Stilp, Louis A. (Berwyn, PA, US)
Anderson, Robert J. (Phoenixville, PA, US)
Ward, Matthew L. (Collegeville, PA, US)

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10/915786

02/03/2005

08/11/2004

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TruePosition, Inc.

342/465

(IPC1-7): G01S007/40

WOODCOCK WASHBURN LLP (ONE LIBERTY PLACE, 46TH FLOOR, 1650 MARKET STREET, PHILADELPHIA, PA, 19103, US)

1. A multiple pass location processing method for use in a wireless location system (WLS) capable of locating a wireless transmitter based on a transmission received from the wireless transmitter, comprising the steps of: (a) producing a first location, lower quality location estimate and providing said first location estimate to a first location application; and (b) subsequently producing a second location, higher quality location estimate for delivery to a second location application, wherein the second location application may be the same as the first location application or different from the first location application.

2. A method as recited in claim 1, wherein the second location estimate is a more accurate estimate than the first location estimate.

3. A method as recited in claim 1, wherein the second location estimate is of a higher confidence than the first location estimate.

4. A method as recited in claim 1, further comprising identifying a plurality of received transmissions as requiring multiple pass location processing.

5. A method as recited in claim 1, further comprising identifying one or more wireless transmitters as requiring multiple pass location processing.

6. A method as recited in claim 1, wherein the received transmission is identified as requiring multiple pass location processing based on the identity of the first location application.

7. A method as recited in claim 1, wherein the received transmission is identified as requiring multiple pass location processing based on a calling number of the wireless transmitter.

8. A method as recited in claim 1, further comprising identifying one or more location applications for which multiple pass location processing is required.

9. A method as recited in claim 1, wherein, in providing the first location estimate, the WLS selects a subset of the received transmission for use in providing a rough estimate of the location of the wireless transmitter.

10. A method as recited in claim 1, further comprising determining a priority level to be associated with location processing for the wireless transmitter.

11. A method as recited in claim 1, further comprising determining a time limit for signal collection for producing the first location estimate.

12. A method as recited in claim 1, further comprising determining a time limit for signal processing for producing the first location estimate.

13. A method as recited in claim 1, further comprising determining a time limit on total latency for providing the first location estimate.

14. A method as recited in claim 1, further comprising determining a signal power threshold.

15. A method as recited in claim 1, further comprising determining a time limit for delivery of a location estimate to the first location application.

16. A method as recited in claim 1, further comprising determining multiple location applications to which the first location estimate is to be provided.

17. A method as recited in claim 1, further comprising determining a time limit for signal collection for the second location estimate.

18. A method as recited in claim 1, further comprising determining a time limit for signal processing for the second location estimate.

19. A method as recited in claim 1, further comprising determining a time limit for total latency for the second location estimate.

20. A method recited in claim 1, further comprising determining a location accuracy threshold for the first location estimate.

21. A method as recited in claim 1, further comprising determining a velocity accuracy threshold for the first location estimate.

22. A method as recited in claim 1, wherein the second location estimate is independent of the first location estimate.

23. A method as recited in claim 1, wherein the first location estimate is used in deriving the second location estimate.

24. A method as recited in claim 1, wherein the first and second location estimates are derived in parallel.

25. A method as recited in claim 1, further comprising providing the second location estimate to the first location application.

26. A method as recited in claim 1, further comprising providing the second location estimate to a second location application, wherein the second location application is different from the first location application.

27. A method as recited in claim 1, further comprising providing multiple location estimates in step (b).

28. A method as recited in claim 1, wherein the first location estimate is provided with an information element identifying it as a lower quality estimate.

29. A method as recited in claim 1, wherein the second location estimate is provided with an information element identifying it as a higher quality estimate.

30. A multiple pass location processing method for use in a wireless location system (WLS) capable of locating a wireless transmitter involved in an emergency services call, based on a transmission received from the wireless transmitter, and routing the call to a call center, comprising the steps of: (a) producing a first location, lower quality location estimate and providing said first location estimate to a first location application; and (b) producing a second location, higher quality location estimate; wherein the first location estimate is less accurate than the second location estimate but is sufficient for call routing purposes.

31. A wireless location system (WLS), comprising: (a) a plurality of geographically separated receiver systems for receiving a transmission from a wireless transmitter; (b) means for producing a first, lower quality location estimate; (c) means for providing said first location estimate to a first location application; (d) means for producing a second, higher quality location estimate; and (e) means for providing said second location estimate to the first or a second location application.

32. A WLS as recited in claim 31, wherein the second location estimate is a more accurate estimate than the first location estimate.

33. A WLS as recited in claim 31, wherein the second location estimate is of a higher confidence than the first location estimate.

34. A WLS as recited in claim 31, further comprising means for identifying a plurality of received transmissions as requiring multiple pass location processing.

35. A WLS as recited in claim 31, further comprising means for identifying one or more wireless transmitters as requiring multiple pass location processing.

36. A WLS as recited in claim 31, wherein the received transmission is identified as requiring multiple pass location processing based on the identity of the first location application.

37. A WLS as recited in claim 31, wherein the received transmission is identified as requiring multiple pass location processing based on a calling number of the wireless transmitter.

38. A WLS as recited in claim 31, further comprising means for identifying one or more location applications for which multiple pass location processing is required.

39. A WLS as recited in claim 31, wherein, in providing the first location estimate, the WLS selects a subset of the received transmission for use in providing a rough estimate of the location of the wireless transmitter.

40. A WLS as recited in claim 31, further comprising means for determining a priority level to be associated with location processing for the wireless transmitter.

41. A WLS as recited in claim 31, further comprising means for determining a time limit for signal collection for producing the first location estimate.

42. A WLS as recited in claim 31, further comprising means for determining a time limit for signal processing for producing the first location estimate.

43. A WLS as recited in claim 31, further comprising means for determining a time limit on total latency for providing the first location estimate.

44. A WLS as recited in claim 31, further comprising means for determining a signal power threshold.

45. A WLS as recited in claim 31, further comprising means for determining a time limit for delivery of a location estimate to the first location application.

46. A WLS as recited in claim 31, further comprising means for determining multiple location applications to which the first location estimate is to be provided.

47. A WLS as recited in claim 31, further comprising means for determining a time limit for signal collection for the second location estimate.

48. A WLS as recited in claim 31, further comprising means for determining a time limit for signal processing for the second location estimate.

49. A WLS as recited in claim 31, further comprising means for determining a time limit for total latency for the second location estimate.

50. A WLS as recited in claim 31, further comprising means for determining a location accuracy threshold for the first location estimate.

51. A WLS as recited in claim 31, further comprising means for determining a velocity accuracy threshold for the first location estimate.

52. A WLS as recited in claim 31, wherein the second location estimate is independent of the first location estimate.

53. A WLS as recited in claim 31, wherein the first location estimate is used in deriving the second location estimate.

54. A WLS as recited in claim 31, wherein the first and second location estimates are derived in parallel.

55. A WLS as recited in claim 31, further comprising means for providing the second location estimate to the first location application.

56. A WLS as recited in claim 31, further comprising means for providing the second location estimate to a second location application, wherein the second location application is different from the first location application.

57. A WLS as recited in claim 31, further comprising means for providing multiple location estimates.

58. A WLS as recited in claim 31, wherein the first location estimate is provided with an information element identifying it as a lower quality estimate.

59. A WLS as recited in claim 31, wherein the second location estimate is provided with an information element identifying it as a higher quality estimate.

60. A WLS as recited in claim 31, wherein the WLS supports multiple pass location processing with multiple pass location records in which a flag is included to indicate a maximum time limit before which a particular application must receive a rough estimate of location and a second maximum time limit in which the same or another application must receive a final location estimate.

61. A WLS as recited in claim 31, wherein a flag is included in a location record to indicate the first pass or second pass status of the location estimate contained in the record.

62. A multiple pass location processing means for use in a wireless location system (WLS) capable of locating a wireless transmitter involved in an emergency services call, based on a transmission received from the wireless transmitter, and routing the call to a call center, comprising: (a) means for producing a first location, lower quality location estimate and providing said first location estimate to a first location application; and (b) means for producing a second location, higher quality location estimate; wherein the first location estimate is less accurate than the second location estimate but is sufficient for call routing purposes.

63. A multiple pass location processing means as recited in claim 62, further comprising a means for identifying transmissions for which a first, lower quality estimate is required, including multiple pass location records in which a flag is included to indicate a maximum time limit before which a particular application must receive a rough estimate of location and a second maximum time limit in which the same or another application must receive a final location estimate.

64. A multiple pass location processing means as recited in claim 62, wherein a flag is included in a location record to indicate the first pass or second pass status of the location estimate contained in the record.

65. A wireless location system (WLS), comprising: (a) a plurality of geographically separated receiver systems for receiving a transmission from a wireless transmitter; and (b) a processor and executable instructions for producing a first, lower quality location estimate, and for subsequently producing a second, higher quality location estimate, wherein said first location estimate may be communicated to a first location application for call routing purposes.

66. -86. (Canceled).

87. A method for use in a Wireless Location System in locating a mobile transmitter involved in an emergency services call and routing the call to the correct call center using a first location attempt bounded by the time to produce a first timely but lower accuracy location estimate for routing and a second location estimate bounded by the accuracy required for the purposes of caller location.

88. A method as recited in claim 87, wherein a site density factor is employed in determining the amount of signal collection and/or signal processing to use to produce the first location estimate.

89. A method as recited in claim 87, wherein a site density factor is employed in determining the scheduling of receiver resources to use to produce the first location estimate.

90. A method as recited in claim 87, wherein a site density factor is employed in determining the amount of signal collection and/or signal processing to use to produce the second location estimate.

