High frequency (HF) radar systems measure the speed and direction of ocean surface currents in near real time. This technology is based on the emission of electromagnetic waves and the study of the echo after reflection by the sea surface. The velocity of the surface current can be derived from the change in frequency between the emitted and reflected signal. This is a result of the Doppler effect, which is the apparent change in frequency of a wave produced by the movement of the source with respect to the observer.
These radars can measure currents over a large region of the coastal ocean, from a few kilometers offshore up to 200 km, and can operate under any weather conditions. They are located near the water's edge, and need not be situated atop a high point of land.
The following map shows the known HF Radar Systems that are or have ever been in operation in the Mediterranean Sea: