SELECTING AN IDEAL GPS AUGMENTATION SYSTEM USING THE

EXPERT CHOICE SOFTWARE

Dang T. Le

March 23, 2011

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Abstract

A trade study on GPS Augmentation is presented to determine which technology best

meets user needs while minimizing cost. This project explores proposed modernization

features for the GPS Block IIF, Block III series, and the iGPS program. Analytic

Hierarchy Process (AHP) developed by Saaty, pairwise comparison, and sensitivity

analysis are employed to determine the ideal system in the area of performance,

implementation, and most cost efficient. The study shows that with respect to

performance, GPS Block III is the ideal system. iGPS best meets user needs with respect

to implementation and is also the most cost efficient system.

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Acknowledgements

The author wishes to thank Dr. Pete McQuade, James Scott, and Gary Stephenson for their

valuable inputs and guidance regarding this work.

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Table of Contents

Abstract ………………………………………………………………………………….. 2Acknowledgements ……………………………………………………………………… 3

I. Introduction ……………………………………………………………………...…… 6 1.1 Motivation …………………………………………………………………... 6 1.2 Research Objectives ……………………………………………………...…. 8 1.3 Methodology ………………………………………………………………... 8

II. Literature Review ……………………………………………………………….....… 9 2.1 Rationale for GPS Augmentation ………………………………………...… 9 2.2 GPS Augmentation Capabilities ……………………………………...…… 10

2.2.1 GPS IIF ……………………………………………………………..…… 102.2.2 GPS III …………………………………………………………………... 112.2.3 iGPS …………………………………………………………………...… 13

III. Methodology ………………………………………………………………………. 153.1 Approach …………………………………………………………………... 153.2 Value Hierarchy ………………………………………………………….... 153.3 Evaluation Measures ………………………………………………………. 16

IV. Analysis and Results ………………………………………………………………. 184.1 Pairwise Comparison ……………………………………………………… 184.2 Sensitivity Analysis ……………………………………………………..… 19

V. Conclusion and Recommendations ……………………………………………….... 23

VI. References …………………………………………………………………………. 24

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List of Figures

Figure 1. GPS Block II/IIA …………………………………………………………...… 6

Figure 2. GPS Block IIR/IIR-M ………………………………………………………… 7

Figure 3. GPS IIF ……………………………………………………………………… 11

Figure 4. GPS IIIA …………………………………………………………………….. 13

Figure 5. Iridium Constellation ………………………………………………………... 15

Figure 6. GPS Augmentation Value Hierarchy ……………………………………….. 16

Figure 7. Evaluation Measures Entered in Expert Choice ………………………….…. 18

Figure 8. Sensitivity Analysis for Best GPS Augmentation System with Respect to

Performance ……………………………………………………………………. 20

Figure 9. Sensitivity Analysis for Best GPS Augmentation System with Respect to

Implementation ……………………………………………………………….... 21

Figure 10. Sensitivity Analysis for Most Cost Efficient GPS Augmentation

System …….....................................................................................................…. 22

List of Tables

Table 1. GPS Augmentation Evaluation Measure Weightings ……………………...… 17

Table 2. Pairwise Comparison of Value Hierarch …………………………………….. 19

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I. Introduction

1.1 Motivation

The history of GPS began with the US Navy's TRANSIT navigation system

developed in the 1960s which relied on six satellites and was designed originally for use

by submarines [14]. In 1973, engineers Ivan Getting and Bradford Parkinson led a

Department of Defense project to provide continuous navigation information known as

NAVSTAR GPS. Five years later, the U.S. Air Force launched its first GPS satellite on

22 February 1978 and completed GPS Block I in 1985. Originally restricted for military

access only, President Reagan made GPS available for nonmilitary users in 1983 after

Soviet fighter jets shot down Korean Air flight 007, a passenger plane that had

accidentally strayed into Soviet airspace, killing all 269 on board. From 1989 to 1997, 28

GPS Block II satellites are launched with the last 19 in the series labeled Block IIA for its

modernized features [1].

