwehr points out in

a discussion of the state of the art in developing secure software:

At present, no technology can assure both

adequate and trustworthy system performance in advance. Those techniques that have

been tried have met with varying degrees of

success, but it is difficult to measure their success objectively, because no good measures exSgPhY9ic~ secu~ty, i.e., control of physical access to comPuting equipment, is a problem as well, generally unsolvable by

technical means and outside the scope of this report.
234

ist for ranking the security of various systems.40

Although there is no quantifiable measure

of the security of a computer system, the DoD

has developed a standard for evaluating the

security of computer systems.41 The evaluation consists of matching the features provided

by a system against those known to be necessary, albeit not sufficient, to provide security.

For example, the second highest rating is given

to those systems that let users label their data

according to its security level, e.g., Confidential or Secret, then protect the labels against

unauthorized modification. Furthermore, the

developer must show the security model used

in enforcing the protection and show that the

system includes a program that checks every

data reference to ensure that it follows the

model. As with fault tolerance, incorporating

security features into software exacts penalties in performance and complexity.

Safety

A software engineering

guished between safety and

Safety and reliability are

journal distinreliability:

often equated,

especially with respect to software, but there

is a growing trend to separate the two concepts. Reliability is usually defined as the

probability that a system will perform its intended function for a specified period of time

under a set of specified environmental conditions. Safety is the probability that conditions

which can lead to an accident (hazards) do not

occur whether the intended function is performed or not. Another way of saying this is

War] Landwehr, “The Best Available Technologies For ~omputer Security,“ IEEE Computer, July 1983, p. 93.

“’’DoD Standard 5200.28, Department of Defense Trusted

Computer System Evaluation Criteria,” (Washington, DC: Department of Defense, Aug. 15, 1983).

that software safety involves ensuring that

the software will execute within a system context without resulting in unacceptable risk.42

Interest in software safety has increased

markedly in recent years. Formal publications

specifically addressing software safety issues

started appearing in the early 1980s.43 As yet,

there are no standard measures or ways of

assessing software safety. Nonetheless, it is

important that BMD software be safe so as

to prevent accidents that are life threatening

and costly. An unsafe BMD system might, for

example, accidentally destroy a satellite, space

station, or shuttle.

Appropriate Measures of Software

Dependability

As should be clear from the preceding discussion, software dependability cannot be captured in any single measure. Correctness,

trustworthiness, safety, security and fault tolerance are all components of dependability. All

should be considered in the development of

software for a BMD system. Attempts to

quantify them in a clear-cut way require specifying too many conditions on the measure to

allow useful generalization. Estimates of the

dependability of BMD software would always

be suspect, since in large part they would always be subjective. Until we can quantify software dependability we cannot know that we

have developed dependable BMD software.

The following sections discuss the factors involved in developing dependable software.

“IEEE Transactions on Software Engineering: Special Issue On Reh”ability And Safety In Real-Time Process Control,

SE-12(9):877, September 1986.

‘

sNancy Leveson and Peter Harvey, “Analyzing Software

Safety,” IEEE Transactions on Softwaml?ngineering, SE-9(?),

September 1983, pp. 569-579, was one of the first papers to discuss software safety.

CHARACTERISTICS OF DEPENDABLE SYSTEMS

Despite the lack of ways of quantifying con- with confidence that they will be reliable when

fidence in software, people trust many com- finished. In this section we discuss why sysputerized systems. Further, people are willing terns come to be trusted and give some examto undertake development of many systems pies of trusted systems. We divide methods
235

of gaining trust into two classes: those based

on observations of the external behavior of the

system, and those based on understanding how

the system operates internally.

Observations of External Behavior

A system, whether containing software or

not, may be considered to be a black box with

connections to the outside world. One may observe the inputs that are sent to the box and

the outputs it produces. The next few sections

discuss methods of gaining confidence in software and systems based on black-box observations of the software.

Extensive Use and Abuse

Perhaps the most important factor inspiring confidence in software is that the software

has been used extensively. A good analogy is

the automobile. Confidence comes from

familiarity with cars in general and frequent

use of one’s own car. Having seen that the engine will start when the key is turned hundreds

of times gives one the feeling that it will start

the next time the key is turned. Automatic

teller machines, electronic calculators, word

processors, and AT&T’s long distance telephone network are all examples of systems controlled by software that are trusted to work

properly. The trust is built on extensive experience: one has high confidence that the telephone will work the next time it is tried because it usually has in the past.

Confidence is considerably enhanced when

a system continues to work even though

abused. A car that starts on cold and rainy

days inspires increased confidence that it will

start on mild and sunny days. Observing that

calls still get through under heavy calling conditions (albeit not as quickly), that dialing a

non-existent number produces a meaningful

response, and that calls can still be made when

major trunk circuits fail boosts one’s confidence that nearly all one’s calls will get through

under normal conditions. Conversely, system

failure detracts from confidence. Having observed that issuing a particular command to

a word processor sometimes results in meaningless text being inserted into a document

leads one to refrain from using that command.

It is important to note that extensive confidence comes from extensive use and not from

testing that incompletely simulates use. No

one would consider granting a license to a pilot who had spent extensive time in a flight

simulator but had never actually flown an airplane. Simulated use inspires confidence to the

degree that the simulation approximates operational conditions. Real-world complications

are often either too expensive or too poorly

understood to simulate. In testing systems,

simulators are useful for convincing ourselves

that the gain from putting the system into its

operating environment is worth the attendant

cost and risks. They allow the jump to actual

use with some confidence that disaster will not

result.

Predictable Environments

Confidence in software also comes from being able to predict the behavior of the software

in its operational environment. If the environment itself is predictable, the job of designing

and testing the software is considerably eased.

For example, engineers can predict and mathematically analyze the number of telephone

calls per hour that a particular switching center will receive at any time of day. The number and type of signals that will be received

on the telephone lines (e.g., the 7 digits in a

local telephone number) are known because

their specifications form part of the requirements for the telephone system and are determined by the designers. The software and hardware may then be designed to cope with the

telephone traffic and the signal types based

on the specifications.

Engineers can observe the system in operation to verify predictions before new software

is placed into operation. Finally, they can observe the behavior of new software in terms

of number of calls handled, number of calls

rerouted, and other parameters for different

traffic loads. Observing that behavior matches

predictions builds confidence in the operation

of the system. Nonetheless, even when the developers have extensive experience with a well-
236

controlled environment, they sometimes make

mistakes in prediction and do not discover

those mistakes until the system goes into use.

Low Cost of a Failure

Although extensive use and environmental

predictability both strongly influence the

amount of confidence placed in a system, they

are not sufficient to induce users to continue

using a system after significant failures. Large,

complicated software systems inevitably experience software failures. Therefore, users

don’t have confidence in the software unless

the risk associated with a failure is smaller than

the gain from using the software. A word processor that loses documents may go unused because the cost of re-creating the document is

greater than the effort saved by using the word

processor.

If, however, an easy method of recovering

from such losses is available, perhaps by including a feature in the word processor that

automatically saves back-up copies of documents, then the cost to the user of the failure

becomes acceptably low: he can recover his document when it is lost. Similarly, the cost of

recovering from a disconnected phone call is

small to the dialer and to the telephone company. (Although a misdialed phone call is not

really a system failure, the same principle applies: users can recover quickly and easily.) The

ability to recover from a failure at low cost increases confidence in and willingness to use

a system.

Systems With Stable Requirements

A desire for flexibility is a prime motive for

using computer systems. The behavior of a

computer can be radically altered by changing its software. Radical changes may be made

to a computer program throughout its entire

lifetime. Because there is no apparent physical structure involved, the impact of change

may not be readily appreciated by those who

demand it without having to implement it. No

one would ask a bridge builder to change his

design from a suspension to an arch-supported

bridge after the bridge was half built without

expecting to pay a high price. The equivalent

is often demanded of software builders with

the expectation of little or no penalty in schedule, cost, or dependability.

An example is the combat system software

for the first of the Navy’s DD 963 class of destroyers. During the development of the software, which cost less than 1 percent of the cost

of building the ship, the customers imposed

major changes on the software developer. The

original requirements specified that the combat system need only provide passive electronic warfare functions. One year into development the buyers added a requirement for

active electronic warfare. A year later they removed the requirement for active electronic

warfare. On the ship’s maiden voyage its commander issued a casualty report on the software: the ship could not perform its function

because of deficiencies in the software. Although the major requirements changes were

probably not the only reason for the deficiencies, they were certainly a prime contributing

factor.

The B-lB bomber is another example of a

system where deficiencies have resulted from

too much change during development. According to a report on the B-l B bomber,

Defense officials blame many of the program’s problems on the decision to begin producing the aircraft at the same time that research and development efforts were under

way, forcing engineers to experiment with

some systems before they were completely developed. 44

Conversely, a system whose requirements

change little during the course of development

is more likely to work properly. Developers

have a chance of understanding the problem

to be solved: they need not continually reanalyze the problem and revise their solution. Stability of requirements is particularly important

for software because of the many decisions involved in software design. Each subdivision

of a program into subprograms involves decisions about the functions to be performed by

the subprograms and about the interfaces be44’’ New Weapon Suffers From Major Defects, ” Washington

Post, Jan. 7, 1987, p. Al.
237

tween them. Writing each subprogram further

involves decisions on the algorithm to be used,

the way data are to be represented, the order

of the actions to be performed, and the instructions to be used to represent those actions.

Decisions made early in the process are more

difficult to change than those made later in the

process because later decisions are often dependent on earlier ones. Furthermore, the process of change is more expensive in later phases

of a project because there are more specifications and other documentation. Using data

from SAFEGUARD software and software

projects at IBM, GTE, and TRW, one expert

has shown that, as a result of the preceding

factors, error correction costs (and costs to

make other software changes) increase exponentially with time. In Boehm’s words:

These factors combine to make the error

typically 100 times more expensive to correct

in the maintenance phase than in the requirements phase.45

Clearly then, for systems where requirements change little during development, not

only can one have increased confidence in the

software, but one can also expect it to cost less.

Among the developers of large, complex systems who attended OTA’s workshop on SDI

software, there was unanimous agreement that

BMD software development could not begin

until there were a clear statement of the requirements of the system.46

Systems Based on Well-Understood

Predecessors

As with other human engineering projects,

successful software systems are generally the

result of slow, evolutionary change. Where radical changes are attempted, failure rates are

high and confidence in performance is low. This

“Barry W. Boehm, Software Engineering llconom”cs (Englewood Cliffs, NJ: Prentice-Hall, 1981), figure 4-2, p. 40.

attendees at the workshop included software developers who

participated in the development of SAFEGUARD, Hard Site

defense, telephone switching systems, digital communication

networks, Ada compilers, and operating systems.

rule can be seen in endeavors such as bridge

building47 as well as software design.48

With the example and experience of a previous solution to a problem, a software developer

can have the confidence that a system to solve

a small variation on the problem can be correctly produced. The structure of the previous

solution and the associated algorithms may

be applied again with small variations. A good

example is the software used by NASA to compute the orientation of unmanned spacecraft.

The orientation, also known as attitude, is computed by ground-based computers while the

spacecraft is in operation. Attitude is determined from the readings of sensors on board

the spacecraft. The sensor readings are telemetered to earth and supplied as input to an attitue determination program for the spacecraft.

The algorithms for computing orientation are

well known and have been used many times.

The design of the attitude determination software that incorporates the algorithms is also

dependable.

The design of an attitude determination program for a new spacecraft starts with the design of an earlier program and consists of modifying the design to take into account sensor

and telemetry changes. Many of the subprograms from the earlier program are reused intact, some are modified, and some new subprograms are written. A typical attitude

determination program of this type is 50,000

to 125,000 lines of code in size and takes about

18 months to produce. It must be produced

before the launch of the associated satellite,

and must work when needed so that the satellite may be maneuvered as necessary. The de47As stated by Henry Petroski in To Engineer Is Human: The

Role of Failure in Successful Design, (New York, NY: St. Martin’s Press, 1985), p. 219.

. . . departures from traditional designs are more likely than not

to hold surprises.

‘Early compilers for the new Ada language have been so slow,

unwieldy to use, and bug-ridden that they have been worthless

for real software development. This situation has occurred despite the fact that compilers for older languages such as FORTRAN, for which there have been compilers since the mid-1950s,

are considered routine development tasks. The main contributing factors were the many features, especially the many new

features, incorporated into Ada.
238

veloper’s confidence in his ability to meet these

criteria is based on the success of the previous attitude determination programs.

The developers of the SAFEGUARD software believed they could solve the problem of

defending a small area from a ballistic missile

attack because similar, but somewhat simpler,

problems had been solved in the past. The history of missile defense systems can be traced

back to World War II anti-aircraft systems,

starting with the T-10 gun director. Next came:

the M-9 gun director, which ultimately attained a 90 percent success rate against the

V-1 flying bombs; the Nike-Ajax missile interceptor system; then, the Nike-Hercules, improved Hercules, Nike-Zeus, Nike-X, and Sentinel ABM systems. 49 Each system typically

involved some mission changes and a change

of one or two components over the previous

one. Although the last few of these were never

used in battle, constraining judgments of success in development, the evolutionary process

is clear.

Note that the evolutionary approach requires the availability of experience gained

from the earlier systems. Experience may take

the form of personal memories or of documentation describing earlier programs. In other

words, most of the problem must be wellunderstood and the solution clearly described.

As Parnas put it, following a series of observations on what makes software engineering

hard,

The common thread in all these observations

is that, even with sound software design principles, we need broad experience with similar

systems to design good, reliable software.50

Observations of Internal Behavior

The above approaches to gaining confidence

in software are based on observing the external behavior of the software without trying to

determine how it behaves internally. That is,

the software is tested by observing the effects

of executing computer programs rather than

the mechanisms by which those effects are

produced. The next few sections discuss methods based on observing the internal behavior

of programs-methods that may be called

“clear box” to denote that the internal mechanism used to produce behavior may now be

observed.

Simple Designs

It is not practical to give mathematical

proofs that software performs correctly. Given

a simple design and a clear specification of requirements, it is sometimes possible to give

a convincing argument that each requirement

is satisfied by some component of the design.

Similarly, a convincing argument can be given

that a simple design is properly implemented

as a program. As with reliability measures,

how convincing the argument is depends on

subjective judgment. Where only a weak argument can be given that the design properly implements the requirements and that the code

properly implements the design, there would

belittle reason to trust the software, especially

in its initial period of operation. As one expert

puts it,

. . . the main principle in dealing with complicated problems is to transform them into simple ones.51

Put another way, each complication in a design makes it less trustworthy. Simplicity, is,

of course, relative to the problem. The inherent complexity of a problem it may require

complex solutions. The designer’s job is to

make the solution as simple as he can. As Einstein said:

Everything should be as simple as possible, but no simpler.52

Disciplined Development

The software development process comprises a variety of activities. Describing software cost estimation techniques, Boehm iden4~he history of missile defense systems given here was supplied in a 1987 personal communication by Victor Vyssotsky,

responsible for development of SAFEGUARD software.

‘“David L. Parnas, op. cit., footnote 1.

51T.C. Jones, Design Metlxxfs, Seeds Of Human Futures, (New

York: John Wiley & Sons, 1980).

‘*Personal communication, P. Neumann.
239

tifies 8 different major activities occurring

during software development and 15 different

cost drivers.53 Other estimators use different

factors. (One early study introduced more than

90 factors influencing the cost of software development.) Fairley lists 17 different factors

that affect the quality and productivity of software.

54 There is general agreement that many

factors affect software development. There is

still considerable doubt over how to identify

the factors that would most significantly affect

anew project—particularly if there is little experience with the development environment,

the personnel involved, or the application. Appendix A describes the typical software development process and some of the complicating

factors.

Development of large, complicated software

must be a carefully controlled process. As the

size and complexity of the software increases,

different factors may dominate the cost and

quality of the resulting product. Based on personal observations, Horning conjectured that:

. . . for every order of magnitude in software

size (measured by almost any interesting metric) a new set of problems seems to dominate.55

Although it is early to expect an accurate

estimate of the size of BMD software, current

estimates of the size of SDI battle management software range from a factor of 2 to a

factor of 30 larger than the largest existing

systems (and the the accuracy of some estimates is judged to be no better than a factor

of 3).56 If Horning’s statement is correct, then

there is reason to suspect that currently unforeseen problems would dominate BMD software development. Solving these problems

“software EngixweringEcononu”cs, op. cit., footnote 45, p. 98.

