Comparison of Real-Time Ionospheric Algorithms for a GPS Wide-Area Augmentation System (WAAS)*

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Abstract
The Federal Aviation Administration (FAA) Satellite Program Office is developing a GPS Wide-Area Augmentation System (WAAS) to support a precision approach capability down to or near the lowest Category I (CAT I) decision height (DH) of 200 ft. In one of the candidate architectures under development, a vector of corrections is sent to the user via geostationary communications satellites (e.g., Inmarsat). This correction vector includes components for ionospheric, clock, and ephemeris corrections. The purpose of this paper is to evaluate the performance of the grid-based algorithms and other real-time ionospheric algorithms that could be implemented at the ground ionospheric reference stations, as well as at the airborne receiver. Results show that all of the ionospheric algorithms used in this paper (grid-based, least-squares, and spherical harmonics) provide roughly equivalent performance. Based on an extensive data collection program, the error in estimating ionospheric delay is derived. An analysis of WAAS accuracy performance is also presented.