CONGRESS OF THE UNITED STATES

CONGRESSIONAL BUDGET OFFICE

CBO

The Global

Positioning System

for Military Users: 

Current 

Modernization Plans 

and Alternatives

OCTOBER 2011
Pub. No. 4192
The Congress of the United States O Congressional Budget Office

A

STUDY

CBO

The Global Positioning System for 

Military Users: Current Modernization 

Plans and Alternatives

October 2011
CBO

Notes

The cover image shows a Global Positioning System Block IIF satellite. The image was provided courtesy of the Air Force.

Unless otherwise indicated, all years referred to in this study are fiscal years and all dollar 

amounts are in 2012 dollars.

Numbers in the text and tables may not add up to totals because of rounding.
Preface

CBO

The U.S. military has come to rely on the Global Positioning System (GPS) to conduct 

many of its operations, both during normal peacetime operations and wartime operations. 

The Department of Defense (DoD) is modernizing the system by purchasing new satellites 

and upgrading the systems that control the satellites. 

This Congressional Budget Office (CBO) study, which was prepared at the request of the former Chairman of the Defense Subcommittee of the House Committee on Appropriations, 

considers the implications of those programs for military users and assesses how those efforts 

are synchronized with DoD’s efforts to improve the capabilities of its GPS receivers. The 

study also examines three alternatives for improving the performance of the GPS for military 

users (primarily by improving the capabilities of military GPS receivers) and estimates the 

budgetary consequences of those options as well as their effect on the ability of the GPS to 

operate in an environment where an opponent is trying to block GPS signals. In keeping with 

CBO’s mandate to provide objective, impartial analysis, this study makes no recommendations.

Frances M. Lussier and Christopher Murphy of CBO’s National Security Division prepared 

the study under the general supervision of David Mosher. Raymond Hall of CBO’s Budget 

Analysis Division produced the cost estimates under the general supervision of Sarah Jennings. Sheila Campbell and Daniel Frisk of CBO and Joseph Post of the Federal Aviation 

Administration provided helpful comments on the report. The assistance of external reviewers 

implies no responsibility for the final product, which rests solely with CBO.

Cynthia Cleveland prepared the manuscript. Christine Bogusz edited the study, and 

Kate Kelly proofread it. Maureen Costantino prepared the report for publication and designed 

the cover. Monte Ruffin printed the initial copies, Linda Schimmel coordinated the print 

distribution, and Simone Thomas prepared the electronic version for CBO’s Web site 

(www.cbo.gov).

Douglas W. Elmendorf

Director

October 2011

Contents

CBO

Summary ix

1 Introduction and Background 1

The Three GPS Segments 1

How GPS Works 3

Interference with GPS Signals 4

Current Status of the System 5

2 DoD’s Plan for Modernizing GPS 9

Description and Cost of DoD’s Plan 9

Increased Capabilities 13

Concerns About the Plan 14

3 Alternatives for Modernizing GPS 17

Overview of the Options 17

Option 1. Improve the Capabilities of Military GPS Receivers 18

Option 2. Enhance GPS Using the Iridium Satellite System 25

Option 3. Improve the Capabilities of Military GPS Receivers and 

Enhance GPS Using the Iridium Satellite System 29

A History of the GPS Program 31

B Examples of Selected Military-Related GPS Components 37
VI THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Tables

S-1. Summary of DoD’s Plan for the Global Positioning System and Three Options x

S-2. Effect of DoD’s Plan and Three Options on GPS Performance Over Time xi

1-1. Signals Transmitted by Different Models of Current GPS Satellites 4

2-1. Costs of DoD’s Plan to Modernize the GPS Satellites, Ground Control System, and 

Receivers 10

2-2. Characteristics of Current and Planned GPS Satellites 11

2-3. Estimated Impact of Planned GPS Improvements on the 

Effective Range of a Jammer 14

3-1. Summary of DoD’s Plan for the Global Positioning System and Three Options 20

3-2. GPS Modernization Costs in DoD’s Plan and Under Three Options 21

3-3. Effect of DoD’s Plan and Three Options on GPS Performance Over Time 24

A-1. Expenditures and Purchases for the GPS Program, 1974 to 2011 32

B-1. Characteristics of GPS Receivers Used by the U.S. Military 38

Figures

S-1. Effect of DoD’s Plan and Three Options on the Effective Range of a 

10-Watt Jammer xiii

1-1. The Three Segments of the Global Positioning System 2

2-1. Effect of DoD’s Plan on the Effective Range of a 10-Watt Jammer 15

3-1. Effect of DoD’s Plan and Three Options on the Effective Range of a 

10-Watt Jammer 23

3-2. GPS Augmentation with the iGPS Program 26

A-1. Structure of Signals Transmitted by GPS Satellites 34

B-1. The Manpack and the Precision Lightweight GPS Receiver 37

B-2. The Precision Lightweight GPS Receiver, Defense Advanced GPS Receiver, and 

MicroDAGR 39

B-3. The Miniature Airborne GPS Receiver and the 3S Shipboard Receiver 39

B-4. Remote Defense Advanced GPS Receiver Antenna and Mounting Strap 40

B-5. Controlled Reception Pattern Antennas (CRPAs) for Use on Military Aircraft 40

B-6. μCRPA 40

B-7. GAJT Antenna 41
CONTENTS THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES VII

CBO

B-8. Precision Lightweight GPS Receiver With and Without Antijam Antenna Retrofit 41

B-9. iGPS Module, Including DAGR 41

B-10. Examples of Notional GPS Jammers 42

B-11. Russian Commercial 4-Watt Jammer 42

B-12. Examples of GPS Jammers 43

Boxes

1-1. GPS Receivers in a Jamming Environment 6

3-1. The Effect of CBO’s Options on Civilian GPS Users 19

Figures (Continued)

CBO

Summary

As the Department of Defense’s (DoD’s) Global 

Positioning System (GPS) satellites reach the end of their 

service lives, the department plans to replace them with 

ones that can counter deliberate interference by generating stronger signals. Analysis by the Congressional 

Budget Office (CBO) indicates that an alternative 

approach—namely, improving military receivers to retain 

the GPS signal even in the presence of such jamming—

would be less expensive than DoD’s plan for upgrading 

its constellation of GPS satellites. Furthermore, the alternative would yield benefits almost a decade earlier than 

DoD’s plan. However, the improvements to military 

receivers could make them larger and heavier (and 

thereby less useful to personnel operating on foot) until 

they could incorporate the substantial gains that have 

been achieved in miniaturization in other applications. 

DoD’s Plan

The GPS uses a constellation of at least 24 satellites, each 

of which transmits precise data on the time and its location. Receivers—both military and civilian—use the data 

transmitted by the satellites to calculate their own position; information from a minimum of 4 satellites is 

required to determine a position accurately in three 

dimensions. Since 1995 (when GPS became fully operational), the U.S. military has come to rely on it to precisely locate both enemy and friendly forces. However, 

because the GPS signal from space is very weak by the 

time it reaches Earth (like the light from a 25-watt lightbulb shining 12,500 miles away), the system can easily be 

swamped by interference. 

In 2000, DoD initiated plans to reduce the system’s susceptibility to intentional interference. As a first step 

toward providing some protection against jamming, 

DoD decided that GPS satellites would transmit additional signals, available only to military users, each of 

which covered a wider range of frequencies than those 

already being transmitted. Those signals, called M-code 

signals, are more difficult for enemy jammers to overwhelm and can improve the ability of military receivers to 

operate in the presence of jammers. Ten satellites capable 

of transmitting M-code signals were already in orbit as of 

August 2011.

To maintain the constellation as existing and new satellites reach the end of their service lives, DoD plans to 

launch a total of 50 satellites through 2030 at an average 

rate of 2 to 3 satellites each year starting in 2012. The 

department has already purchased—but not yet 

launched—10 of those GPS satellites capable of transmitting M-code signals. DoD plans to acquire 40 more satellites—known as GPS III—that are capable of transmitting stronger M-code signals than existing satellites over 

the next 10 to 15 years. 

DoD plans to develop and purchase the new satellites in 

three phases. In the first phase, DoD plans to acquire 8 

GPS IIIA satellites capable of emitting M-code signals 

that are three times stronger than those transmitted by 

current GPS satellites. The first IIIA satellite is scheduled 

to be launched in 2014. In the second phase, DoD plans 

to acquire 16 GPS IIIB satellites with M-code signals that 

are five times stronger than those of current satellites. For 

the final phase, the department’s plan calls for an initial 

purchase of 8 GPS IIIC satellites, which will be equipped 

with a special antenna capable of focusing the M-code 

signals in a “spotbeam”; however, CBO assumes that the 

department would need to purchase an additional 8 IIIC 

satellites in order to have enough IIIC satellites in orbit to 

take advantage of the IIIC’s advanced capabilities. Those 

satellites will transmit signals with the same strength as 

IIIB satellites and will be able to use the spotbeam to illuminate an area with a diameter of 600 miles on the 

Earth’s surface with signals 100 times stronger than those 

of current GPS satellites. In addition, IIIC satellites will 

be equipped with high-speed cross-links, which will allow 
X THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Summary Table 1.

Summary of DoD’s Plan for the Global Positioning System and Three Options

Source: Congressional Budget Office.

Note: DoD = Department of Defense; GPS = Global Positioning System; INS = inertial navigation system; iGPS = High Integrity GPS.

a. Includes 8 IIIC satellites not explicitly included in DoD’s plan but needed, in CBO’s estimation, to attain full capability.

b. Compared with the current capability of the Defense Advanced GPS Receiver and IIF satellite signal.

c. Includes research and development and procurement funds but excludes funds to operate and maintain the system.

d. To attain specified performance.

continuous data updates. As a result, those satellites will 

be able to provide more accurate data to receivers, 

enabling a user’s location to be determined within 

6 inches, instead of 10 feet (using current satellites) or 

3 feet (using IIIA and IIIB models). After the 16th IIIC 

satellite is launched in 2030, the entire constellation 

should be composed of GPS III satellites, 16 of which 

will be IIICs (see Summary Table 1).

Over the next 15 years, DoD also plans to develop software to control the M-code signals and the new GPS III 

satellites and to develop and purchase receivers that are 

capable of processing the M-code signals. Although 

10 satellites capable of transmitting the harder-to-jam 

M-code signals are currently in orbit (the first one since 

2005), no users have been able to benefit from them 

because DoD does not have the ability to monitor or 

control the signals, nor has it fielded receivers to process 

the signals. DoD plans to have a new control system fully 

in place by the end of 2016. To make the entire planned 

system functional, however, additional control capabilities, such as being able to update satellite data transmissions continuously when IIIC satellites enter the constellation and to control their spotbeam antenna, will need 

to be developed. Moreover, to make the planned system 

useful, M-code-capable receivers will need to be fielded as 

well. DoD’s current plan envisions fielding the first such 

receivers in 2017, but because the various armed services 

now field more than 400,000 GPS receivers, it may be 

2030 before all units are fully equipped.

IIIA 8 40 40 40

IIIB 16 0 0 0

IIIC 16 a 000 ___ ___ ___ ___

Total 40 40 40 40

Receivers Are M-Code Capable

Receivers Have Improved Antennas

Receivers Have INS 

iGPS Augmentation

Total Investment Cost, 2012 to 2025

 (Billions of 2012 dollars)c

Year Significantly Improved Capability Is Achieved

Year Full Capability Is Achieved

Receivers Require Added Weight and Powerd

Improved Navigation in Canyons and Mountains

Depends on Commercially Owned System

2030

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

Yes

No

No

2026

Yes

Yes

Yes

No

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

2026

DoD's Plan Option 1 Option 2 Option 3

GPS III Satellites Acquired

Major Improvementsb

Cost and Schedule

Other Considerations

Yes

No

No

No

2026 2018

22.2 19.9 18.9 20.9

2018

2026

2018
SUMMARY THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES XI

CBO

Summary Table 2.

Effect of DoD’s Plan and Three Options on GPS Performance Over Time

Source: Congressional Budget Office.

Note: DoD = Department of Defense; GPS = Global Positioning System; iGPS = High Integrity GPS.

a. The range at which a 10-watt jammer can cause a handheld GPS receiver to lose track of the military signal.

b. Within the focused spotbeam that covers a 600-mile-diameter area from GPS IIIC satellites only.

c. Within the theaters of operation supported by the iGPS program. 

d. The effective range for this option is slightly greater than 130 feet.

e. The values represent the strength of the M-code signal at the receiver.

If the satellites and receivers perform as planned, the 

combination of all of the upgrades proposed by DoD 

would enable military receivers to operate in the presence 

of much stronger jamming signals than they can withstand today. For example, the effective range of a 10-watt 

jammer trying to cause a military receiver within the 

spotbeam of a GPS IIIC satellite to lose the GPS signal 

would be reduced by 96 percent, shrinking from 55 miles 

to about 2 miles (see Summary Table 2). 

Although the planned upgrades to GPS satellites will not 

increase the strength of civilian signals and will not 

improve the performance of civilian receivers in the presence of interference, other planned improvements will 

benefit both military and civilian users. In particular, 

GPS IIIA satellites will transmit signals that will enable 

both types of users to determine their position to within 

3 feet, compared with the 10 feet that is possible with 

signals from current satellites. And once enough IIIC satellites enter the constellation, positioning within 6 inches 

will be possible for all users, according to DoD. 

CBO estimates that it will cost DoD roughly $22 billion 

from 2012 to 2025 to modernize the GPS. That total 

would include the cost from 2012 onward to develop and 

purchase the 40 GPS III satellites (including $3.6 billion 

for the additional 8 IIIC satellites), to develop the software and capability needed to control those satellites and 

their transmissions, and to develop and purchase hundreds of thousands of military receivers capable of receiving and interpreting the M-code signals. 

The Government Accountability Office and the Defense 

Science Board have reviewed DoD’s plan to modernize 

the GPS and raised several concerns, particularly regarding the plan’s focus on improving the satellites rather than 

the receivers and the plan’s lack of coordination in terms 

DoD's Plan 55 55 2.5 b

55 1.8 0.4

55 0.6 0.14 c

55 0.1 0.02 c,d

DoD's Plan 1.6 1.6 160 b

1.6 1.6 5

1.6 1.6 5

1.6 1.6 5

DoD's Plan 10 10 0.5

10 10 3

10 0.7 c 0.7 c

10 0.7 c 0.7 c

Current Capability 2020 2030

Effective Range of a 10-Watt Jammera

 (Miles)

Signal Strength at Receivere (x 10-16 watts)

Option 1

Option 2

Option 3

Accuracy (Feet)

Option 1

Option 2

Option 3

Option 1

Option 2

Option 3
XII THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

of the timing for various capabilities. CBO has developed 

options by which it explores those concerns.

Options

CBO examined three options that would provide antijamming improvements to military users sooner and at a 

lower cost than DoD’s plan. Those options focus more 

on improving the performance of receivers in a jamming 

environment and less on strengthening the signal that the 

GPS satellites transmit. CBO focused on the handheld 

receivers used primarily by the Army and the Marines 

because they are the most widespread throughout the services. (Despite their designation, most such receivers are 

mounted, sometimes permanently, in various military 

vehicles.)

 Option 1 would improve current military GPS receivers by fitting them with better antennas and by adding 

inertial navigation systems.

 Option 2 would capitalize on a DoD research and 

development program by enabling current GPS 

receivers to integrate information received via the Iridium commercial communications satellite network.

 Option 3 would include the improvements of both 

Option 1 and Option 2.

Three items are common to all of the options. First, 

under each option, DoD would purchase 40 GPS III 

satellites (the same number as in DoD’s plan) but confine 

those purchases to the IIIA model. Second, under DoD’s 

plan as well as all the options, DoD would continue to 

develop the ground control system, enabling it to control 

current M-code-transmitting GPS satellites (IIR-M and 

IIF) as well as the newer GPS IIIA satellites. And third, 

DoD would develop and purchase M-code-capable 

receivers in the same numbers under its plan and all three 

options. By CBO’s estimates, the total cost for those three 

common items is $17.9 billion from 2012 through 

2025—which is the amount to maintain, modernize, and 

control the GPS constellation through 2030 and to field 

military receivers that can take full advantage of the 

M-code signal.

All three options would cost less to carry out than DoD’s 

plan and would yield military receivers with greater antijamming capability earlier. CBO’s options would not 

yield similar benefits for civilian users as DoD’s plan, and 

they would forgo some improvements in accuracy for all 

users offered by the GPS IIIC satellites. Because all three 

options would cancel the IIIB and IIIC portions of the 

GPS III program, they would save more than $4 billion 

on satellite and ground control costs from 2012 through 

2025, CBO estimates. Those savings would be partly offset by the cost to make improvements to the receivers 

under the three options, yielding net savings of approximately $2 billion, $3 billion, and slightly more than 

$1 billion, respectively, for Options 1, 2, and 3.

Option 1. Improve the Capabilities of 

Military GPS Receivers

Option 1 would augment military receivers to provide 

users with a better ability to keep track of their location 

in jamming environments. The improvement would 

come from new antennas—capable of rejecting signals 

from jammers—and from the integration of very small 

inertial navigation systems, which would reduce location 

errors introduced by interference and enable users on the 

move to determine their position accurately even after 

losing the GPS signal entirely. 

By increasing the level of noise that receivers could tolerate and still be able to detect and process the GPS signal, 

those augmentations to receivers could reduce the effective range of a wideband noise jammer by 97 percent. 

(The effective range of a 10-watt jammer would be 

decreased from 55 miles to about 2 miles, which means 

that the jammer would need to be within 2 miles of the 

receiver to have an effect.) Because the hardware for the 

improvements in this option has already been developed, 

modifications to existing receivers could begin almost 

immediately, and a significant number of improved 

receivers could be in the field by 2018. The additional 

signal power from the IIIA satellites and the antijamming 

capabilities of M-code receivers would further enhance 

the overall capability of military receivers to operate in 

the presence of jamming. By 2026 (the point at which 

about half the force could be equipped with M-codecapable receivers), the combined improvements under 

this option would reduce the range of a 10-watt jammer 

by 99.3 percent, to 0.4 miles (see Summary Figure 1).

Option 2. Enhance GPS Using the 

Iridium Satellite System

Option 2 would capitalize on a DoD initiative to use an 

existing satellite constellation in low-earth orbit to provide military GPS receivers with information that would
SUMMARY THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES XIII

CBO

Summary Figure 1.

Effect of DoD’s Plan and Three Options on the Effective Range of a 

10-Watt Jammer

(Effective range, in miles)

Source: Congressional Budget Office.

Notes: Improved capability is assumed to have been phased in when half of all fielded receivers are equipped with enhancements or, in the 

case of the Department of Defense’s (DoD’s) plan, when 16 IIIC satellites are scheduled to be in orbit.

The scale for the y-axis is logarithmic, not linear.

enable them to operate better in a jamming environment. 

