Ultra short baseline
Ultra Short Baseline (USBL) is an underwater navigation system based on the transit times of waterborne sound between the object and several reference points. USBL systems are used to determine the position of objects or devices under water.
As a rule, several hydrophones for receiving are installed on board a ship equipped with a USBL system . The underwater object whose position is to be determined has a transmitter (beacon). This sends an acoustic signal that is picked up by the receiving hydrophones on board the ship.
If the speed of sound is known, the distance to the object results from absolute transit times, for which the time of transmission must be known. Exact clocks are used or the command to transmit is transmitted to the transmitter hydrophone, also by sound or electrically via a cable (e.g. when using an ROV ).
The time differences with which the signal reaches the individual hydrophones allow the direction of incidence of the sound waves in relation to the ship to be determined. Changes in this direction are based not only on the actual movements of the object, but in particular also on the pitching and rolling of the ship, which therefore has inertial sensors .
The relative position in relation to the ship can be converted into an absolute position (relative to the ground) if the location and orientation of the ship is known, e.g. B. by satellite navigation .
Due to thermohaline stratification and corresponding variation of the speed of sound, the direction of incidence of the wave fronts deviates from the direction towards the object, and also fluctuates when internal waves pass through . Another fixed hydrophone on the ground improves the accuracy of the method considerably. A large, precisely measured array of hydrophones at the bottom is called a long baseline system (LBL).