The invention consists of a method which comprises a plurality of steps for determining difference in distance from a reference point to at least two sensor modules 10, 20 that are located under water. The sensor modules 10, 20 comprise means for sending and receiving acoustic signals to each other and to a hydrophone 30 at the reference point. Connected to the hydrophone 30 is a calculating unit 40 that utilizes arrival time of the received signals from the sensor modules to determine difference in distance from the sensor modules (10, 20) to the reference point.