The program—known as High Integrity GPS, or iGPS—

would rely on the commercial Iridium satellite communications network to relay data to modified military GPS 

receivers, allowing them to more easily pick up and maintain signals from the GPS satellites located in mediumearth orbit. 

In addition to enabling military receivers to process data 

from the Iridium satellites, this option would integrate 

inertial navigation systems into the receivers in a procedure similar to that in Option 1 (but would not incorporate the improved antennas of that option). The extent of 

the improvements in military receivers’ capability resulting from this option would be somewhat greater than 

that from Option 1 and could be realized just as 

quickly—by 2018. When combined with the contributions from IIIA satellites and M-code-capable receivers, 

the total improvement would reduce the range of a 10-

watt jammer by 99.7 percent, from 55 miles to about 

0.14 miles, by 2026. Unlike Option 1, however, this 

option would have the added advantage of improving the 

accuracy with which military users could determine their 

position. 

Option 3. Combine Options 1 and 2

Option 3 would combine the enhancements in Options 

1 and 2. Specifically, it would provide military receivers 

with improved antennas and inertial navigation systems 

and also enable them to incorporate data relayed by the 

iGPS network. The combined improvements to military 

receivers with M-code capability would reduce the effective range of a typical 10-watt noise jammer by more than 

99.9 percent, from 55 miles to 0.025 miles (or slightly 

more than 130 feet) when all the components of this 

option were fielded.

Advantages and Disadvantages of the 

Options

Compared with DoD’s plan, the options would yield 

greater improvements in reception and would yield 

improvements sooner. Under DoD’s plan, the full benefit 

of the increased M-code signal power of the IIIC satellites 

would not be fully realized until 2030, when the 16th 

IIIC satellite could be in orbit. The earliest benefits 

would probably come once the constellation of 18 GPS 

III satellites—comprised of 8 IIIA and 10 IIIB satellites—is in orbit, scheduled for 2022, but only small 

2010 2015 2020 2025 2030

0.01

0.1

1

10

100

Upgraded Receivers

Fielded to

Half the Force

M-Code Receivers

Fielded to

Half the Force

DoD's Plan

Option 1

Option 2

Option 3
XIV THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

numbers of military receivers capable of processing the 

stronger M-code signals would be in the field then. While 

the IIIC satellites were being placed in orbit, the benefit 

of their stronger signals would be unavailable to users 

until sufficient numbers of M-code-capable receivers 

were fielded, possibly no earlier than 2026. 

In contrast, the technologies included in CBO’s 

options—those for improved antennas for GPS receivers, 

small inertial navigation devices, and iGPS—have already 

been developed. The fielding of ancillary devices to augment existing military GPS receivers could begin in a few 

years, with appreciable numbers of improved receivers in 

the field by 2018. Consequently, the options could 

increase the military’s antijamming capability eight years 

before large numbers of M-code receivers could be in the 

hands of military users under DoD’s plan. 

Additional advantages of Options 2 and 3 come from 

augmenting the GPS constellation with the Iridium satellites in low-Earth orbit. That fuller coverage would virtually ensure that receivers had a line of sight to at least one 

satellite, even in mountainous terrain and urban settings 

where tall buildings block the view of the sky. In addition, because data can be received and updated frequently, receivers using iGPS can determine their position with almost the same accuracy as would be possible 

using data from IIIC satellites—but the receivers would 

have that ability several years earlier.

The options would have several disadvantages when compared with DoD’s plan, however. All of the options 

would require hardware additions to existing receivers: an 

improved antenna and integrated inertial navigation system for Option 1; a module to interpret data relayed 

from Iridium satellites and an inertial navigation system 

for Option 2; and all of the above for Option 3. Each of 

those hardware devices might not be much larger or 

heavier than a typical military handheld receiver, which is 

about six inches long and weighs about a pound, but 

when combined, they would add bulk and requirements 

for additional power. Although that added weight might 

not prove too onerous for military personnel in a vehicle, 

plane, or ship, it could prove troublesome for those on 

foot. The current trend in miniaturization has made it 

possible to integrate such devices into military receivers 

designed specifically for use in munitions (such as cruise 

missiles or small guided bombs) or in the confined space 

of very small unmanned aerial vehicles. But, designing 

and integrating such miniaturized devices for and into 

existing receivers would take time and could entail costs 

not included in CBO’s estimates.

Another disadvantage common to the options is that they 

would forgo the improvements offered by the IIIC satellites, so military users would not benefit from the 

increased power of the M-code signals within the spotbeam. Those signals, which would be roughly 30 times 

stronger than those transmitted by IIIA satellites, could 

be advantageous for users who could not handle the 

added weight and power needed for improved antennas 

or who could not take advantage of the iGPS program. 

Forgoing the IIIC satellites would also mean the loss of 

the ability to determine position to within about 6 inches 

for civilian users under all of the options and for military 

users under Option 1. In those cases, GPS users would 

have to rely on the less accurate signals from the IIIA satellites, allowing them to determine their position to 

within about 3 feet. The addition of iGPS in Options 2 

and 3 would allow military—but not civilian—users to 

determine their position to within 8 inches—almost as 

accurately as would be the case under DoD’s plan. That 

increased accuracy might not be important for most 

applications, but it could be useful when items need to be 

located precisely, such as in the case of land mines or 

unexploded ordnance.

A final disadvantage shared by Options 2 and 3 is the 

dependence on the commercial Iridium satellite constellation and support network, which is not controlled by 

DoD. The future of the Iridium constellation cannot be 

guaranteed by the government, at least not without a cost 

that CBO has not included in its estimates. Although 

apparently financially healthy at the end of 2011, the 

Iridium communications system has had financial setbacks in the past. The risk that DoD might need to 

infuse funds into the system in the future is a disadvantage of relying on iGPS to enhance the ability of GPS 

receivers to operate in a jamming environment.
CHAPTER

CBO

1

Introduction and Background

The Global Positioning System (GPS) is a spacebased system that provides positioning and timing data to 

users worldwide. The data that it provides have become 

essential to the conduct of military operations; they also 

contribute to transportation efficiency and safety in the 

civilian sector and to the timing and recording of financial transactions. The system, which includes a constellation of satellites, was developed and is operated by the 

Department of Defense (DoD) in consultation with the 

Department of Transportation and several other federal 

agencies.1

 The Air Force is responsible for the acquisition 

of GPS components and is in the process of modernizing 

the system to enhance its performance, particularly in 

the presence of deliberate and hostile interference or 

jamming. 

In this study, the Congressional Budget Office (CBO) 

examines DoD’s plan to modernize GPS and assesses the 

cost of that plan and the benefits that the proposed 

improvements would yield to the system’s military users. 

(Although this analysis focuses on military users, it notes 

significant effects on civilian users as well.) CBO also 

examines several options that would improve GPS capabilities for military users at lower cost than would DoD’s 

plan, although those options have some disadvantages relative to DoD’s plan as well.

GPS is a global network composed of three segments: satellites that transmit military and civilian GPS signals, systems on the ground that control the satellites and support 

the signals (ground control systems), and receivers that 

make use of the broadcasted signals (see Figure 1-1). Each 

of those signals includes positioning and timing information that enables users with GPS receivers to determine 

their position and the exact time 24 hours a day, in all 

weather, worldwide. 

GPS began operations with a full constellation of satellites in 1995. (See Appendix A for details of the system’s 

history.) In the years since then, GPS has become vital to 

military operations and is used by all branches of the 

armed services to guide troop movements, integrate logistics support, and synchronize communications networks. 

In addition, U.S. and allied forces use GPS signals to 

guide munitions to their targets and to locate military 

personnel in distress. 

The Three GPS Segments

All three segments of the GPS are necessary to enable 

users to determine their location accurately without interruption. Most GPS users are familiar with both the space 

segment—the constellation of satellites orbiting the 

earth—and the receivers that use the satellites’ signals to 

determine individual locations. The workings of the 

ground control system, which continuously monitors the 

health of the satellites and adjusts their signals to eliminate errors in time and position, are less well known but 

no less necessary for the proper operation of the overall 

system.

Satellites 

The GPS space segment is a constellation of at least 24 

satellites that transmits signals with data on each satellite’s 

position and the time. The satellites transmit at least two 

types of signals with the same time and position information. One set of signals is encrypted and is available only 

to military users. The other, unencrypted civilian signals 

1. For additional information on the roles and responsibilities of the 

various federal agencies in supporting the GPS, see Department of 

Defense, Department of Homeland Security, and Department of 

Transportation, 2010 Federal Radionavigation Plan, DOTVNTSC-RITA-08-02/DoD-4650.05 (Springfield, Va.: National 

Technical Information Service), Chapter 2.
2 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Figure 1-1.

The Three Segments of the Global Positioning System

Source: Congressional Budget Office.

are available to all users.2 The satellites, in orbit 12,500 

miles above the Earth, circle the planet roughly every 12 

hours and pass over the same location on earth about 

once every day. The satellites are arranged in six orbital 

planes and spaced in such a way that a minimum of four 

satellites are in view to users worldwide at any given time. 

That arrangement enables users with an unobstructed 

view of the sky and appropriate receivers to determine 

their position accurately.

Ground Control System 

The ground control system tracks the GPS satellites and 

periodically updates the information that they transmit to 

Earth. This segment includes two master control stations, 

the primary one in Colorado and an alternate one in 

California. In addition, four ground antennas that can 

send commands up to the satellites and six dedicated 

monitoring stations are stationed around the world. 

The monitoring stations receive information from each 

GPS satellite as it orbits the earth roughly twice a day. 

That information is then sent to the master control station, where it is processed to identify inaccuracies in the 

time and position data. Despite the fact that the clocks 

aboard the satellites are very accurate—they have even 

been adjusted to take into account the effects of relativity 

on the clocks aboard the satellite, as opposed to identical 

clocks on earth—very small errors in time can result in 

measurable errors when determining location. (An error 

of 1 billionth of a second can result in location errors of 

1 foot.) Because the clocks can accumulate errors of up 

to 10 billionths of a second per day—creating location 

inaccuracies of up to 10 feet—the Air Force computes 

and uploads time corrections for each satellite daily using 

Ground Control

Segment

Receiver Segment

Satellite Segment

2. The GPS was originally designed in the 1970s primarily for the 

benefit of the U.S. military. To ensure that enemies would not 

have the same navigational advantage as U.S. forces, DoD intentionally degraded the accuracy of the original civilian signal that 

was available to all users. (The accuracy of that signal was less than 

one-third as good as that of the military signal. One source estimated that using the degraded signal, a user would be able to 

determine his or her position to within 250 feet, in contrast to 

within about 70 feet using the military signal.) The more accurate 

data carried by the military signals was encrypted so that only 

U.S. forces and designated allies could benefit from it. Since May 

1, 2000, the accuracy of the civilian signal has not been degraded, 

and users’ position can be determined to roughly the same accuracy using either the civilian or military signals. 
CHAPTER ONE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 3

CBO

the ground control system. In addition, satellites drift 

from their prescribed orbits and, as a result, their actual 

positions differ from predicted ones. Corrections that 

need to be made to an individual satellite’s transmitted 

position are also relayed back up to the satellites once per 

day via the ground antennas.

Receivers 

Military and commercial GPS receivers are installed on 

ships, aircraft, and vehicles and carried by individuals. 

Military GPS receivers are designed to use the encrypted 

GPS signals that are available only to authorized users, 

including military and allied forces and some civilian 

agencies.3

 In contrast, commercial receivers use the civilian GPS signals, which are publicly available worldwide. 

The military fields many types of GPS receivers that have 

been optimized for its use. (See Appendix B for descriptions and pictures of some military GPS receivers.) The 

most widespread receiver in the U.S. military is the handheld version, most of which are mounted, sometimes permanently, in military vehicles. The services field more 

than 350,000 handheld GPS receivers, most of which are 

fielded by the Army. In the past decade, as advances in 

technology have facilitated the development of smaller 

and lighter GPS receivers, it has become possible to 

embed them in precision-guided munitions, such as 

cruise missiles and guided artillery rounds, as well as in 

unmanned aerial vehicles. The latter applications, plus 

the receivers mounted in aircraft and aboard ships, bring 

the total to about 400,000.

How GPS Works

GPS works by timing how long it takes the radio signals 

from its satellites to reach a specific location on earth. 

Each satellite continuously broadcasts the time and its 

own position, and GPS receivers calculate the delay 

between the time when the signal left the satellite and 

when it reaches the receiver. That time delay, when multiplied by the speed of light, determines the receiver’s distance from the satellite. 

A GPS receiver could, in theory, calculate its threedimensional position by measuring its distance from 

three different satellites simultaneously. But, in practice, 

by using the distance measured from a fourth satellite, a 

GPS receiver can calculate its position more accurately. 

Although the Air Force monitors the data that each satellite transmits to ensure its accuracy, errors in determining 

location can be introduced as the satellite’s signal travels 

through the atmosphere and because the clocks on the 

satellite and those in the receiver are not synchronized 

exactly. Satellite geometry is also important because a 

GPS receiver determines its position by triangulation; the 

more widely dispersed the satellites are, the more accurately a receiver will be able to determine its position. 

Several points should be noted about how the system 

works. First, there is no interaction between the satellites 

and the receivers. That is, the satellites send out military 

and civilian signals that are available to all receivers that 

can decode them. The receivers merely process the information received from the satellites; they do not send signals back to the satellites or to other systems. To process 

the data that the satellite or satellites are transmitting, a 

receiver must first “acquire” a signal from one or more 

GPS satellites in view.4

 Once the receiver has acquired 

and identified signals from a GPS satellite, it can more 

easily continue to process the data from—or “track”—

the signal.

Second, because the receivers determine a user’s location 

on the basis of triangulation, any errors introduced into 

the distances calculated from the satellites result in errors 

in determining the location of the receiver. Receivers can 

cancel out any errors introduced by atmospheric interference by using information from signals on two different 

frequencies transmitted by the same satellite.5

 Such a 

capability has been available to military users from the 

system’s inception (because each GPS satellite has always 

transmitted military signals on two different frequencies), 

although not all military receivers were capable of 

3. Military receivers can, in general, also pick up the civilian signals 

that are transmitted on the same frequencies as the military 

signals.

4. The data carried on the military signal are encrypted in a code 

that is a billion times longer than that associated with the civilian 

signal. Consequently, military GPS receivers often acquire the 

civilian signal from a satellite first and then use the information 

gleaned from the civilian code to acquire and track the military 

signal. 

5. The effect of atmospheric anomalies, such as the presence of ions, 

on the speed of a GPS signal varies with the signal’s frequency. By 

comparing the difference in arrival time of two signals of different 

frequency from the same satellite, a GPS receiver can correct for 

the distortion introduced by the signal’s passage through the 

atmosphere.
4 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table 1-1. 

Signals Transmitted by Different Models of Current GPS Satellites

Source: Congressional Budget Office based on Department of Defense data.

Notes: L1, L2, and L5 are commonly used designations for the associated frequencies.

GPS = Global Positioning System; MHz = megahertz.

processing both signals (see Table 1-1).6 Until recently, 

however, civilian receivers did not have that ability, 

because older GPS satellites transmitted the unencrypted 

civilian signal on only a single frequency.7

 GPS satellites 

launched since 2005, however, have transmitted two 

civilian signals on the same frequencies as the military 

signals, enabling civilian receivers to calculate a user’s 

position with greater accuracy. (One source estimates an 

accuracy of 74 feet using a single frequency, as compared 

with 28 feet using two frequencies.)8

Finally, because the receiver calculates its position on the 

basis of triangulation and because data transmitted by 

each satellite include some degree of error, receiving data 

from more satellites enables the receiver to cancel out 

more of the errors and calculate a more accurate position. 

Thus, the greater the number of satellites in view, the 

greater the accuracy of the calculated position. However, 

some locations, such as urban settings and hilly or mountainous terrain, offer only obstructed views of the sky. In 

those circumstances, obtaining signals from even three 

satellites at one time might be difficult. In such situations, the GPS user often must augment the signals with 

information from sources other than GPS to determine 

his or her position accurately.

Interference with GPS Signals

Reception of GPS signals by receivers on Earth can be 

easily disrupted, either intentionally or unintentionally. 

One reason is that the signal from space—which has been 

likened to a 25-watt light bulb shining on the Earth from 

a distance of 12,500 miles—is very weak by the time it 

reaches the Earth. Consequently, the signal can be 

masked unintentionally by other radio frequency signals 

in the vicinity—cell phone traffic and television 

broadcasts, for example—or intentionally by deliberate 

jamming. 

IIA IIR IIR-M IIF

First Launch 1991 1997 2005 2010

Civilian Signals

1575.42 MHz (L1) Yes Yes Yes Yes

1227.60 MHz (L2) No No Yes Yes

1176.45 MHz (L5) No No No Yes

Military Signals

Standard signals

1575.42 MHz (L1) Yes Yes Yes Yes

1227.60 MHz (L2) Yes Yes Yes Yes

M-code signals

1575.42 MHz (L1) No No Yes Yes

1227.60 MHz (L2) No No Yes Yes

Satellite Model

Signals Transmitted

6. Those frequencies are 1575.42 megahertz (MHz) and 1227.6 

MHz. The Army’s first handheld GPS receiver, the Precision 

Lightweight GPS Receiver, could process only the military signal 

that was broadcast at 1575.42 MHz.

7. The initial civilian signal was transmitted at 1575.42 MHz.

8. Michael Shaw, Kanwaljit Sandhoo, and David Turner, “Modernization of the Global Positioning System” (paper presented at the 

32nd Annual Precise Time and Time Interval Meeting, Reston, 

Va., November 28–30, 2000).
CHAPTER ONE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 5

CBO

Jamming is accomplished by generating a signal with 

enough power to overwhelm a weaker signal, in much the 

same way that the headlights from an oncoming car make 

it difficult to see the light reflecting from the dividing line 

in the middle of the road at night. Although the military 

GPS signals are encrypted and are not easy to replicate, 

they can be masked by stronger signals of the appropriate 

frequency rather easily. As an example, a jammer broadcasting 1 watt of power at the appropriate frequencies 

could theoretically prevent a military receiver 40 miles 

away from locating and acquiring a GPS signal. Once the 

receiver has acquired and locked on to the military signal, 

the same 1-watt jammer would need to be within 18 

miles to cause the receiver to lose track of the signal.9

Such a jammer could be as small as a 12-ounce soda can 

and easily be carried by an individual. (See Appendix B 

for pictures and diagrams of some typical jammers.) A 

larger, but still portable, 10-watt jammer could prevent 

the same receiver from acquiring a GPS signal at a distance of 125 miles and could cause the receiver to lose 

track of the signal at 55 miles. (For additional discussion 

of jamming, see Box 1-1.)

Current Status of the System

Thirty working GPS satellites were in orbit at the end of 

August 2011. The satellites were purchased in groups 

known as “blocks”; satellites in the same block have identical capabilities. Each block of satellites is slightly more 

capable than the preceding block. The blocks of existing 

satellites are known as IIA, IIR, IIR-M, and IIF.10

One of the major upgrades that DoD made to the 

satellites was enabling them to transmit new signals—

designated M-code—for military use only. Those signals 

cover a wider frequency range and are separated from the 

civilian signals in order to make jamming more difficult. 

