Comparison of Alternative Methods for Deriving Ground Monitor Requirements for CAT IIIB LAAS

Curtis A. Shively

Abstract:	Standards are being developed to specify the performance of fault monitors needed to ensure the integrity of LAAS when used for CAT IIIb autoland operations. This paper derives fault monitor Pmd limits based on three concepts: 1) integrity of vertical NSE (VPL concept), 2) risk of unsafe landing for the “limit case” of faulted NSE and 3) risk of unsafe landing for “rare faulted” NSE. The first concept applies only to vertical NSE, while the second and third concepts consider the safety of the longitudinal touchdown point including the aircraft FTE. Performance of fault monitors depends on the type of fault (and to some extent so do the Pmd limits). Therefore, the paper addresses faults due to either a single reference receiver or a single satellite. For each type of fault the Pmd limits are compared to provide insight regarding which requirements concept gives the most restrictive limit. For single reference receiver faults, the Pmd limits based on VPL and the limit case are found to be comparable. However, for single satellite faults, the VPL-based Pmd limit is significantly more restrictive. For either type of fault, the Pmd limit based on rare faulted NSE is not restrictive in comparison to the other two limits. Compliance with these Pmd limits is illustrated for representative fault monitor performance. Such monitor performance meets the less restrictive Pmd limit based on the limit case concept with margin for both types of faults. However, detailed performance assessment of specific LAAS monitors for CAT IIIb (especially considering the short time-to-alert of 2 s) remains to be completed.
Published in:	Proceedings of the 2007 National Technical Meeting of The Institute of Navigation January 22 - 24, 2007 The Catamaran Resort Hotel San Diego, CA
Pages:	267 - 284
Cite this article:	Shively, Curtis A., "Comparison of Alternative Methods for Deriving Ground Monitor Requirements for CAT IIIB LAAS," Proceedings of the 2007 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2007, pp. 267-284.
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