Accuracy Assessment in Assisted GPS Positioning

Abstract

Reliable and efficient search windows are provided by allowing the adaptation of the code search window to be dependent on inaccuracy measures of relations between a cellular frame time and a satellite reference time. This inaccuracy is calculated in a positioning node of the cellular communications system, preferably by filtering of measurements received from user equipments. Linear trend Kalman filtering followed by post processing of estimation errors may be utilized. In order to ensure non-ambiguous interpretation of the received time stamps of received satellite signals provided by user equipments, a pseudo propagation delay is computed in both the user equipment and the positioning node based on GPS acquisition assistance data. The GPS time stamp is then defined referring to the determined pseudo propagation delay. In one embodiment, the pseudo propagation delay is assured to be situated within a pre-determined time interval.

1 - 25 . (canceled) 
     
     
         26 . A method for determining a position of a mobile terminal in a cellular communication system, comprising the steps of
 providing, in a positioning node of said cellular communication system, GPS acquisition assistance data comprising code phase, integer code phase, and the GPS bit number for a list of GPS satellites expected at a reference site;   transferring said GPS acquisition assistance data to said mobile terminal;   receiving, at said mobile terminal, signals from a plurality of GPS satellites;   determining, at said mobile terminal, a pseudorange to each of said plurality of GPS satellites and a GPS time stamp for the reception of respective said signals;   transferring said GPS time stamp and truncations of said pseudoranges to said positioning node; and   calculating, in said positioning node, said position of said mobile terminal based on said transferred GPS time stamps and said transferred truncations of said pseudoranges;   said step of determining and said step of calculating both comprising the further step of computing a pseudo propagation delay based on said acquisition assistance data;   said step of determining a GPS time stamp for a first of said plurality of GPS satellites being based on said pseudorange to said first GPS satellite and said pseudo propagation delay;   whereby said step of calculating being further based on said pseudo propagation delay.   
     
     
         27 . The method according to  claim 26 , wherein said computing of said pseudo propagation delay in turn comprises the steps of:
 determining an apparent propagation delay; and   establishing said pseudo propagation delay as said apparent propagation delay when the apparent propagation delay is larger than a predetermined minimum delay, and as said apparent propagation delay plus 80 ms when the apparent propagation delay is smaller than said predetermined minimum delay.   
     
     
         28 . The method according to  claim 27 , wherein said minimum delay is selected in the interval of 7 to 58 ms. 
     
     
         29 . The method according to  claim 28 , wherein said minimum delay is selected in the interval of 30 to 45 ms, 
     
     
         30 . The method according to  claim 26 , wherein: said apparent propagation delay, τ, is calculated as:
     t   t1,ref =GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10 6 , CP≠0
 
     t   t1,ref  =GPSBN·0.02+ICP·0.001, CP=0
 
 
       expressed in s, where GPSBN is the GPS bit number, ICP is the integer code phase and CP is the code phase for said first of said plurality of satellites;
 said GPS time stamp for said first of said plurality of satellites being calculated as:
   GPSTOW=floor{1000*( t   t1 +τ*)}ms,
 
 
 
       expressed in ms, where t t1  is a transmission time of said first of said plurality of satellites and τ* is said pseudo propagation delay. 
     
     
         31 . An arrangement in a cellular communication system node for determining a position of a mobile terminal in a cellular communication system, comprising:
 means for providing GPS acquisition assistance data comprising code phase, integer code phase and the GPS bit number for a list of GPS satellites expected at a reference site;   means for transferring said GPS acquisition assistance data to said mobile terminal;   a receiver for receiving data representing truncations of a pseudorange between a plurality of GPS satellites and said mobile terminal, and a GPS time stamp for the reception of signals from respective ones of GPS satellites in said mobile terminal; and   processor configured to compute a pseudo propagation delay based on said GPS acquisition assistance data;   wherein the processor is also configured to calculate the position of said mobile terminal based on said transferred GPS time stamps, said transferred truncations of said pseudoranges, and said pseudo propagation delay.   
     
