Near-surface ocean current sensors: Problems and performance

When current meters are used to measure mean horizontal currents in the presence of surface gravity wave motions, immunity to the vertical component of flow is important, even though the net vertical flow averages to zero and is normal to the desired horizontal components. A technique is presented for estimating the magnitude of the errors introduced by imperfect rejection of the off-axis flows (cross-talk) from laboratory measurements of the current meter "vertical-cosine-response." The predicted dynamic response is shown to compare favorably with laboratory measurements. The measured steady state vertical-cosine-response functions for several practical current sensors are summarized and used to estimate the magnitude of wave induced errors in horizontal mean current measurements. A new dye technique for evaluating near-surface current meter performance in waves is shown.