Received signal strength location determination of low frequency tags

• US 7,768,392 B1
• Filed: 03/30/2007
• Issued: 08/03/2010
• Est. Priority Date: 03/30/2007
• Status: Active Grant

• Current Assignee: Savi Technology Incorporated
• Original Assignee: Savi Technology Incorporated
• Inventors: Cargonja, Nikola, Ahn, Don, Brand, Timothy K., Nardelli, Albert, Hilliker, Stephen Earl, Skracic, Bruno, Chan, Joseph Siufai
• Primary Examiner(s): Nguyen, Phung
• Application Number:
  US11/694,908
• Time in Patent Office: 1,222 Days
• Field of Search: 340/572.1, 340/825.49, 340/10.4, 340/10.5, 340/825.36, 340/993, 340/572.8, 235/384
• US Class Current: 340/539.13
• CPC Class Codes: G01D 21/00 Measuring or testing not otherwise provided for
• CPC Class Codes: G01S 11/06 using intensity measurements
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

Systems and methods provide for determining a distance between a signpost and a tag and for locating a tag using multiple signposts. The system uses a signal received at a tag from a signpost, and from them measures the signal strength of the signal with respect to one or more antennas on the tag. The system calculates an RSSI using the measured signal strength, from which a distance can be determined between the signpost and the tag. The system also uses signals received from multiple signposts, to measure multiple signal strengths, resulting in multiple distance determinations corresponding to the various signposts, thereby establishing a location for the tag.

Claims

1. A method of establishing a location for a tag using a plurality of signposts, the method comprising:

receiving at the tag a near-field magnetic signal from a first signpost;

receiving at the tag a near-field magnetic signal from a second signpost;

measuring at the tag a signal strength for the received signal from the first signpost with respect to an antenna on the tag;

measuring at the tag a signal strength for the received signal from the second signpost with respect to the antenna;

calculating a received signal strength indication using the measured signal strength of the received signals from the first signpost;

calculating a received signal strength indication using the measured signal strength of the received signals from the second signpost; and

determining a location for the tag using the calculated received signal strength indications from the first and second signposts.

2. The method of claim 1, further comprising:

a second antenna on the tag, the second antenna oriented orthogonal to the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the second antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the second antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the second antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the second antenna.

3. The method of claim 2, further comprising:

a third antenna on the tag, the third antenna oriented orthogonal to the second antenna and the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the third antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the third antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the third antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the third antenna.

4. The method of claim 3, further comprising:

calculating a first signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the first signpost;

calculating a second signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the second signpost; and

wherein determining the location for the tag further comprises using the first signpost eight position vector to establish a first set of eight possible locations for the tag with respect to the first signpost.

5. The method of claim 4, wherein determining the location for the tag further comprises using the second signpost eight position vector to establish a second set of eight possible locations for the tag with respect to the second signpost.

6. The method of claim 5, wherein determining the location for the tag further comprises comparing the first set of eight possible locations to the second set of eight possible locations.

7. The method of claim 6, wherein the information about placement of the tag relative to the first and second signposts comprises an indication that the first and second signposts are stationarily mounted to a structure above the tag, and the eliminated locations comprise locations above the first and second signposts.

8. The method of claim 5, wherein determining the location for the tag further comprises eliminating some of the first set of eight possible locations for the tag with respect to the first signpost and the second set of eight possible locations for the tag with respect to the second signpost using known information about placement of the tag relative to the first and second signposts.

9. The method of claim 1, further comprising:

determining a distance between the tag and the first signpost based on the calculated received signal strength indication for the first signpost; and

determining a distance between the tag and the second signpost based on the calculated received signal strength indication for the second signpost.

10. The method of claim 9, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag; and

determining a location for the tag further comprises determining the intersection of the first sphere and the second sphere.

11. The method of claim 9, further comprising:

receiving at the tag a near-field magnetic signal from a third signpost;

measuring at the tag a signal strength for the received signal from the third signpost with respect to an antenna on the tag;

calculating a received signal strength indication using the measured signal strength of the received signal from the third signpost;

wherein determining the location for the tag further comprises using the calculated received signal strength indication from the third signpost; and

determining a distance between the tag and the third signpost based on the calculated received signal strength indication for the third signpost.

12. The method of claim 11, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag;

the determined distance between the tag and the third signpost comprises a distance defining a radius of a third sphere of possible locations for the tag; and

determining a location for the tag further comprises determining the intersection of the first sphere, the second sphere, and the third sphere.

13. The method of claim 1, further comprising:

comparing the measured signal strength for the received signal from the first signpost to the measured signal strength for the received signal from the second signpost to establish relative signal strengths between the first and second signposts; and

wherein the relative signal strengths are proportional to relative distances between the tag and the first signpost and the tag and the second signpost.

