United States General Accounting Office

GAO Report to the Ranking Minority Member,

Committee on National Security, House

of Representatives

October 1997 THEATER MISSILE

DEFENSE

Significant Technical

Challenges Face the

Airborne Laser

Program

GAO/NSIAD-98-37

GAO United States

General Accounting Office

Washington, D.C. 20548

National Security and

International Affairs Division

B-275849 

October 23, 1997

The Honorable Ronald V. Dellums

Ranking Minority Member

Committee on National Security

House of Representatives

Dear Mr. Dellums:

As you requested, this report discusses our review of the status of the

Airborne Laser (ABL) program. The Department of Defense (DOD) plans to

develop the ABL as its primary program for intercepting theater ballistic

missiles shortly after they have been launched—also known as the boost

phase. The Air Force estimates the life-cycle cost of the ABL program to be

about $11 billion. That estimate includes $1.3 billion for the program

definition and risk reduction phase, $1.2 billion for the engineering and

manufacturing development phase, $3.8 billion for the production phase,

and $4.9 billion for 20 years of operations and support.

This report discusses (1) the way in which the ABL is expected to change

theater missile defense, (2) assurances that the ABL will be able to operate

effectively in the levels of optical turbulence that may be encountered in

the geographical areas in which the system might be used, and (3) the

technical challenges in developing an ABL system that will be compatible

with the unique environment of an aircraft.

Background Operation Desert Storm demonstrated that the U.S. military and other

allied forces have limited capability against theater ballistic missiles. In

fact, U.S. defensive capability is limited to weapons that defend against

missiles nearing the end of their flight, such as the Patriot. No capability

currently exists to destroy missiles in the boost phase. Consequently, DOD

is expending considerable resources to develop the ABL’s capability to

intercept missiles in their boost phase. In simple terms, the ABL program

will involve placing various components, including a powerful

multimegawatt laser, a beam control system, and related equipment, in a

Boeing 747-400 aircraft and ensuring that all the components work

together to detect and destroy enemy missiles in their boost phase.

In November 1996, the Air Force awarded a 77-month program definition

and risk reduction contract to the team of Boeing, TRW, and Lockheed

Martin. Under the contract, Boeing is to produce and modify the 747-400

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B-275849 

aircraft and integrate the laser and the beam control system with the

aircraft, TRW will develop the multimegawatt Chemical Oxygen Iodine

Laser (COIL) and ground support systems, and Lockheed Martin will

develop the beam control system.

The various program components are in the early phases of design and

testing. One prototype ABL will be produced and used in 2002 to shoot

down a missile in its boost phase. If this demonstration is successful, the

program will move into the engineering and manufacturing development

phase in 2003. Production is scheduled to begin about 2005. Initial

operational capability of three ABLs is scheduled for 2006; full operational

capability of seven ABLs is scheduled for 2008.

Results in Brief Although DOD has a long history with laser technologies, the ABL program is

its first attempt to design, develop, and install a multimegawatt laser on an

aircraft. The ABL is also expected to be DOD’s first system to intercept

missiles during the boost phase. To successfully destroy a missile in its

boost phase, the ABL system would have to, within about 30 to 140

seconds, detect a missile shortly after it has been launched several

hundred kilometers away, track the rising missile’s path, and hold a

concentrated laser beam on the missile until the beam’s heat causes the

missile’s pressurized casing to fracture and then explode. This explosion

would then cause a missile’s warhead, along with any nuclear, chemical,

or biological agents it may contain, to fall short of the intended target and

possibly back on the aggressor’s territory.

A key factor in determining whether the ABL will be able to successfully

destroy a missile in its boost phase is the Air Force’s ability to predict the

levels of turbulence that the ABL is expected to encounter. An accurate

prediction of those turbulence levels is needed to define the ABL’s

technical requirements for turbulence. To date, the Air Force has not

shown that it can accurately predict the levels of turbulence the ABL is

expected to encounter or that its technical requirements regarding

turbulence are appropriate.

