Shore-based current meters employ radio antennas and high frequency (HF) Radio Detecting and Ranging systems (radar) to measure surface ocean currents. Following the same premise of the ADCP, these shore-based instruments use the Doppler effect to determine when currents are moving toward or away from the shore. If a wind-driven current is moving toward the shore, the return signal is at a high frequency. If the wind-driven current is moving away from shore, the return signal is at a low frequency. Scientists also use these measurements to determine the velocity of the current. When two or more radar antennas are used, a scientist can calculate an entire field of surface current velocities for thousands of points. Using this data, the scientist can produce a “map” of surface currents for a large coastal area.