(See Appendix A for a discussion of M-code signal structure.) Currently, 10 satellites in orbit have that capability 

(8 IIR-M and 2 IIF), slightly more than half of the 18 

satellites that are needed to provide continuous worldwide coverage. DoD has purchased but not yet launched 

10 additional IIF satellites with M-code capability; they 

are scheduled to be launched as existing satellites fail and 

need to be replaced. However, DoD has neither developed nor fielded any receivers capable of receiving and 

deciphering the M-code signals. 

As part of its plans for modernizing the system, DoD has 

also begun developing a new model of satellite, known as 

GPS III, which is being designed to transmit stronger 

M-code signals. The first two purchases of GPS III satellites are scheduled for 2012.

9. Once a GPS receiver has found and locked on to a signal it can 

continue to track that signal in the presence of interference that is 

much stronger than the level of interference initially needed to 

keep the receiver from finding and acquiring the signal. As an 

example, the Defense Advanced GPS Receiver can maintain track 

of a GPS signal in the presence of jamming that is 10 times stronger than the level of interference that would prevent it from 

acquiring the signal in the first place.

10. All satellites in the first block of operational GPS satellites, Block 

II, have been retired.
6 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Continued

Box 1-1.

GPS Receivers in a Jamming Environment

One measure of a receiver’s ability to acquire and 

retain the signal from a Global Positioning System 

(GPS) satellite in the presence of background noise is 

the maximum ratio of the strength of the background 

noise, or jamming signal (J), to the strength of the 

signal from the satellite (S) at which the receiver can 

continue to process the GPS signal. That ratio, often 

referred to as the jammer-to-signal (J/S) ratio, is significantly greater than 1, even for current military 

GPS receivers, such as the Army’s Defense Advanced 

GPS Receiver (DAGR).

The maximum J/S ratio at which the DAGR can first 

acquire a civilian GPS signal is 250; that is, the jammer signal at the receiver can be up to 250 times 

stronger than the signal from the GPS satellite at the 

receiver and the DAGR can still find the GPS signal.1

For a military GPS signal, the maximum J/S ratio is 

much larger, up to 2,500. Once the DAGR has 

acquired a military GPS signal, it can hold on to it in 

the presence of jamming signals up to 12,600 times 

as strong as the GPS signal (for a J/S ratio equal to 

12,600). But because the GPS signal on earth is so 

weak (1.6 x 10-16 watts), a jamming signal need not 

be very strong to make a GPS receiver lose track; 

based on the DAGR’s capability, the jamming signal 

would need to be greater than approximately 2 x 10-12

watts at the receiver.

Improving the Ability to Track a GPS Signal in a 

Jamming Environment

The ability of a particular receiver to acquire and 

retain the signal from a GPS satellite in the presence 

of jamming could be improved in at least two ways. 

By increasing the strength of the signal from the 

satellite, the receiver would be able to operate in the 

presence of even stronger jamming signals and not 

exceed the maximum allowable J/S ratio of 12,600 

that would cause the DAGR to lose track of the GPS 

signal. For example, a satellite signal that was three 

times stronger than the current signal would allow a 

DAGR to operate in the presence of a jamming signal 

that was three times as strong (up to 6 x 10-12 watts, 

or a threefold improvement). Alternatively, improving the receiver’s ability to filter out jamming noise—

by using sophisticated antennas—could increase the 

maximum allowable J/S ratio 10,000-fold without 

boosting signal strength.2

Relationship Between Increases in the 

J/S Ratio and the Effective Range of a Jammer

The strength of a jammer’s signal decreases as the 

distance from the jammer increases; specifically, the 

strength decreases with the square of the distance.3

 A 

receiver with improvements can operate in the presence of a jamming signal that is stronger than a

1. In general, the DAGR must first acquire a civilian signal 

from a GPS satellite and then use data gleaned from that 

signal to acquire the corresponding military signal. See Rockwell Collins, “Defense Advanced GPS Receiver (DAGR) Fact 

Sheet” (2007) for a detailed list of the DAGR’s performance 

capabilities and specifications (www.rockwellcollins.com/

sitecore/content/Data/Products/Navigation_and_Guidance/

GPS_Devices/Defense_Advanced_GPS_Receiver_ 

DAGR.aspx).

2. See Steve Rounds, “Jamming Protection of GPS Receivers, 

Part II: Antenna Enhancements,” GPS World (February 1, 

2004).

3. For example, if the strength of the jamming signal is 4 watts 

at a distance of 2 miles from the jammer, it will decrease in 

strength to 1 watt at a distance of 4 miles from the jammer. 

In mathematical terms, J at 4 miles = J at 2 miles x (2 miles/

4 miles)2

.
CHAPTER ONE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 7

CBO

Box 1-1. Continued

GPS Receivers in a Jamming Environment

The Jammer-to-Signal Ratio and the Effective Range of a 10-Watt Jammer

(Signal strength of jammer, 10-12 watts)

Source: Congressional Budget Office based on data from the Department of Defense and Rockwell Collins, “DAGR Fact Sheet” 

(2007).

Notes: The maximum jammer-to-signal ratio for the current Defense Advanced GPS Receiver (DAGR) is 12,600. For the improved 

DAGR, the ratio is 1,260,000.

The signal strength on Earth for the IIF satellite is 1.6 x 10-16 watts and for the IIIA model, 5.0 x 10-16 watts.

a. Corresponds to current capability.

receiver without improvements, and it can operate 

closer to the same jammer. 

Consider the example of a 10-watt jammer whose 

signal strength equaled 2 x 10-12 watts at a distance of 

55 miles from the jammer, which is strong enough 

to cause a DAGR to lose track of the current GPS 

signal (see point A on the figure). Improving the 

capability of the receiver 100-fold could enable it to 

withstand a jamming signal 100 times greater, or 

equal to 200 x 10-12 watts. A jammer that generated a 

signal of 2 x 10-12 watts at 55 miles would be able to 

generate a signal of 200 x 10-12 watts only at a much 

shorter range—equal, in fact, to 55 miles divided by 

the square root of 100, or 5.5 miles (see point B on 

the figure). Alternatively, a threefold increase in the 

strength of the GPS signal (which is the increase that 

is planned to be provided by the GPS IIIA satellites) 

would enable the DAGR to retain track of the signal 

in the presence of a jamming signal of up to 6 x 10-12

watts. That improvement would reduce the range of 

a 10-watt jammer to 32 miles (see point C on the 

figure).

0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Effective Range of Jammer (Miles)

B

C A

Lose Track with

Improved DAGR and

IIF Signal

Lose Track with

Current DAGR and IIIA Signal

Lose Track with

Current DAGR and IIF Signala

CHAPTER

CBO

2

DoD’s Plan for Modernizing GPS

The Department of Defense plans to continue 

upgrading all three segments of the Global Positioning 

System, investing $7.3 billion from 2012 through 2016 

and $15 billion over the subsequent nine years (see 

Table 2-1). When fielded, all of DoD’s fully modernized 

GPS components should improve the system’s ability to 

perform, even in a jamming environment. But fully 

implementing all of DoD’s plans could take until 2030.

Description and Cost of DoD’s Plan

During the next decade, DoD plans to purchase enough 

of the next-generation GPS III satellites to eventually 

replace the entire constellation and to upgrade the 

ground control system so that it can fully control existing 

and future satellites. DoD also plans to develop military 

receivers that can decode the new M-code signals and to 

purchase and field those receivers to all military users.

Plans for Satellites

The 2011 President’s Budget (submitted in February 

2010) included a plan for modernizing the GPS. However, the Air Force later outlined a less ambitious modernization program in a September 2010 Cost Analysis 

Requirements Document (CARD). Moreover, at the 

same time that the CARD was being prepared and published, the Air Force was also investigating alternative 

programs for GPS III satellites. The results of that analysis, which was expected to be completed in September 

2011, were not available prior to the publication of this 

study. 

DoD’s plans currently include launching the remaining 

10 (out of 12 total) IIF satellites—the latest model of 

GPS II satellite—and deploying an entirely new generation of GPS III satellites. As of February 2011, the Air 

Force expected to launch the last IIF satellite (which was 

purchased in 2006) in 2014. The GPS III satellites, 

which are designed to have stronger military signals, are 

already under development, although the full rollout of 

that program will not be completed until 2030. According to Air Force plans, the GPS III satellite program will 

be carried out in three stages, with each stage yielding satellites of increasing capability.

IIIA Satellites. In the first stage of the program, the 

Air Force will build 8 GPS IIIA satellites capable of 

transmitting military M-code signals at powers three 

times greater than those transmitted by the IIF satellites 

(see Table 2-2).1

 GPS IIIA satellites will also transmit a 

new civilian signal (referred to as L1C) at 1575.42 megahertz, which will be compatible with signals from Galileo 

satellites that are part of the European navigation system 

under development. The new civilian signal will be no 

stronger than the existing L1 civilian signal at that frequency, but it will be easier to track and have greater 

accuracy because of additional data bits within the L1C 

navigation message. 

The military and civilian signals that the GPS IIIA satellites transmit will allow users to determine their position 

more accurately than current signals allow—to within 

3 feet instead of 10 feet. The first 2 IIIA satellites will be 

built during the development phase, which began in 

2002 and is estimated to cost a total of $2.7 billion.2

Between 2010 and 2018, 6 production IIIA satellites will 

be purchased (2 each in 2012, 2013, and 2014), at a total

1. GPS IIIA satellites will transmit both the M-code and previous 

military signals so that receivers in the field that are incapable of 

interpreting M-code signals will still be operable.

2. Approximately $1.8 billion of the total research and development 

funds for the GPS IIIA satellite program were appropriated before 

2012. In addition, DoD invested $1.3 billion prior to 2012 to 

develop the ground control system needed for the GPS III 

satellites.
10 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table 2-1. 

Costs of DoD’s Plan to Modernize the GPS Satellites, Ground Control System, and 

Receivers 

Source: Congressional Budget Office.

Notes: Numbers may not sum to totals because of rounding.

DoD = Department of Defense; GPS = Global Positioning System; n.a. = not applicable; R&D = research and development.

a. Includes satellites acquired with R&D and procurement funds.

b. Launches from 2012 through 2030 are based on the Air Force Space Command’s launch schedule as of April 2010.

c. Includes R&D and procurement funds but excludes funds to operate and maintain the system.

d. Investment funds are to support the launch of IIF satellites.

e. CBO’s estimated costs are based on the standard weight-based cost models used for satellites and on the Air Force’s estimated costs for 

the GPS IIIA satellites.

f. Costs for the IIIC program include those to acquire 8 IIIC satellites that are not explicitly included in DoD’s plan but are needed, in CBO’s 

estimation, to attain full capability.

g. This is CBO's estimate of the cost to purchase roughly 250,000 handheld receivers and 170,000 cards to be inserted into other types of 

Army receivers to partially fulfill the Army's requirement for 460,000 handheld receivers and 310,000 cards.

cost of $2.0 billion.3 The first IIIA satellite is scheduled 

to be launched in 2014, and the eighth, in 2018.

IIIB Satellites. In the second stage of the GPS III program, DoD plans to acquire a total of 16 IIIB satellites. 

Slightly more capable than the IIIA models, those satellites will be able to transmit somewhat stronger M-code 

signals (see Table 2-2).4

 DoD plans to purchase its first 

IIIB satellites in 2015; the first launch is scheduled for 

2018, and the final launch is planned for 2024. The 

Congressional Budget Office estimates that DoD would

IIFd 0 10 0.2 0 00.2

IIIA 8 8 2.4 0.1 0 2.5

IIIBe 16 16 2.2 3.9 0 6.1

IIICe,f 16 16 0 2.0 5.2 7.2 ____ ____ ___ ___ ___ ____

Total 40 50 4.9 6.0 5.2 16.0

Cost n.a. n.a. 1.6 1.3 1.0 3.9

With Current Capability 35,400 n.a. 0.3 0 0 0.3

With M-Code Capabilityg 250,000 n.a. 0.5 0.8 0.7 2.0 ________ ___ ___ ___ ___

Total 285,400 n.a. 0.8 0.8 0.7 2.3

Total GPS Cost n.a. n.a. 7.3 8.0 7.0 22.2

Be Acquireda Launchesb 2016 2021 2025 2012–2025

(Billions of 2012 dollars)

Investment Fundsc

Receivers

Satellites

Ground Control System

Quantity To 2012– 2017– 2022– Total, Planned

3. Those costs include $300 million for the nuclear detonation 

detection systems that are purchased by the Department of Energy 

and carried on the GPS satellites.

4. According to publicly available documents, the IIIB satellites will 

not transmit civilian signals that are stronger than those transmitted by IIIA or IIF satellites.
CHAPTER TWO THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 11

CBO

Table 2-2. 

Characteristics of Current and Planned GPS Satellites

Source: Congressional Budget Office.

Notes: L1, L2, L5, and L1C are commonly used designations for the associated frequencies and signals. Although the L1C signal will be no 

stronger than the existing L1 signal, it will be easier to track and will transmit more accurate data.

GPS = Global Positioning System; MHz = megahertz.

a. Planned launch date.

b. Strength of the M-code military signal at the GPS receiver.

c. Inside area covered by the spotbeam; otherwise, the strength is the same as with the IIIB.

d. Accuracy with which the GPS receiver can determine location.

e. For transmitting data and messages between satellites.

f. The spotbeam covers an area on Earth of roughly 600 miles in diameter.

need $6.1 billion to develop and purchase the 16 GPS 

IIIB satellites.5

IIIC Satellites. The final stage of DoD’s plans for new 

GPS satellites calls for the acquisition of GPS IIIC satellites. This third model will transmit M-code signals at the 

same strength as the IIIB satellites, but it will also be 

equipped with a large antenna capable of focusing the 

M-code signals in a “spotbeam” that will illuminate specific regions on the earth’s surface. In those regions, 

which could measure roughly 600 miles in diameter, the 

M-code signal is expected to be 100 times stronger than 

that transmitted by the IIF satellites. The IIIC satellites 

will also provide much more accurate time and position 

data because they will carry antennas allowing them to 

transmit and relay data almost instantaneously to all 

other similarly equipped GPS satellites. As a result, the 

ground control system will be able to update all GPS IIIC 

satellites with their correct time and position several 

times per day, compared with the once daily updates 

IIF IIIA IIIB IIIC

First Launch 2010 2014a 2018a 2025a

Signal Strengthb (10-16 watts) 1.6 5 8 160c

Accuracyd (Feet) 10 3 3 0.5

High-Speed Cross-Linkse No No No Yes

Spotbeamf No No No Yes

Civilian Signals

1575.42 MHz (L1) Yes Yes Yes Yes

1227.60 MHz (L2) Yes Yes Yes Yes

1176.45 MHz (L5) Yes Yes Yes Yes

1575.42 MHz (L1C) No Yes Yes Yes

Military Signals

Standard signals

1575.42 MHz (L1) Yes Yes Yes Yes

1227.60 MHz (L2) Yes Yes Yes Yes

M-Code signals

1575.42 MHz (L1) Yes Yes Yes Yes

1227.60 MHz (L2) Yes Yes Yes Yes

Signals Transmitted

Satellite Model

5. CBO’s estimated costs for the IIIB and IIIC satellites are based on 

the standard weight-based cost models used for satellites and on 

the Air Force’s estimated costs for the GPS IIIA satellites. CBO’s 

estimates include costs for the nuclear detonation detection 

systems on all GPS III satellites.
12 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

possible with the current system and IIIA and IIIB satellites. By transmitting data that are very accurate, IIIC satellites will enable both civilian and military users to determine their position within 6 inches, instead of the 3 feet 

that will be possible with GPS IIIA and IIIB satellites. 

Although DoD’s plans as of September 2010 called for 

the purchase of 8 IIIC satellites, that number of IIIC satellites would not be sufficient to ensure improved capability. Indeed, it is hard to gauge what impact such a 

small number of satellites equipped with spotbeams 

would have worldwide. Although it is likely that, once all 

8 IIIC satellites enter the constellation, most parts of the 

world would have 3 IIIC satellites in view at least 50 percent of the time and at least 1 IIIC satellite in view more 

than 95 percent of the time, it is unknown whether that 

limited capability would be sufficient to provide the full 

antijamming benefit of the strong focused spotbeam or 

the improved accuracy possible with continuous updates. 

Achieving the size constellation typically needed for 

worldwide coverage with at least 3 IIIC satellites in view 

almost all of the time would require a total of 16 IIIC satellites.6

 For that reason, CBO extended DoD’s plan to 

allow for the purchase of 16 IIIC satellites.

DoD has not published specific schedules for purchasing 

the IIIC satellites or estimates of their costs. CBO, however, estimates that purchases would be made from 2020 

through 2025 and that $7.2 billion would be needed 

through 2025 to develop and procure the IIIC satellites 

(see Table 2-1).7

 (Of those funds, $3.6 billion would be 

needed to acquire the 8 IIIC satellites in DoD’s plan, and 

the remaining $3.6 billion would pay for the 8 IIIC satellites that CBO assumes would be needed to fill out the 

constellation.) The first IIIC satellite could be launched 

as early as 2025 and the last one in 2030.

Plans for the Ground Control System

More-capable GPS satellites alone cannot provide the 

resistance to jamming that DoD requires. Improvements 

to the ground control system are needed to capitalize on 

the increased capabilities of the modernized satellites. 

The GPS III program will add many capabilities to the 

GPS constellation, including continuous updates and 

focused military signals. To accommodate and control 

those capabilities, DoD will need to upgrade the GPS 

ground control system. 

According to the Air Force’s projections, it may be several 

years before the capabilities of the ground control system 

catch up with those of the satellites. The new control system that is needed to monitor the M-code signals transmitted by satellites that are already in orbit will not be 

fully in place until the end of 2016.8

 Additional capabilities, such as those to update satellite data transmissions 

continuously and to control the spotbeam antenna on the 

IIIC satellites when they enter the constellation, will also 

need to be developed. 

The cost to develop the initial capabilities needed to 

monitor the IIR-M, IIF, and IIIA satellites would be 

$1.6 billion from 2012 through 2016 (see Table 2-1). 

Upgrading the capabilities needed to control the 

IIIC satellites would cost an additional $1.3 billion from 

2017 through 2021, CBO estimates, and $1.0 billion 

thereafter.

Plans for Receivers

At the end of July 2011, the GPS constellation included 

10 satellites capable of transmitting the new harder-tojam M-code signals. However, the services do not have 

any receivers capable of interpreting those signals. To fix 

that situation, DoD plans to invest $500 million from 

2012 through 2016 to begin full-scale development of 

M-code-capable receivers in 2013 and to begin testing 

modules in various platforms in 2016. DoD has directed 

the GPS program office to develop a common module 

that would receive and process the M-code signals and 

6. The plan for the GPS III program outlined in Air Force documents based on the 2011 President’s Budget included a total of 16 

IIIC satellites.