“Richard Fairley, Sofiwam Engineering Concepts, (New York,

NY: McGraw-Hill, 1985).

“Jim Horning, “Computing in Support of Battle Managemerit, ’’ACM SIGSOFT Software En~”neering Notes 10(5):24-

27, October 1985.

“Barry Boehm, author of Soft ware En~”neering Econonu”cs,

and deviser of the most popular analytical software cost estimation model in use today, estimated, in a personal communication, that estimates of the size of SDI battle management

software with which he was familiar could easily be in error by

a factor of 3.

would add to the time and expense involved

in producing the software, and may undermine

judgments of its reliability.

The development process must be geared to

controlling the effects of the dominating factors. An example is the procedures by which

changes are made. Most software development

can be viewed as a process of progressive

change. At every phase, ideas from the previous phase are transformed into the products

of the current phase. For very small projects,

the changes may be kept in the mind of one

person. For moderately small projects, verbal

communication among the project members

may suffice to keep track of changes.

For larger projects, the number of people involved and the length of time of the project

require that changes be approved by small

committees and that written lists of revisions

be distributed to all project personnel at regular intervals. Revised products of earlier phases

are also distributed to those who need them.

For very large projects, formal change control

boards are established and all changes to baseline designs must be approved before they are

implemented. A library of approved documents

and programs is maintained so that all personnel have access to the same version of all

project products. The process of controlling

change becomes a source of considerable overhead, but is necessary so that all project members work from the same assumptions.

Factors Distinguishing DoD

Software Development

There are some similarities between DoD

and commercial software. The environments

where DoD uses software are also found outside of DoD. Commercial and NASA avionics

systems perform many of the same functions

as military avionics, and must also work in lifethreatening situations. Furthermore, the software must ultimately be produced in the same

form, i.e., as a computer program, often in the

same or a similar language for the same or a

similar computer. But the DoD development

process, as described in appendix A, is often
240

quite different from commercial software development.

Several factors, in combination, distinguish

DoD software from commercial software. Elements of all of these factors are found in commercial software applications, but the combination is usually not.

●

●

●

●

●

●

●

Long lifetime. Military command and control software often has a lifetime of 20 or

more years. The Naval Tactical Data System was developed in the early 1960s and

is still in use.

Embedded. New DoD systems must interface with other, existing DoD systems.

The interfaces are not under the control

of the developer, and the need for the interface was often not foreseen when the

existing system was developed. Commercial software developers are generally free

to develop their own interfaces, or build

stand-alone systems.

Operating in Real Time. Command and

control systems must generally respond

to events in the outside world as they are

happening. A delayed response may result

inhuman deaths and damage to material.

Life-critical. Command and Control and

weapon systems are designed to inflict

death or to prevent it from occurring.

Large. DoD systems containing hundreds

of thousands of lines of code are common.

The larger systems contain as many as 3

million lines of code.

Complex. Command and control systems

perform many different functions and

must coordinate the actions of a variety

of equipment based on the occurrence of

external events.

Machine-near. The programmers of command and control systems must understand details of how the computer they

are using works, how the equipment that

it controls works, and what the interface

between the two is. Many such details are

transparent to commercial programmers

because of the standardization of equipment, such as printers, for which already

existing software handles the necessary

details. The same is not true for new weap-

●

ens, sensors, and computer systems specially tailored to particular DoD applications. As an example, the computers used

on board the A-7 aircraft, in both the Navy

and Air Force versions, were designed for

that aircraft and rarely used elsewhere.

The use of non-standard equipment often

means that standard programming languages cannot be used because they pro

tide no instructions that can be used to

control the equipment. The current DoD

trend is toward standardization of computers and languages, but programmers

still must deal with specialized equipment.

Facing Intelligent Adversaries. DoD battle management and command, control,

and communications systems must deal

with intelligent adversaries who actively

seek ways to defeat them.

The DoD software development process is

often characterized as cumbersome and inefficient, but is a significant improvement over

the situation of the early 1970s when there was

no standard development process. It provides

some protection, in the form of required

documentation, against software that is either

unmaintainable or unmaintainable by anyone

except the builders. Minimal requirements for

the conduct of acceptance tests also provides

some protection against grossly inadequate

systems. Nonetheless, the process often still

produces systems that contain serious errors

and are difficult to maintain.57 58 The complexity of BMD software development would probably require significant changes in the process, both in management and technical areas.59

The Fletcher Study concluded that:

Although a strong concern for the development of software prevails throughout the civil

and military data-processing community,

S7For exmpla of problems in such systems ss the SGT YORK

Division Air Defense Gun, see ACM SIGSOFl’Software Engz”-

neering Notes, op. cit., footnote 20.

‘

nUpgrade of the A-7E avionics software, which is small (no

more than 32,000 instructions), but quite complex (to accommodate a new missile cost about $8 million).

‘

eAppendix A contains a further discussion of the DoD software development process and recent technical developments

that might contribute to improving it.
241

more emphasis needs to be placed on the specific problem of BMD:

● Expanded efforts to generate software development tools are needed.

● Further emphasis is needed on simulation

as a means to assist the design of battle

management systems and software.

● Specific work is needed on algorithms related to critical battle management functions.60

Improving the Process

Software development, a labor-intensive

process, depends for its success on many different factors. Improvements tend to come from

better understanding of the process. Furthermore, improvements tend to be made in small

increments because of the many factors influencing the process. To produce a system successfully requires, among other things:

● availability y of appropriate languages and

machines,

● employment of properly trained people,

• ood problem specification,

● stable problem specification, and

● an appropriate methodology.61

Current efforts in software engineering technology development concentrate on providing

automated support for much of the process.

Software engineering tools may contribute to

small incremental improvements in the process and the product. Such tools may help

programmers produce prototypes, write and

check the consistency of specifications, keep

track of test results, and manage development

Software Dependability and

Computer Architecture

Variations in computer design can have a

strong effect on the software dependability.

Some architectures are well-suited to certain

@OJ~es C. Fletcher, Study Chairman and Br@way McMillan, Panel Chairman, op. cit., footnote 11.

elIt is o~y k the l-t few years that the job tit]e ‘Jsoftwtire

engineer” has been used. There is no qualification standard for

software engineers, and no standard curriculum. Few universities or colleges yet offer an undergraduate major in software

engineering, and there is only one educational institution in the

country that offers a master’s degree in software engineering.

applications and make the job of developing

and testing the software easier. As an example, some computer systems allow programs

to act as if they each had their own copy of

the computer’s memory. This feature permits

several programs to execute concurrently without risk that one will write over another’s memory area. The computer detects attempts to

call on memory areas beyond a program’s own

and can terminate the program. The computer

provides the programmer with information

about where in the program the failure

occurred, thus helping him find the error. This

memory sharing technique makes the programmer’s development job easier and allows

the computer to be be used for several different purposes simultaneously.

Other systems permit the programmer to define an area of the computer’s memory whose

contents are sent at regular intervals to an external device. This feature could be used in conjunction with a display device to ensure that

the display is properly maintained without the

progr ammer having to write a special program

to do so. Such a feature simplifies the job of

developing software for graphics applications.

Also, at the cost of added hardware, it improves the performance of the computer system when used with graphic displays.

Features built into the computer may make

the software development job easier, the software more dependable, and the system performance better. The penalty for this approach

may be to make the computer designer’s job

harder and the hardware more expensive. Further, the gain in software dependability is, as

in many other cases, not quantifiable. Chapter 8 contains a more detailed discussion of

various computer architectures and their potential for meeting the computational needs

of BMD.

Software Dependability and

System Architecture

Just as an appropriate computer architecture may lead to improved software dependability, so may an appropriate system architecture. A BMD architecture that simplified
242

coordination and communication needs among

system components, such as different battle

managers, would simplify the software design

and might lead to improved software dependability. As with computer architecture, there

would be a penalty: decreased coordination

usually leads either to decreased efficiency or

to more complex components. The increase in

complexity is caused by the need for each component to compensate for the loss of information otherwise obtained from other components. As an example, if battle managers

cannot exchange track information with each

other, then they must maintain more tracks

individually to do their jobs as efficiently. They

may also have to do their own RV/decoy discrimination. Note that an architecture that requires exchange of a small amount of track information would be nearly as difficult to design

and implement as one that required exchange

of a large amount. The reason is that the communications procedures for the reliable exchange of small quantities of data are about

the same as those for large quantities.

The Eastport group estimated that for an

SDI BMD system the penalty for not exchanging track information among battle managers

during boost phase would be about a 20 percent increase in the number of SBIs needed.62

The improvement in software dependability

that might be obtained by architectural variations is not quantifiable.

Software Dependability and

System Dependability

It is desirable to find some way of combining software and hardware dependability measures. As indicated earlier, MTBF, a traditional

hardware reliability measure, is not appropriate as a sole measure of dependability of BMD

software. Certainly it will still be desirable to

measure hardware reliability in terms of

MTBF in order to schedule hardware maintenance and to estimate repair and replacement

62E&.port Study Group Report, op. cit., footnote 4. The m~Ysis and assumptions behind this claim have not been made

available.

inventory needs. The only components of both

hardware and software dependability for which

there may be some common ground for estimation are trustworthiness and availability.

However, there have been few or no attempts

to estimate trustworthiness for systems that

are composites of hardware and software.

In summary, there are no established ways

to produce a computer (hardware and software)

system dependability measure. Furthermore,

there are few good existing proposals for potential system dependability measures.

Software Dependability and the SDI

Although it is not possible to give a quantitative estimate of achievable software dependability for SDI software, it is possible to

gain an idea of the difficulty of producing BMD

software known to be dependable. We can do

so by comparing the characteristics of a BMD

system with characteristics of large, complex

systems that are considered to be dependable.

In an earlier section those characteristics were

described. We apply them here to potential

SDI BMD systems, using the architecture described in chapter 3 as a reference. Table 9-1

is a summary of the following sections. It

shows whether or not each characteristic can

be applied to SDI software, and provides a

comparison with SAFEGUARD and the AT&T

telephone system software, both often mentioned as comparable to SD I BMD software.63

SAFEGUARD and telephone system software represent different ends of the spectrum

of large systems that could reasonably be compared to SDI BMD systems. The telephone

system:

● is not a weapon system,

● has evolved over a period of a hundred

years,

egcf. Dr. Solomon Buchsbaum, Executive Vice President for

Customer Systems for AT&T Bell Laboratories and former chair

of the Defense Science Board and the White House Science

Council:

. . . most if not all of the essential attributes of the BM/CS system have, 1 believe, been demonstrated in comparable terrestrial systems.

S. Hrg. 99-933, op. cit., footnote 3, p. 275.

The system most applicable to the issue at hand is the U.S.

Public Telecommunications Network.
243

Table 9-1 .—Characteristics of Dependable Systems

Applied to SDI, SAFEGUARD, and

the Telephone System

Telephone

Characteristic SDI SAFEGUARD system

Extensively used & abused No No Yes

Predictable environment. No No Yes

Low cost of a failure No No Yes

Stable requirements. No Yes Yes

Well-understood predecessors No Yes Yes

Simple design ., Unknown 7 Yes

Disciplined development’ Unknown Yes Yes

SOURCE Of ftce of Technology Assessment 1988

• operates in a predictable environment

with well-understood technology,

● is kept supplied with spare hardware parts

that can be quickly installed, and

● is not designed to be resistant to an attack aimed at destroying it (although it

can be reconfigured in hours by its human

operators to circumvent individual damaged switching centers).

The SAFEGUARD system was a missile defense system that used well-understood technology, was never used in battle, would have

had to operate in an environment that was not

easily predictable, and was designed to make

its destruction by an enemy attack costly.

Several other systems lie within the spectrum defined by SAFEGUARD and the telephone system. Examples are NASA flight software systems, such as the Apollo and Space

Shuttle software, and weapon systems such

as AEGIS. All have some of the characteristics of BMD systems. Nearly all are autonomous within clearly defined limits, must operate in real time, and are large. Some that are

viewed as successful developments, such as

AEGIS, have only been used under simulated

and test conditions, but are thought to be sufficiently dependable to be put on operational

status.

None of the examples known to OTA have

been developed under the combined constraints imposed by SD I requirements, i.e., an

SDI system would have to:

● control weapons autonomatically;

● incorporate new technology;

●

●

●

●

●

●

be partly space-based, partly groundbased;

defend itself from active and passive

attacks;

defend against threats whose characteristics cannot be well-specified in advance;

operate in a nuclear environment, whose

characteristics are not well-understood;

be designed so that it can be changed to

meet new threats and add new technology;

and

perform successfully in its first operational use.

Even a system such as AEGIS, which is perhaps DoD’s most technologically advanced deployed system, was not developed under such

stringent constraints, and its success is not

yet fully determined.

Extensively Used and Abused

Although it might undergo considerable testing in a simulated environment, a BMD system cannot be considered to have been used

in its working environment until it has been

used in an actual battle. The working environment for a BMD system would be a nuclear

war. Thus, the first time it would be used would

also likely be the only time. In the telephone

system, components that are put into use even

after extensive testing often fail. A letter to

Congress from designers and maintainers of

AT&T Bell Laboratories switching systems

stated:

Despite rigorous tests, the first time new

equipment is incorporated into the telephone

network, it rarely performs reliably.

Adding new equipment is just the tip of the

iceberg; even the simplest software upgrade

introduces serious errors. Despite our best efforts, the software that controls the telephone

network has approximately one error for every

thousand lines of code when it is initially incorporated into the system. Extensive testing

and simulation cannot discover these errors.64

134A copy of the lettir also appears as “SDI Software, p~t

II: The Software Will Not Be Reliable,” Physics and Society,

16(2), April 1987.
244

Predictable Environment

Two aspects of the BMD battle environment

will remain unpredictable until the outbreak

of war. The first is the effect of the nuclear

background caused by the battle and the second is the type and extent of the countermeasures employed against the system. In contrast,

the telephone system environment is wellknown and predictable. Call traffic can be

measured and compared to mathematical models. Furthermore, much of the environment,

such as the signals used in calling, is controlled

by the designers of the system, so they are wellacquainted with its characteristics. Those who

seek to defeat telephone systems want to use

the environment for their own ends, and generally do not try to disrupt it. Therefore, although countermeasures are not all known in

type and extent, neither are they intended to

destroy the operation of the system.

Low Cost of a Failure

Software errors manifested as failures during a battle would not be repairable until after

the battle. Catastrophic failures could result

in unacceptably high numbers of warheads

reaching their targets; there is no way to guarantee or predict that catastrophic failures will

not occur. Even minor failures may result in

failure to intercept some enemy warheads,

causing loss of human life. Telephone switching centers experiencing catastrophic software

failures generally can be removed from service and the software repaired while calls are

rerouted. Minor failures are at most likely to

cause difficulties for a few subscribers.

Stable Requirements

As new threats arose, new strategies devised, new countermeasures found, and new

technology introduced, the requirements for

BMD systems would change, and change continually. Although some changes could be

planned and introduced gradually, changing

threats and, particularly, countermeasures

would impose changes beyond the control of

the system developers and maintainers. BMD

countermeasures are not subject to close scrutiny by the opposition, and new ones might

appear quickly, requiring rapid response. Because changes in threat and the development

of countermeasures would depend on Soviet

decisionmaking and technology, the rate at

which the U.S. would have to make changes

to its BMD software would partly depend on

Soviet actions. Delays in responding to countermeasures might have serious consequences,

including the temptation for the side that had

anew, effective countermeasure to strike first

before a counter-countermeasure could be devised and implemented.

Well-Understood Predecessors

Earlier BMD systems, such as SAFEGUARD, can be characterized as terminal or

late mid-course defense systems. The terminal and late mid-course defense part of an SD I

BMD system could benefit from experience

with these predecessors. There has been no experience, however, with boost phase and postboost phase, and little experience with early

mid-course defenses.65 They are new problems

that will take new technologies to solve. Most

demanding of all, a system to solve these problems must be trusted to work properly the first

time it is used. There have been approximately

100 years of experience with telephone switching systems. Each new system is a small

change over its predecessor. If a newlyinstalled switching system does not work acceptably, it can be replaced by its predecessor

until it is repaired.

