First and second devices (7, 8) are configured to transmit and receive signals wirelessly and have first and second clocks respectively for determining signal transmission and arrival times. The first device transmits first and second signals (9, 10) and the second device transmits a third signal (1 1 ). By introducing a delay into the first signal, a thief may be able to fool the devices into thinking that they are closer than they really are. To help identify if a delay has been introduced into the first signal, the first device calculates a ratio of clock rates for the first and second clocks and determines whether the calculated ratio falls within a predefined range.