91. A method as recited in claim 90, wherein a site density factor is employed in determining the scheduling of receiver resources to use to produce the second location estimate.

92. -99. (Canceled).

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of U.S. patent application Ser. No. 10/106,081, filed Mar. 25, 2002, entitled “Multiple Pass Location Processing,” which is a continuation of U.S. patent application Ser. No. 10/005,068, filed on Dec. 5, 2001, entitled “Collision Recovery in a Wireless Location System,” which is a divisional of U.S. patent application Ser. No. 09/648,404, filed on Aug. 24, 2000, entitled “Antenna Selection Method for a Wireless Location System,” now U.S. Pat. No. 6,400,320, Jun. 4, 2002, which is a continuation of U.S. patent application Ser. No. 09/227,764, filed on Jan. 8, 1999, entitled “Calibration for Wireless Location System,” now U.S. Pat. No. 6,184,829 B1; Feb. 6, 2001.

FIELD OF THE INVENTION

The present invention relates generally to methods and apparatus for locating wireless transmitters, such as those used in analog or digital cellular systems, personal communications systems (PCS), enhanced specialized mobile radios (ESMRs), and other types of wireless communications systems. This field is now generally known as wireless location, and has application for Wireless E9-1-1, fleet management, RF optimization, and other valuable applications.

BACKGROUND OF THE INVENTION

Early work relating to the field of Wireless Location has been described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System”; and U.S. Pat. No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of Bursty Transmissions.” Both patents are owned by the assignee of the present invention, TruePosition, Inc. TruePosition has conducted extensive experiments with Wireless Location System technology to demonstrate both the viability and value of the technology. For example, several experiments were conducted during several months of 1995 and 1996 in the cities of Philadelphia and Baltimore to verify the system's ability to mitigate multipath in large urban environments. Then, in 1996 the assignee constructed a system in Houston that was used to test the technology's effectiveness in that area and its ability to interface directly with E911 systems. Then, in 1997, the system was tested in a 350 square mile area in New Jersey and was used to locate real 9-1-1 calls from real people in trouble. The system test was then expanded to include 125 cell sites covering an area of over 2,000 square miles. In 1998, the assignee added digital radio capability to the WLS and fielded multiple dual-mode test systems including: a 16 site AMPS/TDMA system in Wilmington Del., a 7 site AMPS/TDMA system in Redmond Wash., a 38 site rural AMPS/TDMA system around Fort Wayne, Indiana, a 19 site AMPS/CDMA system in King of Prussia Pennsylvania, a 33 site dense urban AMPS/CDMA system on Manhattan Island in New York, and a 135 site AMPS/CDMA system in New Jersey, Delaware and Pennsylvania. TruePosition is currently in the process of deploying a commercial 16,000 site nationwide AMPS/TDMA and GSM system. During all of these tests and commercial deployments, various techniques were tested for effectiveness and further developed.

The value and importance of the Wireless Location System has been acknowledged by the wireless communications industry. In June 1996, the Federal Communications Commission issued requirements for the wireless communications industry to deploy location systems for use in locating wireless 9-1-1 callers, with a deadline of October 2001. The location of wireless 9-1-1 callers will save response time, save lives, and save enormous costs because of reduced use of emergency response resources. In addition, numerous surveys and studies have concluded that various wireless applications, such as location sensitive billing, fleet management, and others, will have great commercial value in the coming years.

TruePosition has continued to develop systems and techniques to further improve the accuracy of Wireless Location Systems while significantly reducing the cost of these systems. For example, the following commonly-assigned patents have been awarded for various improvements in the field of Wireless Location:

• • 1. U.S. Pat. No. 6,519,465 B2, Feb. 11, 2003, Modified Transmission Method for Improving Accuracy for E-911 Calls;
  • 2. U.S. Pat. No. 6,492,944 B1, Dec. 10, 2002, Internal Calibration Method for a Receiver System of a Wireless Location System;
  • 3. U.S. Pat. No. 6,483,460 B2, Nov. 19, 2002, Baseline Selection Method for Use in a Wireless Location System;
  • 4. U.S. Pat. No. 6,463,290 B1, Oct. 8, 2002, Mobile-Assisted Network Based Techniques for Improving Accuracy of Wireless Location System;
  • 5. U.S. Pat. No. 6,400,320, Jun. 4, 2002, Antenna Selection Method For A Wireless Location System;
  • 6. U.S. Pat. No. 6,388,618, May 14, 2002, Signal Collection System For A Wireless Location System;
  • 7. U.S. Pat. No. 6,351,235, Feb. 26, 2002, Method And System For Synchronizing Receiver Systems Of A Wireless Location System;
  • 8. U.S. Pat. No. 6,317,081, Nov. 13, 2001, Internal Calibration Method For Receiver System Of A Wireless Location System;
  • 9. U.S. Pat. No. 6,285,321, Sep. 4, 2001, Station Based Processing Method For A Wireless Location System;
  • 10. U.S. Pat. No. 6,334,059, Dec. 25, 2001, Modified Transmission Method For Improving Accuracy For E-911 Calls;
  • 11. U.S. Pat. No. 6,317,604, Nov. 13, 2001, Centralized Database System For A Wireless Location System;
  • 12. U.S. Pat. No. 6,281,834, Aug. 28, 2001, Calibration For Wireless Location System;
  • 13. U.S. Pat. No. 6,266,013, Jul. 24, 2001, Architecture For A Signal Collection System Of A Wireless Location System;
  • 14. U.S. Pat. No. 6,184,829, Feb. 6, 2001, Calibration For Wireless Location System;
  • 15. U.S. Pat. No. 6,172,644, Jan. 9, 2001, Emergency Location Method For A Wireless Location System;
  • 16. U.S. Pat. No. 6,115,599, Sep. 5, 2000, Directed Retry Method For Use In A Wireless Location System;
  • 17. U.S. Pat. No. 6,097,336, Aug. 1, 2000, Method For Improving The Accuracy Of A Wireless Location System;
  • 18. U.S. Pat. No. 6,091,362, Jul. 18, 2000, Bandwidth Synthesis For Wireless Location System;
  • 19. U.S. Pat. No. 5,608,410, Mar. 4, 1997, System For Locating A Source Of Bursty Transmissions; and
  • 20. U.S. Pat. No. 5,327,144, Jul. 5, 1994, Cellular Telephone Location System.
    Other exemplary patents in this field include:
  • 1. U.S. Pat. No. 6,546,256, Apr. 8, 2003, Robust, Efficient, Localization System;
  • 2. U.S. Pat. No. 6,366,241, Apr. 2, 2002, Enhanced Determination Of Position-Dependent Signal Characteristics;
  • 3. U.S. Pat. No. 6,288,676, Sep. 11, 2001, Apparatus And Method For Single Station Communications Localization;
  • 4. U.S. Pat. No. 6,288,675, Sep. 11, 2001, Single Station Communications Localization System;
  • 5. U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System;
  • 6. U.S. Pat. No. 6,108,555, Aug. 22, 2000, Enhanced Time Difference Localization System;
  • 7. U.S. Pat. No. 6,101,178, Aug. 8, 2000, Pseudolite-Augmented GPS For Locating Wireless Telephones;
  • 8. U.S. Pat. No. 6,119,013, Sep. 12, 2000, Enhanced Time-Difference. Localization System;
  • 9. U.S. Pat. No. 6,127,975, Oct. 3, 2000, Single Station Communications Localization System;
  • 10. U.S. Pat. No. 5,959,580, Sep. 28, 1999, Communications Localization System; and
  • 11. U.S. Pat. No. 4,728,959, Mar. 1, 1988, Direction Finding Localization System.

Over the past few years, the cellular industry has increased the number of air interface protocols available for use by wireless telephones. The industry has also increased the number of frequency bands in which wireless or mobile telephones may operate, and has expanded the number of terms that refer or relate to mobile telephones to include “personal communications services”, “wireless”, and others. The changes in terminology and increases in the number of air interface protocols do not change the basic principles and inventions discovered and enhanced by the assignee of the present invention.

As mentioned, there are numerous air interface protocols used for wireless communications systems. These protocols are used in different frequency bands, both in the U.S. and internationally. The frequency band generally does not impact the Wireless Location System's effectiveness at locating wireless telephones.

All air interface protocols use two types of “channels”. The first type includes control channels that are used for conveying information about the wireless telephone or transmitter, for initiating or terminating calls, or for transferring bursty data. For example, some types of short messaging services transfer data over the control channel. In different air interfaces, control channels are known by different terminology but the use of the control channels in each air interface is similar. Control channels generally have identifying information about the wireless telephone or transmitter contained in the transmission. The second type of channel includes voice channels, also known as traffic channels, that are typically used for conveying voice communications over the air interface. These channels are used after a call has been set up using the control channels. Voice channels will typically use dedicated resources within the wireless communications system whereas control channels will use shared resources. This distinction can make the use of control channels for wireless location purposes more cost effective than the use of voice channels, although there are some applications for which regular location on the voice channel is desired. Voice channels generally do not have identifying information about the wireless telephone or transmitter in the transmission.

Some of the differences in the air interface protocols are discussed below:

AMPS—This is the original air interface protocol used for cellular communications in the U.S. In the AMPS system, separate dedicated channels are assigned for use by control channels (RCC). According to the TLA/EIA Standard IS-553A, every control channel block must begin at cellular channel 313 or 334, but the block may be of variable length. In the U.S., by convention, the AMPS control channel block is 21 channels wide, but the use of a 26-channel block is also known. A reverse voice channel (RVC) may occupy any channel that is not assigned to a control channel. The control channel modulation is FSK (frequency shift keying), while the voice channels are modulated using FM (frequency modulation).

