Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of ±30cm (2-σ) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder´s horizontal position was determined with an accuracy of ±8 cm (2-σ) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to ±10 cm (2-σ) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to ±1 m (2-σ). Simulations show that ±30 cm (2-σ) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.