Synthetic-aperture-radar imaging of azimuthally propagating ocean waves.

摘要

Synthetic-aperture-radar (SAR) imaging of ocean waves is investigated using the slightly-rough facet model of the ocean surface. The facets are mapped through the SAR processor individually, and their responses are combined coherently in the image domain to give a simulated image. The statistics of the facets, including expected size and coherence time of their radar backscatter, are determined from an expansion of the well known two-scale scattering model of a rough surface. Analysis of tower-based scatterometry data shows that hydrodynamic modulation of the radar cross-section (RCS) of the surface by the large-scale gravity waves is significant when compared with tilt-modulation effects. Empirically derived RCS profiles along a dominant, azimuthally propagating long gravity wave are included in the image simulations. Dividing the SAR integration time into subwindows allows the full surface-wave spectrum to be included in the simulations deterministically; no sense coherence time is required.; The simulated images accurately show many of the features observed in actual SAR images of ocean waves. When imaging a fully developed, azimuthally directed spectrum of waves, the peak of the image spectrum is shifted toward a lower wave number (when compared with the wave height spectral peak). Moreover, azimuthally directed swells whose wavelengths are large compared to the theoretical SAR resolution are often not imaged. This "azimuthal cutoff" effect depends on the SAR parameters, the local surface wind speed, and the swell wavelength and amplitude, and is due to random smearing of the image response of the individual facets resulting from locally wind-generated intermediate waves. As the wind speed increases, the image response to the swell (compared to the background clutter) is reduced. The image response to the swell does not depend linearly upon the swell amplitude.; The contrast of an image of an azimuthally propagating dominant wave (or swell) can be increased by the adjusting the focus of the SAR processor. The focus adjustment compensates the incoherent propagation of the dominant wave during the integration time of the SAR processor (incoherent propagation, meaning no Doppler shift corresponding to the phase velocity of the long wave is introduced in the radar returns). Thus, the optimal focus adjustment is one half the phase velocity of the dominant wave. The focus adjustment is most successful when imaging low-amplitude waves. With higher wave amplitudes, using multilook processing and shifting the individual looks (by the distance the dominant wave propagates between looks) before incoherently averaging them is more effective.