A method of determining a drift between a local clock of a movable device and a reference clock that is used by a plurality of reference devices, by observing signals transmitted by the respective reference devices, wherein the structure of each signal is known, in the sense that it is possible to determine the relative times of transmission of different parts of the signal, the method comprising: when the movable device is at a known calibration location (P), receiving (S200) by the movable device, at a first time, a first portion of the signal transmitted by one of the reference devices; moving (S205) the movable device to an operating location (Q), at which a satellite positioning fix is not possible; and when the movable device is at the operating location (Q), receiving (S210) by the movable device, at a second time, a second portion of the signal transmitted by another 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 (S250) the time-interval based on the known structure of the one or more signals; obtaining (S230) a model of the signals, describing for each signal at least 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 known calibration location and the operating location; constructing (S260) 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 (S270) the set of one or more equations, to determine the drift. when the movable device is at a known calibration location (P), receiving (S200) by the movable device, at a first time, a first portion of the signal transmitted by one of the reference devices; moving (S205) the movable device to an operating location (Q), at which a satellite positioning fix is not possible; and when the movable device is at the operating location (Q), receiving (S210) by the movable device, at a second time, a second portion of the signal transmitted by another 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 (S250) the time-interval based on the known structure of the one or more signals; obtaining (S230) a model of the signals, describing for each signal at least 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 known calibration location and the operating location; constructing (S260) 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 (S270) the set of one or more equations, to determine the drift. determining (S250) the time-interval based on the known structure of the one or more signals; obtaining (S230) a model of the signals, describing for each signal at least 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 known calibration location and the operating location; constructing (S260) 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 (S270) the set of one or more equations, to determine the drift. <EOS>