Report to the Ranking Minority

Member, Subcommittee on Readiness

and Management Support, Committee

on Armed Services, U.S. Senate

United States General Accounting Office

GAO

August 2001 DEFENSE

SPECTRUM

MANAGEMENT

More Analysis Needed

to Support Spectrum

Use Decisions for the

1755-1850 MHz Band

GAO-01-795
Page i GAO-01-795 Defense Spectrum Management

Letter 1

Results in Brief 3

Background 5

Additional Analysis Is Required for Spectrum Decisions 10

Conclusions 22

Recommendations for Executive Action 23

Agency Comments and Our Evaluation 24

Scope and Methodology 24

Appendix I Spectrum Interference From Third Generation

Systems to DOD Satellites 26

Appendix II Replication of DOD’s and Industry’s IMT-2000 Base

Station Interference Models 36

Appendix III Comments From the Department of Defense 49

Appendix IV Comments From the Department of Commerce 55

Appendix V Comments From the Federal Communications

Commission 56

Tables

Table 1: Results of DOD and Industry Interference Calculations 39

Table 2: Comparison of DOD and Our Interference Results based

upon Our Replication of DOD’s Model 47

Table 3: Comparison of Industry and Our Interference Results

based upon Our Replication of Industry’s Model 47

Contents
Page ii GAO-01-795 Defense Spectrum Management

Abbreviations

BBA Balanced Budget Act

CTIA Cellular Telecommunications and Internet Association

DMSP Defense Meteorological Satellite Program

DOD Department of Defense

DSB Defense Science Board

DSCS Defense Satellite Communications System

DSP Defense Support Program

FCC Federal Communications Commission

FFRDC Federally Funded Research and Development Center

GPS Global Positioning System

IMT-2000 International Mobile Telecommunications-2000

ITFS Instructional Television Fixed Service

ITU International Telecommunication Union

JTCTS Joint Tactical Air Combat Training System

MDS Multipoint Distribution Service

MMDS Multichannel Multipoint Distribution Service

MSE Mobile Subscriber Equipment

NATO North Atlantic Treaty Organization

NDAA National Defense Authorization Act

NTIA National Telecommunication and Information Administration

OBRA Omnibus Budget Reconciliation Act

PSD Power Spectral Density

WARC World Administrative Radio Conference

WRC World Radiocommunication Conference
Page 1 GAO-01-795 Defense Spectrum Management

August 20, 2001

The Honorable James Inhofe

Ranking Minority Member

Subcommittee on Readiness and

 Management Support

Committee on Armed Services

United States Senate

Dear Senator Inhofe:

The use of commercial mobile wireless communications, such as personal

communications services and cell phones, has been escalating worldwide

over the radio frequency spectrum.1

 This spectrum is a scarce and finite

resource, which serves ever increasing and competing public and private

uses. The federal government has supported commercial services by

making spectrum available as these services developed over the years.

Some representatives of the commercial mobile radio service industry

claim additional spectrum is needed to support advanced communications

systems, called third generation systems. Other members of the industry

question the need for, or feasibility of providing, large amounts of

additional spectrum to meet industry requirements. These members point

to the need for increased efficiency in the use of spectrum through new

technologies. Third generation systems are being developed to provide

mobile voice, high-speed data, and Internet capabilities and are expected

to contribute significantly to the economic well-being of the United States.

Access to the radio frequency spectrum is also critically important to

federal, state, and local governments for national security, public safety,

and other governmental functions. Specifically, while the national interest

in a broad sense may be served by a robust commercial mobile wireless

system, national security also requires that the federal government be able

to meet its unique communications requirements to support domestic

training and worldwide military operations. Thus, determining the proper

 1

 Electromagnetic radiation is the propagation of energy that travels through space in the

form of waves. The radio frequency spectrum is the portion of electromagnetic spectrum

that carries radio waves. Frequency is the number of waves traveling by a given point per

unit of time, in cycles per second, or hertz. Radio frequency is usually measured in

thousands of hertz or kilohertz (kHz), millions of hertz or megahertz (MHz), and billions of

hertz or gigahertz (GHz).

United States General Accounting Office

Washington, DC 20548
Page 2 GAO-01-795 Defense Spectrum Management

use of a limited amount of spectrum, today and in the future, is a

challenging and complex task due to competing industry and

governmental demands.

In response to a presidential memorandum released in October of 2000,

the Department of Commerce issued a plan to select spectrum for

potential use by third generation wireless systems in the United States.

This plan designated the National Telecommunication and Information

Administration (NTIA) to study the 1755 to 1850 MHz band. Within the

United States, this band is allocated exclusively to the federal government,

particularly for defense purposes, such as space systems, mobile tactical

communications, and combat training.

To assist the NTIA, the Department of Defense (DOD) issued a report in

February 2001, which focuses primarily on spectrum sharing and

relocation issues in the United States. The report also describes potential

operational impacts and estimates the costs DOD would incur from future

reallocation of this spectrum to private sector users. DOD concluded that

vacating the band could not be accomplished for space systems until at

least 2017, and for most DOD non-space systems until at least 2010. The

report notes that DOD could face significant operational restrictions in any

frequency-sharing situation. In addition, the preliminary results of DOD’s

analysis indicate that comparable spectrum, which is operationally

suitable, may not be available for the relocation of the DOD systems.

NTIA’s final report in March 2001 incorporated DOD’s study results and

concluded that unrestricted sharing of the 1755 to 1850 MHz band is not

feasible and any other sharing option would require considerable

coordination. Further, the report notes that issues involving the

availability of comparable spectrum, reimbursement, and the time required

for federal entities to either modify or replace equipment would need to be

addressed before a decision could be made whether federal users could

share or vacate a band of spectrum.

Because of the importance of this issue to private and public interests in

the United States, you asked us to assess DOD’s study of the 1755 to 1850

MHz band and determine whether the study provides a sound basis for

decisions about the reallocation of this spectrum. As part of this

assessment, we reviewed and analyzed an industry consortium report that
Page 3 GAO-01-795 Defense Spectrum Management

reached different conclusions on space operations than the DOD study.2

We also reviewed a separate ongoing Air Force analysis that has similar

conclusions on space operations to the DOD study.3

 We did not review the

Department of Commerce or the Federal Communications Commission

(FCC) report or each agency’s process to identify and select additional

spectrum for third generation wireless systems.

Spectrum decisions based on either the DOD or the industry study of the

1755 to 1850 MHz band would be premature at this time. Neither study

contains adequate information to make reallocation decisions.4

 In

particular, we found that neither the DOD model nor the industry model is

mature enough to calculate spectrum interference to satellites, and,

therefore, cannot support a near-term decision. DOD recognized in its

report that additional analyses are needed to assess the impacts of any

reallocation or sharing decision. A third model to calculate this

interference, developed by the Air Force, shows promise but is still under

development.5

 All three studies used different assumptions because there

is no single agreed upon methodology or model today to estimate potential

spectrum interference from third generation wireless systems.

 2

 Report of the Industry Association Group on Identification of Spectrum for 3G Services,

an attachment to comments to the Federal Communications Commission’s notice of

proposed rulemaking on 3G submitted by the Industry Association Group. The Industry

Association Group is composed of the Cellular Telecommunications and Internet

Association, the Telecommunications Industry Association, and the Personal

Communications Industry Association.

3

 The Air Force study is being prepared by the Aerospace Corporation, a federally funded

research and development center (FFRDC). These centers are sponsored by government

agencies, but are privately administered by universities and other nonprofit corporations to

ensure objectivity and independence. Aerospace Corporation’s primary customer is the

Space and Missile Systems Center of the Air Force Materiel Command. The primary

responsibility of Aerospace Corporation is to ensure the mission success of national

security space programs.

4

 Potential spectrum interference with DOD satellite operations is discussed later in the

report, and appendixes I and II are devoted to this issue. This focus is the result of a lack of

time for us to conduct similar analyses on other DOD systems and does not mean that

problems are not present in other major tactical radio, air combat training, and precision

guided munitions systems.

5

 Aerospace Corporation reviewed the DOD and industry studies as a starting point and is

making changes in methodology as it deems appropriate. The Corporation will incorporate

these changes into future work.

Results in Brief
Page 4 GAO-01-795 Defense Spectrum Management

If the current schedule to auction spectrum is maintained, the federal

government will make decisions affecting national security without

knowing the full extent of the risks it faces or steps available to reduce

those risks. While the DOD study provides preliminary information about

DOD systems and operational requirements, it does not contain

information on critical considerations, which are necessary for decisions

on DOD’s ability to vacate the entire band or share all or part of the band

with industry. According to DOD officials, the following considerations

(1) were outside the scope of its study or (2) were not included in its study

because of a compressed time schedule and the lack of information on

alternative spectrum and development of third generation systems.

• First, additional technical and operational analysis is required to

determine the impact of third generation systems on military operations.

Without the proper technical and operational analyses, DOD risks a

reduction in military preparedness or a degradation of systems in the 1755

to 1850 MHz band that support mission capabilities. Specifically, DOD

faces an unknown risk of operational degradation to its satellite

operations that could include actual loss of control of its satellites and an

undetermined risk to the warfighter.

• Second, because additional analysis is required to estimate costs, DOD

risks not receiving a fair reimbursement value for the costs the agency

incurs to vacate or share the 1755 to 1850 MHz band. The DOD cost

estimate is potentially understated by billions of dollars because the

Department did not include satellite replacement costs.

• Third, because the DOD report did not fully consider the Department’s

future communication requirements, the Department risks losing spectrum

that may be needed for national security. DOD’s future warfare plans

assume unimpeded communications are readily available to deployed

forces.

• Fourth, DOD’s study process lacked adequate programmatic, budgetary,

technical, and scheduling guidance for command and operational units.

Without this guidance, command and operational units were unsure which

program or technical alternatives to include in the study.

• Finally, DOD did not include in its study the potential impacts of U.S.

spectrum reallocation decisions on international agreements and overseas

military operations. Reallocation of U.S. spectrum could have a negative

impact, such as loss of existing regulatory protection, on these agreements

and operations.
Page 5 GAO-01-795 Defense Spectrum Management

This report contains recommendations to the Secretary of Defense and the

Secretary of Commerce concerning actions needed to complete the

analysis of the impacts from a potential spectrum reallocation of the 1755

to 1850 MHz band. In commenting on a draft of this report, DOD, FCC, and

the Department of Commerce agreed with our findings and

recommendations. The National Security Council provided only technical

comments.

Wireless technologies have become a valuable asset to improve

communications efficiency and reduce costs for industries and

governments around the world. The first generation of wireless

technology, analog cell phones, is still used in many areas of the country.

The second generation of wireless technology added digital “personal

communications services,” such as voice mail, text messaging, and access

to the World Wide Web. The second generation is growing at an

extraordinary rate and is an essential part of the way the world does

business. Today, industry is also developing a third generation of personal

communications, which are expected to give consumers mobile, high data

rate, high-quality multi-media services.

The most technically suitable spectrum for mobile communications, public

and private, is below three gigahertz. This band of spectrum is the best

match for special spectrum propagation characteristics (such as distance,

capacity, and reliability) required by DOD, other federal agencies, and

commercial wireless firms. As a result, this spectrum is the subject of

much competition among the different users. This competition presents

major spectrum management issues for decisions by governmental

organizations within the United States and by international organizations

composed of sovereign nations.

