Performance of an absolute velocity profiler based on acoustic doppler and electromagnetic principles

Abstract

This report describes the performance of a freely-falling velocity profiler called the Absolute Velocity Profiler (AVP) . The AVP is distinguishable from our previously developed velocity profiler the Electro-Magnetic Velocity Profiler (EMVP) in that acoustic Doppler measurements are used, to determine the reference velocity for the EMVP profiles. The AVP contains the essential measurements of the motional electric currents in the sea as implemented in the EMVP and in addition, collects acoustic Doppler measurements of frequencyshifted bottom echoes. The former measurements yield a profile of the horizontal components of velocity relative to a reference velocity, independent of depth, while the latter measurements determine the absolute velocity of the AVP vehicle with respect to the sea floor. The EM profile is obtained from the sea surface to bottom, and the acoustic Doppler measurements are made within about 300 m of the sea floor. The combination of the EM and acoustic Doppler measurements yields an absolute velocity profile throughout the water column. Performance analyses included in this report set method uncertainties of between 1 and 2 cm/s r.m.s. Measurements of temperature and its gradient are also made.