National Academies Press: OpenBook

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

Chapter: Resistance to RF Interference

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Suggested Citation:"Resistance to RF Interference." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 163

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APPENDIX C 163 that at least four satellites must be visible to a user's receiver anywhere on or near the Earth, and the satellites must be providing the required positioning accuracy for the user's application. Some GPS applications, such as static surveying, do not require continuous availability. Others, such as air navigation, can require that GPS signals be available 99.999 percent of the time. The average availability of four or more GPS satellites in view of a given receiver, at SPS accuracy levels, is currently specified as 99.85 percent.32 Continuity of Service Continuity of service, which also is referred to as reliability, is the ability of a navigation system to provide required service over a specified period of time without interruption. The level of continuity is expressed in terms of the probability of not losing the radiated guidance signals.33 Where warranted, continuity of service is achieved by using redundant transmitters and monitors. Continuity of service and availability go together in that availability is the probability that a system will be in service when it needs to be used, and reliability is the probability that the system will continue to provide service. The global average reliability for GPS is specified as 99.97 percent.34 Resistance to RF Interference The accuracy of a GPS receiver can be degraded in the presence of unwanted interfering signals from terrestrial or other sources. In extreme cases, the receiver is unable to provide any useful navigation or positioning capability. Unwanted and unintentional sources of interference exist, such as the third harmonic of some UHF transmitters, which many civilian users may be unaware of. Military users are also concerned with unintentional interference, but they are more concerned with deliberate efforts to prevent the use of navigation signals through jamming. While no receiver can be made entirely immune to interference (intentional or otherwise), steps can be taken in the design of the receiver to 32 This specified value is the average global availability for a 30-day period, assuming that three satellites have been removed from service on 1 of the 30 days, and assuming a total of 4 satellite down days. Depending on the health of the constellation at any given time, and a users location on the globe, observed SPS-level availability may be better or worse than this average. Source: Assistant Secretary of Defense for Command, Control, Communications, and Intelligence, Global Positioning System Standard Positioning Service Signal Specification (Washington, D.C.: U.S. Department of Defense, 8 December 1993), p. B-10. 33 International Civil Aviation Organization (ICAO), International Standards and Recommended Practices, Aeronautical Telecommunications, Annex 10, to the Convention on International Civil Aviation, Volume 1, 22 October 1987. 34 The full set of assumptions used to determine this value can be found in: Assistant Secretary of Defense for Command, Control, Communications, and Intelligence, Global Positioning System Standard Positioning Service Signal Specification, Section 4.0— Service Reliability Characteristics, pp. B-11 through B-14.

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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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