Wireless position determination using adjusted round trip time measurements

Abstract

One method for wirelessly determining a position of a mobile station includes measuring a round trip time (RTT) to a plurality of wireless access points, estimating a first distance to each wireless access point based upon the round trip time delay and an initial processing time associated with each wireless access point, estimating a second distance to each wireless access point based upon supplemental information, combining the first and second distance estimates to each wireless access point, and calculating the position based upon the combined distance estimates. Another method includes measuring a distance to each wireless access point based upon a wireless signal model, calculating a position of the mobile station based upon the measured distance, determining a computed distance to each wireless access point based upon the calculated position of the mobile station, updating the wireless signal model, and determining whether the wireless signal model has converged.

1 . A method for wirelessly determining a position of a mobile station, comprising:
 measuring a round trip time (RTT) to each of a plurality of wireless access points;   estimating a first distance to each wireless access point based upon the round trip time delay and an initial processing time associated with each wireless access point;   estimating a second distance to each wireless access point based upon supplemental information;   combining the first and second distance estimates to each wireless access point; and   calculating the position of the mobile station based upon the combined distance estimates.   
     
     
         2 . The method of  claim 1 , wherein the supplemental information includes received signal strength intensity (RSSI), a previous position estimate, and/or information provided by Bluetooth, beacons, RFID tags, coordinates from a map, and/or Satellite Positioning System (SPS) signals. 
     
     
         3 . The method of  claim 1 , wherein the determining the second distance to each wireless access point further comprises:
 measuring a received signal strength of a packet from each wireless access point; and   estimating a distance to each wireless access point based upon the measured signal strength.   
     
     
         4 . The method of  claim 3 , wherein estimating a distance to each wireless access point based upon the measured signal strength further comprises:
 relating a received signal strength intensity (RSSI) and distance utilizing a model.   
     
     
         5 . The method of  claim 4 , wherein the model provides an average distance, and a variance in this distance, as a function of RSSI. 
     
     
         6 . The method of  claim 4 , wherein the model assumes an indoor environment and provides upper and lower bounds for RSSI as a function of distance between the mobile station and a wireless access point. 
     
     
         7 . The method of  claim 6 , wherein the model further includes a propagation model based upon a map indicating the deployment of each wireless access point. 
     
     
         8 . The method of  claim 4 , wherein the model relates the RSSI to an approximate maximum distance between the mobile station and a wireless access point. 
     
     
         9 . The method of  claim 1 , wherein measuring a round trip time delay to the wireless access point further comprises:
 transmitting a packet from the mobile station to the access point;   recording a first time when the transmitted packet was sent;   receiving a response packet from the wireless access point in response to the transmitted packet;   recording a second time when the response packet was received; and   computing a difference between the second recorded time and the first recorded time.   
     
     
         10 . The method of  claim 9 , wherein the transmitted packet utilizes a unicast packet whereby the mobile station does not associate with the wireless access point. 
     
     
         11 . The method of  claim 9 , wherein the transmitted packet utilizes a packet whereby the mobile station associates with the wireless access point. 
     
     
         12 . The method of  claim 10 , wherein the mobile station and the wireless access points operate in accordance with IEEE 802.11 standards, cellular piconet, cellular femtocell, and/or Bluetooth networking standards. 
     
     
         13 . The method of  claim 1 , further comprising:
 selecting each of the wireless access points based upon a received signal strength indicator.   
     
     
         14 . The method of  claim 1 , wherein the calculating the position of the mobile station using the first and second distances to each wireless access point further comprises:
 determining the combined distance by performing a weighted combination of the first and second distances, wherein the first distance is weighted by a first variance associated with RSSI measurements, and the second distance is weighted by a second variance associated with a round trip time noise; and   performing trilateration using the combined distance associated with each wireless access point.   
     
     
         15 . An apparatus for wireless position determination, comprising:
 a wireless transceiver;   a processor coupled to the wireless transceiver; and   a memory coupled to the processor, wherein the memory stores executable instructions and data for causing the processor to
 measure a round trip time (RTT) to each of a plurality of wireless access points, 
 estimate a first distance to each wireless access point based upon the round trip time delay and an initial processing time associated with each wireless access point, 
 estimate a second distance to each wireless access point based upon supplemental information, 
 combine the first and second distance estimates to each wireless access point, and 
 calculate the position of the mobile station based upon the combined distance estimates. 
   
