Achieving GPS Integrity and Eliminating Areas of Degraded Performance

Paul S. Jorgensen

Abstract:	The Federal Aviation Administration (FAA) is concerned over the safety in navigation with the Global Positioning System (GPS). In particular, they are concerned over the possibility of a sudden degradation in the performance of a GPS satellite and the false navigation it might cause a user to experience. The main worry is the possibility of a sudden change in the output of the atomic frequency standard in the satellite. To circumvent this possibility, the FAA asked Mitre Corporation to look into the idea of an integrity monitoring system that would provide users with a timely warning of a sudden degradation in the performance of a GPS satellite.l The FM requirement for such a monitoring system is quite stringent. It requires that the GPS users be warned within 10 seconds of the onset of abnormal behavior of a satellite. As part of the integrity monitoring system, a communications package on a geostationary satellite is used for relaying this warning to GPS users. The second concern of the FAA is the areas of degraded performance that occur with the baseline 18 satellite constellation. These short-term periods occur mostly at mid-latitudes. Those over CONUS are eliminated with the three on-orbit spares. However, we can't always depend on having all 21 satellites on-orbit. The uncertainties of just when satellites will fail together with the time needed to replace them will bring the constellation below 21 a significant percentage of the time. A GPS navigation package on the geostationary communications satellite provides the additional coverage to eliminate these areas of degraded performance ·over a region of the earth. Thus the geostationary satellite serves as the key to the solution of both problems, the integrity issue and the concern over the areas of degraded performance.
Published in:	Proceedings of the 43rd Annual Meeting of The Institute of Navigation (1987) June 23 - 25, 1987 Dayton, Ohio
Pages:	84 - 92
Cite this article:	Jorgensen, Paul S., "Achieving GPS Integrity and Eliminating Areas of Degraded Performance," Proceedings of the 43rd Annual Meeting of The Institute of Navigation (1987), Dayton, Ohio, June 1987, pp. 84-92.
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