Calibration for wireless location system

• US 6,184,829 B1
• Filed: 01/08/1999
• Issued: 02/06/2001
• Est. Priority Date: 01/08/1999
• Status: Expired due to Term

• Current Assignee: Skyhook Holding, Inc. (Liberty Broadband Corp.)
• Original Assignee: TruePosition Incorporated
• Inventors: Stilp, Louis A.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): PHAN, DAO LINDA
• Application Number:
  US09/227,764
• Time in Patent Office: 760 Days
• Field of Search: 342/165, 342/174, 342/387, 342/444
• US Class Current: 342/387
• CPC Class Codes: G01S 1/026 of associated receivers
• CPC Class Codes: G01S 2205/006 for emergency situations
• CPC Class Codes: G01S 2205/008 using a mobile telephone network
• CPC Class Codes: G01S 2205/06 Emergency
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/0215 Interference
• CPC Class Codes: G01S 5/0218 Multipath in signal reception
• CPC Class Codes: G01S 5/02216 Timing or synchronisation of the receivers
• CPC Class Codes: G01S 5/0244 Accuracy or reliability of position solution or of measurements contributing thereto
• CPC Class Codes: G01S 5/0246 involving frequency difference of arrival or Doppler measurements G01S5/02685 takes precedence
• CPC Class Codes: G01S 5/02521 using a radio-map
• CPC Class Codes: G01S 5/0268 by deriving positions from different combinations of signals or of estimated positions in a single positioning system
• CPC Class Codes: G01S 5/0284 Relative positioning
• 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
• CPC Class Codes: H04W 4/90 Services for handling of emergency or hazardous situations, e.g. earthquake and tsunami warning systems [ETWS]
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management
• CPC Class Codes: H04W 76/50 for emergency connections
• CPC Class Codes: Y02D 30/70 in wireless communication networks

Abstract

Methods and apparatus for calibrating a Wireless Location System to enable the system to make highly accurate TDOA and FDOA measurements are disclosed. An external calibration method in accordance with the present invention comprises the steps of transmitting a first reference signal from a reference transmitter; receiving the first reference signal at first and second receiver systems; determining a first error value by comparing a measured TDOA (or FDOA) value with a theoretical TDOA (or FDOA) value associated with the known locations of the receiver systems and the known location of the reference transmitter; and utilizing the first error value to correct subsequent TDOA measurements associated with a mobile transmitter to be located. An internal calibration method in accordance with the present invention comprises the steps of injecting a comb signal into the first receiver system; utilizing the comb signal to obtain an estimate of the manner in which the transfer function varies across the bandwidth of the first receiver system; and utilizing the estimate to mitigate the effects of the variation of the first transfer function on the time measurements made by the first receiver system.

Claims

1. An external calibration method for a TDOA wireless location system (WLS), comprising the steps of:

transmitting a reference signal from a reference transmitter at a known location, receiving the reference signal at first and second receiver systems, determining an error value, and using the error value to correct subsequent TDOA measurements.

2. A method as recited in claim 1, further comprising transmitting a second reference signal, receiving the second reference signal, determining a second error value, and combining the first and second reference signals for use in making corrections to subsequent TDOA measurements.

3. A method as recited in claim 2, wherein the first and second error values are combined in a weighted average.

4. A method as recited in claim 3, wherein the error values are combined using a time series weighted averaging method that produces a time series weighted average.

5. A method as recited in claim 4, wherein the time series weighted average is provided by a Kalman filter.

6. A method as recited in claim 1, wherein the system monitors a rate of change of the error value and changes a rate of the calibration.

7. A method as recited in claim 6, wherein the rate of calibration is changed by automatically paging reference transmitters.

8. A method as recited in claim 1, wherein the first and second receiver systems have wideband receivers, and the first and second reference signals are narrowband at different carriers.

9. A method as recited in claim 1, wherein the error values are stored for each baseline.

10. A method as recited in claim 1, wherein the error values are weighted by a quality factor prior to correcting subsequent TDOA measurements.

11. A method as recited in claim 10, wherein the quality factor is based on an output of a cross correlation function of reference signals received by first and second receivers.

12. A method as recited in claim 10, wherein the error values are used only if the quality factor exceeds a prescribed threshold value.

13. An external calibration method for use with a wireless location system having first and second receiver systems, wherein the location system estimates velocity using frequency difference of arrival (FDOA), comprising:

transmitting a first reference signal from a reference transmitter at a known location, receiving the reference at the first and second receivers, determining an error value, and utilizing the error value to correct subsequent FDOA measurements.

14. An external calibration method for use with a wireless location system including a receiver system characterized by a time- and frequency-varying transfer function, comprising the steps of:

transmitting a stable, known wideband calibration signal from an external transmitter;

using the wideband calibration signal to estimate the transfer function across a prescribed bandwidth; and

using the estimate to mitigate effects of variation of the transfer function on subsequent time measurements.

