Analysis of Vertical Alert Limit for LAAS CAT III Autoland

C.A. Shively

Abstract:	For the GPS Local Area Augmentation System (LAAS), the vertical alert limit (VAL) represents an error level that can only be exceeded with a small probability (10-9) without annunciation to the user. Previously proposed standards for LAAS CAT III have dictated a minimum VAL of 5.3 m, derived from the vertical accuracy of ILS. However, subsequent analysis has predicted that satellite geometry good enough to achieve such a small VAL may not be available a large enough fraction of time. Consequently, a VAL of 10.0 m (same as for CAT I) has been proposed by Boeing and is under consideration for international acceptance as of this writing. This paper documents a comprehensive and in-depth analysis to base an acceptable VAL for CAT III on autoland airworthiness requirements as currently dictated by the FAA. The analysis focuses on total system longitudinal error (TSE) as composed of the sum of Flight Technical Error (FTE) and Navigation Sensor Error (NSE). Corresponding to these requirements the analysis is divided into the faultfree NSE (accuracy) case and the faulted NSE (integrity) case. The more stringent faulted NSE case is further subdivided into single reference receiver faults (H1 hypothesis) and ranging source faults. For a given size of NSE fault, the integrity risk is computed based on the probability the fault is not detected and the probability that other fault-free errors then cause the TSE to produce an unsafe touch down point. The worst case risk as a function of the NSE fault value is used to conservatively characterize performance. The risk is evaluated as a function of the standard deviation of FTE, which is assumed to be Gaussian. Consider first the largest FTE that achieves acceptable accuracy risk with a VAL = 10.0 m. For that FTE case and a VAL of 10 m, the proposed allocation of total integrity risk is achieved for all types of faults. Consider also the FTE performance that appears to be achieved by state-of-the-art Boeing aircraft. For that FTE case and a VAL of 10 m, the accuracy risk is achieved with a margin factor of 100 and the allocated integrity risk is achieved with a minimum margin factor of 50 for all types of faults. This method of analysis provides a rigorous mathematical basis for justifying a VAL for CAT III that can be adapted to a non-Gaussian error distribution for FTE if desired.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	339 - 351
Cite this article:	Shively, C.A., "Analysis of Vertical Alert Limit for LAAS CAT III Autoland," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 339-351.
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