The Global Positioning System: Assessing National Policies

By Scott Pace; Gerald Frost et al. | Go to book overview
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Chapter One
INTRODUCTION

The Global Positioning System (GPS) is a U.S. military space system operated by the U.S. Air Force. It consists of three segments: The space segment of GPS is a constellation of 24 satellites that broadcast precise time signals. When the satellites are in view of a suitable GPS receiver, these signals can be used to aid position-location, navigation, and precision timing. The control segment consists of a control center and access to overseas command stations, and the user segment includes GPS receivers and associated equipment. The GPS space and ground segments were developed over two decades at the cost of more than $10 billion.1 The purpose of this massive effort was to provide a highly accurate, secure, reliable way for U.S. forces to navigate anywhere in the world, without having to reveal themselves through radio transmissions.

GPS satellites transmit two different signals: the Precision or P-code and the Coarse Acquisition or C/A-code. The P-code is designed for authorized military users and provides what is called the Precise Positioning Service (PPS). To ensure that unauthorized users do not acquire the P-code, the United States can implement an encryption segment on the P-code called anti-spoofing (AS). The C/A-code is designed for use by nonmilitary users and provides what is called the Standard Positioning Service (SPS). The C/A-code is less accurate and easier to jam than the P-code. It is also easier to acquire, so military receivers first track the C/A-code and then transfer to the P-code. The U.S. military can degrade the accuracy of the C/A-code by implementing a technique called selective availability (SA). SA thus controls the level of accuracy available to all users of the Standard Positioning Service.

GPS had its wartime debut during the Persian Gulf War and was one of the most prominent military technologies of the war. Although the entire constellation of satellites was not yet complete, GPS signals were available for many hours a

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1
Senator James Exon, "GPS's Limitless Potential", April 30, 1993 speech on the floor of the U.S. Senate, reprinted in Space News, May 31-June 6,1993, p. 15. The ground segment consists of the master control station and worldwide monitoring stations.

-1-

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