GBAS Differentially Corrected Positioning Service Ionospheric Anomaly Errors Evaluated in an Operational Context

Tim Murphy, Matt Harris, Young Shin Park, Sam Pullen

Abstract:	GBAS is a short-baseline differential GNSS system based on carrier-smoothed code phase measurements. One source of error in the system is the possible decorrelation of ionospheric delay between the ground system and the airborne equipment as the baseline increases. Typically, the ionosphere is well behaved over short distances, and the residual error after application of differential corrections is small. However, during solar storms and in geomagnetic equatorial regions, anomalies in the ionosphere can, on rare occasions, result in significant changes in observed ionospheric delays over relatively short distances. The impact of these anomalous ionospheric conditions has been studied extensively for the precision approach applications of GBAS. The standards for GBAS include two types of service: the precision approach service, which provides deviation information relative to a defined final approach segment path, and the Differentially Corrected Positioning Service (DCPS), which provides differentially corrected latitude and longitude for use by flight management systems for a range of applications, including flying RNAV or RNP operations. Significant work has been done to mitigate the potential impact of errors induced by ionospheric anomalies on the precision approach service. For the DCPS, the potential baseline between the ground station and the user is much longer; consequently the potential errors that could be induced by an ionospheric anomaly are larger. Furthermore, because the DCPS must support many different flight operations, the ground-system geometry screening that is targeted to protect precision approach is insufficient to protect the DCPS. Initial studies of ionospheric anomaly errors and the DCPS led the FAA to withhold approval of the DCPS for the first GBAS ground station to be granted System Design Approval. This study re-evaluates the problem of DCPS errors induced by ionospheric anomalies by examining these errors in the context of how DCPS is anticipated to be used in the terminal area in support of specific RNAV/RNP operations. The study also considers the maximum undetected errors in the context of ADS-B applications given the current proposed separation standards to be supported.
Published in:	Proceedings of the 2010 International Technical Meeting of The Institute of Navigation January 25 - 27, 2010 Catamaran Resort Hotel San Diego, CA
Pages:	394 - 410
Cite this article:	Murphy, Tim, Harris, Matt, Park, Young Shin, Pullen, Sam, "GBAS Differentially Corrected Positioning Service Ionospheric Anomaly Errors Evaluated in an Operational Context," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 394-410.
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