Recently underwater vehicles are typically navigated with an inertial navigation system (INS), a Doppler velocity log (DVL), and an acoustic positioning system (APS). APS are necessary, especially in deep sea observation, because it is absolute positioning method. Super short baseline (SSBL) is frequently used because it is easy to operate. In SSBL, the position of vehicle is obtained on the mother ship. In order to use the positioning result to navigate the vehicle, the result is transmitted to the vehicle with a certain amount of delay.
Authors are developing a test system of new type APS using inverse SSBL (ISSBL) method. In this method, the vehicle is equipped with a receiver array. Arrival direction of acoustic signal from mother ship is detected, and relative position between the mother ship and the vehicle is calculated with the obtained direction and the depth. Information of ship’s position is included in the transmitted acoustic signal, then absolute position of the vehicle can be calculated with the relative position and the included information. The vehicle position can be obtained in the vehicle in real-time and be used directly to navigate. No reply from the vehicle is necessary.
An ocean experiment of this method was conducted in Sagami Bay in Japan. Experimental device was moored on the seabed and the ship cruised with acoustic signal transmission. As a result, this method was available in deep sea area. Demodulation of information in the method was feasible, and positioning of the experimental device was achieved. High rate positioning is useful suppress random error with filtering.