Abstract: Arrangements according to the invention embody a radio frequency receiver for use in a proximity detecting system, the radio frequency receiver comprising at least one antenna coil operable to receive radio frequency signals; tunable receiver circuitry arranged in operative association with the antenna coil and being arranged to modify the frequency at which radio signals are received by the radio frequency receiver; and a signal processor arranged to amplify and filter signals received by the radio frequency receiver, wherein the radio frequency receiver is operable to receive and process radio signals of frequencies between 100 kHz and 10 MHz. By operating in the low frequency range, multi-path problems associated with high frequency equipment (such as reflections from objects located between the path of the transmitter and receiver) are significantly reduced, whilst distance between transmitter and receiver can be identified extremely accurately.

Abstract: A point to multipoint two-way communications systems in which a base station transmits to a remote station ("outstation") on a frequency derived by the base station from the frequency of a previous transmission received by said base station from said outstation.