Geostationary Satellite Reference Station Multipath Characterization: Using Galaxy 15 Failure to Refine the WAAS Multipath Threat

Karl Shallberg, B.J. Potter, Phillip Class

Abstract:	The Federal Aviation Administration´s Wide Area Augmentation System (WAAS) utilizes Geostationary Earth Orbit (GEO) satellites for delivery of messages that provide improved integrity, accuracy and availability to GPS/WAAS capable users. The improved availability is partially satisfied by providing a ranging capability with these GEO satellites. Providing this ranging capability requires all measurement error sources to be appropriately characterized so that integrity bounds corresponding to the ranging signal satisfy WAAS integrity requirements. One of the error sources that has proven challenging to characterize is non-zero mean multipath present with GEO pseudorange measurements. Unlike GPS multipath which generally averages to a near zero mean value over the course of its track, GEO multipath may not. WAAS has observed non-zero mean errors on the order of several meters with GEO pseudorange measurements and attributed these errors to multipath. These errors represent biased measurements and therefore require special consideration in WAAS integrity analysis. Galaxy 15 (CRW) is a GEO satellite used by WAAS that experienced a failure in April 2010, resulting in loss of all telemetry and control to this satellite. Without this control, the satellite drifted from its assigned orbital location of 133W to 97W over the course of a nine month period. Autonomous attitude control by CRW could no longer be maintained in December 2010 and resulted in loss of sun orientation and therefore satellite power. At this point CRW reset which fortuitously allowed recovery of all communications and control for this satellite. The satellite was then maneuvered back to a location close to its original orbital slot. WAAS maintained a ranging signal from this satellite throughout most of this period although integrity bounds associated with this signal were degraded as it moved away from its nominal orbital location. Drift of CRW due to this failure presents a unique opportunity to study multipath error and provide additional insight into this non-zero mean multipath characteristic. This paper characterizes multipath at WAAS reference stations as this satellite drifted and relates this to the observed stationary multipath and the multipath threat used in WAAS integrity processing. This characterization also considers the theoretical basis for non-zero mean multipath and attempts to associate the multipath error at certain reference stations where detailed site characterizations exist. It is anticipated this paper will be of interest to those studying multipath effects as well as other SBAS providers.
Published in:	Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011) September 20 - 23, 2011 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2453 - 2461
Cite this article:	Shallberg, Karl, Potter, B.J., Class, Phillip, "Geostationary Satellite Reference Station Multipath Characterization: Using Galaxy 15 Failure to Refine the WAAS Multipath Threat," Proceedings of the 24th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp. 2453-2461.
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