A signal processing system is provided which describes a method and apparatus to determine the absolute time of arrival of a transmitted signal and to determine the location of a transmitting device by determining the relative difference in time that a transmitted signal is received at three or more receivers which are spaced apart with respect to each other. The method and apparatus is applicable to all systems which transmit discrete signals or signals which comprise composite components. The system comprises a ramp detection component to track the arrival of the signal and to determine, to a high time resolution, the arrival of a signal by identifying a particular feature of the transmitted signal. The time-of-arrival of the signal is determined with reference to a timing circuit. Each receiver further comprises a timing-synchronisation component to maintain a highly accurate clock by using an accurate timing reference such as the timing signals transmitted by the Global Positioning System constellation of satellites. In one embodiment the method is applicable to the transmission of discrete signals according to the protocol of a packet switching network.