Wireless time reference system and method

• US 7,492,316 B1
• Filed: 07/06/2006
• Issued: 02/17/2009
• Est. Priority Date: 12/23/2005
• Status: Active Grant

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: Multispectral Solutions Incorporated (Zebra Technologies Corporation)
• Inventors: Chen, Keming, Richley, Edward A., Fontana, Robert J., Turner, Belinda, Ameti, Aitan
• Primary Examiner(s): Tarcza; Thomas H
• Assistant Examiner(s): Nguyen; Nga X
• Application Number:
  US11/480,982
• Time in Patent Office: 957 Days
• Field of Search: 342/465, 342/387
• 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 clock 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 method of providing normalization of time of local clock circuits of two or more spatially-located receivers of respective monitoring stations, said method comprising:

transmitting at least two RF pulses to define a time reference interval;

detecting said RF pulses at said receivers;

at respective monitoring associated with said receivers, measuring durations of said time reference interval according to said detecting step;

determining a relationship between durations measured at said monitoring stations; and

providing normalization of time of said local clock circuits at two or more monitoring stations according to said relationship.

2. The method of claim 1, wherein said transmitting step further includes transmitting at least two UWB pulses that each comprise a signal burst of a few cycles of RF energy.

3. The method of claim 2, wherein:

said measuring step includes counting unit increments of time to measure said time reference interval between said UWB pulses and latching a clock count to measure said durations; and

said processing step includes normalizing 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 the relationship fixed with respect to a frequency of a local clock of a selected master monitoring station.

4. The method of claim 3, further comprising:

wirelessly receiving at a central processing hub digital information representing clock count information and time-of-arrival measurements of an object tag transmission; and

normalizing said time-of-arrival measurements according to said relationship.

5. The method of claim 2, further comprising providing a ring counter to measure said time reference interval between said UWB pulses and latching a clock count to measure said durations.

6. A method of normalizing independent clocks of respective receivers of remote monitoring stations, said method comprising:

transmitting an ultra wideband (UWB) pulse pair,

determining at first and second monitoring stations a respective clock count indicative of a locally measured time interval between said pulse pair, each said clock count being derived by incremental measurement of time units,

determining a ratio between clock counts of said first and second monitoring stations using programmed processor module, and

utilizing said ratio as a reference to normalize time of local clocks that generate each said clock count.

7. The method of claim 6, wherein said utilizing step includes normalizing time by synchronizing local clocks that generate each said clock count.

8. The method of claim 6, wherein said utilizing step includes normalizing time by applying a correction to time-of-arrival measurements taken at respective monitoring stations.