Simple Design and Disciplined Development

Since the SDI BMD system has not yet proceeded to the point of a system design, much

less a design for battle management or other

software, one cannot judge whether or not the

eSThe Spatm fi99fle, US~ by SA F EG UAR D, cm be considered a late mid-course defense component. However, SAFEGUARD was designed to discriminate reentry vehicles from

aircraft, satellites, aurora, and meteors, but not from decoys

of the types expected to be available for use against BMD systems within the next 10-20 years. The only discriminators available to SAFEGUARD were phased-array radars. Potential countermeasures against modern BMD systems are discussed in

chapters 10 and 11, and discriminators in chapter 4. Options

considered for both include technologies considerably different

from anything available for or against SAFEGUARD.
245

design will be simple. Similarly, one cannot

judge whether or not the development process will be appropriately disciplined.

Development Approaches That

Have Been Suggested

In the middle ground between those who believe that an SDI BMD system could never

be made trustworthy, and those who are sure

that it could, are some software developers who

are unsure about the feasibility. The view some

of them take is that it would be worthwhile

to try to develop BMD software, given that

one were prepared to abandon the attempt if

the system could not be shown to be trustworthy. The approaches they suggest have the following characteristics:

● The purpose of the system would have to

be clearly stated so that the requirements

were known before development started.

●

●

●

●

The development would have to start with

what was best known, i.e., should build

upon the knowledge and results of earlier

U.S. efforts to build BMD systems.

The development would have to be

phased, so that each phase could build

upon the results of the previous one. The

system architecture would have to be consistent with such phasing.

Simulation would be needed at every

stage, and the simulations would have to

be extremely realistic.

Realistic tests would have to be Performed

at each stage of development:

Because failure is a clear possibility, those

who advocate this approach recognize that options to deal with the possibility must be left

open. If this approach were adopted, and failed,

the cost of the attempt, including maintaining other options, could be high.

SUMMARY

Estimating Dependability

Most of the indices of dependability for large,

complex software systems would be missing in

BMD software systems. In particular, the telephone switching system, often cited as an example of a large, complex system, is quite unlike BMD systems.

The characteristics associated with dependability in large, complex systems include:

●

●

●

●

●

●

●

a history of extensive use and abuse,

operation in a predictable environment,

a low cost of failures to the users,

stable requirements,

evolution from well-understood predecessor systems,

a simple design, and

a disciplined development effort.

The absence of many of these factors means

that technology beyond the present state of

the art in software engineering might have to

be developed if there is to be a chance of producing dependable BMD software. It might

be argued that such technology will be invented, but traditionally progress has been

slow in software engineering technology development. It appears that the nature of software causes progress to be slow, and that there

is no prospect for making a radical change in

that nature.

There is no highly reliable way to demonstrate

that BMD software would operate properly when

used for the first time. One of the long-term purposes of the National Test Bed is to provide

a means of simulating operation of BMD software after deployment. Such tests could simulate a variety of threats and countermeasures,

as well as the conditions existing in a nuclear

environment. On the other hand, actual environments often exhibit characteristics not

reproduced in a simulator. Simulations of battles involving BMD would have to reproduce

enemy countermeasures—a particularly difficult task. The usual technique for validating

simulations—making predictions based on the

simulation and then verifying their accuracy—

would be particularly difficult to use. This
246

would especially be true when one considers

the complexity of the atmospheric effects of

nuclear explosions and the speculation involved in determining countermeasures. Repeated failures in simulation tests would demonstrate a lack of dependability. Successful

performance in a simulation would give some

confidence in the dependability of the system,

but neither the dependability nor the confidence could be measured. Subjective judgments based on simulations would probably

be highly controversial.

Traditional Reliability Measures

Traditional measures of reliability, such as

mean time between failure, are insufficient to

characterize dependability of software. Appropriate software reliability measures have yet

to be fully developed. Furthermore, in the debate over BMD software dependability there

is often confusion over the meaning of reliability. Error rate, e.g., number of errors per

KLOC, is often misapplied as a definition of

software reliability. There is no single figure

of merit that would adequately quantify the

dependability of BMD software. A potentially

useful view is that dependability can be considered to be a combination of qualities such

as trustworthiness, correctness, availability,

fault tolerance, security, and safety. Unfortunately, there are no good ways of quantifying

some of these properties and dependability

would have to be a subjective judgment.

Technology for Preventing

Catastrophic Failure

OTA found no evidence that the software engineering technology foreseeable in the near future would make large improvements in the dependability of software for BMD systems. In

particular there would be no way to ensure that

BMD software would not fail catastrophically

when first used. It might be argued that the

most important part of dependability is fault

tolerance, and that there exist large, complex

systems that are fault-tolerant, such as the

telephone switching system. On the other

hand, the fault tolerance of such systems is

small compared to what would be needed for

BMD, since they are not under attack by an

intelligent. adversary interested in destroying

their usefulness. A further complication of the

argument over fault tolerance is that quantifications of software fault tolerance are not easily translated into measures of performance.

At the same time, there is no generally accepted subjective standard of fault tolerance.

Confidence Based on Peacetime

Testing

Confidence in the dependability of a BMD

system would have to be derived from simulated battles and tests conducted during peacetime. Getting a BMD system to the point of

passing realistic peacetime tests would most

likely require a period of stability during which

there were few changes made to the software.

Unfortunately, the system developers and

maintainers would have to respond to changes

in threats and countermeasures put into effect

by the Soviets. That is, the Soviets would

partly control the rate at which changes would

have to be made to the system. As changes

were made, the system would again have to

pass tests in order for the United States to

maintain confidence in it.

Accommodating Changes During Peacetime

Experience with complex systems shows

that changes eventually start introducing errors at a rate faster than they can be removed.

At such a point all changes must be stopped

and new software developed. The extent to

which changes could be made would depend

on the foresight of the developers during the

design of the software. The better the requirements were understood at that time, and the

better the potential changes were predicted,

the more the chance that the software could

accommodate changes as they occurred. The

appearance of an unforeseen threat or countermeasure, or simply the advent of new, unexpected technology, might require redevelopment of all or substantial parts of the software.

In a sense, the useful lifetime of the software

would be determined by how well the software

developers understood the requirements initially.
247

Establishing Goals and Requirements

Explicit performance and dependability

goals for BMD have not been established. Consequently, one cannot set explicit software dependability goals. Even when BMD goals have

been set, it will be difficult to derive explicit

software dependability goals from them; there

is no clear mapping between system dependability and software dependability. All agree

that perfect software dependability is unattainable. Only arguments by analogy, e.g., as

dependable as an automobile or telephone,

have been proposed. There is no common agreement on what the dependability needs to be,

nor how to measure it, except that it must be

high.

There is common agreement that standard

DoD procedures for developing software are

not adequate for producing dependable BMD

software in the face of rapidly changing requirements. There are few convincing proposals as yet on how to improve the procedures. The developers should not be expected

to produce an adequate system on the first try.

As Brooks says in discussing large software

systems:

In most projects, the first system built is

barely usable. It maybe too slow, too big, too

awkward to use, or all three. There is no alternative but to start again, smarting but

smarter, and build a redesigned version in

which these problems are solved. . . . all largesystem experience shows that it will be done.

Where a new system concept or new technology is used, one has to build a system to throw

away, for even the best planning is not so omniscient as to get it right the first time. . . .

Hence plan to throw one away: you will, anyhow.66

BMD software may be an order of magnitude larger than any software system yet

produced. Early estimates of software size for

projects are notoriously inaccurate, often by

a factor of 3 or more. Some argue that the use

of an appropriate systems architecture can

make SDI software comparable in size to the

largest existing systems. On the other hand,

none of the intermediate or far-term architectures yet proposed would appear to have this

effect, and previous experience with large software systems indicates that the size is likely

to be larger than current estimates. Such an

increase in scale could cause unforeseen problems to dominate the development process.

‘Frederick P. Brooks, Jr., The Mythica/ Man-Month; l?ssays

on Software Engin~ring (New

1975), p. 116.

SDIO INVESTMENT IN BATTLE MANAGEMENT,

TECHNOLOGY, AND SOFTWARE

Fork, NY: Addison-Wesl~y,

COMPUTING

SDIO’s battle management program serves

as the focus for addressing many of the communications, computing, and software technology problems discussed in chapters 7,8, and

9. Based on funding and project description

data supplied by SDIO, this section analyzes

how SDIO is spending its money to try to solve

these problems. The battle management program is organized into eight areas:

1. software technology program plan: developing and implementing a software technology program for the SDIO;

2. algorithms: development of algorithms for

solving battle management problems such

as resource allocation, track data handing over, discrumination, and coordination

of actions within a distributed system;

3. communications: identifying the require4.

5.

ments and technology for establishing a

communications system to link SD I components together into a BMD system;

experimental systems: proposing and

evaluating system and battle management architectures and the technologies

for implementing them;

networks: the design and development of

distributed systems and of communications networks that could be used to support BMD;
248

6. The National Test Bed: procurement of

hardware and software needed for the National Test Bed;

7. processors: development of computers

that would be sufficiently powerful, radiation-hardened, fault-tolerant, and secure

for BMD needs, and of the software required to operate them; and

software engineering the technology for

developing and maintaining software for

SDI, including techniques and tools for

requirements specification, design, coding, testing, maintenance, and management of the software life-cycle.

Table 9-2 is a snapshot of the funding for

these areas as of June 1987. Rather than showing the fiscal year 1987 SDIO battle management budget, it shows money that at that time

had been spent since the inception of the program, that was then under contract, or that

was expected soon to be under contract. It is

a picture of how the SDIO was investing its

money to solve battle management problems

over the first few years of the program. Not

shown is money invested by other agencies,

such as the Defense Advanced Research Projects Agency, in joint projects. The leverage

attained by SDIO in some areas is therefore

greater than might appear from the table.

The SDIO battle management program

clearly emphasizes experimental systems. Examination of the individual projects in this area

shows a concentration on the development and

maintenance of simulations and simulation facilities, such as the Army’s Strategic Defense

Command Advanced Research Center Test

Table 9-2.—SDIO Battle Management Investment

Funding Percent of

Area ($M) total

Software technology program plan . . 2.5 1

Algorithms. . . . . . . . . . . . . . . . . . . . . . . 25.3 9

Communications. . . . . . . . . . . . . . . . . . 8.1 3

Experimental systems . . . . . . . . . . . . . 117.5 42

National Test Bed . . . . . . . . . . . . . . . . 13.0 5

Networks . . . . . . . . . . . . . . . . . . . . . . . . 29.6 11

Processors. . . . . . . . . . . . . . . . . . . . . . . 47.1 17

Software engineering. . . . . . . . . . . . . . 32.8 12

Total . . . . . . . . . . . . . . . . . . . . . . . . . . 275.9 100

Bed, used to run battle simulations; on architecture analyses, such as the phase I and

II battle management/C3 architecture studies;

and on the first two Experimental Validation

88 (EV88) experiments.

The funding categories shown in table 9-2

permit considerable overlap; projects in each

category could easily be assigned to a different category. To try to draw clearer distinctions among categories and to try to identify

funding targeted specifically at the problems

discussed in chapters 7 through 9, OTA reorganized the funding data supplied by SDIO.

Table 9-3 shows just those funds aimed at exploring solutions to some of the more significant problems noted in chapters 7 through 9.

It does not include all funds shown in table

9-2, - - -

ing.

1.

2.

but does show percentages of total fundThe categories are defined as follows:

battle management and system simulations: the development of particular simulation algorithms or specialized hardware

for battle management simulations;

simulation technology development: the

development of the hardware and software

for bigger, faster simulations, and for improving techniques for evaluating the results of simulations;

automating existing software engineering

technology: the development of software

and hardware that would be used to improve the software development and maintenance process, which is now based on

existing manual techniques;

Table 9-3.—Funding for OTA Specified Problems

Funding Percent of

Problem ($M)

Battle management and system simulations . . . . . 42.5

Simulation technology development . . . . . . . . . . . . 35.7

Automating existing software engineering

technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2

Computer security. . . . . . . . . . . . . . . . . . . . . . 10,3

Communications networks ... , . . . . . . . . . . . . . 7,8

Software verification . . . . . . . . . . . . . . . . . . . . . 4.6

Fault tolerance (hardware and software) . . . . . . . . 3.1

Software engineering technology development. . . . 2.5

Total. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......120.6

15

13

5

4

3

2

1

1

44

a~rcentage of toul battle management funding, i.e., Of $275.9M

SOURCE: Office of Technology Assessment, 19S8; and SDIO. SOURCE Office of Technology Assessment, 1988, and S010
249

4.

5.

6.

7.

computer security: techniques for detecting and preventing unauthorized access

to computer systems;

communications networks: the organization of computer-controlled communications equipment into a network that could

meet SDI communications requirements;

software verification: the development of

practical techniques for mathematically

proving the correctness of computer

programs;

fault tolerance (hardware and software):

the development of hardware and software

that continues to work despite the occurrence of failures; and

8. software engineering technology developworks, fault tolerance, and new software engineering technology development.

CONCLUSIONS

1.

2.

ment: the development of new techniques

for improving the dependability of software and the rate at which dependable

software can be produced.

Table 9-3 shows that SDIO is investing considerably more in simulations and simulation

technology than any of the other problem areas

in battle management and computing identified by OTA. Of some concern is the smallness

of the investment in especially challenging areas

such as computer security, communications netBased on both the preceding analysis, and

the further exposition in appendix A, OTA has

reached eight major conclusions.

The dependability of BMD software would

have to be estimated subjectively and without the benefit of data or experience from battle use. The nature of software and our experience with large, complex software systems,

including weapon systems, together indicate

that there would always be irresolvable questions about how dependable the BMD software was, and also about the confidence to

be placed in dependability estimates. Political decision-makers would have to keep in

mind that there would be no good technical

answers to questions about the dependability of the software, and no well-founded technical definition of software dependability.

It is important to note that the Soviets

would have similar problems in trying to

estimate the dependability of the software,

and therefore the potential performance of

the system. Technical judgments of dependability would rely on peacetime tests that

would be unlikely to apply to battle conditions. Political judgments about the credibility of the defense provided would therefore rest on very uncertain technical grounds.

No matter how much peacetime testing were

done, there would be no guarantee that the

75-922 0 - 88 - 9

3.

4.

system would not fail catastrophically during battle as a result of a software error. Furthermore, experience with large, complex software systems that have unique requirements

and use technology untested in battle, such

as a BMD system, indicates that there is a

significant probability that a catastrophic failure caused by a software error would occur

in the system’s first battle.

It is possible that an administration and a

Congress would reach the political decision

to “trust” software that passed all the tests

that could be devised in peacetime, despite

the irresolvable doubts about whether such

software might fail catastrophically the first

time it was used in an actual battle. Such

a decision could be based upon the argument

that the purpose of strategic forces—even

defensive strategic forces—is primarily deterrence, and that a defensive system passing all

its peacetime tests would be adequate for deterrence. If deterrence succeeded, we would

never know, and never need to know, whether

the system would function in wartime.

The extent to which BMD software would differ from complex software systems that have

proven to be dependable in the past raises the

possibility that software could not be created

that ever passed its peacetime tests. This a

possibility exacerbated by the prospect of
250

5.

6.

changing requirements caused by Soviet actions. We might arrive at a situation in

which fixing problems revealed by one test

created new problems that caused the software to fail the next test.

No adequate models exist for the development, production, test, and maintenance of

software for full-scale BMD systems. Current

DoD models of the software life-cycle and

methods of software procurement appear inadequate for the job of building software as

large, complex, and dependable as BMD

software would have to be.

The system architecture, the technologies to

be used in the system, and a consistent set

of performance requirements over the lifetime

of the system must be established before starting software development.67 Otherwise, the

system is unlikely even to pass realistic

peacetime tests.

oTNotg that this does not preclude a phased system, with both

capabilities and requirements growing over time, provided that

the final architecture and final performance requirements are

clear before initial software development begins. Even then, considering the uniqueness of BMD defense, one would expect to

spend considerable time finding a workable design.

