Adapting Kriging to the WAAS MOPS Ionospheric Grid

J. Blanch, T. Walter, P. Enge

Abstract:	The most productive way to increase the availability of single frequency users of the Wide Area Augmentation System (WAAS) is by decreasing the Grid Ionospheric Vertical Error (GIVE). Currently the GIVE’s are very conservative, since WAAS has to protect against the worst possible case of ionospheric behavior given the measurements. By characterizing more accurately the vertical ionospheric delay model in nominal conditions and by better defining the ‘well sampled’ regions we can be less conservative while maintaining integrity. It has been shown previously that an ionosphere estimation algorithm based on kriging could address these two issues efficiently. In quiet conditions, the kriging method produces at each location an estimate of the Vertical Ionospheric Delay and a confidence bound on the estimate. The confidence bounds obtained seem to be close to the lower limit of what is possible within the thin shell model –before the storm detector is applied. In addition to that, the particular behavior of the kriging variance at the edge of coverage can be used to mitigate the non-stationarity of Total Electron Content (TEC) during storms. To do that, one can either define a ‘well sampled’ region or use it as a metric of coverage. As it has been described, the algorithm would require the user to know at all times the location of all Ionospheric Pierce Points (IPPs). Unfortunately, there is not enough bandwidth for the user to receive all of this information nor would this be efficient. Instead, the user receives a grid of points, the Ionospheric Grid Points (IGPs), which is updated every 5 minutes. For each satellite, the user interpolates both the delay and the GIVEs from the four closest IGPs. However, kriging gives an optimal estimate only at the IGPs, and the kriging variance is only valid at the IGPs. Therefore the delay computed by the user is not optimal, and the confidence bound will necessarily change as we depart from the IGP. In this study, we present a calculation of the GIVE for the kriging method that protects the user at any location. Results of the algorithm will be shown using ionospheric data collected from the WAAS reference stations. We will show that, even after the increase in confidence bound caused by the grid, we can still generate GIVEs below .6 m inland and 1 m in coastal regions.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	848 - 853
Cite this article:	Blanch, J., Walter, T., Enge, P., "Adapting Kriging to the WAAS MOPS Ionospheric Grid," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 848-853.
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