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

摘要

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 P_md 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 P_md limits). Therefore, the paper addresses faults due to either a single reference receiver or a single satellite. For each type of fault the P_md limits are compared to provide insight regarding which requirements concept gives the most restrictive limit. For single reference receiver faults, the P_md limits based on VPL and the limit case are found to be comparable. However, for single satellite faults, the VPL-based P_md limit is significantly more restrictive. For either type of fault, the P_md limit based on rare faulted NSE is not restrictive in comparison to the other two limits. Compliance with these P_md limits is illustrated for representative fault monitor performance. Such monitor performance meets the less restrictive P_md 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.