Reduction of time to first fix in an SATPS receiver

(i9) United States (12) Reissued Patent

Loomis et al.

A method for fast acquisition, in as little as 6-15 seconds, of signals from a satellite in a Satellite Positioning System (SATPS), such as GPS or GLONASS, that does not require permanent storage of satellite ephemeris information at an SATPS ground station. This SATPS signal acquisition method can be used whenever the "new" station initially powers up or has lost lock on one or more SATPS signals that must be (re)acquired. A reference SATPS station provides the new SATPS station with an estimated reference station location and ephemeris information for one or more identified SATPS satellites visible from the reference station. The new station receives and uses this information to establish carrier frequency ranges to search for the identified SATPS satellite, by limiting the search to a reduced frequency range based upon estimated Doppler shift of SATPS signals received from this satellite. The actual frequency shift may differ from the estimated Doppler shift, due in part to errors in a frequency source used by the new station. When a first SATPS satellite signal is acquired and locked onto by the new station, the error in the new station frequency source is estimated, and the frequency range for searching for an SATPS signal from another satellite is reduced. Acquisition of additional SATPS satellite signals occurs more quickly. This system also allows the use of less accurate timing sources for the new stations. A new station need not store ephemeris information for the SATPS satellites but may call upon and use the ephemeris information available at the reference station.

12 Claims, 4 Drawing Sheets

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