Ionospheric Modeling for WAAS

R. Kumar and P. Munjal

Abstract:	The ionosphere is one of the two major sources of error for WAAS. The other major source of error is the satellite clock bias introduced by intentional and random dithering of the satellite clock for Selective Availability (SA) control. Ionospheric vertical range delays of over 30 meters can occur in Hawaii, and delays of over 20 meters can occur in the CONUS region. The delays at other elevation angles are related to the vertical ionospheric delays via the obliquity factor, and at low elevations the ionospheric delays can be typically three times higher compared to the vertical delays. A network of WRSs (WAAS reference stations) makes measurements of the ionospheric delays in the slant paths to various GPS satellites in view of various WRSs. These measurements are processed by WMS (WAAS Master Station) to arrive at vertical ionospheric delays at a set of grid points across the CONUS. The avionics receiver estimates delays to satellites in its view by appropriate interpolation and processing of these grid point delays. The WAAS network also provides an estimate of the error termed GIVE (grid point ionospheric error) at the grid points from which the user determines the estimates of its own error levels, more specifically the vertical navigation error level. Such an estimate is dependent on the knowledge of the ionospheric spatial and temporal correlation function. In the absence of the knowledge of such a correlation function, a conservative estimate of the vertical navigation error may have to be used, for example, by weighted sum of the GIVES at the pierce points of the satellites in view of the user where the weighting is a function of the satellite direction cosine vector and the obliquity factor. Such a simple procedure may however be overly conservative and therefore result in much reduced availability of the Precision Approach service. To be able to obtain a more precise estimate of the vertical ionospheric error, it is required to obtain an accurate modeling of the ionospheric spatial and temporal correlation function and use this information in the computation of the user vertical navigation error. This paper presents the ionospheric measurements setup and an algorithm to achieve these objectives. Although this setup is independent of WAAS, the experimental results will be of considerable benefit to WAAS.
Published in:	Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997) June 30 - 2, 1997 Albuquerque, NM
Pages:	431 - 439
Cite this article:	Kumar, R., Munjal, P., "Ionospheric Modeling for WAAS," Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997), Albuquerque, NM, June 1997, pp. 431-439.
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