Acoustic Sounders
ADCP
The Acoustic Doppler Current Profiler (ADCP) measures currents beneath a ship while underway. Sound signals sent from the moving ship bounce back to receivers aboard the ship. This provides a profile of water movement relative to the ship-precise modern navigation, allowing the ship's motion to be subtracted from the data. These devices are also used on moorings and profilers and, along with acoustic backscattering, measure animal biomass. Particles in the path of the sound waves, mostly plankton, reflect a small part of the sound energy back toward receivers, allowing researchers to make remote estimates of the sizes and numbers of animals present in the water column. Before the 1970's, the most technologically advanced instrument used to measure water velocity was known as a Doppler speed log. This evolved into the first commercial ADCP, produced in the mid-1970's, which used averaging. Later in the 1970's, the first vessel-mounted ADCP was developed to measure water velocity more accurately and to allow measurement in range cells over a depth profile. As instruments evolved, so did new techniques in Doppler signal processing. Initially, Doppler speed logs used simple analog signal processing methods, which are still used in some commercial speed logs today. However, the first ADCPs incorporated analog-to-digital signal conversion over a narrow communication bandwidth. Around 1990, this technique was developed into broadband signal processing. Since then, broadband ADCPs have been able to generate very accurate, real-time velocity measurements. Additional information available at "http://www.whoi.edu/instruments/viewInstrument.do?id=819"