FCC and NTIA manage the radio spectrum in the United States. NTIA is

the executive branch agency principally responsible for developing and

articulating domestic and international telecommunications policy for the

executive branch. NTIA is also responsible for managing the federal

government’s use of the radio spectrum. FCC, an independent agency of

the federal government, has authority over commercial spectrum use, as

well as the use of spectrum by state and local governments. NTIA and FCC

Background
Page 6 GAO-01-795 Defense Spectrum Management

manage the spectrum through a system of frequency allocations,

allotments, and assignments.6

Previously, Congress directed the reallocation of spectrum from federal to

private sector use under title VI of the Omnibus Budget Reconciliation Act

of 1993 (OBRA 93)7

 and later expanded the reallocation of spectrum under

title III of the Balanced Budget Act of 1997 (BBA 97).8

 Under these laws,

NTIA identified a total of 255 MHz for reallocation.9

Because radio waves transcend national borders and the number of global

services has been increasing, international coordination of spectrum is a

critical component of the spectrum allocation process. The

radiocommunication conferences of the International Telecommunication

Union (ITU)10 are the principal mechanisms for international spectrum

allocation via treaties. At the 2000 World Radiocommunicaton Conference

(WRC-2000), the ITU discussed spectrum and regulatory issues for

advanced mobile applications, including third generation services.

According to NTIA officials, the WRC-2000 and the ITU 1992 World

Administrative Radio Conference (WARC-92) identified a total of 749

 6

 The entire radio spectrum is divided into blocks, or bands, that are allocated for broad

categories of radio services, such as fixed, mobile, broadcasting, or satellite services. FCC

establishes rules that further define the particular types of use that are permitted within

each allocation. For example, a frequency band that is allocated to the mobile service may

be designated in the FCC rules for particular users such as business users, public safety

users, or cellular users. Allotments may be made within certain services, such as TV

broadcasting, where particular channels are provided in each geographic area. Assignment

refers to the final subdivision of the spectrum in which a party gets an assignment or

license to operate a radio transmitter on a specific channel or group of channels at a

particular location under specific conditions.

7

 P.L. 103-66, Aug. 10, 1993.

8

 P.L. 105-33, Aug. 5, 1997.

9

Eight MHz of spectrum was subsequently reclaimed per congressional direction. See

section 1062 of the National Defense Authorization Act for Fiscal Year 2000 (NDAA-2000)

(P.L. 106-65, Oct. 5, 1999). OBRA 93 required FCC to gradually allocate and assign

frequencies over the course of 10 years. The reallocation of the majority of the 235 MHz

identified under that act is still underway. BBA 97 imposed a stricter deadline for NTIA to

identify for reallocation and FCC to reallocate, auction, and assign licenses by September

2002 for the additional 20 MHz of federal spectrum.

10 The International Telecommunication Union is a United Nations specialized agency. The

federal government considers the ITU the principal competent and appropriate

international organization for the purpose of formulating international treaties and

understandings regarding certain telecommunications matters.
Page 7 GAO-01-795 Defense Spectrum Management

MHz11 of spectrum for use by sovereign countries wishing to implement

IMT-2000 (also known as third generation mobile wireless), including the

1755 to 1850 MHz band.12 The United States agreed that it would study

these bands domestically. Because of the significance of the services in

this band of spectrum, the United States did not commit to providing

additional spectrum for third generation systems.

Subsequent to WRC-2000, the President issued an executive memorandum

on October 13, 2000, that outlined a policy to encourage cooperation

among FCC, NTIA, other federal agencies, and the private sector to

determine if additional spectrum could be made available for third

generation wireless systems. In addition, the memorandum specified that

incumbent users of spectrum be treated equitably, taking national security

and public safety into account. The memorandum directed the Secretary

of Commerce to work cooperatively with FCC to develop a plan to select

spectrum for third generation wireless systems and to issue an interim

report on the current spectrum uses and the potential for reallocation or

sharing of the bands identified at WRC-2000. The plan established

spectrum sharing and relocation options to be studied for implementation

in the years 2003, 2006, and 2010. FCC, in conjunction with NTIA, was

expected to identify spectrum by July 2001, and auction licenses to

competing applicants by September 30, 2002.

Under the plan, NTIA studied the 1755 to 1850 MHz band and the FCC

studied the 2500 to 2690 MHz band.13 Within the United States, the 1755 to

1850 MHz band is allocated on an exclusive basis to the federal

government for fixed and mobile services and satellite control. DOD is the

 11 According to NTIA, WARC-92 identified 1885 to 2025 and 2110 to 2200 MHz and WRC2000 identified 806 to 960 MHz, 1710 to 1885 MHz, and 2500 to 2690 MHz.

12 According to NTIA officials, WRC-2000 indicated that identification of these bands does

not preclude the use of these bands by any services to which they are allocated and does

not establish priority for third generation systems in the radio regulations. A DOD official

said “identification” of spectrum for third generation mobile wireless systems does not

create a treaty obligation on each party to a treaty to use particular spectrum for third

generation systems.

13 The 1755 to 1850 MHz band is a federal government band used primarily for fixed point to

point microwave, air and ground mobile uses, and space operations. In the United States,

the 2500 to 2690 MHz band is currently used by the Instructional Television Fixed Service

(ITFS), Multipoint Distribution Service (MDS), and Multichannel Multipoint Distribution

Service (MMDS).
Page 8 GAO-01-795 Defense Spectrum Management

predominant user, although 13 other federal agencies operate extensive

fixed and mobile systems in this band throughout the United States.

To support NTIA’s efforts to study the 1755 to 1850 MHz band, DOD issued

a final report in February 2001. The report was prepared for DOD by the

Office of Spectrum Analysis and Management, which is part of the Defense

Information Systems Agency. In addition, responsibility for assisting in the

development of cost estimates associated with implementing study results

was assigned to the Department’s Cost Analysis Improvement Group.14 The

study addressed whether the Department could share or vacate this band.

The Office of Spectrum Analysis and Management grouped military

systems into five major categories. These categories included satellite

operations, tactical radio relay, air combat training, weapons data links,

and miscellaneous systems. Next, technical analyses were conducted to

calculate the effects of potential spectrum interference between proposed

commercial (third generation) and major military systems.15 For example,

DOD calculated the potential interference between selected DOD satellite

ground stations and anticipated third generation wireless systems (mobile

and fixed-base stations). DOD also calculated the potential interference

from third generation wireless systems on DOD satellite operations. In

another example of technical analysis, DOD selected two air combat

training ranges, which were considered representative of all training areas

in the United States, for analysis of potential spectrum interference.

Finally, the military commands and the operational communities

responsible for each major category of systems used the results of the

technical analysis to estimate the extent of any operational impacts on

their missions. Then, acquisition program officers provided cost estimates

for the options of sharing or vacating the band.

DOD’s spectrum report concluded that loss of access to spectrum, above

and beyond the spectrum already transferred as a result of OBRA 93 and

BBA 97, would jeopardize DOD’s ability to execute its mission.

Specifically, DOD concluded that it is unable to totally vacate the 1755 to

1850 MHz band until at least 2017 for space systems and at least 2010 for

 14 The Cost Analysis Improvement Group provides overall DOD guidance for accuracy of

DOD cost analyses used in program acquisition decisions.

15 DOD officials said their analysis incorporated expected performance characteristics of

third generation mobile wireless systems derived from published sources and coordinated

with NTIA and FCC.
Page 9 GAO-01-795 Defense Spectrum Management

non-space systems. DOD also found that full band sharing is not feasible.

However, DOD said that the compressed schedule, initiated by the

presidential memorandum and the Department of Commerce’s plan, did

not provide time for a thorough analysis and review of these complex

issues.

NTIA issued a spectrum report on March 30, 2001.16 Based in part on the

DOD report, NTIA found that the unrestricted sharing of the 1755 to 1850

MHz band is not feasible and that any other sharing option would require

considerable coordination between industry and DOD before third

generation systems can be operated along with federal systems.

Specifically, NTIA stated that there are several issues that must be

resolved before any spectrum can be made available for reallocation in the

1755 to 1850 MHz band. These include reimbursement issues17 and the

assurance of availability of comparable spectrum if DOD must surrender

spectrum in this band.18

On June 26, 2001, the Chairman of FCC wrote to the Secretary of

Commerce stating that additional time is necessary to allow the

Commission and the executive branch to complete evaluations of the

various options available for advanced wireless services. FCC sought

additional time to identify and schedule the auction of spectrum for third

generation wireless services. FCC stated in its letter that it wanted to work

with the executive branch and appropriate congressional committees to

come up with a revised allocation plan and auction timetable for third

 16 NTIA’s final report examined the potential for accommodating third generation mobile

wireless systems in the broader 1710 to 1850 MHz band.

17 The Strom Thurmond National Defense Authorization Act for the Fiscal Year 1999

(NDAA-99) (P.L. 105-251, Oct. 17, 1998) authorized federal entities to accept compensation

payments when they relocate or modify their frequency use to accommodate non-federal

users of the spectrum. These reimbursement provisions would be applicable to any

spectrum that is or would be reallocated in the 1755 to 1850 MHz band. Both NTIA and FCC

have issued notices of proposed rulemaking that address reimbursement provisions.

18 The National Defense Authorization Act for Fiscal Year 2000 (NDAA-2000) (P.L. 106-65,

Oct. 5, 1999) specified a number of conditions that have to be met if spectrum in which

DOD is the primary user is surrendered. This act requires that NTIA, in consultation with

FCC, identify and make available to DOD for its primary use, if necessary, an alternate

band(s) of frequency as a replacement for the band surrendered. Further, if such bands of

frequency are to be surrendered, the Secretaries of Defense and Commerce and the

Chairman of the Joint Chiefs of Staff must jointly certify to relevant congressional

committees that such alternative band(s) provide comparable technical characteristics to

restore essential military capability.
Page 10 GAO-01-795 Defense Spectrum Management

generation systems that will allow completion of necessary work. In a July

19, 2001, letter responding to the FCC request, the Secretary of Commerce

directed NTIA to work with FCC to develop a new plan for the selection of

spectrum for third generation mobile wireless systems. The Secretary

asked that this work be coordinated with appropriate executive branch

entities, such as the National Security Council, the National Economic

Council, the Office of Management and Budget, and DOD. He also

encouraged participants in this process to consider ways to achieve

flexibility with respect to the statutory auction dates if flexibility is needed

to implement the new plan. For example, the 1710 to 1755 MHz band that

is being considered for third generation purposes is under a statutory

deadline for the auction of licenses by September 2002.19

DOD’s February 2001 study does not provide a basis for decisions about

reallocation of spectrum in the 1755 to 1850 MHz band. The study was

constrained by lack of adequate guidance and by inadequate time and

information. Thus, major considerations either were not addressed or

were not adequately addressed in the final report. These considerations

include complete technical and operational analyses of anticipated

spectrum interference; cost estimates supporting DOD reimbursement

claims, spectrum requirements supporting future military operations;

programmatic, budgeting, and schedule decisions needed to guide

analyses of alternatives; and potential impacts of U.S. reallocation

decisions upon international agreements and operations. As a result,

DOD’s analysis was limited in its ability to adequately describe and

document potential technical, operational, and cost impacts should the

Department be required to vacate the 1755 to 1850 MHz band or to share it

with commercial users.

In addition, neither the industry analysis nor the Air Force analysis of

potential spectrum interference to DOD satellite systems provides an

adequate basis for decisions about reallocation of the 1755 to 1850 MHz

band from governmental to nongovernmental uses. The findings of both

analyses, along with DOD’s analysis of satellite interference, are not

reliable or complete.

 19 Section 3007 of P.L. 105-33 provides that the Commission shall conduct the competitive

bidding in such a manner as to ensure that all proceeds are deposited not later than

September 30, 2002.

Additional Analysis Is

Required for

Spectrum Decisions
Page 11 GAO-01-795 Defense Spectrum Management

Before making reallocation decisions with a significant impact on national

security and the economic welfare of the nation, the federal government

should approach the alternatives with knowledge gained from a sound and

complete analysis. Given an adequate amount of time, information, and

guidance, a study of DOD’s spectrum needs and requirements could

reduce operational and cost risks presented by critical spectrum

reallocation decisions. The alternatives considered for making radio

frequencies available to industry include DOD vacating the entire

spectrum band or sharing all or part of the spectrum band with industry.

