ON THE FOCUSING ISSUE OF SYNTHETIC APERTURE RADAR IMAGING OF OCEAN WAVES

It is now widely accepted that the imaging of ocean surface waves by synthetic aperture radar (SAR) can be adequately described by velocity bunching theory in conjunction with the two-scale wave model. However, it has been conjectured that this theory is incapable of explaining why, under certain conditions, the image contrast of airborne SAR imagery of ocean waves can be enhanced by defocusing the SAR processor. If this were true it would raise serious doubts about the validity of the velocity bunching theory to describe the SAR imaging of ocean waves. In this paper the velocity bunching theory is defended. It is shown that image contrast enhancement by defocusing can also be obtained by this theory, which does not require the introduction of the phase or group velocity of the long ocean waves as a basic element of the SAR imaging theory. These velocities only occur in conjunction with scanning distortions and wave pattern shifts between different incoherent looks. The fact that maximum image contrast is often encountered at focus settings corresponding to the phase or group velocity of the dominant ocean wave is purely accidental. In our theory focus sensitivity results from two motion-induced contributions to the SAR imagery mechanism, which are influenced by changes in focus setting: velocity bunching and azimuthal resolution. Focus sensitivity is demonstrated by carrying out two-dimensional Monte Carlo simulation studies using the SAR parameters of the Jet Propulsion Laboratory airborne L-band SAR and ocean wave parameters encountered during the Tower Ocean Wave and Radar Dependence (TOWARD) experiment, conducted off the coast of California in 1984.