GPS Monitor Alarm Limits for Nonprecision Approaches

Robert Loh

Abstract:	The future navigation accuracy requirement for nonprecision approaches has been estimated to be about 100m. GPS can satisfy this requirement by limiting the 95 percent horizontal position determination accuracy for civil users to 100m. The FAA is investigating GPS signal integrity monitoring and the associated monitoring alarm limits in order to satisfy the integrity response time requirements for nonprecision approaches. This paper documents a rationale for the 100m GPS horizontal accuracy requirement and evaluates the impact of using GPS monitor alarm limits greater than 100 m. The GPS monitor alarm limit must be set above 100 m in order to reduce the number of potential alarms. The significant impact on nonprecision approaches, as defined in the Terminal Instrument Procedures (TERPS), appears to be the size of the obstacle clearance areas (OCAs) and their effect on the minimum descent altitude (MDA). Obstacles in the OCAs will elevate the MDA; consequently, the larger the OCAs, the greater the probability that the MDA will have to be elevated, thereby reducing the utility of the nonprecision approach. For a GPS monitor alarm limit of200 m, the results show the proposed OCAs to be approximately equal to those of a VOR installed in the vicinity of the runway threshold. However, for the majority of cases, the OCAs for GPS nonprecision approaches (approximately 70 percent) will be smaller than the equivalent OCAs for VOR approaches.
Published in:	Proceedings of the 1989 National Technical Meeting of The Institute of Navigation January 23 - 26, 1989 San Mateo, CA
Pages:	57 - 61
Cite this article:	Loh, Robert, "GPS Monitor Alarm Limits for Nonprecision Approaches," Proceedings of the 1989 National Technical Meeting of The Institute of Navigation, San Mateo, CA, January 1989, pp. 57-61.
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