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Figure 1. GPS Block II/IIA [9]

To save 33% in cost while adding capabilities that include in-flight upgrades,

increased satellite autonomy, and radiation hardness [8], the Air Force transitioned to the

GPS Block IIR series with the first launch taking place on 17 January 1997. A setback

occurred however, when malfunctions with the Delta II rocket carrying the first satellite

caused the launch vehicle to explode shortly after liftoff. Another Block IIR satellite

successfully launched half a year later on 22 July 1997. To date, there are 12 operational

GPS Block IIR satellites and 5 Block IIR-Ms, with M designating two new military

signals for improved accuracy, enhanced encryption, anti-jamming capabilities and a

second civilian signal to provide dual frequency capability and improve resistance to

interference.

Figure 2. GPS Block IIR/IIR-M [11]

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GPS receiver sales increased dramatically for the first time in 2000 when its

signals were made more accurate to the public. Recently, GPS World reported that

approximately 65 million GPS units were sold in 2009, 70 million units in 2010, and

sales are expected to reach 93.2 million units by 2012 [13]. Foreign navigation systems

such as Russia’s GLONASS, Europe’s Galileo, and Japan’s Quazi-Zenith Satellite

System (QZSS) are also on-line and are expected to increase the total number of Global

Navigation Satellite Systems (GNSS) to approximately 60 to 100 in orbit [16]. With a

steady rise in the number of consumers every year, GPS modernization and augmentation

programs are necessary to meet this constantly growing demand.

1.2 Research Objectives

A fundamental question regarding GPS Augmentation that must be addressed is:

What are the key drivers for next-generation GPS satellites? This question further break

down to several others that this research attempts to answer: What kind of upgrades will

best meet user needs? While the soldier on the battlefield accesses GPS for a different

mission than the user who is removed from conflict, can the next-generation GPS

satellites reliably deliver the required capabilities to both? Do the benefits of GPS

Augmentation outweigh the risks and cost? Finally, if a budget cut is mandated by

Congress, which augmentation system should be chosen as the primary system to deliver

GPS capabilities for military and civil users through the year 2021?

1.3 Methodology

For this project, a trade study of current GPS Augmentation Systems capabilities

is performed by applying the Analytic Hierarchy Process. A value hierarchy structure is

fisrt created with top-level values decomposed and assigned evaluation measures. Based

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on the evaluation measures, pairwise comparison calculations are derived with results

entered in the Expert Choice software for sensitivity analysis.

II. Literature Review

2.1 Rationale for GPS Augmentation

Remember what life was like before access to GPS technology was made

available to the public? It was characterized by being careful not to wander off the trail

on recreational hikes or relying on a hand-held compass for terrain navigation. On cross-

country trips, it meant taking our eyes off the road every now and then to make sure that

we were still driving on the same freeway, traveling in the right direction, and checking

markers on the side to estimate when we’d get there. All this of course was only possible

if we had a map booklet handy. If not, our navigational skills were solely dependent on

how well the gas station clerk knew what he was talking about when he gave us

directions.

GPS forever changed navigation, military operations, and commercial

transportation applications. Although originally designed as a dual-use system with the

primary purpose of enhancing the effectiveness of military forces, GPS evolved to

become an integral component of an emerging global information infrastructure. GPS

civilian applications range from but are not limited to farming, mapping and surveying,

international air traffic control, cellular networks, and most recently, precise timing for

synchronization of financial institutions [5]. Meteorologists gauge wind speed and other

variables by measuring satellite signals as they pass through the atmosphere; geologists

study earthquakes using GPS receivers placed along fault lines; and technicians

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synchronize computer networks for everything from power grids to financial networks

using the satellite signals' precise timing.

The future of GPS is one that will require a robust modernization program in

order to meet an ever growing demand for more precise and rapid positioning,

navigation, and timing (PNT) from military, civil, commercial, and scientific users.

Augmentation programs are in place to transition the current GPS fleet in orbit to next-

generation models capable of improving navigation accuracy, provide for longer

autonomous satellite operation, while maintaining a military advantage. This project is a

trade study on GPS Block IIF, GPS III, and iGPS, all part of the plans that will deliver

these capabilities in the next decade.

2.2 GPS Augmentation Capabilities

2.2.1 GPS IIF

The first of 12 GPS Block IIF satellites launched into orbit on 27 May 2010,

aboard a Delta IV Medium rocket from Cape Canaveral. Among its technological

advances, the Block IIF series delivers greater accuracy through advanced atomic clock

technology, military signals more resistant to jamming, a new civilian signal, on-board

reprogrammable processor with greater capability to receive updated software on orbit,

and longer design life for reduced operating costs [3].

