Sea-floor positioning with global positioning system-acoustic link system

The present study describes a new system designed and developed for observi ng crustal deformation on the sea floor. The system consists of two parts, the kinematic positioning by global positioning system (GPS) and acoustic r anging techniques. Since the location of a site at the ocean bottom relativ e to a reference site on land cannot be determined directly, the procedure was divided into two steps. First, the position of a vessel was determined using differential and kinematic GPS techniques, and then the position of a reference point at the ocean bottom was located relative to the vessel usi ng the acoustic ranging technique. Thus, the location of the ocean-bottom s tation is determined relative to the reference sites on land in the global reference system. The accuracy was tested in several ways. In one experimen t, a buoy was used as the surface station which linked the positions of a G PS receiver to the acoustic transponder. Assuming a simple sound velocity p rofile of the seawater, the position of an ocean-bottom reference point was estimated with an accuracy of several meters. Thus, with the present syste m, it is difficult to observe ocean-bottom crustal deformations generated b y typical plate motions. Methods are being investigated to improve the obse rvation system for more accurate sea floor positioning.