Determination of the time of flight of a signal

The 1st and 2nd equipment (7, 8) transmits and receives a signal on radio, and it has the 1st and 2nd clocks, respectively in order to decide on signal transmitting time and signal attainment time. The 1st equipment transmits the 1st and 2nd signals (9, 10), and the 2nd equipment transmits the 3rd signal (11). By introducing delay into the 1st signal, equipment may be able to be deceived that it thinks that those pieces of equipment of a thief is actually more near. About the 1st and 2nd clocks, the 1st equipment calculates the ratio of a clock rate and determines whether the calculated ratio goes into the predetermined range so that it may specify whether delay was introduced into the 1st signal and may support.