Advanced atomic clock technology on the GPS IIF series provides precision

timing through cesium and rubidium construction that is capable of keeping time to an

accuracy of 8 nanoseconds a day. Jamming of this satellite is deterred with variable

power capability which allows operators to increase signal power and break through

jamming attempts. GPS IIF also broadcasts the third operational civilian signal, L-5,

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which will be used to increase commercial aviation safety by improving positioning and

navigation accuracy of civilian aircraft to under 1 meter. While GPS IIF is under contract

for a design life of 12 years, the GPS IIA series built by Boeing are still in operation with

some lasting 2 to 3 times their design life. GPS IIF is expected to be the backbone of the

GPS constellation for the next 15-18 years.

Figure 6. GPS IIF [10]

2.2.2 GPS III

On 15 May 2008, the Air Force announced Lockheed Martin as the winner of the

contract to build the next-generation GPS Space System program [7], also known as the

GPS Block III. In addition to carrying the capabilities present in all previous GPS series,

GPS Block III will be constructed with a cross-linked command and control architecture,

allowing the entire GPS constellation to be updated simultaneously from a single ground

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station instead of waiting for each satellite to orbit in view of a ground antenna. The

Block III series will also feature a new spot beam capability for enhanced military (M-

Code) coverage and increased resistance to hostile jamming. This is accomplished by

producing 500 times the transmitter energy flux of the current GPS fleet. Spot beam

capability enables GPS III to shut down its broadcast transmission and selectively

broadcast signals to US and allied forces only.

GPS III will also carry a fourth civilian signal, L1C, designed to be highly

interoperable with the European Galileo satellite navigation system signal and intended to

be fully compatible and interoperable with those signal planned for broadcast on Japan’s

Quasi-Zenith Satellite System (QZSS). The contract for the Block III series specifies

delivery in 3 increments and a total of 32 satellites. GPS Block IIIA will deliver 2

spacecraft and options for up to 10 additional spacecraft with the first launch projected

for 2014. Eight satellites for Block IIIB and 16 for Block IIIC are planned for later

increments, with each series to include additional capabilities based on technical

maturity. By the time GPS Block IIIC is launched, accuracy of the satellite’s signal is

expected be under ¼ of a meter. The Block III series is also designed to have a longer

lifetime than its predecessors to achieve reduced operating costs.

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Figure 7. GPS IIIA [12]

2.2.3 iGPS

Due to their extreme dependency on GPS for navigation and targeting in the past

decade, Air Force Chief of Staff Gen Norton Schwartz suggested that the U.S. military

become less reliant on the system. In a speech delivered January 2010, Gen Schwartz

acknowledges that GPS is a “vulnerable capability and an alternative system should be

developed” [5]. One of the most promising GPS Augmentation systems currently in

development is Boeing’s High Integrity GPS program – known more commonly as iGPS.

In 2006, the Boeing Company was issued a study contract for its proposed idea of

reprogramming Iridium satellites to augment the GPS constellation’s navigation and

timing signals. In July 2008, Boeing received a 3-year, $153.5-million cost-plus-fixed-

fee contract from the U.S. Navy’s Naval Research Laboratory (NRL) to continue its

efforts and develop the software upgrades and ground infrastructure needed for iGPS [6].

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Among its capabilities, iGPS will enable faster acquisition (time to first fix or TTFF) of

GPS satellite signals. This is accomplished through the first milestone when Boeing

completed an enhanced narrowband (ENB) software modification to computers on

Iridium satellites, enabling second-generation GPS-aiding signals to be broadcast through

the entire Iridium constellation. With Iridium’s signal being 10,000 times more powerful

than a GPS signal, jamming of the iGPS system is not practical or would require a large

radio transmitter that could be easily traced and neutralized. More power also allows for

rapid signal acquisition in restrictive environments such as buildings, mountains and

canyons, as well as adverse conditions that include enemy jamming attempts or amid

battlefield radio frequency (RF) noise. Like GPS III, iGPS broadcast signals can be

turned on and off like a light switch, and it can be tailored to only work in a specific

region for a specific amount of time.