7.

8.

As the strategic goals for a BMD system became more stringent, confidence in one’s ability to produce software that would meet those

goals would decrease as a result of the increased complication required in the software

design. Even for modest goals, such as improved deterrence, the United States could

not have high confidence that the software

would not fail catastrophically, whether

faced with a modest threat or a severe

threat. Put another way, there is no good

way of knowing that BMD software would

degrade gracefully rather than fail catastrophically when called on to face increasing levels of threat. Current techniques for

identifying problems and detecting errors,

such as simulations, would not help, although they could help to reduce the failure rate. Furthermore, foreseeable improvements in software engineering technology

would not change this situation.

The SDIO is investing relatively small

amounts of money in software technology research in general, and in software engineering technology, computer security, communications networks, and fault tolerance in

particular. This investment strategy is of

some concern, since particularly challenging BMD software development problems

lie in these areas.
Chapter 10

Non-Destructive Countermeasures

to Ballistic Missile Defense

The classified version of this chapter is available in the classified version of

this report. As of this writing, the unclassified version continues to be withheld

from release by the Department of Defense.

CONTENTS

Introduction

Delivery Vehicles

Penetration Aids

Active Electronic and Electro-Optic Countermeasures

Conclusion
Chapter 11

Defense Suppression and

System Survivability

The classified version of this chapter is available in the classified version of

this report. As of this writing, the unclassified version continues to be withheld

from release by the Department of Defense.

CONTENTS

Introduction

Defense Suppression Threats

Survivability Measures To Counter Defense Suppression

Conclusions on Defense Suppression Threats and Survivability Options
Chapter 12

Defense Suppression Scenarios

The classified version of this chapter is available in the classified version of

this report. As of this writing, the unclassified version continues to be withheld

from release by the Department of Defense.

CONTENTS

Introduction

Survivability Scenarios—Phase-One and Phase-Two Architectures

Longer-term Architectures Survivability Scenarios –

Mutual Occupancy of Space by Comparable Ballistic Missile Defense Systems

Key Survivability Scenario Issues and Conclusions
Appendixes
Appendix A

Technology for Producing

Dependable Software

Introduction

Chapter 9 of this report often refers to the technology of specific phases in the software life cycle.

The application of such technology is known as

software engineering. This appendix describes the

state of the art in software engineering and

prospects for improvements in the state of that

art. It serves as a tutorial for those unfamiliar with

Department of Defense (DoD) software development practices. It provides supporting detail for

the discussions in chapter 9 of the technology available for producing dependable systems.

Origins of Software Engineering

The term “software engineering” originated in

1968. Around that time, computer scientists began to focus on the difficulties they encountered

in developing complicated software systems. A recent definition of software engineering is:

. . . the application of science and mathematics by

which the capabilities of computer equipment are

made useful to man via computer programs, procedures, and associated documentation.1

Another recent definition adds requirements for

precise management and adherence to schedule

and cost:

Software engineering is the technological and

managerial discipline concerned with systematic

production and maintenance of software products

that are developed and modified on time and within

cost estimates.2

The Institute of Electrical and Electronic Engineers (IEEE) defines software engineering as:

The systematic approach to the development,

operation, maintenance, and retirement of

software. 3

Before the late 1960s, managers paid little attention to the systematizing of software construction building. Most software systems were not

complicated enough to occupy large numbers of

people for long periods of time. Existing computers

‘Barry W. Boehm, Software Engineering Econom”cs (Englewood

Cliffs, NJ: Prentice-Hall, 1981), p. 16.

*Richard Fairley, Software Engineering Concepts, (New York:

McGraw-Hill, 1985), p. 2.

‘IEEE Standard Glossay of Software Enp”neering Terminology,

IEEE Standard 729-1983.

were not big or fast enough to solve very complicated problems:

As long as there were no machines, Programming was no problem at all; when we had a few

weak computers, Programming became a mild

problem, and now that we have gigantic computers, Programming has become an equally gigantic problem.4

Software engineering technology has improved

since 1972, but not as quickly as the capabilities

of computers. Studies in software engineering technology transfer show that ideas typically take 18

years to move from research environments, such

as universities and laboratories, to common uses

During this time, considerable experimentation

and repackaging occur.

Advances in software engineering technology

often take the form of better techniques for program design and implementation. Some techniques

demand no more than a pencil and paper and an

understanding of their concepts. Most, however,

become partially or fully automated. The form of

automation is generally a computer program,

known as a “software engineering tool. ” One example is the compiler-which helps to debug other

programs; another is a program that checks the

consistency of software specifications. Because

software engineering tools themselves take the

form of complex computer programs, they are subject to the typical problems involved in producing

complex, trustworthy software. This fact helps explain why software engineering technology lags

hardware engineering technology.

The trend toward use of software engineering

tools seems to be growing, as evidenced by such

projects as:

● DoD’s Software Technology for Adaptable,

Reliable Systems (STARS), whose purpose has

been to produce an integrated set of tools for

DoD software engineers;

4Edsger W. Dijkstra, “The Humble Programmer, ” Communications

of the ACM, 15(10):859-866, 1972.

‘William E. Riddle, “The Magic Number 18 Plus or Minus Three: A

Study of Software Technology Maturation, ” ACM Software Engineering Notes, vol. 9, No. 2, 1984, pp 21-37.

The figure of 18 years for technology transfer is consistent with other

engineering fields during periods of technological innovation, as analyzed in Gerhard O. Mensch, Stalemate in Technolo&Cambridge, MA:

Ballinger, 1979).

259
260

●

●

the Software Productivity Consortium,

formed to produce software engineering tools

for its client members, including many of the

nation’s largest Aerospace companies; and

the Microcomputer and Electronics Consortium, one of whose purposes is to produce better software engineering technology for its client members.

State of the Art in Software

Engineering

This section discusses the current state of the

art in software engineering technology. It considers the application of that technology to systematic approaches to software development. Finally,

it reviews recent proposals for improvements in

software engineering to aid in the development of

ballistic missile defense (BMD) software.

The Software Development Cycle

The process of developing and maintaining software for military use is described in DoD military

standards documents as the “software life cycle. ”

The description here is simplified for the purposes

of this report. The activities described are common

to nearly all DoD projects, though they vary in the

amount of attention paid to each, the products

produced by each, and the number and kind of

subactivities in each.

Furthermore, the initial set of activities described encompasses only development, up to the

point of acceptance of the system by DoD. Our

description lumps activities following development

into the category of “maintenance,” which is discussed in a separate section. Finally, the activities

described here generally conform to the model of

development set forth in the DoD Standard (No.

2167) for software development. Commercial development and advanced laboratory work may follow considerably different procedures (although all

tend to produce documentation similr in intent

to that described in the following sections).

Feasibility Analysis

The first phase of development is an analysis of

the DoD’s operational needs for the proposed system. This phase may start with a series of studies

of the feasibility of meeting those needs. The reported results of the feasibility studies are often

based on computer simulations of the situations

that the system would have to handle. The environment in which the system would operate may

be characterized in terms of quantifiable parameters. This process is analogous to telephone company analyses of the traffic load–the number of

calls per hour expected at different times of the

day, week, and year-to be placed on a new switching center. The feasibility analysis may be performed on a contract basis by systems analysts

who are not software engineers. In the case of the

Strategic Defense Initiative (SDI), systems

analysts who are familiar with BMD have done

much of the feasibility analysis as part of the competitive system architecture contracts (see ch. 3).

Feasibility analysis sometimes includes constructing a software prototype designed to investigate a few specific issues, for example, what the

mode of interaction between human and computer

should be or which tracking algorithms would work

best under different circumstances. Most system

functions are not implemented in prototypes. To

save time, the development of prototype software

does not follow the standard cycle. Software in the

prototype is not usually suitable for reuse in the

actual system. The prototype is usually discarded

at the end of the software development cycle.

The end-product of the feasibility analysis is a

document that describes what functions the new

system needs to perform and how it will work. This

report is sometimes called a “concept of operations” or an “operational requirements” document.

The operational requirements document will form

the basis for a request for proposals to potential

development contractors. Once the contract has

been awarded, the document will underlie the statement of requirements to be met by the system—

that is, the description of what the contractor must

build. The (SDI) battle management software development project is currently in the feasibility

analysis stage.

Software Requirements Analysis

and Specification

Software engineers enter the software development process at the software requirements analysis and specification phase. From their interpretation of the operational requirements document,

they write a requirements or performance “specification” for the software (including how it is to

interface with other systems). Upon government

approval, this specification document supplies the

contractual criteria of acceptability of the software

to the government. The specification differs from

the operational requirements as follows:

● It is more detailed than the operational requirements statement. Where the latter may
261

vaguely describe the functions to be performed, such as “simultaneously track 10,000

missiles, ” the requirements specification will

describe a sequence of subfunctions needed to

implement the operational requirement. The

description of each subfunction will include a

description of the input supplied to the subfunction, the output produced by it, and a brief

explanation of the algorithm by which the inputs are transformed into outputs.

For example, a subfunction of tracking may

be to update the track for a particular missile.

The inputs of that subfunction are the current

file of tracks and a new track report. Its output is an updated track file. The algorithm

might consist of the following steps:

—retrieve the existing track for the missile

from the track database;

—update the position, velocity, acceleration,

and time of last report components of the

missile track; and

—store the updated track entry back into

the database.

● It describes the interfaces between the new

system and all the other systems with which

it must interact. Sometimes these interfaces

are described in a separate document.

● It describes the interfaces to the hardware devices that the software must control, such as

weapons, or from which the software must obtain information, such as navigation devices

like inertial guidance units.

When the requirements specification is complete,

the government holds a “requirements review” at

which it decides what, if any, changes must be

made. The specification becomes a contractually

binding document and is passed on to the software

designers. The review is the last time at which requirements errors can be corrected cheaply: few

assumptions have yet been made about the way

the requirements will be implemented as computer

programs. Procedures known as “configuration

control” are established. These ensure that the

specification is not arbitrarily changed. The specification becomes the “baseline requirements specification. ” All further changes go through a formal

approval cycle, requiring the concurrence of a committee known as a “configuration control board. ”

Design

The purpose of the design phase is to produce

a “program design specification” of how a c om- puter program can be written to satisfy the requirements specification. The design specification usually describes the division of the software into

components. Each component may be subdivided

again, with the subdivision process eventually ending in subcomponents that can be implemented as

individual subprograms or collections of data. The

components resulting from the first subdivision,

sometimes called “configuration items, ” are used

to track the status of the system throughout its

lifetime.

The organization that emerges from the design

process is known as the structure of the software,

and the criteria used are called “structuring criteria.” The relationship among components depends on the criteria used in the subdivision process. For example, if the criterion for subdivision

is function, at the first subdivision a component

called “tracking” might be formed. At the second

level one might find tracking subdivided into functions such as “obtain object track” and “update

object track. ” Such a subdivision is called a “functional decomposition. ”

A very different criterion is type of change. At

the first level one might then see components such

as all decisions that will change if the hardware

changes and all decisions that will change if software requirements change. At the second level one

might find hardware decisions subdivided into decisions about sensor hardware, decisions about

weapons hardware, and decisions about computer

hardware. Such an organization is called an “information hiding decomposition. ”

The structuring criteria are key to understanding the design and the trade-offs it embodies.6

Those who use functional decomposition argue

that it results in software that performs more efficiently. Those who use information hiding argue

that it results in software that is easier to maintain because it is easier to demonstrate correct and

easier to understand. Other criteria optimize for

other factors, for example, fault-tolerance, security,

and ease of use. As might be expected, there is no

single criterion that simultaneously optimizes all

design goals.

An important purpose of the design specification is to describe the “interfaces” among components. The interfaces consist of the data to be

passed from one component to another, the se-

%ood designers find it useful to structure the design in several different ways to permit study of different trade-offs. For example, an

information-hiding decomposition is useful in optimizing for changes

later in the life-cycle of the software. A functional decomposition helps

ensure that all functions are performed and makes it easy to analyze

the efficiency of software. An important problem for the designer is to

be able to represent the different design structures so that they are consistent with each other.
262

quences of events to be used in coordinating the

actions of the components, and the conditions under which the components interact. Once the interfaces are established, individual design teams

may design each component. Since each component

may interact with several others, agreement on the

interfaces is crucial for effective cooperation among

the design teams.

Such agreement is equally important for those

who must later implement the design as a computer program. A mistaken assumption about an

interface will result in an error in the software and

a failure in the operation of the software. A change

in an interface requires agreement among all those

working on the interfacing components, and may

result in redoing weeks or months of work. Getting the interfaces right is generally agreed to be

the most difficult part of developing complex

software.

During the design process several reviews of the

design are held. The purpose of the reviews is to

ensure that the design is feasible and correctly implements the requirements. Early reviews on will

be a “preliminary design review.” When the designers feel that the design specification is sufficiently complete to be turned into computer programs, they hold a final, “critical design review. ”

Errors found at this point can still be corrected

relatively cheaply. Once they become embedded in

programs they are very much more difficult to find

and correct. Each design review results in changes

to the design specification. Once the changes are

completed, the design specification becomes the

basis for producing computer programs. It is then

placed under configuration control, much as the

approved requirements specification is.

Code

The process of translating the program design

specification into computer programs is known as

“coding.” By DoD policy, command and control

systems, weapons, and other software development projects must use a DoD standard programming language.7 Individual programmers work

from the design specification to implement the

components as computer programs. The more completely and precisely the components and their interfaces are defined, the less communication is required among the programmers. They can then

work independently, in parallel. An incomplete or

‘Current DoD policy mandates the use of Ada as a standard programming language, unless the developer of a system obtains a waiver. Prior

to the advent of Ada, each service had its own standard programming

language.

ambiguous specification requires the programmers

to make design decisions, often with incomplete

and unrecorded communication among each other.

Programming is writing instructions for a computer to perform a function described in the design specification. The instructions are packaged

together as a subprogram or a set of subprograms

that cooperate to perform the function. Before it

can be executed, a subprogram must be translated

by a compiler from the programming language into

machine language. Part of the programming job

is to devise and perform tests on each subprogram

to show that it works properly. A programmer usually goes through several cycles of writing, testing, and revising a subprogram before he is ready

to declare it finished. When a programmer is satisfied that his subprograms perform correctly, he

submits it to a test group.

Test

A separate group has the sole responsibility to

devise, perform, and report on the results of tests.

With no knowledge of the design, this group devises tests based on the requirements specification.

It sends components that fail tests back to their

developers with descriptions of failures and no attempts to diagnose the reasons for failures.

Test performance is the primary basis for confidence (or no confidence) that a system behaves as

it is supposed to. A variety of techniques tests

trustworthiness, fault-tolerance, correctness, security, and safety. It is during testing that components of the system first operate together and as

a whole. The following sections describe the steps

in the integration and testing process. At each

step, each of the reliability aspects maybe tested.

The test process resembles a reversal of the design process. Subprograms are first tested individually, then combined into components for integration tests. Components are integrated again

and tested as larger components, the process continuing until all components have been combined

into a complete system. To a large extent, integration testing may be thought of as testing the interfaces between components. It is in integration

testing that mistaken assumptions about how

other programs behave first manifest themselves

as failures.

Early tests in the process often include supplying erroneous input data to components or placing them under atypical operating conditions. Such

conditions might include heavy computational

loads or undesired events that, while abnormal,

might occur. Such “stress” tests are designed to

find out how fault-tolerant the system will be.
263

As integration progresses, the total number of

possible states of the formed component is the

product of the number of states of its constituents.

Combining component A with N states and component B with M states results in component C

with M times N states. If R tests were performed

on A, and T tests on B, combining each test of A

with each test of B would require performing R

times T tests on C. For large systems, it is impractical to perform the number of tests needed: at each

integration stage the number of tests performed

relative to the number of possible states becomes

quite small. The tests performed on the entire integrated system include only a few typical expected scenarios.

The integration procedure described above is

called “bottom-up,” since it starts on the lowest

component level and proceeds upward. A second

integration technique that is becoming more common starts with top-level components. It attempts

to test interfaces as early as possible. This technique requires the writing of dummy subcomponents that simulate only some of the actions of the

actual future subcomponents, but that use the

same interfaces. The dummies, called “stubs,” are

gradually replaced with the actual subcomponents

as system “top-down” integration proceeds.

