In a method for determining absolute position under water of a submersible
vessel (1) having a dead reckoning 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 a 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.