Precise positioning for near-bottom equipment using a relay transponder

The Marine Physical Laboratory, Scripps Institution of Oceanography, has developed an acoustic relay transponder for precise relative positioning of nearbottom instruments and geologic sampling devices. Although specifically designed to locate equipment lowered on standard wire ropes without a need to maintain direct electrical contact with the surface vessel, the relay transponder may be used to track free vehicles, such as deep submersibles, from the surface. The relay transponder is positioned relative to an array of bottom-anchored acoustic transponders. It is interrogated acoustically from the surface vessel; it then sequentially interrogates the bottom transponders which, in turn, reply to the surface vessel. From the measurement of the total travel time involved and assuming (or knowing) the sound velocity of the water, we obtain a relayed range measurement. These relayed ranges, used in conjunction with ship to bottom-transponder ranges, allow us to calculate the position of the relay transponder. A recent application of this technique is described in which several gravity core samples from the crest of Horizon Guyot were positioned with respect to the detailed bathymetry and the geology within the area. The estimated error in positioning the samples is less than 25 m inside a navigational net extending over 100 km square.