The unit then listens for the sound to bounce back and it records the time it took and the direction from which the echo returned.
Depth sounders record the sound bouncing off the sea bottom.
The ADCP however, records the sound that bounces back from small particles floating in the water. The ADCP determines the speed at which the particles are moving by the way the reflected sound is altered. This change in apparent sound is known as Doppler Shift, and if you practiced, you might get fairly good at estimating the speed of the train just by how fast the whistle is increasing or decreasing in pitch. This is similar to how the ADCP determines the current under the ship. It listens for how much and how fast the sound is altered as it reflects off the particles floating in the current.