Utilizing sbas signals to improve gnss receiver performance

Abstract

The present invention provides methods of improving GNSS receivers' satellite signal acquisition and TTFF performances by taking advantage of SBAS signals. Due to a SBAS satellite's geostationary position and typically strong signal, the SBAS satellite signal can be acquired more quickly than a GPS satellite signal. Once a SBAS satellite signal is acquired the Doppler frequency search uncertainty may be reduced for remaining GNSS satellites which are to be acquired. Furthermore, a satellite search list may be optimized to search for satellites close to the line of sight (LOS) of the SBAS satellite for which a signal has been acquired, in receiver “warm” and “hot” start modes.

What is claimed is: 
     
         1 . A method of optimizing a satellite search list in a CDMA signal receiver, comprising:
 acquiring a geostationary satellite signal;   calculating angles between the line of sight for the geostationary satellite and the line of sight for other satellites;   flagging satellites for which said angle is less than a threshold value; and   placing flagged satellites at the beginning of said satellite search list.   
     
     
         2 . A method as in  claim 1 , wherein said geostationary satellite signal is transmitted from a satellite-based augmentation system (SBAS) satellite. 
     
     
         3 . A method as in  claim 1 , wherein said other satellites include a SBAS satellite. 
     
     
         4 . A method as in  claim 1 , further comprising placing the flagged satellites in an order corresponding to the size of the calculated angles, said order having the smallest of the calculated angles first. 
     
     
         5 . A method as in  claim 1 , wherein said threshold value is 0.35 radians. 
     
     
         6 . A method as in  claim 1 , wherein said other satellites are satellites not yet acquired by said CDMA signal receiver. 
     
     
         7 . A method as in  claim 1 , wherein said other satellites include GPS satellites. 
     
     
         8 . A method as in  claim 1 , wherein said other satellites include a second SBAS satellite. 
     
     
         9 . A method in a GPS receiver of extracting GPS time from a satellite-based augmentation system (SBAS) satellite signal and aided time information, said method comprising:
 acquiring said SBAS satellite signal;   identifying a repeating SBAS signal sub-frame preamble;   accepting said aided time information;   calculating one or more candidates for said GPS time; and   determining said GPS time from said one or more candidates.   
     
     
         10 . A method as in  claim 9 , wherein said determining includes identifying the one of said one or more candidates that is within said uncertainty of said aided time. 
     
     
         11 . A method as in  claim 10 , wherein said aided time information provides an aided time with an uncertainty of less than three seconds. 
     
     
         12 . A method as in  claim 9 , wherein said determining includes:
 acquiring multiple satellite signals;   generating a GPS position fix for each of said one or more candidates;   calculating pseudo-range residuals for each of said one or more candidates; and   choosing the one of said one or more candidates with the smallest residual.   
     
     
         13 . A method as in  claim 12 , wherein said aided time information provides an aided time with an uncertainty and wherein there are multiple of said one or more candidates within said uncertainty. 
     
     
         14 . A method as in  claim 13 , wherein said uncertainty is greater than three seconds. 
     
     
         15 . A method as in  claim 12 , wherein said acquiring includes acquiring four satellite signals in addition to said SBAS satellite signal, said four satellite signals and said SBAS satellite signal being from five different satellites. 
     
     
         16 . A method as in  claim 15 , wherein one of said four satellite signals is from a second SBAS satellite. 
     
     
         17 . A method as in  claim 12 , wherein said acquiring includes acquiring three satellite signals in addition to said SBAS satellite signal, said three satellites signals and said SBAS satellite signal being from four different satellites, and wherein the altitude of said GPS receiver is known. 
     
     
         18 . A method as in  claim 17 , wherein one of said three satellite signals is from a second SBAS satellite. 
     
     
         19 . A method as in  claim 12 , further comprising checking the plausibility of each said GPS position fix for each of said one or more candidates, wherein said choosing includes choosing the one of said one or more candidates with the smallest residual and plausible GPS position fix. 
     
     
         20 . A method as in  claim 9 , wherein said SBA signal is transmitted from a wide area augmentation system (WAAS) satellite. 
     
     
         21 . A method as in  claim 9 , wherein said GPS time is time of week (TOW). 
     
     
         22 . A method as in  claim 9 , wherein said aided time information is accepted from a cellular network. 
     
     
         23 . A method as in  claim 9 , wherein said GPS receiver acquires said SBAS satellite signal before any other satellite signal.