Multi-lateration system with automatic calibration and error removal

• US 6,380,894 B1
• Filed: 08/29/2000
• Issued: 04/30/2002
• Est. Priority Date: 08/30/1999
• Status: Expired due to Term

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Wherenet Corporation (Zebra Technologies Corporation)
• Inventors: Boyd, Robert W., Belcher, Donald K., Wohl, Michael A.
• Primary Examiner(s): Hellner, Mark
• Application Number:
  US09/649,646
• Time in Patent Office: 609 Days
• Field of Search: 342/450, 342/451, 342/465
• US Class Current: 342/450
• CPC Class Codes: G01S 13/767 Responders; Transponders teaching or practice apparatus for gun-aiming or gun-laying using reflecting targets or active targets F41G3/26
• CPC Class Codes: G01S 13/82 wherein continuous-type signals are transmitted
• CPC Class Codes: G01S 13/825 with exchange of information between interrogator and responder
• CPC Class Codes: G01S 13/878 Combination of several spaced transmitters or receivers of known location for determining the position of a transponder or a reflector G01S13/874 takes precedence
• CPC Class Codes: G01S 5/02 using radio waves using satellite radio beacon systems for determining position G01S19/00
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence
• CPC Class Codes: G06K 7/0008 General problems related to the reading of electronic memory record carriers, independent of its reading method, e.g. power transfer
• CPC Class Codes: G06K 7/10079 the collision being resolved in the spatial domain, e.g. temporary shields for blindfolding the interrogator in specific directions
• CPC Class Codes: G06K 7/10356 using a plurality of antennas, e.g. configurations including means to resolve interference between the plurality of antennas

Abstract

An asset management radio location system uses time-of-arrival differentiation for random and repetitive spread spectrum, short duration transmissions from object-attached tags, to geolocate objects within a monitored environment. An object location processor is coupled to distributed tag transmission readers over signal transport paths having different transport delays, and carries out time-of-arrival differentiation of first-to-arrive transmissions from a tag to determine where its object is located. Geolocation errors associated with variations in parameters of the signal transport paths are effectively removed by installing one or more ‘reference’ tags, whose geolocations are precisely known. Using a background calibration routine that is exercised at a relatively low cycle rate, emissions from the reference tags are processed and coupled to the geolocation processor. The calculated geolocations of the reference tags are compared with their actual locations. Any offset between the two geolocation values is used to adjust time delay offsets associated with signal transport paths between the readers and the geolocation processor.

Claims

1. A method of controlling the operation of a system for geolocating objects within a monitored environment, wherein transmissions from tags associated with said objects are detected at a plurality of spaced apart monitoring locations containing tag transmission readers, each respective monitoring location containing a plurality of transmission readers respectively employing mutually diverse antennas to minimize multipath interference at said antennas simultaneously, and outputs of said tag transmission readers are coupled over signal transport paths, having associated signal transport delays, to an object location processor which processes said outputs of said tag transmissions to geolocate said tags and thereby their associated objects within said monitored environment, said method comprising the steps of:

(a) providing within said monitored environment at least one ‘

reference’

tag whose geolocation is known, and which is operative to transmit a reference tag signal encoded with information representative of the identification of said reference tag;

(b) receiving said reference tag signal at said transmission readers, and coupling outputs thereof over said signal transport paths to said object location processor for processing thereby to determine the geolocation of said reference tag;

(c) comparing the geolocation of said reference tag as determined in step (b) with the known geolocation of said reference tag; and

(d) controllably adjusting the processing operation carried out by said object location processor to compensate for errors, such as those associated with delays through said signal transport paths, in accordance with a difference between the geolocation of said reference tag as determined in step (b) and the known geolocation of said reference tag.

2. A method according to claim 1, wherein the known geolocation of said reference tag is stored in association with said object location processor.

3. A method according to claim 1, wherein the known geolocation of said reference tag is stored in memory on board said reference tag and included as part of the information contained in the signal transmitted by said reference tag.

4. A method according to claim 1, wherein said object location processor is operative to conduct time-of-arrival differentiation processing of first-to-arrive signals transmitted from a tag to geolocate said tag.

5. A method according to claim 4, wherein said object location processor is operative to carry out said time-of-arrival differentiation of earliest-in-time ones of first-to-arrive signals detected by said tag transmission readers.

6. A method according to claim 5, wherein signals transmitted from said tags comprise spread spectrum RF signals, and wherein a respective tag reader output is coupled to a matched filter correlator having a plurality of matched filter banks containing parallel correlators, which are operative to correlate a received spread spectrum RF signal with successive time offset versions of a reference spreading code corresponding to that of said spread spectrum RF signals.