7. DoD’s schedule for acquiring the 32 GPS III satellites in its plan 

purchases satellites at the rate of 3 per year. CBO extended the 

purchase of the GPS IIIC satellites at the same rate of 3 per year. 

Although Air Force estimates of the rate at which satellites might 

need to be launched over the next 20 years is less than 3 per year, 

DoD purchases satellites in advance of their anticipated need in 

order to purchase them in efficient quantities and to have extras 

on hand in the event of unexpected satellite failure or loss.

8. Government Accountability Office, Global Positioning System: 

Challenges in Sustaining and Upgrading Capabilities Persist, GAO10-636 (September 2010), p. 26.
CHAPTER TWO THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 13

CBO

that could be embedded in different models of receivers 

designed to meet the specific needs of each service.9

According to DoD, the Army could begin fielding the 

lead platform equipped with the modules—the Raven 

unmanned aerial vehicle—in 2017, and the first receivers 

specifically designed to be mounted in aircraft and ships 

sometime after 2019. Handheld receivers would be the 

last to be fielded, with production scheduled to begin in 

2020. Because the various services now field more than 

400,000 GPS receivers—three-quarters of which are the 

handheld variety—it could be 2030 before all U.S. military forces are fully equipped with receivers capable of 

interpreting the M-code signals.10

Total funding required from 2012 to 2025 to develop 

and purchase M-code-capable receivers would be 

$2 billion, CBO estimates (see Table 2-1). DoD has not 

publicly released estimates of the cost to develop the 

receivers after 2016 or the cost to purchase the receivers 

in sufficient numbers to replace those in the field. 

Because the Army fields more than 90 percent of the military’s receivers and because the Army’s requirements for 

replacement receivers are publicly available, CBO estimated the cost to replace the Army’s currently fielded 

receivers on the assumption that this would capture most 

of the cost.11 Assuming that half of those receivers would 

be replaced between 2017 and 2025 (with the remainder 

replaced thereafter), CBO estimates that the cost to the 

Army would be about $1.5 billion during that period.12

Increased Capabilities

The increased capabilities of GPS satellites and receivers 

under DoD’s plan would better enable military users to 

operate in the presence of radio frequency interference—

the main goal of the plan. Likewise, improvements to the 

satellite and ground control capabilities that are included 

in that plan would allow military and civilian users to 

determine their position more accurately than they can 

today.

The planned improvements would enable military GPS 

receivers to pick up signals from satellites and to retain 

those signals in the presence of higher levels of electromagnetic interference. One way to characterize the magnitude of the improvement is to estimate the reduction in 

one type of jammer’s effectiveness. For example, if a 10-

watt jammer can cause a Defense Advanced GPS Receiver 

(DAGR) to lose a GPS signal that it was tracking from a 

IIR-M or IIF satellite at a range of 55 miles, how much 

would that range be diminished after DoD has fielded 

the improvements that it plans for GPS?

The combined effects of all of DoD’s planned improvements would be to reduce the effective range of noise 

jammers by an estimated 96 percent.13 Each component 

of the system would contribute to that overall improvement. A new receiver, for example, that was capable of 

processing the M-code signals from the older-model satellites already in orbit would be able to maintain track of 

the GPS signals as close as 25 miles to the same 10-watt 

jammer (see Table 2-3).14 In other words, fielding receivers capable of decoding the M-code signals would reduce 

the effective range of a noise jammer by an estimated 

55 percent. 

9. To demonstrate performance, the services have each designated 

one platform to receive the initial modules. As of February 2011, 

those platforms were the Army’s Raven unmanned aerial vehicle, 

the Air Force’s F-15E fighter aircraft, and the Navy’s Arleigh 

Burke class destroyer.

10. The Government Accountability Office predicted in 2009 that it 

could take until 2025 to fully equip the services with new receivers 

capable of interpreting the M-code signal. (See Government 

Accountability Office, Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities, GAO09-325, April 2009, p. 27.) Since that report was published, the 

schedule for fielding receivers capable of processing the M-code 

signal has been extended. In the case of handheld GPS receivers, 

the projected date to begin full-rate production has been delayed 

by five years, leading to a possible full fielding date of 2030. 

11. Although the other services would incur costs to replace or 

upgrade their receivers, because the number of M-code-capable 

receivers needed for the other services is much smaller than the 

Army’s requirement, those costs should be much less than those 

needed to replace and augment the Army’s current receivers.

12. That would be the cost to purchase 250,000 new handheld receivers and 170,000 M-code modules to be packaged with other 

devices such as radios and fire control systems, if the cost for each 

was roughly the same as for the current Defense Advanced GPS 

Receiver and Ground-Based GPS Receiver Application Module 

card, respectively. This would represent slightly more than half of 

the Army’s stated need for more than 460,000 handheld receivers 

and 310,000 M-code modules.

13. That includes fielding receivers able to use the M-code signals and 

increasing the strength of those signals 100-fold within the regionally focused signal from the spotbeam antenna on the GPS IIIC 

satellite.

14. That corresponds to a fivefold (or 7 decibel) improvement in the 

ability to resist jamming.
14 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table 2-3. 

Estimated Impact of Planned GPS Improvements on the 

Effective Range of a Jammer

Source: Congressional Budget Office based on data from the Department of Defense.

Note: GPS = Global Positioning System; DAGR = Defense Advanced GPS Receiver.

a. The M-code-capable receiver is a DAGR-type receiver capable of receiving and processing M-code signals.

b. The maximum ratio of jammer signal strength to GPS satellite signal strength at which the receiver can maintain track of the GPS signal.

c. Within the 600-mile-diameter spotbeam.

d. The jammer signal strength that can cause a DAGR-type handheld receiver to lose track of the current military signal or an M-code signal.

e. The range at which a 10-watt noise jammer can cause a DAGR-type handheld receiver to lose track of the current military signal or an 

M-code signal.

The effective range of a 10-watt jammer would be diminished even further—to 14 miles—once GPS IIIA satellites are added to the constellation. That is because those 

satellites will transmit M-code signals that are roughly 

three times stronger than those transmitted by IIR-M and 

IIF satellites (see Table 2-2).15 When GPS IIIC satellites 

are added, they will be capable of focusing the M-code 

signals on to a specific region on Earth, raising the 

strength of the signal within that region to 100 times that 

of the most modern GPS satellites now in orbit.16 By 

doing so, the range at which a 10-watt jammer could 

defeat an M-code-capable receiver would be reduced to 

an estimated 2.5 miles, less than 5 percent of its range 

against current receivers trying to track signals from current satellites (see Table 2-3).

Another improvement in the capability of GPS overall 

would result from the ability to continuously update data 

transmitted by the satellites with the fielding of GPS IIIC 

satellites. Once a sufficient number of satellites with 

high-speed cross-link antennas are in the constellation, 

the ground control system will be able to update hourly 

the time data broadcast by the satellites. That improvement would reduce time and position errors to such an 

extent that military and civilian GPS users would probably be able to determine their location to within less than 

6 inches.

Concerns About the Plan 

Researchers and others have raised three concerns about 

DoD’s plan. First, the fielding of the equipment to counter jamming, including improved receivers and higherpower satellites, would take well into the next decade, 

leaving military GPS users vulnerable to interference for 

many years. Second, although DoD has not published a 

schedule or an estimated cost to complete any portion 

of the GPS program, the details that are known about the 

GPS III satellite program indicate that it is ambitious, 

which raises the risk of cost increases, delays in fielding, 

or both.17 And third, there is a lack of synchronization 

among the upgrades to the various segments of the 

program. 

Attribute

Maximum Jammer-to-Satellite Signal Ratiob (x 1,000) 12.6 63 63 63

Military Signal Strength at Receiver (10-16 watts) 1.6 1.6 5 160 c

Maximum Jammer Signal Toleratedd (10-13 watts) 20 100 320 10,100 c

Maximum Effective Jammer Rangee (Miles) 55 25 14 2.5 c

Capablea

Satellite Model and Receiver

DAGR

and Current

IIR-M, IIF,

and M-Code

III-A 

and M-Code

Capablea Capablea

IIR-M, IIF, IIIC

and M-Code

15. The combined improvement in antijamming capability resulting 

from the M-code signal (a factor of 5) and the stronger signal 

from the IIIA satellite (a factor of 3.16) is equal to the product of 

the individual improvements, or 15.8.

16. GPS IIIB satellites are scheduled to transmit M-code signals that 

are five times the strength of those transmitted by IIF satellites 

and slightly stronger than those transmitted by IIIA satellites.

17. DoD has published the full schedule and cost of the GPS IIIA satellite program, but that represents the cost for only 8 satellites.
CHAPTER TWO THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 15

CBO

Figure 2-1.

Effect of DoD’s Plan on the Effective Range of a 10-Watt Jammer

(Effective range, in miles)

Source: Congressional Budget Office based on Department of Defense data.

Notes: Improved capability is assumed to have been phased in when 18 satellites of improved capability (or 16 IIIC satellites) are in orbit.

The scale for the y-axis is logarithmic, not linear.

DoD = Department of Defense.

a. Improved performance would be available only to the small number of M-code-capable receivers in the field.

b. Half of all fielded receivers would be equipped with M-code capability.

c. Within the 600-mile-diameter area covered by the satellite’s spotbeam.

Timetable and Sufficiency of Full Antijam Capability 

DoD’s planned improvements in the ability of GPS to 

operate in a jamming environment rely heavily on new 

military signals from space. Those include the M-code 

signals, the more powerful M-code signals that will be 

transmitted by IIIA and IIIB satellites, and the focused 

M-code signals from IIIC satellites. The increased resistance to jamming provided by those improvements, 

although significant, will not be available for 10 to 20 

years. 

The antijamming improvements expected from the Mcode signals will not be realized until sufficient numbers 

of M-code-capable receivers are in the field—probably no 

earlier than 2026.18 At that time, the effectiveness of 

enemy jammers could be lessened significantly (see 

Figure 2-1). Similarly, although the first IIIA satellite is 

scheduled to be launched in 2014, 1 satellite alone will 

not provide sufficient coverage to make a difference to 

GPS users. A minimum of 18 satellites need to be in the 

constellation to guarantee that at least 3 will be visible at 

all times. Thus, it could be 2022 before enough satellites—8 IIIA and 10 IIIB—are in the constellation to 

make it likely that strong signals from 3 satellites will be 

available at any given time (see Figure 2-1). And even 

then, only small numbers of M-code-capable receivers 

will have been fielded. Thus, the benefits of stronger signals from the GPS III satellites will be available only to a 

small number of users in 2022. Finally, GPS IIIC satellites promise a 100-fold improvement in the strength of 

M-code signals compared with signals from today’s satellites. However, the stronger signals will not be available 

worldwide until 16 IIIC satellites are in the constellation, 

which is not likely to occur before 2030. 

Even when all of the improvements included in DoD’s 

plan are in place, they might not be sufficient to counter 

the hostile environment that users might face. The 

Defense Science Board (DSB), in a 2005 report, recommended that antijamming improvements be made to 

2010 2015 2020 2025 2030

0.01

0.1

1

10

100

Available to Most Users

Available to the

Small Number of

M-Code Receivers

Fielded

IIIA

Satellites IIIB

Satellites

Significant Number of

M-Code Receivers

Fielded

IIIC

Satellites

b

a a

c

18. For its analysis, CBO assumes that replacing half of all military 

receivers will be sufficient to yield an appreciable increase in overall capability.
16 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

achieve a GPS system that could withstand jamming levels that were 10,000 to 1 million times greater than current levels.19 DoD’s planned enhancements, combined, 

will improve performance in a jamming environment by 

at most a factor of 500, far short of the level of improvement recommended by the DSB.

Uncertainty in Costs and Schedules 

Developing the new satellites, control systems, and 

receivers carries some risks.20 The risks are relatively small 

for the IIIA satellites, because their planned capabilities 

are not that much greater than those of the IIF satellites. 

And the IIIB satellites in DoD’s plan represent only a 

small increase in capability over the IIIA model. Providing the IIIC satellites with high-speed cross-links to communicate with other GPS satellites, in contrast, requires 

new antennas and other hardware whose development 

could carry greater risk. In addition, GPS IIIC satellites, 

which will be capable of focusing the signal on to specific 

regions on Earth, will need to be equipped with a dedicated large and steerable antenna and the hardware 

needed to direct the antenna and signal to the desired 

spot on Earth as a satellite passes overhead. Those significant increases in capability require the development of 

new technologies needed to produce satellites to the 

desired specifications, ensure that they operate correctly, 

and control them from the Earth. In the past, when the 

GPS program tried to increase the capability of its satellites significantly (as was the case with the upgrade from 

the IIR to the IIF model) or its ground control system 

(such as by adding the ability to manage the M-code signals), the program experienced increases in costs and 

delays in the planned delivery of improved capabilities. 

As currently structured, the GPS program could realize 

the same problems, especially for those portions that 

would develop and procure the IIIC satellites.

CBO estimates that the investment needed to complete 

DoD’s planned modernization of the GPS satellites, control system, and user equipment will be slightly greater 

than $22 billion. (DoD has not published those costs.) 

Most of the costs would be attributed to the satellite portion of the program, with investment requirements equal 

to or exceeding $1 billion per year through 2025.

Unsynchronized Improvements 

Another concern that has been highlighted by the 

Defense Science Board and the Government Accountability Office is the lack of synchronization among the 

three segments of the GPS program.21 For example, 

although 10 GPS satellites transmitting M-code signals 

were in orbit at the end of July 2011, DoD has not developed or fielded any receivers capable of processing those 

signals—and the initial fielding of the first common 

ground modules to demonstrate the capability to decode 

the signals is not scheduled until 2017. Because full production of the handheld receivers is not scheduled to 

begin until 2021, after one year of low-rate production, 

large numbers of M-code-capable receivers to replace the 

current DAGRs are not likely to be fielded before 2026. 

And even if some military receivers capable of processing 

the M-code signals could be fielded earlier, the ground 

control system would be incapable before the end of 

2016 of ensuring that the satellites were transmitting the 

correct data on the M-code signal.22 Thus, although satellites with enhanced capabilities are being developed, purchased, and launched, no one will be able to take advantage of those improvements for at least five more years.

19. Defense Science Board, The Future of the Global Positioning System

(October 2005), p. 63. 

20. Recent changes made in the GPS III satellite program are an indication of those risks. The GPS program associated with the 2011 

President’s Budget and documented in the spring of 2010 was 

more ambitious than the current plan. It included fewer IIIB satellites (8 instead of 16) but those IIIB satellites would have been 

equipped with high-speed cross-links, thus enabling the transmission of more accurate data. Furthermore, the spring 2010 plan 

included a total of 16 IIIC satellites, half of the initial 32 GPS III 

satellite constellation. As a result, the constellation resulting from 

the earlier plan would have had more capability than that likely to 

result from the most recent plan described in the September 2010 

CARD. One reason given in the press for deferring the high-speed 

cross-links to the IIIC rather than the IIIB model was to reduce 

risk and increase affordability. See “GPS IIIB Satellites to Add 

Critical New Capabilities,” GPS Daily (July 6, 2011), www.gpsdaily.com/reports/GPS_IIIB_Satellites_to_Add_Critical_New

_Capabilities_999.html.

21. Defense Science Board, The Future of the Global Positioning System, pp. 48, 49, 55, and 56; Government Accountability Office, 

Global Positioning System: Significant Challenges in Sustaining and 

Upgrading Widely Used Capabilities, pp. 26–29; and Government 

Accountability Office, Global Positioning System: Challenges in Sustaining and Upgrading Capabilities Persist, pp. 25–29. 

22. The Air Force projected in February 2011 that the control capability for the M-code signal would not be available until September 2016, nine years later than had originally been planned.
CHAPTER

CBO

3

Alternatives for Modernizing GPS

The Congressional Budget Office examined three 

alternatives to the Department of Defense’s plan that 

would modernize the Global Positioning System, 

improve the system’s performance in a jamming environment, and address some of the concerns about DoD’s 

plan. The alternatives, or options, would reduce the cost 

of modernizing and controlling the GPS satellite constellation, as well as the risk of developing it, by canceling the 

IIIB and IIIC satellite portions of the program. The 

options would also make these changes: 

 Option 1 would improve current GPS receivers by fitting them with better antennas and by adding tightly 

integrated inertial navigation systems (INSs).

 Option 2 would capitalize on a DoD research and 

development program known as iGPS, or High Integrity GPS, by enabling modified GPS receivers to use 

information received via the Iridium commercial communications satellite network to improve performance 

in the presence of jammers.

 Option 3 would carry out the improvements included 

in both Option 1 and Option 2.

All three options would cost less to implement than 

DoD’s plan and would yield greater benefits for military 

users—in terms of antijamming capability—sooner. Each 

option, however, has some disadvantages, such as the 

potentially greater weight of more capable receivers, 

which would reduce their value to personnel operating on 

foot.

Overview of the Options

In contrast to DoD’s modernization plan, which reduces 

GPS’s vulnerability to jamming primarily by increasing 

the satellites’ capabilities, the options emphasize improving the ability of military GPS receivers to operate in a 

hostile environment. That approach was recommended 

by the Defense Science Board in its October 2005 report 

on the future of GPS. The DSB noted that improving 

GPS receivers is the quickest path to making the system 

as a whole less vulnerable to jamming.1

 CBO focused on 

ways to enhance the capabilities of the military’s handheld receivers, which are used primarily by the Army and 

the Marines but are widespread throughout all of the services. (Most of the military’s so-called handheld receivers 

are mounted, sometimes permanently, in various military 

vehicles.)

Although significant improvement would come from 

introducing receivers capable of processing M-code signals earlier than DoD’s plan allows, the Government 

Accountability Office has reported that accelerating the 

fielding of such receivers through the infusion of additional funds is not possible because of technical difficulties.2

 Several different approaches that could yield 

improved receivers in the next 5 to 10 years are possible, 

however. Approaches recommended by the DSB include 

equipping receivers with antennas that block jamming 

signals and integrating miniature inertial navigation systems into receivers to enhance their ability to process the 

GPS signal and to limit interference from jamming signals. Another path to improving the capabilities of GPS 

receivers that DoD has been pursuing for several years 

would use the Iridium commercial low-Earth orbiting 

satellite constellation to provide data to military receivers 

that would enhance their ability to process the GPS signal 

in the presence of jamming.

1. Defense Science Board, The Future of the Global Positioning System

(October 2005), p. 56. 

2. Government Accountability Office, Global Positioning System: Significant Challenges in Sustaining and Upgrading Widely Used Capabilities, GAO-09-325 (April 2009), p. 31; and Government 

Accountability Office, Global Positioning System: Challenges in Sustaining and Upgrading Capabilities Persist, GAO-10-636 (September 2010), p. 28.
18 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

All of the options would rely solely on IIIA satellites. 

