Wireless time reference system and method

• US 20090243934A1
• Filed: 06/15/2009
• Published: 10/01/2009
• Est. Priority Date: 12/23/2005
• Status: Active Grant

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Aitan Ameti, Edward A. Richley, Keming Chen, Robert J. Fontana, Turner Belinda
• Inventors: Chen, Keming, Richley, Edward A., Fontana, Robert J., Turner, Belinda, Ameti, Aitan
• Granted Patent: US 8,063,826 B2
• Time in Patent Office: Days
• US Class Current: 342/465
• CPC Class Codes: G01S 5/0009 Transmission of position information to remote stations involving assistance data G01S5/0236
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location

Abstract

Instead of normalizing time reference of independent spatially-located clocks using a reference tag transmission from known location, the present invention uses an interarrival time interval between a pulse pair of UWB pulses as a timing metric. Thus, a method of synchronizing spatially-located clocks or normalizing time indications thereof comprises transmitting a UWB pulse pair, determining at first and second monitoring stations a respective count value indicative of a locally measured time interval between received pulse pairs, determining a ratio between clock counts of first and second monitoring stations, and utilizing the ratio to determine clock skew, e.g., a timing correction to be applied to respective local clocks of the monitoring stations. A corresponding system comprises a reference tag transmitter that transmits a pulse pair of UWB pulses to define a time reference interval, a first independent receiver that receives, the pulse pair to generate a first count value indicative an interarrival interval between the pulse pair, a second independent receiver that receives the pulse pair to similarly generate a second count value, and a processor hub responsive to the count values to determine a ratio corresponding to the ratio of respective clock frequencies of the first and second receiver clocks. Once the correction is applied, time-of-arrival information from object tag transmissions may be used to determine object location with sub-foot position accuracies.

Claims

1. A system that provides normalization of time of local clock circuits of plural spatially-located monitoring stations, said system comprising:

a reference transmitter to transmit at least two RF pulses that define a time reference interval,

said local clock circuits having respective detection circuits that detect and measure time durations of said time reference interval at receivers of said monitoring stations, and

a processing device that determines a relationship between time durations measured at receivers of at least two monitoring stations and that effects normalization of time of said local clock circuits according to said relationship.

2. The system of claim 1, wherein said RF pulses comprise ultra wideband (UWB) pulses and said local clock circuits measure times-of-arrival of an object tag transmission in an object location system.

3. The system of claim 2, wherein each of said local clock circuits comprises a ring counter that detects and latches a clock count to measure a duration of said time reference interval and said processing device effects normalization of time-of-arrival measurements among receivers of said monitoring stations by controlling the frequency of at least one local clock circuit of a monitoring station to maintain a fixed one-to-one ratio with respect to a frequency of a local clock of a selected other monitoring station.

4. The system of claim 2, wherein each of said local clock circuits comprises a ring counter that detects and latches a clock count to measure the duration of said time reference interval and said processing device (i) wirelessly receives at a central processing hub digital information representing clock count information of the monitoring stations and time-of-arrival measurements of an object tag transmission and (ii) effects normalization of said time-of-arrival measurements according to said relationship.

5. A time reference system for independent clocks of spatially-located wireless receivers of an object locating system that determines the position of an object, said time reference system comprising:

a reference tag transmitter to transmit a pair of short-pulse signals that define a time reference interval therebetween,

a first wireless receiver that receives said short-pulse signals to generate a first count value according to a first local clock indicative of a time interval between pulses of said pair of short-pulse signals,

a second wireless receiver that receives said short-pulse signals to generate a second count value according to a second local clock indicative of a time interval between pulses of said short-pulse signals and

a processor hub responsive to the first and second count values of said first and second receivers to determine an offset between said first and second local clocks whereby to enable determination of object location according to said offset.

6. The time reference system of claim 5, wherein short-pulse signals comprise ultra wideband (UWB) pulses.

7. The time reference system of claim 6, wherein said first and second receivers generate digital representations of said Count values and said processor hub wirelessly receives said digital representations of said first and second count values.