     
     
         16 . The apparatus of  claim 15 , wherein the supplemental information includes received signal strength intensity (RSSI), a previous position estimate, and/or information provided by Bluetooth, beacons, RFID tags, coordinates from a map, and/or Satellite Positioning System (SPS) signals. 
     
     
         17 . The apparatus of  claim 15 , wherein the instructions for determining the second distance to each wireless access point further cause the processor to
 measure a received signal strength of a packet from each wireless access point, and   estimate a distance to each wireless access point based upon the measured signal strength.   
     
     
         18 . The apparatus of  claim 17 , wherein the instructions for estimating a distance to each wireless access point based upon the measured signal strength further cause the processor to
 relate a received signal strength intensity (RSSI) and distance utilizing a model.   
     
     
         19 . The apparatus of  claim 18 , wherein the model provides an average distance, and a variance in this distance, as a function of RSSI. 
     
     
         20 . The apparatus of  claim 18 , wherein the model assumes an indoor environment and provides upper and lower bounds for RSSI as a function of distance between the mobile station and a wireless access point. 
     
     
         21 . The apparatus of  claim 20 , wherein the model further includes a propagation model based upon a map indicating the deployment of each wireless access point. 
     
     
         22 . The apparatus of  claim 18 , wherein the model relates the RSSI to an approximate maximum distance between the mobile station and a wireless access point. 
     
     
         23 . The apparatus of  claim 15 , wherein the instructions for measuring a round trip time delay to the wireless access point further cause the processor to
 transmit a packet from the mobile station to the access point,   record a first time when the transmitted packet was sent,   receive a response packet from the wireless access point in response to the transmitted packet,   record a second time when the response packet was received, and   compute a difference between the second recorded time and the first recorded time.   
     
     
         24 . The apparatus of  claim 23 , wherein the transmitted packet utilizes a unicast packet whereby the mobile station does not associate with the wireless access point. 
     
     
         25 . The apparatus of  claim 23 , wherein the transmitted packet utilizes a packet whereby the mobile station associates with the wireless access point. 
     
     
         26 . The apparatus of  claim 24 , wherein the mobile station and the wireless access points operate in accordance with IEEE 802.11 standards, cellular piconet, cellular femtocell, and/or Bluetooth networking standards. 
     
     
         27 . The apparatus of  claim 15 , further comprising instructions which cause the processor to
 select each of the wireless access points based upon a received signal strength indicator.   
     
     
         28 . The apparatus of  claim 15 , wherein the instructions for calculating the position of the mobile station using the first and second distances to each wireless access point further cause the processor to
 determine the combined distance by performing a weighted combination of the first and second distances, wherein the first distance is weighted by a first variance associated with RSSI measurements, and the second distance is weighted by a second variance associated with a round trip time noise, and   perform trilateration using the combined distance associated with each wireless access point.   
     
     
         29 . An apparatus for wireless position determination, comprising:
 means for measuring a round trip time (RTT) to each of a plurality of wireless access points;   means for estimating a first distance to each wireless access point based upon the round trip time delay and an initial processing time associated with each wireless access point;   means for estimating a second distance to each wireless access point based upon supplemental information;   means for combining the first and second distance estimates to each wireless access point; and   means for calculating the position of the mobile station based upon the combined distance estimates.   
     
     
         30 . The apparatus of  claim 29 , wherein the supplemental information includes received signal strength intensity (RSSI), a previous position estimate, and/or information provided by Bluetooth, beacons, RFID tags, coordinates from a map, and/or Satellite Positioning System (SPS) signals. 
     
     
         31 . The apparatus of  claim 29 , wherein the determining the second distance to each wireless access point further comprises:
 means for measuring a received signal strength of a packet from each wireless access point; and   means for estimating a distance to each wireless access point based upon the measured signal strength.   
     
     
         32 . The apparatus of  claim 31 , wherein estimating a distance to each wireless access point based upon the measured signal strength further comprises:
 means for relating a received signal strength intensity (RSSI) and distance utilizing a model.   
     
     
         33 . The apparatus of  claim 32 , wherein the model provides an average distance, and a variance in this distance, as a function of RSSI. 
     
     
         34 . The apparatus of  claim 32 , wherein the model assumes an indoor environment and provides upper and lower bounds for RSSI as a function of distance between the mobile station and a wireless access point. 
     