Those satellites would transmit M-code signals at higher 

power than current satellites do and would emit a new 

civilian signal; otherwise, they do not include additional 

capabilities that would require significant developmental 

effort. Because the satellites in the GPS constellation 

must be replaced when they can no longer reliably perform their mission (estimated to be after 8 to 12 years in 

orbit), CBO predicts—using Air Force planning factors—that DoD will need to launch 50 or more satellites 

between 2012 and 2030. Because DoD has already 

acquired 10 GPS IIF satellites that it has not yet 

launched, CBO estimates that DoD will need to acquire 

40 satellites in the GPS III program through 2030.3

Under the options, DoD would also acquire a total of 

40 GPS satellites, but they would all be the IIIA model.

CBO did not analyze the effects of its options on civilian 

users of the system in quantitative detail. Because all of 

the options would retain GPS IIIA satellites but cancel 

the programs for IIIB and IIIC satellites, they would have 

the same general effect on civilian users. (See Box 3-1 for 

an expanded discussion.) 

All three of the options that CBO analyzed have several 

features in common in addition to acquiring 40 GPS IIIA 

satellites and canceling the IIIB and IIIC programs (see 

Table 3-1). They would all continue to develop the GPS 

ground control system so that it would be able to control 

the GPS satellites now in orbit that are capable of transmitting M-code signals as well as the newer GPS IIIA satellites. But the options would cancel those portions of the 

modernization program that would have developed control software to support the high-speed cross-links and 

the spotbeam antenna introduced on the IIIC satellites. 

CBO estimates that those reductions in the scope of the 

GPS III satellite program and the improved ground control system program would yield about $2.2 billion in 

savings from 2012 through 2021 and more than $4 billion if the total cost of DoD’s plan through 2025 is taken 

into account.4

 And finally, the options would develop and 

purchase the same number of M-code-capable receivers as 

would DoD’s plan and continue support of the IIF 

satellites.5

The features common to the three options—40 GPS IIIA 

satellites, selected improvements to the ground control 

system, M-code-capable receivers, and the cost to launch 

the remaining IIF satellites—are needed to maintain and 

control the GPS constellation and to take full advantage 

of the satellites (some of which are already in orbit) that 

are capable of emitting the harder-to-jam M-code signals. 

CBO estimates that those features would cost $17.9 billion through 2025 (see Table 3-2). In addition, each of 

the three options would have other costs ranging from 

$1.0 billion (for the second option) to $3.0 billion (for 

the third option).

Option 1. Improve the Capabilities of 

Military GPS Receivers

Option 1 would improve the ability of military GPS 

receivers to provide accurate location data in a jamming 

environment by employing two techniques that, when 

combined, could reduce the effective range of noise jammers by 97 percent.6

 Because the techniques have already 

been researched and developed, it should be possible to 

field large numbers of improved receivers by 2018, significantly sooner than any improved systems acquired under 

DoD’s plan could be fielded. Furthermore, the overall 

cost to carry out this option would be slightly more than 

$2 billion less than the total cost through 2025 to implement DoD’s plan.

Techniques for Improving the Capabilities of 

Receivers

The ability of military receivers to process GPS signals 

could be improved in the next few years through antenna 

enhancements that limit the interference from jamming 

signals and through techniques that eliminate errors that 

are introduced during signal processing. This option 

would enhance receivers’ capabilities through a combina3. In the options, CBO assumes that DoD would purchase IIIA satellites at a rate of 3 per year, matching the rate at which DoD 

plans to purchase IIIB and IIIC satellites.

4. CBO estimates that the procurement of 16 GPS IIIC satellites 

will not be complete until 2025.

5. CBO’s estimate is based on the cost to purchase only the Army’s 

handheld GPS receivers and cards, which are far more numerous 

than those of the other services and account for more than 90 percent of all military receivers. 

6. The basis for comparisons of improved capability resulting from 

implementing CBO’s three options is that of the Army’s current 

DAGR and the signal from the IIF satellite. 
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 19

CBO

Box 3-1.

The Effect of CBO’s Options on Civilian GPS Users

For each of the three options considered in this 

Congressional Budget Office (CBO) analysis, the 

effect on civilian users would be the same and would 

result primarily from the cancellation of the capabilities introduced by the IIIC satellites included in the 

Department of Defense’s (DoD’s) plans. 

All of the options would purchase enough IIIA satellites to maintain the Global Positioning System 

(GPS) constellation as long as DoD’s plan would—

probably until at least 2030. The improvements to 

civilian signals associated with the introduction of the 

IIIA satellites would be retained in all of CBO’s 

options. Those improvements would result from the 

addition of the L1C signal, which, according to 

DoD’s statements, would improve tracking of GPS 

signals by civilian receivers and also enable those 

receivers to determine their position with greater 

accuracy than using signals from current satellites. 

The size of the improvement in accuracy is uncertain 

at this time; if it was similar to that afforded to military users, it would mean an increase in location 

accuracy from 10 feet to 3 feet. 

However, without the constantly updated data that 

would be transmitted by DoD’s planned IIIC satellites, civilian users would not be able to determine 

their position as accurately as they would if DoD’s 

plans were carried out. For military users with access 

to iGPS (High Integrity GPS) in Options 2 and 3, 

that disadvantage would be essentially negated. But 

civilian users would not have access to the iGPS 

network, so they would not benefit from its enhanced 

accuracy. Consequently, civilian users under CBO’s 

options would have the improved accuracy—to 

within roughly 3 feet—resulting from the more modern civilian signal transmitted by IIIA satellites, but 

they would not be able to attain the location accuracy 

of 6 inches promised by the constantly refreshed data 

transmitted by the IIIC satellites.

Moreover, the improvements to performance in the 

presence of interference that would accrue to military 

users in CBO’s options would not pertain to civilian 

users, for several reasons. First, civilian users would 

not have access to the harder-to-jam M-code signal 

that would be available to military users. Second, 

GPS III satellites, including the IIIA model, would 

not broadcast civilian signals at higher strength than 

current GPS satellites. And finally, CBO’s options 

would not include any enhancements to civilian 

receivers similar to those proposed for military receivers. Although the performance of civilian receivers 

could be improved by using enhancements that are 

similar to those proposed for military receivers—that 

is, adding improved antennas and integrating inertial 

navigation systems—such improvements might cost 

as much as, or more than, the receiver itself. (The 

Defense Science Board estimated the cost to upgrade 

a handheld receiver in its 2005 report to be $2,000.)1

1. Defense Science Board, The Future of the Global Positioning 

System (October 2005), p. 96.
20 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table 3-1. 

Summary of DoD’s Plan for the Global Positioning System and Three Options

Source: Congressional Budget Office.

Note: DoD = Department of Defense; GPS = Global Positioning System; INS = inertial navigation system; iGPS = High Integrity GPS.

a. Includes 8 IIIC satellites not explicitly included in DoD’s plan but needed, in CBO’s estimation, to attain full capability.

b. Compared with the current capability of the Defense Advanced GPS Receiver and IIF satellite signal.

c. Includes research and development and procurement funds but excludes funds to operate and maintain the system.

d. To attain specified performance.

tion of those two techniques, each of which could reduce 

a jammer’s effective range by 82 percent.7

Improved antennas for GPS receivers use various types of 

filtering to limit the interference from a jamming signal. 

If the jamming signal is limited to a very narrow frequency range, spectral filters can prevent reception of the 

signal at those frequencies.8

 If the jamming signal covers 

the military signal’s full range of frequencies, however, as 

is the case with commonly used wide-band noise jammers, that technique will not work. To counter wideband jamming signals, advanced antennas that limit 

input on the basis of the signal’s angle of arrival can filter 

out most of the jamming noise. Directional antennas 

block signals from ground-based jammers by receiving 

only those signals that come from satellites that are visible 

IIIA 8 40 40 40

IIIB 16 0 0 0

IIIC 16 a 000 ___ ___ ___ ___

Total 40 40 40 40

Receivers Are M-Code Capable

Receivers Have Improved Antennas

Receivers Have INS 

iGPS Augmentation

Total Investment Cost, 2012 to 2025

 (Billions of 2012 dollars)c

Year Significantly Improved Capability Is Achieved

Year Full Capability Is Achieved

Receivers Require Added Weight and Powerd

Improved Navigation in Canyons and Mountains

Depends on Commercially Owned System

2030

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

Yes

No

No

2026

Yes

Yes

Yes

No

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

2026

DoD's Plan Option 1 Option 2 Option 3

GPS III Satellites Acquired

Major Improvementsb

Cost and Schedule

Other Considerations

Yes

No

No

No

2026 2018

22.2 19.9 18.9 20.9

2018

2026

2018

7. That corresponds to an increase by a factor of roughly 32 (equal 

to 101.5) in receivers’ capability from each of the techniques. 

Together, the resulting improvement is equal to the product of the 

individual factors, equal to 101.5 x 101.5, or 1,000.

8. The original military signals (not the M-code signal) extend out 

10 megahertz (MHz) on either side of the two central frequencies. 

If the jamming signal covers only a small part of that spectrum, as 

would be the case with narrow-band noise jammers, then spectral 

filters can block the receiver from processing that signal. If, however, the jamming signal covers the entire 20 MHz of the military 

signal (in a process known as wide-band noise jamming), then 

spectral filters will not be able to screen out the jamming signal 

without also blocking the signal from the GPS satellite.
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 21

CBO

Table 3-2. 

GPS Modernization Costs in DoD’s Plan and Under Three Options

(Billions of 2012 dollars)

Source: Congressional Budget Office.

Note: GPS = Global Positioning System; DoD = Department of Defense; iGPS = High Integrity GPS.

a. Includes research and development and procurement funds but excludes funds to operate and maintain the system.

b. CBO’s estimates for the cost of IIIB and IIIC satellites are based on the standard weight-based cost models used for satellites and on the 

Air Force’s estimated costs for the GPS IIIA satellites.

c. Includes current models and M-code receivers.

d. Costs for the iGPS program are for acquisition only and do not include the roughly $20 million per year that would be needed starting in 

2014 to maintain the ground stations and to buy time on the Iridium satellite network.

Satellitesb 4.9 6.0 5.2 16.0

Control Systems 1.6 1.3 1.0 3.9

Receiversc 0.8 0.8 0.7 2.3 ___ ___ ___ ____

Total 7.3 8.0 7.0 22.2

Satellites 4.5 4.9 3.7 13.1

Control Systems 1.6 0.5 0.4 2.5

Receiversc 0.8 0.8 0.7 2.3 ___ ___ ___ ____

Total 6.9 6.2 4.8 17.9

Options' Common Cost 6.9 6.2 4.8 17.9

Receiver Improvements 1.0 1.0 0 2.0 ___ ___ ___ ____

Total 7.9 7.2 4.8 19.9

Options' Common Cost 6.9 6.2 4.8 17.9

iGPSd 0.2 0.5 0.3 1.0 ___ ___ ___ ____

Total 7.1 6.7 5.1 18.9

Options' Common Cost 6.9 6.2 4.8 17.9

Receiver Improvements 1.0 1.0 0 2.0

iGPSd 0.2 0.5 0.3 1.0 ___ ___ ___ ____

Total 8.1 7.7 5.1 20.9

Option 3: Improve GPS Receivers and Augment GPS with iGPS

2012–2016 2017–2021 2022–2025 Total, 2012–2025 

Investment Costsa

DoD's Plan

Common to All Options

Option 1: Improve GPS Receivers

Option 2: Augment GPS with iGPS
22 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

at least 10 degrees above the horizon. The Army has 

developed that type of antenna for installation on some of 

its weapon systems.9

More sophisticated directional antennas that are composed of a number of separate elements use a technique 

called nulling to prevent the receiver from being overwhelmed by jamming noise. If one of the elements 

detects a jamming signal, that element is turned off, leaving the rest of the antenna to function normally and 

receive the GPS signal without interference. Those types 

of antennas can block signals from jammers located at 

several different positions around the receiver. DoD has 

developed several versions of those antennas and installed 

them on some military aircraft and ships; newer and 

smaller versions could also be mounted on vehicles or carried in a backpack. (See Appendix B for some examples of 

such antennas.) The use of improved antennas such as 

those described here could prevent 97 percent of jamming noise from reaching the receiver and reduce a jammer’s effective range by 82 percent.10

Even though improved antennas can filter out most of 

the noise and associated errors introduced by jamming 

signals, they would not be able to prevent the introduction of all errors. Augmentations to receivers’ processing 

abilities and components could further improve their performance in hostile jamming environments. One 

improvement studied by DoD would couple a GPS 

receiver with an inertial navigation system, which can 

measure changes in location by accurately monitoring a 

user’s movements. Integrating information from the INS 

enhances a receiver’s ability to process the GPS signal and 

to filter out noise. Tightly integrating an INS with a GPS 

receiver could, by itself, also reduce the effective range of 

enemy jammers by 82 percent.11

Schedule and Cost for Purchasing and Fielding 

Improvements to Receivers

Because the technologies for the improvements included 

in this option have already been developed, it should be 

possible to field receivers incorporating those technologies relatively quickly. The services have already begun to 

develop, and in some cases, field, advanced antennas to 

be used on specific aircraft and weapon systems.12

Because such systems have been designed and tested, 

improved antennas and INSs could be purchased starting 

in 2013 and integrated into existing platforms thereafter.

CBO estimates that the total cost to purchase the needed 

hardware and to integrate it into existing or future GPS 

receivers would be $2 billion. Of that total, $1 billion 

would be needed before 2017 to purchase enough equipment to upgrade roughly half of the GPS receivers currently fielded by the services. An additional $1 billion 

would be needed from 2017 through 2021 to purchase 

the remaining hardware (see Table 3-2 on page 21).

Effect of Option 1 on GPS Performance in a 

Jamming Environment

The ability of military receivers to operate in a jamming 

environment would be significantly improved within just 

9. One example is the GPS Antenna Masking Ring developed for use 

on some Army vehicles equipped with rocket launchers.

10. Such antennas could improve the performance of GPS receivers in 

a jamming environment by a factor of roughly 30 or more. See 

Lt. Col. Bill Hawken, Directorate of Joint Capability Production, 

Chief of Force Development, Canadian National Defense, 

“NAVWAR (Navigation Warfare): Electronic Warfare and the 

Global Positioning System” (presentation to the Ottawa, Ontario, 

chapter of the Armed Forces Communications and Electronics 

Association, March 2007, available at http://afceaottawa.ca/

view_doc_by_id.php?edit=1&inpage=true&id=128); Office of 

the Program Manager, Communications Program Office, Program Executive Office, Command, Control, Communications, 

and Intelligence, “GPS Air Navigation Overview”(presentation to 

the Air Combat Electronics User’s Conference, Reno, Nev., March 

2010); Chuck Andrews, Joint Navigation Warfare Center, 

“JNWC Mission Brief” (presentation to the Program Manager’s 

GPS Integration Conference, Sterling Heights, Mich., September 

2009); Edwin Hogan, Research, Development, and Engineering 

Command, U.S. Army, “GPS Antenna Masking Ring (GAMR) 

Update” (September 2009); and Steve Rounds, “Jamming Protection of GPS Receivers, Part II: Antenna Enhancements,” GPS 

World (February 1, 2004).

11. Tightly integrating an INS unit with a GPS receiver improves the 

receiver’s ability to reject jamming noise. The combined system 

can navigate alone on the inertial measurement unit if the GPS 

signal is totally lost. For more information, see Hawken, 

“NAVWAR (Navigation Warfare): Electronic Warfare and the 

Global Positioning System”; Andrews, “JNWC Mission Brief”; 

Steve Rounds, “Jamming Protection of GPS Receivers, Part I: 

Receiver Enhancements,” GPS World (January 1, 2004); and Brad 

Parkinson, “GPS ‘Big Five’ Contributions to Users’ Needs: Showing Dependence of User Measures of Effectiveness (MOE) on 

GPS System Design” (presentation to the National Space-Based 

PNT Advisory Board Meeting, Washington, D.C., March 27, 

2008). Additional data were provided in a private communication 

to the Congressional Budget Office from the Office of the 

Secretary of Defense, Office of the Under Secretary of Defense 

(Acquisition, Technology, and Logistics), Office of the Director, 

Defense Research and Engineering, August 2010.

12. DoD has developed several types of antennas, for use on military 

aircraft, that are designed to negate jamming signals. The Navy is 

fielding them on its F-18, AV-8, and P-3 aircraft and on its MH60, HH-60H, and MH-53 helicopters. The Army has tested in 

some of its vehicles directional antennas designed to negate signals 

from ground-based jammers. 
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 23

CBO

Figure 3-1.

Effect of DoD’s Plan and Three Options on the Effective Range of a 

10-Watt Jammer

(Effective range, in miles)

Source: Congressional Budget Office.

Notes: Improved capability is assumed to have been phased in when half of all fielded receivers are equipped with enhancements or, in the 

case of the Department of Defense’s (DoD’s) plan, when 16 IIIC satellites are scheduled to be in orbit.

The scale for the y-axis is logarithmic, not linear.

a few years by fielding the enhancements to receivers 

envisioned under Option 1. The combined increase in 

capability afforded by the two enhancements—compared 

with the capability available using the Army’s Defense 

Advanced GPS Receiver and the signal from a IIF satellite—would reduce the range at which an enemy jammer 

broadcasting a 10-watt signal could cause a DAGR to 

lose track of a GPS signal from 55 miles to about 2 miles 

(see Figure 3-1).13 And because the hardware associated 

with the proposed enhancements could be purchased and 

fielded quickly, CBO estimates that roughly half of the 

military’s receivers could be upgraded by 2018.

Additional improvements in capability would follow 

when 18 IIIA satellites were put into orbit and when 

large numbers of M-code-capable receivers were fielded. 

Although 18 IIIA satellites transmitting a stronger 

M-code signal will be in the constellation by 2022, most 

of the GPS receivers in the field at that point, particularly 

handheld versions, would not be capable of processing 

the M-code signal. Only by 2026, CBO estimates, would 

roughly half of military users have M-code-capable 

receivers. At that point, the combined enhancements 

would reduce the effective range of a 10-watt jammer 

against updated receivers even further—to 0.4 miles (see 

Figure 3-1 and Table 3-3). Thus, when all the improvements introduced by this option have been fielded in 

sufficient number, the effective range of enemy jammers 

should be reduced by 99.3 percent, from 55 miles 

to 0.4 miles in the case of a 10-watt jammer.14

Advantages and Disadvantages of Option 1

Option 1 would have three advantages when compared 

with DoD’s plan. First, it would yield greater improvement in the ability of GPS to operate in a jamming environment, reducing the effective range of a 10-watt jammer to 0.4 miles (compared with 2.5 miles under DoD’s 

plan).15 That level of improvement is within the range 

2010 2015 2020 2025 2030

0.01

0.1

1

10

100

Upgraded Receivers

Fielded to

Half the Force

M-Code Receivers

Fielded to

Half the Force

DoD's Plan

Option 1

Option 2

Option 3

13. Those improvements would allow receivers to operate in the presence of jamming signals 1,000 times stronger than would be possible with the receivers that are currently fielded. As a result, the 

effective range of any wide-band noise jammer would be reduced 

by a factor of 32, equal to the square root of 1,000.