N-AMPS—This air interface is an expansion of the AMPS air interface protocol, and is defined in EIA/TIA standard IS-88. The control channels are substantially the same as for AMPS, but the voice channels are different. The voice channels occupy less than 10 KHz of bandwidth, versus the 30 KHz used for AMPS, and the modulation is FM.

TDMA—This interface is also known D-AMPS, and is defined in EIA/TIA standard IS-136. This air interface is characterized by the use of both frequency and time separation. Control channels are known as Digital Control Channels (DCCH) and are transmitted in bursts in timeslots assigned for use by DCCH. Unlike AMPS, DCCH may be assigned anywhere in the frequency band, although there are generally some frequency assignments that are more attractive than others based upon the use of probability blocks. Voice channels are known as Digital Traffic Channels (DTC). DCCH and DTC may occupy the same frequency assignments, but not the same timeslot assignment in a given frequency assignment. DCCH and DTC use the same modulation scheme, known as π/4 DQPSK (differential quadrature phase shift keying). In the cellular band, a carrier may use both the AMPS and TDMA protocols, as long as the frequency assignments for each protocol are kept separated.

CDMA—This air interface is defined by EIA/TIA standard IS-95A. This air interface is characterized by the use of both frequency and code separation. However, because adjacent cell sites may use the same frequency sets, CDMA is also characterized by very careful power control. This careful power control leads to a situation known to those skilled in the art as the near-far problem, which makes wireless location difficult for most approaches to function properly (but see U.S. Pat. No. 6,047,192, Apr. 4, 2000, Robust, Efficient, Localization System, for a solution to this problem). Control channels are known as Access Channels, and voice channels are known as Traffic Channels. Access and Traffic Channels may share the same frequency band but are separated by code. Access and Traffic Channels use the same modulation scheme, known as OQPSK.

GSM—This air interface is defined by the international standard Global System for Mobile Communications. Like TDMA, GSM is characterized by the use of both frequency and time separation. The channel bandwidth is 200 KHz, which is wider than the 30 KHz used for TDMA. Control channels are known as Standalone Dedicated Control Channels (SDCCH), and are transmitted in bursts in timeslots assigned for use by SDCCH. SDCCH may be assigned anywhere in the frequency band. Voice channels are known as Traffic Channels (TCH). SDCCH and TCH may occupy the same frequency assignments but not the same timeslot assignment in a given frequency assignment. SDCCH and TCH use the same modulation scheme, known as GMSK. The GSM General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE) systems reuse the GSM channel structure, but can use multiple modulation schemes and data compression to provide higher data throughput.

Within this specification, a reference to control channels or voice channels shall refer to all types of control or voice channels, whatever the preferred terminology for a particular air interface. Moreover, there are many more types of air interfaces (e.g., IS-95 CDMA, CDMA 2000, and UMTS WCDMA) used throughout the world, and, unless the contrary is indicated, there is no intent to exclude any air interface from the inventive concepts described within this specification. Indeed, those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.

The preferred embodiments of the inventions disclosed herein have many advantages over other techniques for locating wireless telephones. For example, some of these other techniques involve adding GPS functionality to telephones, which requires that significant changes be made to the telephones. The preferred embodiments disclosed herein do not require any changes to wireless telephones, and so they can be used in connection with the current installed base of over 65 million wireless telephones in the U.S. and 250 million wireless telephones worldwide.

SUMMARY OF THE INVENTION

The present invention provides a multiple pass location processing method for use in a wireless location system (WLS). In an exemplary implementation, the inventive method comprises identifying a received transmission as requiring multiple pass location processing whereby the WLS produces a first, lower quality location estimate and subsequently produces a second, higher quality location estimate. The WLS provides the first location estimate to a first location application, and then produces the second location estimate. The second location estimate may be a more accurate estimate than the first location estimate and/or of a higher confidence than the first location estimate. This method is suitable, but not limited, for use in connection with locating a wireless transmitter involved in an emergency services call and routing the call to a call center.

In illustrative embodiments and implementations of the invention, the second location estimate is a more accurate estimate and/or of a higher confidence than the first location estimate. In addition, specific embodiments may include the steps or functions of: identifying a plurality of received transmissions as requiring multiple pass location processing; identifying one or more wireless transmitters as requiring multiple pass location processing; identifying the received transmission as requiring multiple pass location processing based on the identity of the first location application; identifying the received transmission as requiring multiple pass location processing based on a calling number of the wireless transmitter; identifying one or more location applications for which multiple pass location processing is required, and/or identifying a first and second location technology to most efficiently meet the demanded quality-of-service parameters.

Further exemplary features may include selecting a subset of the received transmission for use in providing a rough estimate of the location of the wireless transmitter; determining a priority level to be associated with location processing for the wireless transmitter; determining a time limit for signal collection for producing the first location estimate; determining a time limit for signal processing for producing the first location estimate; determining a time limit on total latency for providing the first location estimate; determining a signal power threshold; determining a time limit for delivery of a location estimate to the first location application; determining multiple location applications to which the first location estimate is to be provided; determining a time limit for signal collection for the second location estimate; determining a time limit for signal processing for the second location estimate; determining a time limit for total latency for the second location estimate; determining a location accuracy threshold for the first location estimate; determining a velocity accuracy threshold for the first location estimate; making the second location estimate independent of the first location estimate; using the first location estimate in deriving the second location estimate; deriving the first and second location estimates in parallel; providing the second location estimate to the first location application; providing the second location estimate to a second location application; providing the first location estimate with an information element identifying it as a lower quality estimate; and/or providing the second location estimate with an information element identifying it as a higher quality estimate.

In another illustrative embodiment of the invention, when a multipass location request is received, a current cell and sector (the “primary”) is compared to a list of possible cooperating sites in the network to compute a site density factor, or alternatively a cell site density factor may be maintained for each primary site. Based on this site density factor, the collection time, signal processing methods, and the number of cooperating antennas can be determined for each of the estimated locations, first and subsequent. Because the amount of required collection time and signal processing, and the number of cooperating antennas vary as a function of site density, knowledge of the site density permits the correct amount of system resources to be used for each pass, thereby optimizing system accuracy and capacity.

As with Automatic Synchronous Tuning (see below, and see co-pending application Ser. No. 10/106,089, filed Mar. 25, 2002, “Automatic Synchronous Tuning of Narrowband Receivers of a Wireless Location System for Voice/Traffic Channel Tracking,” which is hereby incorporated by reference in its entirety), scheduling of scarce resources (e.g., receivers, processors) is used to increase capacity of the WLS. With given qualities of service (time, confidence, accuracy, for example) for the first and second location estimates and the WLS's knowledge of the RF environment, a model can be used to determine the probable duration of the signal collection phase of the location estimation process. With this calculated probable duration and a latency or time limit parameter in the first pass quality-of-service constraints, the WLS may compute a priority level for the request and schedule the request to optimize the capacity of the WLS.

If the system determines from the model that sufficient resources are available to meet the first and second quality-of-service parameter lists with a single collection phase, then the WLS can abort the second pass and deliver in the first pass the quality of location estimate requested in the first pass.

If the WLS as installed has both TDOA and angle of arrival (AoA) capabilities, then the multipass location scenario could be used to deliver a TDOA measurement followed by an AoA or TDOA/AoA hybrid location estimate to meet accuracy or time requirements demanded by the quality-of-service for the location estimate.

The WLS may store a detailed model of RF propagation characteristics for each cell and sector of the WLS service area. This model when used with carrier network supplied or mobile station supplied cell/sector, timing and power measurements allows the WLS to calculate a first location estimate in a useful form. These more useful forms include a latitude and longitude, a latitude and longitude with error radius and confidence factor, or a polygon with a confidence factor. These location estimates derived from external (to the WLS) data can be compared to the first set of quality-of-service requirements for goodness of fit.

In a simplest form, the cell/sector density or cell/sector size stored by the WLS can be compared to the first set of quality-of-service parameters. If the cell/sector derived location is itself sufficient to achieve the accuracy required in the first-pass quality-of-service parameters, the system can return that value. If the location estimate yielded by cell and sector is not sufficiently accurate, the WLS system can combine the cell and sector with carrier network-supplied or mobile station-supplied timing information (round-trip-delay, timing advance in TDMA systems such as IS-54, IS-136 and GSM or PN-offset in CDMA systems) or carrier network-supplied or mobile station-supplied power measurements with WLS stored radio environmental and propagation information to generate a first location estimate that can be compared to the first set of quality-of-service parameters to find if any exceed the first set of quality-of-service parameters. A first location estimate derived in this manner may be delivered to the requesting or specified destination location application.

If the first pass location method, whatever the technology used, exceeds the first and second set of quality-of-service parameters, then the second pass may be aborted, saving location receiver resources. If the second pass is aborted, then the requesting application can either receive just the first pass response or both the second and first pass responses. For example, the system may use a flag or other indicator in the first pass response to inform the requesting application that a second response will not be delivered.

A further aspect of the present invention includes the notion that multipass location processing may be divided into three parts: (1) selection of multipass processing, (2) performance of multipass processing, and (3) identification. These are discussed further below.

Other details of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 1A schematically depict a Wireless Location System.

FIG. 2 schematically depicts a Signal Collection System (SCS) 10.

FIG. 2A schematically depicts a receiver module 10-2 employed by the Signal Collection System.

FIGS. 2B and 2C schematically depict alternative ways of coupling the receiver module(s) 10-2 to the antennas 10-1.

FIG. 2C-1 is a flowchart of a process employed by the Wireless Location System when using narrowband receiver modules.

FIG. 2D schematically depicts a DSP module 10-3 employed in the Signal Collection System.

FIG. 2E is a flowchart of the operation of the DSP module(s) 10-3, and FIG. 2E-1 is a flowchart of the process employed by the DSP modules for detecting active channels.

