ISLAND CONNECTED SEA-BED SIGNATURES OBSERVED BY MULTIFREQUENCY SYNTHETIC-APERTURE RADAR

Multifrequency X-, C-, and L-band synthetic aperture radar (SAR) image s of the northern sea area off the isle of Heligoland in the German Ei ght of the North Sea have been analysed. The data were collected durin g the SAR and X-band Ocean Nonlinearities Research Platform North Sea Experiment (SAXON-FPN) which was carried out in November, 1990. Differ ent oceanographic phenomena are visible on simultaneously obtained SAR images. Wind streaks and a vortex street can be identified only on th e X- and C-band SAR images. Elongated streaks of predominantly low rad ar return are related to nearshore reefs and are imaged on all availab le radar scenes. The imaging mechanism of these submarine ridges is in vestigated and discussed using with some modifications the simple Brag g relaxation model proposed by Alpers and Hennings. The improved model differs from the original version in the representation of the unpert urbed wave-energy spectral density. Also the advection term and the ph ase modulation (velocity bunching) have been included in the model. Du e to the improvements it is now possible to simulate the radar cross-s ection modulation with the same order of magnitude at 0.4 GHz (X-band) and 5.3 GHz (C-band) as well as at 1.3 GHz (L-band). However, the sim ulated radar cross-section modulation is still underestimated compared with the SAR data, but the phase of the calculated radar cross-sectio n modulation agrees quite well with the SAR measurements.