Radar signatures of sea surface films of different origin are investigated, which have been acquired by airborne and spaceborne multi-frequency/multi-polarisation microwave sensors during the
Spaceborne Imaging Radar-C/X-Band Synthetic Aperture Radar
(SIR-C/X-SAR), missions in 1994, as well as by the ERS SAR in 1996–98. During SIR-C/XSAR, controlled surface film experiments were performed in the German Bight by deploying various quasi-biogenic substances and mineral oil on the sea surface, in order to study the radar signatures caused by surface films of different visco-elastic properties. In general, our results show that multi-frequency capabilities, rather than multi-polarisation capabilities, are needed for a radar-based system for the discrimination of marine surface films. We show that, under high wind conditions (> 10 m s
), a discrimination between the different kinds of surface films is very difficult, whereas at low to moderate wind speeds (≤ 5 m s
) a discrimination seems to be possible. This finding is supported analytically by means of a new model for the wave number-dependent radar contrast at high wind speeds (> 10 m s
) and statistically through the analyses of more than 700 ERS SAR images. In addition, results of laboratory experiments are presented that were carried out in the wind-wave tank of the University of Hamburg. At certain wind-speed ranges a different damping of bound and freely propagating surface waves by monomolecular surface films is observed, which may explain the high radar contrasts measured by the microwave sensors.