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