Precise acoustic ranging and positioning: a progress report

The authors carried out an experiment in April 1999 by using a 2 m-square surface buoy and the RV Tansei-maru, Ocean Research Institute, University of Tokyo. Three GPS antennae were fixed on the top of the buoy and an acoustic transducer on the bottom. GPS reference station for kinematic GPS positioning from the study area was set at Aburatsubo, about 35 km away. Both the reference station and the GPS receivers installed on the buoy were recorded at 1 Hz sampling. An acoustic ranging system was developed under the Ocean Hemisphere Project, but the resolution of the acoustic ranging was 10 cm. Three acoustic transponders were deployed on the seafloor at about 1400 m water depth. The authors carried out acoustic positioning every 20 s for 30-60 minutes. The effect of motion of the buoy was only removed using the data of the GPS receivers; the problem was that timing of acoustic measurements was not so precise. The repeatability of the buoy's positioning is estimated to be 20-30 cm. The objective was to establish sea-floor reference points for the measurement of tectonic plate movements and crustal deformations.