Received signal strength distance determination of low frequency tags

• US 8,111,141 B1
• Filed: 03/30/2007
• Issued: 02/07/2012
• Est. Priority Date: 03/30/2007
• Status: Active Grant

• Current Assignee: Savi Technology Incorporated
• Original Assignee: Savi Technology Incorporated
• Inventors: Ahn, Don, Brand, Timothy K., Hilliker, Stephen Earl, Chan, Joseph Siufai, Nardelli, Albert, Cargonja, Nikola, Skracic, Bruno
• Primary Examiner(s): BROWN, VERNAL U
• Application Number:
  US11/694,905
• Time in Patent Office: 1,775 Days
• Field of Search: 340/10.5, 340/539.13, 340/992, 340/10.3, 342/47
• US Class Current: 340/10.5
• CPC Class Codes: H04B 17/318 Received signal strength
• CPC Class Codes: H04B 5/0056 for use in interrogation, identification or read/write systems record carriers G06K7/00, G06K19/00; for railways B61L3/12
• CPC Class Codes: H04B 5/0081 with antenna coils loop aerials H01Q7/00
• CPC Class Codes: H04B 5/26 using coils
• CPC Class Codes: H04B 5/77 for interrogation

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 determining a distance between a signpost and a tag, the method comprising:

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

measuring at the tag a first signal strength of the near-field magnetic signal received from the signpost with respect to a first antenna on the tag;

calculating a received signal strength indication using the first signal strength of the near-field magnetic signal received from the signpost at the first antenna;

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

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

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

determining location information for the tag using the measured signal strengths from the first antenna, the second antenna, and the third antenna, wherein determining the location information comprises calculating an eight position vector using the measured signal strength of the signals received from the signpost at the first antenna, the second antenna, and the third antenna to yield eight possible locations for the tag.

2. The method of claim 1, further comprising:

establishing a signal strength qualification level for the signal received from the signpost at the first antenna;

comparing the first signal strength to the signal strength qualification level; and

validating the signal received with respect to the first antenna if the first signal strength meets the signal strength qualification level.

3. The method of claim 2, wherein determining the distance between the signpost and the tag is in response to validating the signal received at the first antenna.

4. The method of claim 2, further comprising rejecting the signal received with respect to the first antenna as spurious if the first signal strength is less than the signal strength qualification level.

5. The method of claim 2, wherein the signal strength qualification level is configurable by the signpost and provided to the tag by the signpost signal.

6. The method of claim 1, further comprising:

establishing a signal strength qualification level for the signal received from the signpost with respect to the first antenna;

comparing the calculated signal strength indication to the signal strength qualification level; and

validating the signal received with respect to the first antenna if the calculated signal strength indication meets the signal strength qualification level.

7. The method of claim 1, wherein:

the second antenna is oriented orthogonal to the first antenna; and

the received signal strength indication is calculated using the signals received at the first and the second antennas.

8. The method of claim 7, wherein the calculating comprises taking a square root of a sum of squares of the measured signal strength of the signals received at the first and the second antennas.

9. The method of claim 7, wherein:

the third antenna is oriented orthogonal to the second antenna and the first antenna; and

the received signal strength indication is calculated using the signals received at the three antennas.

10. The method of claim 9, wherein the calculating comprises taking a square root of a sum of squares of the measured signal strength of the signals received at the three antennas.

11. The method of claim 1, wherein the location information determination comprises determining two intersecting planes comprising possible locations for the tag using the measured signal strength from the first antenna and the second antenna.

12. The method of claim 1, 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 signpost.

13. The method of claim 1, wherein determining the distance between the signpost and the tag takes place at the tag.

14. The method of claim 1, further comprising transmitting the received signal strength indication to a site server, wherein determining the distance between the signpost and the tag takes place at the site server.