The turbulence that the ABL will encounter is referred to as optical

turbulence. This type of turbulence can be measured either optically or

non-optically. Optical measurements are taken by transmitting laser beams

from one aircraft to instruments on board another aircraft at various

altitudes and distances. Non-optical measurements of turbulence are taken

by radar or by temperature probes mounted on balloons or on an aircraft’s

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B-275849 

exterior. The Air Force has taken both optical and non-optical

measurements for the ABL program. However, because the ABL is an optical

weapon system, only optical measurements of turbulence can measure the

turbulence that will actually be encountered by the ABL laser beam along

its path. The Air Force has no plans to take additional optical

measurements. Instead, it plans to take additional non-optical

measurements to predict the severity of optical turbulence the ABL will

encounter. Therefore, to ensure that the non-optical measurements can be

validly applied to the ABL program, the Air Force must determine whether

the non-optical measurements can be correlated to optical measurements.

A senior-level ABL oversight team has expressed concern about the

absence of such a correlation. In response, the Air Force has indicated

that it plans to determine, in late 1997, whether a correlation exists

between optical and non-optical measurements.

Until the Air Force can verify that its predicted levels of optical turbulence

are valid, it will not be able to validate the ABL’s design specification for

overcoming turbulence. The Air Force has established a design

specification for the ABL that is based on Air Force modeling techniques.

However, data collected by the program office indicate that the levels of

turbulence the ABL may encounter could be four times greater than the

levels in which the system is being designed to operate. According to DOD

officials, if the higher levels of optical turbulence are encountered, the

effective range of the ABL system would decrease, and the risk that the ABL

system would be underdesigned for its intended mission would increase.

DOD officials also indicated that a more realistic design may not be

achievable using current state-of-the-art technology.

In addition to the challenges posed by turbulence, developing and

integrating a laser weapon system into an aircraft pose many technical

challenges for the Air Force. The Air Force must build a new laser that is

able to contend with size and weight restrictions, motion and vibrations,

and other factors unique to an aircraft environment and yet be powerful

enough to sustain a killing force over a range of at least 500 kilometers.

Also, the Air Force must create a beam control system that must

compensate for the optical turbulence in which the system is operating

and control the direction and size of the laser beam. The beam control

system will consist of complex software programs, moving telescopes, and

sophisticated mirrors. To date, the Air Force has not demonstrated how

well a beam control system of such complexity can operate on an aircraft.

Because these challenges will not be resolved for several years, it is too

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B-275849 

early to accurately predict whether the ABL program will evolve into a

viable missile defense system.

The ABL Is a New

Weapon Concept

The ABL is a complex laser weapon system that is expected to detect an

enemy missile shortly after its launch, track the missile’s path, and destroy

the missile by holding a concentrated laser beam on it until the beam’s

heat causes the pressurized missile casing to crack, in turn causing the

missile to explode and the warhead to fall to earth well short of its

intended target.

The ABL’s opportunity to shoot down a missile lasts only from the time the

missile has cleared the cloud tops until its booster burns out.1

 That

interval can range from 30 to 140 seconds, depending on missile type.

During that interval, the ABL is expected to detect, track, and destroy the

missile, as shown in figure 1.

1

The missile’s booster is under pressure only while it is burning. This pressure causes the missile to

explode after heat from the laser fractures the casing.

Page 4 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

Figure 1: ABL Missile Engagement

120,000

100,000

80,000

60,000

40,000

20,000

Cloud Tops

Altiude (feet)

Missile 

destroyed

Laser Dwell

Destruction

Tracking

Detection

Window of opportunity

30 to 140 seconds

Booster burnout

747

Page 5 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

The first step—detection—is to begin when the ABL’s infrared search

sensor detects a burst of heat that could be fire from a missile’s booster.2

Because clouds block the view of the infrared search sensor, the sensor

cannot detect this burst of heat until the missile has broken through the

cloud tops—assumed to be at about 38,500 feet. The sensor detects the

heat burst about 2 seconds after the missile has cleared the cloud tops. (In

the absence of clouds, detection can occur earlier.) The ABL would then

use information from the sensor to verify that the heat burst is the plume

of a missile in its boost phase and would then move the telescope located

in the nose of the aircraft toward the coordinates identified by the infrared

sensor.

The second step—tracking—is to be performed sequentially and with

increasing precision by several ABL devices. The first of these tracking

devices, the acquisition sensor, is to take control of the telescope, center

the plume in the telescope’s field of view, and hand off that information to

the next device, the plume tracker.

