GNSS Satellite Ephemeris Fault Risk Concepts and Analysis for CAT I GBAS

C.A. Shively

Abstract:	A GNSS satellite ephemeris fault occurs when the actual satellite position in space differs from the position based on the broadcast ephemeris parameters by more than the nominal error. For Category I (CAT I) precision approach procedures using a GNSS ground based augmentation system (GBAS), the ephemeris fault risk is mitigated by monitoring in the GBAS ground facility in conjunction with an error bound, VPBApr_e, computed in the aircraft. This paper describes a concept for analyzing the probability of loss of integrity (PLOI) resulting from the combined operation of the ground and airborne mitigations. Loss of integrity occurs when: 1) an ephemeris fault occurs (probability Pfault), 2) the fault goes undetected (design probability Pmd_req at minimum detectable error (mde)) and 3) the total vertical error exceeds VPBApr_e (design probability PPLe_req at ephemeris error bias = mde). Limiting values of PLOI are derived as a function of X, the distance from the GBAS ground facility. For intermediate values of X, the lower limit for PLOI(X) involves all three probabilities: Pfault x Pmd_req x PPLe_req / 2. However, for extremes of X, PLOI(X) is limited to much larger values involving only Pfault with either Pmd_req or PPLe_req, but not both. For X within CAT I GBAS coverage (X <= 43 km), the PLOI(X) upper limit for very large X (Pfault x Pmd_req) is not reached. However, the PLOI(X) upper limit for X = 0 (Pfault x PPLe_req) restricts the choice of the Kmd_e_CATI parameter that corresponds to PPLe_req in VPBApr_e. The analysis shows that this restriction can be somewhat relaxed by considering the geometry limiting provided by the fault-free protection level (VPLH0(X)) also computed in the aircraft. Neither VPLH0(X) nor VPBApr_e(X) is permitted to exceed the vertical alert limit (VAL). Furthermore, VPLH0(X) is shown to be larger than VPBApr_e(X) for small values of X. Therefore, the actual risk of exceeding VAL due to ephemeris faults can be made less than a required value (PLOI_req) even though the design risk that the total error due to ephemeris fault exceeds only VPBApr_e is larger than PLOI_req.
Published in:	Proceedings of the 2011 International Technical Meeting of The Institute of Navigation January 24 - 26, 2011 Catamaran Resort Hotel San Diego, CA
Pages:	473 - 488
Cite this article:	Shively, C.A., "GNSS Satellite Ephemeris Fault Risk Concepts and Analysis for CAT I GBAS," Proceedings of the 2011 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2011, pp. 473-488.
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