Improving Availability of Ionospheric Corrections in the Low Magnetic Latitude Region

Takeyasu Sakai, Keisuke Matsunaga and Kazuaki Hoshinoo

Abstract:	The current MSAS, Japanese version of SBAS/WAAS, ionospheric correction procedure is based on the algorithm called as ‘planar fit’. It estimates the propagation delays at IGPs and broadcasts them to users within the service area. The first order estimator, relates to ‘plane’, is used in the algorithm. The previous study has shown that the algorithm works relatively well over Japan although the absolute delay is large. The availability of ionospheric corrections for MSAS is, however, relatively low due to insufficient measurements for performing planar fit. The storm detector often trips even for quiet ionosphere conditions. And potential problem is the fact that the distribution of residual errors after planar fit removal is not Gaussian, which is assumed in the overbounding algorithm of MSAS/WAAS. The authors will introduce zeroth order fit to improve availability of ionospheric corrections at IGPs. It will be used only when the standard planar fit cannot be applied due to trip of the storm detector or insufficient number of IPPs. Even with small number of IPPs, zeroth fit does provide robust estimates of vertical ionospheric delays at IGPs with necessary overbounding information. Quadratic fit is also introduced to improve the performance of estimation process. It needs good geometry of the distribution of IPPs being used for fit. The authors have tested four candidate metrics to determine if quadratic fit can be applied or not. It will be shown that HDOP and condition number metrics are functional for this purpose. Overbounding of residual errors will also be discussed. With performing zeroth fit, 5-6% of IGPs were saved from the unavailable states for a storm condition observed in October 2003. The RMS of sigma values which overbounded the largest residual error was 1.79 meters. Quadratic fit reduced about 12% of residual error in sense of RMS for 70% of IGPs. These results can be applied for the future (phase-II) MSAS implementation.
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:	569 - 579
Cite this article:	Sakai, Takeyasu, Matsunaga, Keisuke, Hoshinoo, Kazuaki, "Improving Availability of Ionospheric Corrections in the Low Magnetic Latitude Region," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 569-579.
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