Infrared Laser Pulse Scattering from Randomly Rough Surfaces

Analytical expression for the two-frequency mutual coherence function of the infrared laser pulse scattering from randomly rough surfaces is presented based on the Kirchhoff approximation. Scattered pulse shapes are calculated at 10.6μm as the Fourier transform of the two-frequency mutual coherent function. It is shown that the root-mean-square height of rough surface greatly influences on results and the scattered pulse power mainly comes from the specular direction with small rms height.     

Bistatic Scattering From a Three-Dimensional Object Over a Randomly Rough Surface Using the FDTD Algorithm

A numerical finite-difference time-domain approach for electromagnetic scattering from an object over a randomly rough surface in three-dimensional (3-D) model is developed. Rough surface is truncated in numerical simulation by using the periodic surface extension, and one period of the rough surface with dependence of the correlation length is used for scattering computation. Generation of the incident wave upon the rough surface is presented. A numerical model and algorithm of a single object on or above a rough surface are developed. Polarized bistatic scattering from the object and rough surface is obtained based on numerical distribution of the near zone fields. Comparison with conventional two-dimensional model is also discussed.     

随机表面光散射的研究和应用综述

随机表面光散射的研究在过去10年中取得了一系列重要的进展,在现代军事、遥感、辐射测量和工业在线检测等领域的应用也日益广泛,本文在简要回顾早期研究成果的同时,侧重介绍了近年来面散射研究的方法、成果和应用。     

一种改进的随机粗糙地表的双尺度微波散射模型研究(英文)

本文提出一种新的双尺度(Modified Two-Scale Model, MTSM)粗糙度地表微波散射模型,采用小波包变换的方法将面上的粗糙度(z(x, y))变换到频域进行处理,粗糙度频谱的低频部分代表大尺度分量,高频部分代表小尺度分量,然后分别用基尔霍夫近似和小扰动模型来分别模拟大小尺度的粗糙度,考虑到小尺度的粗糙度是叠于大尺度粗糙度之上,模型在计算小尺度粗糙度的后向散射贡献时还考虑了大尺度起伏造成的几何倾斜。MTSM的解为大尺度粗糙度的基尔霍夫解加上几何倾斜修正后的小尺度粗糙度的小扰动解。最后采用改进的积分方程模型(AIEM)对MTSM做了初步的验证,结果表明在入射角i30,粗糙度较小(ks=0.354)的时候,MTSM有比较好的精度。      

Modeling of the Bistatic Electromagnetic Scattering From Sea Surfaces Covered in Oil for Microwave Applications

This paper describes the influence of oil pollution over sea surfaces on the height spectrum and the height autocorrelation function of rough surfaces. An oil slick damps the capillarity waves of the surface height spectrum and reduces the root mean square slope of the surface. These modified functions then have an influence on the radar cross section (RCS) from contaminated sea surfaces. The bistatic RCS of the contaminated sea surface is then presented by comparison with a clean sea: results from a benchmark numerical model are presented and compared with a new semiempirical model using the geometric optics approximation and then the first-order smallslope approximation.     

随机粗糙面上目标散射差值计算的快速互耦迭代方法

用粗糙面上方有目标和无目标时空间散射场的差值计算的雷达散射截面,称为差场雷达散射截面.本文推导TE波入射下电场积分方程(EFIE),直接求解散射差场.本文提出目标与粗糙面之间的互耦迭代的计算方法,散射场纳入了目标与粗糙面之间复杂的相互作用,给出了迭代过程中纳入的粗糙面长度的选择.用Monte-Carlo方法,计算了P-M谱粗糙海面上方二维圆柱和方柱的散射,说明目标的几何结构对散射方向图的影响.     

Bidirectional Analytic Ray Tracing for Fast Computation of Composite Scattering From Electric-Large Target Over a Randomly Rough Surface

The bidirectional analytic ray tracing (BART) method is developed to rapidly calculate composite scattering from three-dimensional (3D) electrically large complex targets above a randomly rough surface. Ray tracing is carried out both along the incident (forward) direction and converse direction of scattering (backward) recording different orders of ray illumination on each facet or edge of the target and surface. Once a pair of forward and backward rays meet on a facet/edge, a scattering term is constructed using the diffused scattering/diffraction of this facet/edge and all reflections occurred on the tracing paths. The rough surface is modeled with “rough facets” including coherent scattering and diffused incoherent scattering, which can be directly calculated according to the IEM (integral equation method) of a randomly rough surface. Analytic tracing of polygon ray tubes is developed to precisely calculate the illumination and shadowing of facets, which exempt large patches of the target from any finer meshing. It significantly reduces the complexity relevant to the target electric-size. Higher orders of scattering and, in particular, interactions between the target and rough surface are then taken into account. The accuracy and performance of BART is validated and evaluated by comparing with exact computational electromagnetic methods for electrically small targets. Numerical examples of angularly composite scattering from a three-dimensional electrically large, e.g., a ship-like target over a randomly rough surface are presented and discussed.      

