Ocean surface current measurements by an L band two-frequency microwave scatterometer

Real time current measurements with an L band pulsed two-frequency microwave scatterometer (2FS) were carried out from the German North Sea Research Platform (Forschungsplattform Nordsee) during the 1979 Marine Remote Sensing (Marsen) experiment. The principle of this remote sensing technique consists of measuring the phase velocity of a long ocean wave by resonant scattering of a beat wave produced by two microwave signals. The difference between the measured phase velocity and the phase velocity, as calculated from the dispersion relation for still water, represents a weighted average of the ocean current near the surface. Current measurements over full tidal cycles in various wind conditions (up to 18 ms^−-1) are presented. At high wind speeds, pronounced differences are found between the 2FS measurements and the in situ current measurements made at depths of 9 m and 23 m. These differences can be explained as the effect of the wind stress acting on the upper layer of the ocean. The measurements indicate that the interaction of the tide- and wind-driven currents strongly depends on the parallel and antiparallel orientation of the currents. The accuracy of the measurements reported here is approximately ±0.12 m s^−-1, but this can, in principle, be improved.