1. A method of locating the source of an unknown signal received by a plurality of signal relays, the method including the steps of: (a) arranging for a plurality of receivers ( 18 ) to receive replicas of the unknown signal via respective signal relays ( 14, 16 ); and (b) subjecting the replicas to correlation processing; characterised in that correlation processing in step (b) is performed with a complex correlation function (CCF) at least partly compensated for change in the replicas' Differential Frequency Offset (DFO) with time due to relay motion relative to the source and receivers.