LAAS Integrity Risk Due to Satellite Ephemeris Faults

Curtis A. Shively

Abstract:	Standards are under development for use of the FAA’s Local Area Augmentation System (LAAS) for both Category I (CAT I) precision approach and Differentially Corrected Positioning Service (DCPS) area navigation applications. An ephemeris fault occurs when the actual satellite position in space differs from the position based on the broadcast ephemeris parameters by more than the usual tolerance. Of particular interest is the total integrity risk ensured by an ephemeris fault detection process of the LGF in conjunction with protection level (PL) equations that limit geometry of satellites used in the position solution of the aircraft. This paper uses a novel approach to develop a mathematical expression for the resulting risk that the applicable (vertical, lateral or horizontal) alert limit will be exceeded due to undetected ephemeris error in conjunction with other fault-free satellite ranging errors. As usual, the fault detection process in the LGF is characterized by a minimum detectable error (mde) with an associated probability of missed detection Pmdreq. The ephemeris protection level is also characterized by an allocation for the fault-free errors with associated probability Pffo. Because the noise in the LGF detection process is independent of the other fault-free errors, the overall risk is predicted to be nearly as small as the product PfaultPmdreqPffo, where Pfault is the a priori probability of an ephemeris fault. The allowable risk due to ephemeris faults is derived for comparison to the predicted risk. These derivations and analyses are first presented in the context of the CAT I application. Predicted risk for the DCPS application is then obtained merely by changing the value of a single parameter in the CAT I expression. For both CAT I and DCPS predicted risk meets prospective requirements for Pffo = 10 -3 and Pmdreq = 10 -4 assuming noise characteristics observed in the ephemeris error detection process. The paper also shows the risk resulting when a separate ephemeris protection level equation is not applied in the aircraft. In that case acceptable risk is provided if the mde of the ground test can be made sufficiently small. For all cases analyzed the risk is mathematically independent of the alert limit, since the alert limit determines both the worst geometry allowed and the maximum tolerable position error.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	1711 - 1737
Cite this article:	Shively, Curtis A., "LAAS Integrity Risk Due to Satellite Ephemeris Faults," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 1711-1737.
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