7. A method according to claim 1, further including, at each monitoring location, a plurality of auxiliary ‘

phased array’

signal processing paths coupled to said mutually diverse antennas to which associated tag transmission readers are coupled, and wherein each auxiliary phased array path is configured to sum energy received from said antennas in a prescribed phase relationship, and to couple a respective energy sum to an associated tag transmission reader, and wherein a further reader output processor is coupled to said associated tag transmission readers, and is operative to process the energy sum supplied to said associated tag transmission reader and to generate a further first-to-arrive signal, and wherein said object location processor is operative to carry out time-of-arrival differentiation of earliest-in-time ones of signals provided by said tag transmission readers.

8. An arrangement for controlling the operation of a system for geolocating objects within a monitored environment, wherein transmissions from tags associated with said objects are detected at a plurality of spaced apart monitoring locations containing tag transmission readers, each respective monitoring location containing a plurality of transmission readers respectively employing mutually diverse antennas to minimize multipath interference at said antennas simultaneously, and outputs of said tag transmission readers are coupled over signal transport paths, having associated signal transport delays, to an object location processor which processes said outputs of said tag transmissions to geolocate said tags and thereby their associated objects within said monitored environment, said arrangement comprising:

at least one ‘

reference’

tag disposed within said monitored environment and whose geolocation is known, and being operative to transmit a reference tag signal encoded with information representative of the identification of said reference tag, said reference tag signal being received at said transmission readers, outputs of which are coupled over said signal transport paths to said object location processor for processing thereby to determine the geolocation of said reference tag;

a calibration mechanism executed by said object location processor and being operative to compare the determined geolocation of said reference tag with the known geolocation of said reference tag, and to controllably compensate for delays through said signal transport paths, in accordance with a difference between the determined geolocation of said reference tag and the known geolocation of said reference tag.

9. An arrangement according to claim 8, wherein the known geolocation of said reference tag is stored in association with said object location processor.

10. An arrangement according to claim 8, wherein the known geolocation of said reference tag is stored in memory on board said reference tag and included as part of the information contained in the signal transmitted by said reference tag.

11. An arrangement according to claim 8, wherein said object location processor is operative to conduct time-of-arrival differentiation processing of first-to-arrive signals transmitted from a tag to geolocate said tag.

12. An arrangement according to claim 11, wherein said object location processor is operative to carry out said time-of-arrival differentiation of earliest-in-time ones of first-to-arrive signals detected by said tag transmission readers.

13. An arrangement according to claim 12, wherein signals transmitted from said tags comprise spread spectrum RF signals, and wherein a respective reader output is coupled to a matched filter correlator having a plurality of matched filter banks containing parallel correlators, which are operative to correlate a received spread spectrum RF signal with successive time offset versions of a reference spreading code corresponding to that of said spread spectrum RF signals.

14. A method for calibrating a system for geolocating objects within a monitored environment, wherein transmissions from tags associated with said objects as detected by tag transmission readers at a plurality of spaced apart monitoring locations are processed to identify first-to-arrive signals transmitted from a tag, each respective monitoring location containing a plurality of transmission readers respectively employing mutually diverse antennas to minimize multipath interference at said antennas simultaneously, and outputs of said tag transmission readers are coupled over signal transport paths, having associated signal transport delays, to an object location processor which processes said outputs of said tag transmission to geolocate said tags and thereby their associated objects within said monitored environment, said method comprising the steps of:

(a) providing within said monitored environment at least one ‘

reference’

tag whose geolocation is known, and which is operative to transmit a reference tag signal encoded with information representative of the identification of said reference tag;

(b) receiving said reference tag signal at said plurality of tag transmission readers, and coupling outputs thereof over said signal transport paths to said object location processor for processing thereby to determine the geolocation of said reference tag;

(c) comparing the geolocation of said reference tag as determined in step (b) with the known geolocation of said reference tag; and

(d) controllably adjusting the processing operation carried out by said object location processor to compensate for delays through said signal transport paths based upon a difference between the geolocation of said reference tag as determined in step (b) and the known geolocation of said reference tag.

15. A method according to claim 14, wherein the known geolocation of said reference tag is stored in association with said object location processor.

16. A method according to claim 14, wherein the known geolocation of said reference tag is stored in memory on board said reference tag and included as part of the information contained in the signal transmitted by said reference tag.

17. A method according to claim 14, wherein said object location processor is operative to conduct time-of-arrival differentiation processing of first-to-arrive signals transmitted from a tag to geolocate said tag.

18. A method according to claim 17, wherein said object location processor is operative to carry out said time-of-arrival differentiation of earliest-in-time ones of first-to-arrive signals detected by said tag transmission readers.