Mitigation of Ionospheric Gradient Threats for GBAS to Support CAT II/III

T. Murphy, M. Harris

Abstract:	The Ground Based Augmentation System (GBAS) is based on carrier smoothed code phase differential processing. The use of a long time constant for the carrier smoothing filters reduces the magnitude of some measurement errors and leads to improved accuracy. The divergence of the code and carrier as the signal travels through the ionosphere coupled with a temporal change in the ionospheric delay introduces a bias into the output of the smoothing filter. Because the filters used by the ground based receivers and airborne receivers are matched, this iono induced error will normally be eliminated through the differential processing. However, a significant gradient in the ionosphere which results in a different delay and delay rate being observed by the airborne and ground reference receivers can result in an error that will not be removed by the differential processing. If unmitigated, this error can grow to be significantly larger than the noise and multipath effects the carrier smoothing is intended to reduce. Large ionospheric gradients that can induce such spatial/temporal decorrelation errors are apparently relatively rare in the mid latitudes but potentially more common in polar or equatorial regions. Although rare, these phenomena must be considered if GBAS is to provide integrity sufficient to support operations in CAT II/III conditions. Significant work has been done to characterize this threat and to develop a relevant threat space for mid-latitudes. The previous work focuses primarily on the characterization and mitigation of the threat for GBAS in the context of CAT I operations. This paper builds on the previous work by considering the potential mitigations for the threat in the context of GBAS supporting CAT II/III operations. This paper investigates several proposed monitoring schemes intended to protect GBAS based operations from the effects of large ionospheric gradients. These schemes involve some form of monitoring in both the ground and airborne segments of GBAS. This paper describes several proposed airborne monitoring algorithms intended to detect large gradients by observing differences between the code and carrier over time. The ability of the airborne receiver to detect ionospheric gradients in this manner depends on the magnitude and spectral characteristics of thermal noise and multipath errors experienced by the receiver. To assess the potential performance of the airborne monitors, the thermal noise and multipath observed during a previous extensive flight test program was analyzed. To the extent possible the proposed airborne monitoring algorithms were applied to the flight test data and the ability of the monitors to detect iono gradients was demonstrated. The paper also discusses appropriate thresholds for the monitors and shows how the total monitoring scheme mitigates the ionospheric gradient threat.
Published in:	Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006) September 26 - 29, 2006 Fort Worth Convention Center Fort Worth, TX
Pages:	449 - 461
Cite this article:	Murphy, T., Harris, M., "Mitigation of Ionospheric Gradient Threats for GBAS to Support CAT II/III," Proceedings of the 19th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2006), Fort Worth, TX, September 2006, pp. 449-461.
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