The plume tracker, having taken control of the telescope, is to track and

determine the shape of the missile plume and use this information to

estimate the location of the missile’s body and project a beam from the

track illuminator laser to light up the nose cone of the missile. The plume

tracker is then to hand its information, and control of the telescope, to the

final tracking device, the fine tracker.

The fine tracker is to measure the effects of turbulence and determine the

aimpoint for the beacon laser and, ultimately, for the COIL laser. The

reflected light from the illuminator laser provides information that is to be

used to operate a sophisticated mirror system (known as a fast-steering

mirror) that helps to compensate for optical turbulence by stabilizing the

COIL beam on the target. The reflected light from the beacon laser provides

information that is to be used to operate deformable mirrors that will

further compensate for turbulence by shaping the COIL beam.3

 With the

illuminator and beacon lasers still operating, the fine tracker is to

determine the aimpoint for the COIL laser. The COIL laser is to be brought to

full power and focused on the aimpoint.

2

Even though the ABL’s surveillance system will be the primary means for detecting missiles, the ABL

will also have the capability to accept missile detection information from other DOD sensor systems.

3

A deformable mirror is a flexible reflective surface mounted to an array of actuators, or pistons, that

can rapidly (up to 1,000 times per second) alter the shape of the mirror. In effect, the mirror’s shape is

altered to predistort an outgoing laser beam, which is then refocused by the turbulence through which

the beam travels on its way to the target.

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At this point, the final step in the sequence—missile destruction—is to

begin. During this final step, a lethal laser beam is held on the missile. The

length of time that the beam must dwell on the missile will depend on

turbulence levels and the missile type, hardness, range, and altitude.

Throughout the lethal dwell, the illuminator and beacon lasers are to

continue to operate, providing the information to operate the fast-steering

and deformable mirrors. Under the intense heat of the laser beam, which is

focused on an area about the size of a basketball, the missile’s pressurized

casing fractures, and then explodes, destroying the missile.

The ABL is expected to operate from a central base in the United States and

be available to be deployed worldwide. The program calls for a

seven-aircraft fleet, with five aircraft to be available for operational duty at

any given time. The other two aircraft are to be undergoing modifications

or down for maintenance or repair. When the ABLs are deployed, two

aircraft are to fly, in figure-eight patterns, above the clouds at about 

40,000 feet. Through in-flight refueling, which is to occur between 25,000

and 35,000 feet, and rotation of aircraft, two ABLs will always be on patrol,

thus ensuring 24-hour coverage of potential missile launch sites within the

theater of operations. The ABLs are intended to operate about 

90 kilometers behind the front line of friendly troops but could move

forward once air superiority has been established in the theater of

operations. When on patrol, the ABLs are to be provided the same sort of

fighter and/or surface-to-air missile protection provided to other

high-value air assets, such as the Airborne Warning and Control System

and the Joint Surveillance Target Attack Radar System.

ABL’s Operational

Effectiveness Is

Currently Unknown

A key factor in determining whether the ABL will be able to successfully

destroy a missile in its boost phase is the Air Force’s ability to predict the

levels of turbulence that the ABL is expected to encounter. Those levels are

needed to define the ABL’s technical requirements for turbulence. To date,

the Air Force has not shown that it can accurately predict the levels of

turbulence the ABL is expected to encounter or that its technical

requirements regarding turbulence is appropriate.

Correlation Between

Non-Optical and Optical

Turbulence Measurements

Is Needed

The type of turbulence that the ABL will encounter is referred to as optical

turbulence. It is caused by temperature variations in the atmosphere.

These variations distort and reduce the intensity of the laser beam. Optical

turbulence can be measured either optically on non-optically. Optical

measurements are taken by transmitting laser beams from one aircraft to

Page 7 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

instruments on board another aircraft at various altitudes and distances.

Non-optical measurements of turbulence are taken by radar or by

temperature probes mounted on balloons or on an aircraft’s exterior.

The Air Force’s ABL program office has not determined whether

non-optical measurements of turbulence can be mathematically correlated

with optical measurements. Without demonstrating that such a correlation

exists, the program office cannot ensure that the non-optical

measurements of turbulence that it is collecting are useful in predicting

the turbulence likely to be encountered by the ABL’s laser beam.

