An instrument used to measure ocean currents. It transmits high frequency acoustic signals which are backscattered from plankton, suspended sediment, and bubbles, all of which are assumed to be traveling with the mean speed of the water. The ADCP estimates horizontal and vertical velocity as a function of depth by using the Doppler effect to measure the radial relative velocity between the instrument and scatterers in the ocean.

Three acoustic beams in different directions are the minimum number required for measuring the three velocity components, with a fourth adding redundancy and an error estimate. A ping is transmitted from each transducer about once per second, with the echo returning over an extended period. Echos from shallow depths return before those from greater depths. Profiles are produced by range-gating the echo signal, i.e. breaking the echo into successive segments called depth bins corresponding to successively deeper depth ranges. The noisy velocity estimates from each ping are vector-averaged into 1- to 10-minute ensembles, and the resulting relative velocities are rotated from the transducer's to the earth's reference frame using the ship's gyrocompass.

A navigation calculation is performed to obtain absolute currents, which are obtained by subtracting the average of the ship velocity relative to a reference layer (i.e. ADCP velocities) from the absolute ship velocity over the ground (from GPS navigation). The raw absolute current velocities relative to the reference layer are then smoothed to reduce the effect of noise in the position fixes, and combined with the navigation data to obtain the best estimates of ship positions and velocities. Thus, absolute currents at any depth can be determined from the ship navigation data and the relative ADCP measurements.

The ADCP measures the ocean current velocity continuously over the upper 300 m of the water column, usually in 8 m depth increments. It is also used to estimate the abundance and distribution of biological scatterers over the same depth range and in the same depth increments.

ADCP data collection requires that four instruments work together. These are the ADCP itself, the ship's gyrocompass, a GPS receiver, and a GPS Attitude Determination Unit (ADU).