Weighted RAIM for WAAS Category I Precision Approach

John H. Kraemer, Gerald Y. Chin, and Giau C. Nim

Abstract:	This paper deals with the potential use of Receiver Autonomous Integrity Monitoring (RAIM) to supplement the FAA’s Wide Area Augmentation System (WAAS). Integrity refers to the capability of a navigation or landing system to provide a timely warning when the system no longer meets specifications and should not be used. WAAS will provide the integrity for Category I precision approach and RAIM, if used, will serve in a backup role. RAIM can protect against local error sources (tropospheric and ionospheric effects, and possible interference), not observable to the WAAS. RAIM will not, however, be required for an approach to be made. If RAIM is available and indicates that there is an integrity problem, the approach will not be made. The following issues are treated in this paper: a) on-line determination of RAIM protection levels for a weighted Differential GPS (DGPS) solution, b) a comparison of static and dynamic missed alert rates, and c) RAIM (detection) availability for WAAS DGPS corrected pseudorange measurements. The term protection level refers to the navigation accuracy which can be protected. While RTCA defines protection level from an operational standpoint, it is left to equipment manufacturers to determine how they wish to compute its value. Although this is satisfactory for equipment testing, a general method of determining protection level is needed to compute system availability. Integrity is typically specified for worst-case conditions. This usually means that a worst-case failure (a bias error) is placed on the satellite where it is the hardest to detect. By worst-case we mean that the error magnitude is chosen so as to maximize the likelihood of a missed alert. Monte Carlo simulations run with a fixed, worst-case bias constitute a static experiment. Algorithm testing is done both off-line (for example on a PC), and on-line with the algorithm operating within the avionics. A ramp bias error is used for RAIM testing. This constitutes a dynamic experiment as distinguished from a static one. The two experiments are different and produce different results. It is therefore necessary to relate the results of a static experiment (worst-case bias) to those of a dynamic one in order to compute protection levels.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	499 - 509
Cite this article:	Kraemer, John H., Chin, Gerald Y., Nim, Giau C., "Weighted RAIM for WAAS Category I Precision Approach," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 499-509.
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