DOD based its analysis of potential operational and cost impacts on the

premise that it could not accept any degradation of current mission

capability from a complete or partial reallocation of its spectrum to other

users. DOD also stated in its report that unrestricted sharing of the entire

band with third generation mobile systems would place unacceptable

operational restrictions on both DOD and commercial users of the band.

These factors caused DOD to establish critical conditions that it believes

must be met before it vacates or shares the 1755 to 1850 MHz band. These

conditions include (1) alternative spectrum must be provided comparable

to what the Department loses; (2) cost reimbursement must be timely as

required under current law; (3) the Department must receive the same

regulatory protection in any new spectrum as it now enjoys in the 1755 to

1850 MHz band; (4) defense systems must receive timely certification to

operate in any new spectrum band; and (5) new commercial users in the

1755 to 1850 MHz band must be prohibited from interfering with DOD’s

legacy systems while they migrate into their new spectrum. The

Department concluded in its report that these conditions could not be met

in the short term and that it must have continued access to the 1755 to

1850 MHz band until at least the year 2017 for satellite systems and until

approximately 2010 for other systems.

In its June 26, 2001, letter to NTIA, the Chairman of the FCC stated that the

entire federal government faces a challenging set of issues in addressing

how best to make available sufficient U.S. spectrum for advanced wireless

services and that the public interest would be best served by additional

time for informed consideration of these issues. DOD’s February 2001

report also recognizes that additional analyses are needed to fully assess

operational impacts and develop estimates of costs resulting from any

spectrum reallocation or sharing.

Importance of Analyzing

DOD’s Spectrum Needs

and Requirements
Page 12 GAO-01-795 Defense Spectrum Management

Based on our experience and previous work on spectrum management

issues,20 we believe an analysis of spectrum reallocation affecting DOD

would be more comprehensive if it included the following considerations:

• Additional technical and operational analyses to more completely reveal

the impact of third generation wireless systems on military systems and

any potential operational degradation of DOD systems.

• Additional analysis to estimate the cost of vacating or sharing the

frequency band and the level of reimbursement.

• Identification of the expanding future communication requirements to

allow DOD to include those requirements necessary for the envisioned

warfare strategies, which rely heavily on wireless communications.

• Appropriate programmatic, budgeting, schedule, and technical guidance to

the services and units conducting the analyses of the individual systems to

define the scope and breadth of the analysis and prepare an accurate

assessment of operational and cost impacts.

• Consideration of the impact of reallocation decisions on international

agreements and operations. A national spectrum strategy could give DOD,

FCC, and NTIA a guiding framework for decisions affecting training

operations with allies, overseas deployments, and international treaty

obligations. Spectrum reallocation impacts all of these areas.

Due to time and information constraints on DOD’s initial technical and

operational analyses, further study is required in these areas. For example,

the DOD report predicts interference to satellite operations from third

generation wireless systems by the year 2006 and states that this

interference could impede command and control of DOD satellites,

especially low-earth orbit satellites. However, officials from the

telecommunications industry have a different view. An industry analysis

states that interference from third generation mobile wireless systems to

DOD satellite receivers will be at acceptable levels and that sharing

between these systems and DOD satellites is possible without any efforts

to mitigate interference levels from the commercial systems. Our review of

 20 Defense Communications: Federal Frequency Spectrum Sale Could Impair Military

Operations (GAO/NSIAD-97-131, June 17, 1997).

Additional Technical and

Operational Analyses Are

Required for Spectrum Use

Decisions
Page 13 GAO-01-795 Defense Spectrum Management

the DOD and industry reports, as described below, however, found that

the two parties used different assumptions to calculate the extent of

potential interference and the impact this interference will have on

satellites. 21

A third view of the potential interference from the commercial systems on

DOD satellites is being developed by the Aerospace Corporation, a

federally funded research and development center, for the Air Force.

Aerospace officials said they are using many of the same assumptions

industry used in its analysis but that they are also using techniques and

assumptions not included in either the DOD or industry analysis. These

officials told us that their results to date confirm the DOD position that

third generation mobile wireless in the 1755 to 1850 MHz band could

interfere with DOD operation of its satellites. However, Aerospace

officials said they disagree with DOD on which satellites will be affected.

According to these officials, the satellites affected by spectrum

interference from third generation mobile wireless systems will be

medium-earth orbit (20,000 kilometers) and high-earth orbit (36,000

kilometers) satellites, not the satellites in low-earth orbit as forecasted by

DOD. When we spoke to Aerospace officials, they were starting to assess

the effects of spectrum interference on the operations of specific types of

satellites. These operational analyses are important for understanding the

full effect of potential spectrum interference on satellite performance. For

example, our review of Aerospace data suggests that estimated

interference levels from third generation mobile wireless systems are high

enough to adversely affect successful contact with the Global Positioning

System (GPS).22

Each analysis led to a different conclusion because, while certain general

engineering principles apply to estimating spectrum interference, no single

methodology or model exists today to estimate potential spectrum

interference to DOD satellite operations from third generation mobile

wireless systems. As a result, each party used different methodologies and

assumptions. In addition, our preliminary analysis of the DOD and

 21 See also appendix I.

22 GPS provides worldwide navigation and timing data to both military and civilian users.

The civilian market is estimated at approximately $10 billion dollars annually.
Page 14 GAO-01-795 Defense Spectrum Management

industry analyses indicates that questionable assumptions,23 inadequate

information, and a compressed schedule negatively impacted their

analyses. One questionable assumption that was used in DOD and industry

analyses was the assumption that cities would generate most of the

spectrum interference to satellites and that contribution from suburban

areas would be marginal. DOD officials said that including interference

from suburban and rural areas or along interstate highways increases the

projected amount of interference from third generation systems on the

satellites. They said this interference could be severe enough to disrupt the

command links to many satellites. However, DOD officials did not have

time to revise their published analysis to incorporate this new information.

Aerospace officials said they included an estimate for suburban and rural

interference that neither industry nor DOD recognized in their reports.

With respect to inadequate information, all three analyses lacked essential

information from industry about its plans for building and deploying third

generation systems. The lack of good information about future industry

plans for geographic coverage and density of third generation mobile

wireless systems creates a very high level of uncertainty about the levels

of energy from these systems that can cause interference with satellite

operations and lessens the reliability of all three estimates. NTIA officials

said this information was requested from industry representatives during a

series of government-led industry outreach meetings between November

2000 and February 2001. NTIA officials said, however, that industry

representatives refused to provide such information because it is

proprietary and could not be shared with competitors. In addition, as we

describe in appendix I, DOD may have significantly underestimated

potential interference to control of its satellites because it incorrectly

estimated the size of cities in its population database.

Another example of an incomplete technical and operational analysis is

the DOD assessment of third generation systems on airborne, precision

guided weapons training programs. These training programs use radio

spectrum for data links between the aircraft and the air launched weapons

within both the 1710 to 1755 and 1755 to 1850 MHz bands. The 1710 to 1755

MHz band was designated for reallocation from federal governmental to

non-governmental use pursuant to congressional direction, but federal

 23 We analyzed the DOD and industry models to the point that we could replicate their

estimates of the level of spectrum interference generated by the worldwide build-out of

third generation systems. We reviewed the Air Force's model but did not attempt to

replicate its findings.
Page 15 GAO-01-795 Defense Spectrum Management

operations can continue in the band within 16 protected zones.24 According

to NTIA officials, these sites were initially established by NTIA to protect

DOD ground and air training functions.25 However, in its March 2001

report, NTIA proposed, among other options, eliminating the 16 protection

zones and relocating all systems in the 1710 to 1755 MHz band to the 1755

to 1850 MHz band or a higher band. According to NTIA, this proposal was

necessary because no other solution was available to share the 1710 to

1755 MHz band with industry. The NTIA report noted that this proposal

was not in the FCC and Commerce plan to identify spectrum for potential

reallocation and had not been evaluated by DOD. Therefore, NTIA did not

know how this proposal would affect DOD operations. DOD had not

completed a review of this proposal at the time of our review.

The Air Force identified concerns to us about loss of spectrum in either

band. Air Force officials said the frequencies in the 1710 to 1755 MHz and

the 1755 to 1850 MHz bands operate as a pair,26 and loss of either

frequency would adversely affect the training operations in the other

frequency. For example, Air Force officials told us the existing 16

protection zones in the 1710 to 1755 MHz band are already too small in

land area to simulate realistic combat conditions, but eliminating them

entirely would stop all operationally realistic training at these sites. In

addition, an Air Force official stated that many training missions now

flown on low level training routes over most of the continental United

States would be severely degraded if further spectrum is lost in either

band.

However, Air Force officials said that no new spectrum has been selected

for training on precision guided weapons and that the total cost and

operational impact of changing frequency bands have not been fully

assessed. These officials stated that, at this time, any studies or analyses

have been based on numerous assumptions and, thus, study results are

preliminary. The cost of changing frequency bands could be substantial.

Recognizing that these are preliminary estimates, Air Force officials

believe that loss of spectrum in either band and moving to a higher band

 24 The Federal Power Administrations and public safety fixed links will also be protected.

25 FCC disagreed with NTIA. According to FCC technical comments, Air Force operations

were not identified to remain in this spectrum. FCC said retaining such operations would

have a detrimental impact on any significant use of the spectrum for nongovernmental

operations.

26 Frequencies in both bands are used for the same missions.
Page 16 GAO-01-795 Defense Spectrum Management

could cost up to $580 million in new equipment development and take up

to 10 years to complete.

Air Force and Navy officials managing precision guided weapons

programs also cited lack of time and information as the main reasons for

their inability to perform a detailed analysis. For example, these officials

stated that they did not have adequate time to receive input from training

bases and obtain technical information from the commercial providers of

the weapons systems to determine the feasibility of band sharing or

segmentation. DOD also said that it could not determine the amount of

new engineering work required for the communications components of

the weapons without knowledge of a new operating band.

The problems of a compressed time schedule and a lack of information

make it difficult for federal agencies to reach a reasonable decision about

reallocation of the 1755 to 1850 MHz band to nongovernmental uses. For

example, a decision to exclude third generation mobile systems from this

band because of potential interference to satellite or other operations

could mean economic loss to industry. On the other hand, allowing these

systems into the 1755 to 1850 MHz band when in fact they could interfere

with satellite or training operations could mean a reduction in military

preparedness, degradation of satellite performance, or even loss of

satellites in orbit.

We found that DOD—within time constraints, extensive programmatic

uncertainty, and available guidance—produced reasonable cost estimates

for the assumptions used in the studies. The cost estimates in DOD’s

report range from at least $2.8 billion to relocate major defense

communications systems from segments of the band to in excess of

$4.3 billion to fully vacate the band. However, the Department’s cost

estimates are incomplete because of program, budget, and technical

uncertainties and could be underestimated by billions of dollars.

The DOD report acknowledges that its cost estimates are preliminary and

states that they are not conclusive. According to the report, all of the cost

estimates are sensitive to many complex technical and budgetary

unknowns. For example, the report notes that implementation of

interference mitigation measures can greatly enhance opportunities for

spectrum sharing, but employing any of these techniques would require a

new cost assessment that could dramatically alter the cost estimates in the

report. In addition, Air Force officials told us they did not determine the

cost of replacing entire satellite systems to make room for third generation

Additional DOD Work

Required to Estimate Costs
Page 17 GAO-01-795 Defense Spectrum Management

mobile wireless systems before the year 2017. The DOD report states that

the 1755 to 1850 MHz band is used to control over 120 satellites in orbit

and that loss of this band before the year 2017 means it could no longer

control satellites in orbit and would have to replace them. According to Air

Force officials, they assumed continued access to the band for the life of

existing satellites.27 They said satellite systems, including spacecraft and

related ground infrastructure, costing billions of dollars, would become

useless if DOD were forced to vacate the 1755 to 1850 MHz band before

the year 2017. While replacing these satellite systems would cost billions

of dollars, Air Force officials also questioned whether industrial base or

launch facilities exist to build and launch significant numbers of new

satellites before the year 2017. The total system costs of these satellite

systems suggest that the replacement costs would be significant.28 For

example, DOD estimates total GPS program costs at $18.4 billion over a

43-year period–fiscal years 1974 through 2016.29 In a second example, the

Defense Meteorological Satellite Program (DMSP) has a total program

cost estimate through the year 2012 of $2.4 billion.30 Therefore, the cost

estimates could increase significantly if DOD is forced to vacate the 1755

to 1850 MHz band before 2017 and had to replace existing satellites before

the end of their normal life cycle.31

Finally, DOD cost estimates on vacating the 1755 to 1850 MHz band cannot

be completed until the alternative spectrum for DOD is identified. To date,

NTIA has not been able to identify alternative and comparable spectrum

available for federal use to replace the 1755 to 1850 MHz band. DOD’s

report states that relocation costs could vary depending upon the bands

selected as replacements for lost spectrum in the 1755 to 1850 MHz band.

