Real-Time Ionospheric Monitoring System Using the GPS

Anthea J. Coster, E. Michael Gaposchkin, Lorraine E. Thornton

Abstract:	In satellite tracking using ground based radars, an estimate of the total electron content along the path to the satellite is required to measure accurately the satellite’s range. This is because the radar wave travels at a slower speed as it propagates through the ionosphere. The range error, AR, that is introduced is dependent on the radar frequency, f, and on the electron content (TEC) along the propagation path, AR (meters) = (no possible to copy Formula here) where N. is the local electron density and R the radar range. The total electron content can vary significantly with time of day, geomagnetic activity, and look direction. A real-time synoptic ionospheric monitoring system has been developed using data acquired from a TI4OOO GPS receiver for use at the Millstone Hill satellite tracking radar. The TI4100 GPS receiver can track up to four GPS satellites at any one time. Each GPS satellite transmits signals at two different L-band frequencies, Ll (1575.42 MHz) and L2 (1227.6 MHz). The total electron content along the path to each satellite can be determined by combining both frequencies using the pseudo-range and the phase data. At Millstone, the TEC is measured every three seconds for each GPS satellite in view. This data is input into a Kalman filter which is used to predict the coefficients of a simple TEC model with azimuth and elevation dependence. This model takes advantage of the real-time knowledge provided by the GPS data of the variations in TEC around the Millstone location. The coefficients for this model are then sent to the satellite tracking computer, and the model is applied in real-time to account for the ionospheric path delay to whatever satellite is currently in track. The preliminary results of using this ionospheric monitoring system at Millstone will be discussed. The zenith value of the TEC predicted by our GPS model will be compared with another ionospheric measurement, the fof2 obtained from the collocated University of Lowell Digisonde. The TEC values predicted by our GPS model during both geomagnetically quiet and disturbed time periods will be discussed, as well as those associated with high solar flux time periods. Finally, the improvement in our radar system calibration due to the use of the new GPS model will be demonstrated. This improvement is evident in observations of the average range residual over a pass of the Lagoes satellite. The standard deviation of these residuals drops from approximately 20 TEC units to less than 5 TEC units when the new GPS model is used to estimate the ionospheric refraction correction. It is our opinion that, outside of upgrading all satellite tracking radars to dual frequency capability, the GPS is currently the best monitoring system of the TEC available.
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:	299 - 308
Cite this article:	Coster, Anthea J., Gaposchkin, E. Michael, Thornton, Lorraine E., "Real-Time Ionospheric Monitoring System Using the GPS," Proceedings of the 4th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1991), Albuquerque, NM, September 1991, pp. 299-308.
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