When the developer deems the system ready for

delivery, a contractually-specified formal test procedure is performed to ensure that the system is

acceptable to the government. This “acceptance

test” consists of running several scenarios, and it

may test endurance and handling of stress. Acceptance tests are performed under government observation under conditions as closely approximating

real use as possible. If included, an endurance test

consists of continuous simulated use of the system for a minimum of 24 hours. Endurance tests

are important for systems that are expected to

operate continuously once placed in service. Once

a system has passed its acceptance tests, it is delivered to the government and enters the remaining phase of its life cycle, known as maintenance.

Despite elaborate test procedures, all complex

software systems contain errors when delivered.

As previously noted, software tests cannot be

exhaustive and cannot be relied upon to find all

errors. As an example, during the operational

evaluation of the AEGIS system on the U.S.S.

Ticonderoga, 20 target missiles were fired while

the ship was at sea under simulated battle conditions. Some of the target missiles were fired simultaneously into the area scanned by the AEGIS

combat system; thus, the 20 targets constituted

fewer than 20 scenario tests. The tests revealed

several software errors, costing approximately

$450,000 to fix.8

Since errors do remain in software after acceptance testing, the correction of errors continues as

a major activity after a system has been delivered

and put into use.

Maintenance

Unlike hardware, software contains no physical

components and does not wear out as a result of

continuing use. Maintenance is really a misnomer

when applied to software:

In the hardware world, maintenance means the

prevention and detection of component failure

caused by aging and/or physical abuse. Since programs do not age or wear out, maintenance in the

software world is often a euphemism for continued

test and debug, and modification to meet changing requirements.9

Errors emerging during system use must be corrected, and the system must be retested to ensure

that the corrections work properly and that no new

errors have been introduced. Correcting an error

may entail reanalyzing some requirements and doing some redesign; it almost certainly demands rewriting some code. Accordingly, all of the development activities also occur during maintenance.

Even when requirements appear to be complete

and consistent, as users gain experience with a system they may change their minds about the performance they desire from it. The automobile is a

case in point. Drivers’ behavior and expectations

about the performance of their vehicles changed

as new possibilities for travel emerged and as new

technology became available. Behavior also

changed as economic and political situations

changed. For example, wartime conditions affected

the cost and availability of cars and auto parts,

and oil production decisions affected the price of

fuel. Similarly, as writers have switched from typewriters to word processors, both their writing

habits and the features they expect in a word processor have changed.

Maintenance costs are now becoming the major

component of software life-cycle costs. Some data

show that by 1978, 48.8 percent of data processing costs were spent on maintenance activities.10

‘Discussion of the results of the Ticonderoga operational evaluation

testsmaybe found in U.C. Congress, Senate Committee on Armed Servicesthe record of the hearings before the Committee on Armed Services, United States Senate, 98th Congress, second session on S.2414

part 8, Sea Power and Force Projection, Mar. 14, 18, 29, Apr. 5, II,

May 1, 1984.

“D. David Weiss, U.S. Naval Research Laboratory, ‘‘The MUDD

&port: A Case Study of Navy Software Development Practices, ” NRL

Report 7909, May 1975.

IOBoehm, Softwsre Engineering Econo~”cs, op. cit., footnote 1, figure

3-2, p. 18.
264

New technology, new strategies, and new computational algorithms would cause envisioned BMD

systems to evolve over many years, Their longlifetime, complexity, and evolutionary nature will

magnify the general trend towards relatively larger

software maintenance costs.

Interaction Among the Phases

In the preceding sections the different phases of

the software life cycle are described as if they occur in a strict sequence. In fact, there is considerable feedback among the phases. Changes in requirements, design, and code occur continually.

Large systems may be subdivided into subsystems, for each of which there is a separate requirements specification and a separate development

cycle. These separate developments may proceed

in parallel or sequentially.

The first planned delivery in a sequential development is called the initial operating capability

(IOC). Sequencing the delivery of different versions

of the system over time permits faster delivery of

some capabilities, but it introduces additional problems into all of the development phases. The system must be designed so that added capabilities

do not require large changes to existing design or

code. In particular, interfaces must be designed to

take into account potential future changes in capabilities. This is another example of a problem in

large, complex systems that does not occur in small

systems, where the entire system is delivered at

once.

The requirements analysis and design phases

usually consume about 40 percent of the development effort, the coding phase about 20 percent, and

the testing phase about 40 percent. For long-lived

systems, the maintenance phase consumes 60-80

percent of the total lifecycle cost. For this reason,

the trend in large, long-lived systems is to try to

develop the software so as to make the maintenance job easier. Since the principal activity in

maintenance is change, an important development

consideration is how to make change easier.

Software Engineering Technology

Software engineering technology research and

development tend to focus on particular phases of

the software life cycle. For example, work on improving design techniques is often independent of

work on improving techniques for translating highlevel languages to machine code. One reason is that

the different phases present very different problems for the software engineer.

The next few sections briefly describe the state

of the art, the state of the practice, and the direction in which software technology is currently moving, particularly within DoD. Much of the work in

the last few years has concentrated on creating

automated support for software development techniques. Such support usually consists of one or

more programs, called tools.ll

Constructing Prototypes

During the feasibility analysis and requirements

specification stages, software engineers sometimes

quickly produce prototypes to help feasibility analysis and to explore different ways in which users

might interact with the completed system. Such

rapid prototype are intended to allow exploration

of only a few issues: they are not intended to be

models of the final software. They are often executed in different languages, on different computers, and using a different development process

than the final software. Usually less than 10 percent of development effort is spent on such prototypes.

Sometimes software engineers do full prototyping. Full prototyping means building a complete

prototype system and then discarding it. This approach has been advocated by Brooks:

Inmost projects, the first system built is barely

usable. It may be too slow, too big, too awkward

to use, or all three. There is no alternative but to

start again, smarting but smarter, and build a redesigned version in which these problems are

solved. . . . all large-system experience shows that

it will be done. Where a new system concept or new

technology is used, one has to build a system to

throw away, for even the best planning is not so

omniscient as to get it right the first time....

Hence plan to throw one away: you will, anyhow.”

Specifying Requirements

Software requirements specifications constitute

an agreement between the customer and the software developers. In stating what the software

must do, the specifications must be unambiguous,

precise, internally consistent, complete, and cortlRather th~ trying to describe even the few most notable example%

this appendix just indicates general trends. For a more detailed survey

of software technology that maybe applicable to SDI, the reader should

see the following Institute for Defense Analysis report on the subject

prepared for the SDIO: Samuel T. RedWine, Jr., Sarah H. Nash, et al.,

SDI Prelirnimuy Software Technology Integration Plan, IDA paper P1926, July 1986.

1~Frederi& p. Br~ks, Jr., The Mythical lkf~-lfonth: ESSZ?YS On Software En~”neering (New York, NY: Addison-Wesley, 1975).
265

rectly representive of the customer’s desires and

developers’ intent. By systematizing the process,

methodologies for analyzing and expressing requirements are intended to help the analyst be

complete, consistent, and clear. Some tools partly

automate the process—sometimes by providing

mechanisms to support a particular methodology,

sometimes just by providing storage and retrieval

of documentation.

Before 1975, nearly all requirements were manually produced. Although they might contain

mathematical equations, they used no formalisms

or notations tailored to the job of writing software

specifications. The underlying methodology focused on describing functions the software had to

perform and specified the input to and the output

from each function. By about 1980, two or three

new methodologies had appeared, incorporating

novel methods of decomposition and corresponding notations and formalisms.

Also appearing were several tools representing

somewhat clumsy attempts to automate the processes of storing and retrieving requirements specifications and of performing internal completeness

and consistency checks. More advanced tools have

added some simulation capability, enabling the requirements analyst to run a simulation of his system based on the description stored by the tool.

The early tools enjoyed a brief popularity that has

not been sustained.

Recent technology includes attempts to combine

automated support for methodologies with microcomputer systems, resulting in so-called “softwareengineering workstations. ” Simpler workstations

may use word processors to automate the text

maintenance and production process. More complex workstations use document control systems

on minicomputers to manage the entry, maintenance, and production of requirements documentation; such systems may feature version control

and graphics support. The more tedious jobs of producing and maintainingg text have been automated,

but the more difficult jobs of assuring that requirements are complete, consistent, and feasible to implement have not yet been much affected.

Design

Design technology is in a similar state to that

of requirements specification. There is still little

agreement on the appropriate techniques for representing and specifying designs. A few design

methodologies have become popular in the last 10

years, and there are a few supporting tools that

help the designer. There are a few serious attempts

to integrate requirements specification and design

support technology, but they have not been very

successful. DoD has concentrated on finding design techniques that are compatible with Ada (the

recently adopted standard DoD programming language), then developing tools that support those

techniques.

Recent software design technology is on a par

with requirements specification technology: the development of workstations and personal computer

tools aimed at supporting the designer’s job has

followed the development of similar tools on larger

computers.

Validating and Verifying

Requirements and Design

According to one estimate, errors in large projects

are 100 times more expensive to correct in the

maintenance phase than in the requirements

phase.19 Supporting data suggest that the relationship between the relative cost to fix an error to the

phase in which the error is detected and corrected

is exponential. Accordingly, products of the software development phases undergo some kind of

validation and verification several times during

each phase and at the end of each phase.

Although simulation is used to verify the results

of feasibility and requirements analyses, much of

the verification and validation of requirements and

design is done by review. A review is a laborintensive process. Users, designers, system engineers, and others scrutinize a specification for errors, usefulness, and other properties. Then, in a

series of meetings, they discuss comments and objections to the specification. There is little automated support for reviews, and there have been

few advances in the past 10 years in the way they

have been conducted. Although many of the clerical aspects of such reviews are ripe for automation,

the more difficult parts are likely to remain highly

labor-intensive.

Coding

Coding activities generally consume about 10-

20 percent of the effort in large-scale software development, but they have been more highly automated than any other part of the process. Perhaps

the largest advance was the development of compilers that translate high-level languages into sequences of machine instructions. In addition, there

is a continuing stream of new tools that help the

coder to enter, edit, and debug his code.

ljB~hm, ~ftwue Enp”neering Economics, op. cit., footnote 1, P. 40.
266

Advanced coding tools include editors, compilers, and debuggers that:

● incorporate syntactic knowledge of the language being used,

• allow the programmer to move freely between

editing and debugging programs, and

• provide him with powerful means of browsing

through the text of a program and analyzing

the results of its execution.

Such tools generally reside either on a time-shared

computer or on a workstation that is at the sole

disposal of the programmer.

Current practice varies widely, from compilers

used on batch machines (i.e., noninteractively, with

little or no editing or debugging tools and with

programmers relying principally on printouts for

information), to state-of-the-art systems.

Showing Correctness and Utility of Code

Because code is the means of directing a computer’s actions, it is the realization of the requirements and the implementation of the design. Although earlier stages in the development process

might conceivably be reduced in scope and effort—

or even eliminated-code to implement the system

must still be written. To show that it is the accurate

realization of the desired system, the code must

be demonstrated to execute correctly and usefully.

Technology to support such demonstrations has

followed several different approaches.

The traditional approach is to test the software

over a range of inputs that are deemed adequate

to demonstrate correctness and usefulness. (The

criteria for adequacy are generally determined by

those responsible for accepting the software as adequate.) Testing technology is discussed in the next

section.

Code reviews, similar to design and requirements

reviews in structure, function, and labor-intensiveness, are also generally used during the coding

process to find errors. As with other reviews, the

nonclerical aspects of the process are unlikely to

be automated.

Correctness proofs based on mathematical techniques are discussed in chapter 9 of this report.

Although work in automating proofs of program

correctness and finding and applying techniques

that work for large programs started about 20

years ago, the technology is still inadequate for

large, complex programs. There are no current

signs of ideas that may lead to rapid progress.

Testing

Although there are different types of testing for

different situations, the principles underlying

different tests are the same: the program is executed using different sets of inputs and its behavior, particularly its output, is observed. Testing technology has advanced to the point where

test inputs can often be automatically generated,

test output can be automatically compared with

desired output, and the parts of the program that

have been executed during the test can be automatically identified. As with coding, the current

practice varies widely. For simple, noncritical systems, none of the process maybe automated. For

critical systems, considerable investment is often

made in automating tests. For such systems, it is

often important that test results be made visible

and understandable to nontechnical users. As an

example, elaborate computer-driven simulations

are used in pre-flight testing of aircraft flight software. A pilot can test the behavior of the flight

software without any knowledge of the code.

Integrated Support

Early tools designed to support software development or maintenance were aimed at solving specific problems, such as translating high-level languages to machine instructions, and were designed

either to work alone or in cooperation with one or

two other tools. Requirements and, particularly,

design support tools did not interface well with coding and testing tools. More recent attempts at automating the software development and maintenance processes are aimed at developing software

engineering environments: tools that are compatible with each other, that make it easy for the software engineer to switch his attention among different tasks in different phases of the software life

cycle, and that support the entire software life cycle. Such environments are still in the development

stage.

In recent years, efforts to provide automated

management support have appeared. Such support

might consist of providing an automated database

containing information about the progress of a

software development or maintenance project. Efficient tools for providing integrated management

support should be appearing on the market shortly.

One area that has enjoyed automated support for

some time is change control, that is, keeping track

of changes that have been proposed and made to
267

a system during its lifetime, long recognized as an

important management need. Automated support

systems designed just for change control have been

available for at least 10 years.

Incremental Development

To avoid the problems associated with attempting to develop a large, complex system at one time,

an incremental development technique is often

used. Systematic approaches for incremental development have been described in the literature for

more than 10 years; example variations are iterative enhancement, and program family development. 14 More recently, incremental development

has been incorporated into a risk-based approach

to development called the spiral approach.15 In this

approach, each developmental increment is accompanied by risk analyses. When deemed worth the

risk, a complete development cycle, which maybe

similar to the one described in the preceding, is

used. Incremental development has been used by

DoD in a variety of forms for a number of years,

and should not be expected to result in a major

improvement in software dependability or productivity.

Other Paradigms

The preceding discussion is oriented towards the

standard DoD software life cycle. Other paradigms

for the software life cycle have been suggested.

Some expand on the life cycle, such as a recently

proposed model by Boehm that incorporates riskassessment and incremental development. Others

attempt to eliminate or merge existing steps, such

as object-oriented programming using languages

like Smalltalk that lend themselves to rapid

change. Some introduce new or improved technology to change the nature of existing steps, such

as the Cleanroom method.

Object-Oriented Programming

Object-oriented programming is based on several different ideas that are used differently by advocates of the technique.

“V.R. Basili and A.J. Turner, “Iterative Enhancement: A Practical

Technique for Software Development, ” IEEE Transactions on Software

Engineering SE-1(4):390-396, December 1975. See also D. L. Parnas,

“Designing Software for Ease of Extension and Contraction, ” IEEE

Transactions on Software Enp”neering 5(2), March 1979.

“B. Boehm, TRW Corp., A Spiral Model of Software Development

and Enhancement, TRW technical report 21-371-85, 1985.

The term object-oriented programming has bee n

used to mean different things, but one thing these

languages have in common is objects. Objects are

entities that combine the properties of procedures

and data since they perform computations and

save local states.16

In many versions of the object-oriented model,

the role of formal requirements and design specification is reduced in favor of quickly producing

different versions of a program until one is attained

that exhibits the desired behavior. Although this

technique appears to work well on a small-scale,

it has yet to be tried on large-scale programs that

require the cooperation of many programmers and

that are to be long-lived. Most likely, some of the

ideas and tools that facilitate change in languages,

like Smalltalk, will be incorporated into the software engineering environments under development

for the standard DoD life cycle, where they will

help make a modest improvement in productivity.

The Cleanroom Method

The Cleanroom method is an approach to software engineering recently developed at IBM.17 The

method requires programmers to verify their programs, using mathematically-based functional verification methods developed at IBM. Programmers

are not permitted to debug or test their own programs; testing is done by a separate test group.