14. The capability of receivers to operate in a jamming environment 

should increase by a factor of 15,800.

15. This option could enable receivers to operate in the presence of 

jamming signals that were roughly 3,000 times stronger than 

would be possible under DoD’s plan.
24 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table 3-3. 

Effect of DoD’s Plan and Three Options on GPS Performance Over Time

Source: Congressional Budget Office.

Note: DoD = Department of Defense; GPS = Global Positioning System; iGPS = High Integrity GPS.

a. The range at which a 10-watt jammer can cause a handheld GPS receiver to lose track of the military signal.

b. Within the focused spotbeam that covers a 600-mile-diameter area from GPS IIIC satellites only.

c. Within the theaters of operation supported by the iGPS program. 

d. The effective jammer range for this option is slightly greater than 130 feet.

e. The values represent the strength of the M-code signal at the receiver.

recommended by the DSB in its 2005 report.16 Second, 

the improvements from Option 1 would be available to 

users 8 years earlier than any significant improvements 

resulting from DoD’s plan (see Figure 3-1). Third, the 

cost to carry out Option 1—about $19.9 billion through 

2025—would be more than $2 billion (or 10 percent) 

lower than the cost of DoD’s plan (see Table 3-2).

By forgoing the improvements provided by the GPS IIIC 

satellite program, however, Option 1 would have two 

major disadvantages when compared with DoD’s plan. 

First, under this option, users would not benefit from the 

greatly improved accuracy that could be realized with 

the introduction of the high-speed cross-link antennas on 

the IIIC satellites. The ability to update all GPS satellites 

every hour that comes with those antennas and the 

upgraded ground control system would be forfeit, along 

with the much more accurate data that could be transmitted by the satellites. Thus, the GPS program under 

Option 1, by relying on IIIA technology and once-daily 

satellite updates, would yield accuracies in determining 

location of about 3 feet, rather than the 6 inches that 

might be possible under DoD’s plan. Such pinpoint accuracy might not be important for most applications, but it 

could be useful when trying to establish the exact position of landmines or unexploded ordnance. 

The second disadvantage, which all three of CBO’s 

options would have, results from the loss of the planned 

spotbeam signal on the IIIC satellites. That higher power 

focused signal—100 times stronger than the signal transmitted by current IIF satellites—would improve the ability of GPS receivers to operate in a jamming environment 

even without the improvements to receivers considered in 

DoD's Plan 55 55 2.5 b

55 1.8 0.4

55 0.6 0.14 c

55 0.1 0.02 c,d

DoD's Plan 1.6 1.6 160 b

1.6 1.6 5

1.6 1.6 5

1.6 1.6 5

DoD's Plan 10 10 0.5

10 10 3

10 0.7 c 0.7 c

10 0.7 c 0.7 c

Option 1

Option 2

Option 3

Accuracy (Feet)

Option 1

Option 2

Option 3

Option 1

Option 2

Option 3

Current Capability 2020 2030

Effective Range of a 10-Watt Jammera

 (Miles)

Signal Strength at Receivere

 (x 10-16 watts)

16. The DSB recommended improvements by factors of 10,000 to 

1 million. The relationship between the effective range of the jammer and the improvement factor is explained in Box 1-1.
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 25

CBO

this option. Thus, for receivers that cannot incorporate a 

larger antenna or an INS because they are limited by size 

or power constraints (such as those for use by personnel 

operating on foot or for use in small munitions or 

unmanned vehicles), the antijamming advantage provided by the IIIC’s stronger signal would not be available. 

That drawback might be mitigated, however, by the 

widespread availability on the commercial market of 

small versions of improved antennas and inertial measurement units and navigation systems. One commercial 

vendor offers antijam antennas for the DAGR’s predecessor, the Precision Lightweight GPS Receiver, and another 

offers inertial measurement units small enough to fit in 

munitions.

Option 2. Enhance GPS Using the 

Iridium Satellite System 

The High Integrity GPS program seeks to improve the 

ability of GPS receivers to acquire and track GPS signals 

in a strong jamming environment by using the Iridium 

communications network. Funded by the Office of the 

Assistant Secretary of Defense for Research and Engineering (OASD(R&E)) and managed primarily by the Office 

of Naval Research, the iGPS program is scheduled to 

complete development in 2011.17 Under this option, 

DoD would fully implement the concept developed in 

the iGPS program.

The ability of GPS receivers to operate in a jamming 

environment would be improved by using data transmitted by the strong signals from the commercial Iridium 

satellite network. Data relayed by the Iridium satellites 

would tell GPS receivers where in the sky to look for GPS 

satellites. The receivers’ ability to process information 

from the GPS signal would also be enhanced, and it 

would be augmented with an inertial navigation system 

similar to the one included in Option 1. This option 

would require some hardware and software additions to 

the Iridium satellite network and to GPS receivers. To 

reduce costs, this option would scale back the scope of 

the GPS III satellite program (as would Option 1), purchasing 40 IIIA satellites but no IIIB or IIIC satellites. As 

in all of CBO’s options, this option would acquire GPS 

receivers capable of taking advantage of the M-code signal. Compared with costs under DoD’s plan, Option 2 

would save $1.5 billion from 2012 through 2021 and 

more than $3 billion from 2012 through 2025.

Improving the Capabilities of GPS Using the 

Iridium Network 

Iridium Communications Inc. is a mobile satellite communications company that operates a constellation of 

66 active satellites and a number of in-orbit spares.18 The 

Iridium constellation operates from a low-Earth orbit at 

an altitude of 485 miles (in contrast with the GPS constellation, which operates at medium-Earth orbit at an 

altitude of about 12,500 miles). Satellite launches for the 

Iridium network began in 1997 and continued through 

2002. Iridium has recently obtained financing to launch a 

replacement constellation called Iridium NEXT.19 A total 

of 72 Iridium NEXT satellites—enough to replace all 66 

working satellites and some of the spares in the present 

constellation—will be launched starting in 2015, and the 

network will start operating by 2017.

The Iridium network provides a high-power signal that 

can help specially modified GPS receivers acquire and 

lock on to much weaker GPS signals. The Iridium satellites receive data regarding the location of GPS satellites 

within view of a receiver from an iGPS reference station 

located within 750 miles of the receiver. The Iridium satellites can then relay that data to the GPS receiver, which 

uses the data to shorten the time it needs to locate the 

GPS satellites and lock on to their signals. 

The iGPS program has four main components (see 

Figure 3-2):

 The Iridium satellite network operations center in 

Leesburg, Virginia, which feeds Iridium data to the

17. A total of $232 million (in 2012 dollars) has been appropriated 

for research and development of the iGPS program. From 2007 

through 2011, OASD(R&E) provided a total of $217 million 

for that program. In 2008 and 2009, iGPS also received funding 

of $15 million through Special Operations Technology 

Development. 

18. In September 2009, the Iridium constellation included 7 in-orbit 

spares.

19. The overall cost of the project—not to be borne by DoD and in 

current-year dollars—is expected to be about $2.9 billion. That 

total includes $2.1 billion for Thales Alenia Space of France to 

build 81 satellites, and $492 million for SpaceX to launch 72 satellites into orbit on its Falcon 9 rockets. Iridium has announced 

that funding for that project will come from a combination of 

internal cash and $1.8 billion in financing from a syndicate 

of nine banks.
26 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Figure 3-2.

GPS Augmentation with the iGPS Program

Source: Congressional Budget Office based on an image from the Department of Defense, Office of the Assistant Secretary of Defense for 

Research and Engineering.

Note: GPS = Global Positioning System; iGPS = High Integrity GPS.

iGPS reference stations. Those data, known as ephemeris data, include information about where the Iridium satellites are located at a particular point in time. 

The iGPS reference stations also receive similar data 

about the location of the GPS satellites.

 An iGPS reference station, located within 750 miles of 

the GPS ground user, which transmits navigation 

data, differential corrections, and time references to 

the Iridium satellite;20

 An Iridium satellite within the field of view of the 

GPS ground user that can provide GPS satellite location data via its high-power signal;

 The ground user operating a GPS receiver that has 

been modified to detect the Iridium signal and augiGPS Reference

Station 

Satellite Network

Operations Center,

Leesburg, Virginia 

Iridium Satellite

Polar Orbit 

Iridium

Footprint, 

750-Mile

Radius

Active Iridium

Satellite 

GPS Spacecraft

Data 

Uplink 

Data

Ranging

Link 

iGPS User

20. The requirement that users be within 750 miles of a reference station—which are unmanned but must be in a secure location—

somewhat limits the geographic areas where iGPS could be useful. 

However, the extent of the coverage of a single reference station is 

such that most geographic areas of interest could be covered by 

placing reference stations in countries that are friendly to the 

United States. For example, one station in Seoul, South Korea, 

would provide coverage for all of Japan, both North and South 

Korea, and some of eastern China. And as few as six reference stations may be sufficient to cover the area of operations for the U.S. 

Central Command (that is, eastern Africa, the Middle East, and 

central Asia).
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 27

CBO

mented with a miniature INS module. For military 

users, the receiver would be a modified version of the 

DAGR, which would include the addition of an iGPS 

receiver, a closely integrated INS, and an iGPS 

antenna. (See Appendix B for a picture of the earlyproduction version of the iGPS module.)

To operate, iGPS first establishes a one-way data link 

from the network of Iridium satellites to the iGPS 

ground user. The transmitted data provide initial geolocation information to the iGPS user on the basis of the 

location of the nearest Iridium satellite. The satellite also 

transmits data to the iGPS user that aid in finding the 

best path to acquire the GPS signals. That information is 

then used to help the modified DAGR acquire and lock 

on to the GPS signals. Once that has happened, GPS 

tracking begins. Information transmitted by the Iridium 

satellite also helps the receiver to maintain that tracking 

in the presence of jamming by continuing to provide 

information about the location of GPS satellites in the 

receiver’s field of view. 

Schedule and Cost for Fielding iGPS

The iGPS program has had several demonstrations of signal acquisition and tracking by GPS receivers under jamming conditions. On the basis of those demonstrations, 

OASD(R&E) claims that iGPS capability will attain 

operational status during 2012. However, to provide that 

capability to military users, several components of 

iGPS—primarily the network of reference stations and 

modifications to Defense Advanced GPS Receivers—

would have to be fielded. DoD has already purchased 55 

reference stations—enough, according to OASD(R&E), 

to provide global coverage—and plans to begin deploying 

them worldwide in 2012.21 Although the reference stations are unmanned, they must still be placed in secure 

locations and maintained. OASD(R&E) plans to develop 

the ability to cross-link the signals from the reference stations in the future so as to reduce the overall number of 

stations required.

To receive the Iridium signal, DAGRs would need to be 

modified. Those modifications would require the addition of software and hardware, including an iGPS core 

module and an antenna to receive the Iridium signal and 

the GPS signals. OASD(R&E) had proposed that the 

modifications to DAGRs could be made when the 

upgrade to receive M-code signals occurred, but such an 

upgrade might not happen for 10 or more years, according to DoD’s current schedule. If iGPS capability is to be 

fielded in the next few years, then the necessary hardware 

needs to be produced and fielded before any planned Mcode upgrades to current receivers are made. At present, 

OASD(R&E) estimates that approximately $3,000 

would be needed to purchase one iGPS module and 

upgraded DAGR that could receive and process the Iridium signal. Because the current iGPS module is significantly larger than the DAGR itself, this option would 

limit the iGPS upgrades to the 200,000 or so DAGRs 

mounted in vehicles.

CBO estimates that the total investment cost to carry out 

Option 2 would be $18.9 billion from 2012 through 

2025, a net savings of more than $3 billion compared 

with DoD’s plan. The acquisition costs to implement 

iGPS, which would pay for improvements to the GPS 

receivers, would be approximately $1 billion from 2012 

through 2025. CBO’s estimate includes research and 

development funds needed to convert the prototype 

receivers into fieldable iGPS units as well as the cost to 

purchase 200,000 modified receivers so that they can take 

advantage of the signals from the Iridium satellites. 

For iGPS to operate, the user must have access to the 

Iridium satellite network. At the end of September 2011, 

Iridium had more than 500,000 subscribers who pay various rates for its communications service. The U.S. government, including the Department of Defense, 

accounted for just under a quarter of Iridium’s revenues 

in calendar year 2010 from that service. For the iGPS 

program, payment for use would be on a per-minute 

basis, although the usage rate would not be based on a 

per-user fee. Rather, all iGPS users in a particular region 

of approximately 1,500 miles in diameter would receive 

the same iGPS signal and be covered by the blanket perminute charge. Overall integration of iGPS would be 

managed through the Iridium ground-based controller. 

To achieve a center with continuous operation, 

OASD(R&E) estimates that a full-time staff of approximately 10 people would be needed. Even though the ultimate objective is to have a fully automated system, personnel would be needed to maintain and upgrade the 

system. 

21. This is based on OASD(R&E)’s expectation that operations will 

be possible with regional reference stations at distances of up to 

750 miles from a receiver, despite the fact that most of the demonstrations of iGPS-enabled user equipment during development 

were carried out with reference stations located within 140 miles 

of the receiver. 
28 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

CBO estimates that, starting in 2014, the costs for time 

on the Iridium satellite network would be between 

$5 million and $10 million per year, and the costs to 

operate and maintain the system would be $10 million 

per year. All told, such annual operating costs could total 

$240 million over the 2012–2025 period. However, in 

totaling the costs of this option and comparing the three 

options, CBO did not include the recurring operating 

costs. 

Effect of Option 2 on GPS Performance in a 

Jamming Environment

Demonstrations of iGPS during the OASD(R&E) program have shown that it can reduce the effectiveness of 

jammers against the DAGR. Using an iGPS-augmented 

receiver, a user could lock on to and track a GPS signal in 

a jamming environment within approximately 120 seconds of turning on the receiver.22 Furthermore, according 

to data provided by the iGPS program office, integrating 

a DAGR with iGPS and an inertial navigation system 

would reduce the effectiveness of enemy jammers by 

99 percent.23 That improvement in capability would 

mean that the range at which a 10-watt jammer could 

cause the iGPS-augmented DAGR to lose track of the 

signal would drop from 55 miles with current equipment 

to about 0.6 miles (see Figure 3-1). 

Additional improvements in capability would follow 

when 18 IIIA satellites were put into orbit and when large 

numbers of M-code-capable receivers were fielded. By 

2026, when CBO estimates that a full constellation of 

IIIA satellites would be in orbit and roughly half of military receivers would be M-code-capable (including those 

attached to iGPS modules), those enhancements would 

reduce the effective range of a 10-watt jammer even further—to 0.14 miles (see Figure 3-1 and Table 3-3). Thus, 

when all the improvements introduced by this option are 

in the field in sufficient number, the effective range of 10-

watt enemy jammers should be reduced by 99.7 percent, 

from 55 miles to 0.14 miles.

Advantages and Disadvantages of Option 2

Option 2 would have several advantages over DoD’s plan. 

One advantage is a greater improvement in the ability of 

GPS to operate in a jamming environment. Several other 

advantages stem from the capabilities of iGPS. First, 

iGPS can help navigate in locations where the direct line 

of sight to the GPS satellite is limited, such as in urban 

canyons and mountainous terrain. Because the Iridium 

satellites are operating at a lower orbit and thus move 

more quickly than GPS satellites across the sky, they are 

“seen” more often by modified GPS receivers. Second, 

because location data are received from both the Iridium 

and the GPS satellite systems, spoofing—intentionally 

transmitting false GPS data that appear authentic to the 

receiver—is much more difficult to achieve. Third, the 

iGPS program has the potential to provide greater accuracy in determining the receiver’s position—on the order 

of about 8 inches—compared with the accuracy of 

approximately 10 feet achievable with today’s GPS 

receivers and satellites. That amount of improvement is 

comparable to the accuracy of 6 inches that might be 

achievable under DoD’s plan. Finally, the cost to carry 

out Option 2—$18.9 billion through 2025—would be 

more than $3 billion (or 15 percent) lower than that necessary to implement DoD’s plan.

Some considerations regarding iGPS could limit this 

option’s potential as an alternative to DoD’s plan, however. Modifications to the DAGR to achieve iGPS capability may add weight to the unit and require more 

power. The early-production iGPS module that is being 

used to augment the DAGR is 7 inches by 11 inches by 

3.5 inches, weighs 4.5 pounds, and draws 25 watts (see 

Appendix B). Although the size, weight, and power 

requirements of the module could diminish appreciably 

in the next few years, the development efforts needed to 

achieve such reductions would have associated costs. 

An additional drawback of Option 2 is that operation of 

iGPS requires the use of two separate satellite systems, 

GPS and Iridium, the latter of which is not owned by the 

U.S. government. Even though the Iridium satellite network has been operating for more than a decade and the 

next generation of satellites is scheduled to begin operating in 2017, the company’s continued operation is not 

guaranteed. The company that launched the original 

Iridium constellation, Iridium LLC, went public in 1997 

but declared bankruptcy two years later. As a result of the 

bankruptcy, there was discussion of destroying the Iridium satellite constellation by de-orbiting it. In 2001, the 

22. For comparison, the average time from when the DAGR is first 

turned on in a nonjamming environment to when it is able to 

acquire the GPS signal and compute an initial position is less than 

100 seconds.

23. That corresponds to an improvement in the receiver’s ability to 

maintain its track in the presence of jamming by a factor of 104

, or 

10,000.
CHAPTER THREE THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 29

CBO

company was acquired from bankruptcy, and that entity 

formed the foundation of the present Iridium Communications Inc. 

Shortly after Iridium emerged from bankruptcy, DoD 

announced a two-year $72 million contract for satellite 

phone service from Iridium (worth roughly $90 million 

in 2012 dollars), which at the time amounted to 40 percent of the cost to operate the Iridium network. At present, Iridium Communications Inc., with its core business 

of satellite phone communication, is profitable, and it has 

announced secured funding for its Iridium NEXT satellite constellation. If, however, Iridium runs into financial 

difficulty and stops operating its satellite network or is 

bought by a hostile party that could deny service to the 

U.S. government, iGPS would no longer function. 

Although DoD could avoid that situation by providing 

more financial support or buying out Iridium, doing so 

would increase the costs associated with this option.

Option 3. Improve the Capabilities of 

Military GPS Receivers and Enhance 

GPS Using the Iridium Satellite System

The third option would combine the improvements 

included in the two previous options. The resulting 

increase in the ability of GPS receivers to operate in a 

jamming environment would be significant, exceeding 

that recommended by the Defense Science Board in 2005 

and reducing the effective range of enemy jammers by 

more than 99.9 percent.24 CBO estimates that this option 

would cost about $1.3 billion less than DoD’s plan. It 

would have the same disadvantages as Option 1 and 

Option 2, however, including additional weight and 

greater power requirements for receivers, the loss of the 

stronger and more accurate military signals transmitted 

by the canceled IIIB and IIIC satellites, and reliance on 

the commercial Iridium satellite system.

