Radar operating at wavelengths of 10–100 m with the received signal being scattered by ocean waves of half the radar wavelength. The received signal is Doppler shifted by an amount corresponding to the phase speed of this ocean wave component. The surface current in the radial direction can be computed from the difference between the observed Doppler shift and that which is expected from the surface-wave dispersion relation. See also Doppler radar, marine radar, microwave radar, synthetic aperture radar.