GPS/GLONASS RAIM augmentation to WAAS for CAT 1 precision approach
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GPS/GLONASS RAIM augmentation to WAAS for CAT 1 precision approach

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      This paper deals with the potential use of Receiver Autonomous Integrity Monitoring @AIM) 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. The focus of this paper is on Category I precision approach where WAAS is the primary source for the integrity function and RAIM, if used, will serve as an augmentation. 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. Since RAIM serves as a back-up to WAAS, the missed alert probability requirement can be relaxed significantly. The analysis in this paper is an extension of work presented earlier this year [I] which demonstrated that RAIM is available for CAT I precision approach less than 80% of the time with a constellation of 24 GPS satellites and three geostationary satellites. This study examines the improvement in RAIM availability for CAT 1 if GLONASS satellites are incorporated into the solution in addition to GPS and WAAS satellites. RAIM detection availability results are presented as a function of missed alert rate for various combinations of GPS, GLONASS and WAAS geostationary ranging satellites. Previous RAIM studies have been based upon the explicitly stated assumption that only one satellite will fail at any given time. Hence RAIM need only contend with single-satellite failures when operating outside of WAAS coverage. When RAIM is applied to differentially- corrected pseudoranges, however, the single-failure assumption is no longer valid. In this case, the notion of a failing satellite gives way to the notion of the inability of the WAAS corrections to suitably correct locally- measured pseudoranges. Reasons for this include the possible presence of local propagation effects not observable to WAAS e.g. local tropospheric or ionospheric effects or multipath. This paper addresses the situation where multiple differentially-corrected pseudoranges are corrupted.
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