The Iridium constellation is currently the largest commercial spacecraft system in

the world with 66 operational satellites. Iridium’s cross-link capability will enhance the

integrity and reliability of iGPS for users. Boeing also plans to reprogram Iridium to be

interoperable with the military M-Code signal broadcast by current GPS satellites in orbit

assisting U.S. and allied forces. Field demonstrations for the iGPS system are due this

year and Boeing is optimistic that iGPS services will be available for military and civilian

applications by the end of 2011.

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Figure 8. Iridium Constellation [17]

III. Methodology

3.1 Approach

The Analytic Hierarchy Process (AHP) is applied using the educational version of

Expert Choice to determine the best GPS Augmentation system, assuming a budget cut

occurs and the Air Force is to select one navigation system with an operational lifetime

spanning the years 2011 to 2021. The AHP is a useful decision-making tool for

prioritizing alternatives when many criteria are being considered. AHP is an approach to

structuring a problem as a hierarchy. The goal of this study is to select the best

augmentation system. The criteria are performance and implementation. The

alternatives are GPS IIF, GPS III, and iGPS.

3.2 Value Hierarchy

A value hierarchy of the GPS Augmentation system is created as shown in Figure 9.

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Figure 9. GPS Augmentation Value Hierarchy

The hierarchy is maximized to 4 levels to fully utilize the educational version of the

Expert Choice software. Top-level criteria for Performance are decomposed to functional

requirements that include Anti-jam, Cross-link Enabled, and Accuracy. Top-level criteria

for Implementation are supported by Schedule and Cost. The lowest level of the

hierarchy (denoted by green boxes) defines supporting capabilities for the Anti-jam and

Cross-link Enabled functional requirements and also defines Availability and Mission

Life, the key components that make up Schedule.

3.3 Evaluation Measures

Weighted relationships between values within the hierarchy are then assigned and

reflected in Table 1. These measures are assigned by the author with priority given to

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GPS Augmentation

Performance

Anti-jam

Signal StrengthSelective Broadcast Capability

Cross-link Enabled

On-orbit Reprogram Capability

Interoperability

Accuracy

Implementation

Schedule

Availabilty Mission Life

Cost

capabilities that are deemed important to users in next-generation GPS Augmentation

systems.

Performance Implementation0.4 0.6

Anti-jam Crosslink EnabledAccurac

ySchedule

Cost

0.33 0.33 0.33 0.7 0.3Signal Strengt

h

Selective Broadcast Capability

On-orbit Reprogram Capability

Interoperability

Availability

Mission Life

0.70 0.30 0.70 0.30   0.80 0.20

Table 1. GPS Augmentation Evaluation Measure Weightings

Implementation is weighted more than Performance because Implementation is

schedule and not cost driven. Functional requirements are equally ranked among Anti-

jam, Crosslink Enabled, and Accuracy. Signal Strength and On-orbit Reprogram carry

more weight than Selective Broadcast and Interoperability to reflect a current user need

for these upgraded capabilities in the next-generation GPS systems. Availability of the

system is weighted more than Mission Life due to the urgent nature of delivering required

GPS capabilities to users within a time constraint. Below are evaluation measures as

inputed into the Expert Choice software.

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Model Name: GPS Augmentation

Treeview

Goal: Best GPS Augmentation System

Performance (L: .400)

Anti-jam (L: .333)

Signal Strength (L: .700)

Selective Broadcast Capability (L: .300)

Crosslink Enabled (L: .333)

On-orbit Reprogram Capability (L: .700)

Interoperability (L: .300)

Accuracy (L: .333)

Implementation (L: .600)

Schedule (L: .700)

Avalability (L: .800)

Mission Life (L: .200)

Cost (L: .300)

Alternatives

.260

.303

.437

* Ideal mode

Figure 10. Evaluation Measures entered in Expert Choice

IV. Analysis and Results

4.1 Pairwise Comparison

Pairwise comparison results are depicted in Table 2. The results are calculated

based on evaluation measure weightings assigned (Table 1). For example, since

implementation is weighted more than performance, implementation is 0.6/0.4 or 1.5

times more preferred than performance on the pairwise comparison matrix. This means

that its reciprocal judgement, performance, is 0.4/0.6 or 0.67 preferrred and this result

entered in row 2. Entries along the main diagonal of the matrix are always 1 since each

value is equally preferred to itself.