FIG. 2F schematically depicts a Control and Communications Module 10-5.

FIGS. 2G-2J depict aspects of the presently preferred SCS calibration methods. FIG. 2G is a schematic illustration of baselines and error values used to explain an external calibration method. FIG. 2H is a flowchart of an internal calibration method. FIG. 2I is an exemplary transfer function of an AMPS control channel and FIG. 2J depicts an exemplary comb signal.

FIGS. 2K and 2L are flowcharts of two methods for monitoring performance of a Wireless Location System.

FIG. 3 schematically depicts a TDOA Location Processor 12.

FIG. 3A depicts the structure of an exemplary network map maintained by the TLP controllers.

FIGS. 4 and 4A schematically depict different aspects of an Applications Processor 14.

FIG. 5 is a flowchart of a central station-based location processing method.

FIG. 6 is a flowchart of a station-based location processing method.

FIG. 7 is a flowchart of a method for determining, for each transmission for which a location is desired, whether to employ central or station-based processing.

FIG. 8 is a flowchart of a dynamic process used to select cooperating antennas and SCS's 10 used in location processing.

FIG. 9 is diagram that is referred to below in explaining a method for selecting a candidate list of SCS's and antennas using a predetermined set of criteria.

FIG. 9A is a flowchart of a multiple pass location processing method.

FIGS. 10A and 10B are flowcharts of alternative methods for increasing the bandwidth of a transmitted signal to improve location accuracy.

FIGS. 11A-11C are signal flow diagrams and FIG. 11D is a flowchart, and they are used to explain an inventive method for combining multiple statistically independent location estimates to provide an estimate with improved accuracy.

FIGS. 12A and 12B are a block diagram and a graph, respectively, for explaining a bandwidth synthesis method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The Wireless Location System (WLS) operates as a passive overlay to a wireless communications system, such as a cellular, PCS, or ESMR system, although the concepts are not limited to just those types of communications systems. Wireless communications systems are generally not suitable for locating wireless devices because the designs of the wireless transmitters and cell sites do not include the necessary functionality to achieve accurate location. Accurate location in this application is defined as accuracy of 100 to 400 feet RMS (root mean square). This is distinguished from the location accuracy that can be achieved by existing cell sites, which is generally limited to the radius of the cell site. In general, cell sites are not designed or programmed to cooperate between and among themselves to determine wireless transmitter location. Additionally, wireless transmitters such as cellular and PCS telephones are designed to be low cost and therefore generally do not have locating capability built-in. The WLS is designed to be a low cost addition to a wireless communications system that involves minimal changes to cell sites and no changes at all to standard wireless transmitters. The WLS is passive because it does not contain transmitters, and therefore cannot cause interference of any kind to the wireless communications system. The WLS uses only its own specialized receivers at cell sites or other receiving locations.

Overview of Wireless Location System

As shown in FIG. 1, the Wireless Location System has four major kinds of subsystems: the Signal Collection Systems (SCS's) 10, the TDOA Location Processors (TLP's) 12, the Application Processors (AP's) 14, and the Network Operations Console (NOC) 16. Each SCS is responsible for receiving the RF signals transmitted by the wireless transmitters on both control channels and voice channels. In general, each SCS is preferably installed at a wireless carrier's cell site, and therefore operates in parallel to a base station. Each TLP 12 is responsible for managing a network of SCS's 10 and for providing a centralized pool of digital signal processing (DSP) resources that can be used in the location calculations. The SCS's 10 and the TLP's 12 operate together to determine the location of the wireless transmitters, as will be discussed more fully below. Digital signal processing is the preferable manner in which to process radio signals because DSP's are relatively low cost, provide consistent performance, and are easily re-programmable to handle many different tasks. Both the SCS's 10 and TLP's 12 contain a significant amount of DSP resources, and the software in these systems can operate dynamically to determine where to perform a particular processing function based upon tradeoffs in processing time, communications time, queuing time, and cost. Each TLP 12 exists centrally primarily to reduce the overall cost of implementing the WLS, although the techniques discussed herein are not limited to the preferred architecture shown. That is, DSP resources can be relocated within the WLS without changing the basic concepts and functionality disclosed.

The AP's 14 are responsible for managing all of the resources in the WLS, including all of the SCS's 10 and TLP's 12. Each AP 14 also contains a specialized database that contains “triggers” for the WLS. In order to conserve resources, the WLS can be programmed to locate only certain pre-determined types of transmissions. When a transmission of a pre-determined type occurs, then the WLS is triggered to begin location processing. Otherwise, the WLS may be programmed to ignore the transmission. Each AP 14 also contains applications interfaces that permit a variety of applications to securely access the WLS. These applications may, for example, access location records in real time or non-real time, create or delete certain type of triggers, or cause the WLS to take other actions. Each AP 14 is also capable of certain post-processing functions that allow the AP 14 to combine a number of location records to generate extended reports or analyses useful for applications such as traffic monitoring or RF optimization.

The NOC 16 is a network management system that provides operators of the WLS easy access to the programming parameters. For example, in some cities, the WLS may contain many hundreds or even thousands of SCS's 10. The NOC is the most effective way to manage a large system, using graphical user interface capabilities. The NOC will also receive real time alerts if certain functions within the WLS are not operating properly. These real time alerts can be used by the operator to take corrective action quickly and prevent a degradation of location service. Experience with trials of the WLS show that the ability of the system to maintain good location accuracy over time is directly related to the operator's ability to keep the system operating within its predetermined parameters.

Readers of U.S. Pat. Nos. 5,327,144 and 5,608,410 and this specification will note similarities between the respective systems. Indeed, the system disclosed herein is significantly based upon and also significantly enhanced from the system described in those previous patents. For example, the SCS 10 has been expanded and enhanced from the Antenna Site System described in U.S. Pat. No. 5,608,410. The SCS 10 now has the capability to support many more antennas at a single cell site, and further can support the use of extended antennas as described below. This enables the SCS 10 to operate with the sectored cell sites now commonly used. The SCS 10 can also transfer data from multiple antennas at a cell site to the TLP 12 instead of always combining data from multiple antennas before transfer. Additionally, the SCS 10 can support multiple air interface protocols thereby allowing the SCS 10 to function even as a wireless carrier continually changes the configuration of its system.

The TLP 12 is similar to the Central Site System disclosed in U.S. Pat. No. 5,608,410, but has also been expanded and enhanced. For example, the TLP 12 has been made scaleable so that the amount of DSP resources required by each TLP 12 can be appropriately scaled to match the number of locations per second required by customers of the WLS. In order to support scaling for different WLS capacities, a networking scheme has been added to the TLP 12 so that multiple TLP's 12 can cooperate to share RF data across wireless communication system network boundaries. Additionally, the TLP 12 has been given control means to determine the SCS's 10, and more importantly the antennas at each of the SCS's 10, from which the TLP 12 is to receive data in order to process a specific location. Previously, the Antenna Site Systems automatically forwarded data to the Central Site System, whether requested or not by the Central Site System. Furthermore, the SCS 10 and TLP 12 combined have been designed with additional means for removing multipath from the received transmissions.

The Database Subsystem of the Central Site System has been expanded and developed into the AP 14. The AP 14 can support a greater variety of applications than previously disclosed in U.S. Pat. No. 5,608,410, including the ability to post-process large volumes of location records from multiple wireless transmitters. This post-processed data can yield, for example, very effective maps for use by wireless carriers to improve and optimize the RF design of the communications systems. This can be achieved, for example, by plotting the locations of all of the callers in an area and the received signal strengths at a number of cell sites. The carrier can then determine whether each cell site is, in fact, serving the exact coverage area desired by the carrier. The AP 14 can also now store location records anonymously, that is, with the MIN and/or other identity information removed from the location record, so that the location record can be used for RF optimization or traffic monitoring without causing concerns about an individual user's privacy.

As shown in FIG. 1A, a presently preferred implementation of the WLS includes a plurality of SCS regions each of which comprises multiple SCS's 10. For example, “SCS Region 1” includes SCS's 10A and 10B (and preferably others, not shown) that are located at respective cell sites and share antennas with the base stations at those cell sites. Drop and insert units 11A and 11B are used to interface fractional T1/E1 lines to full T1/E1 lines, which in turn are coupled to a digital access and control system (DACS) 13A. The DACS 13A and another DACS 13B are used in the manner described more fully below for communications between the SCS's 10A, 10B, etc., and multiple TLP's 12A, 12B, etc. As shown, the TLP's are typically collocated and interconnected via an Ethernet network (backbone) and a second, redundant Ethernet network. Also coupled to the Ethernet networks are multiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminal server 15. Routers 19A and 19B are used to couple one WLS to one or more other Wireless Location System(s).

Signal Collection System 10

Generally, cell sites will have one of the following antenna configurations: (i) an omnidirectional site with 1 or 2 receive antennas or (ii) a sectored site with 1, 2, or 3 sectors, and with 1 or 2 receive antennas used in each sector. As the number of cell sites has increased in the U.S. and internationally, sectored cell sites have become the predominant configuration. However, there are also a growing number of micro-cells and pico-cells, which can be omnidirectional. Therefore, the SCS 10 has been designed to be configurable for any of these typical cell sites and has been provided with mechanisms to employ any number of antennas at a cell site.

The basic architectural elements of the SCS 10 remain the same as for the Antenna Site System described in U.S. Pat. No. 5,608,410, but several enhancements have been made to increase the flexibility of the SCS 10 and to reduce the commercial deployment cost of the system. The most presently preferred embodiment of the SCS 10 is described herein. The SCS 10, an overview of which is shown in FIG. 2, includes digital receiver modules 10-2A through 10-2C; DSP modules 10-3A through 10-3C; a serial bus 10-4, a control and communications module 10-5; a GPS module 10-6; and a clock distribution module 10-7. The SCS 10 has the following external connections: power, fractional T1/E1 communications, RF connections to antennas, and a GPS antenna connection for the timing generation (or clock distribution) module 10-7. The architecture and packaging of the SCS 10 permit it to be physically collocated with cell sites (which is the most common installation place), located at other types of towers (such as FM, AM, two-way emergency communications, television, etc.), or located at other building structures (such as rooftops, silos, etc.).