15. The method of claim 1, wherein measuring at the tag the signal strength of the signal received from the signpost with respect to the first antenna takes place at a first time, further comprising:

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

calculating a received signal strength indication using the measured signal strength of the signal received at the second time;

determining a distance between the signpost and the tag at the second time based on the calculated signal strength indication at the second time; and

thereby determining a direction of movement of the tag relative to the signpost over time.

16. A tag, comprising:

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

a measure module for measuring at the tag a first signal strength of the near-field magnetic signal received from the signpost with respect to a first antenna on the tag, measuring at the tag a second signal strength of a signal received from the signpost with respect to a second antenna on the tag, measuring at the tag a third signal strength of a signal received from the signpost with respect to a third antenna on the tag, the measure module enabled to receive signals from the receive module;

a distance module for determining a distance between the signpost and the tag based on the first signal strength, the distance module enabled to receive signals from the measure module; and

a location module for determining location information for the tag using the measured signal strengths from the first antenna, the second antenna, and the third antenna, wherein determining the location information comprises calculating an eight position vector using the measured signal strength of the signals received from the signpost at the first antenna, the second antenna, and the third antenna to yield eight possible locations for the tag.

17. The tag of claim 16, further comprising:

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

wherein determining the distance between the signpost and the tag is based on the calculated received signal strength indication, the distance module enabled to receive signals from the calculate module.

18. The tag of claim 16, wherein:

the second antenna is oriented orthogonal to the antenna on the tag; and

the calculate module is further configured to calculate the received signal strength indication using the signals received at the first and the second antennas.

19. The tag of claim 18, wherein the calculating comprises taking a square root of a sum of squares of the measured signal strength of the signals received at the first and the second antennas.

20. The tag of claim 18, wherein:

the third antenna oriented is orthogonal to the second antenna and the antenna on the tag; and

wherein the calculate module is further configured to calculate the received signal strength indication using the signals received at the three antennas.

21. The tag of claim 18, wherein the calculating comprises taking a square root of a sum of squares of the measured signal strength of the signals received at the three antennas.

22. The tag of claim 16, further comprising:

a validation module enabled to receive signals from the measure module, configured for;

establishing a signal strength qualification level for the signal received from the signpost with respect to the first antenna;

comparing the calculated signal strength indication to the signal strength qualification level; and

validating the signal received with respect to the first antenna if the calculated signal strength indication meets the signal strength qualification level.

23. The tag of claim 22, wherein determining the distance between the signpost and the tag is in response to validating the signal received with respect to the first antenna, the distance module enabled to receive signals from the validation module.

24. The tag of claim 22, wherein the validation module is further configured for rejecting the signal received with respect to the first antenna as spurious if the first signal strength is less than the signal strength qualification level.

25. The tag of claim 22, wherein the signal strength qualification level is configurable by the signpost and provided to the tag by the signpost signal.

26. The tag of claim 16, further comprising:

a validation module enabled to receive signals from the calculate module, configured for;

establishing a signal strength qualification level for the signal received from the signpost with respect to the first antenna;

comparing the calculated signal strength indication to the signal strength qualification level; and

validating the signal received with respect to the first antenna if the calculated signal strength indication meets the signal strength qualification level.

27. The tag of claim 16, wherein measuring at the tag the signal strength of the signal received from the signpost with respect to the first antenna takes place at a first time, wherein:

the measure module is further configured for measuring at the tag a signal strength of a signal received from the signpost with respect to the first antenna at a second time;

the calculate module is further configured for calculating a received signal strength indication using the measured signal strength of the signal received at the second time; and

the distance module is further configured for determining a distance between the signpost and the tag at the second time based on the calculated signal strength indication at the second time, thereby determining a direction of movement of the tag relative to the signpost over time.

