Carrier Phase Airborne and Ground Monitors for Ionospheric Front Detection for Category III LAAS

L. Gratton, 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 an approach. During days of normal ionospheric activity, the LAAS Ground Facility (LGF) broadcasts a conservative standard deviation of the spatial ionospheric gradient (óvig) to LAAS users. Under these normal circumstances, navigation integrity is ensured by incorporating óvig into the computation of position domain protection levels. However, anomalies exhibiting abrupt changes in the ionospheric gradient have been observed during ionospheric storms. Therefore, monitoring algorithms are necessary for LAAS to detect these hazardous ionospheric anomalies. A three parameter (front width, gradient, and front speed) ionospheric threat model has been proposed and significant research has been devoted to the development and analysis of LGF and airborne code-carrier divergence monitors in the past. However, 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. In previous work a differential carrier phase Receiver Autonomous Integrity Monitoring (RAIM) was introduced evaluating its effectiveness for Category I approaches. This work extends the analysis to Category III approaches, for the whole spectrum of fronts within the threat model, determining what combinations of widths and gradients are detectable, and what is the associated availability cost to ensure integrity. Recent adjustments to the Threat model were taken into account. New algorithms to reduce computation time were also introduced, as the more stringent requirements for Category III made the algorithm running time prohibitive. The availability loss incurred by the implementation of the carrier phase RAIM monitor is presented for different locations. It is shown that this loss is negligible for many width-gradient combinations. It is also shown it is tolerable for all width-gradients combinations if the monitor only operates during a storm alarm. Basic ideas on how this alarm could be triggered are mentioned in the paper.
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:	359 - 366
Cite this article:	Gratton, L., Pervan, B., "Carrier Phase Airborne and Ground Monitors for Ionospheric Front Detection for Category III LAAS," 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. 359-366.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In