SIR-B Observations of Ocean Waves in the NE Atlantic

Synthetic-aperture radar (SAR) imagery from SIR-B was obtained over a deep-water site in the NE Atlantic Ocean. Selected images were Fourier analyzed in order to compare SAR measurements of ocean waves with simultaneous data obtained from wave buoys deployed in the site. The wave heights were not large enough to be detected in the first pass over the site, but larger wave amplitudes were present in two subsequent passes that were almost orthogonal and separated by 6 h. The first of these passes showed a system of waves traveling close to the range direction, and secondary wave systems were present in some spectra. Azimuth-traveling waves were seen clearly on only one of the spectra from the second of these passes. All the spectra had the appearance expected from a linear imaging theory, and no significant discrepancies were found between the SAR and buoy measurements of the wavelengths and directions of the dominant wave systems. However, the difficulty in detecting azimuth-traveling waves here may be hard to explain in terms of existing theories, but this is not a firm conclusion because large variations (factors of about 4) in spectral intensity were observed across distances as small as 20 km. This variability means that it is very difficult to determine whether SIR-B was observing the true sea state. Procedures to obtain more accurate tests of sea-imaging theories are discussed.