National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: Impact of A-S on Military Users

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Suggested Citation:"Impact of A-S on Military Users." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
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Page 84

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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 84 (5) The technology for developing direct Y-code receivers is currently available and the development and initial deployment of these receivers could be accomplished in a short period of time if adequately funded. (6) The FAA's WAAS, the Coast Guard's differential system, and GLONASS are expected to be fully operational in the next 1 to 3 years. The Coast Guard's DGPS network and the WAAS will provide accuracies greater than that available from GPS with SA turned to zero and GLONASS provides accuracies that are comparable to GPS without SA. At the same time, other local DGPS capabilities are likely to continue to proliferate. Selective Availability should be turned to zero immediately and deactivated after three years. In the interim, the prerogative to reintroduce SA at its current level should be retained by the National Command Authority. Anti-Spoofing The purpose of A-S is to protect military receivers from an adversary transmitting a spoofed P-code signal and to deny the precision to an adversary through encryption.12 When A-S is turned on, the P-code modulation on both the L1 and L2 carriers is replaced with a classified known as the Y-code that has the same chipping rate and correlation properties as the P-code. (C/A-code is not affected by Y-code transmission.) Except for special arrangements to turn off A-S for specific requirements, it has remained on continuously since January 31, 1994. Impact of A-S on Military Users PPS receivers are able to track the Y-code through the use of a security module that employs National Security Agency cryptographic techniques, and requires the manual distribution of encryption keys. There are compelling reasons to retain the A-S feature. If the recommendation to remove SA is implemented and potential adversaries have access to the resulting more accurate C/A-code on the L1 frequency, the reasons to retain A-S become still more compelling. In addition to its anti-spoofing feature, A-S forces adversaries to use the C/A-code on the L1 frequency, which can be denied by jamming techniques (without impacting L2). The NRC committee believes that denying L1 to an enemy through jamming, while employing only L2 for its own forces, should be the basis of a new military doctrine for the use of GPS. However, this doctrine will require U.S. military receivers to acquire the Y-code rapidly without the C/A-code. Military receivers also should be able to provide accurate 12 The process of sending incorrect information to an adversary's radio equipment (in this case a GPS receiver) without their knowledge, using mimicked signals, is known as spoofing.

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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