Timing Generation

The Wireless Location System depends upon the accurate determination of time at all SCS's 10 contained within a network. Several different timing generation systems have been described in previous disclosures, however the most presently preferred embodiment is based upon an enhanced GPS receiver 10-6. The enhanced GPS receiver differs from most traditional GPS receivers in that the receiver contains algorithms that remove some of the timing instability of the GPS signals, and guarantees that any two SCS's 10 contained within a network can receive timing pulses that are within approximately ten nanoseconds of each other. These enhanced GPS receivers are now commercially available, and further reduce some of the time reference related errors that were observed in previous implementations of wireless location systems. While this enhanced GPS receiver can produce a very accurate time reference, the output of the receiver may still have an unacceptable phase noise. Therefore, the output of the receiver is input to a low phase noise, crystal oscillator-driven phase locked loop circuit that can now produce 10 MHz and one pulse per second (PPS) reference signals with less than 0.01 degrees RMS of phase noise, and with the pulse output at any SCS 10 in a Wireless Location System network within ten nanoseconds of any other pulse at another SCS 10. This combination of enhanced GPS receiver, crystal oscillator, and phase locked loop is now the most preferred method to produce stable time and frequency reference signals with low phase noise.

The SCS 10 has been designed to support multiple frequency bands and multiple carriers with equipment located at the same cell site. This can take place by using multiple receivers internal to a single SCS chassis, or by using multiple chassis each with separate receivers. In the event that multiple SCS chassis are placed at the same cell site, the SCS's 10 can share a single timing generation/clock distribution circuit 10-7 and thereby reduce overall system cost. The 10 MHz and one PPS output signals from the timing generation circuit are amplified and buffered internal to the SCS 10, and then made available via external connectors. Therefore a second SCS can receive its timing from a first SCS using the buffered output and the external connectors. These signals can also be made available to base station equipment collocated at the cell site. This might be useful to the base station, for example, in improving the frequency re-use pattern of a wireless communications system.

Receiver Module 10-2 (Wideband Embodiment)

When a wireless transmitter makes a transmission, the Wireless Location System must receive the transmission at multiple SCS's 10 located at multiple geographically dispersed cell sites. Therefore, each SCS 10 has the ability to receive a transmission on any RF channel on which the transmission may originate. Additionally, since the SCS 10 is capable of supporting multiple air interface protocols, the SCS 10 also supports multiple types of RF channels. This is in contrast to most current base station receivers, which typically receive only one type of channel and are usually capable of receiving only on select RF channels at each cell site. For example, a typical TDMA base station receiver will only support 30 KHz wide channels, and each receiver is programmed to receive signals on only a single channel whose frequency does not change often (i.e. there is a relatively fixed frequency plan). Therefore, very few TDMA base station receivers would receive a transmission on any given frequency. As another example, even though some GSM base station receivers are capable of frequency hopping, the receivers at multiple base stations are generally not capable of simultaneously tuning to a single frequency for the purpose of performing location processing. In fact, the receivers at GSM base stations are programmed to frequency hop to avoid using an RF channel that is being used by another transmitter so as to minimize interference.

The SCS receiver module 10-2 is preferably a dual wideband digital receiver that can receive the entire frequency band and all of the RF channels of an air interface. For cellular systems in the U.S., this receiver module is either 15 MHz wide or 25 MHz wide so that all of the channels of a single carrier or all of the channels of both carriers can be received. This receiver module has many of the characteristics of the receiver previously described in U.S. Pat. No. 5,608,410, and FIG. 2A is a block diagram of the currently preferred embodiment. Each receiver module contains an RF tuner section 10-2-1, a data interface and control section 10-2-2 and an analog to digital conversion section 10-2-3. The RF tuner section 10-2-1 includes two full independent digital receivers (including Tuner #1 and Tuner #2) that convert the analog RF input from an external connector into a digitized data stream. Unlike most base station receivers, the SCS receiver module does not perform diversity combining or switching. Rather, the digitized signal from each independent receiver is made available to the location processing. The present inventors have determined that there is an advantage to the location processing, and especially the multipath mitigation processing, to independently process the signals from each antenna rather than perform combining on the receiver module.

The receiver module 10-2 performs, or is coupled to elements that perform, the following functions: automatic gain control (to support both nearby strong signals and far away weak signals), bandpass filtering to remove potentially interfering signals from outside of the RF band of interest, synthesis of frequencies needed for mixing with the RF signals to create an IF signal that can be sampled, mixing, and analog to digital conversion (ADC) for sampling the RF signals and outputting a digitized data stream having an appropriate bandwidth and bit resolution. The frequency synthesizer locks the synthesized frequencies to the 10 MHz reference signal from the clock distribution/timing generation module 10-7 (FIG. 2). All of the circuits used in the receiver module maintain the low phase noise characteristics of the timing reference signal. The receiver module preferably has a spurious free dynamic range of at least 80 dB.

The receiver module 10-2 also contains circuits to generate test frequencies and calibration signals, as well as test ports where measurements can be made by technicians during installation or troubleshooting. Various calibration processes are described in further detail below. The internally generated test frequencies and test ports provide an easy method for engineers and technicians to rapidly test the receiver module and diagnose any suspected problems. This is also especially useful during the manufacturing process.

One of the advantages of the Wireless Location System described herein is that no new antennas are required at cell sites. The Wireless Location System can use the existing antennas already installed at most cell sites, including both omni-directional and sectored antennas. This feature can result in significant savings in the installation and maintenance costs of the Wireless Location System versus other approaches that have been described in the prior art. The SCS's digital receivers 10-2 can be connected to the existing antennas in two ways, as shown in FIGS. 2B and 2C, respectively. In FIG. 2B, the SCS receivers 10-2 are connected to the existing cell site multi-coupler or RF splitter. In this manner, the SCS 10 uses the cell site's existing low noise pre-amplifier, band pass filter, and multi-coupler or RF splitter. This type of connection usually limits the SCS 10 to supporting the frequency band of a single carrier. For example, an A-side cellular carrier will typically use the band pass filter to block signals from customers of the B-side carrier, and vice versa.

In FIG. 2C, the existing RF path at the cell site has been interrupted, and a new pre-amplifier, band pass filter, and RF splitter has been added as part of the Wireless Location System. The new band pass filter will pass multiple contiguous frequency bands, such as both the A-side and B-side cellular carriers, thereby allowing the Wireless Location System to locate wireless transmitters using both cellular systems but using the antennas from a single cell site. In this configuration, the Wireless Location System uses matched RF components at each cell site, so that the phase versus frequency responses are identical. This is in contrast to existing RF components, which may be from different manufacturers or using different model numbers at various cell sites. Matching the response characteristics of RF components reduces a possible source of error for the location processing, although the Wireless Location System has the capability to compensate for these sources of error. Finally, the new pre-amplifier installed with the Wireless Location System will have a very low noise figure to improve the sensitivity of the SCS 10 at a cell site. The overall noise figure of the SCS digital receivers 10-2 is dominated by the noise figure of the low noise amplifiers. Because the Wireless Location System can use weak signals in location processing, whereas the base station typically cannot process weak signals, the Wireless Location System can significantly benefit from a high quality, very low noise amplifier.

In order to improve the ability of the Wireless Location System to accurately determine TDOA for a wireless transmission, the phase versus frequency response of the cell site's RF components are determined at the time of installation and updated at other certain times and then stored in a table in the Wireless Location System. This can be important because, for example, the band pass filters and/or multi-couplers made by some manufacturers have a steep and non-linear phase versus frequency response near the edge of the pass band. If the edge of the pass band is very near to or coincident with the reverse control or voice channels, then the Wireless Location System would make incorrect measurements of the transmitted signal's phase characteristics if the Wireless Location System did not correct the measurements using the stored characteristics. This becomes even more important if a carrier has installed multi-couplers and/or band pass filters from more than one manufacturer, because the characteristics at each site may be different. In addition to measuring the phase versus frequency response, other environmental factors may cause changes to the RF path prior to the ADC. These factors require occasional and sometimes periodic calibration in the SCS 10.

Alternative Narrowband Embodiment of Receiver Module 10-2

In addition or as an alternative to the wideband receiver module, the SCS 10 also supports a narrowband embodiment of the receiver module 10-2. In contrast to the wideband receiver module that can simultaneously receive all of the RF channels in use by a wireless communications system, the narrowband receiver can only receive one or a few RF channels at a time. For example, the SCS 10 supports a 60 KHz narrowband receiver for use in AMPS/TDMA systems, covering two contiguous 30 KHz channels. This receiver is still a digital receiver as described for the wideband module, however, the frequency synthesizing and mixing circuits are used to dynamically tune the receiver module to various RF channels on command. This dynamic tuning can typically occur in one millisecond or less, and the receiver can dwell on a specific RF channel for as long as required to receive and digitize RF data for location processing.