 27 Air Force officials assumed that all satellites launched after the year 2010 would be

capable of using the Unified S- band (2025 to 2110 MHz band). However, as of June 2001 Air

Force officials told us no decision had been made to move to this band.

28 The Acting Assistant Secretary of Defense for Command, Control, Communications, and

Intelligence testified on July 31, 2001, before Congress that the more than 120 satellites

represent a cumulative investment of about $100 billion.

29 The GPS system is totally DOD funded, about $9.0 billion has been invested to date and

about $9.4 billion is planned to be invested.

30 The DMSP system is totally DOD funded and provides weather information primarily for

the military.

31 The National Security Council said that if relocation costs were incurred because of

international developments, the federal government may not be entitled to full

reimbursement.
Page 18 GAO-01-795 Defense Spectrum Management

A change in spectrum bands for weapons data links, for example, could

require either an extensive engineering redesign of antennas and other

radio equipment on both weapons and the aircraft delivering the weapons

or essentially a new major systems development program – depending on

the new spectrum band selected. In another example, changes to

frequencies used by existing satellites awaiting launch could delay the

launch by years in order to develop and manufacture key components for

the new frequencies.

Without complete cost estimates for the reallocation of spectrum, the

Department cannot ensure that it is receiving a fair reimbursement value

for the costs the Department incurs to vacate or share the 1755 to 1850

MHz band. Ultimately, if the Department is not fully reimbursed for the

costs of reallocation, the government would be responsible for the funds

needed to ensure that national defense is not degraded.

DOD’s report does not describe future spectrum requirements necessary

to meet the Department’s growing communications needs. The Defense

Science Board’s (DSB) November 2000 study on spectrum issues

concluded that the Department’s need for spectrum is escalating rapidly as

“information superior” forces become a reality and deploy.32 DSB said

wireless communication is particularly critical for the type of

geographically dispersed warfare contemplated in future concepts of the

individual services, such as the Marine Expeditionary Forces.33 The

Board’s study stated that the Department requires a proactive, needsbased strategy supported by detailed knowledge of DOD’s spectrum

requirements. DSB recommended the Department expand an ongoing

internal requirements study into an inventory of current and future

defense spectrum needs linked to military capabilities.

The Department’s spectrum report acknowledged that it is highly likely

that new defense requirements for this band and other military spectrum

bands will arise. However, the DOD spectrum report does not discuss

future spectrum requirements in any depth and does not attempt to

 32 Joint Vision 2010 establishes the DOD warfighting vision and defines “information

superiority” as the key enabler of the operational concepts of Joint Vision 2010.

33 A Marine Expeditionary Force is a warfighting arm of the Marine Corps that is composed

of about 45,000 personnel from a Marine division, various support activities, and a Marine

aircraft wing.

Future Military Spectrum

Requirements Not

Considered
Page 19 GAO-01-795 Defense Spectrum Management

quantify the requirements for the new systems. A Joint Chiefs of Staff

official said that an analysis of future requirements was outside the scope

of the report. After the report was issued, DOD provided us general

forecast information about fixed and mobile spectrum requirements. This

information projected an increase in mobile spectrum requirements, below

3 gigahertz, of 92 percent by the year 2005; fixed requirements increasing

by 60 percent by the year 2007; as well as more than 600 MHz of spectrum

for training by the year 2005. According to the information provided, any

sharing arrangement with third generation mobile wireless users in the

1755 to 1850 MHz band may not be workable in the longterm, unless DOD

freezes its spectrum requirements in this band.

We recognize that DOD’s requirements are likely to change as new

systems, technologies, and strategies are developed for the nation’s future

warfighting force. However without a better understanding of future

requirements, DOD increases its risk of losing access to bands of spectrum

necessary for future mission needs.

We also found that key programmatic, budgeting, and schedule decisions

had not been provided to appropriate command and operational units to

help prepare DOD operational and cost estimates. These decisions are

necessary to guide the Department’s analyses of alternative courses of

action to either share the spectrum or vacate the spectrum band.

For example, Army Mobile Subscriber Equipment (MSE) program officials

were uncertain about how band sharing would be accomplished for

operational and cost analyses. Because of this uncertainty and the time

constraints to complete the study, they chose a general, high-level

approach that did not consider important factors with operational and

cost implications in their assessment of the MSE program.34 Under this

high-level approach, the Army did not include the operational and cost

impacts of relocating reserve units to accommodate training requirements

or costs at individual bases to implement band sharing. In addition, the

Army based its analysis on the assumption that the accelerated

development and production of the High Capacity Line of Sight radio

would replace the MSE radio. However, Army assumptions about

accelerating production of the replacement radio have not been approved

in the DOD budget and would require additional funding and

 34 The MSE system consists of line of sight trunk radios linking switching centers.

DOD Analysis Requires

Additional Programmatic,

Budgeting, and Schedule

Guidance
Page 20 GAO-01-795 Defense Spectrum Management

reprogramming of funds to earlier years. In addition, replacing the MSE

radio with a new radio may not solve the Army’s tactical radio

communication problems should DOD have to share the 1755 to 1850 MHz

band with third generation mobile wireless users.

In another example of inadequate program and budget guidance, the Joint

Program Office for the Joint Tactical Air Combat Training System (JTCTS)

based its operational analysis on a plan to accelerate development and

fielding of that system,35 but Navy and Air Force sponsors of JTCTS had

not formally reviewed or approved this accelerated plan. In July 2001,

DOD officials told us the contract for this program had been cancelled

because of schedule and performance problems. According to these

officials, no schedule has been approved for a new program, and any

equipment from a new program could not be fielded until at least the

year 2014.

Without programmatic, budgetary, and scheduling guidance, command

and operational units risk uncertainty when assessing alternatives and

making assumptions in their analyses. Thus, the resulting operational and

cost estimates will also be uncertain.

The DOD report does not recognize or discuss planned development of

commercial wireless systems in other countries. Instead, it focuses on

potential operational degradation that may be caused by sharing the 1755

to 1850 MHz band with commercial wireless systems in the continental

United States. The services may need to operate on the same frequencies

overseas as they train on in the United States, and spectrum allocation

decisions in foreign nations could prevent the United States from using

these frequencies in other countries. According to a Joint Chiefs of Staff

official, the Joint Staff has produced several documents advocating greater

flexibility through the use of multiple frequencies in new defense systems

for overseas operations. However, the official said an analysis of overseas

operations was outside the scope of the DOD report.

Omission of any discussion of foreign spectrum developments creates an

unrecognized risk for DOD overseas operations, particularly DOD’s

 35 JTCTS is the next generation of aircrew training systems displaying weapon and aircraft

information in real-time. It is intended to support “rangeless” training that current systems

cannot support.

Reallocation Effects on

International Agreements

and Overseas Operations

Not Described
Page 21 GAO-01-795 Defense Spectrum Management

control of its satellites. The International Telecommunication Union has

identified the 1710 to 1885 MHz band as one of several bands for possible

use by third generation mobile wireless systems worldwide. Overseas

development in this band over the long term by current second generation

commercial wireless systems or future development of this band by third

generation wireless systems could result in spectrum interference with

U.S. satellites.36 Thus, DOD may have problems in the future controlling its

medium- to higher-earth orbit satellites from ground stations in the United

States because spectrum interference from overseas development of

commercial wireless systems in this band could be visible to these

satellites.37 This interference could occur even if the United States does not

allow commercial use of the 1755 to 1850 MHz band in this country.

Unilateral reallocation of the 1755 to 1850 MHz band by the United States

could also have a potential negative impact on U.S. international spectrum

agreements and overseas military operations. For example, DOD provides

communications support to Great Britain and the North Atlantic Treaty

Organization (NATO) within the 1755 to 1850 MHz band in the United

States. Further, military air combat training systems in this same band are

used by allied nations during training in the United States. The National

Telecommunication and Information Administration Organization Act, as

amended,38 requires, among other things, that NTIA, before acting on a

petition for relocation, determine that any proposed use of spectrum

frequency to which a federal entity will be relocated is consistent with

obligations undertaken by the United States in international agreements,

national security and public safety interests. Also, the proposed use must

be suitable for the technical characteristics of the band.

The November 2000 DSB study noted that the United States does not have

a national spectrum strategy that addresses international issues. The

Board said that international spectrum usage by DOD is governed by

treaty, status of forces agreements,39 and other arrangements with allied

 36 NTIA officials said the ITU identified several other bands for potential third generation

wireless use but that such identification does not preclude use of any of these bands by any

services to which they are now allocated and that no priority is accorded to third

generation wireless systems in the radio regulations.

37 NTIA officials expect deployment overseas will not occur first in the 1710 to 1885 MHz

band but in other bands recently auctioned throughout the world.

38 47 U.S.C. sec. 923(g)(2)(D).

39 Status of forces agreements and other arrangements govern interactions of U.S. forces

deployed overseas with foreign governments.
Page 22 GAO-01-795 Defense Spectrum Management

nations. The Board also recognized that other nations are asserting their

sovereign rights to manage their own spectrum, complicating deployments

of U.S. forces abroad. The Board recommended that the National Security

Council develop a national strategy given the increasing domestic and

international private sector demand for spectrum and the importance of

spectrum to national security. NTIA officials told us that federal agencies

requiring spectrum in other countries must work directly with foreign

countries because NTIA does not have responsibility or authority to

develop plans for federal use of foreign spectrum. For example, they said

DOD must press its strategic plans in NATO via country-to-country

alliances and develop international support for its requirements at

international radio conferences.

In the United States, there is a national allocation table wherein some 45

radiocommunication services are allocated spectrum. According to NTIA,

the allocation table and existing spectrum management processes

constitute a basic U.S. strategic spectrum plan, which covers all cases of

spectrum use. However, the national allocation table only reflects the

current landscape of spectrum use and does not provide a framework to

guide spectrum decisions into the future.

The Chairman of the FCC has also expressed support for overall

improvement of spectrum planning through a more coherent, nationally

harmonized spectrum policy. He said FCC is trying to improve

overarching coordination of the many existing policies within the

Commission, but critical spectrum is also controlled by other parts of the

federal government, each with its own area of responsibility. He said the

administration and Congress both play critical roles in allocating scarce

spectrum resources to the highest and best uses.

Original plans for identifying spectrum to support third generation mobile

wireless systems by July 30, 2001, and to auction licenses by September

30, 2002, were premature. We agree with FCC and the Department of

Commerce that delaying the identification of spectrum and the auction of

licenses for third generation wireless systems could serve the public

interest. Adequate information is not currently available to fully identify

and address the uncertainties and risks of reallocation. Thus, DOD and the

federal government could make decisions affecting national security

without knowing the full extent of risks they face or the steps available to

reduce those risks.

