Ding Nie

Study of Microwave Backscattering From Two-Dimensional Nonlinear Surfaces of Finite-Depth Seas

This paper presents a study of the microwave backscattering from 2-D time-evolving nonlinear surfaces of a sea with finite depth by using the second-order small-slope approximation. According to the shallow-water dispersion relation, the revised nonlinear hydrodynamic choppy wave model in connection with an experiment-verified sea spectrum for finite-depth water is employed to construct the wave profiles in the finite-depth sea. The numerical results show that the discrepancy between the choppy surfaces of the infinite-depth sea and their finite-depth counterparts for monostatic normalized radar cross section is much smaller than that between the linear surfaces and the nonlinear choppy surfaces. Furthermore, the comparison of the Doppler spectra of the backscattered echoes from the linear and nonlinear choppy sea surfaces shows that the nonlinear hydrodynamic features significantly impact the Doppler spectrum. In particular, the Doppler spectrum for nonlinear finite-depth sea presents much higher second-order peaks and increased spectral amplitudes in the frequency range around the Doppler peak frequency, which reiterates the importance of the role that the nonlinear hydrodynamic effect of waves played in the interpretation of backscattering from finite-depth nearshore seas from the qualitative point of view.

Robust Semi-Deterministic Facet Model for Fast Estimation on EM Scattering from Ocean-Like Surface

A robust semi-deterministic facet model for the computa- tion of the radar scattering cross section from the ocean-like surface is presented. As a facet-based theory, it is a more comprehensive model which can re∞ect the specular and difiuse conflgurations, as well as the mono- and bistatic features. Signiflcant computational e-ciency and good agreement with experimental data are observed, which makes the proposed facet model well suitable for fast estimation on EM scatter- ing and synthetic aperture radar (SAR) imagery simulation of marine scene.

EM scattering calculation of large sea surface with SSA method at S, X, Ku, and K bands

Abstract The practical method, small slope approximation is employed to analyze EM scattering from sea surface. Considering the simulation of EM scattering from actual scene, the size of sea surface is very large and it should be much larger than the target’s size if a target exists. However, the memory consumption is huge and always larger than the existing memory of personal computer (PC) when EM frequency is at X band or higher band. Here, the spectral decomposition modeling (SDM) is proposed to decrease memory consumption without reducing efficiency. The SDM method uses quite less memory to keep the data of large sea surface, so that the memory insufficient problem can be avoided. The scattering coefficients and Doppler spectra of the sea surfaces generated by the proposed method fit well with the results calculated by general method when interpolation method with four points is applied. Moreover, higher efficiency can be obtained by applying the compute unified device architecture-based parallel method for the SDM method involves less data to transmit than the general method.

Facet-Based Investigation on Microwave Backscattering From Sea Surface With Breaking Waves: Sea Spikes and SAR Imaging

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.

Spectral Decomposition Modeling Method and Its Application to EM Scattering Calculation of Large Rough Surface With SSA Method

The small slope approximation (SSA) method is a practical method to calculate the electromagnetic (EM) scattering from rough surfaces. However, the SSA method requires that the interval for sampling surfaces must be small enough, such as less than one-tenth of incident wavelength. This constraint condition will cause the problem of huge memory consumption and insufficient memory when the EM scattering of large rough surfaces is calculated. Although the hard disk has large space to keep data and can solve the insufficient memory problem, its read/write speed is still too slow. In addition, massive data transmission will reduce the computational efficiency for the compute unified device architecture (CUDA) parallel computation under some conditions. In this paper, the main idea of the spectral decomposition modeling method is that the whole spectrum of rough surface is divided into several parts and these parts can be used to generate different-scale rough surfaces. Then, by analyzing the different-scale rough surfaces, the large rough surface can be achieved and applied to the calculation of EM scattering with the SSA method. Due to the small memory consumption of different-scale rough surfaces, it takes less time to translate data for the different-scale rough surfaces than that for the standard large surface. Thus, the spectral decomposition modeling method could readily be applied to CUDA parallel computation.

Study on Doppler spectra of two-dimensional skewed sea surfaces

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.

A versatile composite surface model for electromagnetic backscattering from seas

A versatile composite surface model (VCSM) is presented for estimation of the electromagnetic backscattering coefficient of the sea. Taking into account the statistical characteristics of the sea surface and the validity conditions of component models for the small-scale and large-scale surfaces in the composite surface model (CSM), a method for the two-scale decomposition of sea surfaces is introduced. On this basis, the cutoff wavenumber with wind speed dependence and incident wave frequency dependence is applied to separate the sea spectrum into large- and small-scale components at different sea states with increased accuracy. Then, numerical results of the backscattering coefficient are evaluated and discussed in the case of different wind speeds, polarizations as well as incident frequencies. Finally, the VCSM is verified through the comparisons with the available experimental data, and the comparisons of the VCSM results and the classical CSM results also show that the VCSM behaves better.

Improved GO/PO method and its application to wideband SAR image of conducting objects over rough surface

Abstract To simulate the multiple scattering effect of target in synthetic aperture radar (SAR) image, the hybrid method GO/PO method, which combines the geometrical optics (GO) and physical optics (PO), is employed to simulate the scattering field of target. For ray tracing is time-consuming, the Open Graphics Library (OpenGL) is usually employed to accelerate the process of ray tracing. Furthermore, the GO/PO method is improved for the simulation in low pixel situation. For the improved GO/PO method, the pixels are arranged corresponding to the rectangular wave beams one by one, and the GO/PO result is the sum of the contribution values of all the rectangular wave beams. To get high-resolution SAR image, the wideband echo signal is simulated which includes information of many electromagnetic (EM) waves with different frequencies. Finally, the improved GO/PO method is used to simulate the SAR image of targets above rough surface. And the effects of reflected rays and the size of pixel matrix on the SAR image are also discussed.

Spectral Investigation of Doppler Signals From Surfaces With a Mixture of Wind Wave and Swell

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.

Characterization of Electromagnetic Scattering From Water-Depth-Changed Nearshore Sea Surface

A novel type of sea surface taking the influence of changed water depth toward the shore into account is presented based on the newly proposed depth-limited spectrum, and characterization of electromagnetic (EM) scattering from such nearshore shallow sea surface is performed by using the second-order small-slope approximation theory (SSA-II), which is proved to be suitable for studying EM scattering from electrically large dynamic sea surface. A deep-sea surface model is applied to verify the SSA-II model by comparing to exact numerical solution. Then, the coherent and incoherent components of bistatic scattering from water-depth-changed sea surface are evaluated, and relevant numerical results for deep sea and shallow sea with constant water depth are both included to contrastively show the scattering characteristics of sea waves in depth-changed marine environment.