System and method for distributing accurate time and frequency over a network

• US 8,055,288 B2
• Filed: 11/03/2008
• Issued: 11/08/2011
• Est. Priority Date: 11/02/2007
• Status: Active Grant

• Current Assignee: Novatel Incorporated (Hexagon AB)
• Original Assignee: Novatel Incorporated (Hexagon AB)
• Inventors: Ladd, Jonathan, Fenton, Patrick C.
• Primary Examiner(s): Lee; John J
• Application Number:
  US12/263,940
• Publication Number:
  US 20090117928A1
• Time in Patent Office: 1,100 Days
• Field of Search: 455/502, 455/456.1, 455/67.11, 455/3.02, 455/3.01, 455/414.2, 455/452.1, 455/509, 455/63.1, 455/67.16, 455/3.03, 342/463, 370/485, 370/480, 370/204, 370/265, 375/261, 375/259, 375/325
• US Class Current: 455/502
• CPC Class Codes: G04R 20/06 Decoding time data; Circuits therefor
• CPC Class Codes: H02J 3/36 Arrangements for transfer of electric power between ac networks via a high-tension dc link
• CPC Class Codes: H04B 7/2662 Arrangements for Wireless System Synchronisation
• CPC Class Codes: Y02E 60/60 Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

A system to distribute accurate time and/or frequency over a network utilizing signals of opportunity transmitted by one or more local transmitters with known locations, the system includes a base receiver with a clock synchronized to a reference time scale such as GNSS or UTC time that saves a series of samples of the signals of opportunity and time tags the series with a calculated time of broadcast. A remote receiver saves samples of the signals of opportunity and correlates the series with the saved samples. The remote receiver calculates a time of transmission of saved samples that correspond to the series, determines a time offset as a difference in the time of broadcast calculated at the remote receiver and the time of broadcast calculated at the base receiver, and determines the time offset with respect to the base receiver. The base receiver further or instead phase locks to the signal opportunity and at predetermined intervals determines phase measurements of the integrated carrier frequency of the signal of opportunity and provides the phase information to the remote receiver. The remote receiver, which is also phase locked to the same signal of opportunity, uses the phase measurement information to frequency lock its clock to the base receiver clock by determining a frequency error based on the rate of change of phase measurements made at the base receiver and the remote receiver.

Claims

1. A system for transferring time over a network utilizing signals of opportunity transmitted by one or more transmitters with known locations, the system comprising:

a base receiver with a clock synchronized to a reference time that samples the signals of opportunity, time tags the samples with a calculated time of broadcast, and sends a series of the time-tagged samples over a communications network, the base receiver calculating the time of broadcast based on the time of receipt of the signal that corresponds to the series of samples and a time delay associated with a base line from the transmitter to the base receiver;

a remote receiver that receives information over the communications network, the remote receiver

saving and time-tagging samples of the signals of opportunity,

correlating the time-tagged series of samples received over the communications network with the saved time-tagged samples,

calculating a time of broadcast of the saved time-tagged samples that correspond to the received series of samples,

determining a time offset as a difference in the time of broadcast calculated at the remote receiver and the time of broadcast calculated at the base receiver, and

determining the time with respect to the reference time at base receiver based on the time offset.

2. The system of claim 1 wherein the remote receiver calculates the time of broadcast based on the time of receipt of the signal that corresponds to the series of samples at the remote receiver and a time delay associated with a baseline from the remote receiver to the transmitter.

3. The system of claim 1 further including

the base receiver synchronizing clock frequency to a reference source, determining carrier phase measurements associated with the broadcast signals of opportunity and providing phase information to the remote receiver, and

the remote receiver having a clock and determining carrier phase measurements associated with the broadcast signals of opportunity, the remote receiver determining a frequency offset of the clock from the base receiver clock frequency by the differences between the phase measurements of the base and remote receivers, and

the remote receiver using the frequency offset measurements to adjust and align its clock frequency to that of the base.

4. The system of claim 3 wherein

the base and remote receivers determine the phase measurements by phase locking to the broadcast signals of opportunity and integrating the frequency to produce a wave count that represents an accumulating phase of the broadcast frequency associated with the transmitter and measuring the wave count at predetermined times according to the reference frequencies of the respective receivers.

5. The system of claim 3 wherein

the base and remote receivers determine the phase measurements by downconverting the broadcast signals of opportunity using a down converting frequency source generated from the respective receivers reference frequencies and integrating the residual frequency to produce wave counts that represent the frequency offset of the signal of opportunity from the down converting frequency as measured at predetermined times according to the reference frequency.

6. The system of claim 1 further including additional remote receivers that determine a relative time as time offsets from the base receiver time.

7. The system of claim 1 wherein the base and remote receivers utilize signals of opportunity from a plurality of the transmitters.

8. The system of claim 1 wherein one or both of the base and remote receivers include configurable front end filters that scan the broadcast signals to select one or more signal of opportunity for use.

9. The system of claim 1 wherein the communication network provides a list of usable signals of opportunity and one or both of the base and remote receivers selects one or more signals of opportunity from the list for use.

10. The system of claim 1 wherein the remote receivers adjust a phase of a sample clock based on non-symmetrical earlier and later correlation values determined during correlation of the series of samples to the saved signal samples.

