Method for determining distance between underwater acoustic devices

A method for determining distance from a reference point to at least two sensor modules (10, 20) when these have the same distance to the reference point and all of which are located under water, wherein the sensor modules (10, 20) are in signal connection with each other in that they comprise means for sending and receiving acoustic signals, and wherein the reference point comprises a hydrophone (30) with means for receiving acoustic signals from the sensor modules (10, 20), the method comprising the following steps:

• a) sending an acoustic signal from the first sensor module (10) to the second sensor module (20);
• b) sending an acoustic signal from the second sensor module (20) to the first sensor module (10) and the hydrophone (30) either immediately, or following set time delays after the second sensor module (20) receives the said first acoustic signal sent from the first sensor module (10);
• c) measuring in the first sensor module (10) the time T1 it takes from when the acoustic signal is sent from the first sensor module (10) until the first sensor module (10) receives the said acoustic signal sent from the second sensor module (20);
• d) sending an acoustic signal, comprising the said time T1, from the first sensor module (10) to the hydrophone (30) either immediately, or following a set time delay after the first sensor module (10) receives the said acoustic signal sent from the second sensor module (20);
• e) measuring arrival times and travel times of received acoustic signals sent from the first and second sensor module (10, 20) to the hydrophone (30) at the reference point;
• f) determining the difference in distance from the reference point to each of the sensor modules (10, 20) by calculating this difference in a calculating unit (40) connected to the hydrophone (30) at the reference point based on measured arrival times of acoustic signals on the hydrophone (30), and on the time T1 and optional set time delays, characterised in performing the following additional steps:
• g) adjusting and positioning the distance to the sensor modules (10, 20) in relation to the reference point such that calculated difference in distance approaches zero;
• h) logging over time values of received acoustic signals within a set uncertainty for calculated differences for distances approaching zero;
• i) determining distance from the reference point to the at least two sensor modules (10, 20) by using measured travel times of the acoustic signals that are included in said logged values.