     
         35 . The apparatus of  claim 34 , wherein the model further includes a propagation model based upon a map indicating the deployment of each wireless access point. 
     
     
         36 . The apparatus of  claim 32 , wherein the model relates the RSSI to an approximate maximum distance between the mobile station and a wireless access point. 
     
     
         37 . The apparatus of  claim 29 , wherein measuring a round trip time delay to the wireless access point further comprises:
 means for transmitting a packet from the mobile station to the access point;   means for recording a first time when the transmitted packet was sent;   means for receiving a response packet from the wireless access point in response to the transmitted packet;   means for recording a second time when the response packet was received; and   means for computing a difference between the second recorded time and the first recorded time.   
     
     
         38 . The apparatus of  claim 37 , wherein the transmitted packet utilizes a unicast packet whereby the mobile station does not associate with the wireless access point. 
     
     
         39 . The apparatus of  claim 37 , wherein the transmitted packet utilizes a packet whereby the mobile station associates with the wireless access point. 
     
     
         40 . The apparatus of  claim 38 , wherein the mobile station and the wireless access points operate in accordance with IEEE 802.11 standards, cellular piconet, cellular femtocell, and/or Bluetooth networking standards. 
     
     
         41 . The apparatus of  claim 29 , further comprising:
 means for selecting each of the wireless access points based upon a received signal strength indicator.   
     
     
         42 . The apparatus of  claim 29 , wherein the calculating the position of the mobile station using the first and second distances to each wireless access point further comprises:
 means for determining the combined distance by performing a weighted combination of the first and second distances, wherein the first distance is weighted by a first variance associated with RSSI measurements, and the second distance is weighted by a second variance associated with a round trip time noise; and   means for performing trilateration using the combined distance associated with each wireless access point.   
     
     
         43 . A method for wirelessly determining a position of a mobile station using signals provided by a plurality of wireless access points, comprising:
 measuring a distance to each wireless access point based upon a wireless signal model;   calculating a position of the mobile station based upon the measured distance;   determining a computed distance to each wireless access point based upon the calculated position of the mobile station;   updating the wireless signal model based upon the measured and computed distances to each wireless access point; and   determining whether the wireless signal model has converged.   
     
     
         44 . The method according to  claim 43 , further comprising:
 repeating the method of  claim 43  when the wireless signal model has not converged.   
     
     
         45 . The method according to  claim 43 , further comprising:
 determining that the wireless signal model has not converged;   refining the distance measurement to each wireless access point based upon the updated wireless signal model;   calculating a refined position of the mobile station based upon the refined distance measurement;   determining another computed distance to each wireless access point based upon the refined position of the mobile station; and   updating the wireless signal model based upon the refined measurement and computed distances to each wireless access point.   
     
     
         46 . The method according to  claim 43 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to a round trip time (RTT) of the wireless signal. 
     
     
         47 . The method according to  claim 46 , further comprising:
 measuring a round trip time (RTT) to each of the wireless access points;   determining a first distance to each wireless access point based upon the round trip time delay;   determining a second distance to each wireless access point based upon supplemental information;   calculating the position of the mobile station using the first and second distances to each wireless access point; and   updating the initial processing time for each of the wireless access points based upon the calculated position of the mobile station.   
     
     
         48 . The method according to  claim 47  further comprising:
 determining that at least one of the updated processing times have not converged;   measuring the round trip time delay to each of the plurality of wireless access points;   determining a revised first distance to each wireless access point based upon the round trip time delay and the updated processing time associated with each wireless access point;   calculating a subsequent position of the mobile station using the revised first distance and the second distance to each wireless access point; and   refining the updated processing time for each of the wireless access points based upon the subsequent position of the mobile station.   
     
     
         49 . The method according to  claim 47 , wherein the supplemental information may be derived from RSSI models, a previous position estimate, information provided by Bluetooth, beacons, RFID tags, coordinates from a map, and/or Satellite Positioning System (SPS) signals. 
     
     
         50 . The method according to  claim 43 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to received signal strength intensity (RSSI) of the wireless signal. 
     
     
         51 . The method of  claim 50 , wherein the model assumes an indoor environment and further comprises:
 receiving an initial model of the indoor environment;   calculating an initial position of the mobile station based upon the initial distance to each wireless access point; and   updating the initial model using the calculated initial position and the received signal strength of the packet.   
     
