Because of the advantage of acoustic signal traveling in the water, most techniques of precisely locating AUVs adopt GPS and acoustic measurements. The development of modern GNSS techniques, attitude measuring techniques and precise underwater ranging techniques make GPS-acoustic technology possible for accurate localization of an AUV. Because the transmit angles are unknown for transducers, it is complicated for us to solve the coordinates of the AUV. In order to rapidly and accurately obtain the positions of AUV at each epoch, in this paper, an algorithm for locating an AUV in an absolute reference frame is presented. It uses three surface buoys or vessels to range and adopts the iterative resection to solve the position of the AUVs. Acoustic ranges are calculated using the equivalent sound speed profile and travel times of sound rays. This simple equivalent SSP is usually adopted to have a constant gradient in the total water column. Under some approximate conditions, the initial Snell constant can be deduced so that the linear geometric distances can be obtained through ray trace after the Snell constant is iteratively solved. Finally, the precise position of the AUV at each epoch is calculated using the precise geometric distances. It is proved by simulation that this method is capable of real-time positioning of the AUV with an accuracy of ± 20 cm in the inner cover area of three transducers at a depth of 500 m supposing the error of sound speed is 0.1 m/s. The precision can be improved further if kalman filter is adopted.
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