Performance of an Autonomous Underwater Vehicle borne acoustic doppler current profiler

The currents measured by a SonTek Acoustic Doppler Current Profiler (ADCP) mounted on an Iver2 Autonomous Underwater Vehicle (AUV) are compared to those measured by a bottom-mounted Acoustic Wave and Current Meter (AWAC), taken as a reference, in an attempt to evaluate the AUV´s performance as a current meter instrument. In October 2013, the AUV performed 3 short missions navigating over the AWAC at ~1.5 m depth and ~1.5 ms^-1 velocity while profiling currents down to about 22 m depth, where the AWAC was mounted. The AUV spent about a 16% of mission time on the wavy surface, where its ADCP continued profiling. A 3D AUV currents data processing approach is used to compensate for AUV attitude and is validated by averaging entire mission data, showing absolute errors lower than 0.05 ms^-1 for a large part of the depth range. An AUV data averaging time of 600 s yields horizontal velocity errors of about 0.07 ms^-1 while the 90 s averaging time is found to be a good compromise between error (slightly larger than the one obtained with a 600 s time window) and temporal resolution. Results are promising but more testing in different environmental and operational conditions is needed to validate the AUV as a current meter.