WI-FI POSITION FIX

• US 20130082878A1
• Filed: 09/30/2011
• Published: 04/04/2013
• Est. Priority Date: 09/30/2011
• Status: Active Grant

• Current Assignee: Qualcomm technologies international, ltd. (Qualcomm, Inc.)
• Original Assignee: Cambridge Silicon Radio Limited (Qualcomm, Inc.)
• Inventors: Tarlow, Ben, Jarvis, Murray
• Granted Patent: US 8,994,590 B2
• Time in Patent Office: Days
• US Class Current: 342/458
• CPC Class Codes: G01S 5/02 using radio waves using satellite radio beacon systems for determining position G01S19/00
• CPC Class Codes: G01S 5/0278 involving statistical or probabilistic considerations G01S5/0252, G01S5/0294 take precedence
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

A method of forming an estimate of the two-dimensional position of a radio receiver relative to a plurality of radio transmitters each having an associated position estimate and a position uncertainty expressible as an uncertainty ellipse having major and minor axes, the method comprising using the uncertainty vectors describing the uncertainty ellipses of the radio transmitters in a predetermined coordinate system to define a new compound coordinate basis, and forming an estimate of the two-dimensional position of the radio receiver in the compound coordinate basis by projecting the major and minor axes of each uncertainty ellipse onto the new compound coordinate basis and calculating the position of the radio receiver by means of a weighted centroid that uses weighting values calculated in the new compound coordinate basis and position estimates of the plurality of the radio transmitters expressed in the compound coordinate basis.

Claims

1. A method of forming an estimate of the two-dimensional position of a radio receiver relative to a plurality of radio transmitters each having an associated position estimate and a position uncertainty expressible as an uncertainty ellipse having major and minor axes, the method comprising:

receiving at the radio receiver signals from the plurality of radio transmitters and forming in dependence on those received signals a set of transmitter weighting values each expressing a measure of the distance of a respective radio transmitter from the radio receiver;

for each of the plurality of radio transmitters, scaling an uncertainty vector describing the orientation of the respective uncertainty ellipse in a predetermined coordinate system by a scaling value dependent on the respective transmitter weighting value and rotating that uncertainty vector by multiplying its argument by a factor of four;

summing the scaled and rotated vectors of the plurality of radio transmitters so as to form a total vector;

dividing the argument of the total vector by a factor of four and using the resulting rotated total vector to define a compound coordinate basis comprising first and second coordinate axes; and

forming an estimate of the two-dimensional position of the radio receiver in the compound coordinate basis by;

projecting the major and minor axes of each uncertainty ellipse onto the first and second coordinate axes so as to form a set of projected components for each axis;

summing the projected components of each axis so as to form a total uncertainty along each axis;

for each uncertainty ellipse, forming first and second coordinate axis weighting values from the respective transmitter weighting values scaled such that the first/second coordinate axis weighting value decreases as the total uncertainty along that first/second coordinate axis increases; and

calculating the position of the radio receiver by means of a weighted centroid using the first and second coordinate axis weighting values and the position estimates of the plurality of the radio transmitters expressed in the compound coordinate basis.

2. A method as claimed in claim 1, further comprising transforming the calculated position of the radio receiver into the predetermined coordinate system for use as an estimate of the two-dimensional position of a radio receiver.

3. A method as claimed in claim 1, wherein each uncertainty ellipse is represented by information expressing the length of the major and minor axes of the uncertainty ellipse and its orientation according to the predetermined coordinate system.

4. A method as claimed in claim 1, wherein the argument of the uncertainty vector describing the orientation of a respective uncertainty ellipse is the angle between a predetermined one of the major and minor axes of that uncertainty ellipse and a predetermined direction defined in the predetermined coordinate system.

5. A method as claimed in claim 1, wherein each scaling value used in the step of scaling the uncertainty vectors of the plurality of radio transmitters is further dependent on a measure of the anisotropy of the respective uncertainty ellipse.

6. A method as claimed in claim 5, wherein the measure of the anisotropy of each uncertainty ellipse ratio is given by a ratio of the major axis length to the minor axis length of the respective uncertainty ellipse.

7. A method as claimed in claim 1, wherein each scaling value is further dependent on a measure of the length of the minor axis of the respective uncertainty ellipse relative to the first predetermined length such that each scaling value varies from zero when the length of the respective minor axis is equal to the first predetermined length up to a maximum when the length of the respective minor axis is zero.

