ADCP bias and stokes drift in AUV-based measurements

We consider the effects of surface gravity waves on AUV-based acoustic Doppler current profiler (ADCP) measurements. We present data from AUV hydrodynamic surveys in conjunction with current velocity measurements from bottom mounted ADCPs in coral reefs off the coast of Oahu, Hawaii. Transect averaged (100 m) along- and cross-track velocity measurements were calculated on a vehicle-based reference frame and compared with fixed ADCP measurements. Water velocity measurements obtained in a range of wave conditions confirm the presence of a wave-induced bias (Vbw) that is closely related to the Stokes drift. Moreover, AUV-based water velocity measurements are also subject to a wave-induced alias that results from a Doppler shifting of the measured wave velocity field. Waveinduced aliasing can enhance or suppress the bias by an amount that is proportional to the ratio of the vehicle velocity relative to the wave velocity. In addition to the wave-induced bias and consistent with a previously reported behavior, the data suggests the presence of an additional bias in the direction of the vehicle motion.