Concern about turbulence measurements was expressed by a DOD

oversight office nearly 1 year ago. In November 1996, during its 

milestone 1 review of the ABL program,4

 the Defense Acquisition Board

directed the program office to develop a plan for gathering additional data

on optical turbulence and present that plan to a senior-level ABL oversight

team for approval. The Board also asked the program office to

“demonstrate a quantifiable understanding of the range and range

variability due to optical turbulence and assess operational implications.”

This requirement was one of several that the Air Force has been asked to

meet before being granted the authority to proceed with development of

the ABL. That authority-to-proceed decision is scheduled for June 1998.

In February 1997, the program office presented to the oversight team a

plan for gathering only non-optical data. The oversight team accepted the

plan but noted concern that the plan was based on a “fundamental

assumption” of a correlation between non-optical and optical

measurements. If that assumption does not prove to be accurate,

according to the oversight team, the program office will have to develop a

new plan to gather more relevant (i.e., optical rather than non-optical)

measurements. Accordingly, the oversight team required that the program

office include in its data-gathering plan a statement agreeing to

demonstrate the correlation between the non-optical and optical

measurements. Program officials said they plan to demonstrate that

correlation in the summer of 1997.

To establish that a correlation exists, the program office plans to use

optical and non-optical turbulence measurements taken during a 1995 Air

Force project known as Airborne Laser Extended Atmospheric

Characterization Experiment (ABLE ACE). Optical measurements were made

by transmitting two laser beams from one aircraft to instruments aboard

4

This review is the point in time at which a new acquisition program is approved.

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B-275849 

another aircraft at distances from 13 to 198 kilometers and at altitudes

from 39,000 to 46,000 feet. These measurements provided the data used to

calculate the average turbulence strengths encountered by the beams over

these distances.

The ABLE ACE project also took non-optical measurements of turbulence

using temperature probes mounted on the exterior of one of the aircraft.

Rather than taking measurements over the path of a laser beam between

two aircraft, as with the optical measurements, the probes measured

temperature variations of the air as the aircraft flew its route.

Opinions vary within DOD about whether a correlation between optical and

non-optical turbulence measurements can be established. Some

atmospheric experts, who are members of the program office’s Working

Group on Atmospheric Characterization, criticized the program office’s

plan for collecting additional atmospheric data because it did not include

additional optical measurements. Minutes from a Working Group meeting

indicated that some of these experts believed that “current scientific

understanding is far too immature” to predict optical effects from

non-optical point measurements. In contrast, the chief scientist for the ABL

program said it would be surprising if the two measurements were not

directly related; he added that evaluations at specific points in the ABLE ACE

tests have already indicated a relationship. According to the chief scientist,

it would be prudent for the program office to continue to collect

non-optical data while it completes its in-depth analysis of the ABLE ACE

data.

According to a DOD headquarters official, because the ABL is an optical

weapon, gathering non-optical data without first establishing their

correlation to optical data is risky. The official concluded that, if the

program office cannot establish this correlation, turbulence data will have

to be gathered through optical means.

Technical Requirements

for Overcoming

Turbulence May Be

Understated

The ABL program office also has not shown that the turbulence levels in

which the ABL is being designed to operate are realistic. Available optical

data on optical turbulence indicate that the turbulence the ABL may

encounter could be four times greater than the design specifications.

These higher levels of optical turbulence would decrease the effective

range of the ABL system.

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B-275849 

The ABL program office set the ABL’s design specifications for optical

turbulence at a level twice that, according to a model, the ABL would likely

encounter at its operational altitude. This model was based on research

carried out in 1984 for the ground based laser/free electron laser program,

in which non-optical measurements were taken by 12 balloon flights at the

White Sands Missile Range in New Mexico. Each of the 12 flights took

temperature measurements at various altitudes. These measurements

were then used to develop a turbulence model that the program office

refers to as “clear 1 night.”

The clear 1 night model shows the average turbulence levels found at

various altitudes. The ABL is being designed to operate at about 

40,000 feet, so the turbulence expected at that level became the starting

point for setting the design specifications. To ensure that the ABL would

operate effectively at the intended ranges, for design purposes, the

program office doubled the turbulence levels indicated by its clear 1 night

model. The program office estimated that the ABL could be expected to

encounter turbulence at or below that level 85 percent of the time. This

estimate was based on the turbulence measured by 63 balloon flights made

at various locations in the United States during the 1980s.

