Long Basline Ranging Acoustic Positioning System

Abstract

A long-baseline (LBL) underwater acoustic communication and localization system was developed for the Virginia Tech Underwater Glider (VTUG). Autonomous underwater vehicles, much like terrestrial and aerial robots require an effective positioning system, like GPS to perform a wide variety of guidance, navigation and control operations. Sea and freshwater attenuate electromagnetic waves (sea water is worse due to higher conductivity) within very few meters of striking the water surface. Since radio frequency communications are unavailable, many undersea systems use acoustic communications instead. Underwater acoustic communication is the technique of sending and receiving data below water. Underwater acoustic positioning is the technique of locating an underwater object. Among the various types of acoustic positioning systems, the LBL acoustic positioning method offers the highest accuracy for underwater vehicle navigation. A system consisting of three acoustic 'beacons which are placed on the surface of the water at known locations was developed. Using an acoustic modem to excite an acoustic transducer to send sound waves from an underwater glider, the range measurements to each of the beacons was calculated. These range measurements along with data from the attitude heading and reference system (AHRS) on board the glider were used to estimate the position of the underwater vehicle. Static and dynamic estimators were implemented. The system also allowed for underwater acoustic communication in the form of heartbeat messages from the glider, which were used to monitor the health of the vehicle.