     
         52 . The method of  claim 51 , further comprising:
 updating the model of the indoor environment based upon subsequent positions and received signal strength measurements as the mobile station moves within the indoor environment.   
     
     
         53 . The method of  claim 51 , wherein the initial model is generated based upon ray tracing a map of the indoor environment. 
     
     
         54 . The method of  claim 51 , wherein the initial model is generated based upon generic models of indoor environments. 
     
     
         55 . The method of  claim 53 , wherein the generic models of indoor environments include office environments, warehouse environments, and/or shopping mall environments. 
     
     
         56 . The method of  claim 51 , wherein calculating an initial position of the mobile station further comprises:
 determining an initial distance to each wireless access point based upon the initial model; and   performing trilateration using the initial distances.   
     
     
         57 . The method of  claim 51 , wherein calculating an initial position of the mobile station further comprises:
 matching observed RSSI values with a fingerprinting database.   
     
     
         58 . The method of  claim 43 , wherein the determining the second distance to each wireless access point further comprises:
 determining a mean RTT noise from the supplemental information;   subtracting the initial processing time and the mean RTT noise from the RTT to determine an adjusted time value; and   converting the adjusted time to the second distance.   
     
     
         59 . The method of  claim 58 , wherein the mean RTT noise value is based upon a received signal strength intensity value associated with each wireless access point. 
     
     
         60 . The method of  claim 43 , wherein it is determined that the wireless signal model has converged, further comprising:
 storing parameters associated with the wireless signal model.   
     
     
         61 . An apparatus for wireless position determination of a mobile station using signals provided by a plurality of wireless access points, comprising:
 a wireless transceiver;   a processor coupled to the wireless transceiver; and   a memory coupled to the processor, wherein the memory stores executable instructions and data for causing the processor to
 measure a distance to each wireless access point based upon a wireless signal model, 
 calculate a position of the mobile station based upon the measured distance, 
 determine a computed distance to each wireless access point based upon the calculated position of the mobile station, 
 update the wireless signal model based upon the measured and computed distances to each wireless access point, and 
 determine whether the wireless signal model has converged. 
   
     
     
         62 . The apparatus according to  claim 61 , further comprising instructions causing the processor to
 repeat the instructions of  claim 61  when the wireless signal model has not converged.   
     
     
         63 . The apparatus according to  claim 61 , further comprising instructions causing the processor to
 determine that the wireless signal model has not converged,   refine the distance measurement to each wireless access point based upon the updated wireless signal model,   calculate a refined position of the mobile station based upon the refined distance measurement,   determine another computed distance to each wireless access point based upon the refined position of the mobile station, and   update the wireless signal model based upon the refined measurement and computed distances to each wireless access point.   
     
     
         64 . The apparatus according to  claim 61 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to a round trip time (RTT) of the wireless signal. 
     
     
         65 . The apparatus according to  claim 64 , further comprising instructions causing the processor to
 measure a round trip time (RTT) to each of the wireless access points,   determine a first distance to each wireless access point based upon the round trip time delay,   determine a second distance to each wireless access point based upon supplemental information,   calculate the position of the mobile station using the first and second distances to each wireless access point, and   update the initial processing time for each of the wireless access points based upon the calculated position of the mobile station.   
     
     
         66 . The apparatus according to  claim 65  further comprising instructions causing the processor to
 determine that at least one of the updated processing times have not converged,   measure the round trip time delay to each of the plurality of wireless access points,   determine a revised first distance to each wireless access point based upon the round trip time delay and the updated processing time associated with each wireless access point,   calculate a subsequent position of the mobile station using the revised first distance and the second distance to each wireless access point, and   refine the updated processing time for each of the wireless access points based upon the subsequent position of the mobile station.   
     
     
         67 . The apparatus according to  claim 65 , wherein the supplemental information may be derived from RSSI models, a previous position estimate, information provided by Bluetooth, beacons, RFID tags, coordinates from a map, and/or Satellite Positioning System (SPS) signals. 
     
     
         68 . The apparatus according to  claim 61 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to received signal strength intensity (RSSI) of the wireless signal. 
     