Conclusions
Page 23 GAO-01-795 Defense Spectrum Management

Extending the current schedule for identification and auction of licenses

for this portion of the spectrum would allow DOD to complete technical

and operational assessments and to consider future spectrum

requirements of DOD systems. In addition, a delay would allow the federal

government and DOD to further consider the adequacy of existing national

spectrum strategies affecting international agreements and DOD overseas

military operations, to modify these strategies as necessary, and to

incorporate these strategies into a DOD long-range spectrum plan. Also,

identifying potential alternative bands of spectrum would provide DOD

with needed information to complete its technical, operational, and cost

assessments. Including relevant national and DOD strategies into

spectrum reallocation decisions would allow a more informed decision

that balances national security interests and private interests. These

strategies could also guide detailed studies that develop adequate

measures to reallocate that spectrum and develop detailed cost estimates

before any auction is scheduled.

To more accurately assess the potential impacts to DOD if the 1755 to 1850

MHz band is selected for third generation systems in the United States, we

recommend that the Secretary of Defense

• complete a system-by-system analysis to determine existing and future

spectrum needs and requirements of systems in the 1755 to 1850 MHz

band;

• prepare a long-range spectrum plan and make programmatic decisions

necessary to carry out that plan; and

• complete the technical, operational, and cost assessments of satellite

systems in the 1755 to 1850 MHz band and review and complete

assessments of other systems as necessary.

To provide DOD with adequate time and guidance to complete its plans

and analysis, we recommend that the Secretary of Commerce

• incorporate a sufficient amount of time into the new NTIA plan to select

spectrum for third generation mobile wireless systems to address the

issues discussed in this report, specifically with respect to satellite

operations;

• direct NTIA, in conjunction with FCC, to identify comparable alternative

spectrum for use by the DOD systems before a decision is made to

Recommendations for

Executive Action
Page 24 GAO-01-795 Defense Spectrum Management

reallocate the 1755 to 1850 MHz band, should such an action be

contemplated; and

• coordinate with appropriate executive branch agencies to review existing

national spectrum management plans and policies, and, if necessary, to

establish a clearly defined national spectrum strategy reflecting DOD

requirements for international agreements and spectrum requirements to

operate overseas.

In commenting on a draft of this report, DOD, FCC, and the Department of

Commerce agreed with our findings and recommendations. All three

agencies and the National Security Council also provided technical

comments to ensure completeness and accuracy, and to provide clarity

and balance. These comments were reviewed and incorporated into our

report as necessary.

We also clarified our recommendation that the Secretary of Commerce

identify alternative spectrum for DOD use before any reallocation

decision. We included DOD's suggestion that NTIA work in conjunction

with FCC to identify possible nongovernmental spectrum for DOD use.

DOD, the Department of Commerce, and FCC comments are reprinted in

appendixes III, IV, and V, respectively.

To determine whether the DOD process to define defense spectrum needs

in the 1755 to 1850 MHz band experienced material constraints and

whether the final analysis and findings of the DOD report are complete or

need further work, we reviewed the Department’s data collections plans

and visited selected field locations, commands, and program offices to

review implementation of these plans. We also discussed data collection

issues and related cost issues with officials from the Office of the Assistant

Secretary of Defense for Command, Control, Communications, and

Intelligence; the Joint Chiefs of Staff Communications Division; the Office

of Spectrum Analysis and Management; and individual service spectrum

management offices. In addition, we reviewed findings and supporting

material in the Department’s February 2001 spectrum report. We reviewed

selected issues within the Department’s technical analysis and a separate

technical analysis prepared by a communications industry working group.

We also met with DOD and industry officials responsible for preparing

these technical analyses to obtain further information and to discuss our

findings on these issues. We reviewed material presented by Aerospace

officials and conducted a telephone conference with those officials.

Agency Comments

and Our Evaluation

Scope and

Methodology
Page 25 GAO-01-795 Defense Spectrum Management

(See appendix II for details of our analysis.) In addition, we met with NTIA

and FCC officials to discuss proposed rulemaking actions on spectrum

related issues.

We conducted our work from November 2000 through June 2001 in

accordance with generally accepted auditing standards.

We are sending copies of this report to appropriate congressional

committees. We are also sending this report to the Secretary of Defense,

the Secretary of the Air Force, the Secretary of the Army, the Secretary of

the Navy, the Secretary of Commerce, the Chairman of the Federal

Communications Commission, and the National Security Council. We will

make copies available to others upon request. The report will also be

available on our homepage at http://www.gao.gov.

If you or your staff have any questions concerning this report, please call

me on (202) 512-4841 or Charles Rey on (202) 512-4174. Other major

contributors to this report were Rahul Gupta, Arthur Fine, Robert Hadley,

Judy Lasley, Gary Middleton, Keith Rhodes, Joseph Rizzo, Jay Tallon, and

Dr. Hai Tran.

Sincerely yours,

Allen Li

Director

Acquisition and Sourcing Management
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 26 GAO-01-795 Defense Spectrum Management

While certain general engineering principles apply to any estimation of

spectrum interference, no single agreed-upon methodology or model

exists today to estimate potential interference to Department of Defense

(DOD) satellite operations from third generation mobile wireless systems.

DOD satellite operations control more than 120 satellites and their

payloads, including launch and early orbit operations, transmission of

mission data, on-orbit operations, and emergency and end-of-life

operations. These satellites provide for missile warning, navigation,

military communications, weather, and other defense missions. Specific

satellites controlled or served by DOD include (1) the Global Positioning

System, which provides navigational data and precise time transfer

capability to military and civilian users worldwide and (2) the Defense

Satellite Communications System, which provides essential command and

control communications. DOD controls these satellites through a set of

common and dedicated Air Force Space Command and Navy satellite

control sites both inside and outside the United States. This control is

exercised through the Space Ground Link Subsystem of the Air Force

Satellite Control Network or through other dedicated (mission specific)

satellite control networks. Agencies and other government users served

include the National Command Authority, DOD, Combatant Commanders,

the Federal Aviation Administration, and National Aeronautics and Space

Administration. Because some DOD satellites-most notably the Global

Position System-broadcast information on an unrestricted basis, DOD

satellite control also supports civil and commercial interests.

Developing a standard methodology to estimate potential spectrum

interference to DOD satellite operations from third generation mobile

wireless systems is particularly challenging because key assumptions must

be made not only about the technical characteristics of mobile wireless

systems to be fielded many years in the future, but also about the extent to

which these systems will be deployed worldwide in urban, suburban, and

rural environments. Unlike other DOD systems, satellites are unique in

that they may be exposed to the aggregate level of interference emanating

from significant portions of the globe. Moreover, because some DOD

satellites may need to operate within the 1755 to 1850 MHz band until at

least 2017 (and perhaps as late as 2030), interference estimates must

consider the extent of third generation mobile wireless systems

(IMT-2000) build-out over the next several decades. We reviewed, in

detail, two models–a DOD model and an industry model–that took

different approaches to estimating the level of interference DOD satellites

could experience from this worldwide build-out. The level of interference

predicted by these two models differed significantly. We also performed a

limited review of a third Air Force model being developed by the

Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 27 GAO-01-795 Defense Spectrum Management

Aerospace Corporation, a federally funded research and development

center. The Aerospace Corporation is developing its model in support of

the Air Force’s Space and Missile Systems Center and Air Force Space

Command to better understand why the DOD and industry models

produced such different results and to attempt to develop a trusted and

credible interference model to generate results that all stakeholders can

agree upon and that can be adopted as the standard in support of key

decisions.1

We found that neither the DOD nor industry model is fully mature and that

they differ strikingly from one another in many of their key assumptions.

In our opinion, neither model is mature enough to support a near-term

decision by the United States regarding whether DOD satellites can share

spectrum with third generation mobile wireless systems, and both

incorporate questionable assumptions that drastically influence their

predicted level of spectrum interference. Moreover, because IMT-2000

will be a worldwide system and because the International

Telecommunication Union has identified the 1710 to1855 MHz band

(among other bands) for use by countries wishing to implement IMT-2000,

some degree of spectrum sharing could become unavoidable regardless of

any reallocation decisions made within the United States. If the aggregate

level of the non-U.S. interference in the 1755 to 1850 MHz band proves to

be significant, DOD satellites could be adversely affected by overseas

deployment of IMT-2000.

As a result of this uncertainty, DOD faces an unquantified risk of

operational degradation to its satellite operations if a decision is made to

allow industry into the 1755 to 1850 MHz band in the United States before

more accurate information is available upon which to base a decision.

However, DOD may face such a risk irrespective of that decision due to

overseas build-out of IMT-2000.

 1

 We analyzed the DOD and industry models to the point that we could reproduce their

estimates of the level of spectrum interference generated by the worldwide build-out of

third generation systems. We reviewed the Air Force model but did not attempt to

replicate its findings.

Summary
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 28 GAO-01-795 Defense Spectrum Management

DOD and industry studies do not agree about how to predict the level of

potential interference to satellite receivers from the worldwide build-out

of third generation wireless systems or about the effect of this interference

on satellite operations. DOD predicts interference levels significant

enough to affect satellite operations by the year 2006, particularly for lowearth-orbit satellites. Industry, on the other hand, predicts interference

levels that are too low to adversely affect DOD satellite operations until at

least the year 2015 and perhaps never. The Air Force model incorporates

features more closely aligned with industry’s methods. However, the Air

Force model’s worst-case results for operating under nominal conditions

agree with the DOD report’s overall conclusion that third generation

wireless systems present potential interference problems to DOD

satellites. The Air Force model’s analysis suggests, however, that

interference is much less likely to occur with low earth-orbit satellites

(as DOD had concluded) than with medium and high earth-orbit satellites.

For example, the Air Force model’s analysis suggests that, under certain

modeling conditions, the Air Force may experience difficulty

communicating effectively with Global Positioning System and Defense

Support Program satellites because of interference from the build-out of

third generation systems.

A number of key data elements are necessary to finalize estimates of

potential third generation wireless systems interference to DOD satellite

operations. First, the locations and sizes of the geographic areas most

likely to be served by the mobile wireless industry must be properly

estimated, and translated into an estimate of the number of third

generation base stations needed to serve each geographic area. Second,

the amount of power needed by each base station to communicate with

mobile units within its area of coverage (typically referred to as a mobile

wireless base station’s “cell”) must be calculated. Third, the fraction of

that power that will reach the satellite must be estimated. In addition,

assuming some level of consensus can be reached regarding the best

assumptions and methodology for predicting the level of interference at

satellite orbital altitudes from the worldwide build-out of third generation

wireless systems, the operational impact of this interference on DOD’s

ability to communicate with particular types of satellites from specific

satellite control sites (both within and outside the United States) under

specific operating conditions would need to be fully analyzed and

presented to key decisionmakers before reasonable decisions can be

made.

Different Analytical

Approaches
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 29 GAO-01-795 Defense Spectrum Management

While they disagree on the specifics of the interference estimates, both

DOD and industry agree that low-power mobile stations are much less

likely to cause significant interference with DOD satellite operations.

However, they disagree about the potential interference to satellite

operations from much higher-powered base stations. Each base station

services a cell of a given size-up to about 10 kilometers in radius

(or 314 square kilometers), although smaller base stations are more likely

to be used within dense urban environments. Both DOD and industry

agree that the first step in determining the interference generated by

IMT-2000 base stations is to estimate the area of each geographic region

likely to be served by third generation mobile wireless systems. However,

neither DOD nor industry was able to locate a comprehensive database of

the actual areas (geographic dimensions) of the world’s population

centers. The only information readily available to both was the geographic

location and population of the world’s largest urban centers-generally

those with a population of 100,000 or more. Thus, both DOD and industry

used the population of each urban center to estimate how much land area

that population lives on. Those area estimates were then used to

determine the interference contribution from base stations within that

area. DOD’s approach did not explicitly estimate the number of base

stations required to service a given area. Rather, DOD assumed that the

power radiated per unit area from IMT-2000 base stations would be a

constant derived from reports and recommendations of the International

Telecommunication Union, and then multiplied this constant by the

estimated size of each urban area to determine the power radiated from

that area. Industry’s methodology used their estimated size of each service

area to calculate the number of maximum-sized (10 kilometer radius) base

stations that would be needed both to service that area, and to serve as an

“upper bound” on the estimate of total power radiating from that area if

smaller, more numerous base stations were used.

