Determining clock-drift using signals of opportunity

• US 9,681,408 B2
• Filed: 04/29/2015
• Issued: 06/13/2017
• Est. Priority Date: 04/30/2014
• Status: Active Grant

• Current Assignee: U-Blox AG (u-blox Holding AG)
• Original Assignee: U-Blox AG (u-blox Holding AG)
• Inventors: Marshall, Christopher Brian
• Primary Examiner(s): Nguyen, Steven H
• Application Number:
  US14/700,086
• Publication Number:
  US 20150319725A1
• Time in Patent Office: 776 Days
• Field of Search: None
• CPC Class Codes: G01S 19/23 Testing, monitoring, correcting or calibrating of receiver elements
• CPC Class Codes: G01S 19/246 involving long acquisition integration times, extended snapshots of signals or methods specifically directed towards weak signal acquisition
• CPC Class Codes: G01S 19/42 Determining position
• CPC Class Codes: G01S 5/02 using radio waves using satellite radio beacon systems for determining position G01S19/00
• CPC Class Codes: G04R 40/06 by computing the time value implied by the radio signal
• CPC Class Codes: H04W 56/0015 one node acting as a reference for the others

Abstract

A method and apparatus for determining a drift between a local clock of a movable device and a reference clock that is used by one or more reference devices. The local clock may be calibrated at a calibration location and then the movable device may be moved to an operating location that is different from the calibration location. The drift is determined by observing one or more signals transmitted by the respective one or more reference devices, when the device is at the calibration location and when it is at the operating location. These observations are used together with a model of the signals to determine the drift.

Claims

1. A method, for a movable device having a local clock, of determining a drift between the local clock and a reference clock that is used by one or more reference devices, by observing one or more signals transmitted by the respective one or more reference devices, the method comprising:

when the movable device is at a calibration location, receiving at the movable device, at a first time, a first portion of the signal transmitted by one of the reference devices;

moving the movable device to an operating location; and

when the movable device is at the operating location, receiving at the movable device, at a second time, a second portion of the signal transmitted by one of the reference devices, the first portion and the second portion having been transmitted with a time-interval between them, according to the reference clock;

the method further comprising;

determining the time-interval;

obtaining a model of the one or more signals, describing for each signal at least one of;

a direction of propagation of that signal, and

the location of the respective reference device;

determining local time information comprising;

the first time and the second time, or a time difference between them, according to the local clock;

defining position variables comprising;

the calibration location and the operating location, or

a displacement vector between them;

constructing a set of one or more equations, which relates the position variables, the model, the local time information, the time-interval, and the drift;

solving the set of one or more equations, to determine the drift; and

correcting the clock of the movable device based on the determined drift.

2. The method of claim 1, wherein the first portion and the second portion are different portions of the signal transmitted by one reference device.

3. The method of claim 1, wherein:

the first portion and the second portion are transmitted by different reference devices;

the model comprises the location of each of the reference devices; and

the calibration location is known.

4. The method of any of claim 1, wherein each of the one or more reference devices is stationary.

5. The method of claim 1, further comprising:

when the movable device is at the calibration location, determining the first time according to the reference clock; and

when the movable device is at the operating location, determining the current time according to the reference clock, based on;

the first time according to the reference clock;

the first time according to the local clock;

the current time according to the local clock; and

the drift.

6. The method of claim 5, wherein:

the reference clock is synchronised with a clock of a satellite positioning system;

the movable device comprises a satellite positioning receiver for use with the satellite positioning system; and

the step of determining the first time according to the reference clock comprises determining, using the satellite positioning receiver, the first time according to the clock of the satellite positioning system.

7. The method of claim 1, wherein the movable device comprises a satellite positioning receiver for determining, using a satellite positioning system, the position of the movable device,

and the method further comprises determining the calibration location using the satellite positioning receiver.

8. The method of claim 1, further comprising determining the operating location based on the signals transmitted by the one or more reference devices.