8. A method as claimed in claim 1, wherein the step of foaming each first and second coordinate axis weighting value from the transmitter weighting values is performed such that each first/second coordinate axis weighting value is given by the respective transmitter weighting value scaled by a factor that ranges from a predetermined minimum value when the total uncertainty along that first/second coordinate axis is at least a second predetermined length up to a maximum when the total uncertainty along that first/second coordinate axis is zero.

9. A method as claimed in claim 7, wherein the first and second predetermined lengths are approximately 60 metres.

10. A method as claimed in claim 8, wherein the predetermined minimum value is ⅛

.

11. A method as claimed in claim 1, wherein each uncertainty ellipse is defined with respect to the predetermined coordinate system.

12. A method as claimed in claim 1, wherein the first coordinate axis is represented by a normalised vector coincident with the rotated total vector and the second coordinate axis is represented by a normalised vector non-coincident with the rotated total vector.

13. A method as claimed in claim 1, wherein the first and second coordinate axes are defined so as to form an orthogonal basis set.

14. A method as claimed in claim 1, wherein the radio receiver is configured to receive the position estimate and position uncertainty of each of the plurality of radio transmitters by means of one or more of:

radio signals transmitted from one or more of the plurality of radio transmitters;

a database stored at the radio receiver; and

a database remotely accessible to the radio receiver.

15. A method as claimed in claim 1, wherein each uncertainty vector describing the orientation of a respective uncertainty ellipse is a unit vector defining the direction of the major or minor axis of that uncertainty ellipse in the predetermined coordinate system.

16. A method as claimed in claim 1, wherein the predetermined coordinate system is one in which two-dimensional positions are expressed with respect to magnetic north or a geodetic coordinate system.

17. A method as claimed in claim 1, wherein each transmitter weighting value is formed in dependence on the signal strength of the signals received from the respective radio transmitter, a transmitter weighting value being greater for a greater received signal strength.

18. A method as claimed in claim 17, wherein each transmitter weighting value is scaled in dependence on the power of the respective radio transmitter such that the transmitter weighting values are approximately independent of transmitter power, the transmitter power of each radio transmitter being signalled to the radio receiver by the respective radio transmitter.

19. A method as claimed in claim 1, wherein the step of summing the projected components of each axis so as to form a total uncertainty along each axis is performed such that the square of the total first/second axis uncertainty is given by the sum of the squares of the respective projected components.

20. A method as claimed in claim 1, further comprising, for each of the first and second coordinate axes:

calculating a correlated confidence estimate by means of a weighted centroid using the respective coordinate axis weighting values and the projected components of the major and minor axes of each uncertainty ellipse along that respective coordinate axis;

calculating an uncorrelated confidence estimate by;

forming for each uncertainty ellipse the product of the respective coordinate axis weighting value and the sum of the projected components along the respective coordinate axis of the major and minor axes of that uncertainty ellipse;

summing over the set of uncertainty ellipses the squares of those products; and

, dividing by the sum over the set of uncertainty ellipses of the squares of the respective coordinate axis weighting values;

forming a compound confidence estimate whose square is a weighted combination of the uncorrelated confidence estimate and the square of the correlated confidence estimate; and

using the compound confidence estimate as an estimate of the uncertainty of the position of the radio receiver due to the uncertainty in the position estimates of the plurality of radio transmitters.

21. A method as claimed in claim 20, further comprising:

forming an estimate of a weighting uncertainty in the position of the radio receiver due to the mechanism by which the transmitter weighting values are formed and optionally systematic errors due to the position of the radio receiver being calculated by means of a weighted centroid; and

combining the compound confidence estimate and the weighting uncertainty in quadrature so as to form an estimate of the total uncertainty in the estimated position of the radio receiver.

22. A method as claimed in claim 1, wherein the plurality of radio transmitters are Wi-Fi access points and the radio receiver is a portable Wi-Fi device.

23. A radio receiver operable to estimate its position relative to a plurality of radio transmitters, the radio receiver having access to information defining, for each radio transmitter, a position estimate and position uncertainty expressible as an uncertainty ellipse having major and minor axes, and the radio receiver comprising:

an antenna for receiving signals from the plurality of radio transmitters;

a weighting determination unit configured to form in dependence on signals received from the radio transmitters a set of transmitter weighting values each expressing a measure of the distance of a respective radio transmitter from the radio receiver; and

a position calculation unit configured to estimate the position of the radio receiver by;

for each of the plurality of radio transmitters, scaling an uncertainty vector describing the orientation of the respective uncertainty ellipse in a predetermined coordinate system by a scaling value dependent on the respective transmitter weighting value and rotating that uncertainty vector by multiplying its argument by a factor of four;

summing the scaled and rotated vectors of the plurality of radio transmitters so as to form a total vector;

