Location system and method that achieves time synchronized network performance using unsynchronized receiver clocks

• US 20050280578A1
• Filed: 11/03/2004
• Published: 12/22/2005
• Est. Priority Date: 11/07/2003
• Status: Active Grant

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Wherenet Corporation (Zebra Technologies Corporation)
• Inventors: Boyd, Robert W.
• Granted Patent: US 7,190,271 B2
• Time in Patent Office: Days
• US Class Current: 342/465
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence

Abstract

A system for locating an object within a monitored environment includes a tag transmitter that transmits an RF signal to receivers having unsynchronized clocks. A processor is operative with each receiver and determines clock timing relationships for the clocks based on one of at least propagation or processing delays in the receivers to synchronize receiver performance.

Claims

1. A system for locating an object within a monitored environment comprising:

a tag transmitter that transmits an RF signal and associated with an object;

a plurality of receivers within the monitored environment for receiving the RF signal from the tag transmitter, said receivers having unsynchronized clocks;

a processor operative with each receiver for determining clock timing relationships for the clocks based on one of at least the propagation or processing delays in the receivers for synchronizing receiver performance, wherein said processor is operative for conducting differentiation of the time of arrival signals received from a tag transmitter to locate an object associated with the tag transmitter.

2. A system according to claim 1, wherein said tag transmitter is positioned at a fixed location.

3. A system according to claim 1, and further comprising a pair of tag transmitters which transmit RF signals to receivers with known propagation delay.

4. A system according to claim 1, wherein the processing delay within each receiver is substantially constant with each other.

5. A system according to claim 1, wherein the processor is operative for processing two time separated signals for determining a differential clock rate.

6. A system according to claim 5, wherein the processor is operative for processing three time separated signals to indicate differential rate change.

7. A system according to claim 6, wherein the processor is operative for maintaining a model of timing relationships of clocks at the receivers.

8. A system according to claim 1, wherein said receivers are positioned at known locations.

9. A system according to claim 1, wherein said RF signal transmitted from a tag transmitter includes data indentifying an object associated with the tag transmitter.

10. A system according to claim 1, wherein said RF signal further comprises a pseudo random spread spectrum RF signal.

11. A system according to claim 1, wherein said processor is operative to relate the clocks of each receiver to a system time.

12. A system according to claim 1, wherein said processor is operative for forming a tracking loop to track clock relationships that occur naturally over time.

13. A system according to claim 1, which further comprises determining an equation relating the indicated time of one clock at a respective receiver to another receiver clock and tracking changes in that relationship over time.

14. A system for time synchronizing a network comprising:

a tag transmitter positioned at a known location that transmits an RF signal;

a plurality of receivers operative as network nodes for receiving the RF signal from the tag transmitter, said receivers having unsynchronized clocks;

a processor operative with each receiver for determining clock timing relationships for the clocks at a receiver based on one of at least the propagation or processing delays in the receivers for synchronizing receiver performance.

15. A system according to claim 14, wherein said tag transmitter is positioned at a fixed location.

16. A system according to claim 14, and further comprising a pair of tag transmitters which transmit RF signals to receivers with known propagation delay.

17. A system according to claim 14, wherein the processing delay within each receiver is substantially constant with each other.

18. A system according to claim 14, wherein the processor is operative for processing two time separated signals for determining a differential clock rate.

19. A system according to claim 18, wherein the processor is operative for processing three time separated signals to indicate differential rate change.

20. A system according to claim 19, wherein the processor is operative for maintaining a model of timing relationships of clocks at the receivers.

21. A system according to claim 14, wherein said receivers are positioned at known locations.

22. A system according to claim 14, wherein said RF signal transmitted from a tag transmitter includes data indentifying an object to be located and associated with the tag transmitter.

23. A system according to claim 14, wherein said RF signal further comprises a pseudo random spread spectrum RF signal.

24. A system according to claim 14, wherein said processor is operative to relate the clocks of each receiver to a system time.

25. A system according to claim 14, wherein said processor is operative for forming a tracking loop to track clock relationships that occur naturally over time.

26. A system according to claim 14, which further comprises determining an equation relating the indicated time of one clock at a respective receiver to another receiver clock and tracking changes in that relationship over time.

27. A method for locating an object within a monitored environment, which comprises:

transmitting an RF signal from a tag transmitter associated with an object to be located;

receiving the RF signal at a plurality of receivers having unsynchronized clocks and positioned within the monitored environment;

transmitting to a processor the time of arrival of the received RF signals based on the unsynchronized clock time at each respective receiver;

determining clock timing relationships for the clocks based on one of at least the propagation or processing delays in the receivers for synchronizing receiver performance; and

conducting differentiation of the time of arrival signals received from the tag transmitter to locate an object associated with the tag transmitter.

28. A method according to claim 27, which further comprises transmitting RF signals from a tag positioned at fixed location.

34. A method according to claim 27, which further comprises positioning the receivers at known locations.

35. A method according to claim 27, which further comprises transmitting an RF signal from the tag transmitter having data indentifying the object to which the tag is associated.

36. A method according to claim 27, which further comprises transmitting a pseudo random spread spectrum RF signal from the at least one tag.

37. A method according to claim 27, which further comprises processing first-to-arrive RF signals at each receiver.

38. A method according to claim 27, which further comprises forming a tracking loop to track clock relationships that occur naturally over time.

39. A method according to claim 27, which further comprises determining an equation relating the indicated time of one clock at a respective receiver to another clock at another receiver and tracking changes in that relationship over time.

29. A method according to claim 29, which further comprises transmitting RF signals from a pair of tag transmitters to a receiver with known propagation delay.

30. A method according to claim 29, wherein the processing delay within each receiver is substantially constant with each other.

31. A method according to claim 29, which further comprises receiving two time separated signals for determining differential clock rate.

32. A method according to claim 31, which further comprises receiving three time separated signals to indicate differential rate change.

33. A method according to claim 32, which further comprises maintaining a model of timing relationships of clocks at the receivers.

40. A method for time synchronizing a network which comprises:

transmitting an RF signal from a tag transmitter positioned at a known location;

receiving the RF signal at a plurality of receivers operative as network nodes and having unsynchronized clocks;

transmitting to a processor the time of arrival of any received RF signal based on an unsynchronized clock time; and

determining clock timing relationships for the clocks based on one of at least the propagation or processing delays in the receivers for synchronizing receiver performance.

41. A method according to claim 40, which further comprises transmitting RF signals from a pair of tag transmitters to receivers with known propagation delay.

42. A method according to claim 40, wherein the processing delay within each receiver is substantially constant with each other.

43. A method according to claim 40, which further comprises receiving two time separated signals for determining differential clock rate.

44. A method according to claim 43, which further comprises receiving three time separated signals to indicate differential rate change.

45. A method according to claim 44, which further comprises maintaining a model of timing relationships of clocks at the receivers.

46. A method according to claim 40, which further comprises positioning the receivers at known locations.

47. A method according to claim 40, which further comprises associating a tag with an object to be located.

48. A method according to claim 40, which further comprises transmitting an RF signal from the at least one tag having data indentifying the object to which the tag is associated.

49. A method according to claim 40, which further comprises transmitting a pseudo random spread spectrum RF signal from the at least one tag.

50. A method according to claim 40, which further comprises processing first-to-arrive RF signals at each receiver.

51. A method according to claim 40, which further comprises forming a tracking loop to track clock relationships that occur naturally over time.

52. A method according to claim 40, which further comprises determining an equation relating the indicated time of one clock at a respective receiver to another clock at another receiver and tracking changes in that relationship over time.