A New Sound Velocity Measurement System

Abstract

At present, ranging with acoustic signals is the primary means for determining positions on the sea floor. Converting travel time measurements into distances requires accurate knowledge of the effective sound velocity along the propagation path. In fact, marine positioning accuracy is presently most limited by the uncertainty in determinations of the ocean’s sound velocity field. The best sea-going sound velocity meters have accuracies of a part in 104, which translates into position uncertainties of 100 centimeters in 10 kilometers. A recently developed sound velocity meter improves this accuracy by a factor of ten, using a time-averaged, phase com-parison process. A description of this sound velocity meter and a discussion of its calibration and accuracy are presented here. Ultimately, knowledge of sound velocity in the ocean is limited by the variability of the ocean’s temperature and salinity structure. Therefore, the requirement is to measure sound velocity accurately on time and length scales appropriate for the position measurement being made, and a brief discussion of statistical methods of measurement in the ocean in order to adequately model the spatial coherence and temporal variability of sound velocity follows.