The purpose of the narrowband receiver is to reduce the implementation cost of a Wireless Location System from the cost that is incurred with wideband receivers. Of course, there is some loss of performance, but the availability of these multiple receivers permits wireless carriers to have more cost/performance options. Additional inventive functions and enhancements have been added to the Wireless Location System to support this new type of narrowband receiver. When the wideband receiver is being used, all RF channels are received continuously at all SCS's 10, and subsequent to the transmission, the Wireless Location System can use the DSP's 10-3 (FIG. 2) to dynamically select any RF channel from the digital memory. With the narrowband receiver, the Wireless Location System must ensure a priori that the narrowband receivers at multiple cell sites are simultaneously tuned to the same RF channel so that all receivers can simultaneously receive, digitize and store the same wireless transmission. For this reason, the narrowband receiver is generally used only for locating voice channel transmissions, which can be known a priori to be making a transmission. Since control channel transmissions can occur asynchronously at any time, the narrowband receiver may not be tuned to the correct channel to receive the transmission.

When the narrowband receivers are used for locating AMPS voice channel transmissions, the Wireless Location System has the ability to temporarily change the modulation characteristics of the AMPS wireless transmitter to aid location processing. This may be necessary because AMPS voice channels are only FM modulated with the addition of a low level supervisory tone known as SAT. As is known in the art, the Cramer-Rao lower bound of AMPS FM modulation is significantly worse than the Manchester encoded FSK modulation used for AMPS reverse channels and “blank and burst” transmissions on the voice channel. Further, AMPS wireless transmitters may be transmitting with significantly reduced energy if there is no modulating input signal (i.e., no one is speaking). To improve the location estimate by improving the modulation characteristics without depending on the existence or amplitude of an input modulating signal, the Wireless Location System can cause an AMPS wireless transmitter to transmit a “blank and burst” message at a point in time when the narrowband receivers at multiple SCS's 10 are tuned to the RF channel on which the message will be sent. This is further described later.

The Wireless Location System performs the following steps when using the narrowband receiver module (see the flowchart of FIG. 2C-1):

• • a first wireless transmitter is a priori engaged in transmitting on a particular RF channel;
  • the Wireless Location System triggers to make a location estimate of the first wireless transmitter (the trigger may occur either internally or externally via a command/response interface);
  • the Wireless Location System determines the cell site, sector, RF channel, timeslot, long code mask, and encryption key (all information elements may not be necessary for all air interface protocols) currently in use by the first wireless transmitter;
  • the Wireless Location System tunes an appropriate first narrowband receiver at an appropriate first SCS 10 to the RF channel and timeslot at the designated cell site and sector, where appropriate typically means both available and collocated or in closest proximity;
  • the first SCS 10 receives a time segment of RF data, typically ranging from a few microseconds to tens of milliseconds, from the first narrowband receiver and evaluates the transmission's power, SNR, and modulation characteristics;
  • if the transmission's power or SNR is below a predetermined threshold, the Wireless Location System waits a predetermined length of time and then returns to the above third step (where the Wireless Location System determines the cell site, sector, etc.);
  • if the transmission is an AMPS voice channel transmission and the modulation is below a threshold, then the Wireless Location System commands the wireless communications system to send a command to the first wireless transmitter to cause a “blank and burst” on the first wireless transmitter;
  • the Wireless Location System requests the wireless communications system to prevent hand-off of the wireless transmitter to another RF channel for a predetermined length of time;
  • the Wireless Location System receives a response from the wireless communications system indicating the time period during which the first wireless transmitter will be prevented from handing-off, and if commanded, the time period during which the wireless communications system will send a command to the first wireless transmitter to cause a “blank and burst”;
  • the Wireless Location System determines the list of antennas that will be used in location processing (the antenna selection process is described below);
  • the Wireless Location System determines the earliest Wireless Location System timestamp at which the narrowband receivers connected to the selected antennas are available to begin simultaneously collecting RF data from the RF channel currently in use by the first wireless transmitter;
  • based upon the earliest Wireless Location System timestamp and the time periods in the response from the wireless communications system, the Wireless Location System commands the narrowband receivers connected to the antennas that will be used in location processing to tune to the cell site, sector, and RF channel currently in use by the first wireless transmitter and to receive RF data for a predetermined dwell time (based upon the bandwidth of the signal, SNR, and integration requirements);
  • the RF data received by the narrowband receivers are written into the dual port memory;
  • location processing on the received RF data commences, as described in U.S. Pat. Nos. 5,327,144 and 5,608,410 and in sections below;
  • the Wireless Location System again determines the cell site, sector, RF channel, timeslot, long code mask, and encryption key currently in use by the first wireless transmitter;
  • if the cell site, sector, RF channel, timeslot, long code mask, and encryption key currently in use by the first wireless transmitter has changed between queries (i.e. before and after gathering the RF data) the Wireless Location System ceases location processing, causes an alert message that location processing failed because the wireless transmitter changed transmission status during the period of time in which RF data was being received, and re-triggers this entire process;
  • location processing on the received RF data completes in accordance with the steps described below.

The determination of the information elements including cell site, sector, RF channel, timeslot, long code mask, and encryption key (all information elements may not be necessary for all air interface protocols) is typically obtained by the Wireless Location System through a command/response interface between the Wireless Location System and the wireless communications system.

The use of the narrowband receiver in the manner described above is known as random tuning because the receivers can be directed to any RF channel on command from the system. One advantage to random tuning is that locations are processed only for those wireless transmitters for which the Wireless Location System is triggered. One disadvantage to random tuning is that various synchronization factors, including the interface between the wireless communications system and the Wireless Location System and the latency times in scheduling the necessary receivers throughout the system, can limit the total location processing throughput. For example, in a TDMA system, random tuning used throughout the Wireless Location System will typically limit location processing throughput to about 2.5 locations per second per cell site sector.

Therefore, the narrowband receiver also supports another mode, known as automatic sequential tuning, which can perform location processing at a higher throughput. For example, in a TDMA system, using similar assumptions about dwell time and setup time as for the narrowband receiver operation described above, sequential tuning can achieve a location processing throughput of about 41 locations per second per cell site sector, meaning that all 395 TDMA RF channels can be processed in about 9 seconds. This increased rate can be achieved by taking advantage of, for example, the two contiguous RF channels that can be received simultaneously, location processing all three TDMA timeslots in an RF channel, and eliminating the need for synchronization with the wireless communications system. When the Wireless Location System is using the narrowband receivers for sequential tuning, the Wireless Location System has no knowledge of the identity of the wireless transmitter because the Wireless Location System does not wait for a trigger, nor does the Wireless Location System query the wireless communications system for the identity information prior to receiving the transmission. In this method, the Wireless Location System sequences through every cell site, RF channel and time slot, performs location processing, and reports a location record identifying a time stamp, cell site, RF channel, time slot, and location. Subsequent to the location record report, the Wireless Location System and the wireless communications system match the location records to the wireless communications system's data indicating which wireless transmitters were in use at the time, and which cell sites, RF channels, and time slots were used by each wireless transmitter. Then, the Wireless Location System can retain the location records for wireless transmitters of interest, and discard those location records for the remaining wireless transmitters.

Digital Signal Processor Module 10-3

The SCS digital receiver modules 10-2 output a digitized RF data stream having a specified bandwidth and bit resolution. For example, a 15 MHz embodiment of the wideband receiver may output a data stream containing 60 million samples per second, at a resolution of 14 bits per sample. This RF data stream will contain all of the RF channels that are used by the wireless communications system. The DSP modules 10-3 receive the digitized data stream, and can extract any individual RF channel through digital mixing and filtering. The DSP's can also reduce the bit resolution upon command from the Wireless Location System, as needed to reduce the bandwidth requirements between the SCS 10 and TLP 12. The Wireless Location System can dynamically select the bit resolution at which to forward digitized baseband RF data, based upon the processing requirements for each location. DSP's are used for these functions to reduce the systemic errors that can occur from mixing and filtering with analog components. The use of DSP's allows perfect matching in the processing between any two SCS's 10.

A block diagram of the DSP module 10-3 is shown is FIG. 2D, and the operation of the DSP module is depicted by the flowchart of FIG. 2E. As shown in FIG. 2D, the DSP module 10-3 comprises the following elements: a pair of DSP elements 10-3-1A and 10-3-1B, referred to collectively as a “first” DSP; serial to parallel converters 10-3-2; dual port memory elements 10-3-3; a second DSP 10-3-4; a parallel to serial converter; a FIFO buffer; a DSP 10-3-5 (including RAM) for detection, another DSP 10-3-6 for demodulation, and another DSP 10-3-7 for normalization and control; and an address generator 10-3-8. In a presently preferred embodiment, the DSP module 10-3 receives the wideband digitized data stream (FIG. 2E, step S1), and uses the first DSP (10-3-1A and 10-3-1B) to extract blocks of channels (step S2). For example, a first DSP programmed to operate as a digital drop receiver can extract four blocks of channels, where each block includes at least 1.25 MHz of bandwidth. This bandwidth can include 42 channels of AMPS or TDMA, 6 channels of GSM, or 1 channel of CDMA. The DSP does not require the blocks to be contiguous, as the DSP can independently digitally tune to any set of RF channels within the bandwidth of the wideband digitized data stream. The DSP can also perform wideband or narrow band energy detection on all or any of the channels in the block, and report the power levels by channel to the TLP 12 (step S3). For example, every 10 ms, the DSP can perform wideband energy detection and create an RF spectral map for all channels for all receivers (see step S9). Because this spectral map can be sent from the SCS 10 to the TLP 12 every 10 ms via the communications link connecting the SCS 10 and the TLP 12, a significant data overhead could exist. Therefore, the DSP reduces the data overhead by companding the data into a finite number of levels. Normally, for example, 84 dB of dynamic range could require 14 bits. In the companding process implemented by the DSP, the data is reduced, for example, to only 4 bits by selecting 16 important RF spectral levels to send to the TLP 12. The choice of the number of levels, and therefore the number of bits, as well as the representation of the levels, can be automatically adjusted by the Wireless Location System. These adjustments are performed to maximize the information value of the RF spectral messages sent to the TLP 12 as well as to optimize the use of the bandwidth available on the communications link between the SCS 10 and the TLP 12.

