A method for determining the distance from a reference point to at least two sensor modules (10, 20) when these are placed approximately at the same distances from the reference point, all of which are located under water, wherein the sensor modules (10, 20) are in connection with each other in that they comprise means for sending and receiving acoustic signals, and wherein a hydrophone with means for receiving acoustic signals from the sensor modules (10, 20) is placed at the reference point, the method comprising the steps of a) sending an acoustic signal from the first sensor module to the second sensor module; b) sending an acoustic signal from the second sensor module to the first sensor module and to the hydrophone either immediately or following set time delays after the second sensor module receives the acoustic signal sent from the first sensor module; c) measuring in the first sensor module the time T 1 it takes from when the acoustic signal is sent from the first sensor module until the first sensor module receives the acoustic signal sent from the second sensor module; d) sending an acoustic signal from the first sensor module to the hydrophone either immediately or following a set time delay after the first sensor module receives the acoustic signal sent from the second sensor module; e) measuring arrival times and travel times of the acoustic signals sent from the first and second sensor module (10, 20) to the hydrophone; f) using a calculating unit connected to the hydrophone for determining the difference in distance from the reference point to each of the sensor modules (10, 20) based on the measured arrival times of the acoustic signals on the hydrophone, on the time T 1 and on the optional set time delays, g) adjusting the distance and positioning the sensor modules (10, 20) in relation to the reference point such that the calculated difference in distance approaches zero; h) logging over time the values of the acoustic signals within a set uncertainty for which the calculated differences in distance approach zero; i) determining the distance from the reference point to the at least two sensor modules (10, 20) by using the measured travel times of the acoustic signals that are included in said logged values. a) sending an acoustic signal from the first sensor module to the second sensor module; b) sending an acoustic signal from the second sensor module to the first sensor module and to the hydrophone either immediately or following set time delays after the second sensor module receives the acoustic signal sent from the first sensor module; c) measuring in the first sensor module the time T 1 it takes from when the acoustic signal is sent from the first sensor module until the first sensor module receives the acoustic signal sent from the second sensor module; d) sending an acoustic signal from the first sensor module to the hydrophone either immediately or following a set time delay after the first sensor module receives the acoustic signal sent from the second sensor module; e) measuring arrival times and travel times of the acoustic signals sent from the first and second sensor module (10, 20) to the hydrophone; f) using a calculating unit connected to the hydrophone for determining the difference in distance from the reference point to each of the sensor modules (10, 20) based on the measured arrival times of the acoustic signals on the hydrophone, on the time T 1 and on the optional set time delays, g) adjusting the distance and positioning the sensor modules (10, 20) in relation to the reference point such that the calculated difference in distance approaches zero; h) logging over time the values of the acoustic signals within a set uncertainty for which the calculated differences in distance approach zero; i) determining the distance from the reference point to the at least two sensor modules (10, 20) by using the measured travel times of the acoustic signals that are included in said logged values.