# US 9,520,860 B2 · Time distribution switch

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- Number: US 9,520,860 B2
- Document ID: US9520860B2
- Status: Granted
- Kind: B2
- Application number: 14/057,803
- Filed: 2013-10-18
- Published: 2014-04-24
- Granted: 2016-12-13
- Earliest claimed: 2012-10-19
- Assignees: Schweitzer Engineering Laboratories, Inc.
- Applicants: Schweitzer Engineering Laboratories, Inc.
- Inventors: David E. Whitehead; Shankar V. Achanta; Henry Loehner
- Web page: https://patentgrep.com/patent/US9520860B2

## CPC

- H04J 3/0688: {Change of the master or reference, e.g. take-over or failure of the master}
- G06F 1/14: Time supervision arrangements, e.g. real time clock
- H03K 3/01: Details
- H04J 3/0644: {External master-clock}
- G01S 19/00: Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- H02H 7/261: {involving signal transmission between at least two stations (transmission of signals in general H02H1/0061)}
- H02H 7/28: for meshed systems
- H04J: MULTIPLEX COMMUNICATION (transmission in general H04B; peculiar to transmission of digital information H04L5/00; systems for the simultaneous or sequential transmission of more than one television signal H04N7/08; in exchanges H04Q11/00; stereophonic systems H04S)
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)
- H03K: PULSE TECHNIQUE (measuring pulse characteristics G01R; modulating sinusoidal oscillations with pulses H03C; transmission of digital information H04L; discriminator circuits detecting phase difference between two signals by counting or integrating cycles of oscillation H03D3/04; automatic control, starting, synchronisation or stabilisation of generators of electronic oscillations or pulses where the type of generator is irrelevant or unspecified H03L; coding, decoding or code conversion, in general H03M)
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- H02H: EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS (indicating or signalling undesired working conditions G01R, e.g. G01R31/00, G08B; locating faults along lines G01R31/08; emergency protective devices H01H)

## 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

### Claim 1

A method comprising:
receiving, at a time distribution device, two or more consecutive time signals from a first external time source;
determining a first duration between each of the two or more consecutive time signals from the first external time source, wherein the first duration is based on an internal oscillator;
receiving, at the time distribution device, two or more consecutive time signals from a second external time source;
determining a second duration between each of the two or more consecutive time signals from the second external time source, wherein the second duration is based on the internal oscillator;
comparing the first duration and the second duration to determine a time source with the lowest variation in the duration between the two or more consecutive time signals;
selecting the time source with the lowest variation as a best available time source; and
distributing the best available time source to one or more consuming devices.

### Claim 2 (depends on 1)

The method of claim 1, wherein the internal oscillator comprises a high accuracy oscillator.

### Claim 3 (depends on 2)

The method of claim 2, wherein the high accuracy oscillator comprises one of: an oven controlled crystal oscillator (OCXO); a temperature compensated crystal oscillator (TCXO); a voltage controlled crystal oscillator (VCXO); a rubidium oscillator; or an atomic oscillator.

### Claim 4 (depends on 1)

The method of claim 1, wherein the first and second time sources include one or more of: Inter-Range Instrumentation Group (IRIG) protocol, a global navigation satellite system (GNSS), a National Institute of Science and Technology (NIST) broadcast, an Institute of Electrical and Electronics Engineers (IEEE) 1588 protocol, a network time protocol (NTP), a simple network time protocol (SNTP), or a precision time protocol.

### Claim 5

A time distribution device comprising:
a plurality of receivers configured to receive consecutive time signals from each of first and second external corresponding precision time sources;
an output configured to provide a precision time signal to an intelligent electronic device (IED), wherein the precision time signal is determined from a best available time source of the first and second external corresponding precision time sources; and
a time quality module configured to:
determine, for the first external precision time source, a first duration between two or more consecutive time signals of the first external precision time source, wherein the first duration is based on an internal oscillator;
determine, for the second external precision time source, a second duration between two or more consecutive time signals of the second external precision time source, wherein the second duration is based on the internal oscillator;
compare the duration between the two or more consecutive time signals for each time source to determine a time source with the lowest variation in the duration between the two or more consecutive time signals; and
select the precision time source with the lowest variation as the best available time source.

### Claim 6 (depends on 5)

The time distribution device of claim 5, wherein the internal oscillator comprises a high accuracy oscillator.