dividing the argument of the total vector by a factor of four and using the resulting rotated total vector to define a compound coordinate basis comprising first and second coordinate axes; and

forming an estimate of the two-dimensional position of the radio receiver in the compound coordinate basis by;

projecting the major and minor axes of each uncertainty ellipse onto the first and second coordinate axes so as to form a set of projected components for each axis;

summing the projected components of each axis so as to form a total uncertainty along each axis;

for each uncertainty ellipse, forming first and second coordinate axis weighting values from the respective transmitter weighting values scaled such that the first/second coordinate axis weighting value decreases as the total uncertainty along that first/second coordinate axis increases; and

calculating the position of the radio receiver by means of a weighted centroid using the first and second coordinate axis weighting values and the position estimates of the plurality of the radio transmitters expressed in the compound coordinate basis.

24. A method of forming an estimate of the two-dimensional position of a radio transmitter relative to a plurality of radio receiver positions each having a corresponding position uncertainty expressible as an uncertainty ellipse having major and minor axes, the method comprising:

receiving at the radio receiver positions signals from the radio transmitter and forming in dependence on those received signals a set of receiver weighting values each expressing a measure of the distance of the radio transmitter from the radio receiver positions;

for each of the plurality of radio receiver positions, scaling an uncertainty vector describing the orientation of the respective uncertainty ellipse in a predetermined coordinate system by a scaling value dependent on the respective receiver weighting value and rotating that uncertainty vector by multiplying its argument by a factor of four;

summing the scaled vectors of the plurality of radio receiver positions so as to form a total vector;

dividing the argument of the total vector by a factor of four and using the resulting rotated total vector to define a compound coordinate basis comprising first and second coordinate axes; and

forming an estimate of the two-dimensional position of the radio transmitter in the compound coordinate basis by;

projecting the major and minor axes of each uncertainty ellipse onto the first and second coordinate axes so as to form a set of projected components for each axis;

summing the projected components of each axis so as to form a total uncertainty along each axis;

for each uncertainty ellipse, forming first and second coordinate axis weighting values from the respective transmitter weighting values scaled such that the first/second coordinate axis weighting value decreases as the total uncertainty along that first/second coordinate axis increases; and

calculating the position of the radio transmitter by means of a weighted centroid using the first and second coordinate axis weighting values and the radio receiver positions expressed in the compound coordinate basis.

25. A method as claimed in claim 24, further comprising transforming the calculated position of the radio transmitter into the predetermined coordinate system for use as an estimate of the two-dimensional position of a radio transmitter.

26. A method as claimed in claim 24, wherein the argument of the uncertainty vector describing the orientation of a respective uncertainty ellipse is the angle between a predetermined one of the major and minor axes of that uncertainty ellipse and a predetermined direction defined in the predetermined coordinate system.

27. A method as claimed in claim 24, wherein each uncertainty ellipse is represented by information expressing the length of the major and minor axes of the uncertainty ellipse and its orientation according to the predetermined coordinate system.

28. A method as claimed in claim 24, wherein the steps of the method are performed at a radio receiver arranged to move between the plurality of radio receiver positions so as to receive said signals from the radio transmitter at those radio receiver positions.

29. A method as claimed in claim 28, wherein the radio receiver is configured to determine its position and its position uncertainty as an uncertainty ellipse having major and minor axes.

30. A method as claimed in claim 29, wherein the radio receiver is configured to determine its position and its position uncertainty by means of a GPS receiver.

31. A method as claimed in claim 28, wherein the radio transmitter is a Wi-Fi access point and the radio receiver is a scanning vehicle supporting a Wi-Fi receiver and a GPS receiver.

32. A method as claimed in claim 24, wherein a different radio receiver is provided at each of the radio receiver positions so as to collectively receive said signals from the radio transmitter at those radio receiver positions.

33. A method as claimed in claim 32, wherein the radio receivers are configured to exchange their respective receiver weighting values so as to allow one or more of the radio receivers to form an estimate of the two-dimensional position of the radio transmitter.

34. A method as claimed in claim 32, wherein the estimate of the two-dimensional position of the radio transmitter is formed at a calculation server coupled to the plurality of radio receivers and configured to receive their respective receiver weighting values.

35. A method as claimed in claim 24, wherein each scaling value used in the step of scaling the uncertainty vectors of the plurality of radio receivers is further dependent on a measure of the anisotropy of the respective uncertainty ellipse.

36. A method as claimed in claim 24, wherein each scaling value is further dependent on a measure of the length of the minor axis of the respective uncertainty ellipse relative to the first predetermined length such that each scaling value varies from zero when the length of the respective minor axis is equal to the first predetermined length up to a maximum when the length of the respective minor axis is zero.