     
         32 . The arrangement according to  claim 31 , wherein said processor is arranged for performing said computing of said pseudo propagation delay by:
 determining an apparent propagation delay; and   establishing said pseudo propagation delay as said apparent propagation delay when the apparent propagation delay is larger than a predetermined minimum delay, and as said apparent propagation delay plus 80 ms when the apparent propagation delay is smaller than said predetermined minimum delay.   
     
     
         33 . The arrangement according to claim  2 , wherein said minimum delay is selected in the interval of 7 to 58 ms. 
     
     
         34 . The arrangement according to  claim 33 , wherein said minimum delay is selected in the interval of 30 to 45 ms. 
     
     
         35 . The arrangement according to  claim 32 , wherein said apparent propagation delay, τ, is calculated as:
     t   t1,ref =GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10 6 , CP≠0
 
     t   t1,ref =GPSBN·0.02+ICP·0.001, CP=0
 
 
       expressed in s, where GPSBN is the GPS bit number, ICP is the integer code phase, and CP is the code phase for said first of said plurality of satellites. 
     
     
         36 . The arrangement according to  claim 35 , wherein said calculation of said relations is based on a transmission time calculated as:
     t   t1 =[GPSTOW−floor{1000*(δ t   t1 +τ*)}]/1000 +δt   t1  
   
       expressed in s, where at m  is a truncated transmission time calculated according to the relation:
   δρ 1   =c ·(0.001−δ t   t1 )
 
 
       where δρ 1  is said truncated pseudorange, GPSTGW sv1  is said GPS time stamp and τ* is said pseudo propagation delay. 
     
     
         37 . (canceled) 
     
     
         38 . The arrangement communication system node according to  claim 31 , wherein said cellular communication system node is a radio network controller or a serving mobile location center (SMLC). 
     
     
         39 . A mobile terminal for mobile terminal assisted positioning using the global positioning system (GPS), comprising:
 a first receiver for receiving GPS acquisition assistance data comprising code phase, integer code phase, and the GPS bit number for a list of GPS satellites expected at a reference site;   a processor for computing a pseudo propagation delay based on said acquisition assistance data;   a second receiver for receiving signals from a plurality of GPS satellites;   said processor being configured to determine a pseudorange to each of said plurality of GPS satellites;   said processor being further configured to determine a GPS time stamp for a first of said plurality of GPS satellites based on said pseudorange to said first GPS satellite and said pseudo propagation delay; and   means for transferring said GPS time stamp and truncations of said pseudoranges to a positioning node in said cellular communications system;   wherein the positioning node utilizes the GPS time stamp and truncations of the pseudoranges to determine the position of the mobile terminal.   
     
     
         40 . The mobile terminal according to  claim 39 , wherein said processor is configured to compute the pseudo propagation delay by:
 determining an apparent propagation delay; and   establishing said pseudo propagation delay as said apparent propagation delay when the apparent propagation delay is larger than a predetermined minimum delay, and as said apparent propagation delay plus 80 ms when the apparent propagation delay is smaller than said predetermined minimum delay.   
     
     
         41 . The mobile terminal according to  claim 40 , wherein said minimum delay is selected in the interval of 7 to 58 ms. 
     
     
         42 . The mobile terminal according to  claim 41 , wherein said minimum delay is selected in the interval of 30 to 45 ms. 
     
     
         43 . The mobile terminal according to  claim 40 , wherein said apparent propagation delay, τ, is calculated as:
     t   t1,ref =GPSBN·0.02+ICP·0.001+(1023−CP)/1.023·10 6 , CP≠0
 
     t   t1,ref =GPSBN·0.02+ICP·0.001, CP=0
 
 
       expressed in s, where GPSBN is the GPS bit number, ICP is the integer code phase, and CP is the code phase for said first of said plurality of satellites;
 said GPS time stamp for said first of said plurality of satellites being calculated as:
   GPSTOW=floor{1000*( t   t1 +τ*)}ms,
 
 
 
       expressed in ms, where 4 1  is a transmission time of aid first of said plurality of satellites and τ* is said pseudo propagation delay. 
     
     
         44 . (canceled) 
     
     
         45 . (canceled)