After conversion, each block of RF channels (each at least 1.25 MHz) is passed through serial to parallel converter 10-3-2 and then stored in dual port digital memory 10-3-3 (step S4). The digital memory is a circular memory, which means that the DSP module begins writing data into the first memory address and then continues sequentially until the last memory address is reached. When the last memory address is reached, the DSP returns to the first memory address and continues to sequentially write data into memory. Each DSP module typically contains enough memory to store several seconds of data for each block of RF channels to support the latency and queuing times in the location process.

In the DSP module, the memory address at which digitized and converted RF data is written into memory is the time stamp used throughout the Wireless Location System and which the location processing references in determining TDOA. In order to ensure that the time stamps are aligned at every SCS 10 in the Wireless Location System, the address generator 10-3-8 receives the one pulse per second signal from the timing generation/clock distribution module 10-7 (FIG. 2). Periodically, the address generator at all SCS's 10 in a Wireless Location System will simultaneously reset themselves to a known address. This enables the location processing to reduce or eliminate accumulated timing errors in the recording of time stamps for each digitized data element.

The address generator 10-3-8 controls both writing to and reading from the dual port digital memory 10-3-3. Writing takes places continuously since the ADC is continuously sampling and digitizing RF signals and the first DSP (10-3-1A and 10-3-1B) is continuously performing the digital drop receiver function. However, reading occurs in bursts as the Wireless Location System requests data for performing demodulation and location processing. The Wireless Location System may even perform location processing recursively on a single transmission, and therefore requires access to the same data multiple times. In order to service the many requirements of the Wireless Location System, the address generator allows the dual port digital memory to be read at a rate faster than the writing occurs. Typically, reading can be performed eight times faster than writing.

The DSP module 10-3 uses the second DSP 10-3-4 to read the data from the digital memory 10-3-3, and then performs a second digital drop receiver function to extract baseband data from the blocks of RF channels (step S5). For example, the second DSP can extract any single 30 KHz AMPS or TDMA channel from any block of RF channels that have been digitized and stored in the memory. Likewise, the second DSP can extract any single GSM channel. The second DSP is not required to extract a CDMA channel, since the channel bandwidth occupies the full bandwidth of the stored RF data. The combination of the first DSP 10-3-1A, 10-3-1B and the second DSP 10-3-4 allows the DSP module to select, store, and recover any single RF channel in a wireless communications system. A DSP module typically will store four blocks of channels. In a dual-mode AMPS/TDMA system, a single DSP module can continuously and simultaneously monitor up to 42 analog reverse control channels, up to 84 digital control channels, and also be tasked to monitor and locate any voice channel transmission. A single SCS chassis will typically support up to three receiver modules 10-2 (FIG. 2), to cover three sectors of two antennas each, and up to nine DSP modules (three DSP modules per receiver permits an entire 15 MHz bandwidth to be simultaneously stored into digital memory). Thus, the SCS 10 is a very modular system than can be easily scaled to match any type of cell site configuration and processing load.

The DSP module 10-3 also performs other functions, including automatic detection of active channels used in each sector (step S6), demodulation (step S7), and station based location processing (step S8). The Wireless Location System maintains an active map of the usage of the RF channels in a wireless communications system (step S9), which enables the Wireless Location System to manage receiver and processing resources, and to rapidly initiate processing when a particular transmission of interest has occurred. The active map comprises a table maintained within the Wireless Location System that lists for each antenna connected to an SCS 10 the primary channels assigned to that SCS 10 and the protocols used in those channels. A primary channel is an RF control channel assigned to a collocated or nearby base station which the base station uses for communications with wireless transmitters. For example, in a typical cellular system with sectored cell sites, there will be one RF control channel frequency assigned for use in each sector. Those control channel frequencies would typically be assigned as primary channels for a collocated SCS 10.

The same SCS 10 may also be assigned to monitor the RF control channels of other nearby base stations as primary channels, even if other SCS's 10 also have the same primary channels assigned. In this manner, the Wireless Location System implements a system demodulation redundancy that ensures that any given wireless transmission has an infinitesimal probability of being missed. When this demodulation redundancy feature is used, the Wireless Location System will receive, detect, and demodulate the same wireless transmission two or more times at more than one SCS 10. The Wireless Location System includes means to detect when this multiple demodulation has occurred and to trigger location processing only once. This function conserves the processing and communications resources of the Wireless Location System, and is further described below. This ability for a single SCS 10 to detect and demodulate wireless transmissions occurring at cell sites not collocated with the SCS 10 permits operators of the Wireless Location System to deploy more efficient Wireless Location System networks. For example, the Wireless Location System may be designed such that the Wireless Location System uses much fewer SCS's 10 than the wireless communications system has base stations.

In the Wireless Location System, primary channels are entered and maintained in the table using two methods: direct programming and automatic detection. Direct programming comprises entering primary channel data into the table using one of the Wireless Location System user interfaces, such as the Network Operations Console 16 (FIG. 1), or by receiving channel assignment data from the Wireless Location System to wireless communications system interface. Alternatively, the DSP module 10-3 also runs a background process known as automatic detection in which the DSP uses spare or scheduled processing capacity to detect transmissions on various possible RF channels and then attempt to demodulate those transmissions using probable protocols. The DSP module can then confirm that the primary channels directly programmed are correct, and can also quickly detect changes made to channels at base station and send an alert to the operator of the Wireless Location System.

The DSP module performs the following steps in automatic detection (see FIG. 2E-1):

• • for each possible control and/or voice channel which may be used in the coverage area of the SCS 10, peg counters are established (step S7-1);
  • at the start of a detection period, all peg counters are reset to zero (step S7-2);
  • each time that a transmission occurs in a specified RF channel, and the received power level is above a particular pre-set threshold, the peg counter for that channel is incremented (step S7-3);
  • each time that a transmission occurs in a specified RF channel, and the received power level is above a second particular pre-set threshold, the DSP module attempts to demodulate a certain portion of the transmission using a first preferred protocol (step S7-4);
  • if the demodulation is successful, a second peg counter for that channel is incremented (step S7-5);
  • if the demodulation is unsuccessful, the DSP module attempts to demodulate a portion of the transmission using a second preferred protocol (step S7-6);
  • if the demodulation is successful, a third peg counter for that channel is incremented (step S7-7);
  • at the end of a detection period, the Wireless Location System reads all peg counters (step S7-8); and
  • the Wireless Location System automatically assigns primary channels based upon the peg counters (step S7-9).

The operator of the Wireless Location System can review the peg counters and the automatic assignment of primary channels and demodulation protocols, and override any settings that were performed automatically. In addition, if more than two preferred protocols may be used by the wireless carrier, then the DSP module 10-3 can be downloaded with software to detect the additional protocols. The architecture of the SCS 10, based upon wideband receivers 10-2, DSP modules 10-3, and downloadable software permits the Wireless Location System to support multiple demodulation protocols in a single system. There is a significant cost advantage to supporting multiple protocols within the single system, as only a single SCS 10 is required at a cell site. This is in contrast to many base station architectures, which may require different transceiver modules for different modulation protocols. For example, while the SCS 10 could support AMPS, TDMA, and CDMA simultaneously in the same SCS 10, there is no base station currently available that can support this functionality.

The ability to detect and demodulate multiple protocols also includes the ability to independently detect the use of authentication in messages transmitted over the certain air interface protocols. The use of authentication fields in wireless transmitters started to become prevalent within the last few years as a means to reduce the occurrence of fraud in wireless communications systems. However, not all wireless transmitters have implemented authentication. When authentication is used, the protocol generally inserts an additional field into the transmitted message. Frequently this field is inserted between the identity of the wireless transmitter and the dialed digits in the transmitted message. When demodulating a wireless transmission, the Wireless Location System determines the number of fields in the transmitted message, as well as the message type (i.e. registration, origination, page response, etc.). The Wireless Location System demodulates all fields and if extra fields appear to be present, giving consideration to the type of message transmitted, then the Wireless Location System tests all fields for a trigger condition. For example, if the dialed digits “911” appear in the proper place in a field, and the field is located either in its proper place without authentication or its proper place with authentication, then the Wireless Location System triggers normally. In this example, the digits “911” would be required to appear in sequence as “911” or “*911”, with no other digits before or after either sequence. This functionality reduces or eliminates a false trigger caused by the digits “911” appearing as part of an authentication field.

The support for multiple demodulation protocols is important for the Wireless Location System to successfully operate because location processing must be quickly triggered when a wireless caller has dialed “911”. The Wireless Location System can trigger location processing using two methods: the Wireless Location System will independently demodulate control channel transmissions, and trigger location processing using any number of criteria such as dialed digits, or the Wireless Location System may receive triggers from an external source such as the carrier's wireless communications system. The present inventors have found that independent demodulation by the SCS 10 results in the fastest time to trigger, as measured from the moment that a wireless user presses the “SEND” or “TALK” (or similar) button on a wireless transmitter.

Control and Communications Module 10-5

The control and communications module 10-5, depicted in FIG. 2F, includes data buffers 10-5-1, a controller 10-5-2, memory 10-5-3, a CPU 10-5-4 and a T1/E1 communications chip 10-5-5. The module has many of the characteristics previously described in U.S. Pat. No. 5,608,410. Several enhancements have been added in the present embodiment. For example, the SCS 10 now includes an automatic remote reset capability, even if the CPU on the control and communications module ceases to execute its programmed software. This capability can reduce the operating costs of the Wireless Location System because technicians are not required to travel to a cell site to reset an SCS 10 if it fails to operate normally. The automatic remote reset circuit operates by monitoring the communications interface between the SCS 10 and the TLP 12 for a particular sequence of bits. This sequence of bits is a sequence that does not occur during normal communications between the SCS 10 and the TLP 12. This sequence, for example, may consist of an all ones pattern. The reset circuit operates independently of the CPU so that even if the CPU has placed itself in a locked or other non-operating status, the circuit can still achieve the reset of the SCS 10 and return the CPU to an operating status.

