Abstract: A subsea measurement system (400) comprises a first reference beacon (110) disposed at a first known location, a second reference beacon (112) disposed at a second known location, and an acoustic communications module (302) for coupling to a subsea element to be monitored. The acoustic communications module (302) is capable of moving, when in use, relative to the first reference beacon (110) and the second reference beacon (112). The acoustic communications module (302) also comprises a processing resource and is arranged to determine first range-related data to the first reference beacon (110) in response to receipt of a first signal by the acoustic communications module (302) from the first reference beacon (110) and second range-related data to the second reference beacon (112) in response to receipt by the acoustic communication module (302) of a second signal from the second reference beacon (112).

Abstract: A subsea measurement system (400) comprises a first reference beacon (110) disposed at a first known location, a second reference beacon (112) disposed at a second known location, and an acoustic communications module (302) for coupling to a subsea element to be monitored. The acoustic communications module (302) is capable of moving, when in use, relative to the first reference beacon (110) and the second reference beacon (112). The acoustic communications module (302) also comprises a processing resource and is arranged to determine first range-related data to the first reference beacon (110) in response to receipt of a first signal by the acoustic communications module (302) from the first reference beacon (110) and second range-related data to the second reference beacon (112) in response to receipt by the acoustic communication module (302) of a second signal from the second reference beacon (112).

Abstract: In a method for determining absolute position under water of a submersible vessel (1) having a dead recloning navigation system and receiving acoustic signals from a reference station (19), signals are received from one reference station in several positions (15-18) of the vessel. Estimated absolute positions of the vessel are calculated using range data and relative position data. Range rate data derived from the signals are preferably utilised. In a method for scanning an underwater survey area, the absolute position of a vessel (1) is intermittently being determined according to said method. The reference station may be placed at a fixed absolute position (19), or on the surface of the water, preferably in a buoy or a vessel. A system for determining the absolute position under water of vessel comprises: acoustic communication means in a reference station and on board the vessel; a dead reckoning navigation system on board the vessel; and computing means.