Modified Ionospheric Correction Algorithm for the SBAS Based on Geometry Monitor Concept

Takeyasu Sakai, Keisuke Matsunaga, Kazuaki Hoshinoo and Todd Walter

Abstract:	The ionospheric correction procedure of the current MSAS, Japanese version of SBAS/WAAS, is built on the algorithm called as ‘planar fit’. It estimates the ionospheric propagation delays at IGPs using the first order estimator, relates to ‘plane’, and the storm detector is implemented to determine if there is storm condition or not to protect users from large correction errors. The previous study has shown that the algorithm works relatively well over Japan although the absolute delay is large due to geomagnetic effects. A potential problem is, however, the fact that the distribution of residual errors after planar fit removal is not Gaussian, which is assumed in the storm detector algorithm of MSAS/WAAS. During storm conditions, the availability of ionospheric corrections for MSAS tends to lower significantly due to trip of storm detector even the actual error is small. Another problem is that the current storm detector determines ionospheric conditions based on the observed residuals. This approach, dependent upon observations, cannot avoid undersampling threat; We never observe where observation does not exist. In fact, chi-square statistics implicitly expects a good geometry of observations. A small chi-square value does not guarantee that the actual error distribution is normal due to a possible undersampling situation. The authors will introduce geometry monitor concept to overcome these problems. Using an appropriate metric indicating goodness of the geometry of observations, it is possible to bound the actual error by a function of the metric. The relative centroid metric (RCM) and condition number metric (CNM) are evaluated for this purpose. These metrics can also be applied to determine which model, or the order of fit, is better for fit. Employing an adaptive fit algorithm based on the geometry monitor, the availability of the ionospheric corrections improves from 70-80% to over 99%. Missed detection conditions are eliminated with little enlargement of GIVE values.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	735 - 747
Cite this article:	Sakai, Takeyasu, Matsunaga, Keisuke, Hoshinoo, Kazuaki, Walter, Todd, "Modified Ionospheric Correction Algorithm for the SBAS Based on Geometry Monitor Concept," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 735-747.
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