Description, Schedule, and Cost

Option 3 would make the improvements to the GPS 

included in both Option 1 and Option 2. Specifically, it 

would acquire and integrate improved antennas and inertial navigation systems into military GPS receivers, and it 

would augment the current GPS receivers with information provided through the Iridium satellite network. This 

option would acquire sufficient hardware to ensure that 

all military GPS receivers from all services were integrated 

with improved antennas that could filter out jamming 

signals and with INSs to compensate for errors introduced by jammers. In addition, this option would fully 

integrate the Iridium satellite system with the GPS so 

that it could, when called upon, provide GPS users with 

additional data that would improve the performance of 

appropriately modified receivers in difficult environments. To take advantage of the information transmitted 

by the Iridium satellites, this option would upgrade 

slightly more than 200,000 Army DAGRs so that they 

could receive and process the iGPS data; the remaining 

DAGRs would still benefit from improved antennas and 

inertial navigation systems.

The schedule for acquiring and fielding the hardware 

needed to carry out this option is the same as that outlined in each of the previous options. If purchases of 

improved antennas and inertial navigation systems began 

in 2013, then half of the services’ receivers would be 

upgraded by 2018. Similarly, if upgrades to Army 

DAGRs to enable them to receive data from the Iridium 

satellites began in 2014, then roughly 100,000 units 

could be using iGPS data by 2018. The total cost for 

those improvements would be $3.0 billion from 2012 

through 2025, CBO estimates, with almost all of the cost 

realized between 2012 and 2021 (see Table 3-2).25

Effect of Option 3 on GPS Performance in a 

Jamming Environment

The combined effects of the various improvements and 

augmentations made to the Global Positioning System 

under Option 3 would greatly enhance the system’s ability to perform in the presence of jammers. Excluding the 

contributions from M-code-capable receivers and stronger signals from IIIA satellites, the enhancements in this 

option—improved antennas, integrated INSs, and augmentation with iGPS—would reduce the effective range 

of enemy jammers by 99.8 percent.26 The effective range 

of a 10-watt jammer would decrease from 55 miles with 

24. The improvement from this option—roughly a factor of 5 million 

compared with current capability, or 10,000 times the improvement resulting from DoD’s plan—exceeds the 10,000 to 1 million–fold improvement recommended by the DSB in its October 

2005 report.

25. Those costs, and those in Table 3-2, do not include annual recurring costs of about $20 million needed to maintain the iGPS network after 2014.

26. That corresponds to a more than 300,000-fold improvement in 

the ability of receivers to operate in the presence of jammers.
30 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

current DAGRs to slightly less than 0.1 mile. And 

because the hardware needed to make the improvements 

has already been developed, many receivers with 

enhanced performance could be in the field by 2018 (see 

Figure 3-1). Additional improvements in capability 

would be realized as IIIA satellites and M-code-capable 

receivers were fielded. All told, the effective range of 

enemy jammers could be reduced by 99.96 percent, 

exceeding the DSB’s recommendations for improvement.27 Ultimately, by 2026, when 28 IIIA satellites 

could be in orbit and large numbers of M-code-capable 

receivers fielded, the effective range of a jammer that 

today could cause a DAGR to lose track of a signal at 55 

miles would be reduced to roughly 130 feet.

Advantages and Disadvantages of Option 3

Of the three options that CBO considered, this one 

would yield the greatest improvement in GPS performance and do so at a lower cost than DoD’s plan. Compared with DoD’s plan, savings under Option 3 would 

total $1.3 billion (or slightly less than 6 percent) from 

2012 through 2025. The improvement in performance in 

a jamming environment would be substantial relative to 

today’s capabilities, reducing the effective range of jammers to roughly 130 feet. As a result, jammers would be 

only 1 percent as effective under this option as they 

would be under DoD’s plan (see Table 3-3 and Figure 3-

1).28 (Improvements in accuracy would be roughly equivalent under both plans.) Another advantage of this option 

is that it would rely on several different methods to defeat 

an enemy’s jamming efforts. If one method should 

become unavailable—such as the signal from the Iridium 

satellites—then the antijamming capability provided by 

the upgraded GPS receivers would still yield improvements over today’s Global Positioning System and even 

that planned by DoD.

The disadvantages of adopting this option include those 

common to all of the options. Improvements to the 

receivers’ capabilities would require greater weight and 

power given current technology, which would make usage 

more difficult for personnel operating on foot. Although 

military suppliers are rapidly developing improved receivers that are much smaller and require less power than the 

DAGR, it may be several years before such miniaturized 

models are widely available. In addition, canceling the 

IIIB and IIIC satellites forfeits planned improvements in 

signal strength that would be particularly beneficial in situations where it was not feasible to increase the weight of 

and power available to the receivers. This option also 

shares with Option 2 the disadvantage of relying on the 

commercially owned Iridium satellite system. That reliance carries the risk that the service will not be there 

when needed or, alternatively, that DoD will have to bear 

most or all of the cost to support it.

27. A military receiver’s capability would be improved by a factor of 5 

million.

28. The comparison is between an effective range for a 10-watt jammer of 0.025 miles under Option 3 and 2.5 miles under DoD’s 

plan.
APPENDIX

CBO

A

History of the GPS Program

The Global Positioning System (GPS) program 

began development in 1974 and, even before it became 

fully operational in 1995, was providing navigation data 

to military users during the first Persian Gulf War in 

1991. Since then, the system has become a mainstay for 

military navigation as both its satellites and receivers have 

become more capable and sophisticated. By 2011, the 

Department of Defense (DoD) had received a total of 

more than $24 billion to develop and purchase components of the GPS, including satellites, ground control 

systems, and receivers (see Table A-1).

1974 to 1999

The first 25 years of the GPS program saw the rapid 

development and deployment of a system that has 

become worldwide in its applications. During that 

period, DoD developed and purchased the first GPS satellites and receivers, investing $7.3 billion for satellites 

and associated ground control systems and $6.7 billion 

for receivers for all four branches of the armed services.1

GPS Satellites

Although the program officially began in 1974 (with the 

start of scientific and technical development), full-scale 

development did not begin until 1979. A production 

contract to build the first 28 satellites was signed in 

1983, and the full GPS constellation of 24 satellites was 

declared fully operational in 1995. The cost to develop 

and purchase the initial GPS constellation of satellites 

(Blocks II and IIA), including 4 spare units, was 

$4.6 billion.

Because all GPS satellites have lives of finite length, 

maintaining a constellation of fixed size requires the purchase and launch of replacement satellites. Two of the 

components that are necessary for the operation of GPS 

satellites and are likely to stop performing as required as 

the satellite ages are the electrical system—particularly 

the solar panels—and the atomic clocks. The ability of 

solar panels to produce electricity declines over time as 

the photoelectric cells are damaged by radiation and collisions with space particles. And, although the first GPS 

satellites each carried four atomic clocks as insurance 

against the failure of any one of them, each clock has a 

finite life and without a working clock a GPS satellite is 

worthless.2

Because many years elapse between when a satellite is 

ordered and when it is available to be launched, the Air 

Force, as DoD’s agent, purchases satellites long before it 

expects to need them to replace inoperable ones. The initial GPS satellites were designed to last 71⁄2 years in space. 

The first GPS satellite was launched in 1989, and in 

order to ensure that no satellites remained in orbit longer 

than 8 years, replacement satellites were planned to be 

launched beginning in 1997. After that, roughly 3 satellites were planned to be launched annually. In line with 

those planning factors, the Air Force began to purchase 

replacement satellites as soon as the first GPS satellite was 

launched. Between 1989 and 1999, the Air Force developed and purchased 27 replacement satellites in two 

blocks at a total cost of $2.7 billion. The first 21 replacement satellites, known as Block IIR, were purchased from 

1992 to 1996, and the first GPS IIR satellite was 

launched in 1997. A total of 6 satellites in the next block, 

IIF, were purchased in 1997 and 1998. Satellites in each 

block were slightly more capable than those in the preceding block. 1. Because most of the funding for the development of ground control systems through 2011 was included with the funding for the 

satellites, the Congressional Budget Office cannot easily identify 

spending devoted exclusively to the development of ground control systems for that period.

2. Atomic clocks have become more reliable since the program 

began, so modern GPS satellites carry only three of them.
32 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table A-1. 

Expenditures and Purchases for the GPS Program, 1974 to 2011

Source: Congressional Budget Office.

Note: GPS = Global Positioning System; n.a. = not applicable.

a. Investment funds attributed to satellites include those for the ground control systems.

b. The first attempted launch of a IIR satellite was unsuccessful and resulted in the loss of the satellite; the second launch was successful.

c. Funds include those to upgrade eight IIR satellites that were purchased before 2000 but launched later.

d. Funds include those to upgrade six IIF satellites that were purchased before 2000 but launched later.

e. Includes $2.1 billion to acquire GPS III satellites and $1.3 billion to develop the ground control system for those satellites.

f. Of the 61 satellites purchased, 50 had been launched successfully by the end of July 2011 and 10 were awaiting launch.

g. Excludes upgrades and retrofits purchased during this period.

As late as December 1997, the Air Force planned to continue buying GPS satellites at a rate of 3 per year from 

2000 through 2016. Those plans changed significantly, 

however, in the subsequent two years. Although the Air 

Force plans its purchases of replacement satellites on the 

basis of the expected lifetime of those satellites, it 

launches them only when they are needed to replace satellites that are no longer working correctly. After the 24-

satellite GPS constellation was established in 1995, the 

Air Force launched several more GPS satellites in the next 

two years in order to have several spares in orbit in case 

any of the essential 24 should fail. After that, the satellites 

in orbit continued to operate long after the end of their 

design lives. Consequently, the need for replacements was 

much lower than the Air Force had projected. By the end 

of 1999, the Air Force had purchased a total of 55 GPS 

satellites but had launched only 30, and the demonstrated lifetime of the original blocks of GPS satellites 

(II and IIA) had increased by as much as four years over 

the eight-year service life the Air Force had originally 

anticipated.3

 The longer lifetimes and the surplus of purchased but unlaunched satellites allowed the Air Force in 

1999 to halt further purchases of GPS satellites, pending 

the results of reviews that were initiated in the late 1990s 

by the White House and DoD in response to concerns 

1974 to 1999

II and IIA 4.6 28 28

IIR 1.9 21 1 b

IIF 0.8 6 0 ___ ___ ___

Subtotal 7.3 55 29

2000 to 2011

IIR and IIR-M 1.4 c 0 19

IIF 3.2 d 6 2

III 3.4 e 0 0 ___ __ ___

Subtotal 8.0 6 21

Total 15.3 61 f 50 f

1974 to 1999 6.7 n.a.

2000 to 2011 2.6 n.a. ____

 Total 9.3 n.a.

103,600

 292,000 g

395,600

________

Receivers

Satellitesa

Investment Funds

(Billions of 2012 dollars)

Quantity

Purchased Successful Launches

3. One of the IIR satellites was lost on launch when the rocket carrying it failed.
APPENDIX A THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 33

CBO

about the safety of civilian aviation and the vulnerability 

of the GPS signal to interference. In response to those 

reviews, the GPS program was subsequently restructured. 

GPS Receivers

Development of GPS receivers also began in 1974, with 

efforts undertaken by all three military departments. A 

total of $2.7 billion was invested from 1974 to 1999 to 

develop an array of receivers that could be installed in airplanes, vehicles, and ships or carried by individuals. By 

1999, another $4.0 billion had been invested in purchases of receivers, bringing the total to $6.7 billion. The 

roughly 103,600 receivers purchased with those funds 

were sufficient to equip all 435 of the Navy’s major ships 

and submarines, as well as more than two-thirds of the 

services’ aircraft and armored vehicles. The capabilities of 

GPS receivers were also part of DoD’s review of the program in the late 1990s and, as a consequence, additional 

large purchases of handheld receivers were suspended for 

several years after that.

Major Program Changes 

In response to changes in both civilian and military goals 

that resulted from several reviews of GPS performance 

conducted from 1997 through 1999, the GPS modernization program went through a major restructuring 

between December 1997 and February 2000. 

Civilian Concerns

Concerns raised about the reliance of transportation 

systems—particularly aviation—on data from GPS led 

the Clinton Administration to initiate two changes to the 

civilian signals transmitted by GPS satellites.4

 ThenVice President Gore announced in March 1998 that, to 

make GPS more useful for civilian users worldwide, 

future GPS satellites would transmit a second civilian signal at the same frequency as the second military signal.5

The addition of a signal at a second frequency would 

enable civilian receivers to make more effective corrections for the distorting effects of the Earth’s atmosphere 

on signals from GPS satellites. Less than a year later, the 

White House announced that future GPS satellites would 

also be required to transmit a third civilian signal to meet 

critical “safety of life” issues associated with civil (nonmilitary) aviation.6

Military Concerns

On the basis of reviews conducted in the late 1990s, 

DoD decided to take several measures to make it harder 

for hostile forces to prevent military users from taking 

advantage of GPS signals in the future. Those measures 

included:

 Enabling future GPS satellites to transmit new military signals (designated M-code) that would cover a 

wider frequency range and be separated in frequency 

from the civilian signals (see Figure A-1), and

 Developing satellites capable of transmitting military 

signals at higher power.

The purpose of those measures was to make it more difficult for hostile forces to use jammers capable of masking 

GPS signals from significant distances. The jammers 

work by broadcasting noise that covers the same frequency range as the signal from the satellite.7

 To generate 

noise over a wider frequency spectrum, the enemy is 

forced to use more powerful jammers to attain the same 

effective range. Such jammers require larger and bulkier 

sources of electric power, which makes them larger and 

more expensive. (See Appendix B for descriptions and 

pictures of some typical jammers.) By transmitting a 

stronger and broader GPS signal from space, DoD hoped 

to force enemies intent on preventing reception of the 

GPS signal by U.S. forces to transmit stronger jamming 

signals, which would make them easier to locate and 

attack.

At the same time, DoD announced that it would improve 

the ability of military GPS receivers to take advantage of

4. White House Commission on Aviation Safety and Security, Final 

Report to President Clinton (February 12, 1997).

5. Office of the Vice President, “Vice President Gore Announces 

Enhancements to the Global Positioning System That Will Benefit Civilian Users Worldwide” (March 30, 1998).

6. Office of the Vice President, “Vice President Gore Announces 

New Global Positioning System Modernization Initiative” (January 25, 1999). That third signal provides data for civil aviators at a 

frequency that is approved for use by civil aviation.

7. Although the jamming signal may not be structured in the same 

way as the signal from the satellite, it effectively masks the satellite’s signal because it is much stronger.
34 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Figure A-1.

Structure of Signals Transmitted by GPS Satellites

Source: Congressional Budget Office based on The Interagency GPS Executive Board, GPS L1 Civil Signal Modernization (L1C), 

July 30, 2004.

Notes: GPS = Global Positioning System; nW/Hz = nanowatts per hertz; MHz = megahertz; 1 nanowatt= 10-9 watt.

The scale for the y-axis is logarithmic, not linear.

all of the GPS military signals and to operate in a jamming environment.8

 To that end, DoD initiated programs to:

 Develop and field improved antennas and processors 

for military GPS receivers to make them able to filter 

out jamming noise, and

 Develop and field handheld receivers capable of 

receiving the military signals being transmitted on two 

separate frequencies.

2000 to 2011

Between 2000 and 2011, the GPS program maintained 

and expanded the satellite constellation and implemented 

some of the changes recommended by the White House 

Commission on Aviation Safety and Security and various 

DoD review panels.9

 Satellites capable of transmitting 

three civilian signals, two standard military signals, and 

two M-code military signals were launched. And handheld receivers with the ability to receive military signals 

on both frequencies (although not the M-code signals) 

were purchased and fielded. Some of the planned 

improvements, however, particularly those that will 

increase the strength of the signal transmitted by the satellites, will have to wait for the completion of new programs after 2011.

GPS Satellites

DoD continued to maintain a robust GPS constellation 

and improve the quality and consistency of the signals 

available to users worldwide. During this period, DoD 

launched 21 satellites, all of which had been purchased 

before 1999. Because the original block of GPS satellites 

continued to operate long past the end of their expected 

1,000

100

10

1

-20 -15 -10 -5 0 20 5 10 15

Civilian Signal Civilian Signal

Existing

Military Signal New

Military Signal

(M-Code)

Power Spectral Density (nW/Hz)

Offset from Center Frequency (MHz)

1,000

100

10

1

-20 -15 -10 -5 0 20 5 10 15

Original Signal Structure

(1990 to 2005)

Enhanced Signal Structure

(After 2005)

Military Signal

8. Department of Defense, Plan for Development of an Enhanced 

Global Positioning System: A Report to Congress (July 1999).

9. See White House Commission on Aviation Safety and Security, 

Final Report to President Clinton. In June 1999, DoD’s Joint 

Requirements Oversight Council approved recommendations 

from the Air Force’s Space Command and Air Combat Command 

for improvements in GPS capabilities, including the ability to 

operate in the presence of jamming.
APPENDIX A THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 35

CBO

lifetimes and even after replacements had been launched 

and were operating, the size of the constellation grew to 

include between 29 and 31 satellites by 2005. With so 

many satellites transmitting usable signals, the enlarged 

constellation meant that users often had more than four 

satellites in view, enabling them to calculate their position 

with increased accuracy.

To make at least some of the improvements deemed necessary in the late 1990s available as soon as possible, DoD 

and the Department of Transportation (DoT) decided to 

retrofit existing, but not yet launched, satellites with as 

many upgrades as feasible before they were needed to 

replace failing satellites.10 DoD had more than 20 

unlaunched satellites on hand or on order by the end of 

1999—in particular, IIR and IIF satellites that had been 

purchased in the 1990s but were not yet needed. In 2000, 

those circumstances led the Air Force to begin modifying 

IIR satellites by adding the ability to transmit a second 

civilian signal and the new M-code signals. (Upgraded 

IIR satellites were designated IIR-Ms.) The Air Force 

upgraded a total of 8 IIR satellites to the IIR-M configuration between 2000 and 2011. The Air Force also modified the first 6 IIF satellites that had been purchased in 

1997 and 1998 so that they could transmit a second and 

third civilian signal as well as the M-code signals and purchased an additional 6 IIF satellites in 2005 and 2006 

with that upgraded capability. The cost to develop the 

capabilities needed to meet the revised requirements, to 

retrofit them to the IIR and IIF satellites already purchased, and to buy 6 new IIF satellites was $4.6 billion 

from 2000 to 2011—$1.4 billion for the IIR satellites 

and $3.2 billion for the IIF satellites (see Table A-1). 

