An Acoustic Doppler Current Profiler (ADCP) measures ocean currents using the principle of “Doppler shift.” If you have heard a train whistle in the distance, you are familiar with Doppler effect. As the train gets closer, the whistle pitch gets higher. As the train moves away, the whistle pitch gets lower. The change in pitch is proportional to the speed of the train.
An ADCP follows the premise of the Doppler effect. It emits a series of high-frequency pulses of sound that bounce off of moving particles in the water. If the particle is moving away from the instrument, the return signal is at a lower frequency. If the particle is moving toward the instrument, the return signal is at a higher frequency. Because the particles move at the same speed as the water that carries them, the speed of the water’s current can be determined.
An ADCP is usually equipped with four acoustic transducers that emit and receive signals from four different directions. This allows the instrument to measure currents at different depths simultaneously. On large research vessels, the ADCP is often permanently mounted on the ship’s outer hull and operates continuously. (Source: NOAA Ocean Explorer Web Site)
NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) uses Doppler profilers to collect ocean current information in various ports and harbors throughout United States territorial waters. Instruments, such as ADCPs, are anchored on the ocean floor for more than 30 days so that the tidal currents can be measured accurately. These measurements are used to update predictions in Tidal Current Tables that mariners are required to carry on board their vessels.