Ionospheric Estimation using Extended Kriging for a Low Latitude SBAS

Juan Blanch, Todd Walter, Per Enge

Abstract:	The ionosphere causes the most difficult error to mitigate in Satellite Based Augmentation Systems (SBAS). The problem has been solved for the mid latitude regions using the thin shell approximation. There, it is very accurate on quiet days and allows the augmentation system so send the information in a two dimensional grid with a five by five degree resolution. However, even during quiet days, this approximation does not model correctly the ionosphere in the low latitudes: the decorrelation of the projected ionospheric vertical delays over the thin shell is very large. Several ionospheric estimation methods have been proposed to decrease the User Ionospheric Vertical Error (UIVE), among which are the ‘conical domain’ approach, tomography and extended kriging. The conical domain approach requires several measurements from the same satellite to work properly and in tomography the equation to solve is underdetermined, leading to artificial constraints and very large estimation errors at the edge of coverage. Extended kriging was developed to avoid these problems. The idea is to use kriging with several layers and an average vertical density profile to define the covariance between measurements (unlike in previous applications of kriging, where only one layer, the thin shell at 350 km, is used). Early results show that extended kriging gives estimation errors 30% to 50 % lower than the planar fit using the thin shell model. As a consequence this method has the potential to reduce the UIVEs by the same amount, thus increasing the availability of the augmentation system. In this paper we will first recall the basics of extended kriging and the assumptions needed. Then we will present a new error analysis more adapted to disturbed ionospheric conditions and apply it to real ionospheric delay measurements taken at reference stations over Brazil. Based on this error analysis, we will propose a new Vertical Position Level equation and evaluate it using an SBAS simulation tool. The results show that, even under severe ionospheric disturbances, the 95th percentile of the Vertical Protection Level is to not too far from 50 meters.
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:	387 - 391
Cite this article:	Blanch, Juan, Walter, Todd, Enge, Per, "Ionospheric Estimation using Extended Kriging for a Low Latitude SBAS," 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. 387-391.
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