A bidirectional dialogue is carried out between a portable code emitter (15) and a base station (1) for access to a motor vehicle. Both signals are correlated with each other and the dialogue repeated at an altered carrier frequency. The distance of the code emitter (15) from the base station (11) is determined from the difference in two phase difference measurements between the sent and received signal in one station (11 or 15). Access to, or use of the motor vehicle (10) is only permitted when the code emitter (15) is within a certain distance from the motor vehicle (10).