Evaluating Ionospheric Effects on SBAS in the Low Magnetic Latitude Region

Takeyasu Sakai, Keisuke Matsunaga, and Kazuaki Hoshinoo, Todd Walter

Abstract:	The signal propagation delay caused by ionosphere is now recognized as a major error source for satellite navigation systems. Because ionosphere is generated by solar radiation and affected by the geomagnetic field, the associated signal delay is very large and has significant variance in the low magnetic latitude region. The ionospheric delay problem is currently the major concern for MSAS program (Japanese version of SBAS/WAAS) which includes the near equatorial region in its service area and therefore might experience significant performance degradation. The current SBAS ionospheric correction procedure is based on the algorithm called as ‘planar fit’. It estimates the propagation delays at IGPs located at every five by five degrees in geographic latitude and longitude, and then broadcasts them to users within the coverage. The first order estimator, relates to ‘plane’, is used in the algorithm. This procedure is designed based on the observations and experiences over US CONUS, but we do not know how well this works in the low magnetic latitude region. At first the authors have analyzed the actual ionospheric effects over Japan during quiet and storm conditions. According to this analysis, although the absolute signal delay and its variance are relatively large, the planar fit could describe the spatial distribution of vertical ionospheric delays very well, and resulted in variance of residuals, sigma-decorr, of around 35-40 cm, which agrees with the value in CONUS. Then we have evaluated the performance of the current SBAS ionospheric correction procedure over Japan. The first order (planar fit) correction does, fortunately, work well although the absolute delay is large. We also tried the second order correction but it seemed not practical. The authors also have demonstrated the performance of the storm detector currently employed for MSAS. Even for nominal condition, most IGPs will be determined as storm conditions around Japan by the current detector. This fact implies that the detector might need to be modified slightly for improving availability of MSAS. Three candidate algorithms for alternative storm detector have been tested, and all responded with low false alarm rate relative to the current algorithm.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	1318 - 1328
Cite this article:	Sakai, Takeyasu, Matsunaga, Keisuke, Hoshinoo, Kazuaki, Walter, Todd, "Evaluating Ionospheric Effects on SBAS in the Low Magnetic Latitude Region," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 1318-1328.
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