     
         69 . The apparatus of  claim 68 , wherein the model assumes an indoor environment and the apparatus further comprises instructions causing the processor to
 receive an initial model of the indoor environment,   calculate an initial position of the mobile station based upon the initial distance to each wireless access point, and   update the initial model using the calculated initial position and the received signal strength of the packet.   
     
     
         70 . The apparatus of  claim 69 , further comprising instructions causing the processor to
 update the model of the indoor environment based upon subsequent positions and received signal strength measurements as the mobile station moves within the indoor environment.   
     
     
         71 . The apparatus of  claim 69 , wherein the initial model is generated based upon ray tracing a map of the indoor environment. 
     
     
         72 . The apparatus of  claim 69 , wherein the initial model is generated based upon generic models of indoor environments. 
     
     
         73 . The apparatus of  claim 71 , wherein the generic models of indoor environments include office environments, warehouse environments, and/or shopping mall environments. 
     
     
         74 . The apparatus of  claim 69 , wherein calculating an initial position of the mobile station further comprises instructions causing the processor to
 determine an initial distance to each wireless access point based upon the initial model, and   perform trilateration using the initial distances.   
     
     
         75 . The apparatus of  claim 69 , wherein calculating an initial position of the mobile station further comprises instructions causing the processor to
 match observed RSSI values with a fingerprinting database.   
     
     
         76 . The apparatus of  claim 61 , wherein the determining the second distance to each wireless access point further comprises instructions causing the processor to
 determine a mean RTT noise from the supplemental information,   subtract the initial processing time and the mean RTT noise from the RTT to determine an adjusted time value, and   convert the adjusted time to the second distance.   
     
     
         77 . The apparatus of  claim 76 , wherein the mean RTT noise value is based upon a received signal strength intensity value associated with each wireless access point. 
     
     
         78 . The apparatus of  claim 61 , wherein it is determined that the wireless signal model has converged, further comprising instructions causing the processor to
 store parameters associated with the wireless signal model.   
     
     
         79 . An apparatus for wireless position determination of a mobile station using signals provided by a plurality of wireless access points, comprising:
 means for measuring a distance to each wireless access point based upon a wireless signal model;   means for calculating a position of the mobile station based upon the measured distance;   means for determining a computed distance to each wireless access point based upon the calculated position of the mobile station;   means for updating the wireless signal model based upon the measured and computed distances to each wireless access point; and   means for determining whether the wireless signal model has converged.   
     
     
         80 . The apparatus according to  claim 79 , further comprising:
 means for repeating the functions of  claim 79  when the wireless signal model has not converged.   
     
     
         81 . The apparatus according to  claim 79 , further comprising:
 means for determining that the wireless signal model has not converged;   means for refining the distance measurement to each wireless access point based upon the updated wireless signal model;   means for calculating a refined position of the mobile station based upon the refined distance measurement;   means for determining another computed distance to each wireless access point based upon the refined position of the mobile station; and   means for updating the wireless signal model based upon the refined measurement and computed distances to each wireless access point.   
     
     
         82 . The apparatus according to  claim 79 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to a round trip time (RTT) of the wireless signal. 
     
     
         83 . The apparatus according to  claim 82 , further comprising:
 measuring a round trip time (RTT) to each of the wireless access points;   means for determining a first distance to each wireless access point based upon the round trip time delay;   means for determining a second distance to each wireless access point based upon supplemental information;   means for calculating the position of the mobile station using the first and second distances to each wireless access point; and   means for updating the initial processing time for each of the wireless access points based upon the calculated position of the mobile station.   
     
     
         84 . The apparatus according to  claim 83  further comprising:
 means for determining that at least one of the updated processing times have not converged;   means for measuring the round trip time delay to each of the plurality of wireless access points;   means for determining a revised first distance to each wireless access point based upon the round trip time delay and the updated processing time associated with each wireless access point;   means for calculating a subsequent position of the mobile station using the revised first distance and the second distance to each wireless access point; and   means for refining the updated processing time for each of the wireless access points based upon the subsequent position of the mobile station.   
     
     
         85 . The apparatus according to  claim 79 , wherein the wireless signal model relates the distance between the mobile station and each wireless access point to received signal strength intensity (RSSI) of the wireless signal. 
     
     
         86 . The apparatus of  claim 85 , wherein the model assumes an indoor environment and further comprises:
 means for receiving an initial model of the indoor environment;   means for calculating an initial position of the mobile station based upon the initial distance to each wireless access point; and   means for updating the initial model using the calculated initial position and the received signal strength of the packet.   
     