11. The system of claim 10 wherein the remote receivers filter the earlier and later correlation values before making the adjustment.

12. A method for transferring relative time over a network utilizing signals of opportunity transmitted by one or more local transmitters with known locations, with signal service areas covering all or a portion of the network, the method comprising the steps of

synchronizing a clock to a reference time scale,

saving series of samples of the signals of opportunity at the base receiver, time tagging the series with a time of broadcast calculated using the base receiver clock, and providing the time tagged series of samples to a remote receiver, the base receiver calculating the time based on the time of receipt of the signals at the base receiver and a time delay associated with a base line from the transmitter to the base receiver;

saving samples of the signals of opportunity at the remote receiver and correlating the received series of samples with the saved signal samples;

calculating a time of broadcast of the signal samples that correspond to the series of samples and determining a time offset as a difference in the time of broadcast calculated at the remote receiver and the time of broadcast calculated at the base receiver, and

determining the relative time difference between the remote and base receivers based on the time offset to determine time at the remote receiver.

13. The method of claim 12 wherein the base receiver synchronizes its clock to a global reference.

14. The method of claim 13 wherein the base receiver synchronizes its clock to one of GNSS or UTC time.

15. The method of claim 12 wherein the step of calculating the time of broadcast at the remote receiver includes calculating the time based on the time of receipt of the signals that correspond to the series of samples and a time delay associated with a base line from the remote receiver to the transmitter.

16. The method of claim 12 further including

determining phase measurements associated with the carrier frequency of the broadcast signals of opportunity at the base receiver and providing phase measurement information to the remote receiver, and

determining phase measurements associated with the broadcast signals of opportunity at the remote receiver and determining a frequency offset for which the phase measurements determined at the remote receiver correspond to the phase error determined at the base receiver, and

aligning the frequency of the clock at the remote receiver with a clock at the base receiver based on the frequency offset.

17. The method of claim 16 wherein

the step of determining phase measurements at the base and remote receiver includes phase locking to the broadcast signal of opportunity, integrating the received carrier frequency of the signal of opportunity and periodically sampling the integrating value based with a time interval derived from the reference frequency.

18. The method of claim 16 wherein

the step of determining the phase measurements at the base and remote receiver includes downconverting the broadcast signal of opportunity to a lower frequency using a down converting frequency signal derived from the respective receiver reference frequencies integrating the down converted frequency and periodically sampling the integrating value based on a time interval derived from the respective receiver reference frequencies.

19. The method of claim 12 further including additional remote receivers performing the steps to determining relative time synchronization from time offsets from the base receiver time.

20. The method of claim 12 further including utilizing signals of opportunity from a plurality of transmitters at the base receiver and the remote receiver.

21. The method of claim 12 further including one or both of the base and remote receivers scanning the broadcast signals to select a signal of opportunity for use.

22. The method of claim 12 further including one or both of the base and the remote receiver selecting the signals of opportunity for use from a list of usable signals of opportunity.

23. The method of claim 12 wherein the step of sampling the signal at the remote receivers includes adjusting the phase of sampling based on non-symmetrical earlier and later correlation values determined during correlation of the snapshot to the saved signal samples.

24. The system of claim 23 wherein the step of adjusting includes filtering the earlier and later correlation values before making the adjustment.

25. A system for transferring frequency between two or more receivers over a network utilizing signals of opportunity transmitted by one or more transmitters at known locations, with signal service areas covering all or a portion of the network, the system comprising:

a base receiver with a clock synchronized to a reference frequency that phase locks to the signal of opportunity, makes phase measurements at predetermined intervals, and sends the phase measurements over a communications network;

a remote receiver that has a clock and receives information over the communications network, the remote receiver

phase locking to the signal of opportunity and making phase measurements,

determining a frequency offset of the remote receiver clock as a rate of change in the phase measurements made at the base receiver and at the remote receiver, and

adjusting the frequency of the remote receiver clock to remove the measured offset.

26. The system of claim 25 further including additional remote receivers that are synchronized in frequency based on frequency errors determined using phase measurements received from the base receiver.

27. The system of claim 25 wherein the base and remote receivers utilize signals of opportunity from a plurality of the transmitters.

28. The system of claim 25 wherein one or both of the base and remote receivers include configurable front end filters that scan the broadcast signals to select a signal of opportunity for use.

29. The system of claim 25 wherein the communication network provides a list of usable signals of opportunity and one or both of the base and remote receivers selects one or more signal of opportunity from the list for use.

30. The system of claim 25 wherein

the base and remote receivers determine the phase measurements by phase locking to the broadcast signals of opportunity, integrating the frequency to produce a wave count that represents an accumulating phase of the broadcast frequency associated with the transmitter and measuring the wave count at predetermined times according to the reference frequencies of the respective receivers.

31. The system of claim 25 wherein

the base and remote receivers determine the phase measurements by downconverting the broadcast signals of opportunity using a down converting frequency source generated from the respective receivers reference frequencies and integrating the residual frequency to produce wave counts that represent the frequency offset of the signal of opportunity from the down converting frequency as measured at predetermined times according to the reference frequency.