37. A method as claimed in claim 24, wherein the step of forming each first and second coordinate axis weighting value from the receiver weighting values is performed such that each first/second coordinate axis weighting value is given by the respective receiver weighting value scaled by a factor that ranges from a predetermined minimum value when the total uncertainty along that first/second coordinate axis is at least a second predetermined length up to a maximum when the total uncertainty along that first/second coordinate axis is zero.

38. A method as claimed in claim 24, wherein each uncertainty ellipse is defined with respect to the predetermined coordinate system.

39. A method as claimed in claim 24, wherein the first coordinate axis is represented by a normalised vector coincident with the rotated total vector and the second coordinate axis is represented by a normalised vector orthogonal to the rotated total vector.

40. A method as claimed in claim 24, wherein each uncertainty vector describing the orientation of a respective uncertainty ellipse is a unit vector defining the direction of the major or minor axis of that uncertainty ellipse in the predetermined coordinate system.

41. A method as claimed in claim 24, wherein each receiver weighting value is formed in dependence on the signal strength of the signals received at the respective radio receiver position from the radio transmitter, a receiver weighting value being greater for a greater received signal strength.

42. A method as claimed in claim 24, wherein the step of summing the projected components of each axis so as to form a total uncertainty along each axis is performed such that the square of the total first/second axis uncertainty is given by, the sum of the squares of the respective projected components.

43. A method as claimed in claim 24, further comprising, for each of the first and second coordinate axes:

calculating a correlated confidence estimate by means of a weighted centroid using the respective coordinate axis weighting values and the projected components of the major and minor axes of each uncertainty ellipse along that respective coordinate axis;

calculating an uncorrelated confidence estimate by;

forming for each uncertainty ellipse the product of the respective coordinate axis weighting value and the sum of the projected components along the respective coordinate axis of the major and minor axes of that uncertainty ellipse;

summing over the set of uncertainty ellipses the squares of those products; and

, dividing by the sum over the set of uncertainty ellipses of the squares of the respective coordinate axis weighting values;

forming a compound confidence estimate whose square is a weighted combination of the uncorrelated confidence estimate and the square of the correlated confidence estimate; and

using the compound confidence estimate as an estimate of the uncertainty of the position of the radio transmitter due to the uncertainty in the position estimates of the plurality of radio receiver positions.

44. A method as claimed in claim 43, further comprising:

forming an estimate of a weighting uncertainty in the position of the radio transmitter due to the mechanism by which the receiver weighting values are formed and optionally systematic errors due to the position of the radio transmitter being calculated by means of a weighted centroid; and

combining the compound confidence estimate and the weighting uncertainty in quadrature so as to form an estimate of the total uncertainty in the estimated position of the radio transmitter.

45. A method as claimed in claim 24, wherein the predetermined coordinate system is one in which two-dimensional positions are expressed with respect to magnetic north or a geodetic coordinate system.

46. A radio receiver operable to estimate the position of a radio transmitter, the radio receiver being arranged to move between a plurality of radio receiver positions and being operable to determine its position uncertainty at each of those positions as an uncertainty ellipse having major and minor axes, and the radio receiver comprising:

an antenna for receiving signals from the radio transmitter;

a weighting determination unit configured to form in dependence on signals received from the radio transmitter a set of transmitter weighting values each expressing a measure of the distance of the radio transmitter from the radio receiver positions; and

a position calculation unit configured to estimate the position of the radio transmitter by;

for each of the plurality of radio receiver positions, scaling an uncertainty vector describing the orientation of the respective uncertainty ellipse in a predetermined coordinate system by a scaling value dependent on the respective receiver weighting value and rotating that uncertainty vector by multiplying its argument by a factor of four;

summing the scaled vectors of the plurality of radio receiver positions so as to form a total vector;

dividing the argument of the total vector by a factor of four and using the resulting rotated total vector to define a compound coordinate basis comprising first and second coordinate axes; and

forming an estimate of the two-dimensional position of the radio transmitter in the compound coordinate basis by;

projecting the major and minor axes of each uncertainty ellipse onto the first and second coordinate axes so as to form a set of projected components for each axis;

summing the projected components of each axis so as to form a total uncertainty along each axis;

for each uncertainty ellipse, forming first and second coordinate axis weighting values from the respective transmitter weighting values scaled such that the first/second coordinate axis weighting value decreases as the total uncertainty along that first/second coordinate axis increases; and

calculating the position of the radio transmitter by means of a weighted centroid using the first and second coordinate axis weighting values and the radio receiver positions expressed in the compound coordinate basis.

47. (canceled)