The application of a 3-D empirical ionosphere model to WAAS simulation

The Wide Area Augmentation System (WAAS) is designed to augment the Global Positioning System (GPS). One of the services provided by WAAS is a broadcast ionospheric correction based upon dual-frequency measurements. These observations are used to construct a grid of expected vertical ionospheric delays. To date, the broadcast single frequency model has been used to project vertical delay observations onto the predefined grid points. The performance of the WAAS ionospheric grid could be improved by incorporating a more realistic description of the ionosphere than this nominal transport model. In this paper we explore the use of IRI90 for this purpose by means of a software-based WAAS simulator. The quality of the broadcast WAAS ionospheric corrections is tested by comparing the ionospheric delays predicted by the WAAS grid algorithm with those derived from a truth source. Results to date indicate that using a transport model derived from IRI90 yields a small improvement over the broadcast model.