Acoustic Doppler Current Profiler
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An Acoustic Doppler Current Profiler (ADCP), is a type of sonar that attempts to produce a record of water current velocities over a range of depths.
The most popular ADCP uses a scheme of four ceramic transducers, which work in water similar to loud speakers in air. These transducers are aimed in such a way that the monofrequency, sound pulse they produce travels through the water in four different, but known directions. If the ADCP is looking down into the water, you can imagine each transducer as being aligned at 12, 3, 6 and 9 o’clock positions, facing away from perimeter of the clock (i.e. 90 degrees apart in Azimuth) and then tilted down 60 degrees in elevation below the horizon. As the echo of the sound is returned by scatterers in the water, it is shifted in frequency due to the Doppler effect.
In addition to the transducers, an ADCP typically has a receiver, amplifier, 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. Trigonometry, averaging and some critical assumptions are used, to calculate the Horizontal 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.
There are a number of factors that effect accuracy, resolution, profile range, profile sizes and error checking.
Recently, phased array ADCP sonars have been commercialized, these ADCPs produce and listen for the return "Acoustic" signals with an array of transducers, allowing for economical production of both smaller and larger sized ADCPs that are tailered for specific frequency from the tens of kilo-Hertz to several mega-Hertz.
The ADCP has been commercially available for about 25 years. It is now considered an indispensable aid for oceanography, estuary, river and stream flow current measurement, even in weather forecasting. ADCPs are used in diverse ways, from locating underwater "tornados" that might damage deep water oil drilling activity, to measuring water flow through sewer pipes, or even hanging up side down under an iceberg and measuring the flow of freshwater melting off of the iceberg. Even harbor managers now use ADCPs to help them take advantage of tides and currents and optimize the flow of shipping in a busy port.
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 of the bottom and can determine the velocity vector of a subsea vehicle moving across the seafloor. This information can be combined with a starting fix (typically by use of a Kalman Filter) to calculate the position of the vehicle. DVLs are used to help navigate submarines, autonomous underwater vehicles, and ROVs for precise positioning in an environment where GPS, and other navigational aids, don't work.
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 to from the ADCP memory to a computer and displayed using a variety of graphical and text base software to observe the water current profiles. Or ADCPs can be connected to RS232, RS422, SDI-12 or USB connections to provide real time, "live", monitoring of their output.
