Deep water bottom-track ship´s velocities from an acoustic correlation current profiler

Acoustic Doppler logs have a typical bottom-tracking range of 500 m; whilst this is perfectly adequate for use on continental shelves, it is clearly inadequate for use in the deep ocean. Doppler logs use narrow beams requiring large transducers at the low frequencies necessary for long range. In contrast acoustic correlation logs can use smaller, lighter and less expensive low frequency transducers while delivering long range performance as they only require wide beams (~20°). In this paper we present results from a 22 kHz acoustic correlation current profiler (ACCP) installed on RRS Discovery from the eastern North Atlantic and the western Mediterranean Sea in water depths of 110 to 3000 m. A deficiency in the bottom tracking algorithm resulted in the instrument recording false bottom echoes for extended periods. Nevertheless, the ACCP is a valuable instrument for measuring the speed of a vessel over the ocean floor with proven performance between 500 and 3000 m. As a velocity log on an autonomous underwater vehicle it would significantly augment an inertial or dead-reckoning autopilot. Simulating a 30 hour AUV mission using our ship data at an average speed of 7.6 km hour^-1, involving several changes of course, resulted in an integrated position error of less than 2 km over a 227 km track