### Claim 7 (depends on 6)

The time distribution device of claim 6, wherein the high accuracy oscillator comprises one of: an oven controlled crystal oscillator (OCXO); a temperature compensated crystal oscillator (TCXO); a voltage controlled crystal oscillator (VCXO); a rubidium oscillator; or an atomic oscillator.

### Claim 8 (depends on 5)

The time distribution device of claim 5, wherein the first and second precision time sources include one or more of: Inter-Range Instrumentation Group (IRIG) protocol, a global navigation satellite system (GNSS), a National Institute of Science and Technology (NIST) broadcast, an Institute of Electrical and Electronics Engineers (IEEE) 1588 protocol, a network time protocol (NTP), a simple network time protocol (SNTP), or a precision time protocol.

### Claim 9 (depends on 5)

The time distribution device of claim 5, wherein the time quality module is further configured to:
detect a failure of the selected precision time source; and
determine a backup precision time source for use as the best available time source, wherein the process to determine the best available time source is repeated to determine the backup time source.

### Claim 10 (depends on 9)

The time distribution device of claim 9, wherein to detect a failure of the selected precision time source the time quality module is configured to:
train an unlocked oscillator with a time signal from the best available time source;
compare the time signal from the best available time source to a drift rate of the unlocked oscillator; and
detect a failure of the best available time source in response to the comparing showing that the time signal from the best available time source varies from the drift rate of the unlocked oscillator by a defined margin.

### Claim 11 (depends on 9)

The time distribution device of claim 9, wherein the time quality module is further configured to indicate a failure condition in response to detecting a failure of the best available time source.

### Claim 12 (depends on 9)

The time distribution device of claim 9, wherein to detect a failure of the selected precision time source the time quality module is configured to:
receive a location based on a plurality of Global Navigational Satellite System (GNSS) signals received at the time distribution device;
compare the received location with a known location of the time distribution device; and
detect a failure of the best available time source in response to the received location and the known location differing by a defined margin.

### Claim 13 (depends on 12)

The time distribution device of claim 12, wherein the known location is calculated using the plurality of GNSS signals.

### Claim 14 (depends on 12)

The time distribution device of claim 12, wherein the known location is entered into the time distribution device at setup.

### Claim 15

A non-transitory computer readable storage medium having instructions stored thereon, which when executed by a processor, cause the processor to perform a method for determining a best available time source, the method comprising:
receiving, at a time distribution device, two or more consecutive time signals from a first external time source;
determining, for the first external time source, a first duration between each of the two or more consecutive time signals, wherein the first duration is based on an internal oscillator;
receiving, at the time distribution device, two or more consecutive time signals from a second external time source;
determining, for the second external time source, a second duration between each of the two or more consecutive time signals, wherein the second duration is based on the internal oscillator;
comparing the first duration and the second duration for each time source to determine a time source with the lowest variation in the duration between the two or more consecutive time signals; and
selecting the time source with the lowest variation as the best available time source.

### Claim 16 (depends on 15)

The non-transitory computer readable storage medium of claim 15, wherein the internal oscillator comprises a high accuracy oscillator.

### Claim 17 (depends on 16)

The non-transitory computer readable storage medium of claim 16, wherein the high accuracy oscillator comprises one of: an oven controlled crystal oscillator (OCXO); a temperature compensated crystal oscillator (TCXO); a voltage controlled crystal oscillator (VCXO); a rubidium oscillator; or an atomic oscillator.

### Claim 18 (depends on 15)

The non-transitory computer readable storage medium of claim 15, wherein the first and second precision time sources include one or more of: Inter-Range Instrumentation Group (IRIG) protocol, a global navigation satellite system (GNSS), a National Institute of Science and Technology (NIST) broadcast, an Institute of Electrical and Electronics Engineers (IEEE) 1588 protocol, a network time protocol (NTP), a simple network time protocol (SNTP), or a precision time protocol.

### Claim 19 (depends on 15)

The non-transitory computer readable storage medium of claim 15, wherein the method further comprises distributing the best available time source to one or more consuming devices.

### Claim 20 (depends on 15)

The non-transitory computer readable storage medium of claim 15, wherein the method further comprises:
detecting a failure of the selected time source; and
determining a backup time source for use as the best available time source, wherein the method for determining the best available time source is repeated to determine the backup time source.