Improved Method for Satellite Acquisition

Samuli Pietilä and Jari Syrjärinne

Abstract:	When a GPS satellite positioning system receiver is turned on, it must first acquire and then track signals from at least four satellites to be able to solve three- dimensional user position and GPS time. Currently, there are 28 operational satellites in orbit. This paper focuses on how to effectively find the required four visible satellites above the horizon among the 28 possible ones. A typical GPS receiver acquires satellites sequentially. When the receiver has information about the satellite orbits (i.e. almanac), coarse time (minute level accuracy), and coarse location (kilometre level accuracy), it can compute which satellites are currently visible. In that case acquisition can concentrate on the known visible satellites only. However, if any of these three pieces of information is missing, visible satellites cannot be determined, and the receiver must start to search through all 28 possible satellites until at least four satellites are found. After that, almanac and ephemeris information is received from the satellite data broadcast, and position and time can be solved. Thereafter, the remaining visible satellites can be determined. In this paper we describe a method that makes fast acquisition of the first four visible satellites possible even when the receiver does not know which satellites are actually visible. By utilising the almanac information it is possible to adaptively search satellites, and minimise the search time wasted on satellites below the horizon. This will considerably improve the time to first fix, which is one of the major performance measures of a GPS receiver. Computer simulations with real almanac data have been carried out to verify the proposed method. Different search scenarios are tested, and simulation results show that savings of up to 50% in total acquisition time can be achieved compared to traditional methods. This reduction in acquisition time converts directly to reduction in time to first fix.
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:	1957 - 1961
Cite this article:	Pietilä, Samuli, Syrjärinne, Jari, "Improved Method for Satellite Acquisition," 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. 1957-1961.
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