RESSAC - A NEW AIRBORNE FM/CW RADAR OCEAN WAVE SPECTROMETER

We present the French radar system RESSAC, used from aircraft platforms for measuring directional spectra of ocean waves in the gravity domain (wavelengths from 30 to 400 m). The instrument consists of a C-band (5.35 GHz) FM/CW radar system. The transmitting and receiving antennas are looking toward the surface at a low incidence angle (almost-equal-to 14-degrees for the center of the antenna beam), and rotate around a vertical axis. When processing the data, one removes the known antenna gain pattern from the recorded signal, thus making it possible to estimate the sea surface slope variance, which in turn is used to determine the tilt modulation transfer function, without the need of any external wind measurement. Fully normalized spectra obtained from RESSAC are presented and compared to other data sets (nondirectional frequency spectra from buoy measurements, directional spectra from a ship radar and from a sea-state model). The consistency is in general very satisfactory, and the estimates of sea surface slope variance inferred from RESSAC are consistent with the wind observations and with previous results found in the literature.