9. The method of claim 1, wherein three stationary reference devices are synchronised with the reference clock and the method comprises:

when the movable device is at the calibration location, receiving, from each of the three reference devices, at respective first times, a first portion of the signal transmitted by that reference device;

when the movable device is at the operating location, receiving, from each of the three reference devices, at respective second times, a second portion of the signal transmitted by that reference device, each first portion and second portion having been transmitted by the respective reference device with a respective predetermined time-interval between them, according to the reference clock;

wherein the local time information comprises;

the first times and the second times, or respective time differences between them, according to the local clock.

10. The method of claim 1 wherein the step of solving the set of equations to determine the drift is assisted by satellite positioning signals received by the movable device at the operating location.

11. The method of claim 1, wherein the model comprises the direction of propagation of each of the one or more signals and the direction of propagation of at least one signal is different from the straight-line direction to the respective reference device from which the signal was transmitted.

12. The method of claim 2, wherein each of the one or more reference devices is stationary.

13. The method of claim 3, wherein each of the one or more reference devices is stationary.

14. A non-transitory computer-readable medium containing a computer program comprising computer program code adapted to control a physical computing device to perform all the steps of a method of determining a drift between a local clock and a reference clock, if said program is run on the physical computing device,

wherein the local clock is a clock of a movable device,

the reference clock is used by one or more reference devices, and the method comprises observing, at the movable device, one or more signals transmitted by the respective one or more reference devices,

the method comprising:

when the movable device is at a calibration location, receiving at the movable device, at a first time, a first portion of the signal transmitted by one of the reference devices;

moving the movable device to an operating location; and

when the movable device is at the operating location, receiving at the movable device, at a second time, a second portion of the signal transmitted by one of the reference devices, the first portion and the second portion having been transmitted with a time-interval between them, according to the reference clock;

the method further comprising;

determining the time-interval;

obtaining a model of the one or more signals, describing for each signal at least one of;

a direction of propagation of that signal, and

the location of the respective reference device;

determining local time information comprising;

the first time and the second time, or a time difference between them, according to the local clock;

defining position variables comprising;

the calibration location and the operating location, or

a displacement vector between them;

constructing a set of one or more equations, which relates the position variables, the model, the local time information, the time-interval, and the drift;

solving the set of one or more equations, to determine the drift; and

correcting the local clock of the movable device based on the determined drift.

15. A movable device comprising:

a local clock (1225);

a receiver (1210), adapted to receive one or more signals transmitted by respective one or more reference devices, wherein the reference devices use a reference clock; and

a processor (1220), adapted to determine a drift between the local clock and the reference clock,

wherein the receiver (1210) is adapted to;

receive, at a calibration location (P) at a first time, a first portion of the signal transmitted by one of the reference devices; and

receive, at an operating location (Q) at a second time, a second portion of the signal transmitted by one of the reference devices, the first portion and the second portion having been transmitted with a time-interval between them, said time-interval being defined according to the reference clock,

and wherein the processor (1220) is adapted to;

determine the time-interval;

obtain a model of the one or more signals, describing for each signal at least one of;

a direction of propagation of that signal, and the location of the respective reference device;

determine local time information comprising;

the first time and the second time, or a time difference between them, according to the local clock;

define position variables comprising;

the calibration location and the operating location, or a displacement vector between them;

construct a set of one or more equations, which relates the position variables, the model, the local time information, the time-interval, and the drift;

solving the set of one or more equations, to determine the drift; and

correct the local clock of the movable device based on the determined drift.

16. The device of claim 15, wherein the device comprises at least one of:

a cellular base station, preferably an indoor cellular base station;

a wireless-network access point;

one of a set of wireless reference beacons, each of which is adapted to transmit a beacon signal with a predetermined timing relationship to the other beacons, for a remote device receiving the beacon signals to calculate its position by trilateration; and

one of a set of wireless reference receivers, each of which is adapted to receive a signal from a remote device and determine the time or arrival of the signal, for calculating the position of that remote device by trilateration.

17. The device of claim 15, wherein the one or more reference devices include at least one of:

a cellular base station;

a wireless-network access point;

one of a synchronised set of wireless reference beacons;

one of a synchronised set of wireless reference receivers;

a terrestrial digital television or digital audio broadcast transmitter; and

a geostationary communications satellite.