The first problem we identified is that both the DOD and industry models

are likely to have significantly underestimated the total number and size of

areas (urban, suburban, and rural) that are potential markets for third

generation systems. This problem exists, in part, because no standard

definitions exist of the most likely market areas for third generation

mobile wireless systems. The International Telecommunication Union

identified potential uses of third generation systems for urban, suburban,

and rural markets, but detailed forecasts of IMT-2000 build-out are not

available. Thus, both models are based on incomplete and inconsistent

databases as a source to calculate the number of urban centers and other

potential areas for service delivery.

Third Generation Mobile

Wireless Base Station

Coverage Areas
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 30 GAO-01-795 Defense Spectrum Management

For example, DOD used a database of 2,763 urban centers worldwide, of

which approximately 320 were urban centers within the United States. 2

Industry used a database of 3,312 urban centers worldwide, of which

approximately 209 were urban centers within the United States. However,

both of the databases included only capitals and urban centers with

populations over 100,000. The databases excluded many urban centers

below 100,000, as well as suburban and rural areas. The Air Force model

uses the same database of 2,763 urban centers used by DOD. However,

the Air Force’s model also includes an estimate of rural geographic areas

that could be served by third generation systems. The Air Force is working

to improve the overall quality of its population database. A 1990 U.S.

Census Bureau report indicates that, within the United States, there are

224 urbanized areas3

 with a population of more than 100,000, and 396

urbanized areas with a population of more than 50,000; many of the

additional 172 urbanized areas with a population between 50,000 and

100,000 are not included in any of the three models.

The population estimates for both the DOD and industry models are also

incomplete because they do not include any estimate of population

growth. The Air Force’s model includes an estimation of the population

growth based on population databases from 2 different years. DOD

estimates the lifetime of a number of satellites may extend until 2017 and

beyond, and perhaps until as late as the year 2030. Another indicator that

service areas may be significantly underestimated is the FCC report on

broadband (high data rate) use in the United States. This report

documents high speed Internet access in 59 percent of the postal zip codes

in the United States; according to the report, 91 percent of the U.S.

population is in these zip code areas. We believe it is reasonable to expect

that these counties would be candidates for third generation systems.

The second problem is that the DOD and industry models used different

equations to estimate the geographic area covered by their urban centers.

Both models analyzed 1990-era census data for a number of urban centers

whose areas are known to estimate the size of the rest of urban centers in

their databases. However, each model used different coefficients to

 2

 Best estimate based on urban center locations; the database of urban centers used by

DOD was not coded by country. Moreover, the origins of this database were not well

documented.

3

 The United States’ urbanized areas are defined by the U.S. Census Bureau. Each consists

of at least one central city or place and its urban fringe.
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 31 GAO-01-795 Defense Spectrum Management

calculate urban center size from population. Industry urban center sizes

were bigger than DOD urban center sizes, but the Air Force model

estimates urban center sizes are bigger than either DOD or industry

estimates. In addition, DOD used only three urban centers in the United

States, and five urban centers outside the United States, to obtain an

average and appears to have used incorrect numbers for urban center size

for two of the three U.S. urban centers. Thus, DOD calculated an average

urban center size of 144.2 square kilometers per 1 million of population.

Using 1990 U.S. census data, we calculated an average size for the 396

urbanized areas of about 1,000 square kilometers per one million people,

or about seven times as much area for a population of the same size.

DOD recognizes that its database may significantly underestimate

potential interference coming from rural and interstate highway base

stations and smaller urban areas that will likely be served by third

generation systems. According to DOD officials, their preliminary

calculations suggest that even partial coverage of rural U.S. interstate

highways alone will result in the deployment of thousands of additional

high-power base stations. The industry model did not include any

estimate for rural interference. The Air Force’s model included estimates

of interference from urban, suburban, and rural areas. According to

Aerospace Corporation representatives, results from this model showed

that significant interference came from rural areas.

Once the number of base stations is determined, the next required data

element is how much power each base station must use to service mobile

stations within its cell. DOD and industry disagree markedly in their

estimate of power radiating from the area covered by an individual

10-kilometer radius cell served by a single base station. Industry placed an

effective upper bounds of 30 watts of power for this 314 square kilometer

area.4

 Using this set of assumptions, DOD’s ITU-based method for

calculating power from this 314 square kilometer area would be about

2,600 watts of power, or nearly 100 times as much power. As noted above,

DOD’s analysis did not explicitly label this area as the service area of a

large base station. The Air Force’s model calculated the level of power

required to service a geographic area, taking into consideration the

propagation model suitable to a particular area. Their results showed that

 4

 Based on IMT-2000 technology over a 200 kHz bandwidth.

Power Requirements for

Each Base Station
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 32 GAO-01-795 Defense Spectrum Management

power radiating from an urban area would be significantly greater than

predicted by industry, but considerably less than predicted by DOD.

According to Aerospace Corporation representatives, the Air Force’s

model uses a higher power level than industry because their investigation

of second generation wireless systems suggests that power loss from each

base station is higher in urban areas than industry assumed. Buildings in

urban areas significantly lower distance covered by base stations. Industry

assumed that the power radiated from IMT-2000 base stations would be

proportional to the square of the base station cell’s radius irrespective of

the propagation environment. For example, a 314 square kilometer rural

area, served by a single 10-kilometer base station, would radiate

essentially the same amount of power as an urban area of the same size,

served by 100 1-kilometer base stations. According to Aerospace

Corporation representatives, the Air Force’s model relies upon

calculations of the specific distances that a base station could complete a

call to a mobile unit, resulting in significantly higher estimate of power

levels required of urban base stations within each area served by those

base stations.

The third required data element is a calculation of the amount of power

from all base stations that reaches the satellites. Power from third

generation systems reaching a DOD satellite is the sum of contributions

from all of the base stations in view of that satellite at any given moment

in time, and that contribution is the power each station generates times

the fraction of the amount of that power that arrives in space at the

satellite’s orbital altitude. After attempting to determine the number of

base stations and the power level required by urban base stations, the next

issue is to determine how much of the power generated by each base

station actually reaches DOD satellites. The International

Telecommunication Union provides little guidance for estimating

interference from IMT-2000 ground base stations to satellite operations.

The answer to this issue depends on two factors: (1) the way in which a

base station’s antenna concentrates the power it transmits in certain

preferred directions (generally referred to as the antenna’s gain pattern),

and (2) how much power radiated by a base station’s antenna is lost to

atmospheric and other environmental effects before reaching orbital

altitude. DOD assumed that each base station’s antenna radiates power

isotropically–that is, equally in all directions, but that 90 percent of this

power would be lost due to environmental factors. Industry, on the other

hand, employed an antenna gain pattern that assumes that the power

Base Station Power

Radiated Into Space
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 33 GAO-01-795 Defense Spectrum Management

radiated from a base station would be greatest at low elevation angles

(directed just below the horizon), and that only a small fraction of the base

station’s power would be directed toward satellites that are not at or near

the horizon. Industry further assumed that, at low elevation angles,

90 percent of the base station’s power transmitted in that direction would

be lost to the environment, but that most or all of the power radiated at

high elevation angles would reach space. However, as noted above,

overall the power radiated at high elevation angles represents a very small

portion of the base station’s radiated power. For intermediate elevation

angles, industry assumed that power loss would fall between these two

values – 0 and 90 percent. The Air Force’s model employed the same

antenna gain pattern as industry, but assumed that, other than at very low

elevation angles, only a small fraction of this power would be lost to the

environment before reaching space. As noted earlier, the Air Force’s

model still found potential problems from third generation wireless

systems to satellite operations.

Calculating power levels reaching satellites is also difficult because data

do not exist today to prepare a baseline, which can be compared to power

levels projected for third generation systems. Commercial systems now

operate in the 1755 to 1850 MHz band overseas, and DOD has military

systems operating in this band within the United States and overseas.

However, DOD officials said they have no data on existing second

generation system base stations and deployment patterns. DOD officials

told us that no interference has been experienced to date with satellite

operations because overseas mobile unit densities on a given satellite

control channel are much lower than those expected for third generation

systems.

Assuming some consensus can be reached regarding how much power

from IMT-2000 systems is likely to reach orbital altitudes, agreement

would still be required on how this level of interference is likely to affect

satellite operations. This analysis is complicated by the fact that, under

any modeling assumptions, IMT-2000 interference is likely to vary greatly

over the orbital “shell” at a given latitude, particularly for low-earth orbits.

Moreover, each type of satellite can be affected differently by interference

from third generation wireless systems because (1) any specific type of

satellite can follow a different orbit, (2) the technical characteristics of

satellites are different, and (3) each type of satellite has different abilities

to respond to DOD commands in the presence of interference because the

mission of each satellite type is different.

Operational Impact of

Potential Interference
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 34 GAO-01-795 Defense Spectrum Management

Neither DOD nor industry attempted a technical or an operational analysis

on each satellite to determine how specific satellites can be affected by

third generation interference from specific satellite control sites.5 Instead,

they both assumed a standard response irrespective of specific satellite

characteristics. However, DOD does acknowledge that parameters that

can affect its ability to communicate vary from satellite to satellite. As a

result, accurate and complete information is not available to determine if

operational workarounds can be planned to avoid loss of control or

degraded satellite operations.

The Air Force’s model suggests potential interference problems from third

generation base stations for DOD satellites. The Air Force model’s results

were, in fact, close to the DOD results, even though its methodology was

closer to that of industry. However, because it recognized the differences

between satellite types, the Air Force is also using its model to analyze the

potential effects from third generation wireless systems on specific types

of satellite operations. The Air Force’s analysis calculates interference

values for several types of satellites through their entire orbits, and

calculated probability of successful contacts for those satellites at any

given point in their orbits. For example, the Air Force model’s output data

show that interference levels for the Global Positioning Satellite were high

enough, under certain modeling assumptions, to prevent successful

contact, with an acceptable margin of safety, for approximately 60 percent

of an orbit. The Air Force model’s output data, however, has not yet

presented interference levels for the period of time that the satellite is in

sight of a specific ground station. Thus, the probability of successful

contact is not available for each satellite at each ground station. Such an

analysis is necessary to determine if alternative satellite contact plans can

be devised to enable successful contacts.

The Air Force study recognizes that specific operational impact analyses

should be performed for all ground stations and DOD satellites. The study

recommends that the Air Force model be evolved to include (1) improved

population modeling, (2) modeling of satellite contacts from specific

 5

 We recognize that industry may lack the information to perform such a detailed analysis.
Appendix I: Spectrum Interference From

Third Generation Systems to DOD Satellites

Page 35 GAO-01-795 Defense Spectrum Management

ground stations (3) analysis of operations under both nominal and adverse

conditions, and (4) inclusion of more types of DOD satellites.
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 36 GAO-01-795 Defense Spectrum Management

DOD and industry each developed models to predict the level of

interference to DOD satellites that could arise from the worldwide buildout of IMT-2000 base stations. The level of interference predicted by these

two models differed significantly. In order to determine whether DOD’s

and industry’s interference calculations could be reproduced and to obtain

a better understanding of the factors contributing to the differences

between DOD’s and industry’s results, we replicated DOD’s and industry’s

interference models. We also reviewed a third IMT-2000 interference study

being developed for the Air Force by the Aerospace Corporation, but did

not attempt to reproduce its findings. This appendix briefly describes

DOD’s and industry’s approaches to modeling IMT-2000 interference and

our approach to reconstructing those two models, and offers several

observations regarding the differing assumptions used by DOD and

industry for various components of their interference models.

