A new observation system was developed for GPS/Acoustic seafloor positioning. The newly developed acoustic system for medium water depth (about 3, 000 m) simultaneously measures ranges from three precision acoustic transponders (PXPs) on the seafloor. Precise ranging is achieved with cross-correlation analysis between transmitted and received acoustic signals. Another important feature of the observation system is a 1.5 m square towed buoy used for the positioning. The buoy is fairly stable, and three GPS antennas on the buoy can measure the position and attitude of the buoy. The buoy towed from a survey vessel with a rope and cables of 100 m is efficient to reduce the underwater acoustic noises generated from the thrusters of the vessel. We deployed and located 3 PXPs off Miyagi Prefecture in August 2003, and carried out the positioning of the PXP array center twice in August 2004, each time for about one day. Although estimated observation error is large, the results obtained twice agree within 2 cm.
