A Study of Nominal Ionospheric Gradient for GBAS (Ground-based Augmentation System) in Japan

T. Yoshihara, S. Saito, N. Fujii

Abstract:	In GBAS (Ground-Based Augmentation System), a large spatial gradient often produces a significant range error through carrier smoothing processing. GBAS broadcasts a parameter of sigma vertical ionospheric gradient (?vig) as an ionospheric evaluation parameter for computing a protection level (PL) at aircraft. In safety design of GBAS, it is required to design and determine an upper bound for ionospheric spatial gradients which should be considered by protection levels although the remained ionospheric risk should be regarded as anomaly. Therefore, it is important to investigate statistical characteristics of ionospheric gradients under nominal conditions. We investigated nominal ionospheric gradients in activities for GBAS safety design in Japan using data set of a dense GPS network called as GEONET (GPS earth observation network) with a period from 1998 to 2009. Kyoto University provides Total Electron Contents (TECs) database using GEONET observational data with a correction of inter frequency biases. We examined ionospheric gradients at dense areas by applying a linear fitting method to TECs for a common GPS satellite at four stations within a horizontal scale of about 40 - 90 km. Then, we used TEC data under some conditions, which were a cut-off elevation angle of 30 degrees and an exclusion of data for magnetic storm periods with Kp index of more than 3 in order to investigate characteristics of occurrence distribution curves under nominal conditions. We calculated ionospheric gradients for each South to North (S-N) and West to East (W-E) direction. The center part of a Probability Density Function (PDF) of ionospheric gradients shows dependencies on solar activity. Additionally, a variation range of the center part in the S-N direction is larger than the W-E direction due to the equatorial anomaly, which produces ionospheric gradients toward the south direction. Because it is also important to evaluate tail parts of occurrence distribution curves, a bounding sigma for a PDF with Gaussian curve is also investigated. The bounding sigmas at a probability of 10-3 of PDFs change by a factor of three or more with solar activity. They also tend to be larger at the southern regions than the northern and the results are consistent with characteristics of the equatorial anomaly. On the other hand, tendencies of latitudinal variations among bounding sigmas at a probability of 10-4 are not so clear. Because large gradients at tail parts include erroneous data such as effects of estimated error in inter frequency biases, it is also required to carefully investigate and extract true ionospheric gradients with a distinction from such errors.
Published in:	Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010) September 21 - 24, 2010 Oregon Convention Center, Portland, Oregon Portland, OR
Pages:	2689 - 2694
Cite this article:	Yoshihara, T., Saito, S., Fujii, N., "A Study of Nominal Ionospheric Gradient for GBAS (Ground-based Augmentation System) in Japan," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2689-2694.
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