Seafloor positioning system with GPS-acoustic link for crustal dynamics observation - a preliminary result from experiments in the sea

Many active plate boundaries, such as mid-ocean ridges and trenches, are un der the sea. Seafloor crustal deformation data will contribute significantl y to understanding the nature of the tectonic processes at these plate boun daries. We have developed a seafloor positioning system with a GPS-acoustic link. This system consists of two main components; (1) the surface positio ning by differential GPS to on-land reference and (2) the precise acoustic ranging using the M-sequence between the surface and seafloor references. T he position and attitude of the surface GPS-acoustic link unit are determin ed from the GPS observations. Simultaneously, the acoustic ranging between the surface unit and seafloor references are carried out. The positions of the seafloor references are determined from these observations and a sound- speed structure model of the seawater. We performed preliminary seafloor po sitioning experiments. In these experiments, simple 1-D structure models ar e assumed for the sound-speed in the sea. The results show that the positio ns of the seafloor references are estimated with an accuracy on order of 10 cm. The residuals for acoustic ranging imply that there are systematic dif ferences between the assumed and real sound-speed structure. It is necessar y to reduce the uncertainties of sound-speed structures for more accurate p ositioning.