HF radio waves are used to measure surface ocean currents. The outgoing radio wave, wavelength , resonantly Bragg-scatters from sea-surface waves of wavelength /2 (upper sketch). If no current is present, the backscattered radio waves are Doppler-shifted by the phase-speed of the surface waves toward or away from the transmitter, (middle sketch). If there is an ocean surface current toward or away from the transmitter, it imposes an additional Doppler shift on the backscattered wave. This additional shift is what indicates the magnitude and direction of the current, either toward or away from the transmitter (lower sketch). Range-gating is used to profile currents vs distance from the transmitter. Compact multi-element receive antennas, with peak sensitivity in orthogonal directions, to allow direction-finding to assign an azimuth to each returning frequency band (i.e. each current estimate). Maps of surface current vectors are constructed by combining simultaneous measurements from two separated sites.