14. The method of claim 13, wherein establishing the location for the tag further comprises using the relative distances between the tag and the first signpost and the tag and the second signpost.

15. The method of claim 1, wherein determining a location for the tag using the calculated received signal strength indications from the first and second signposts takes place at the tag.

16. The method of claim 1, further comprising:

transmitting the measured signal strengths for the received signals from the first and second signposts to a site server; and

wherein determining a location for the tag using the calculated received signal strength indications from the first and second signposts takes place at the site server.

17. The method of claim 1, wherein the tag further comprises a second antenna, the second antenna oriented orthogonal to the antenna on the tag, further comprising:

measuring at the tag a signal strength of the signal received from the first signpost with respect to the second antenna;

measuring at the tag a signal strength of the signal received from the second signpost with respect to the second antenna; and

determining location information for the tag using the measured signal strengths from the antenna on the tag and the second antenna.

18. The method of claim 17, wherein the location information determination further comprises:

determining two intersecting planes comprising possible locations for the tag using the measured signal strength from the antenna on the tag; and

determining two intersecting planes comprising possible locations for the tag using the measured signal strength from the second antenna.

19. The method of claim 18, wherein determining the location for the tag further comprises comparing the possible locations for the tag using the measured signal strength from the antenna on the tag to the possible locations for the tag using the measured signal strength from the second antenna.

20. The method of claim 1, wherein the tag further comprises a second antenna and a third antenna, the second antenna oriented orthogonal to the antenna on the tag, the third antenna oriented orthogonal to the second antenna and the antenna on the tag further comprising:

measuring at the tag a signal strength of the signal received from the first signpost with respect to the second antenna;

measuring at the tag a signal strength of the signal received from the second signpost with respect to the second antenna;

measuring at the tag a signal strength of the signal received from the first signpost with respect to the third antenna;

measuring at the tag a signal strength of the signal received from the second signpost with respect to the third antenna;

determining location information for the tag using the measured signal strengths from the antenna on the tag, the second antenna, and the third antenna.

21. The method of claim 20, wherein the location information determination comprises calculating an eight position vector using the measured signal strength of the signals received from the first signpost at the antenna on the tag, the second antenna, and the third antenna to yield eight possible locations for the tag with respect to the first signpost.

22. The method of claim 21, wherein the location information determination further comprises eliminating some of eight possible locations for the tag with respect to the first signpost using known information about placement of the tag relative to the first signpost.

23. The method of claim 21, wherein the location information determination further comprises calculating an eight position vector using the measured signal strength of the signals received from the second signpost at the antenna on the tag, the second antenna, and the third antenna to yield eight possible locations for the tag with respect to the second signpost.

24. The method of claim 23, wherein determining the location for the tag further comprises comparing the eight possible locations for the tag with respect to the first signpost to the eight possible locations for the tag with respect to the second signpost.

25. A tag, comprising:

a receive module for receiving at the tag a near-field magnetic signal from a first signpost and a near-field magnetic signal from a second signpost;

a measure module for measuring at the tag a signal strength for the received signal from the first signpost and a signal strength for the received signal from the second signpost with respect to an antenna on the tag, the measure module enabled to receive signals from the receive module;

a calculate module for calculating a received signal strength indication using the measured signal strength of the received signals from the first signpost and calculating a received signal strength indication using the measured signal strength of the received signals from the second signpost, the calculate module enabled to receive signals from the measure module; and

a location module for determining a location for the tag using the calculated received signal strength indications from the first and second signposts, the location module enabled to received signals from the calculate module.

26. The tag of claim 25, further comprising:

a distance module for determining a distance between the tag and the first signpost based on the calculated received signal strength indication for the first signpost and for determining a distance between the tag and the second signpost based on the calculated received signal strength indication for the second signpost, the distance module enabled to receive signals from the calculate module.

27. The tag of claim 26, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag; and

the distance module is further configured for determining the intersection of the first sphere and the second sphere.

28. The tag of claim 26, wherein:

the receive module is further configured for receiving at the tag a near-field magnetic signal from a third signpost;

the measure module is further configured for measuring at the tag a signal strength for the received signal from the third signpost with respect to an antenna on the tag;

the calculate module is further configured for calculating a received signal strength indication using the measured signal strength of the received signal from the third signpost;

wherein determining the location for the tag further comprises using the calculated received signal strength indication from the third signpost; and

the distance module is further configured for determining a distance between the tag and the third signpost based on the calculated received signal strength indication for the third signpost.

29. The tag of claim 28, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag;

the determined distance between the tag and the third signpost comprises a distance defining a radius of a third sphere of possible locations for the tag; and

the location module is further configured for determining a location for the tag further comprises determining the intersection of the first sphere, the second sphere, and the third sphere.

30. The tag of claim 25, further comprising:

a second antenna on the tag, the second antenna oriented orthogonal to the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the second antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the second antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the second antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the second antenna.

