Analysis of GPS RAIM Continuity and Availability for Oceanic RNP Operations

Young C. Lee and J. P. Fernow

Abstract:	The FAA recently published Technical Standard Order (TSO) C-129, Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS). The TSO requires use of receiver autonomous integrity monitoring (RAIM) to alert a possible failure condition. Upon the occurrence of such an alert or when the RAlM function is not available due to a satellite geometry limitation, the aircraft can fall back on a non-GPS navigation system which is assumed to exist for supplemental use. If GPS is to provide a required navigation performance (RNP) capability, RAIM performance must be enhanced in two ways. First, the total observable alarm rate must be reduced. Second, availability and continuity of the integrity function must be increased. To increase availability and continuity, fault isolation function is highly desirable for RNP. Since the isolation function requires much more stringent geometric criteria than the RAIM detection function alone, the availability and continuity of the receiver autonomous fault detection and isolation (FDI) function should be carefully evaluated to determine if the system can meet requirements. If a combination of GPS and some other system is to be used for RNP, the integration scheme must also be analyzed. This paper first compares quantitatively two leading FDI algorithms in terms of their performance in RNP operation. It then analyzes continuity and availability of RAIM functions as a function of satellite constellation, mask angle, and aiding with altimeter input or clock coasting. The paper also discusses the difference between avionics certilication requirements and operational approval requirements. Alternative methods of achieving suitability for oceanic RNP operations with unaugmented GPS such as dispatch requirements and contingency plans are suggested. Finally, the paper describes trade-offs for some alternatives for integrating GPS and inertial systems for use in oceanic flight.
Published in:	Proceedings of the 49th Annual Meeting of The Institute of Navigation (1993) June 21 - 23, 1993 Royal Sonesta Hotel Cambridge, MA
Pages:	287 - 304
Cite this article:	Lee, Young C., Fernow, J. P., "Analysis of GPS RAIM Continuity and Availability for Oceanic RNP Operations," Proceedings of the 49th Annual Meeting of The Institute of Navigation (1993), Cambridge, MA, June 1993, pp. 287-304.
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