Our replication of DOD and industry models was based on (1) the

mathematical descriptions of the models provided in the referenced

studies, (2) copies of the population databases used in each study (which

we obtained from DOD and industry representatives), and (3) equations

adopted from a standard space mission planning textbook that describe

the geometrical relationship between a point on the surface of the earth

and a point in space at a satellite’s orbital altitude. To clarify certain

matters regarding calculations used in these models, we also met and

corresponded with DOD and industry representatives on several

occasions. As shown in tables 2 and 3, we were able to reproduce both

DOD’s and industry’s estimates for peak IMT-2000 interference levels to

within about 0.1 dB for most orbital altitudes.

Each analysis led to a different conclusion because, while certain general

engineering principles apply to estimating spectrum interference, no single

methodology or model exists today to estimate potential spectrum

interference to DOD satellite operations from third generation mobile

wireless systems. DOD’s IMT-2000 interference model is described in

DOD’s final report on accommodating IMT-2000 within the 1755 to 1850

Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station Interference

Models

Summary

DOD’s and Industry’s

Approaches to

IMT-2000 Interference

Modeling
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 37 GAO-01-795 Defense Spectrum Management

MHz band.1 Industry’s model is described in the Report of the Working

Group on Satellite Control Systems.2

Each model incorporated a database describing the physical location

(longitude and latitude) and population of the world’s largest urban

centers, assumed that IMT-2000 service would be provided within the 1755

to 1850 MHz band at each of these locations, and then approximated the

size (geographic extent) of each urban center using a model-specific

parametric equation to relate an urban center’s population to its

geographic area. For each geographic area, both models then computed

the aggregate power spectral density (PSD) that would radiate from all of

the IMT-2000 base stations needed to provide service within that

geographic area. To reduce the computational complexity of this

calculation, both models assumed that, for typical orbital altitudes, the

aggregate PSD received from all of the IMT-2000 base stations within a

given urban area could be closely approximated by a point source at the

center of the geographic area whose PSD is the sum of the PSD from each

IMT-2000 base station operating within the area served. Both models then

employed link budget calculations to compute the interference

contribution from each urban center at any given point S on an “orbital

shell” defined by the altitude of a class of satellites. In logarithmic form,

the interference contribution for each visible urban center is given by:

R T T km MHz LE I = 10× log(I ) + G −32.44 − 20× log(r ) − 20× log( f ) − [1]

where:

RI = power spectral density received at S from the urban center in decibel

watts per Hertz

 1 Department of Defense: Investigation of the Feasibility of Accommodating the

International Mobile Telecommunications (IMT) 2000 Within the 1755-1850 MHz Band;

(Feb. 9, 2001).

2 Evaluation of Sharing between International Mobile Telecommunications (IMT) 2000

Technology and Satellite Control Systems Operating in the Band 1755-1850 MHz

(Feb. 19, 2001) [Note: This report was filed with the Federal Communications Commission

as Attachment II of the Report of the Industry Association Group on Identification of

Spectrum For 3G Services (Feb. 22, 2001) in response to the Commission’s notice of

proposed rulemaking on 3G.]
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 38 GAO-01-795 Defense Spectrum Management

TI = power spectral density transmitted from the urban center in watts per

Hertz

GT = gain of the transmitting antenna relative to an isotropic antenna (dBi)

km r = slant range from the urban center to the point S in kilometers

MHz f = operating frequency in megahertz

LE = environmental loss in dB

Note that specific assumptions about the gain of the satellite receiver’s

antenna are suppressed in equation [1] in order to consider the

interference environment generated by IMT-2000 base stations irrespective

of a particular satellite receiver’s antenna specifications. However, it is

assumed the satellite receiver’s antenna is gain-limited.

Calculating the interference contribution for each urban center required

analyzing the particular earth-space geometry between the point S and the

location of each urban center. Specifically, only those urban centers

visible from the point S contribute to the aggregate interference level.

Thus, the elevation angle of the point S as seen from each urban center

had to be calculated to determine whether that urban center was visible

(above the local horizon). Moreover, for the industry model, the

transmitting antenna’s gain, and the environmental loss were functions of

this local elevation angle. In addition, the slant range between each urban

center and the point S had to be calculated to complete the interference

calculation. After obtaining solutions for equation [1] for each visible

urban center, both models then summed these contributions to arrive at

the aggregate interference level at the point S.

Finally, both models calculated the aggregate interference at each point S ́

on a lattice that spanned the orbital shell from –180 degrees to 180 degrees

longitude, and from –90 degrees to 90 degrees latitude to find the

approximate location and value of the highest predicted interference level

worldwide. This interference level was then used to examine the “worst

case” effects of IMT-2000 interference on satellite receivers. Each model

performed this analysis for four orbital shells representing four typical

satellite orbital altitudes: (1) 250 km, the typical orbital altitude of the

space shuttle; (2) 833 km, the orbital altitude of DOD’s Defense

Meteorological Satellite Program (DMSP) satellites; (3) 20,200 km, the
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 39 GAO-01-795 Defense Spectrum Management

orbital altitude of DOD’s Global Positioning System (GPS) satellites; and

(4) 35,784 km,3

 the orbital altitude of several types of geosynchronous

satellites including the Defense Support Program (DSP) and the Defense

Satellite Communications System (DSCS). Each step in this calculation is

described in some detail later in this appendix.

DOD’s and industry’s results for peak IMT-2000 interference differed

significantly. As shown in table 1, DOD’s peak PSD levels are about 5 dB

higher than industry’s levels for each of the orbital shells analyzed. For

both models, values in the table refer to the PSD at the front end of the

satellite’s antenna and are, therefore, independent of the specific

characteristics of this antenna.4

Table 1: Results of DOD and Industry Interference Calculations

Satellite altitude

(km)

DOD peak PSD

(dBw/Hz)

Industry peak PSD

(dBW/Hz)

Difference

(dB)

250 - 161.5 - 166.2 4.7

833 - 166.6 - 173.2 6.6

20,200 - 186.7 - 191.9 5.2

35,784a- 191.1 - 195.8 4.7

a

Industry used a slightly smaller value for this geosynchronous earth orbit altitude; the use of DOD’s

slightly larger value would likely decrease industry’s peak PSD for this altitude by less than 0.01 dB

In order to determine whether DOD’s and industry’s interference

calculations could be reproduced and to obtain a better understanding of

the factors contributing to the differences between DOD’s and industry’s

results, we replicated DOD’s and industry’s interference models. In

general, sufficient information was available in the published DOD and

industry reports to recreate the two models once we had obtained the

models’ urban center population databases. Both the DOD and industry

reports provided their versions of equation [1] used to calculate the

 3

 Industry used a slightly smaller assumed value for its analysis of the geosynchronous

orbital shell.

4

 It should be noted that industry’s report on IMT-2000 interference to satellites shows

interference results that are 5 dB lower in each case than those given in table 1 because

industry’s interference levels were calculated at the front end of the satellite receiver after

accounting for the (assumed) – 5 dB gain of the satellite receiver’s antenna. Because DOD’s

reported PSD levels are taken at the front end of the satellite antenna, industry’s numbers

have been adjusted here to make possible an “apples-to-apples” comparison.

Results of DOD and

Industry Interference

Calculations

Analysis of DOD’s and

Industry’s IMT-2000

Interference Calculations
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 40 GAO-01-795 Defense Spectrum Management

interference from a single urban center to a point S on the orbital shell at a

given slant range and local elevation angle. To clarify certain matters

regarding calculations used in these models, we also met and

corresponded with DOD and industry representatives on several

occasions. We developed versions of DOD’s and industry’s models that

incorporated all of the salient features of the original models.

DOD’s and industry’s databases containing the longitude, latitude and

population of the world’s largest urban centers were obtained from DOD

and industry representatives and incorporated into our models. For each

model, we first developed a “single-point” PSD calculation module that

could calculate the aggregate PSD from all of the urban centers in the

model’s database within view of any single point S (defined by its

longitude, latitude and orbital altitude). We applied slant range and

satellite viewing angle formulas from a standard textbook on space

mission planning.5

 Specifically, we applied suitable forms of equations

from this source in our replication of DOD’s and industry’s models.

Equations derived from the DOD and industry reports that calculated an

urban center’s geographic area, the aggregate PSD radiating from an urban

center, the base station antenna gain, and base station PSD environmental

losses were also incorporated into our models. Finally a suitable form of

equation [1] was incorporated into our models to calculate the

interference contribution from each visible urban center at the point S.

To calculate the PSD for a lattice of points that spanned the entire orbital

shell, we incorporated our “single-point” PSD calculation modules into an

iterative routine that calculated and stored the location information

(longitude and latitude) and associated PSD value for each point on the

orbital shell’s lattice. The data generated were then analyzed to determine

the value and location of the peak PSD for the orbital shell. We carried out

calculations over a 2-degree by 2-degree lattice using the same four orbital

shells analyzed by DOD and industry. Each key element of the modeling

exercise is discussed briefly below. In addition, selected observations on

DOD’s and industry’s assumptions with respect to those elements of the

models are also included.

 5 Space Mission Analysis and Design; 2nd. ed. Wiley J. Larson and James R. Wertz

(editors); Kluwer Academic Publishers (1992); Chapter 5; “Space Mission Geometry”;

pp. 110-111.
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 41 GAO-01-795 Defense Spectrum Management

1. IMT-2000 Base Station Service Area for Each Urban Center – DOD and

industry both used databases describing the location (longitude and

latitude) and population of the world’s largest urban centers, and then

approximated the size (geographic extent) of each urban center using a

model-specific parametric equation to relate an urban center’s population

to its geographic area.

1.1 Population Databases – DOD’s model used a database describing

the physical location (longitude and latitude) and population of 2,763

of the world’s most populous urban centers, with a total population of

1.33 billion. Data were obtained from a variety of sources, including

United Nations statistical data and U.S. Census Bureau data. Industry

used a similar (though not identical) database of 3,312 of the world’s

most populous urban centers obtained from United Nations data with

a total population of 1.64 billion. Both of these databases were

generally limited to urban centers with populations of 100,000 or more,

and both were based on 1990-era census data. In contrast, the United

Nations Population Division has estimated the world’s urban

population at about 2.85 billion in 2000 and projects that this figure

will grow to about 3.82 billion by 2015.

1.2 Urban Center Area Estimation – Both DOD and industry

approximated the size (geographic extent) of the urban centers in their

respective database by applying a parametric equation of the form:

β RP = α × P [2]

to each urban center where:

RP = radius of the urban center with population P

P = urban center’s population

α = constant of proportionality

β = scaling factor

DOD used data on the population and land area of 8 large urban

centers around the world (including 3 urban centers in the United

States) to calculate an average inverse population density of 144.2 km2

per million people, and applied this constant of proportionality to all

2,763 urban centers in its database. Assuming that the area of each
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 42 GAO-01-795 Defense Spectrum Management

urban center is a circle, this relationship can be expressed in the

parametric form of equation [2] with:

.006775

1,000,000

144.2 ≈ × = π

α DOD [2A]

β DOD = 0.5 [2B]

Industry used a more extensive sampling of data from the 1990 U.S.

census to obtain a “best fit” to the parametric equation described in

equation [2] for cities within the United States, and used United

Nations data to obtain a “best fit” for cities outside the United States.

For cities within the United States, industry calculated values of:

= 0.035 α Industry [2C]

= 0.44 βIndustry [2D]

According to industry’s report, for cities outside the United States, this

scaling factor remains unchanged, but a smaller constant of

proportionality was calculated to account for the fact that urban

centers outside the United States tend to be more densely populated

than urban centers within the United States. However, the principal

author of industry’s study informed us that he had, in fact, used the

single, more conservative constant shown in equation [2C].

