Vertical Protection Levels for a Local Airport Monitor for WAAS

Jason Rife, Sam Pullen, Todd Walter and Per Enge

Abstract:	"Certification challenges and higher than anticipated development costs for the Local Area Augmentation System (LAAS) have motivated the investigation of alternative approaches for achieving Category I precision approach and landing service. One proposed alternative is the Local Airport Monitor (LAM). This concept would rebroadcast Wide Area Augmentation System (WAAS) differential corrections in LAAS format while using an on-airport receiver to monitor the WAAS corrections and tighten the broadcast error bounds to the level required for a Category I approach. The LAM proposal would thus merge the capabilities of WAAS and LAAS to achieve Category I with an architecture similar to that proposed for precision approach with the Ground-Based Regional Augmentation System (GRAS). This paper examines a method for implementing the LAM using a range-domain concept for the LAM ground facility. The cornerstone to the implementation is a modified Vertical Protection Level (VPL) equation that takes into account the discrepancy between the WAAS pseudorange corrections and the locally measured pseudorange corrections. This modified protection level equation can be implemented without requiring any changes to existing airborne receiver equipment. Specifically, through a careful manipulation of the VHF Data Broadcast (VDB) message, the aircraftÕs existing protection levels are transformed into the desired LAM protection levels. Simulation indicates that a baseline LAM implementation achieves reasonable integrity and availability performance even without a specialized multipath limiting antenna. However, baseline availability and continuity are severely degraded for a Vertical Alert Limit (VAL) below 12 m. Two optional modifications are thus introduced to augment availability in support of a 10 m VAL. The first technique exploits prior knowledge of the WAAS error distribution, and the second computes continuity risk on an ensemble basis, rather than a specific (worst-case) basis. Simulations of these modifications suggest they provide acceptable availability, even for a VAL of 10 m."
Published in:	Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005) June 27 - 29, 2005 Royal Sonesta Hotel Cambridge, MA
Pages:	745 - 758
Cite this article:	Rife, Jason, Pullen, Sam, Walter, Todd, Enge, Per, "Vertical Protection Levels for a Local Airport Monitor for WAAS," Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005), Cambridge, MA, June 2005, pp. 745-758.
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