Mitigating Ionospheric Threat Using a Dense Monitoring Network

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

Abstract:	The ionospheric correction procedure of the current MSAS, the Japanese version of SBAS/WAAS, is built on the algorithm named the ‘planar fit.’ It estimates the ionospheric propagation delays at IGPs using a first order estimator, and the associated confidence bound broadcast for computation of protection levels, based on the formal variance of the estimate. In such an estimation process, there are threats regarding mismodeling of the ionosphere and undersampling so it is necessary to inflate the confidence bound (GIVE values) to ensure that the integrity requirement is met. This inflation leads to relatively large GIVE values and thus lowers availability of the system for flight modes with vertical guidance. In Japan, we have a continuous GPS observation network, known as the GEONET, operated by the Geographical Survey Institute of Japan. GEONET has over one thousand realtime GPS stations distributed inside Japan with an average separation of 20-30 km. This paper investigates integrating GEONET with MSAS for generation of ionospheric corrections. The authors have developed a new ionospheric correction algorithm optimizing ionospheric delay residuals based on the observations obtained by GEONET. The current planar fit algorithm is not optimal for such a situation as the algorithm must be capable of handling a very large number of IPP observations. Our analysis is currently based on the GPS observation dataset obtained at all available GEONET sites with 30-second sampling. It allows us to conduct analyses not only for the recent conditions when the ionosphere may be quiet but also for historical storm events because GEONET maintains its database for a period of more than ten years. The ionospheric corrections generated from GEONET improved correction performance, and more importantly, could reduce the GIVE values which impact the uncertainty of ionospheric corrections and are used for computation of protection levels. Due to the high density of monitor stations and IPP observations, these corrections could reduce the inflation factor and the spatial threat term, thus minimizing GIVE values. According to our simulation using prototype SBAS software, the proposed algorithm possibly enables APV-II flight mode (VAL = 20 m) at most Japanese airports including the single frequency system. The current MSAS provides APV-I flight mode only at the center of mainland of Japan.
Published in:	Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007) September 25 - 28, 2007 Fort Worth Convention Center Fort Worth, TX
Pages:	927 - 938
Cite this article:	Sakai, Takeyasu, Matsunaga, Keisuke, Hoshinoo, Kazuaki, Ito, Ken, Walter, Todd, "Mitigating Ionospheric Threat Using a Dense Monitoring Network," Proceedings of the 20th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2007), Fort Worth, TX, September 2007, pp. 927-938.
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