GPS RAIM: Screening Out Bad Geometries Under Worst-Case Bias Conditions

Gerald Y. Chin and John H. Kraemer, R. Grover Brown

Abstract:	GPS Receiver Autonomous Integrity Monitoring @AIM) has received considerable attention at navigation meetings over the past few years. This is understandable because a GPS Integrity Channel (GIG) is still a few years away, and the GPS user community will have to rely on RAIM for some time. There are two parts to any RAIM algorithm. On the one hand, there is the decision-rule part which formally gives the procedure for detection of a navigation failure. On the other hand, there must be a procedure for screening out bad satellite geometries. There are geometries that may be satisfactory for navigation purposes but that cannot provide failure detection within the required specifications. These are the cases of bad integrity geometries where there is a relatively high probability of missed detection of navigation failure induced by a satellite failure with a gradually increasing bias. Declaring such geometries as inadmissible diminishes the RAIM availability, so the algorithm must not do the screening indiscriminately. This paper concentrates on the screening function of RACM. The study compares the effectiveness of the older I-IDOP,, criterion and a newer criterion called Approximate Radial-error Protected (ARP). The ARP criterion is shown to be a much more reliable measure of navigation failure detection than HDOPmax. The results of extensive Monte Carlo simulations are presented. In the process of establishing reliable ceiling levels for the ARP figure-of-merit for various phases of flight, it was found necessary to determine empirically the worst-case satellite bii that was applicable in each case. Results of this study are also presented. The ARP method is further investigated and its properties illuminated. The main contribution is a reliable criterion for screening out bad detection geometries. This screening criterion is straight-forward and can be readily implemented online in a GPS receiver.
Published in:	Proceedings of the 48th Annual Meeting of The Institute of Navigation (1992) June 29 - 1, 1992 ANA Westin Hotel Dayton, OH
Pages:	63 - 72
Cite this article:	Chin, Gerald Y., Kraemer, John H., Brown, R. Grover, "GPS RAIM: Screening Out Bad Geometries Under Worst-Case Bias Conditions," Proceedings of the 48th Annual Meeting of The Institute of Navigation (1992), Dayton, OH, June 1992, pp. 63-72.
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