An Evaluation of the GPS Single Frequency User Ionospheric Time-Delay Model

Susan G. Stephens, William A. Feess

Abstract:	Signal propagation time-delay caused by the ionosphere is an important consideration in satellite-ranging accuracy of the Global Positioning System (GPS). The GPS two-frequency user equipment takes advantage of the fact that the ionosphere is a dispersive medium and computes the ionospheric contribution to range error by measuring the difference in time delays between the two operating frequencies. For single frequency users (basically all non-defense users), uncompensated ionospheric propagation time-delay is the largest source of range-measurement error. To improve navigation accuracy for these users, a numeric model of ionospheric range error was incorporated into the GPS system. The GPS model consists of an ionospheric time-delay algorithm to model the diurnal behavior (J. A. Klobuchar, 1976) which uses the positive portion of a cosine wave during daylight hours with a constant night-time offset. While night-time and cosine phasing terms are held constant, third-order polynomials are used to depict cosine amplitude and period as a function of user geomagnetic latitude. The polynomial coefficients are transmitted to the user via the GPS navigation message. These coefficients are chosen daily from sets of constants to reflect the sensitivity to solar flux and seasonal variation. Pointwise comparisons were made between computed algorithm values and dual-frequency ionospheric measurements. Intervals of high, low, and gradient solar flux were selected from the years of 1980-1982, which cover the peak of an eleven year solar flux cycle. Data were obtained from the four monitoring stations
Published in:	Proceedings of the 1986 National Technical Meeting of The Institute of Navigation January 21 - 23, 1986 Long Beach, CA
Pages:	129 - 130
Cite this article:	Stephens, Susan G., Feess, William A., "An Evaluation of the GPS Single Frequency User Ionospheric Time-Delay Model," Proceedings of the 1986 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1986, pp. 129-130.
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