A complete facet model for the backscattering from rough sea surface with breaking waves is proposed in this paper. In consideration of the spatial distribution of breaking waves on sea surface, the model is able to give a good interpretation to the “super events” under high sea states, such as sea spikes and high polarization ratios. Based on the proposed model, normalized radar cross section plots of sea surface under backscattering configuration are calculated and compared with measured data. The comparisons show that the proposed model is tractable to estimate the scattering from electrically large ocean surface under high sea states with accuracy and efficiency. In addition, the non-Gaussian statistics and spatial correlation properties of sea clutter are analyzed under different range resolutions and incident angles. The results show that high kurtosis value, which means a sea spike phenomenon, mostly happens in lower grazing angle and higher range resolution cases. The comparisons of simulated SAR images of sea surface with and without breaking waves also reveal that sea spikes and high polarization ratios are caused by breaking waves. All the simulation results prove that our model not only is able to explain the physical mechanism of the scattering but also can be applied to the analyses of statistical properties of sea clutter under high sea states.

Traditional wake detection methods have been successfully applied to the detection of a simple linear ship wake. However, they cannot effectively detect nonlinear wake and weak wake under high sea state conditions, whereas the deep-learning-based detection method could play to its strengths in this respect. Due to the lack of sufficient measured SAR images of ship wake to meet the training requirement for deep learning method, this paper explores the method to detect the nonlinear ship wake by combining electromagnetic scattering model with deep learning technique. The composite scene model of the sea surface and its wake is established first, then the facet scattering distribution of the ship wake and the sea background is analyzed with the help of the electromagnetic scattering model, and the simulation of the wake SAR images under the sea background is finally accomplished based on the modulation model. Combined with the simulation results and measured wake SAR images, the sample database is constructed. The You Only Look Once Version five algorithm (YOLOv5) based on deep learning techniques is applied to detect the wake target in complex conditions such as different sea states, multiple targets, curvilinear wakes, and weak wakes. The result show that the YOLOv5 leads to an obvious higher detection efficiency with satisfactory accuracy. Moreover, the comparison between YOLOv5 and the traditional Radon transform method for detecting nonlinear wakes in a strong noise background shows that the proposed method is better than the traditional object detection model. Thus, the proposed scheme would be a practical tool to deal with the detection of nonlinear ship wake and weak wake in complex scenarios, which will be helpful to the further remote sensing investigation of the ship.

The mixture of wind wave and swell is simulated based on a bimodal spectrum and nonlinear hydrodynamic theory, and the investigation of the Doppler spectra of backscattered signals from such sea surfaces in time-varying marine environment is performed by applying the second-order small-slope approximation model. The results are also compared with some popular data sets, and based on the comparison it can be found that the variation trends of Doppler shift and Doppler spectral bandwidth are different for mixed waves with various weights, which reflect qualitatively the distinct ways of influences that wind wave components and swell ones exert on Doppler signature.

Small slope approximation (SSA) is a widely accepted approach in sea surface electromagnetic (EM) scattering studies. Nevertheless, the spatial sample interval used for sea surface should be around or even smaller than one-eighth of the incident wavelength to ensure EM scattering calculation accuracy, which requires a huge amount of computation, creating an obstacle to scattering numerical simulation, especially for high microwave band incident waves and large sea surface scenes. In this paper, a novel realization approach for SSA is proposed to significantly decrease the computation demands and computer memory requirements in sea surface scattering simulation. First, the sea surface is decomposed into two scales, and each scale has its own spatial sample interval. Then, the inclination state of the large-scale sea surface is determined under a specific wind speed. After that, scattering calculations of a typical surface cell with a finely sampled structure are completed and saved in all possible situations. Finally, scattering results for all the cells of a concrete sea surface are extracted from the saved cell scattering data base. From the different kinds of scattering result comparisons, it is demonstrated that this novel SSA realization approach can attain almost similar scattering results to exact SSA. This approach can be broadly applied in composite scattering studies, and remote sense imaging simulation of large sea surfaces with multiple targets.

This paper focuses on improving the understanding of the electromagnetic (EM) scattering from water waves modulated by shoaling effect arising from different upslope submarine topography in the nearshore zone. Simulation of EM bistatic scattering from water waves in the nearshore zone where the water depth becomes gradually shallower is presented. The shallow water waves with periodic sinusoidal type are simulated by the model that is based on Boussinesq approximation theory, which can accurately construct the detailed wave structure in the nearshore zone, and the analytical model-the second-order small slope approximation is applied to modeling the EM bistatic scattering from the shallow water waves. Comparisons of the bistatic scattering coefficient (BSC) between water waves corresponding to different submarine plane-upslope topography in the nearshore zone have shown that the angular position of harmonics of BSC will change as upslope varies: higher-order harmonics for plane-upslope cases would be far away from the specular peaks for flat topography case. Moreover, the wave height of the shallow water waves and nonlinear effect will greatly impact changing trend of the BSC. Simulations for TMA spectrum-based water waves also support the fact that the transfer of wave energy from low frequency to higher frequency wave components would be intensified in the shallow water zone with the upslope submarine topography. The results in this paper indicate that shallow water waves would reflect some interesting scattering features that traditional deep-sea waves didnt show, which will be helpful to the further quantitative investigation of scattering characteristics of nearshore waves.

This paper deals with the effects of horizontal skewness on Doppler spectra. A.K. Fung et al. pointed out that the crests of the sea waves are tilted towards the wind direction, which is referred to as horizontal skewness. Although the nonlinear sea surface is more realistic than the linear one, it cannot embody the statistical front-back asymmetry of real sea waves due to the influence of the wind. Thus, the impact of horizontal skewness must be considered. The first-order modified Lagrange model for asymmetric ocean waves is combined with Monte Carlo method to produce the two-dimensional (2-D) time-varying skewed sea surfaces which are nonlinear and skewed simultaneously. Moreover, the second-order small-slope approximation method is used to calculate the scattering field and study the Doppler spectral characteristics from skewed sea surfaces. Horizontal skewness makes Doppler spectra broader in upwind case, but narrower in downwind case. In addition, the impact of horizontal skewness on Doppler spectra is dependent on wind direction and incident elevation angle.