System and method for estimating positioning error within a WLAN-based positioning system
In one embodiment, a technique is provided for estimating and using an expected error of a position estimate of a wireless local area network (WLAN)-enabled mobile device. The WLAN-enabled mobile device received signals transmitted by a plurality of WLAN access points that are in range. A position of the WLAN-enabled mobile device is estimated based on the signals from one or more of the plurality of WLAN access points. An expected error of the position estimate is estimated based on a combination of correlated parameters, such as a spatial spread of geographic positions of the one or more WLAN access points, the signal coverage areas of the one or more WLAN access points, and a number of the one or more WLAN access points. Based on the expected error, the position estimate may be weighted or excluded from use in deriving a position, speed, or direction of travel.
1. A method of estimating and using an expected error of a position estimate of a wireless local area network (WLAN)-enabled mobile device, the method comprising:
receiving, by the WLAN-enabled mobile device, signals transmitted by a plurality of WLAN access points in range of the WLAN-enabled mobile device, wherein each of the WLAN access points has an associated signal coverage area;
estimating a position of the WLAN-enabled mobile device based on the signals from one or more of the plurality of WLAN access points in range of the WLAN-enabled mobile device;
estimating the expected error of the position estimate of the WLAN-enabled mobile device based on a combination of correlated parameters, the correlated parameters including two or more parameters selected from the group consisting of: a spatial spread associated with geographic positions of the one or more WLAN access points, a signal coverage areas of the one or more WLAN access points, and a number of the one or more WLAN access points, wherein the expected error predicts a relative accuracy of the position estimate of the WLAN-enabled mobile device; and
based on the expected error, weighting the position estimate or excluding the position estimate from use in deriving a position, speed, or direction of travel of the WLAN-enabled mobile device.
2. The method of claim 1 , wherein the expected error is represented in terms of distance.
3. The method of claim 1 , wherein the correlated parameters include the spatial spread associated with the geographic positions of the one or more WLAN access points.
4. The method of claim 3 , wherein a component of the expected error that is based on spatial spread is directly proportional to a square of a standard deviation of the spatial spread of the geographic positions of the WLAN access points used to estimate the position of the WLAN-enabled mobile device.
5. The method of claim 1 , wherein the correlated parameters include the signal coverage areas of the one or more WLAN access points.
6. The method of claim 5 , further comprising:
calculating a component of the expected error that is based on signal coverage areas using a smallest signal coverage area of the one or more WLAN access points used to estimate the position of the WLAN-enabled mobile device.
7. The method of claim 6 , wherein a component of the expected error based on the signal coverage areas is directly proportional to the smallest signal coverage area of the one or more WLAN access points used to estimate the position of the WLAN-enabled mobile device.
8. The method of claim 6 , wherein the signal coverage area for each WLAN access point used to estimate the position of the WLAN-enabled mobile device is estimated by:
determining locations at which a WLAN-enabled mobile device receives a signal from the WLAN access point;
determining a standard deviation of the signal coverage area of the WLAN access point based on the determined locations; and
estimating the signal coverage area of the WLAN access point based on the standard deviation of the coverage area.
9. The method of claim 1 , wherein the correlated parameters include the number of the one or more WLAN access points.
10. The method of claim 1 , wherein a component of the expected error that is based on the number of the one or more WLAN access points is inversely proportional to a square root of the number of WLAN access points.
11. The method of claim 10 , wherein the component of the expected error that is based on the number of the one or more WLAN access points is equal to an inverse of a square root of the number of WLAN access points.
12. The method of claim 11 , wherein the constant is empirically determined based on samples in a signal coverage area.
13. The method of claim 12 , wherein the method excludes the position estimate of the WLAN-enabled mobile device, wherein a given position estimate of the WLAN-enabled mobile device is used only if the expected error of the given position estimate is lower than a predetermined threshold.
14. The method of claim 12 , wherein the method weights the position estimate of the WLAN-enabled mobile device, wherein a weighted average of position estimates is produced using the weighted position estimate.
15. The method of claim 14 , wherein the weighted average is produced also using at least one other position estimate provided by a global positioning system (GPS)-based positioning system.
16. A system for estimating and using an expected error of a position estimate of a wireless local area network (WLAN)-enabled mobile device, the system comprising:
a WLAN-enabled mobile device configured to receive signals transmitted by a plurality of WLAN access points in range of the WLAN-enabled mobile device;
positioning software residing on the WLAN-enabled mobile device, the positioning software when executed by one or more processors operable to:
estimate the position of the WLAN-enabled mobile device based on received signals from one or more of the plurality of WLAN access points in range of the WLAN-enabled mobile device,
estimate an expected error of the position estimate of the WLAN-enabled mobile device based on a combination of correlated parameters, the correlated parameters including two or more parameters selected from the group consisting of: a spatial spread associated with geographic positions of the one or more WLAN access points, a signal coverage areas of the one or more WLAN access points device, and a number of the one or more WLAN access points, and
use the expected error of the position estimate of the WLAN-enabled mobile device to weight the position estimate or exclude the position estimate from use in deriving a position, speed, or direction of travel of the WLAN-enabled mobile device.
17. The system of claim 16 , wherein the correlated parameters include the spatial spread associated with geographic positions of the one or more WLAN access points.
18. The system of claim 16 , wherein the correlated parameters include the signal coverage areas of the one or more WLAN access points.
19. The system of claim 16 , wherein the correlated parameters include the number of the one or more WLAN access points.
20. Software stored in a non-transitory device-readable medium that when executed by one or more processors is operable to:
based on signals from a plurality of wireless local area network (WLAN) access points in range of a WLAN-enabled mobile device, estimate a position of the WLAN-enabled mobile device;
estimate an expected error of the position estimate of the WLAN-enabled mobile device based on a combination of correlated parameters, the correlated parameters including two or more parameters selected from the group consisting of: a spatial spread associated with the geographic positions of the plurality of WLAN access points, the signal coverage areas of the plurality of WLAN access points used, and a number of the plurality of WLAN access points, wherein the expected error predicts a relative accuracy of the position estimate of the WLAN-enabled mobile device; and
use the expected error when providing one or more location-based services to a user of the WLAN-enabled mobile device.