Our models replicate these calculations to obtain estimates for the

geographic area of each of the urban centers. It should be noted that

industry’s model assumed that urban centers are 4 to 7 times larger

than DOD assumed. For example, while DOD assumed that an urban

center of 1 million people would have a total land area of 144.2 km2

,

industry assumed that this urban center would have a total land area of

733.2 km2

—about 5 times as large. We also compared these values with

the average inverse population density calculated from our analysis of

data from the 1990 U.S. Census Bureau report on the population and
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 43 GAO-01-795 Defense Spectrum Management

land area of the 396 urbanized areas in the United States.6 We found

that the average inverse population density for these 396 urbanized

areas was about 1,000 km2/million people.7

2. PSD Radiated by IMT-2000 Base Stations from an Urban Center – Once

the geographic area of each urban center had been approximated, DOD

and industry models each calculated the aggregate PSD for all of the

IMT-2000 base stations that were assumed to be operating within the

boundaries of each area. DOD’s approach applied a constant PSD per unit

area derived from reports and recommendations of the International

Telecommunication Union. Industry’s approach was based upon

consideration of the technical characteristics of IMT-2000 base stations

employing UWC-136 (TDMA) technology. Because DOD’s approach was

much simpler than industry’s, the two approaches are described separately

here.

2.1 DOD’s Approach — DOD’s approach to calculating the PSD

radiated by IMT-2000 base stations from a given urban center was very

simple. DOD assumed that IMT-2000 base stations serving all urban

land areas would radiate a constant PSD per unit area of

41 μwatts/Hz/km2

, or - 43.9 dBW/Hz/km2. This figure was derived from

one report and one recommendation of the International

Telecommunication Union (ITU).8

 DOD did not attempt to explicitly

calculate the number of base stations to which this PSD value

corresponded. Rather, DOD’s model multiplies this value by the size of

the urban center calculated using equation [2]. We incorporated a

suitable form of this calculation into our model.

 6 1990 Census of Population and Housing: Supplementary Reports – Urbanized Areas of

the United Sates and Puerto Rico (1990 CPH-S-1-2). U.S. Department of Commerce;

Economics and Statistics Administration; Bureau of the Census (December 1993); table 2,

“Rank of Urbanized Areas by 1990 Population; Housing Units, Area Measurements and

Density: 1990.” [Note: Nearly identical information on the 1990 population and land area of

these 396 urbanized areas was also found on a commercial website –

www.demographia.com].

7

 Calculated as {Total land area for the 396 urbanized areas} × 1,000,000/{Total population

of the 396 urbanized areas}.

8

 Recommendation ITU-R M.687-2 (1990) estimated a PSD of 38μW/Hz/km2 for IMT-2000

base and mobile stations. DOD updated this figure using information from ITU-R Report

M.2023. We did not attempt to replicate DOD’s calculation but accepted the 41μW/Hz/km2

as the input PSD for our reproduction of DOD’s model.
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 44 GAO-01-795 Defense Spectrum Management

2.2 Industry’s Approach — Industry’s approach to calculating the PSD

radiated from IMT-2000 base stations from a given urban center was

more complex. Industry first estimated the number of base stations of

a particular size and deployment penetration factor required to serve

the urban center’s area (as calculated using equation [2].) Industry

assumed that each base station would serve a circular area of radius

Rh . Industry then calculated that while the relative mix of differentsized base stations might change from one urban center to the next,

the maximum number of any particular size of base stations of service

radius Rh , needed to fully serve an urban center could be calculated

using equation (6) from industry’s report. Finally, industry assumed

that the PSD radiated from a base station with a smaller service radius

Rh relative to the PSD of a base station with the largest assumed

service radius (10 km) Rh−max would be directly proportional to the

square of ratio of these two radii, that is:

2

max

max ( ) ( ) 



 



 =

−

−

h

h

h h R

R P R P R [3]

Industry argued that adding 2 to the number of maximum-sized base

stations calculated using equation (6) from the industry report would

result in an aggregate PSD for each urban center that was as least as

large as the aggregate PSD from any other relative mix of 3 differentsized base stations. Industry’s “upper bound” method is described in

more detail in its published report. We applied suitable forms of

equations derived from industry’s “upper bond” method to our model.

Unlike DOD, industry did not assume that each urban center in its

database would be fully “built out.” Rather, industry developed a

population density-dependent deployment penetration factor to

account for the fact that less densely populated urban centers may not

achieve the same level of IMT-2000 penetration as more densely

populated urban centers. Industry’s deployment penetration factor

curve appears as figure 2 of its report. However, because the industry

report did not explain, in detail, its assumed values for this curve, we

did not initially have enough data to replicate this curve precisely for

all population densities. Consequently, we approximated this curve by

visually extracting data points from the published curve and then

fitting this data to a curve that would adequately reproduced the
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 45 GAO-01-795 Defense Spectrum Management

published curve. To approximate industry’s penetration curve, we used

a four-parameter equation of the form:

( ) (1 exp( )) b e

P η D = − aD + cD [4]

where:

(D) ηP = IMT-2000 deployment penetration factor

D = urban center’s population density (people/km2).

a,b,c,e are a set of “best fit” parameters

During subsequent correspondence with the principal author of the

industry study, we were provided with the table of values for

population density versus deployment penetration factor used in the

industry model. However, a comparison between the estimated total

number of base stations worldwide predicted using our “best fit”

penetration curve and the total number predicted using industry’s

approach showed less than a 0.1 percent difference. Consequently, we

did not adjust this factor in our replication of industry’s model.

We noted that industry’s introduction of a deployment penetration

factor into its model ultimately made very little difference to the

predicted aggregate PSD level because most urban centers in

industry’s database had relatively large (calculated) population

densities, resulting in penetration factors close to one, and because

industry’s “upper bound” method tended to suppress the effects of the

generally lower penetration factors calculated for the smaller urban

centers in industry’s database. For example, we calculated that the

3,312 urban centers in industry’s database would be served by a total

of 11,974 base stations based upon our “best fit” penetration curve, but

that using a 100 percent penetration factor for all urban centers

increased this total by only 203 base stations (to 12,177) or by about

1.7 percent.

3. PSD Received at Point S – Once the aggregate PSD radiated from each

urban center is calculated, equation [1] is used to calculate the

contribution from each visible urban center at the point S. This requires

consideration of the IMT-2000 base stations vertical antenna gain pattern

and elevation-dependent environmental losses.
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 46 GAO-01-795 Defense Spectrum Management

3.1 IMT-2000 Base Station Antenna Gain Pattern – DOD’s model

assumed that IMT-2000 base stations would radiate power

isotropically. Thus, DOD assumed that in calculating an urban center’s

interference contribution using equation [1], GT = 0 in all cases. Our

reproduction of DOD’s model incorporates this assumption. Industry’s

model used an antenna gain pattern based upon an ITU document9

 to

calculate an elevation angle-dependent antenna gain (ε) GT , which is

described by equations (11a) and (11b) in industry’s report. We

incorporated a suitable form of these equations into our reproduction

of industry’s model.

3.2 IMT-2000 Environmental Losses – DOD’s model assumed that 90

percent of the power radiated from IMT-2000 base stations would be

lost to the environment before reaching satellite orbital altitudes

irrespective of the local elevation angle; thus DOD assumed that in

calculating an urban center’s interference contribution using equation

[1], LE = 10 dB in all cases. Industry’s model assumed an elevation

angle-dependent environmental loss factor as follows:

LE (ε) =10 dB(for 0 ε ≤ 20 ) [5A]

(60 20 )

(60 ) ( ) 10 0 0

0

−

− = ×

ε

ε LE dB(for 0 0 20 < ε ≤ 60 ) [5B]

LE (ε) = 0 dB(for 0 ε > 60 ) [5C]

We incorporated a suitable form of equation [5] into our reproduction

of industry’s model. We noted that neither DOD nor industry made

conservative assumptions regarding environmental losses. For

example, during our limited review of the Air Force’s IMT-2000

interference model, we discussed environmental losses with

Aerospace Corporation representatives. The Air Force model assumed

environmental losses would be much less than either DOD or industry

assumed.

 9

 Recommendation ITU-R F.1336 with k = 0.
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 47 GAO-01-795 Defense Spectrum Management

We were generally able to reproduce the value of the peak IMT-2000

interference levels predicted by DOD’s and industry’s original models for

each orbital shell to within about 0.1 dB. In addition, the locations of those

peaks generally corresponded well with those predicted by DOD and

industry. It should be noted, however, that because DOD calculated PSD

values for every point on a 1-degree longitude by 1-degree latitude lattice,

whereas our PSD values were calculated over a 2-degree longitude by

2-degree latitude lattice, some minor differences exist in the location of

the peak PSD value on the orbital shell. Tables 2 and 3 summarize these

results for DOD and industry, respectively.

Table 2: Comparison of DOD and Our Interference Results based upon Our

Replication of DOD’s Model

Satellite altitude

(km)

DOD peak PSD

(dBw/Hz)

Replication of DOD’s

peak PSD

(dBW/Hz)

Difference

(dB)

250 - 161.5

(at +39 lat./+117 long.)

- 161.5

(at +38 lat./+118 long.)

+ 0.0

833 - 166.6

(at +36 lat./+119 long.)

- 166.6

(at +36 lat./+120 long.)

+ 0.0

20,200 - 186.7

(at +72 lat./+48 long.)

- 186.7

(at +72 lat./+48 long.)

+ 0.0

35,784 - 191.1

(at +73 lat./+16 long.)

- 191.0

(at +72 lat./+18 long.)

- 0.1

Table 3: Comparison of Industry and Our Interference Results based upon Our

Replication of Industry’s Model

Satellite altitude

(km)

Industry’s peak PSD

(dBw/Hz)

Replication of

Industry’s peak PSD

(dBW/Hz)

Difference

(dB)

250 - 166.2

(at +52 lat./-2 long.)

- 166.2

(at +52 lat./-2 long.)

+ 0.0

833 - 173.2

(at +52 lat./+4 long.)

- 173.2

(at +52 lat./+4 long.)

+ 0.0

20,200 - 191.9

(at +64 lat./-6 long.);

(at +64 lat./-4 long.)

- 191.3

(at +64 lat./-2 long.)

- 0.6

35,784 - 195.8

(at +54 lat./+8 long.);

(at +56 lat./+4 long.);

(at +62 lat./-18 long.)

- 195.8

(at +56 lat./+4 long.)

+ 0.0

Results From the

Replication of DOD’s

and Industry’s

IMT-2000 Base Station

Interference Models
Appendix II: Replication of DOD’s and

Industry’s IMT-2000 Base Station

Interference Models

Page 48 GAO-01-795 Defense Spectrum Management

With respect to table 3, we did not attempt to resolve the –0.6 dB

discrepancy with industry representatives. As discussed above, we used a

slightly different approach to calculating the IMT-2000 base station

penetration factor, which might account for part of this difference.

Moreover, we did not attempt to review, in detail, industry’s assumptions

regarding values used for fundamental physical constants, or rounding of

intermediate calculations. While additional work would likely have

resolved this discrepancy, the – 0.6 dB difference is not large enough to

justify that exercise.
Appendix III: Comments From the

Department of Defense

Page 49 GAO-01-795 Defense Spectrum Management

Appendix III: Comments From the

Department of Defense
Appendix III: Comments From the

Department of Defense

Page 50 GAO-01-795 Defense Spectrum Management
Appendix III: Comments From the

Department of Defense

Page 51 GAO-01-795 Defense Spectrum Management
Appendix III: Comments From the

Department of Defense

Page 52 GAO-01-795 Defense Spectrum Management

Now on p. 24.

Now on pp. 23-24.

Now on p. 23.
Appendix III: Comments From the

Department of Defense

Page 53 GAO-01-795 Defense Spectrum Management
Appendix III: Comments From the

Department of Defense

Page 54 GAO-01-795 Defense Spectrum Management
Appendix IV: Comments From the Department

of Commerce

Page 55 GAO-01-795 Defense Spectrum Management

Appendix IV: Comments From the

Department of Commerce
Appendix V: Comments From the Federal

Communications Commission

Page 56 GAO-01-795 Defense Spectrum Management

Appendix V: Comments From the Federal

Communications Commission

(120017)
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