When the ABL design specifications were established, the program office

had very little data on turbulence. However, more recent data,

accumulated during the ABLE ACE program, indicated that turbulence levels

in many areas were much greater than those the ABL is being designed to

handle. According to DOD officials, if such higher levels of turbulence are

encountered, the effective range of the ABL system would decrease, and the

risk that the ABL system would be underdesigned for its intended mission

would increase. DOD officials also indicated that a more realistic design

may not be achievable using current state-of-the-art technology.

ABLE ACE took optical measurements in various parts of the world,

including airspace over the United States, Japan, and Korea. According to

the program office and Office of the Secretary of Defense (OSD) analyses of

optical measurements taken during seven ABLE ACE missions, overall

turbulence levels exceeded the design specifications 50 percent of the

time. For the two ABLE ACE missions flown over Korea, the measurements

indicated turbulence of up to four times the design specifications.

Additionally, according to officials in OSD, ABLE ACE data were biased

toward benign, low-turbulent, nighttime conditions. According to these

officials, turbulence levels may be greater in the daytime.

Page 10 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

Developing and

Integrating ABL

Components Pose

Many Technical

Challenges

Developing and integrating a weapon-level laser, a beam control system,

and the many associated components and software systems into an

aircraft are unprecedented challenges for DOD. Although DOD has

integrated a weapon-level laser and beam control system on the ground at

White Sands Missile Range, it has not done so in an aircraft environment.

Therefore, it has not had to contend with size and weight restrictions,

motion and vibrations, and other factors unique to an aircraft

environment.

The COIL is in the early development stage. The Air Force must build the

laser to be able to contend with size and weight restrictions, motion and

vibrations, and other factors unique to an aircraft environment, yet be

powerful enough to sustain a killing force over a range of at least 

500 kilometers. It is to be constructed in a configuration that links

modules together to produce a single high-energy beam. The laser being

developed for the program definition and risk reduction phase will have

six modules. The laser to be developed for the engineering and

manufacturing development phase of the program will have 14 modules.

To date, one developmental module has been constructed and tested.

Although this developmental module exceeded its energy output

requirements, it is too heavy and too large to meet integration

requirements. The module currently weighs about 5,535 pounds and must

be reduced to about 2,777 pounds. The module’s width must also be

reduced by about one-third. To accomplish these reductions, many

components of the module may have to be built of advanced materials,

such as composites.

The ABL aircraft, a Boeing 747-400 Freighter, will require many

modifications to allow integration of the laser, beam control system, and

other components. A significant modification is the installation of the

beam control turret in the nose of the aircraft. The beam control turret is

to be used for acquisition, tracking, and pointing actions used in

destroying a missile. Consequently, the location of the turret is critical to

the success of the ABL. Issues associated with the turret include the

decreased aircraft performance resulting from the additional drag on the

aircraft; the interaction of the laser beam with the atmosphere next to the

turret, which can cause the laser beam to lose intensity; and vibrations

from the operation of the aircraft that affect the accuracy of pointing the

Page 11 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

beam control turret. The contractor has conducted wind tunnel tests of

these expected effects for three different turret locations and found that

installing the turret in the nose of the aircraft would cause the fewest

negative effects. However, the operational effectiveness of the beam

control turret will not be known until it undergoes additional testing in

2002 in an operationally realistic environment.

The laser exhaust system is another critical modification. The system must

prevent the hot corrosive laser exhaust from damaging the bottom of the

aircraft and other structural components made of conventional aluminum.

The exhaust created by the laser will reach about 500 degrees Fahrenheit

when it is ejected through the laser exhaust system on the bottom of the

aircraft. This exhaust system must also undergo additional testing on the

aircraft in 2002 to determine its operational effectiveness.

