Abstract: A method for determining a mutual distance between two objects in a pipeline. The method includes the steps of: providing a pipeline which is filled with a medium having an acoustic impedance different from that of the material of the pipeline and/or a medium present outside the pipeline; providing each of the two objects with a clock; synchronizing the clocks; periodically transmitting an acoustic signal from a first to a second one of the two objects; measuring the reception time of the acoustic signal; determining the delay time from the first to the second object; and calculating a traveled distance of the signal between the two objects on the basis of a propagation speed of the acoustic signal in the medium and the measured delay time of the acoustic signal.