A radio system is provided which comprises a radio receiver and a processing system, wherein the radio receiver is configured to detect radio signals transmitted from a radio transmitter on a plurality of frequency channels, and to measure respective signal strengths of the radio signals for each of the plurality of frequency channels. The processing system is configured to evaluate a measure of statistical dispersion of the respective signal strengths over the plurality of frequency channels, and to use the measure of statistical dispersion to determine information relating to a proximity of the radio transmitter to the radio receiver.