Integrating the beam control system with the aircraft also poses a

challenge for the Air Force. The Air Force must create a beam control

system, consisting of complex software programs, moving telescopes, and

sophisticated mirrors, that will compensate for the optical turbulence in

which the system is operating and control the direction and size of the

laser beam. In addition, the beam control system must be able to tolerate

the various kinds of motions and vibrations that will be encountered in an

aircraft environment. In deciding the on-board location of the beam

control system’s components, the Air Force used data gathered by an

extensive study of aircraft vibrations on the 747-400 Freighter. The beam

control components are expected to be located in those areas of the

aircraft that experience less intense vibrations and, to the extent possible,

be shielded from vibrations and other aircraft motion. To date, the 

Air Force has not demonstrated how well a beam control system of such

complexity can operate on an aircraft. The contractor has modeled the

ABL’s beam control system on a brassboard but has not tested it on board

an aircraft.5

Conclusions and

Recommendations

The ABL program is a revolutionary weapon system concept. Although DOD

has a long history with laser technologies, the ABL is its first attempt to

design, develop, and install a multimegawatt laser on an aircraft. As such,

the concept faces a number of technological challenges. A fundamental

challenge is for the Air Force to accurately and reliably predict the level of

optical turbulence that the ABL will encounter and then design the system

to operate effectively in that turbulence. The Air Force will not have

5

A brassboard is an experimental device (or group of devices) used to determine feasibility and

develop technical and operational data. It may resemble the end item but is not intended for use as the

end item.

Page 12 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

resolved that challenge until it has demonstrated whether there is a

reliable correlation between its non-optical and optical turbulence

measurements, or, should such a correlation not exist, gather additional

optical data, which may delay the ABL program. Whether relevant and

reliable data are confirmed through correlation or by additional optical

measurements, the data are critical in assessing the appropriateness of the

design specifications for turbulence. If the specifications need to be set

higher, that should be done as soon as possible.

Therefore, we recommend that the Secretary of Defense direct the

Secretary of the Air Force to take the following actions:

• Demonstrate as quickly as possible, but no later than the time when DOD

decides whether to grant the ABL program the authority to proceed

(currently scheduled for June 1998), the existence of a correlation

between the optical and non-optical turbulence data. If a correlation

between optical and non-optical data cannot be established, the Air Force

should be required to gather additional optical data to accurately predict

the turbulence levels the ABL may encounter, before being given the

authority to proceed with the program as planned.

• Validate the appropriateness of the design specification for turbulence

based on reliable data that are either derived from a correlation between

optical and non-optical data or obtained through the collection of

additional optical data.

Agency Comments DOD concurred with both of our recommendations. DOD’s comments are

reprinted in appendix I. DOD also provided technical comments that we

incorporated in this report where appropriate.

Scope and

Methodology

We reviewed and analyzed DOD, Air Force, ABL program office, and

contractor documents and studies regarding various aspects of the ABL

program. We discussed the ABL program with officials of the Office of the

Under Secretary of Defense (Comptroller); the Office of the Under

Secretary of Defense (Acquisition and Technology); the Air Combat

Command; the ABL program office; the Air Force’s Phillips Laboratory; and

the ABL Contractor team of Boeing, TRW, and Lockheed Martin. We also

discussed selected aspects of the ABL program with a consultant to the ABL

program office.

Page 13 GAO/NSIAD-98-37 Theater Missile Defense
B-275849 

We conducted our review from September 1996 to August 1997 in

accordance with generally accepted government auditing standards.

We are sending copies of this report to the congressional committees that

have jurisdiction over the matters discussed and to the Secretary of

Defense; the Secretary of the Air Force; and the Director, Office of

Management and Budget. We will make copies available to others on

request.

Please contact me at (202) 512-4841 if you or your staff have questions

concerning this report. Major contributors to this report were 

Steven Kuhta, Ted Baird, Suzanne MacFarlane, and Rich Horiuchi.

Sincerely yours,

Louis J. Rodrigues

Director, Defense Acquisitions Issues

Page 14 GAO/NSIAD-98-37 Theater Missile Defense
Page 15 GAO/NSIAD-98-37 Theater Missile Defense
Appendix I 

Comments From the Department of Defense

Page 16 GAO/NSIAD-98-37 Theater Missile Defense
Appendix I 

Comments From the Department of Defense

Now on p. 13.

Page 17 GAO/NSIAD-98-37 Theater Missile Defense
Appendix I 

Comments From the Department of Defense

Now on p. 13.

(707210) Page 18 GAO/NSIAD-98-37 Theater Missile Defense
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