Performance and Availability Analysis of a Simple Local Airport Position Domain Monitor for WAAS

Curtis A. Shively, Rick Niles and Thomas T. Hsiao

Abstract:	The FAA’s LAAS Program Office has proposed to investigate the feasibility of a Local Airport Monitor (LAM) that would use the Wide Area Augmentation System (WAAS) to enable LAAS avionics to be used for Category I (CAT I) precision approach applications. This paper describes and analyzes a particular simple LAM concept which may generally be described as position domain monitoring on the ground. In this concept the correction components broadcast by WAAS are received by the LAM, lumped together and retransmitted over a LAAS VHF data broadcast (LAAS VDB). Ground monitoring consists of simply computing a position solution applying the received WAAS corrections to locally measured pseudoranges for a given set of satellites. If the difference between this position and the surveyed position of the LAM reference antenna exceeds a threshold, the WAAS corrections are declared unusable for a CAT I approach with that satellite set. The monitor threshold is set to meet CAT I continuity requirements. Such a test is applied to all sets of satellites the LAAS avionics could consider for use. If any test fails, CAT I service using WAAS is declared unavailable at that time by removing corrections for all visible satellites from the VDB. An availability analysis for 140 CAT I locations in CONUS predicts a vertical alert limit (VAL) of 10 m can be provided with daily average availability of 0.995 for the optimal 24 GPS constellation and 0.9997 for the current (12/31/04) GPS constellation. Operation of the method on real WAAS corrections and local range measurement data indicates 100% availability and no monitor false alarms with a current satellite constellation and a quiet ionosphere. Further work includes evaluation of monitor performance under disturbed and stormy ionospheric conditions, as well as different tropospheric conditions. This method of simple position domain monitoring on the ground may be extensible to GNSS augmentation systems other than WAAS.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2837 - 2854
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
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