Satellite Autonomous Integrity Monitoring and its Role in Enhancing GPS User Performance

Logi Vioarsson, Sam Pullen, Gaylord Green and Per Enge

Abstract:	One of the difficulties inherent in use of Global Navigation Satellite Systems (GNSS) for safety-critical applications is the need to insure user integrity in the face of an unbounded array of possible failures in the GNSS satellite signals. GPS satellites, for example, are generally operated until failure, and some of those failures will cause ranging errors which impact user integrity. Ground-based satelliteoperator solutions to this problem are complex and costly and have difficulty alerting users of the failure quickly. Models for possible satellite failures have been developed but are imperfect because of the limited statistical information available to the satellite operators and to the civil user community. As a result, it is difficult to support the single-failure assumption made by Receiver Autonomous Integrity Monitoring (RAIM) or to certify that Space Based Augmentation Systems (SBAS) and Ground Based Augmentation Systems (GBAS) safely detect all threatening satellite failures. In addition, when satellite failures occur, they must be detected and sorted out from an array of possible failures in SBAS and/or GBAS ground systems, or else continuity will be sacrificed unnecessarily. These difficulties would be greatly lessened if integrity monitoring were conducted within the satellite constellation itself so that immediate warnings could be transmitted to users. We have developed and are prototyping a method for Satellite Autonomous Integrity Monitoring (SAIM) that could be applied to future GNSS satellites such as GPS III. The processing demands on SAIM are much lighter than those on a LAAS or GBAS ground system because each satellite’s SAIM function monitors the ranging signals of the satellite it is attached to rather than the 12 or more satellites that must be simultaneously handed by SBAS and GBAS. This paper describes our SAIM concept in detail and presents test results from a SAIM software prototype now under development. This prototype has been tested against nominal satellite signals (to confirm that fault-free alarms are rare enough to support civil aviation continuity requirements) and several classes of failed signals. Practical implementation issues such as satellite multipath and receiver clock calibration will also be addressed.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	690 - 702
Cite this article:	Vioarsson, Logi, Pullen, Sam, Green, Gaylord, Enge, Per, "Satellite Autonomous Integrity Monitoring and its Role in Enhancing GPS User Performance," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 690-702.
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