Possibilities for GPS Time Recovery with GSM Network Assistance

Jari Syrjarinne

Abstract: Exact knowledge of time and accurate timing are the key factors in GPS positioning. Exact time is needed in calculation of satellite positions and accurate timing is naturally needed in ranging. In a standalone GPS receiver, both of these functions are accomplished by utilizing broadcasted satellite signals. Satellite signals transmit information about accurate GPS time and they also contain the means to measure ranges with extreme accuracy. Getting exact time and timing are in this case totally dependent on the receiver's ability to receive the satellite signals and to demodulate the information contained in them. Should either of these functions fail, the same will of course happen to the positioning ability of the receiver. Failures usually occur in urban canyons, indoors or in shaded areas where the received signals are too noisy to be demodulated without tolerable amount of errors or are too attenuated or distorted by the multipath components to be received and used reliably in ranging. Fortunately, difficulties in getting the exact time can be compensated for by providing assistance to the receiver, e.g. in the form of absolute reference time. Assistance can be delivered, e.g., through a cellular network if the GPS receiver is integrated into a cellular phone. Unfortunately, the problems relating to multipath mitigation are not directly removable by external assistance. In this paper, the time delivery and time recovery methods using the assistance data available from GSM networks are presented and discussed. The paper also explains what kind of information has to be delivered to the integrated GPS receiver to enable accurate time recovery and positioning. The ideas given here are of course directly applicable also in other type of cellular networks. The GSM network and the related standard (ETSI) provides a good reference since the GSM standard supports both the (mobile station) MS-based and MS- assisted GPS methods.
Published in: Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000)
September 19 - 22, 2000
Salt Palace Convention Center
Salt Lake City, UT
Pages: 955 - 966
Cite this article: Syrjarinne, Jari, "Possibilities for GPS Time Recovery with GSM Network Assistance," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 955-966.
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