High reduction of acquisition and tracking thresholds of GPS spaceborne receivers

Jean-Luc Issler, Jean Fourcade, Laurent Lestarquit, Christian Mehlen, Gilles Gamier

Abstract:	The spaceborne GPS receivers are classically used in low Earth orbit, with good GPS visibility conditions. However, some space missions require a GPS receiver operating with poor link budgets. This concern, for instance: - Reentry capsule or shuttle, making roll reversal maneuvres after the radio black out, and needing a quick reacquisition of the GPS satellites. - Geostationary satellite, during station keeping or transfer orbits, from GTO (Geostationary Transfer Orbit) to GDO (Geostationary Drifting Orbit). This concern also any satellite at high altitude. - GPS attitude determination, needing better GPS visibility conditions than GPS orbital navigation. - Degraded pointing modes of orbital platforms. - Radio occultalions, with important propagation losses due to long paths through the Earth's atmosphere. 3 types of solutions are possible to bypass a potential lack of GPS satellites visible with a good link budget: - Use of a GNSS receiver (GPS and/or GLONASS and/or GEOs) instead of a GPS receiver. - Use of directive antennas, instead of omnidirectional antennas, to improve the link budget. - Reduce the acquisition and tracking thresholds. CNES is considering these 3 types of solutions for its future projects. The third solution seems to be promising, since a new patented technique enable to reduce the thresholds below 20 dBHz, without any external aiding. This threshold improvement is obtained with a tight coupling between an orbital navigator (such as the DIOGENE navigator, developped at CNES) and the signal processing of the receiver. This enable to use one or two GPS omnidirectional antennas on a Geostationary platform to perform GPS navigation from GTO to GDO. These techniques and the associated performances are presented, versus the signal processing parameters of the code and carrier loops of the GPS receiver.
Published in:	Proceedings of the 1998 National Technical Meeting of The Institute of Navigation January 21 - 23, 1998 Westin Long Beach Hotel Long Beach, CA
Pages:	123 - 131
Cite this article:	Issler, Jean-Luc, Fourcade, Jean, Lestarquit, Laurent, Mehlen, Christian, Gamier, Gilles, "High reduction of acquisition and tracking thresholds of GPS spaceborne receivers," Proceedings of the 1998 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1998, pp. 123-131.
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