     
         87 . The apparatus of  claim 86 , further comprising:
 means for updating the model of the indoor environment based upon subsequent positions and received signal strength measurements as the mobile station moves within the indoor environment.   
     
     
         88 . The apparatus of  claim 86 , wherein calculating an initial position of the mobile station further comprises:
 means for determining an initial distance to each wireless access point based upon the initial model; and   means for performing trilateration using the initial distances.   
     
     
         89 . The apparatus of  claim 79 , wherein the determining the second distance to each wireless access point further comprises:
 means for determining a mean RTT noise from the supplemental information;   means for subtracting the initial processing time and the mean RTT noise from the RTT to determine an adjusted time value; and   means for converting the adjusted time to the second distance.   
     
     
         90 . A method for wirelessly determining a position of a mobile station, comprising:
 measuring a round trip time delay to each of a plurality of wireless access points;   estimating an initial processing time for each of the wireless access points;   calculating the position of the mobile station based upon the measured round trip time delays and estimated processing times; and   updating the estimated processing time for each of the wireless access points based upon the calculated position of the mobile station.   
     
     
         91 . The method of  claim 90 , further comprising:
 determining that the updated processing times have not converged;   measuring the round trip time delay to each of the plurality of wireless access points;   calculating a subsequent position of the mobile station using the measured round trip time delays and updated estimated processing times; and   refining the updated processing time for each of the wireless access points based upon the subsequent position of the mobile station.   
     
     
         92 . The method of  claim 90 , further comprising:
 determining that the estimated processing times have converged;   determining that the mobile station has changed position;   measuring a round trip time delay to each of a plurality of wireless access points; and   calculating a position of the mobile station using the measured round trip time delays and updated estimated processing times.   
     
     
         93 . The method of  claim 90 , wherein measuring a round trip time delay to the wireless access point further comprises:
 transmitting a packet from the mobile station to the access point;   recording a first time when the transmitted packet was sent;   receiving a response packet from the wireless access point in response to the transmitted packet;   recording a second time when the response packet was received; and   computing a difference between the second recorded time and the first recorded time.   
     
     
         94 . The method of  claim 93 , wherein the transmitted packet utilizes a unicast packet whereby the mobile station does not associate with the wireless access point. 
     
     
         95 . The method of  claim 93 , wherein the transmitted packet utilizes a packet whereby the mobile station associates with the wireless access point. 
     
     
         96 . The method of  claim 93 , wherein the mobile station and the wireless access points operate in accordance with IEEE 802.11 standards, cellular piconet, cellular femtocell, and/or Bluetooth networking standards. 
     
     
         97 . The method of  claim 90 , wherein the estimating the initial processing time further comprises:
 determining received signal strength of a packet from each wireless access point;   estimating a distance to each wireless access point based upon the determined signal strength;   calculating the initial processing time for each wireless access point based upon the estimated distance.   
     
     
         98 . The method of  claim 97 , further comprising:
 bracketing the distance to each wireless access point within an interval based upon the received signal strength.   
     
     
         99 . The method according to  claim 98 , wherein it is known that the processing times for at least two wireless access points are substantially different, the method further comprising:
 estimating the initial processing time for each wireless access point to lie in the midpoint of its respective interval.   
     
     
         100 . The method according to  claim 98 , wherein it is known that the processing times for the wireless access points are substantially similar, the method further comprising:
 estimating the initial processing time to lie in the midpoint of an intersection of the intervals of the wireless access points.   
     
     
         101 . The method of  claim 90 , wherein the calculating the position of the mobile station further comprises:
 selecting the plurality of wireless access points based upon received signal strengths;   determining positions of each of the selected wireless access points;   calculating a distance between the mobile station and each wireless access point using the measured round trip time delay and the estimated processing time; and   performing trilateration based upon the calculated distance and a position for each wireless access point.   
     
     
         102 . The method of  claim 101 , wherein the position of each of the selected wireless access points are defined in a standard coordinate system. 
     
     
         103 . The method of  claim 90 , wherein the updating the estimated processing time for each of the wireless access points further comprises:
 calculating a distance to each wireless access point based the calculated position of the mobile station and positions of the wireless access points; and   computing a new processing time for each wireless access point based upon the measured round trip time delay to each wireless access point and the calculated distance to each wireless access point.   
     
