Estimating SBAS Ionospheric Delays Without Grids: The Conical Domain Approach

L. Sparks, A. Komjathy, and A.J. Mannucci

Abstract:	To ensure the accuracy and integrity of user position estimates based upon Global Positioning System (GPS) measurements, satellite-based augmentation systems (SBAS) require accurate calibration of ionospheric delays. In an SBAS such as the United States’ Wide Area Augmentation System (WAAS), slant ionospheric delay error and confidence bounds are derived from interpolated estimates of vertical ionospheric delay modeled on a grid at regularly spaced intervals of latitude and longitude. Estimation of the vertical delay at each ionospheric grid point relies on the standard thin shell model of the ionosphere to project measured slant delays to vertical. This paper presents an alternative model of slant delay measurements that allows direct computation of the user’s slant delay estimate without the intervening use of a vertical delay grid. Instead of evaluating fits at points on a grid, each fit is performed in a conical domain with a satellite at the cone vertex. The objective of this approach is to reduce or eliminate sources of error that arise both from the use of the thin shell model and from the grid interpolation. It is motivated, in part, by the need to develop alternatives for modeling slant delays at low latitudes, where complex ionospheric structure, high delay values, and large electron density gradients pose severe challenges for SBAS systems based upon the thin shell model. We present results comparing the accuracy of the conical domain approach with that achieved using the standard thin shell model. The root mean square (RMS) of the residual differences between slant delay measurements and estimates based upon model fits is reduced by up to 40% when the conical domain model is employed.
Published in:	Proceedings of the 2004 National Technical Meeting of The Institute of Navigation January 26 - 28, 2004 The Catamaran Resort Hotel San Diego, CA
Pages:	530 - 541
Cite this article:	Sparks, L., Komjathy, A., Mannucci, A.J., "Estimating SBAS Ionospheric Delays Without Grids: The Conical Domain Approach," Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2004, pp. 530-541.
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