Algorithms for Airborne Ionospheric Front Detection in LAAS Using Carrier Phase and INS Measurements

L. Gratton, F. Chan and B. Pervan

Abstract:	The Federal Aviation Administration is currently developing the Local Area Augmentation System (LAAS) to transition from the current instrument landing system to satellite based navigation. Due to the single frequency nature of the current LAAS architecture, spatial ionospheric decorrelation contributes significantly to the differential ranging error. During days of normal ionospheric activity, the LAAS Ground Facility (LGF) broadcasts a conservative standard deviation of the spatial ionospheric gradient (svig) to LAAS users. Under these normal circumstances, navigation integrity is ensured by incorporating svig into the computation of position domain protection levels. However, anomalies exhibiting abrupt changes in the ionospheric gradient have been observed during ionospheric storms in October and November 2003. Therefore, monitoring algorithms are necessary for LAAS to detect these hazardous ionospheric anomalies without affecting the system’s availability and continuity significantly. A three parameter (front width, gradient, and front speed) ionospheric threat model has been proposed by Stanford University and significant research has been devoted to the development and analysis of LGF and airborne codecarrier divergence monitors. There are limitations on the effectiveness of these monitors, as they depend on the rate of change of the delay with time. The most hazardous threat in this regard is a static ionospheric wave front and this is the case considered. Two new airborne ionospheric integrity-monitoring algorithms are investigated. The first one uses double difference carrier phase measurements in a Receiver Autonomous Integrity Monitoring (RAIM) type algorithm to detect ionospheric wave fronts. The second one incorporates inertial sensor measurements to address certain regions of the threat space. Ground monitors are also discussed. They are based on detecting the front gradient using the baselines formed by the very precise Integrated Multipath Limiting Antennas (IMLA) already existing in the present LGF configuration. This paper shows wider fronts can be detected regardless of their gradient (within the threat model), and the ideas for dealing with thinner fronts are also presented.
Published in:	Proceedings of the 2005 National Technical Meeting of The Institute of Navigation January 24 - 26, 2005 The Catamaran Resort Hotel San Diego, CA
Pages:	131 - 139
Cite this article:	Gratton, L., Chan, F., Pervan, B., "Algorithms for Airborne Ionospheric Front Detection in LAAS Using Carrier Phase and INS Measurements," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 131-139.
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