This module now also has the ability to record and report a wide variety of statistics and variables used in monitoring or diagnosing the performance of the SCS 10. For example, the SCS 10 can monitor the percent capacity usage of any DSP or other processor in the SCS 10, as well as the communications interface between the SCS 10 and the TLP 12. These values are reported regularly to the AP 14 and the NOC 16, and are used to determine when additional processing and communications resources are required in the system. For example, alarm thresholds may be set in the NOC to indicate to an operator if any resource is consistently exceeding a preset threshold. The SCS 10 can also monitor the number of times that transmissions have been successfully demodulated, as well as the number of failures. This is useful in allowing operators to determine whether the signal thresholds for demodulation have been set optimally.

This module, as well as the other modules, can also self-report its identity to the TLP 12. As described below, many SCS's 10 can be connected to a single TLP 12. Typically, the communications between SCS's 10 and TLP's 12 is shared with the communications between base stations and MSC's. It is frequently difficult to quickly determine exactly which SCS's 10 have been assigned to particular circuits. Therefore, the SCS 10 contains a hard coded identity, which is recorded at the time of installation. This identity can be read and verified by the TLP 12 to positively determine which SCS 10 has been assigned by a carrier to each of several different communications circuits.

The SCS to TLP communications supports a variety of messages, including: commands and responses, software download, status and heartbeat, parameter download, diagnostic, spectral data, phase data, primary channel demodulation, and RF data. The communications protocol is designed to optimize Wireless Location System operation by minimizing the protocol overhead and the protocol includes a message priority scheme. Each message type is assigned a priority, and the SCS 10 and the TLP 12 will queue messages by priority such that a higher priority message is sent before a lower priority message is sent. For example, demodulation messages are generally set at a high priority because the Wireless Location System must trigger location processing on certain types of calls (i.e., E9-1-1) without delay. Although higher priority messages are queued before lower priority messages, the protocol generally does not preempt a message that is already in transit. That is, a message in the process of being sent across the SCS 10 to TLP 12 communications interface will be completed fully, but then the next message to be sent will be the highest priority message with the earliest time stamp. In order to minimize the latency of high priority messages, long messages, such as RF data, are sent in segments. For example, the RF data for a full 100-millisecond AMPS transmission may be separated into 10-millisecond segments. In this manner, a high priority message may be queued in between segments of the RF data.

Calibration and Performance Monitoring

The architecture of the SCS 10 is heavily based upon digital technologies including the digital receiver and the digital signal processors. Once RF signals have been digitized, timing, frequency, and phase differences can be carefully controlled in the various processes. More importantly, any timing, frequency, and phase differences can be perfectly matched between the various receivers and various SCS's 10 used in the Wireless Location System. However, prior to the ADC, the RF signals pass through a number of RF components, including antennas, cables, low noise amplifiers, filters, duplexors, multi-couplers, and RF splitters. Each of these RF components has characteristics important to the Wireless Location System, including delay and phase versus frequency response. When the RF and analog components are perfectly matched between the pairs of SCS's 10, such as SCS 10A and SCS 10B in FIG. 2G, then the effects of these characteristics are automatically eliminated in the location processing. But when the characteristics of the components are not matched, then the location processing can inadvertently include instrumental errors resulting from the mismatch. Additionally, many of these RF components can experience instability with power, time, temperature, or other factors that can add instrumental errors to the determination of location. Therefore, several inventive techniques have been developed to calibrate the RF components in the Wireless Location System and to monitor the performance of the Wireless Location System on a regular basis. Subsequent to calibration, the Wireless Location System stores the values of these delays and phases versus frequency response (i.e. by RF channel number) in a table in the Wireless Location System for use in correcting these instrumental errors. FIGS. 2G-2J are referred to below in explaining these calibration methods.

External Calibration Method

Referring to FIG. 2G, the timing stability of the Wireless Location System is measured along baselines, where each baseline is comprised of two SCS's, 10A and 10B, and an imaginary line (A-B) drawn between them. In a TDOA/FDOA type of Wireless Location System, locations of wireless transmitters are calculated by measuring the differences in the times that each SCS 10 records for the arrival of the signal from a wireless transmitter. Thus, it is important that the differences in times measured by SCS's 10 along any baseline are largely attributed to the transmission time of the signal from the wireless transmitter and minimally attributed to the variations in the RF and analog components of the SCS's 10 themselves. To meet the accuracy goals of the Wireless Location System, the timing stability for any pair of SCS's 10 are maintained at much less than 100 nanoseconds RMS (root mean square). Thus, the components of the Wireless Location System will contribute less than 100 feet RMS of instrumentation error in the estimation of the location of a wireless transmitter. Some of this error is allocated to the ambiguity of the signal used to calibrate the system. This ambiguity can be determined from the well-known Cramer-Rao lower bound equation. In the case of an AMPS reverse control channel, this error is approximately 40 nanoseconds RMS. The remainder of the error budget is allocated to the components of the Wireless Location System, primarily the RF and analog components in the SCS 10.

In the external calibration method, the Wireless Location System uses a network of calibration transmitters whose signal characteristics match those of the target wireless transmitters. These calibration transmitters may be ordinary wireless telephones emitting periodic registration signals and/or page response signals. Each usable SCS-to-SCS baseline is preferably calibrated periodically using a calibration transmitter that has a relatively clear and unobstructed path to both SCS's 10 associated with the baseline. The calibration signal is processed identically to a signal from a target wireless transmitter. Since the TDOA values are known a priori, any errors in the calculations are due to systemic errors in the Wireless Location System. These systemic errors can then be removed in the subsequent location calculations for target transmitters.

FIG. 2G illustrates the external calibration method for minimizing timing errors. As shown, a first SCS 10A at a point “A” and a second SCS 10A at a point “B” have an associated baseline A-B. A calibration signal emitted at time To by a calibration transmitter at point “C” will theoretically reach first SCS 10A at time T₀+T_AC. T_AC is a measure of the amount of time required for the calibration signal to travel from the antenna on the calibration transmitter to the dual port digital memory in a digital receiver. Likewise, the same calibration signal will reach second SCS 10B at a theoretical time T₀+T_BC. Usually, however, the calibration signal will not reach the digital memory and the digital signal processing components of the respective SCS's 10 at exactly the correct times. Rather, there will be errors e1 and e2 in the amount of time (T_AC, T_BC) it takes the calibration signal to propagate from the calibration transmitter to the SCS's 10, respectively, such that the exact times of arrival are actually T₀+T_AC+e1 and T₀+T_BC+e2. Such errors will be due to some extent to delays in the signal propagation through the air, i.e., from the calibration transmitter's antenna to the SCS antennas; however, the errors will be due primarily to time varying characteristics in the SCS front end components. The errors e1 and e2 cannot be determined per se because the system does not know the exact time (T₀) at which the calibration signal was transmitted. The system can, however, determine the error in the difference in the time of arrival of the calibration signal at the respective SCS's 10 of any given pair of SCS's 10. This TDOA error value is defined as the difference between the measured TDOA value and the theoretical TDOA value τ₀, where τ₀ is the theoretical differences between the theoretical delay values T_AC and T_BC. Theoretical TDOA values for each pair of SCS's 10 and each calibration transmitter are known because the positions of the SCS's 10 and calibration transmitter, and the speed at which the calibration signal propagates, are known. The measured TDOA baseline (TDOA_A-B) can be represented as TDOA_A-B=τ₀+ε, where ε=e1−e2. In a similar manner, a calibration signal from a second calibration transmitter at point “D” will have associated errors e3 and e4. The ultimate value of Eto be subtracted from TDOA measurements for a target transmitter will be a function (e.g., weighted average) of the E values derived for one or more calibration transmitters. Therefore, a given TDOA measurement (TDOA_measured) for a pair of SCS's 10 at points “X” and “Y” and a target wireless transmitter at an unknown location will be corrected as follows:
TDOA_X-Y=TDOA_{measur ed}ε
ε=k1ε1+k2ε2+ . . . kNεN,
where k1, k2, etc., are weighting factors and ε1, ε2, etc., are the errors determined by subtracting the measured TDOA values from the theoretical values for each calibration transmitter. In this example, error value ε1 may the error value associated with the calibration transmitter at point “C” in the drawing. The weighting factors are determined by the operator of the Wireless Location System, and input into the configuration tables for each baseline. The operator will take into consideration the distance from each calibration transmitter to the SCS's 10 at points “X” and “Y”, the empirically determined line of sight from each calibration transmitter to the SCS's 10 at points “X” and “Y”, and the contribution that each SCS “X” and “Y” would have made to a location estimate of a wireless transmitter that might be located in the vicinity of each calibration transmitter. In general, calibration transmitters that are nearer to the SCS's 10 at points “X” and “Y” will be weighted higher than calibration transmitters that are farther away, and calibration transmitters with better line of sight to the SCS's 10 at points “X” and “Y” will be weighted higher than calibration transmitters with worse line of sight.

Each error component e1, e2, etc., and therefore the resulting error component ε can vary widely, and wildly, over time because some of the error component is due to multipath reflection from the calibration transmitter to each SCS 10. The multipath reflection is very much path dependent and therefore will vary from measurement to measurement and from path to path. It is not an object of this method to determine the multipath reflection for these calibration paths, but rather to determine the portion of the errors that are attributable to the components of the SCS's 10. Typically, therefore, error values e1 and e3 will have a common component since they relate to the same first SCS 10A. Likewise, error values e2 and e4 will a