15. A method as recited in claim 14, wherein the external transmission is of short duration and low power so as not to cause interference to a wireless system hosting the location system.

16. A method as recited in claim 14, wherein the receiver system is synchronized with the external transmitter so that the receiver system is programmed to receive and process the entire wideband of the calibration signal only at the time that the calibration signal is being sent.

17. A method as recited in claim 16, wherein the receiver system will not perform calibration processing at any other time except when in synchronization with the external calibration transmissions.

18. A method as recited in claim 16, wherein the receiver system and external calibration transmitter are synchronized using GPS timing units.

19. A method as recited in claim 18, wherein a wireless communications link is used between the receiver system and the external calibration transmitter to exchange commands and responses.

20. A method as recited in claim 14, wherein the external transmitter uses a directional antenna to direct the wideband signal only at antennae of the receiver system.

21. A method as recited in claim 20, wherein the directional antenna is a Yagi antenna.

22. A method as recited in claim 20, wherein the method includes making the external transmission only when the directional antenna is aimed at the receiver system antennae and the risk of multipath reflections is reduced.

23. A method as recited in claim 14, wherein the transfer function includes antennae, filters, amplifiers, and cabling associated with the receiver system.

24. An external calibration method for calibrating a location system having a first receiver system and a second receiver system, wherein the location system is operative to estimate the location of a mobile transmitter by, in part, determining a difference in time of arrival of a signal transmitted by said mobile transmitter and received by said first and second receiver systems, and wherein the accuracy of the location estimate is dependent, in part, upon the accuracy of time difference of arrival (TDOA) measurements made by the location system, the method comprising the steps of:

(A) transmitting a first reference signal from a reference transmitter;

(B) receiving said first reference signal at said first and second receiver systems;

(C) determining a first error value by comparing a measured TDOA value with a theoretical TDOA value associated with the known locations of the receiver systems and the known location of the reference transmitter; and

(D) utilizing said first error value to correct subsequent TDOA measurements associated with a mobile transmitter to be located.

25. A method as recited in claim 24, further comprising the steps of transmitting a second reference signal from a second reference transmitter;

receiving said second reference signal at said first and second receiver systems;

determining a second error value by comparing a second measured TDOA value with a second theoretical TDOA value associated with the known locations of the receiver systems and the known location of the second reference transmitter; and

utilizing said second error value in combination with said first error value to correct subsequent TDOA measurements associated with said mobile transmitter to be located.

26. A method as recited in claim 25, wherein said first and second error values are combined in a weighted average.

27. A method as recited in claim 25, wherein said first and second receiver systems include wideband receivers and said first and second reference signals are narrowband signals with different carrier frequencies.

28. A method as recited in claim 24, wherein the error values are stored in tabular form for each baseline in the location system.

29. A method as recited in claim 25, wherein the error values are stored in tabular form for each baseline in the location system.

30. A method as recited in claim 26, wherein the error values are combined in a time series weighted averaging method prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.

31. A method as recited in claim 30, wherein the time series weighted averaging method is based on a Kalman filter.

32. A method as recited in claim 24, wherein the error values are weighted by a quality factor prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.

33. A method as recited in claim 25, wherein the error values are weighted by a quality factor prior to being used to correct subsequent TDOA measurements associated with a mobile transmitter to be located.

34. A method as recited in claim 32, wherein the quality factor is based upon the output of a cross-correlation function of a reference signal received by the first and second receivers.

35. A method as recited in claim 32, wherein the error values are used by the location system only if the quality factor exceeds a prescribed threshold value.

36. A method as recited in claim 30, wherein the location system monitors the rate of change of the error values and changes the rate of calibration, or time interval between calibrations, to exceed the rate of change of the error values.

37. A method as recited in claim 35, wherein the rate of calibration is controlled by automatically paging the reference transmitters.

38. An external calibration method for calibrating a location system having a first receiver system and a second receiver system, wherein the location system is operative to estimate the location and velocity of a mobile transmitter by, in part, determining a difference in frequency of arrival of a signal transmitted by said mobile transmitter and received by said first and second receiver systems, and wherein the accuracy of the location and velocity estimate is dependent, in part, upon the accuracy of frequency difference of arrival (FDOA) measurements made by the location system, the method comprising the steps of:

(A) transmitting a first reference signal from a reference transmitter;

(B) receiving said first reference signal at said first and second receiver systems;

(C) determining a first error value by comparing a measured FDOA value with a theoretical FDOA value associated with the known locations of the receiver systems and the known location of the reference transmitter; and

(D) utilizing said first error value to correct subsequent FDOA measurements associated with a mobile transmitter to be located.