Quality of precision time sources

• US 9,709,680 B2
• Filed: 09/04/2013
• Issued: 07/18/2017
• Est. Priority Date: 09/08/2012
• Status: Active Grant

• Current Assignee: Schweitzer Engineering Laboratories, Incorporated
• Original Assignee: Schweitzer Engineering Laboratories, Incorporated
• Inventors: Loehner, Henry, Achanta, Shankar V., Whitehead, David E.
• Primary Examiner(s): McGue, Frank J
• Application Number:
  US14/017,522
• Publication Number:
  US 20140250972A1
• Time in Patent Office: 1,413 Days
• Field of Search: 34235758
• CPC Class Codes: G01S 19/03 Cooperating elements; Interaction or communication between different cooperating elements or between cooperating elements and receivers
• CPC Class Codes: G01S 19/13 Receivers
• CPC Class Codes: G01S 19/215 issues related to spoofing
• CPC Class Codes: H04J 3/0688 Change of the master or reference, e.g. take-over or failure of the master

Abstract

Systems and methods for detecting the failure of a precision time source using an independent time source are disclosed. Additionally, detecting the failure of a GNSS based precision time source based on a calculated location of a GNSS receiver is disclosed. Moreover, the system may be further configured to distribute a time derived from the precision time source as a precision time reference to time dependent devices. In the event of a failure of the precision time source, the system may be configured to distribute a time derived from a second precision time source as the precision time signal during a holdover period.

Claims

1. A method comprising:

receiving, at a time distribution device, a first time signal from a first precision time source via a first time receiver;

distributing, by the time distribution device a time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for a precision time reference;

receiving, at the time distribution device, a second time signal from a second precision time source via a second time receiver, the second time signal independent from the first precision time source;

comparing, by a time quality component of the time distribution device, the first time signal with the second time signal;

detecting, by the time quality component of the time distribution device, a failure of the first precision time source in response to the comparing showing that a variation of the first time signal from the second time signal exceeds a defined margin; and

,

upon detection of the failure of the first precision time source, the time distribution device distributing to the time-dependent device the time signal based on the second time signal.

2. the method of claim 1, wherein the first precision time source is a global navigation satellite system (GNSS) time source and the first time signal is a GNSS pulse per-second (PPS) signal.

3. The method of claim 1, wherein the second precision time source is a WWVB time source and the second time signal is a WWVB PPS.

4. The method of claim 1, further comprising:

in response to detecting a failure of the first precision time source, relying on the second precision time source.

5. A system comprising:

a first receiver of a time distribution device, configured to receive a first signal including a first precision time signal;

a second receiver of the time distribution device configured to receive a second signal including a second precision time signal independent from the first precision time signal, wherein the second precision time signal is relatively less precise than the first precision time signal;

an output of the time distribution device, configured to provide an output time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for a precision time reference; and

a time quality module of the time distribution device, configured to

compare the first precision time signal with the second precision time signal;

detect an error condition of the first precision time signal in response to a variation of the first precision time signal from the second precision time signal exceeding a defined threshold;

distribute to the time-dependent device a time signal based on the first precision time signal; and

,

upon detection of the error condition of the first precision time signal, distribute to the time-dependent device a time signal based on the second precision time signal; and

,

the time-dependent device in communication with the time distribution device, configured to receive the output time signal from the time distribution device.

6. the system of claim 5, wherein the first signal is a global navigation satellite system (GNSS) signal.

7. The system of claim 5, wherein the second signal is a WWVB signal.

8. The system of claim 5, wherein the second signal is a network time signal.

9. The system of claim 5 further comprising, an unlocked oscillator trained to the first precision time signal, wherein the second precision time signal comprises a time signal from the unlocked oscillator.

10. The system of claim 5, wherein, in response to detecting an error condition, the output is configured to provide the second precision time signal to the IED.

11. A method comprising:

receiving a global navigation satellite system (GNSS) signal, including a GNSS time signal, at a time distribution device;

distributing, by the time distribution device, a time signal to a time-dependent device, wherein the time-dependent device is dependent on the time distribution device for the time signal;

determining, by the time distribution device, whether the GNSS has failed;

in response to determining that the GNSS has failed, indicating to a user an error condition;

in response to determining that the GNSS has not failed, distributing to the time-dependent device the time signal based on the GNSS time signal; and

,

in response to determining that the GNSS has failed, distributing to the time-dependent device a time signal not based on the GNSS.

12. The method of claim 11, wherein determining whether the GNSS has failed comprises:

comparing a location determined from the GNSS signal with a known location of the time distribution device; and

determining that the GNSS has failed in response to the GNSS location varying from the known location by more than a defined threshold.

13. The method of claim 11, wherein determining whether the GNSS has failed comprises:

comparing the GNSS time signal with an independent time signal; and

determining that the GNSS has failed in response to the GNSS time signal varying from the independent time signal by more than a defined threshold.

14. The method of claim 13, wherein the independent time signal is a National Institute of Science and Technology (NIST) broadcast signal.

15. The method of claim 13, wherein the independent time signal is generated by an oscillator.

16. The method of claim 13, wherein the independent time signal is received via a network protocol.

17. The method of claim 13, further comprising:

in response to determining that the GNSS has failed, distributing to the time-dependent device a time signal based on the independent time signal.

18. The method of claim 11, wherein determining whether the GNSS has failed comprises:

calculating a location drift rate based on the GNSS signal;

comparing the location drift rate to a defined threshold; and

determining that the GNSS has failed in response to the location drift rate exceeding the defined threshold.

19. The method of claim 11, wherein determining whether the GNSS has failed comprises:

monitoring instantaneous and average GNSS signal strength; and

determining that the GNSS has failed in response to the instantaneous signal strength exceeding a defined threshold for a set number of samples.

20. The method of claim 11, wherein determining whether the GNSS has failed comprises:

monitoring satellite constellation; and

determining that the GNSS time source has failed in response to detecting a change in satellite constellation.