How does Doppler sonar measure the speed of upper-ocean currents?

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World map showing major ocean current systems, with arrows indicating the direction and speed of warm and cold currents in the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. Warm currents are marked in red and cold currents in blue, with currents labeled by name and symbols denoting those that reverse direction seasonally. A legend explains current speed categories and color codes. — ocean current Major ocean current systems of the world.

A Doppler-sonar system measures the speed of upper-ocean currents by transmitting a narrow beam of sound into the water. This beam scatters off drifting plankton and other organisms present in the uppermost strata of the ocean.1

When these particles move with the water, they act as moving obstacles that reflect the sound waves. The frequency of the backscattered sound undergoes a Doppler shift—increasing if the particles move toward the transmitter and decreasing if they move away. By analyzing this frequency shift, the system determines the component of water velocity parallel to the beam. This method allows for precise measurements at ranges up to 1,400 metres from the transmitter.21