SBAS Ionospheric Correction with Minimalization of the Ionospheric Threat

Takeyasu Sakai, Mitsunori Kitamura, Takahiro Aso, and Kazuaki Hoshinoo

Abstract:	Ionospheric delay is recognized as a major error source for satellite navigation systems. The ICAO SBAS (satellite-based augmentation system), defined by SARPs (international standards and recommended practices) documents, has a capability to make a correction to ionospheric delay effects on GNSS in addition to satellite clock and orbit. It broadcasts users the vertical ionospheric delays in meters at the grid points (IGP; ionospheric grid point) located at every five by five degrees in latitude and longitude. The SBAS is the international standard system for global and seamless satellite-based navigation, so it would be used even in the equatorial or low latitude regions, while in fact the ionosphere has the significant activities in such regions. The equatorial anomalies affect on the large-scale structure of electron density of ionosphere which perhaps might be difficult to be corrected by SBAS ionospheric correction messages. SBAS providers who serve to the magnetic equator and the low magnetic latitude regions need to investigate ionospheric effects in such regions in terms of SBAS ionospheric error correction. Actually Japan has been operating its own SBAS system called MSAS (MTSAT-based satellite augmentation system). It is covering wide range of latitude and the lowest magnetic latitude in the coverage is nearly ten degrees magnetic north, where is very close to the equatorial region. Especially for predicting and improving actual performance of MSAS, we have been investigating the ionospheric effects around Japan. According to the analysis of MSAS shows that some ionospheric correction has a large uncertainty due to less number of observations around the IGP which induces large protection levels and lowers availability of the system. The central mechanism of this behavior is due to the response of the storm detector which is applied to detect the condition that the assumption that the distribution of residual errors is the normal (Gaussian) may not be valid. While this mechanism contributes for protection of users from an invalid estimation of ionosphere, it is shown that the storm detector makes a lot of false trips with the ionosphere in the low magnetic latitude; It is important that false trips of storm detector lower the availability of MSAS. In order to avoid false trips of the storm detector, the authors have investigated some algorithms to make some adjustment to the fitting radius of planar fit. According to simulation results, Storm Avoidance Algorithm reduces the uncertainty of estimation while Metric Reduction Algorithm slightly improves residual error of the fit. In the paper, the authors discuss the problem to be solved, the basic idea of the proposed method, and some additional requirements for the threat model. Then practical results simulated with the actual data are shown. The proposed method would contribute to the performance improvement of the SBAS at the equatorial and low latitude regions.
Published in:	Proceedings of the 2017 International Technical Meeting of The Institute of Navigation January 30 - 2, 2017 Hyatt Regency Monterey Monterey, California
Pages:	1049 - 1056
Cite this article:	Sakai, Takeyasu, Kitamura, Mitsunori, Aso, Takahiro, Hoshinoo, Kazuaki, "SBAS Ionospheric Correction with Minimalization of the Ionospheric Threat," Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, January 2017, pp. 1049-1056. https://doi.org/10.33012/2017.14943
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