Furthermore, the test process is based on statistical methods that permit statistically valid estimates of mean time to failure to be calculated from

test results.18 Reported Cleanroom experience includes three projects, the largest containing 45,000

lines of code. There is no reported accumulated

operational experience with software developed

using this technique. Proponents believe the technique will scale up to programs of the size and complexity needed for SDI.

Automatic Programming

In another suggested paradigm that would eliminate much of the requirements and design phases,

programs would be automatically generated

directly from a requirements specification language that might read like a mathematized version of English. This idea is not new; the term auto1mM~k s~fik ~d Dtiel G. Bobrow, “Object-Oriented programming:

Themes and Variations,” The AI Magszine 6(4):40-62, winter 1986.

ITH~l~ D. Mfi, “Cle~wm Software Engineering, ” to be published

in IEEE Transactions on Software Engineering

‘*P. Allen, Michael Dyer, and D. Harlan, “Certifying the Reliability

of So ftware, ” IEEE Transactions on Software Engineering, SE-12(l):3-

11, January 1986.
268

matic programming was applied in discussions of

programming languages as early as 1948.19 As programming languages became more powerful, the

level of expectation for automatic programming

rose. The technology to implement this paradigm

in such a way that design specifications, as now

used, would be unneeded, is still well beyond the

state of the art.

Artificial Intelligence

Artificial intelligence (AI) is sometimes asserted

to be a technology that would be needed to build

the software for an SDI BMD system. In a critique

of AI as applied to SDI, David Parnas points out

two different definitions of AI that are currently

used:

AI-1: The use of computers to solve problems

that previously could only be solved by applying

human intelligence.

AI-2: The use of a specific set of programming

techniques known as heuristic or rule-based programming. In this approach, human experts are

studied to determine what heuristics or rules of

thumb they use in solving problems. Usually they

are asked for their rules. These rules are then encoded as input to a program that attempts to behave in accordance with them. In other words the

program is designed to solve a problem the way

that humans seem to solve it. 20

Much of the investment in AI technology today

seems to be based on AI-2. The result is likely to

be several systems that work well in limited applications where the rules for solving a problem are

well-known, relatively few in number, and consistent with each other. For battle management and

other complex SDI computing problems, such an

approach is unlikely to apply: the rules for conducting a battle in space against an opponent, who may

use unforeseen strategy or tactics, are not well

known.

Since AI-1 may be considered as a set of problems, such as writing a computer program that can

translate English to Russian, it cannot be truly

characterized as having an underlying technology.

Solutions to such problems may or may not use

AI-2, or any other technology. Accordingly, the

state of the art in AI-1 can only be considered on

a problem-by-problem basis, and a technological

assessment cannot be made. Since it lacks a unifyISFOT a discussion of automatic progr amming, See David L. Parnas,

“Can Automatic Progr amming Solve The SDI Software Problem, ” in

“Software Aspects of Strategic Defense Systems,” American &“entist,

September-Oct&er 1985, pp.432-40.

*“David L. Parnas, “Artificial Intelligence and the Strategic Defense

Initiative, ” ibid.

ing concept or technology, there is not much sense

in talking about “applying’ AI-1 to SDI battle

management problems until a specific set of battle management ’problems and their solutions is

specified.

Technology Summary

Much of the current software technology work

may be viewed as consolidation: the development

of tools to support existing methodologies. This

view is especially true for DoD, whose recent software technology investments are aimed at providing automated support for software to be developed in the Ada language. Both within and without

DoD, particular emphasis is being given to software engineering environments: tools that are compatible with each other, that make it easy for the

software engineer to switch his attention among

different tasks in different phases of the software

life cycle, and that support the entire software life

cycle. This emphasis is likely a result of a growing

recognition by software engineers that although

they have spent considerable time helping to automate other industries, they have been slow to

automate the software development and maintenance industry.

The difficult problems of how to go about creating, analyzing, specifying, and validating requirements and design, and validating that implementations satisfy requirements, are still open research

problems on which progress is slow.

Measuring Improvement

Because the quality of software depends so

strongly on the quality of the software development process, both the process and the product

need to be measured to understand where process

improvements are needed and what their effect is.

As previously noted, increases in product scale result in a shift in the factors determining success.

Measurements made on small scale developments

cannot be generalized to large scale developments.

As a result, laboratory-style measurements are of

little help in trying to determine the factors affecting the development of BMD software. To be useful, measurements must be made of the actual production process, with the attendant risks of

affecting the process. Since data from such measurements gives considerable insight into the practices used by a company, it is considered by most

companies to be sensitive and is rarely available

for study outside of the company. As a result, there

is little to chance to separate the effects of differ-
ent factors by comparing data from different development environments. Outside of internal company studies, the few studies of software available

from measurements in production environments

come either from NASA’s Software Engineering

Laboratory, or from the Data and Analysis Center for Software, supported by the Rome Air Development Center.

Scaling up to the size estimated for the SDI battle management software means that new developmental problems will be encountered and that

existing measurements will not apply well. Esti269

mates for the size of the SDI battle management

software range from 7 million lines of code to 60

million lines of code, depending on the estimator

and system architecture. The largest operational

systems today that could be said to be similar to

BMD systems contain about 3-4 million lines of

code. 21

“The AEGIS software is in this category. The software for AT&T’s

5ESS TM switching system, although not a good model for BMD software, is also in this size range. The SAFEGUARD system, not currently

operational, was slightly smaller.
Appendix B

Glossary

Glossary of Acronyms and

Abbreviations

ABM —Anti-ballistic Missile

ALS –Advanced Launch System

AOA –Airborne Optical Adjunct

AOS —Airborne Optical System

ASAT –Anti-satellite Weapon

ATA –Advanced Test Accelerator

ATH –Above The Horizon (Sensor)

ATS –Advanced Test Stand (particle

beam accelerator)

BM/C3–Battle Management/Command,

Control, and Communications

BMD —Ballistic Missile Defense

BMEWS –Ballistic Missile Early Warning

System

BSTS –Boost Surveillance and

Tracking System

CCD –Charge-coupled Device

CELV –Complementary Expendable

Launch Vehicle

–Centimeter

CONUS –Continental United States

CV –Carrier Vehicle

DANASAT –Direct Ascent Nuclear Antisatellite Weapon

DANNASAT —Direct Ascent Non-nuclear AntiDARPA

DEW

DF

DIPS

DoD

DSAT

DSP

DST

EML

EMP

ERINT

ERIS

ETA

eV

FBB

FEL

FLAGE

FLIR

270

satellite Weapon

–Defense Advanced Research

Projects Agency

–Directed-Energy Weapon

–Deuterium Fluoride (Laser)

–Dynamic Isotope Power System

–Department of Defense

—Defensive Satellite Weapon

–Defense Support Program (U.S.

Military Satellite)

–Defense Suppression Threat

–Electromagnetic Launcher

–Electromagnetic Pulse

–Extended Range Interceptor

–Exe-atmospheric Re-entry

Interceptor System

–Experimental Test Accelerator

–Electron Volt

—Fast-burn Booster

—Free Electron Laser

–Flexible, Lightweight, Agile,

Guided Experiment

–Forward Looking Infrared

(Sensor)

FLOPS

FOC

FSED

FTV

g

GBFEL

GOPS

GSTS

HALO

HEDI

HF

HLLV

HOE

IBC

ICBM

IOC

ISAR

J

KEW

kg

kJ

KKV

km

kW

LAMP

LCC

LEAP

LOC

LWIR

m

MaRV

MeV

MHD

MIPS

MIRACL

MIRV

MOPS

MOSFET

–Floating Point Operations per

Second

–Full Operating Capability

–Full-scale Engineering

Development

–Flight Test Vehicle; Functional

Technical Validation

–Gram

—Ground-based Free Electron

Laser

–Billion (Giga-) Operations per

Second

–Ground-based Surveillance and

Tracking System (Pop-up Probe)

—High Altitude Large Optics

—High Endo-atmospheric Defense

Interceptor

–Hydrogen Fluoride (Laser)

–Heavy Lift Launch Vehicle

–Homing Overlay Experiment

–Impurity Band Conductor

–Intercontinental Ballistic

Missile

–Initial Operating Capability

—Inverse Synthetic Aperture

Radar

–Joule

—Kinetic Energy Weapon

–Kilogram

–Kilojoule

–Kinetic Kill Vehicle

–Kilometer

–Kilowatt

—Large Advanced Mirror

Program

–Life Cycle Cost

—Lightweight Exe-atmospheric

Advanced Projectiles

–Lines of Code

–Long-wave Infrared

–Meter

–Maneuverable Reentry Vehicle

–Million Electron Volts

–Magnetohydrodynamics

–Million Instructions per Second

–Mid-infrared Advanced

Chemical Laser

–Multiple Independentlytargetable Reentry Vehicle

–Million Operations per Second

–Metallic Oxide Semiconductor

Field Effect Transistor
271

MTBF

MTTR

MW

MW/sr

MWIR

Nd:YAG

NPB

NTB

PAR

PBV

RTG

RV

SBHEL

SBI

SBKKV

SDI

SDIO

SDS

SGEMP

SLBM

—Mean Time Between Failures

—Mean Time to Repair

–Megawatt

—Megawatts per Steradian

–Medium-wave Infrared

—Neodymium Yttrium-AluminumGarnet (Laser)

–Neutral Particle Beam

–National Test Bed (for

computer simulations)

–Phased-array Radar

–Post-boost Vehicle

–Radioisotope Thermal

Generator

–Reentry Vehicle

—Space-based High Energy Laser

—Space-based Interceptor

–Space-based Kinetic Kill Vehicle

–Strategic Defense Initiative

–Strategic Defense Initiative

Organization

–Strategic Defense System

–System-Generated

Electromagnetic Pulse

–Submarine-launched Ballistic

Missile

SS-18, SS-19,

SS-24, SS-25 —U.S. designators for various

Soviet ICBMS

SS-20 –U.S. designator for a Soviet

Medium Range Ballistic Missile

SSME –Space Shuttle Main Engine

SSTS –Space Surveillance and

Tracking System

STAS —Space Transportation

Architecture Study

SWIR –Short-wave Infrared

TDI –Time Delay and Integrate (data

processing technique for

sensors)

TIR –Terminal Imaging Radar

TREP –Thrusted Replica (decoy)

UHF –U1tra-high Frequency

U V –Ultra-violet

VHSIC –Very High Speed Integrated

Circuitry

VLSI –Very Large-Scale Integrated

(Circuit)

w –Watt

W/sr –Watts per Steradian

Glossary of Terms

Ablative Shield: A shield that evaporates when

heated, thereby absorbing energy and protecting the underlying structure from heat damage.

Such shields are used to protect reentry vehicles

from damage during atmospheric reentry. They

could also be used to shield boosters against

lasers.

Absentee Ratio: In a constellation of orbiting

weapon platforms, the ratio of the number of

platforms not in position to participate in a battle to the number that are. Typical absentee ratios for kinetic energy weapons systems are

around 10 to 30, depending on details of the satellite orbits and the assumed battle.

Absorption Bands: Frequency ranges in the electromagnetic spectrum that are highly absorbed

by the atmosphere, thus restricting transmission

between Earth and space. Absorption bands of

interest to BMD are found in the ultraviolet, the

infrared, and at microwave frequencies.

Accelerator Test Stand (ATS): An accelerator at

Los Alamos National Laboratory that is a prototype for a high intensity neutral particle beam

accelerator.

Acquisition: The search for, detection of, and maintenance of contact with a potential target by the

sensors of a weapon system.

Actitiv Discrimination: The electromagnetic irradiation of a potential target in order to determine

from the characteristics of the reflected radiation whether it is a threatening object or a decoy. Radar and laser radar (ladar) are examples

of active discrimination tools.

Adaptive Preferential Defense: A BMD tactic designed to increase the value of the defense. The

defense determines in the early mid-course phase

of ballistic missile trajectories the intended target of each RV that has survived the first defensive layers. Those targets that the defense

wishes to protect are defended by preferentially

attacking those warheads aimed at these targets.

To optimize the number of surviving targets,

those that have the fewest warheads aimed at

them are defended first.

Advanced Launch System (ALS): A rocket proposed for deployment in the mid-1990s that

would have a large payload-to-orbit capacity (50

tonnes or more) and that would be the primary
272

launch vehicle for deploying the thousands of

tonnes required for an early deployment of the

Strategic Defense System (q.v.).

Advanced Test Accelerator (ATA): A high-current

50 MeV accelerator at Lawrence Livermore National Laboratory that is being used for free electron laser experimentation and development.

AEGIS: The anti-missile system currently in use

by the U.S. Navy, designed to defend carrier

groups against attack by rockets, aircraft, or airbreathing missiles.

Agility: The ability of a pointing system for a sensor or a weapon to shift rapidly and accurately

from one target to another.

Air Defense Initiative (ADI): A Department of

Defense research program that is designed to

counter air-breathing threats to the United

States, such as aircraft and cruise missiles. Designed to supplement the SDI.

Airborne Optical Adjunct (AOA): An experimental

aircraft with sensors being prepared for testing

in the late 1980s as a part of the development

program for the AOS.

Airborne Optical System (AOS): A set of optical

elements mounted on aircraft and intended to

provide tracking information on approaching

warheads and decoys and then to hand off this

information to the terminal phase of a missile

defense system.

Algorithm: A precise description of a method for

solving a particular problem, using operations

or actions from a well-understood repertoire.

(Definition by J. Shore)

Alpha Laser: A megawatt-class chemical laser being developed by TRW as a prototype for a

space-based anti-missile laser weapon.

Anti-ballistic Missile (ABM): A missile designed

to destroy a ballistic missile.

Anti-satellite (ASAT) Weapon: A weapon designed

to destroy satellites.

Anti-simulation: The tactic of trying to disguise

a military asset as a decoy.

Architecture: The overall design of a system: its

elements, their functions, and their interrelationships.

ARPA Network: The computer network set up by

the Department of Defense’s Advanced Research Projects Agency to facilitate data and information exchange and thus to aid programming and research.

Artificial Intelligence: The ability of a computing

system to learn from experience and to act in

other ways indistinguishably from a sentient

being.

Atmospheric Compensation: The physical distortion or modification of the components of an optical system for the purpose of compensating for

the distortion of light waves as they pass

through the atmosphere and the optical system.

Atmospheric Heave: Raising a large volume of the

upper atmosphere to a substantially higher altitude (hundreds of kilometers) by means of a nuclear detonation within the atmosphere. This

could have several different effects on the capability of a missile defense system: for example,

nuclear background radiation problems could be

substantially worsened for the defense and some

directed-energy weapons could be partially neutralized; on the other hand, offensive decoys

could become detectable and offensive targeting

could become more difficult.

Atmospheric Turbulence: Variations in atmospheric density that cause small changes in refractivity of light that passes through the air. In the

context of BMD sensors, turbulence causes distortions of plane wavefronts that could, if not

compensated for, weaken the coherence and effectiveness of a high power laser beam.

Ballistic Missile: A rocket-driven missile that

burns out relatively early in its trajectory and

then follows a ballistic path in the Earth’s

gravitational field to its target.

Ballistic Missile Defense (BMD): A weapon system designed to destroy ballistic missiles or their

warheads at any or all points in their trajectories, from launch until just before target impact.

Battle Management: The analysis of data on the

state of a battle and decisions regarding weapon

aiming and allocation. Subtasks include command and communication, kill assessment, maintaining knowledge of the state and positions of

all elements of the defense system and calculation of target track files.

Beam Control: The system that maintains the

desired pointing, tracking, and quality of a laser or particle beam.

Beam Director: A system that focuses and points

a laser or particle beam in the desired direction.

Beam Neutralizer: Device located at the exit of a

particle accelerator that neutralizes the charged
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ion beam in order to produce a neutral particle

beam, usually by stripping off extra electrons

(charged beams can be accelerated and focused

by a particle accelerator while neutral beams cannot). The device may be a gas cell or foil or may

utilize an appropriate laser beam.

Bistatic Radar/Ladar: Radar or ladar using transmitters and receivers separated by substantial

distances. Reasons for separation may include

enhanced survivability or the ability to overcome

the countermeasure of retroreflectors.

Blinding: Destroying elements of an optical sensor by overloading them with radiation.