8. The time reference system of claim 7, wherein said processor hub determines location of an object according to time of arrival of an object tag signal as measured by said local clocks adjusted by said offset.

9. A time reference system for independent clocks of spatially-located wireless receivers of an object locating system that determines the position of an object, said time reference system comprising:

a reference tag transmitter to transmit a pair of short-pulse signals that define a time reference interval therebetween

a first wireless receiver to receive said short-pulse signals to generate a first count value according to a first local clock in order to measure a time interval between pulses of said pair of short-pulse signals,

a second wireless receiver to receive said short-pulse signals to generate a second count value according to a second local clock in order to measure a time interval between pulses of said short-pulse signals, and

a processor responsive to the first and second count values of the first and second receivers to determine an offset and to use said offset to effect adjustment of the frequency of at least one of said first and second local clocks to maintain relative synchronization thereof.

10. The time reference system of claim 9, wherein at least one receiver transmits a count value and said processor is colocated with the other of said first and second receivers to receive the transmitted count value.

11. A method of normalizing timing references of spatially-located receivers in a geoposition system comprising utilizing a UWB pulse pair transmission to determine offsets between clocks of said receivers.

12. A method calibrating independent, spatially-located clocks of a geoposition system in order to geolocate an object having an associated object tag, said method comprising:

transmitting an RF pulse pair,

receiving the pulse pair at multiple locations,

utilizing respective frequencies of first and second spatially-located clocks to produce count values to effect measurement of an interarrival interval at each of multiple locations,

determining a ratio of count values relative to said first and second spatially-located clocks, and

utilizing said ratio to calibrate time indications of said clocks.

13. The method of claim 12, further comprising utilizing a ratio of an initial pulse pair to maintain synchronization of said clocks during subsequent pulse pair transmissions.

14. The method of claim 13, further comprising wirelessly transmitting said count values to a central processing hub that determines object position according to said ratio.

15. An object location system to locate an object equipped with an object tag transmitter, said system comprising:

a reference transmitter to transmit at least two UWB pulses comprising short bursts of RF energy,

multiple monitoring stations positionable at known locations, each including a UWB receiver and a local clock that responds to receipt of said UWB pulses to determine a clock count based on an interarrival interval,

each said monitoring station further including transmitter to transmit a digital representation of said clock count to a central processing hub, and

said central processing hub including a processor to compute a ratio of count values relative to first and second ones of said multiple monitoring stations and to utilize said ratio to align indications of timing references of said monitoring stations in order to determine the position of said object.

16. The object location system of claim 15, wherein said processor computes a second count value ratio between first and third ones of said monitoring stations and utilizes said first and second count values to align respective local clocks to determine the position of said object.

17. The object location system of claim 16, wherein said processor utilizes a clock count value of one of the monitoring stations as a common reference in order to compute ratios with respect to clock count values of other monitoring stations, and said processor determines object location by aligning local clocks of other monitoring stations with said first monitoring station.

18. The object location system of claim 15, wherein said reference transmitter transmit multiple pulse pair groups, and said processor corrects indications of clock drifts among said monitoring stations in order to maintain synchronization of local clocks whereby to determine the location of object based upon subsequent object tag transmissions.

19. The object location system of claim 18, wherein said processor corrects local clocks relative to higher order anomalies.

20. The object location system of claim 15, wherein said digital representation further includes a station identifier to identify respective ones of said monitoring stations.

21. The object location system of claim 15, wherein said processing hub is co-located with one of said monitoring stations.

22. The object location system of claim 15, wherein the monitoring stations and processing hub are remotely located, and said monitoring stations transmit said count value ratios to said central processing hub via an over-the-air RF transmission.

23. The object location system of claim 15, further including a free-running cyclic ring counter that is indexed by said local clock of each said monitoring station in order to measure respective clock count values during said interarrival interval of said UWB pulses.