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Pairwise Comparison: Performance & Implementation  Performance ImplementationPerformance  1.00 1.50Implementation  0.67  1.00

Pairwise Comparison: Antijam, Crosslink Enabled, Accuracy  Antijam Crosslink Enabled AccuracyAntijam  1.00 1.00 1.00Crosslink Enabled  1.00 1.00  1.00Accuracy  1.00 1.00  1.00

   Pairwise Comparison: Performance => Antijam

  Signal StrengthSelective Broadcast

CapabilitySignal Strength  1.00 0.43Selective Broadcast Capability  2.33  1.00

Pairwise Comparison: Performance => Crosslink Enabled

 On-orbit Reprogram

Capability InteroperabilityOn-orbit Reprogram Capability  1.00 0.43Interoperability 2.33  1.00 

Pairwise Comparison: Schedule & Cost  Schedule CostSchedule  1.00 0.43Cost  2.33  1.00

Pairwise Comparison: Implementation => Schedule  Availability Mission LifeAvailability  1.00 0.25Mission Life  4.00  1.00

Table 2. Pairwise Comparison of Value Hierarchy

4.2 Sensitivity Analysis

Expert Choice results for sensitivity analysis based on evaluation measures and

pairwise comparison inputs above are depicted in Figures 10 – 13 for Performance,

Implementation, as well as the top-level evaluation criteria for most cost efficient GPS

Augmentation system.

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Performance Sensitivity for nodes below: Goal: Best GPS Augmentation System > Performance (L:.400)

.00

.10

.20

.30

.40

.50

.60

.70

.80

.90

.00

.10

.20

.30

.40

.50

.60Crit% Alt%

GPS IIF

iGPS

GPS III

Anti-jam Crosslink En Accuracy OVERALL

Objectives Names

Alternatives Names

Figure 10. Sensitivity Analysis for Best GPS Augmentation System with Respect to Performance

With respect to Performance, when functional values for Anti-jam, Cross-link

Enabled, and Accuracy are set equal, sensitivity analysis results yields GPS III as the

favored augmentation system (Figure 10). This is due to GPS III being more

technologically advanced than iGPS in the areas of Selective Broadcast Capability and

Interoperability. GPS III also out-performs GPS IIF in On-orbit Reprogram Capability

and Accuracy.

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Performance Sensitivity for nodes below: Goal: Best GPS Augmentation System > Implementation (L:.600)

.00

.10

.20

.30

.40

.50

.60

.70

.80

.90

.00

.10

.20

.30

.40

.50

.60

.70

.80Crit% Alt%

GPS III

GPS IIF

iGPS

Schedule Cost OVERALL

Objectives Names

Alternatives Names

Figure 11. Sensitivity Analysis for Best GPS Augmentation System with Respect to Implementation

With respect to Implementation, sensitivity analysis yields iGPS as the favored

augmentation system (Figure 11). The outcome for this analysis is a result of the weight

of Availability making up 80% of Schedule in the value hierarchy. The Iridium-based

iGPS program currently has 66 satellites in its constellation and is therefore readily

available for immediate service. Iridium is expected to remain operational for the next 4

years and with relative ease of replacing a dead Iridium satellite, the cost of iGPS is

significantly less than that of GPS IIF and GPS III [17].

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Performance Sensitivity for nodes below: Goal: Best GPS Augmentation System

.00

.10

.20

.30

.40

.50

.60

.70

.80

.90

.00

.10

.20

.30

.40

.50Crit% Alt%

GPS IIF

GPS III

iGPS

Performance Implementati OVERALL

Objectives Names

Alternatives Names

Figure 12. Sensitivity Analysis for Most Cost Efficient GPS Augmentation System

Finally, with the goal of selecting the most cost efficient GPS Augmentation

system, sensitivity analysis results depict iGPS as the system of choice (Figure 12).

While the performance of iGPS is lower than that of GPS III, iGPS scores higher than

both the GPS IIF and GPS III augmentation systems in Implementation. The

Implementation value of the value hierarchy is supported by Schedule and Cost where

both is considerably favorable to the Air Force for the iGPS system in the event of a

budget cut.

