An Ionosphere Estimation Algorithm for WAAS Based on Kriging

Juan Blanch

Abstract:	GPS alone cannot provide the integrity needed for air navigation. Several error sources deteriorate the precision of the position estimate. One of the largest and more unpredictable sources of error for single frequency users is the ionosphere. For this reason, ionospheric behavior drives the performance of the Wide Area Augmentation System (WAAS). At any given time, the only information we have of the ionosphere is a limited amount of Total Electron Content (TEC) measurements. As a consequence, in order to estimate the ionospheric delay and get a confidence bound on such an estimate, we need to understand the spatial structure of the ionosphere over the region of interest. Using the thin shell model framework, where each TEC measurement is identified as a location on the thin shell, labeled the Ionospheric Pierce Point (IPP), the problem is reduced to a 2-dimensional problem. Once we have a good description of a nominal ionosphere, there are two questions that need to be answered before estimating the delay at a given IPP: Are the IPP measurements compatible with the assumed nominal model of the ionosphere? How relevant are the IPP measurements to the location we need to estimate? To answer the first one, an accurate characterization of the ionosphere in nominal conditions is needed. The large observed stationarity violations make this latter question very difficult. A worst case based method to determine the spatial structure of the nominal ionosphere in terms of the variogram, or, equivalently, the covariance is presented. The technique called "kriging" produces at each location an estimate and a confidence bound on the estimate, the kriging variance. The particular behavior of the kriging variance at the edge of coverage allows us to intuitively define the "well sampled" region. We show that a carefully designed estimation algorithm based on kriging could provide confidence bounds on the ionospheric delay corrections allowing WAAS to meet the GNSS Landing System requirements.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	816 - 823
Cite this article:	Blanch, Juan, "An Ionosphere Estimation Algorithm for WAAS Based on Kriging," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 816-823.
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