Doppler current meters
The ADCP (Acoustic Doppler Current Profiler) is composed of four acoustic transducers attached to the ship's hull. They are oriented in four different directions and act as transceivers.
Signals are sent downward, following beams whose axes are slanted by 30° with respect to the vertical. These signals then bounce off micro-organisms and suspended particles in the water. The acoustic transducers receive them with a frequency lag proportional to the radial speed of particles compared to the transmission frequency. This is the Doppler effect.
Sampling over time provides measurement of the Doppler lags in the water columns at regularly distributed depths.
Combining the signals received provides a representation of the ship's speed with respect to each water column.
The transmission frequencies go from 38 to 150 kHz with respective ranges from 1,000 to 400 m. This delivers data on the speeds and direction of the water layers accurate to 1%.
How does it work ? (access to the SHOM server)