Fast Acquisition Techniques For G.P.S. Receivers

Stephane Lannelongue, Pedro Pablos

Abstract:	For the time being, performances of navigation space systems in urban environment are limited. Because of the presence of buildings and trees, the availability of the service is quite poor. Furthermore limitations of actual receivers in terms of acquisition time do not allow users to take advantage of satellites available for a very short period of time. Therefore, decreasing the time necessary for GPS receivers to measure pseudo-ranges should improve the accuracy, availability and continuity of a navigation space system in urban environment. This article describes different concepts aiming at improving the performance of GPS receivers in terms of acquisition speed. It deals mainly with the presentation of a positioning process able to give a fix with a few milliseconds of signal. It consists of a code acquisition of the GPS signal in open loop in order to get the delays of the GPS satellite codes in front of the local codes. Those parameters, called Times of Arrival (ToA), are then processed with an existing algorithm to get a fix with neither bit nor frame synchronisation. More specifically, the performance of GPS receivers in open loop is studied. Different acquisition methods in the frequency domain, which allow speeding up the process, are assessed. Results on code acquisition time and ToA measurement accuracy are presented. Next, theoretical concepts of the existing algorithm are explained and tested with software simulations. Solutions to improve its stability and its robustness are proposed. Furthermore, a new technique aiming at increasing its convergence performances is explained. The third part is dedicated to the feasibility of this new process with real data. GPS signals are generated either by the real GPS constellation or by a GPS RF simulator. This signal is down converted, sampled and stored in real time. Then the data are post processed. The results foreseen in simulations are confirmed. The possibility of getting fixes without bit or frame synchronisation is demonstrated. As this process allows reducing the acquisition time of one order of magnitude (from 50 seconds to a few seconds), it makes possible in good conditions, the use of space navigation systems in urban environment.
Published in:	Proceedings of the 54th Annual Meeting of The Institute of Navigation (1998) June 1 - 3, 1998 The Adams Mark Hotel Denver, CO
Pages:	261 - 269
Cite this article:	Lannelongue, Stephane, Pablos, Pedro, "Fast Acquisition Techniques For G.P.S. Receivers," Proceedings of the 54th Annual Meeting of The Institute of Navigation (1998), Denver, CO, June 1998, pp. 261-269.
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