Experimental Study of Laser Bistatic Scattering from Random Deeply Rough Surface and Backscattering Enhancement

The laser bistatic scattering from some deeply rough plane samples are measured by using of the automated scattering measurement system. We observe the backscattering enhancement, which is confined to a narrow cone around the antispecular direction, and discuss the influences of roughness and dielectric properties on them.     

A Two-Step Procedure for Characterizing Obstacles Under a Rough Surface From Bistatic Measurements

A two-step electromagnetic detection procedure is proposed to characterize a dielectric obstacle buried at low depth under a rough surface from single-frequency and multistatic data. First, we have developed, in the framework of the small perturbation theory, a correlation procedure of the scattered field, which enables us to recover an estimation of the roughness profile. This method is tested for various cases with synthetic data provided by a rigorous boundary integral solver. Second, the obtained surface profile is introduced into the numerical simulation due to a finite-element code. An iterative process is then used, based on a level-set formulation, to obtain the shape of the buried target. The influence of the prior step on the accuracy of the reconstruction of the target is studied via various criteria and for different configurations.     

Electromagnetic Scattering From Multilayer Rough Surfaces With Arbitrary Dielectric Profiles for Remote Sensing of Subsurface Soil Moisture

Radar remote sensing of soil moisture content at low frequencies requires an accurate scattering model of realistic soils, which often involves multilayer rough surfaces and dielectric profiles. In this paper, a hybrid analytical/numerical solution to two-dimensional scattering from multilayer rough surfaces separated by arbitrary dielectric profiles based on the extended boundary condition method (EBCM) and scattering matrix technique is presented. The reflection and transmission matrices of rough interfaces are constructed using EBCM. The dielectric profiles are modeled as stacks of piecewise homogeneous dielectric thin layers, whose scattering matrices are computed by recursively cascading reflection and transmission matrices of individual dielectric interfaces. The interactions between the rough interfaces and stratified dielectric profiles are taken into account by applying the generalized scattering matrix technique. The scattering coefficients are obtained by combining the powers computed from the resulting Floquet modes of the overall system. The bistatic scattering coefficients are validated against existing analytical and numerical solutions. Field-collected soil moisture data are then used for numerical simulations to investigate the penetration capability at different frequencies and to address the potential of low-frequency radar systems in estimating deep soil moisture. In particular, soil moisture profiles during dry ground, wet ground, and wet subsurface layer conditions are examined. The results show that both backscattering coefficients and copolarized phase difference at low frequencies are sensitive to the roughness of subsurface interfaces and deep soil moisture. Also, much larger depth sensitivity can be achieved using copolarized phase difference than scattering coefficients     

粗糙表面的多路径散射模型研究

要提高雷达低角跟踪目标的精度,必须建立精确的多路径反射模型.采用小平面模型描述自然散射表面,利用基尔霍夫近似理论建立了适用于任意粗糙表面的低空多路径散射模型,考虑了阴影和多次散射对多径回波信号的影响.基于此模型仿真研究了多路径回波信号与波长及粗糙表面参量间的各种关系;分析了雷达接收到的直达信号和反射信号之间的相关性.     

Bistatic Scattering and Emissivities of Lossy Dielectric Surfaces With Exponential Correlation Functions

Bistatic scattering and emissivities of surfaces with exponential correlation functions are studied numerically for 2-D geometries in a numerical Maxwell model with 2-D simulations. Surfaces with exponential correlation functions are important for the active and passive microwave remote sensing of land surfaces. Because of the fine-scale features with large slopes of such surfaces, numerical accuracy, which is particularly important for the calculation of emissivity in passive remote sensing, is ensured by a variety of procedures in this paper. The rooftop function and Galerkin's method with numerical integration of near-field impedance matrix elements are used. Cubic spline interpolation is employed to connect knots on random rough surfaces. Numerical accuracy convergence tests are performed for numerical solutions of Maxwell equations by varying the number of points from 13 to 103 points per wavelength in the dielectric medium corresponding to 50-400 points per free wavelength. Surface lengths of up to 100 and 200 free wavelengths and root mean square heights of up to 0.4 and 0.8 free wavelengths, respectively, are used at 5 and 10 GHz to capture all the essential features. Because of the large number of surface unknowns (up to 80 000), the multilevel UV method is further used to accelerate the matrix equation solver. Numerical results are illustrated for both bistatic scattering and emissivities as functions of frequencies and incidence and scattering angles for cases of interests in microwave remote sensing. Comparisons are made with the second-order small perturbation method and Kirchhoff's approximation to reestablish the regimes of validity of these methods     