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V. Conclusion and Reccomendations

GPS Augmentation is the means of delivering next-generation capabilities to the

warfighter and civil users. Do the benefits of GPS Augmentation outweigh the risks and

cost? Yes, requirements for a more robust system with greater anti-jam measures through

stronger signal strength ensure continued military operations under intentional jamming

environment. Selective broadcast ability will enable GPS to send its signals to our troops

and friendly forces only. Requirements for cross-link features through on-orbit

reprogramming and interoperability allow GPS to interface not only within its

constellation but also with Russia’s GLONASS, Europe’s Galileo system, and Japan’s

QZSS. Interoperability of GPS will provide a myriad of options for future GPS users.

The aforementioned requirements all serve as key drivers to upgrade capabilities and

reliably deliver military and civil users with new augmentation features.

It would be interesting to perform additional trade studies to compare current

Augmentation Systems with foreign GPS systems. Understanding the missions for

GLONASS, Galileo, and QZSS would offer insight into user needs that drove the design

for the system. With cross-link and interoperability capabilities as a standard for

augmentation, another interesting study would be to prove if the current number of

satellites required for an accurate fix can be reduced from 4 satellites.

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VI. References

1. Aerospace. “GPS Timeline”, http://www.aero.org/education/primers/gps/gpstimeline.html, accessed Fall Semester 2010.

2. Air Force Space Command. “Evolved Expendable Launch Vehicle”, http://www.afspc.af.mil/library/factsheets/factsheet.asp?id=3643, accessed Fall Semester 2010.

3. Air Force Space Command. “GPS IIF-1 Introduces a Host of New Capabilities to Users”, http://www.afspc.af.mil/news/story.asp?id=123229701, released by AFSPC on November 5, 2010, accessed Fall Semester 2010.

4. Brinton, Turner. “Boeing and Iridium Hope to Sell GPS Enhancement Service”, http://www.spacenews.com/military/100412-boeing-iridium-gps-enhancement-service.html, released by Space News on April 12, 2010, accessed Fall Semester 2010.

5. DeGryse, Donald G. “GPS Modernization and the Path Forward: Bringing New Capabilities to Military and Civil Users Worldwide”, High Frontier-The Journal for Space & Missile Professionals. Volume 4, Number 3, May 2008.

6. Gibbons, Glen. “Boeing Wins NRL Contract to Continue Iridium/GPS Development”, http://www.insidegnss.com/node/745, released by Inside GNSS on July 28, 2008, accessed Fall Semester 2010.

7. Global Security. “GPS Block III”, http://www.globalsecurity.org/space/systems/gps_3.htm, accessed Fall Semester 2010.

8. Global Security. “GPS Block IIR”, http://www.globalsecurity.org/space/systems/gps_2r.htm, accessed Fall Semester 2010.

9. GPS Image Library, GPS II/IIA, http://www.gps.gov/multimedia/images/II-IIA.jpg, accessed Fall Semester 2010.

10. GPS Image Library, GPS IIF, http://www.gps.gov/multimedia/images/IIF.jpg, accessed Fall Semester 2010.

11. GPS Image Library, GPS IIR/IIR-M, http://www.gps.gov/multimedia/images/IIR-M.jpg, accessed Fall Semester 2010.

12. GPS Image Library, GPS III, http://www.gps.gov/multimedia/images/GPS-III-A.jpg, accessed Fall Semester 2010.

13. GPS World. “GPS Maniac Media Kit 2009”, http://www.gpsworld.com/gps/gps-maniac-media-kit-2009-6921, accessed Fall Semester 2010.

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14. James, Randy. “A Brief History of GPS”, http://www.time.com/time/nation/article/0,8599,1900862,00.html, released by Time on May 26, 2009, accessed Fall Semester 2010.

15. Liberatore, Matthew and Nydick, Robert, “Decision Technology Modeling, Software, and Applications, (2003). John Wiley & Sons, Inc. ISBN 0471-41712-2.

16. Murrett, Robert B. “NGA: GPS Consumer and Contributor”, High Frontier-The Journal for Space & Missile Professionals.Volume 4, Number 3, May 2008.

17. World Communication Center. “Iridium System Overview”, http://www.wcclp.com/index.asp?pgid=11, accessed Fall Semester 2010.

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