Wi-Fi position fix

Patent metadata

Field	Value
Publication number	US-8994590-B2
Application number	US-201113249948-A
Country	US
Kind code	B2
Filing date	Sep 30, 2011
Priority date	Sep 30, 2011
Publication date	Mar 31, 2015
Grant date	Mar 31, 2015

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.

First claim

The invention claimed is:

1. A method of forming an estimate of a two-dimensional position of a radio receiver relative to a plurality of radio transmitters each radio transmitter having an associated position estimate and a position uncertainty expressed as an uncertainty ellipse having major and minor axes, respective lengths and orientations of the major and minor axes defining the uncertainty ellipse, the method comprising:
receiving the position estimates and position uncertainties of the plurality of radio transmitters;
receiving at the radio receiver respective signals from the plurality of radio transmitters and forming, by a weighting determination unit, in dependence on the received signals respective transmitter weighting values each weighting value being inversely proportional to a measure of distance between a respective radio transmitter of the plurality of radio transmitters and the radio receiver;
for each of the plurality of radio transmitters, scaling, by a position calculation unit, an uncertainty vector having an argument describing an orientation of the respective uncertainty ellipse in a predetermined coordinate system, defined relative to the major and minor axes of the uncertainty ellipse, by a scaling value, wherein the scaling value is proportional to the respective transmitter weighting value, and rotating the scaled uncertainty vector by multiplying its argument by a factor of four, wherein the argument of the uncertainty vector represents an angle that the uncertainty vector makes relative to a predetermined direction in the predetermined coordinate system;
summing, by the position calculation unit, the scaled and rotated vectors of the plurality of radio transmitters so as to form a total vector;
dividing, by the position calculation unit, 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, by a position calculation unit, 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 of each of the respective transmitters onto the first and second coordinate axes so as to form a set of projected components for each of the major and minor axes of each of the uncertainty ellipses;
summing the projected components of each of the respective major and minor axes of the uncertainty ellipses, so as to form respective total uncertainty values along each of the first and second coordinate axes;
for each uncertainty ellipse, forming first and second coordinate axis weighting values from the respective transmitter weighting values scaled such that each of the first and second coordinate axis weighting values for each uncertainty ellipse decreases as the total uncertainty along the first and second coordinate axis increases; and
calculating the position of the radio receiver by computing 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 the orientation of at least one of the major and minor axes 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 the predetermined direction.

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 a first predetermined length such that each scaling value varies from a value of zero when the length of the respective minor axis is equal to the first predetermined length up to a maximum value when the length of the respective minor axis is zero.

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

9. A method as claimed in claim 8 , wherein the first and second predetermined lengths are approximately 60 meters.

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 one of 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 a signal strength of the respective signal received from the respective radio transmitter, each 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 transmission power of the respective radio transmitter such that, prior to the scaling, the transmitter weighting values are approximately i

Assignees

• Jarvis Murray
• Tarlow Ben
• Cambridge Silicon Radio Ltd

Inventors

• Jarvis Murray
• Tarlow Ben

Classifications

• G01S5/02

  using radio waves (using satellite radio beacon systems for determining position G01S19/00) · CPC title

• G01S5/0278 Primary

  involving statistical or probabilistic considerations (G01S5/0252, G01S5/0294 take precedence) · CPC title

• G01S5/14

  Determining absolute distances from a plurality of spaced points of known location · CPC title