     
         104 . An apparatus for wirelessly determining a position of a mobile station, comprising:
 a wireless transceiver;   a processor coupled to the wireless transceiver; and   a memory coupled to the processor, wherein the memory stores executable instructions and data for causing the processor to
 measure a round trip time delay to each of a plurality of wireless access points, 
 estimate an initial processing time for each of the wireless access points, 
 calculate the position of the mobile station based upon the measured round trip time delays and estimated processing times, and 
 update the estimated processing time for each of the wireless access points based upon the calculated position of the mobile station. 
   
     
     
         105 . The apparatus of  claim 104 , further comprising instructions causing the processor to
 determine that the updated processing times have not converged,   measure the round trip time delay to each of the plurality of wireless access points,   calculate a subsequent position of the mobile station using the measured round trip time delays and updated estimated processing times, and   refine the updated processing time for each of the wireless access points based upon the subsequent position of the mobile station.   
     
     
         106 . The apparatus of  claim 104 , further comprising instructions causing the processor to
 determine that the estimated processing times have converged,   determine that the mobile station has changed position,   measure a round trip time delay to each of a plurality of wireless access points, and   calculate a position of the mobile station using the measured round trip time delays and updated estimated processing times.   
     
     
         107 . The apparatus of  claim 104 , wherein measuring a round trip time delay to the wireless access point further comprises instructions causing the processor to
 transmit a packet from the mobile station to the access point,   record a first time when the transmitted packet was sent,   receive a response packet from the wireless access point in response to the transmitted packet,   record a second time when the response packet was received, and   compute a difference between the second recorded time and the first recorded time.   
     
     
         108 . The apparatus of  claim 107 , wherein the transmitted packet utilizes a unicast packet whereby the mobile station does not associate with the wireless access point. 
     
     
         109 . The apparatus of  claim 107 , wherein the transmitted packet utilizes a packet whereby the mobile station associates with the wireless access point. 
     
     
         110 . The apparatus of  claim 107 , wherein the mobile station and the wireless access points operate in accordance with IEEE 802.11 standards, cellular piconet, cellular femtocell, and/or Bluetooth networking standards. 
     
     
         111 . The apparatus of  claim 104 , wherein the estimating the initial processing time further comprises instructions causing the processor to
 determine received signal strength of a packet from each wireless access point,   estimate a distance to each wireless access point based upon the determined signal strength, and   calculate the initial processing time for each wireless access point based upon the estimated distance.   
     
     
         112 . The apparatus of  claim 111 , further comprising instructions causing the processor to
 bracket the distance to each wireless access point within an interval based upon the received signal strength.   
     
     
         113 . The apparatus according to  claim 112 , wherein it is known that the processing times for at least two wireless access points are substantially different, the apparatus further comprises instructions causing the processor to
 estimate the initial processing time for each wireless access point to lie in the midpoint of its respective interval.   
     
     
         114 . The apparatus according to  claim 112 , wherein it is known that the processing times for the wireless access points are substantially similar, the apparatus further comprises instructions causing the processor to
 estimate the initial processing time to lie in the midpoint of an intersection of the intervals of the wireless access points.   
     
     
         115 . The apparatus of  claim 104 , wherein the calculating the position of the mobile station further comprises instructions causing the processor to
 select the plurality of wireless access points based upon received signal strengths,   determine positions of each of the selected wireless access points,   calculate a distance between the mobile station and each wireless access point using the measured round trip time delay and the estimated processing time, and   perform trilateration based upon the calculated distance and a position for each wireless access point.   
     
     
         116 . The apparatus of  claim 104 , wherein the updating the estimated processing time for each of the wireless access points further comprises instructions causing the processor to
 calculate a distance to each wireless access point based the calculated position of the mobile station and positions of the wireless access points, and   compute a new processing time for each wireless access point based upon the measured round trip time delay to each wireless access point and the calculated distance to each wireless access point.   
     
     
         117 . An apparatus for wirelessly determining a position of a mobile station, comprising:
 means for measuring a round trip time delay to each of a plurality of wireless access points;   means for estimating an initial processing time for each of the wireless access points;   means for calculating the position of the mobile station based upon the measured round trip time delays and estimated processing times; and   means for updating the estimated processing time for each of the wireless access points based upon the calculated position of the mobile station.