A data communication systems including a main channel that includes means for sending a signal, means for transmitting the signal and means for receiving the signal, the sending means sending signals at a known frequency. The communication system includes a stand-by channel that includes the following devices: a device for temporarily stopping the sent signal at instants known as stopping instants for a constant length of time that corresponds to the sending of a first determined number of sent signals, the stopping instants corresponding to temporal coding of a stand-by signal; a device for summing the amplitudes of the received signals, the summing being carried out on a second determined number of received signals; a device for temporally determining the instants corresponding to the minima of the summed signal, the determined instants having the same temporal coding as the stopping instants.