In addition, DoD began developing a new model of GPS 

satellite, known as GPS III, that would be able to transmit stronger M-code signals. Specifically, DoD invested 

$2.1 billion to develop and begin to purchase the new 

satellites and an additional $1.3 billion to develop a new 

ground control system to monitor and control the GPS 

III satellites and their signals, for a total cost of $3.4 billion.11

GPS Receivers

DoD invested considerable funds from 2000 to 2011 to 

develop and buy improved GPS receivers. During that 

period, the Army purchased more than 260,000 

improved handheld receivers—known as the Defense 

Advanced GPS Receiver (DAGR)—capable of receiving 

and decoding military signals on two frequencies. (See 

Appendix B for descriptions and pictures of the DAGR 

and its predecessor, the Precision Lightweight GPS 

Receiver.) That capability greatly improves the accuracy 

with which the receiver can compute position. The 

Air Force also invested funds to develop—but not 

purchase—receivers capable of decoding the military 

M-code signal as well as to improve the ability of its 

existing receivers to locate GPS signals in a jamming 

environment. All told, DoD invested $2.6 billion during 

that period to improve its receivers (see Table A-1).

Status at the End of 2011

By the end of 2006, DoD had purchased several satellites 

with the improved capabilities that had been identified as 

critical, including 20 satellites capable of transmitting the 

new M-code signals. Only a limited amount of that capability had actually made it into the field by the end of 

2011, however. The first IIR-M satellite was launched in 

September 2005, and the last of 8 was launched in 

August 2009. The first 2 IIF satellites were launched in 

May 2010 and July 2011, but only 1 was fully operational on October 1, 2011. Therefore, by the end of 

10. DoD and DoT have been jointly responsible for developing plans 

for U.S. radionavigation, including GPS, since 1980. DoD is 

responsible for the overall development, acquisition, security, and 

continued modernization of GPS. The Secretary of Defense has 

designated the Air Force as the lead agency responsible for all 

aspects of acquisition, although other military departments and 

organizations within DoD are responsible for some aspects of 

developing receivers and for funding various parts of the program. 

DoT has the lead responsibility for coordinating civilian requirements from all civilian departments and agencies. The Department of State leads negotiations with foreign governments and 

international organizations on GPS matters. In addition, the 

Coast Guard, which is now part of the Department of Homeland 

Security, operates the Navigation Center, which provides up-todate GPS operations advisories, advisories to GPS users, and 

notices of potential interference with GPS reception from U.S. 

government testing, among other things. For additional information on the roles and responsibilities of the various agencies in 

matters pertaining to GPS, see Department of Defense, Department of Homeland Security, and Department of Transportation, 

2010 Federal Radionavigation Plan, DOT-VNTSC-RITA-08-02/

DoD-4650.05 (Springfield, Va.: National Technical Information 

Service), Chapter 2.

11. The control system for the GPS III satellites is being developed 

under a contract separate from that for the development of the 

satellites. Therefore, its costs are separate from those of the satellites themselves and, unlike those for the ground control system 

for the GPS II satellites, easily identifiable.
36 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

2011, only 10 GPS satellites capable of transmitting the 

harder-to-jam M-code signal were in orbit, and only 8 

were fully operational, far short of the 18 needed to provide continuous worldwide coverage.12 In addition, the 

upgrades to the ground control system needed to monitor 

and control the M-code signal were not complete by the 

end of 2011, and, according to DoD, will not be available for several more years. Finally, the other satellite 

enhancements needed to meet the new military goals—

primarily the ability to transmit M-code signals at higher 

power—were deferred to be installed on a future model 

GPS satellite as part of the GPS III program.

Some of the improvements to GPS receivers that DoD 

had viewed as necessary in its report to the Congress in 

1999 have been made. In particular, more than 250,000 

Defense Advanced GPS Receivers were purchased from 

2000 through 2010, and funds for an additional 13,000 

receivers have been requested. However, no receivers 

capable of receiving and deciphering the M-code signals 

have been fielded, even though several satellites are in 

orbit transmitting that signal. 

12. One IIR-M satellite, launched in March 2009, was never declared 

fully operational and was ultimately decommissioned in May 

2011.
APPENDIX

CBO

B

Examples of Selected 

Military-Related GPS Components

This appendix describes the Global Positioning 

System (GPS) receivers that are commonly used by the 

U.S. military, typical accessories—such as antennas—that 

complement those receivers, and some jammers that can 

be used to interfere with the military’s use of the system.

Military GPS Receivers 

Although hundreds of different types of GPS receivers are 

available in the commercial market, the U.S. military 

includes a much smaller number in its standard inventory. Since the first receivers were fielded in the 1980s, 

subsequent versions have increased in capability and 

decreased in size and weight. The following is an overview of the receivers that are most widely fielded by the 

military today.

Handheld Receivers

The most widely fielded receiver in all four armed services is the handheld variety. Handheld receivers are used 

by individuals on foot and also are installed in many 

types of vehicles.

Precision Lightweight GPS Receiver. The Precision Lightweight GPS Receiver (PLGR) was developed to fill a need 

for a truly handheld receiver. Designed to replace the 

much larger and heavier Manpack portable GPS receiver, 

the much smaller and lighter PLGR was initially fielded 

in 1994 (see Figure B-1 and Table B-1). The Army 

bought and fielded a total of almost 113,000 PLGRs 

from 1993 through 2005.

Defense Advanced GPS Receiver. The Defense Advanced 

GPS Receiver (DAGR) is an improved handheld receiver 

designed to replace the PLGR (see Figure B-2). The 

DAGR can be operated using only one hand, is smaller 

than the PLGR, and requires less power—four lithium 

AA batteries as compared with the PLGR’s eight batteries. 

(See Table B-1 for a comparison of the size and weight of 

the two models.) Unlike the PLGR, the DAGR is able to 

process both of the military signals that are transmitted 

on two different frequencies from all of the GPS satellites 

that are in view. 

MicroDAGR. The microDAGR is a very small GPS 

receiver that was commercially developed for rapid fielding with U.S. forces. It is less than half the size of the 

DAGR, weighs only 6.5 ounces, and needs just two AA 

batteries (see Figure B-2). Initially fielded with the ability 

to receive GPS signals on only one frequency, it otherwise 

has capabilities similar to those of the DAGR. Rockwell

Figure B-1.

The Manpack and the Precision 

Lightweight GPS Receiver

Source: Congressional Budget Office based on images from the 

Department of Defense and from the Smithsonian 

Institution’s National Museum of American History.

Notes: Images are not to scale.

GPS = Global Positioning System.

Manpack

Precision Lightweight

GPS Receiver
38 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Table B-1. 

Characteristics of GPS Receivers Used by the U.S. Military 

Source: Congressional Budget Office.

Note: GPS = Global Positioning System; DAGR= Defense Advanced GPS Receiver; * = unknown.

a. The original Miniature Airborne GPS Receiver, introduced in 1990, displayed similar physical characteristics but had five channels and thus 

could process data from only five satellites at once.

b. The original 3S receiver was introduced in the late 1980s and displayed similar physical characteristics.

c. Number of satellites whose signals can be processed simultaneously.

Collins, the manufacturer, stated in the spring of 2011 

that it plans to upgrade the receivers to add the ability to 

process the military signal on the second frequency in the 

near term. 

Airborne and Shipboard Receivers 

The miniature airborne GPS receiver (MAGR) was developed in 1990 and was smaller and more capable than the 

first airborne GPS receiver—the 3A—that was fielded in 

the 1980s. At 11 pounds and almost 280 cubic inches, 

the MAGR is much larger and heavier than the handheld 

DAGR (see Figure B-3). A more capable version, the 

MAGR 2000, with replaceable receiver cards in the same 

overall package, was introduced in 2000.

The 3S receiver, also designated the WRN-6, was 

designed for shipboard use and is significantly larger than 

all other military GPS receivers (see Figure B-3 and 

Table B-1). It has five channels and contains features that 

enhance its ability to support shipboard requirements for 

the Navy’s surface ships and submarines, primarily the 

ability to accommodate the power fluctuations common 

on a ship. Upgrades have been made over the years, 

including the integration of the more capable GPS cards 

in 1996. But because there has been no need to reduce 

the receiver’s size and power requirements, the dimensions of the receiver have remained relatively constant 

since it was first fielded.

Accessories 

The capabilities of the military’s GPS receivers can be 

augmented through the use of external antennas and 

other auxiliary devices. Most of those accessories are 

designed to improve the signal’s reception, and, in some 

cases, to limit interference and the adverse effects of 

jamming.

Antennas

To filter out jamming signals, the Department of Defense 

(DoD) has developed external antennas for use with the 

DAGR and larger antennas designed to be used on aircraft and vehicles. 

Application

First Introduced

Power Requirement (Watts)

Signal-Processing Capacity

Number of frequencies

Number of channels

Number of satellitesc

Weight (Pounds)

Size (Inches)

Width

Length

Height

Volume (Cubic inches) 3,420

2.75

*

70

12

19

15

2.6

1.4

3.9

14

11

3.2

6.8

12.8

279

0.4

9.5

4.1

2.6

101

1

2

12

0.94

6.4

3.5

1.6

36

5

1

5

5 12

2011

Airborne

2000a

*

Shipboard

1996b

0.7

1

12

24

2

12

150

2

5

Receiver

Precision Defense Miniature

 GPS Receiver (WRN-6)

Lightweight

GPS Receiver

Advanced

MicroDAGR

Airborne

GPS Receiver 2000

3S

Handheld

1994

Handheld

2005

Handheld

12
APPENDIX B THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 39

CBO

Figure B-2.

The Precision Lightweight GPS 

Receiver, Defense Advanced GPS 

Receiver, and MicroDAGR

Source: Congressional Budget Office based on images from 

Rockwell Collins.

Note: GPS = Global Positioning System.

DAGR Antennas. Although the DAGR includes a built-in 

antenna, the unit can also be used with a remote antenna 

mounted on a vehicle or even on a soldier’s helmet (see 

Figure B-4). Those antennas are small—commercially 

available versions are roughly 3 inches in diameter and 

less than 1 inch thick. Antennas designed to be mounted 

on a vehicle’s roof include magnets that are strong enough 

to keep the antenna in place at speeds of up to 70 miles 

per hour. (A slightly modified version can be mounted 

permanently with bolts.) The same antenna can be 

strapped to a soldier’s helmet using a specially designed 

holder. 

Spatial Filtering Antennas. The Army has developed 

antennas to mount on weapons platforms (such as rocket 

launchers) that are designed to filter out interference 

from ground-based jammers. Those ground-masking 

antennas limit the jamming effects of all ground-based 

jammers regardless of the signal’s angle of arrival. One 

example of such an antenna developed by the Army is 

roughly 4 inches high, weighs 1 pound, requires 2 watts 

of power, and can be easily mounted on a vehicle’s roof.

Controlled Reception Pattern Antennas. Controlled 

reception pattern antennas (CRPAs) have been developed 

to provide antijamming protection, primarily for aircraft. 

The antenna has the capability to form “nulls” in the 

reception pattern in the direction from which the jammer’s energy is arriving, thereby reducing the effectiveness 

of jamming signals. (The number of separate elements 

that make up the CRPA determines the number of different jammers—equal to the number of elements minus 

one—that can be nullified at one time.) The 7-element 

CRPAs currently used on aircraft weigh 4.5 pounds, and 

one model is roughly 14 inches in diameter (see Figure B5). The conformal version of the antenna—the CCRPA—reduces radar reflections, is 14 inches on each 

side, and can be mounted on the outside of an aircraft. 

The smaller 4-element CRPA is about half the size of the 

7-element model—it has a 7-inch diameter and weighs 

1.5 pounds—but can negate only half as many jammers 

as the larger antennas. 

Figure B-3.

The Miniature Airborne GPS Receiver 

and the 3S Shipboard Receiver

Source: Congressional Budget Office based on images from the 

Department of Defense.

Notes: Images are not to scale.

GPS = Global Positioning System.

Miniature Airborne 

GPS Receiver 3S Shipboard Receiver
40 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Figure B-4.

Remote Defense Advanced GPS 

Receiver Antenna and Mounting Strap

Source: Congressional Budget Office based on images from 

Rockwell Collins.

Note: GPS = Global Positioning System.

Efforts are under way to develop very small CRPAs, and 

several electronics manufacturers offer different versions 

for various applications. For example, Honeywell’s 

μCRPA—measuring 5.8 inches in diameter and 1.45 

inches in height and weighing 1.5 pounds—is capable of 

negating up to five spatially separated jammers (see 

Figure B-6). According to Honeywell, the μCRPA provides GPS satellite signal reception and antijam capability 

that rival the performance of larger diameter CRPAs. 

An example of a CRPA for use with ground vehicles is 

one that is offered by Canadian and British manufacturers NovAtel and QinetiQ. They have produced the GPS 

Anti-Jam Technology (GAJT) antenna designed specifically for military land vehicles. The antenna includes 

seven elements, is less than 12 inches in diameter, and 

weighs roughly 17 pounds. GAJT is a commercial 

off-the-shelf product that can be integrated into new 

vehicle platforms or retrofitted with GPS receivers and

Figure B-5.

Controlled Reception Pattern Antennas 

(CRPAs) for Use on Military Aircraft

Source: Congressional Budget Office based on images from the 

Department of the Navy.

Note: Images are not to scale.

navigation systems on existing military vehicles by adding 

appropriate cables and access to a 20-watt power supply 

(see Figure B-7). 

Antijam Antenna for the PLGR. The electronics industry 

has developed antijam antennas that can be retrofitted to 

the Army’s PLGR. The new antenna, which is only 

slightly bigger than the original antenna, can boost 

protection against jammers by a factor of 300 (see 

Figure B-8).

Other Auxiliary Devices

The Department of Defense and the electronics industry 

have developed various devices that are designed to 

improve the performance of GPS receivers.

Figure B-6.

μCRPA

Source: Congressional Budget Office based on an image from 

Honeywell.

Note: CRPA = controlled reception pattern antenna.

Inertial Navigation Systems and Inertial Measurement 

Units. Manufacturers are making inertial measurement 

units small enough to be easily integrated with handheld 

receivers or into munitions. These units typically include 

an extremely small gyroscope that is based on the idea of 

the Foucault pendulum but uses a vibrating element, 

known as a Micro Electro-Mechanical System, instead of 

an actual pendulum.

iGPS Module. The adjunct equipment needed to implement the High Integrity GPS (iGPS) augmentation using 

the Iridium satellite communications network (see 

Chapter 3 for details) is currently most appropriate 

for vehicular mounting (see Figure B-9). The early

Antenna

(Top)

Antenna

(Bottom)

Strap for

Mounting on Helmet

7-Element CRPA

7-Element

Conformal Antenna 4-Element CRPA
APPENDIX B THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 41

CBO

Figure B-7.

GAJT Antenna

Source: NovAtel, Mitigating the Threat of GPS Jamming: Anti-Jam 

Technology for Land Vehicles (white paper, June 2011).

Note: GAJT = GPS Anti-Jam Technology; GPS = Global Positioning System.

Figure B-8.

Precision Lightweight GPS Receiver 

With and Without Antijam Antenna 

Retrofit

Source: Congressional Budget Office based on images from the 

Institute of Navigation (left image) and Honeywell (right 

image).

Note: GPS = Global Positioning System.

production iGPS module available in May 2011 was 

7 inches deep by 11 inches wide by 3.5 inches high, 

weighed 4.5 pounds, and required 25 watts of power. 

Efforts were under way at that time, however, to make the 

equipment smaller. In fact, a prototype version suitable 

for integrating into the Joint Direct Attack Missile and 

the Small Diameter Bomb has been demonstrated. 

Although adding iGPS capability to the navigation unit 

would not significantly boost the size of such weapons, it 

would increase the power requirements of the unit from 

7 watts to 13 watts.

Jammers

The most common jammers likely to affect GPS reception are those emitting radio signals over a relatively 

broad band of frequencies. Because the GPS signals are so

Figure B-9.

iGPS Module, Including DAGR

Source: Congressional Budget Office based on an image from the 

Department of Defense, Office of the Assistant Secretary 

of Defense for Research and Engineering.

Note: GPS = Global Positioning System; iGPS = High Integrity 

GPS; DAGR = Defense Advanced GPS Receiver.

weak by the time they reach the Earth, very weak jammer 

signals can mask the signals from the GPS satellites. Jammers that generate signals of higher power can adversely 

affect GPS receivers at greater ranges. Because more powerful jammers need larger power supplies, however, they 

are more expensive and easier to detect and attack (see 

Figure B-10). For example, although a 1-watt jammer 

may operate for 12 hours using only 2 pounds of alkaline 

batteries, a 10-watt jammer would need 10 times as many 

batteries, weighing roughly 20 pounds, to operate for the

Receiver

Battery

GAJT

With Standard

Antenna

With Antijam

Antenna
42 THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES

CBO

Figure B-10.

Examples of Notional GPS Jammers

Source: Congressional Budget Office based on an image from 

Lt. Col. Bill Hawken, “NAVWAR (Navigation Warfare): 

Electronic Warfare and the Global Positioning System” 

(presentation to the Ottawa, Ontario, chapter of 

the Armed Forces Communications and Electronics 

Association, March 2007).

Note: GPS = Global Positioning System; ERP = effective radiated 

power; W = watts, kW = kilowatts (1,000 watts).

same amount of time.1 Larger jammers may rely on different sources for power—such as gasoline-powered 

generators. But those power sources also carry weight 

penalties. A generator for an 80-watt jammer, for example, might weigh 30 pounds and run for only five hours 

on one gallon of gasoline.

In 1999, a Russian manufacturing company displayed 

a portable GPS jammer at the Paris Air Show (see 

Figure B-11). Its accompanying placard advertised an 

output of 4 watts and a transmitter weight of 17.6 

pounds to 22 pounds, without the accompanying power 

source needed to supply 25 watts of power. Images of 

other jammers have appeared in public presentations, 

although, in some cases, without mention of how effective they might be (see Figure B-12).

Figure B-11.

Russian Commercial 4-Watt Jammer

Source: Congressional Budget Office based on an image from 

Lt. Col. Bill Hawken, “NAVWAR (Navigation Warfare): 

Electronic Warfare and the Global Positioning System” 

(presentation to the Ottawa, Ontario, chapter of 

the Armed Forces Communications and Electronics 

Association, March 2007).

1. John A. Volpe National Transportation Systems Center, Vulnerability Assessment of the Transportation Infrastructure Relying on the 

Global Positioning System: Final Report (report prepared for the 

Office of the Assistant Secretary for Transportation Policy, U.S. 

Department of Transportation, August 29, 2001), p. 31.

50 kW ERP

$5 Million

1 W ERP

$500 

3"

100 W ERP

$100,000

1 W ERP

$500

1,000 W ERP

$100,000

100 kW ERP

$1.5 Million
APPENDIX B THE GLOBAL POSITIONING SYSTEM FOR MILITARY USERS: CURRENT MODERNIZATION PLANS AND ALTERNATIVES 43

CBO

Figure B-12.

Examples of GPS Jammers

Source: James V. Carroll, “Vulnerability Assessment” (briefing to 

the DOT/OST Outreach Meeting, October 5, 2001).

Note: GPS = Global Positioning System.

Russian Jammer

1-Watt Jammer