However, additional Doppler shift due to the orbital velocity of sea waves would cause the velocity bunching (VB) effect, which results in azimuth displacement and smear in the SAR images, and the measured SAR image of the ocean waves would thus not represent the actual ocean wave scene. The high-resolution in the azimuth direction is obtained based on the Doppler history of the target signal. SAR obtains high range resolution through the pulse compression technique. By contrast, synthetic aperture radar (SAR) has been applied widely in ocean remote sensing because of its capability to provide high-resolution and wide-swath images of the sea surface in all-day and all-weather conditions. Traditional measurement methods of ocean waves such as moored buoys and shipboard radar are limited in their observation area. Ocean wave observation has a pivotal role in ocean engineering development, marine transport, aquaculture, and so on. From the simulated SAR image spectrum, it can be found that the azimuth wavenumber is cut off when the VB effect is considered in the simulation process, and the azimuth cut-off wavelength increases with the range-to-velocity ratio. The imaging results indicate that the texture feature of the wind waves would be severely damaged due to the VB effect, while the texture of swells in the simulated SAR images may not be damaged or even becomes clearer. Based on the numerical simulation method proposed by us, SAR images of ocean waves for different marine environments and radar platform parameters are simulated. The comparisons demonstrate that the characteristics of simulated SAR images, such as the intensity distribution and the image spectra, are consistent with those of actual RADARSAT-2 SAR images. To verify the reliability of the simulation method, the simulated SAR images using the parameters of the RADARSAT-2 SAR, the corresponding wind wave information measured by an in-situ buoy, and the reanalysis wave spectra have been compared with the actual RADARSAT-2 SAR images.
Not only are the modulation of ocean waves, speckle noise, and temporal decorrelation of the small-scale waves considered, but the velocity bunching (VB) effect caused by the motion of large-scale waves is also effectively added to the simulation of the SAR echo signal. Based on the simulated signal, a numerical simulation method of synthetic aperture radar (SAR) imaging for time-varying sea surfaces is proposed, which is helpful to study the SAR imaging mechanism of time-varying sea surfaces so as to better extract ocean wave parameters from SAR images.
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