An Algorithm for Inter-frequency Bias Calibration and Application to WAAS Ionosphere Modeling

Yi-chung Chao, Yeou-Jyh Tsai, Todd Walter, Changdon Kee, Per Enge, and Brad Parkinson

Abstract:	The Wide Area Augmentation System (WAAS) was invented to improve both the differential GPS accuracy as well as the integrity monitoring over a large geographical region. [Kee et al, 1990, Walter et al, 19941 The ionosphere model parameters are one of the most important messages for the single frequency WAAS users. However, the embedded inter-frequency biases in both GPS satellites and dual-frequency Wide Area Reference Station (WRS) receivers make the WAAS generate biased ionospheric delay measurements. As the airborne single frequency avionics retrieve this WAAS ionospheric corrections, a systematic error will be transformed into user’s navigation solution. These biases must be reduced to obtain the best possible performance of WAAS: Furthermore, the biased ionospheric delay measurement will also offset the ionosphere-free pseudorange estimates. Thus these biases not only offset the ionosphere model but will also corrupt the WAAS corrections for satellite clocks and ephemerides. Without appropriate modeling, the bias can be as large as several meters. An alternative software calibration methodology has been developed at Stanford University for estimating these inter-frequency biases. This method allows a constant monitoring and a preliminary report has been presented earlier[Chao, et al, 19951. Since then the algorithm has been enhanced in several aspects: 1) The data from several stations are processed together rather than the previous single station processing. 2) The spherical harmonics model order is increased from 1 to 2. 3)The process is more efficient and more automatic. The new results show a better than half meter consistency. When the new biases are incorporated into Stanford’s Master Station algorithm, the ionospheric model shows greater agreement with the corrected dual-frequency measurements. Most importantly, as will be shown later, new corrections significantly improve the positioning accuracy for a single frequency user. These lower and more zero-centered navigation fixes will allow us to further analyze the system in more detail as well as monitor with better integrity.
Published in:	Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995) September 12 - 15, 1995 Palm Springs, CA
Pages:	639 - 646
Cite this article:	Chao, Yi-chung, Tsai, Yeou-Jyh, Walter, Todd, Kee, Changdon, Enge, Per, Parkinson, Brad, "An Algorithm for Inter-frequency Bias Calibration and Application to WAAS Ionosphere Modeling," Proceedings of the 8th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1995), Palm Springs, CA, September 1995, pp. 639-646.
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