Patent ID: 8472971

Claim:
A method of determining whether a mobile terminal has moved outside a given locale, said terminal including a receiver able to measure relative receive time offsets of the signals received by the receiver from one or more transmission sources of an unsynchronized transmission system, the relative receive time offsets corresponding to differences in the times of receipt of signals at the receiver from a plurality of the transmission sources relative to at least one of the signals or to a reference signal, the method comprising the steps of: (a) obtaining at a first time a first set of one or more relative receive time offsets corresponding to said receiver located at the centre of the locale; (b) measuring, at a second time when it is desired to determine whether the mobile terminal has moved outside said locale, a second set of one or more relative receive time offsets of the signals from at least one of the transmission sources, the relative receive time offset of which is included in the first set; (c) calculating one or more movement indicators by subtracting the corresponding relative receive time offsets in the first and second sets from one another; (d) if the transmission sources are unsynchronised, obtaining the change or changes, in transmission time offsets of the signals transmitted by at least one transmission source relative to a reference or to another transmission source, between the first and second times, and adding or subtracting the change or changes in obtained transmission time offsets from the corresponding movement indicators in order to correct the movement indicators for said change or changes in transmission time offsets; and (e) comparing the movement indicators with one or more threshold values in order to determine whether the mobile terminal has moved outside said locale.