31. The tag of claim 30, further comprising:

a third antenna on the tag, the third antenna oriented orthogonal to the second antenna and the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the third antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the third antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the third antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the third antenna.

32. The tag of claim 31, wherein the calculate module is further configured for:

calculating a first signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the first signpost;

calculating a second signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the second signpost; and

the location module is further configured for using the first signpost eight position vector to establish a first set of eight possible locations for the tag with respect to the first signpost.

33. The tag of claim 32, wherein the location module is further configured for using the second signpost eight position vector to establish a second set of eight possible locations for the tag with respect to the second signpost.

34. The tag of claim 33, wherein the location module is further configured for comparing the first set of eight possible locations to the second set of eight possible locations.

35. The tag of claim 33, wherein the location module is further configured for eliminating some of the first set of eight possible locations for the tag with respect to the first signpost and the second set of eight possible locations for the tag with respect to the second signpost using known information about placement of the tag relative to the first and second signposts.

36. A computer program product comprising a computer-readable storage medium having computer program instructions embodied thereon for establishing a location for a tag using a plurality of signposts, the computer program product comprising:

a receive module for receiving at the tag a near-field magnetic signal from a first signpost and a near-field magnetic signal from a second signpost;

a measure module for measuring at the tag a signal strength for the received signal from the first signpost and a signal strength for the received signal from the second signpost with respect to an antenna on the tag;

a calculate module for calculating a received signal strength indication using the measured signal strength of the received signals from the first signpost and calculating a received signal strength indication using the measured signal strength of the received signals from the second signpost; and

a location module for determining a location for the tag using the calculated received signal strength indications from the first and second signposts.

37. The computer program product of claim 36, further comprising:

a distance module for determining a distance between the tag and the first signpost based on the calculated received signal strength indication for the first signpost and for determining a distance between the tag and the second signpost based on the calculated received signal strength indication for the second signpost.

38. The computer program product of claim 37, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag; and

the distance module is further configured for determining the intersection of the first sphere and the second sphere.

39. The computer program product of claim 38, wherein:

the receive module is further configured for receiving at the tag a near-field magnetic signal from a third signpost;

the measure module is further configured for measuring at the tag a signal strength for the received signal from the third signpost with respect to an antenna on the tag;

the calculate modules is further configured for calculating a received signal strength indication using the measured signal strength of the received signal from the third signpost;

wherein determining the location for the tag further comprises using the calculated received signal strength indication from the third signpost; and

the distance module is further configured for determining a distance between the tag and the third signpost based on the calculated received signal strength indication for the third signpost.

40. The computer program product of claim 39, wherein:

the determined distance between the tag and the first signpost comprises a distance defining a radius of a first sphere of possible locations for the tag;

the determined distance between the tag and the second signpost comprises a distance defining a radius of a second sphere of possible locations for the tag;

the determined distance between the tag and the third signpost comprises a distance defining a radius of a third sphere of possible locations for the tag; and

the location module is further configured for determining a location for the tag further comprises determining the intersection of the first sphere, the second sphere, and the third sphere.

41. The computer program product of claim 36, further comprising:

a second antenna on the tag, the second antenna oriented orthogonal to the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the second antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the second antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the second antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the second antenna.

42. The computer program product of claim 41, further comprising:

a third antenna on the tag, the third antenna oriented orthogonal to the second antenna and the antenna on the tag;

wherein measuring the signal strength for the received signal from the first signpost is further measured with respect to the third antenna;

wherein measuring the signal strength for the received signal from the second signpost is further measured with respect to the third antenna;

wherein calculating the received signal strength indication using the measured signal strength of the received signals the first signpost further comprises calculating with respect to the third antenna; and

wherein calculating the received signal strength indication using the measured signal strength of the received signals the second signpost further comprises calculating with respect to the third antenna.

43. The computer program product of claim 42, wherein the calculate module is further configured for:

calculating a first signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the first signpost;

calculating a second signpost eight position vector using the measured signal strength of the signals received from the signpost at the antenna on the tag, the second antenna, and the third antenna at the second signpost; and

the location module is further configured for using the first signpost eight position vector to establish a first set of eight possible locations for the tag with respect to the first signpost.

44. The computer program product of claim 43, wherein the location module is further configured for using the second signpost eight position vector to establish a second set of eight possible locations for the tag with respect to the second signpost.

45. The computer program product of claim 44, wherein the location module is further configured for comparing the first set of eight possible locations to the second set of eight possible locations.

46. The computer program product of claim 44, wherein the location module is further configured for eliminating some of the first set of eight possible locations for the tag with respect to the first signpost and the second set of eight possible locations for the tag with respect to the second signpost using known information about placement of the tag relative to the first and second signposts.