An Acoustic Doppler Current Profiler is 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).

From: Wikipedia:

An '''Acoustic Doppler Current Profiler''' ('''ADCP or ADP''') is a sonar that attempts to produce a record of water current velocities for a range of depths.  ADCPs can be configured in many ways:  side-listening, into rivers and canals for long term continuous discharge measurements, downward-listening and mounted on boats for instantaneous surveys in the ocean or rivers, and mounted on moorings, or the seabed for long term current & wave studies. 

Depending on the field application, an ADCP may use one or more ceramics, (or other piezo materials) for transducers, which work in water similar to directional loudspeakers in air. These transducers are aimed such that the sound pulse travels through the water in different, but known directions.  As the sound energy leaves and arrives at the transducer face it is shifted in frequency, known as the Doppler effect, by the relative velocity of the water.  As that sound energy is returned (echo) by scatterers in the water the sound may also be shifted in frequency if there is relative velocity of water to scatterer.   Trigonometry, averaging and some critical assumptions are used to calculate the velocity of the group of echoing scatters in a volume of water.  By repetitive sampling of the return echo, and by “gating” the return data in time, the ADCP can produce a "profile" of water currents over a range of depths. Phased array techniques are also used to aim the sound (acoustic) energy  allowing for economical production of smaller ADCPs to accommodate a range of frequencies from 38 kHz to several megahertz.  

In addition to the transducers, an ADCP typically has an electronic amplifier, receiver, mixer, oscillator, accurate clock, temperature sensor, compass, pitch and roll sensor, analog-to-digital converters, memory, digital signal processor and instruction set. The analog-to-digital converters (ADCs) and digital signal processor (DSP) are used to sample the returning signal, determine the Doppler shift, and sample the compass and other sensors in order to calculate range and a velocity vector relative to a known orientation. 

There are a number of factors that affect accuracy, resolution and profile range. Most notable are: absorption, spreading, speed of sound in water, bandwidth of the sound energy, signal strength of the transmitted pulse and echo, size of transducer, beam width (sic) of the energy pulse travelling through the water, frequency, and a host of limitations associated with the signal processing techniques and hardware, including clock/oscillator accuracy.

The ADCP, commercially available for about 25 years, is currently used for oceanography, estuary, river and stream flow measurement, even in weather forecasting.  ADCPs are used in diverse ways, from locating underwater "tornadoes" that might damage deep water oil drilling activity, to measuring water flow through sewer pipes, or hanging up side down under an iceberg and measuring the flow of freshwater melting off the iceberg.  Some harbor managers now use ADCPs to help them take advantage of tides and currents and optimize the flow of shipping in a busy port. 

ADCPs can be self-contained and operate from batteries for many years under the sea or remotely in a river or stream. Some time later they are retrieved and the historical current data is transferred from the ADCP memory to a computer and displayed using a variety of graphical and text-based software to observe the water current profiles. Or ADCPs can be connected to RS232, RS422, RS485, SDI-12, USB, Ethernet, Fiber, Modbus (SCADA) connections to provide real time, "live", monitoring of their output.

An ADCP can also be an acoustic '''Doppler Velocity Log''' ('''DVL''') if it is programmed with the correct signal processing logic.  The DVL bounces sound off the bottom (or a reference layer of water) and can determine the velocity vector of a subsea vehicle (or surface vessel) moving across the sea floor.  This information can be combined with a starting fix, compass heading, and acceleration sensors (typically by use of a Kalman Filter) to calculate the position of the vehicle.  DVLs are used to help navigate surface vessels, submarines, autonomous underwater vehicles, and ROVs for precise positioning in an environment where GPS, and other navigational aids, don't work.

Further Reading:

• Physical Oceanography Index
• Joint Archive for Shipboard ADCP