Boost Phase: The part of the trajectory of a ballistic missile during which the rocket booster fires.

This phase lasts for about 3 to 5 minutes for current ICBMs.

Booster: The rocket that places a ballistic missile

in its trajectory towards a target or that

launches a satellite or space vehicle into orbit.

Booster Clustering: Locating boosters relatively

near one another (within hundreds of kilometers)

in order to force a space-based BMD to a higher

absentee ratio and therefore to increase the required number of space-based interceptors.

Booster Decoys: Rockets that would imitate the

early phase of booster plume and trajectory in

order to draw fire from the defense, but that

would not be armed intercontinental missiles and

would cost substantially less than ICBMs.

Bus: The rocket-propelled final stage of an ICBM

that, after booster burn-out, places warheads

and, possibly, decoys on ballistic paths towards

their targets. Also known as “post-boost vehicle” (PBV).

Carrier Vehicle (CV): A vehicle in Earth orbit that

carries the space-based interceptors of a BMD.

It may also carry some sensors.

Chaff: Small, light bits of matter deployed in quantity to confuse radar or ladar by scattering radiation and concealing targets.

Charge-coupled Devices: Solid state devices that

convert photons of incoming electromagnetic radiation to electric charge, usually in a proportional manner. The charge is then detected by

attached electronic circuits and the resulting information is analyzed to provide information

about the original photons (direction of the

source, wavelength, time distribution, number).

Chemical Laser: A laser that uses chemical reactions to pump energy into a lasing medium,

thereby creating the inverted state population

needed for lasing. An example of a high power

chemical laser is the hydrogen fluoride laser, in

which a hydrogen-fluorine reaction produces lasing in a hydrogen fluoride medium.

Coherence: The maintenance of constant phase

relationships between adjacent rays of a beam

of electromagnetic radiation. Coherent radiation

is able to transfer energy much more efficiently

and in a more collimated beam than is incoherent radiation. Laser light is coherent.

Command Guidance: The technique of remotely

controlling the trajectory of a rocket.

Complementary Expendable Launch Vehicle

(CELV): The Titan IV; a new U.S. rocket for

launch-to-orbit, to become operational in the late

1980s and with a payload, to low-Earth orbits,

of about 30,000 to 40,000 pounds.

Compulsator: A hollow-core generator able to produce large amounts of electrical energy, but substantially lighter than homopolar generators. It

might be used to produce large currents suitable

for electromagnetic guns.

Computer Simulation: The representation on a

computer of a chain of physical events, using

physical calculations. In the context of BMD,

it will be attempted to reproduce, in computer

memory and on screens, all aspects of a large engagement between a BMD system and the opposing offensive force. The behavior of all elements of both systems will presumably be

included, as well as effects generated by nuclear

explosions in space and in the atmosphere. The

results of many such simulations are supposed

to show the effectiveness, robustness, and survivability of the BMD system under various sets

of assumptions.

Constellation: The number and orbital disposition

of a set of space-based weapons forming part of

a defensive system.

Cost-effective at the margin: In the context of

BMD effectiveness, the requirement that each

additional increment of defensive capability cost

the defense less than each corresponding offensive increment deployed by the offense. In the

context of survivability, it corresponds to the

requirement that the incremental cost of defending one space asset be less expensive than the

incremental cost of adding sufficient defense

suppression capability to destroy that asset.

Cost-exchange Ratio: In the BMD context, the ratio of the cost of an item to the defense to the

cost of an equal and opposing item to the offense.

For example, the cost-exchange ratio of a terminal defense system would be the cost to the defense of the number of rockets (plus the pro-rated

cost of support sensors) needed to kill one incom-
274

ing warhead to the cost to the offense of putting

that warhead and associated decoys in the terminal phase of flight.

Countermeasure (Non-destructive and Defense

Suppression): A tactic used by the offense to oppose defensive measures or by the defense to oppose offensive measures. Non-destructive countermeasures are those that are intended to

nullify the capability of the opposing system by

means other than direct attack. Defense suppression countermeasures include means of attacking defensive system elements.

Cryogenic cooler: The equipment used to cool substances to very low temperatures.

Dazzling: The temporary blinding of a sensor by

hostile incoming radiation (e.g., radiation from

lasers, generators, nuclear explosions).

Debris: In the BMD context, large quantities of

relatively small particles. Debris could be used

as an intentional defense suppression measure;

debris is also generated when objects in space

are destroyed.

Decoy: An object intended to fool the adversary’s

sensors into thinking it is a more expensive and

more threatening object. It could be a decoy reentry vehicle, a decoy ASAT warhead, or a decoy satellite belonging either to the offense or

defense.

Defense Suppression: Destroying the elements of

a defensive system.

Delta 160: An experiment in September 1986 that

successfully took some radar and infrared measurements in space as well as the coordination

among a large number of individual elements in

space and on the ground. See text for more detail.

Detector: A device that can sense and report on

radiation originating from a remote object. Detectors for BMD purposes are usually sensitive

to electromagnetic radiation, but some may be

particle detectors, able to sense, for example,

electrons, neutral beams, or neutrons.

Deterrence: The prevention of war or other undesired acts by a military posture threatening

unacceptable consequences to an aggressor.

Dielectric Coating: On a mirror, a thin (usually fractions of a wavelength) coating of non-conducting

materials designed to maximize reflectivity at

the operating wavelength.

Diffraction: The spreading out over distance of

even a perfectly focused beam of electromagnetic

radiation. It provides an upper limit on the intensity that can be obtained. A perfectly focused

beam will spread out at an angle (in radians)

equal to slightly more than the ratio of the wavelength to the diameter of the final focusing

element.

Direct Ascent Non-nuclear Anti-satellite Weapon

(DANNASAT): A ground-based rocket with a

homing, non-nuclear warhead designed to destroy satellites.

Direct Ascent Nuclear Anti-satellite Weapon

(DANASAT): A ground-based rocket with a nuclear warhead designed to destroy satellites.

Discrimination: The process of determining which

of a set of targets in space (usually reentry vehicles) are real and which are decoys.

Distributed Architecture: A system design whose

primary functions are dispersed and repeated in

numerous nodes at diverse locations. Each node

has a large amount of autonomy. This has the

effect of increasing the survivability of the system, since the loss of a few nodes will not cause

the system to fail catastrophically. It may, however, cause redundant actions, and thus require

more system elements than a more efficient architecture.

Distributed Battle Management: A battle management system that relies on a distributed architecture with numerous, largely autonomous nodes.

Doppler Imaging: The use of radar or ladar to produce reflected doppler-shifted electromagnetic

signals from different parts of an object. This

technique can provide an image of the object if

it is spinning or tumbling. Since the doppler shift

depends on the velocity of the object with respect

to the observer, reflections from those parts of

the object receding from the observer will have

different shifts from those moving towards the

observer.

Dynamic Isotope Power System (DIPS): A device

for producing electric power for satellites that

utilizes the heat generated by a quantity of a

radioactive isotope as a power source and then

converts the heat energy to electricity by means

of a dynamic heat engine.

E-beam: An electron beam.

Eastport Study Group: A group of computer software and hardware experts convened by the

SDIO in 1986 to provide independent advice on

the problems associated with designing and producing a battle management system for BMD

and the associated software. The group reached

the conclusion that insufficient attention had

been thus far paid to software problems and that

any BMD system should be designed around

battle management software, not vice-versa. It
275

also advocated the use of distributed systems

and concluded that a successful battle management system for BMD could be designed.

Electro-optic Countermeasures: Countermeasures

designed to confuse the sensors of one’s adversary by jamming, blinding, or dazzling, or by reducing the radiations and reflections produced

by one’s own assets, or by using decoy targets

in conjunction with real targets.

Electromagnetic Launcher: A device that can accelerate an object to high velocities using the electromotive force produced by a large current in

a transverse magnetic field.

Electromagnetic Pulse (EMP): A large pulse of electromagnetic energy, effectively reaching out to

distances of hundreds of kilometers or more; induced by the interactions of gamma rays that

are produced by a nuclear explosion with atoms

in the upper atmosphere.

Endo-atmospheric Interceptor: An interceptor

rocket that attacks incoming reentry vehicles in

their terminal phase within the atmosphere.

Ephemeris: The orbital parameters of a satellite

or planet.

Excimer Laser: A laser that produces the necessary population inversion of molecular energy

states by an electric discharge in a lasant consisting of a noble gas and a halogen (e.g. argon

and fluorine, xenon and chlorine). The two elements of the lasant form a metastable excited

molecular state, and, upon decaying to the

ground state and emitting a photon, separate

once more into their component atoms.

Exe-atmospheric Interceptor: An interceptor

rocket that destroys incoming reentry vehicles

above the atmosphere during the late midcourse

phase.

Experimental Test Accelerator (ETA): A 5 MeV

accelerator at Livermore Laboratory that

produced free electron lasing in the microwave

band in 1985.

Fast-burning Booster (FBB): A booster for a ballistic missile that burns out more rapidly than

the current minimum time of about three minutes. Down to about 80 seconds, there are no anticipated serious technical difficulties in developing such rockets. For shorter times, significant

developmental work may be necessary.

Fault-tolerance: The ability of hardware or software to continue to function despite the occurence of failures.

FLAGE Experiment: A set of experiments involving a low altitude interceptor rocket for missile

defense.

Fletcher Report: The report of the Defensive Technologies Study Team, headed by Dr. James

Fletcher. This report was requested by President

Reagan in 1983 to investigate the technical feasibility of ballistic missile defenses. The report

unclassified conclusions were optimistic.

Fluorescence: Light or other electromagnetic radiation from excited atoms that have previously

been raised to excited states by radiation of a

higher frequency.

Focal Plane Array: A set of radiation-sensitive devices located at the focal plane of an optical train.

The array then provides an image of objects located in front of the optics. The resolution of the

array depends on the number of elements in it

and on the quality and size of the optics.

Free Electron Laser: A laser that takes energy

from a transversely-oscillating electron beam

and adds it to a coupled beam of electromagnetic

radiation. The wavelength of the radiation depends primarily on the distance of oscillation of

the electron beam and on its energy.

Frond: A proposed countermeasure to doppler imaging by radar or ladar: long, thin strips of metal,

coated with a material that sublimates in space,

are attached to a possible target. The random

motion of many of these objects on the same target could confuse attempts to image the target

by doppler measurements over its surface.

Full-scale Engineering Development (FSED): A

stage in the acquisition of a military system that

is intended to produce several working prototypes.

Geostationary Orbit: A circular orbit about 35,000

kilometers above the Earth’s equator with a

period of one day; a satellite in this orbit thus

remains above one point on the equator.

“Hard” Kill: The destruction of an object in a way

easily detectable from a distance, usually by

fragmenting it or by changing its velocity radically. Distinguished from “soft” kill (q.v.).

Hard Targets: Ground targets, such as missile silos

or deeply buried command centers, that could

survive a nuclear blast unless it were to hit

within a few hundred meters.

Heavy Lift Launch Vehicle (HLLV): Currently referred to as the Advanced Launch System (q.v.);

a projected vehicle capable of lifting tens of

tonnes to orbit and ready for use by the mid

1990s. A requirement for placing space-based elements of the Phase One Strategic Defense System into orbit within the timelines planned.

HF/DF Laser: A chemical laser, fueled by a

hydrogen-(or deuterium-)fluoride reaction, that
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produces lasing at 2.7 (3.8) microns. The most

promising candidate for space-based chemical

lasers.

Hierarchical Architecture: An architecture that

has several layers of a hierarchy; an element may

take direction from an element in a higher layer

but not from one in the same or a lower layer.

High Endo-atmospheric Interceptor (HEDI): A

proposed interceptor for ballistic missile defense

that could be available sometime in the 1990s.

Hit-to-kill Vehicle: A kinetic vehicle that destroys

its target by hitting it directly and thereby transferring a lethal amount of kinetic energy to it.

Homing: In the context of missile defense, the ability of an interceptor to locate its target and to

modify its trajectory to insure that it approaches

its target close enough to destroy it. May be

based on infra-red detectors, radar, or ladar.

Homing Overlay Experiment (HOE): An experiment carried out in summer 1984, in which a

modified Minuteman I rocket, launched from

Kwajelein Atoll in the Pacific, was able to home

in on and destroy a simulated reentry vehicle

launched from Vandenberg Air Force Base in

California.

Homopolar Generator: An electric generator that

employs a rotating cylinder to store large

amounts of electrical energy. Maybe used in conjunction with electromagnetic launchers.

Imaging Synthetic Aperture Radar (ISAR): A radar technique that uses the motion of targets

to reconstruct an image with high resolution.

Resolution is limited by the distance the target

moves between successive radar pulses.

Impulse Tagging: A possible technique for interactive discrimination. A high-powered laser

strikes an object, ablating a small amount of surface material. The object recoils, and the speed

of the recoil is inversely proportional to its mass.

Impurity Band Conductor (IBC): Recently perfected semiconductors that may be used for

charge-coupled devices and have a high degree

of radiation resistance.

Induction Linac: One of the candidate types of particle accelerator for a free electron laser. The

ETA and ATA at Lawrence Livermore Laboratory are induction linacs. The electromagnetic

impulse that accelerates the electron beam

originates in a changing magnetic field produced

by a series of loops that surrounds the beam

rather than by a radiofrequency-generated electric potential that is maintained along the beam

just ahead of the accelerated particles.

Inertial Guidance: Guidance provided from onboard a rocket. Inertial gyros sense acceleration

and use that information to alter the rocket’s

propulsion to maintain a predetermined course.

Interactive Discrimination: Techniques for discrimination that involve perturbing a target and

observing its reactions to the perturbation. Examples are neutral particle beams (producing

electrons or neutrons in the target), impulse tagging with a laser (causing the object to recoil),

and laser thermal tagging (heating an object and

observing the temperature rise).

Keep-out Zone: A volume of space around a satellite that is declared to be forbidden to entry by

unauthorized intruders; enforcement of such a

zone is intended to protect space assets against

attack, particularly by space mines (q.v.).

Kill Assessment: Determiningg with remote sensors

whether an attacked target has been destroyed.

Kinetic Energy Weapons (KEW): Weapons that kill

by transferring a fraction of their kinetic energy

to a target.

Kinetic Kill Vehicle (KKV): A rocket that homes

in on its target and kills it by striking it or by

hitting it with a fragmentation device.

Klystron: A high-powered vacuum tube used to

produce the radio frequency waves that accelerate particles in an rf accelerator.

Ladar: Laser radar; a form of radar that uses laser

light instead of microwave radiation as a radar

signal.

Lethal Radius: The maximum distance from an exploding (nuclear or non-nuclear) warhead, within

which a target would be destroyed.

Lethality: The ability of a weapon to destroy a

target.

Lethality Enhancer: A device, used by a kinetic

kill vehicle, that explodes or expands at the point

of closest approach to the target, sending fragments of material into the target and destroying it.

Life Cycle Costs (LCC): The total cost of a military system, including operation and maintenance over its anticipated lifetime.

Limited Test Ban Treaty: A treaty signed by the

US, the UK, and the USSR in 1963, restricting

nuclear tests to underground sites. Over one hundred nations have become signatories.

Lines of Source Code (LOC): The number of lines

in a computer program in the highest (most removed from the computer) level language used.

Machine Language: The lowest level of computer

language that directly manipulates the bistable
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states in a computer’s memory, effectively moving around numbers and performing arithmetic

operations upon them.

Magnetohydrodynarnics (MHD): In the BMD context, a technique for converting a hot plasma to

electric energy bypassing it through a magnetic

field. In space, it might be used to generate electrical energy from a large rocket exhaust.

Maneuvering Boosters: Boosters that can change

course in a pre-programmed way; they might be

used in an effort to avoid attack during the boost

phase by space-based interceptors.

Maneuvering Reentry Vehicle (MaRV): A reentry

vehicle that maneuvers in the late midcourse or

terminal phase, either to enhance accuracy or to

avoid anti-ballistic missiles. Maneuvers within

the atmosphere are usually accomplished by aerodynamic means; maneuvers in space could be

accomplished by small rockets.

Mass Fraction: The fraction of a rocket stage’s

mass that is taken up by fuel. The remaining

mass is structure and payload.