28. A signpost, comprising:

a transmit module for transmitting to a tag a near-field magnetic signal;

a receive module for receiving from the tag information regarding a first signal strength measured with respect to a first antenna on the tag, a second signal strength measured with respect to a second antenna on the tag, and a third signal strength measured with respect to a third antenna on the tag;

a calculate module for calculating a received signal strength indication using the first signal strength;

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

a location module for determining location information for the tag using the measured signal strengths from the first antenna, the second antenna, and the third antenna, wherein determining the location information comprises calculating an eight position vector using the measured signal strength of the signals received from the signpost at the first antenna, the second antenna, and the third antenna to yield eight possible locations for the tag.

29. The signpost of claim 28, wherein a signal strength qualification level is configurable by the signpost and provided to the tag by the near-field magnetic signal.

30. A system, comprising:

a signpost for transmitting a near-field magnetic signal; and

a tag for receiving the near-field magnetic signal from the signpost and for determining a distance between the signpost and the tag using a signal strength of the received signal, the tag comprising;

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

a measure module for measuring at the tag a first signal strength of the near-field magnetic signal received from the signpost with respect to a first antenna on the tag, measuring at the tag a second signal strength of a signal received from the signpost with respect to a second antenna on the tag, measuring at the tag a third signal strength of a signal received from the signpost with respect to a third antenna on the tag, the measure module enabled to receive signals from the receive module;

a distance module for determining a distance between the signpost and the tag based on the first signal strength, the distance module enabled to receive signals from the measure module; and

a location module for determining location information for the tag using the measured signal strengths from the first antenna, the second antenna, and the third antenna, wherein determining the location information comprises calculating an eight position vector using the measured signal strength of the signals received from the signpost at the first antenna, the second antenna, and the third antenna to yield eight possible locations for the tag.

31. A computer program product comprising a computer-readable medium having computer program instructions embodied thereon for determining a distance between a signpost and a tag, the computer program product comprising:

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

a measure module for measuring at the tag a first signal strength of the near-field magnetic signal received from the signpost with respect to a first antenna on the tag, measuring at the tag a second signal strength of a signal received from the signpost with respect to a second antenna on the tag, measuring at the tag a third signal strength of a signal received from the signpost with respect to a third antenna on the tag, the measure module enabled to receive signals from the receive module;

a distance module for determining a distance between the signpost and the tag based on the first signal strength, the distance module enabled to receive signals from the measure module; and

a location module for determining location information for the tag using the measured signal strengths from the first antenna, the second antenna, and the third antenna, wherein determining the location information comprises calculating an eight position vector using the measured signal strength of the signals received from the signpost at the first antenna, the second antenna, and the third antenna to yield eight possible locations for the tag.

32. The computer program product of claim 31, further comprising:

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

wherein determining the distance between the signpost and the tag is based on the calculated received signal strength indication, the distance module enabled to receive signals from the calculate module.

33. The computer program product of claim 31, further comprising:

a validation module configured for;

establishing a signal strength qualification level for the signal received from the signpost with respect to the first antenna;

comparing the calculated signal strength indication to the signal strength qualification level; and

validating the signal received with respect to the first antenna if the calculated signal strength indication meets the signal strength qualification level.

34. The computer program product of claim 33, wherein determining the distance between the signpost and the tag is in response to validating the signal.

35. The computer program product of claim 33, wherein the validation module is further configured for rejecting the signal received with respect to the first antenna as spurious if the first signal strength is less than the signal strength qualification level.

36. The computer program product of claim 33, wherein the signal strength qualification level is configurable by the signpost and provided to the tag by the signpost signal.

37. The computer program product of claim 31, wherein measuring at the tag the signal strength of the signal received from the signpost with respect to the first antenna takes place at a first time, wherein:

the measure module is further configured for measuring at the tag a signal strength of a signal received from the signpost with respect to the first antenna at a second time;

the calculate module is further configured for calculating a received signal strength indication using the measured signal strength of the signal received at the second time; and

the distance module is further configured for determining a distance between the signpost and the tag at the second time based on the calculated signal strength indication at the second time, thereby determining a direction of movement of the tag relative to the signpost over time.