GBAS GAST-D (CAT IIIB) Aircraft Monitor Performance Requirements for Single Referene Receiver Faults

Curtis A. Shively

Abstract:	FAA, industry and the user community are currently developing fault monitor performance standards for application of the GPS Ground Based Augmentation System (GBAS) to provide Category (CAT) IIIB / GBAS Approach Service Type (GAST)-D. This paper analyzes three monitor concepts to mitigate a fault in a single ground reference receiver (RR). All three concepts are based on B-values computed in the ground station to estimate the possible fault error by comparing consistency of the differential corrections among multiple RRs. The baseline concept (similar to that used for CAT I / GASTC) provides an indirect monitor by comparing an error bound (VPLH1) using the B-value error estimates to the vertical alert limit (VAL) in the aircraft. Another alternative concept uses the VPLH1 monitor in conjunction with taking credit for monitoring of the B-values in the ground station before broadcast. In the third concept the error estimate in the aircraft is compared directly to a threshold (lower than VAL). All three methods are compared to new CAT IIIB monitor requirements based on safety of landing (rather than vertical error). Monitor performance is characterized in terms of probability of missed detection necessary to meet both “limit-case” and “malfunction-case” requirements. These requirements are based on representative aircraft characteristics and glide path angle values of 2.5 deg (most demanding) and 3.0 deg (more typical). The analysis also characterizes the worst, and better than worst, satellite geometries needed to meet fault-free error integrity requirements. Any additional restrictions on satellite geometry needed for proper monitor performance are also considered. On the basis of these analyses, the new “direct” monitor in the aircraft is recommended for its ability to meet even the most demanding performance requirements without restricting availability of satellite geometry and with minimal change from the CAT I / GAST-C architecture.
Published in:	Proceedings of the 2009 International Technical Meeting of The Institute of Navigation January 26 - 28, 2009 Disney's Paradise Pier Hotel Anaheim, CA
Pages:	903 - 916
Cite this article:	Shively, Curtis A., "GBAS GAST-D (CAT IIIB) Aircraft Monitor Performance Requirements for Single Referene Receiver Faults," Proceedings of the 2009 International Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2009, pp. 903-916.
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