Determining clock-drift using signals of opportunity

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

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