Mean Time Between Failures (MTBF): The average time between element failure in a system,

usually composed of many elements.

Mean Time to Repair (MTTR): In a multi-element

system, the average time required to repair the

system in the event of a failure.

Midcourse Phase: The phase in a ballistic missile

trajectory after the warheads are dispensed from

post-boost vehicles and before their reentry into

the atmosphere, in which the reentry vehicles

(and decoys) coast in ballistic trajectories.

Milstar: A U.S. strategic communications satellite

under development.

Mirror System: In the BMD context, a BMD system that one side builds in response to its adversary that contains similar elements and has

similar missions.

Missile Silo: A usually hardened protective container, buried in the ground, in which land-based

long-range ballistic missiles are placed for

launching.

Monostatic Radar/Ladar: A radar/ladar in which

the transmitter and receiver are located together.

Multiple Independently-targetable Reentry Vehicle (MIRV): One of several reentry vehicles carried on the same booster that are sent to diverse

targets by a post-boost vehicle (q.v.).

Mutual Assured Destruction: The strategic situation now existing in which either superpower

could inflict massive nuclear destruction on the

other, no matter who struck first.

Mutual Assured Survival: The hypothetical strategic situation obtaining wherein each superpower would know that it would survive a nuclear attack launched by the other with only

minimal casualties because of the high effectiveness of its defensive systems.

National Test Bed (NTB): The nationwide computing network, with a center in Colorado, to be

organized by SDIO to test and simulate software

and, to a degree, the hardware that might be used

in a BMD system.

Network Topology: In computing, the elements,

lines of communication between the elements,

and the hierarchical structure of a computing

network.

Neutral Particle Beam (NPB): A beam of uncharged atomic particles, produced by a particle accelerator and beam neutralizer, that could

be used to strike and destroy an object or to “interrogate” it, ascertaining some information

about it from the characteristics of the resulting emitted radiation from the object.

Noctilucent Clouds: Naturally occurring clouds in

the upper atmosphere (about 60-80 km altitude)

resulting from the accumulation of ice crystals

about fine particles of meteoric dust. Such clouds

may also be susceptible to artificial creation, and

thus might be useful for possible countermeasures to ground-or air-based infrared sensors trying to look through the Earth’s atmosphere.

Nuclear Background: The background of infrared,

visible, microwave and nuclear radiation caused

by a nuclear explosion in space or in the atmosphere. Such backgrounds could pose significant

problems for many proposed sensors of a BMD

system.

Nuclear Precursor: A nuclear explosion detonated

near an adversary’s sensors or weapons shortly

before the arrival of a number of nuclear warheads on nearby targets. The aim would be to

prevent the adversary from launching his own

weapons or from using his sensors because of

the background or debris produced by the precursor.

Outer Space Treaty: A treaty signed by the US,

the UK, and the USSR in 1967, and acceded to

by many other nations, that bans the stationing of nuclear weapons in space.

Packet-switched Network: A computing network

in which information and data are distributed

and retrieved in “packets”; the size and repetition rate of the packets depend on the computational and communication status of the system

at the time of data transfer.

Particle Accelerator: A device that accelerates
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charged nuclear and sub-nuclear particles by

means of changing electromagnetic fields. Particles are accelerated to energies of millions of

electron-volts and much higher. Accelerators for

basic research have reached over 1012 electron

volts of energy. Those useful for BMD missions,

such as NPBs or FELs, need only reach between

108 and 109 electron volts, but generally require

higher currents.

Passive Discrimination: Discrimination of decoys

from real targets by observing infrared, optical,

or other emissions from the object. In BMD, passive infrared observations may be used in early

versions of a system to attempt to discriminate

between reentry vehicles and simple decoys.

Passive Sensor: A sensor that passively observes

radiation coming from an object at a distance.

Usually refers to infrared or visible sensors.

Payload: On a rocket, the part of the total mass

that is used to accomplish the rocket’s mission,

apart from the rocket fuel and structure. Examples are a satellite launched to orbit or a nuclear

warhead.

Penetration Aid: A device, such as a decoy or chaff,

that would make it easier for a reentry vehicle

to penetrate a BMD system by confusing the

system’s sensors.

Phase One: In the context of missile defense, the

first phase of a deployed Strategic Defense System (q.v.), based on space- and ground-based interceptors, and planned for deployment by the

mid- to late-1990s.

Phase Three: A later phase of missile defense deployment, including directed-energy weapons

and interactive discrimination.

Phase Two: The phase of missile defense deployment, following Phase One; would possibly include more interceptors, some enhanced discrimination capability and other innovations.

Phased Deployment: In the BMD context, the deployment of successively more effective systems

as they are developed.

Pixel: One unit of image information on a screen,

corresponding to the smallest unit of the image;

the more pixels, the higher the potential resolution of the system.

Platform: In the BMD context, a satellite in space

used for weapons, sensors, or both.

Platform Decoy: A relatively inexpensive object

intended to fool an adversary’s sensors into

deciding that the object is really a much more

valuable platform.

Plume: The hot, bright exhaust gases from a

rocket.

Pop-up: The use of rapidly reacting, groundlaunched elements of a ballistic missile defense.

These elements may be sensors (e.g. for midcourse tracking and discrimination) or weapons

(e.g. X-ray lasers).

Post-boost Phase: The phase of a ballistic missile

trajectory in which reentry vehicles and associated decoys are independently released on ballistic trajectories towards various ground

targets.

Post-boost Vehicle (PBV): The rocket-propelled vehicle that dispenses reentry vehicles and associated decoys on independent ballistic trajectories

towards various ground targets.

Preferential Defense: The defensive tactic of protecting some targets strongly while leaving

others less strongly, or not at all defended. This

allocation of defense resources is to be hidden

from the offense, thus requiring it to waste resources and conferring a strategic advantage on

the defense.

Probe: In the context of SDS, a ground-based set

of sensors that would be rapidly launched into

space on warning of attack and then function as

tracking and acquisition sensors to support

weapon allocation and firing by BMD weapons

against enemy ICBMs and RVs.

Radiation Hardness: The ability of electronics to

function in high fields of nuclear radiation. Techniques for increasing hardness include semiconductors less susceptible to radiation upset,

shielding, reduction in size, and redundancy.

Radio Frequency Linac: A particle accelerator that

uses a traveling electromagnetic wave at radio

frequencies to accelerate charged particles. The

wave is positioned at a relatively constant phase

relative to the particle position as both travel

down a tube, providing an electric field that furnishes the accelerating force.

Radioisotope Thermoelectric Generator (RTG): An

electric generator that uses the heat produced

by the decay of intense radioactive sources to

produce electricity by the intermediary of thermoelectric devices.

Railgun: A device that uses the electromotive force

experienced by a moving current in a transverse

magnetic field to accelerate small objects rapidly to high velocities. See electromagnetic

launcher.

Range Gate: An electronic gate placed by a radar

system with a timing intended to include expected return signals. The radar would then only

look at and analyze those return signals within

the gate.
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Reentry Vehicle (RV): The shielded nuclear warhead of a ballistic missile that reenters the

Earth’s atmosphere to strike a target on the

ground.

Responsive Threat: The Soviet ballistic missile

force and the Soviet countermeasures to a U.S.

ballistic missile defense that could be expected

to be deployed by the Soviet Union in an effort

to oppose a U.S. missile defense.

Retargeting: Re-aiming a directed-energy weapon

from one target to another.

Retroreflector: A mirror, usually a corner reflector, that returns light or other electromagnetic

radiation in the direction from which it comes.

Robustness: The ability of a system to fulfill its

mission in the face of non-destructive countermeasures or a direct attack.

Rubber Mirror: An informal name for mirrors that

can be distorted by electromechanical actuators

in order to compensate for atmospheric distortions or changes in the geometry of the optical

system, and thereby restore a desired wave front.

SAFEGUARD: The anti-ballistic missile system

deployed in North Dakota in 1975 and then dismantled in 1976 because of low cost-effectiveness.

Salvage-fused: In a warhead, a design including

a fuse that would detonate the warhead if it were

struck or attacked from a distance by another

weapon.

Sensor: A device that detects electromagnetic radiation or particles emanating from a source at

a distance.

Shoot-back: The defense tactic of shooting at attacking objects, trying to destroy them before

they can destroy the targeted asset.

Shroud (RV, PBV): In the BMD context, a thin

envelope that would enclose a reentry vehicle,

interfering with the infrared radiation that it

would emit; also, a loose conical device that

would be positioned behind a PBV to conceal the

deployment of reentry vehicles and decoys from

outside observers.

Sidelobe: In radar, a solid angle away from the aiming direction of an antenna, that contains a significant fraction of the energy radiated. The size,

shape, and location of sidelobes depend on the

wavelength and on the antenna geometry. Energy can also be inserted into the radar receiver

through a sidelobe, providing an opportunity for

jamming.

Signal Processing: The analysis (often rapid and

in near-real time) of complex incoming data in

order to refine and simplify the information received to a form that is useful to the user. For

sensors, signal processing will be needed to separate real signals (e.g. objects and their positions)

from spurious and unwanted background information.

“Soft” Kill: A kill of an object, usually by a particle beam, against electronics, such that an outside observer cannot detect any physical change

in the target from a distance, but in which the

target is nevertheless unable to perform its

mission.

Soft Targets: In nuclear strategy, any target that

cannot be hardened in order to survive a nearby

nuclear detonation (e.g. people, cities, airfields,

factories).

Software Engineering: The technology for developing and maintaining software.

Software Engineering Environment: Tools for developing software that are mutually compatible,

that enable the software engineer to deal in rapid

succession with different phases of the software

life cycle and that support the entire software

life cycle. Such environments are in the development stage.

Software Security: The resistance of software to

unauthorized use, theft of data, and modification of programs.

Software Trustworthiness: The probability that

there are no errors in the software that will cause

the system to fail catastrophically after it has

undergone testing.

Software Verification: The development of techniques for mathematically proving the correctness of computer programs.

Source Code: A computer program.

Space Mine: A satellite with an explosive (either

nuclear or non-nuclear) charge that is designed

to position itself within lethal range of a target

satellite and detonate upon preprogrammed

command, remote command or upon being itself

attacked.

Space Transportation Architecture Study: An interagency (SDIO, USAF, NASA) study undertaken in 1986 to investigate future US space

transportation needs for military and civilian

missions and to propose methods to meet those

needs.

Space-based Interceptor (SBI): A kinetic kill rocket

based in space.

Spartan: Long-range nuclear-tipped missile used

as part of the SAFEGUARD missile defense system in 1975.

Specific Impulse: A measure of the efficiency of

a rocket fuel: the ratio of the thrust produced

to the rate of fuel burning. It is measured in sec-
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ends; good liquid fuels today range from 300 to

400 seconds, and solid fuels from about 200 to

300 seconds.

Sprint: High acceleration nuclear-tipped short

range interceptors used as part of the SAFEGUARD missile defense system in 1975.

Stability: Resistance to rapid and precipitous

change in an international situation. Crisis stability refers to the ability of a situation to resist

deterioration to war in times of crisis. Arms race

stability refers to the ability of the situation to

resist deterioration into a spiraling arms race

without apparent limits.

Stable Transition: In missile defense, refers to a

hypothetical transition from today’s offensedominated strategic relationship between the superpowers to a regime based on defense dominance, without passing through a period of crisis instability.

Stealth: Means of hiding one’s own asset from an

adversary’s sensors, usually by reducing the radiation emitted or reflected by the asset.

Steradian: The solid angle subtending a unit area

on a unit sphere.

Stimulated Raman Scattering (SRS): A multiphoton interaction between a beam of photons

and the molecules of the medium through which

the beam passes. If intense enough, the photons

can interact with rotational states of the

molecules, producing coherent outgoing beams

of frequency equal to the sum (and difference)

of the frequency initial beam and (from) the

equivalent frequency of the rotational state. In

practice, regarding BMD, a very intense laser

beam might interact with gas molecules in the

atmosphere and produce SRS—the result could

be a dispersal of the original beam, reducing its

weapons potential.

Strategic Defense: Defense against long range nuclear weapons.

Strategic Defense Initiative: A Department of Defense research program, initiated by President

Reagan in 1983, with the purpose of investigating methods of defending against the ballistic

missile threat to the United States.

Strategic Defense System: The proposed Phase

One system for ballistic missile defense, planned

for deployment in the mid- to late-1990s.

Surveillance: In space, the observation, tracking,

and cataloging of objects in Earth orbit and of

objects being launched from the Earth.

Survivability: The ability of a system to continue

to fulfill its mission in the face of a direct attack

upon it.

Survivability Enhancement Option: One of several

methods to help a missile defense system survive a direct attack (e.g., shootback, stealth, maneuver, shielding).

System-Generated Electromagnetic Pulse

(SGEMP): An electromagnetic pulse in a satellite, generated by gamma rays and x-rays from

a nuclear explosion. These rays strike metallic

surfaces of the satellite, causing electrons to flow

along conductors and inducing large currents in

the circuitry within the satellite.

Terminal Imaging Radar (TIR): A radar intended

for missile defense use in the terminal phase by

endo-atmospheric interceptors that need high

resolution and discrimination information. The

radar may be ready for deployment in the 1990s.

Terminal Phase: The final phase of a ballistic

trajectory in which the reentry vehicles pass

through the atmosphere to their targets. This

phase lasts one minute or less.

Theater Defense: Defense against nuclear weapons on a regional level (i.e., Europe, Japan, Israel) rather than at the strategic level (globally

or the United States and the U.S.S.R.).

Thermal Blooming: The dispersion of a highpowered laser beam within the atmosphere,

caused by heat transfer from the beam to the

atmosphere. The center of the beam will be hotter, resulting in a less dense zone with a lower

index of refraction. The beam is then refracted

radially outward.

Thermal Tagging: A discrimination technique in

which a high-powered laser heats up an object;

a subsequent measure of its temperature could

help indicate whether the object were light

(higher temperature) or massive (lower temperature). The higher temperatures could be detected and used as a discriminant by other sensors later in the trajectory.

Threat Tube: A narrow geometrical volume of

space, usually over the arctic, from Soviet missile fields to U.S. targets, within which there

would be a high density of RVs and decoys during a Soviet nuclear attack.

Thrusted Replica: A decoy reentry vehicle that has

a small rocket which is used upon reentry into

the atmosphere. The rocket thrust compensates

for atmospheric drag, making it more difficult

to discriminate the decoy from a real RV.

Time Delay and Integrate: The technique of integrating (essentially, adding) signals from several sensors so that better photon statistics can

be obtained, helping the signal processor find a

signal above background.
Tracking Denial: Denying an adversary’s sensors

the ability to detect and keep track of a space

asset.

Trajectory: The path followed by a moving object.

Warhead: An explosive used by a missile to destroy

its target.

X-band Radar: Radar operating in the frequency

band around 10 GHz.

281

X-ray Laser: A laser that produces radiation in the

X-ray band (around a few angstroms). In the

BMD context, such lasers would derive their

energy from a nearby nuclear explosion, and

would function only for microseconds or less.

Z: The number of electrons (or protons) in the atoms

of a given element.
Related OTA Reports

● Ballistic Missile Defense Technologies.

OTA-ISC-254, 9/85; 336 pages, GPO stock #052-003-01008-9 $12.00

NTIS order #PB 86-182 961/AS, Free summary available.

● Anti-Satellite Weapons, Countermeasures, and Arms Control.

OTA-ISC-281, 9/85; 160 pages, GPO stock #052-003-01009-7 $6.00

NTIS order #PB 86-182 953/AS.

● Directed Energy Missile Defense in Space—Background Paper.

OTA-BP-ISC-26, 4/84; 100 pages, GPO stock #052-003-00948-0 $4.50

NTIS order #PB 84-210 ill/AS.

. Space Launch and Mission Operations: New Technologies and Practices for Cost Reduction —

forthcoming

● Space Transportation Options for the Future: A Buyer’s Guide—forthcoming

NOTE: Reports are available through the U.S. Government Printing Office, Superintendent of Documents, Washington, DC

20401-9325, (202)783-3238; and the National Technical Information Service, 5285 port Royal Road, Springfield, VA

22161-0001, (703)487-4650.