The Effects of Ionospheric Errors on Single-Frequency GPS Users

Richard L. Greenspan, Avram K. Tetwsky, James I. Donna, John A. Klobuchar

Abstract:	Single-frequency GPS users can compensate for over 50% (rms) of the ionospheric ranee errors in precise position determination by using a simple eight coefficient model of the ionosphere specifically designed for that purpose. The ionospheric correction coefficients are transmitted as part of the satellite message. However, there is a consistent body of experimental data from different sources, including results presented in this paper, which shows mat navigation errors resulting from ionospheric delays are smaller than one might predict, and that this trend holds even if the ionospheric correction is not applied to the timedelay measurements. These results could be interpreted to suggest that there are some GPS single-frequency users whose navigation accuracy would not suffer if they omitted the ionospheric delay correction. This paper argues that this is not the proper conclusion, except when GPS selective availability is turned on. A simple mathematical analysis is developed to show how the excess propagation delay is weighted into the navigation solution. This result is interpreted to show that it is the variation of delays from one satellite to another that drives the navigation error. Therefore, to the extent that modeling reduces the residual variability of the corrected delay measurements relative to the variability of the uncorrected delays, it results in a more accuratenavigation solution. Thisanalysisalsosuggests a new metric for obtaining the coefficients in the ionospheric model. We recommend thatGPS monitoring site data for which dual-frequency ionospheric measurements are available, be reprocessed to evaluate the effectiveness of the metric.
Published in:	Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991) September 11 - 13, 1991 Albuquerque, NM
Pages:	291 - 298
Cite this article:	Greenspan, Richard L., Tetwsky, Avram K., Donna, James I., Klobuchar, John A., "The Effects of Ionospheric Errors on Single-Frequency GPS Users," Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991), Albuquerque, NM, September 1991, pp. 291-298.
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