Reduction of time to first fix in an SATPS receiver

Abstract

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.

Description

FIELD OF THE INVENTIONBACKGROUND OF THE INVENTIONWhen a Satellite Positioning System (SATPS) receiver / processor powers up, or when the receiver / processor experiences SATPS signal interruption, if the receiver / processor has no almanac that indicates the present location of the visible SATPS satellites, the receiver / processor and associated SATPS antenna will perform a blind satellite search to find a sufficient number of SATPS satellites, usually three or more, to begin establishing the antenna's SATPS-determined location and / or proper time. The SATPS antenna and receiver / processor will usually select SATPS satellite numbers at random for the search. This procedure will often consume several tens of seconds before "lock" on an adequate number of SATPS satellite signals is achieved. For an SATPS receiver / processor with five or more channels, this time to first fix ("TTFF") is of the order of 60-100 seconds, or longer. An SATPS signal receiver / processor with fewer channels will often h...

AI Technical Summary

Benefits of technology

The invention focuses on initial acquisition (at the time of power-up), reacquisition (after loss of lock) and identification of visible SATPS satellites by an SATPS station that includes an SATPS signal antenna and SATPS receiver / processor. Receipt of differential SATPS signals from another already-operative SATPS (reference) station allows the "new" SATPS station that is searching for satellite lock to reduce the Time To First Fix by as much as an order of magnitude, allows use of less expensive timing sources, and allows receipt and storage by the new station of ephemerides and almanac data that will be needed later. This allows quicker acquisition of the visible SATPS satellites upon (re) acquisition of satellite lock.

Limiting the search for SATPS satellite signals to the reference / visible SAPS satellites for which relevant SATPS information is available provides the following benefits. First, as each reference / visible satellite signal is acquired and identified, the frequency range that needs to be searched for that satellite can be narrowed substantially, using an estimate of the Doppler shift for signals emitted from that satellite that is stored at the new station or that is also broadcast by the reference station transmitter. Second, the current ephemerides, almanac, ionosphere and / or time information is periodically loaded into the new station SATPS receiver / processor so that such information is already available when (re)acquisition of satellite lock is attempted. Third, an inexpensive time base source for a new station SAPS receiver / processor can be used without incurring a large penalty in satellite signal acquisition time. After one satellite signal is locked onto, the new station SATPS receiver / processor can correct for the (relatively large) frequency error of the associated time base and can search over a smaller frequency range that covers the appropriate Doppler shifted frequencies received.

Problems solved by technology

This procedure will often consume several tens of seconds before "lock" on an adequate number of SATPS satellite signals is achieved.

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