一维随机粗糙表面散射特征的数值分析

本文在Ｋｉｒｃｈｈｏｆｆ近似基础上，利用数值积分方法分析了一维随机粗糙表面的散射特征。结果显示随着粗糙度的增大，单次散射峰值由连续地向后向移动，其强度不断降低。峰值宽度增加。在一个较窄的粗糙度范围内，散射场对称于表面法线呈近似余弦分布而接近于理想朗伯体。当粗糙度超过这一范围时，散射峰值将由线向后向移动，并在粗糙度很大时稳定在后向附近。     

A Radar Signature Model for Partially Coherent Scattering from Irregular Surfaces

The radar signature of irregular surfaces, relevant to remote sensing of the earth, represents the characterstics of the electromagnetic field components associated with monostatic radar backsattering. These characteristics are expressable in terms of amplitude, polarization, phase and time dependence. The irregular nature of the objects of interest caues the scattered field components to be random functions. The majority of experimental and theoretical studies of radar signatures deal therefore with the estimated ensenble average of the scattered power rather than the actual electromagnetic field components, the average power being expressed by the radar cross section per unit area. Normalizing the radar cross section with respect to unit area elements implicitly asumes that the average backscattered power from individual surface elements adds incoherently. The use of the radar cross section per unit area concept is therefore limited to those caes where it is ascertained that the uminated area is much larger than the width of the spatial autocorrelation function of the random surfaces involved. However, with present day high resdution radars for monitoring land and sea as well as with short range scatterometry this requirement will not aways be satisfied. Therefore, partial coherence needs to be considered for these caes. In the present paper an electromagnetic scattering model is presented in which spatial correlation beyond the contour of the iluminated area is taken into account. Specifically, two scale surface roughness is considered in which small scale imregulaities are riding on gentle undulations.     

随机表面上的多次光散射与交叉退偏

本文根据Ｋｉｒｃｈｈｏｆｆ近似下的多次散射理论和Ｍｏｎｔｅ Ｃａｒｌｏ数值模拟方法,分析了一维Ｇａｕｓｓ分布强粗糙表面散射场的偏振特征。数值结果说明,单次散射仅在散射角很大时产生非常有限的交叉退偏;后向附近的交叉退偏完全是在多次散射的过程中,场的两个正交分量发生不同位相变化而产生的。     

粗糙表面电磁散射系数数字方法分析与比较

回顾了随机粗糙表面电磁散射特性计算方法的特点,分析对比了各类积分方程法和微分方程法的核心算法,着重讨论了矩阵分裂算法的计算效率,以及时域有限差分法求解色散粗糙面宽带散射特性的有关问题,指出了粗糙表面散射系数的计算和选择方法。     

考查一维随机分布表面高次散射程度的随机分布闪耀光栅近似法

本文提出了将Ｇａｕｓｓ－起伏,Ｇａｕｓｓ－相关的一维随机粗糙散射表面近似成连续三角槽型的面型分布的数值分析方法;通过粗糙度值、高度概率密度分布函数及相关函数等诸参量的模拟统计分析,验证了此方法的可靠性;给出了此方法在解决起伏剧烈的大粗糙度随机表面上形成的散射场分布之数值模拟计算方面的应用前景。     

On the Scattering of Electromagnetic Waves by Periodic Rough Dielectric Surfaces: A BOA Solution

A new approach for the scattering of electromagnetic (EM) waves from periodic dielectric rough surfaces is addressed. The method is an extension of the buried object approach (BOA), which is developed for rough surfaces of infinite extend, to the present problem. The BOA allows to model the original problem as the scattering of EM waves from cylindrical objects located in a two-half-space medium with planar interface. Then, the problem is reduced to the solution of a Fredholm integral equation of second kind through the periodic Green's function of two-half-space medium. The periodic Green's function of two-half-space medium is calculated via the Floquet mode expansion, whose numerical evaluation can be accelerated by using effective methods. The method can also be used to solve the scattering problems of rough surfaces of infinite extend and having a localized roughness. Numerical simulations show that the method yields effective and accurate results for surfaces of arbitrary variation.      

一体化双站散射图形算法

研究了图形算法计算双站RCS的方法，并将RCS分析与目标外形设计相结合，提出了一种一体化方法，将图形算法集成到CAD造型软件中，能够在目标外形设汁阶段随时分析其双站散射特性。计算实例说明．这种方法不但能够满足计算精度要求，而且可以简化RCS的前处理工作，提高目标设计的总体效率。     

A Physical-Optics Model for Double-Bounce Scattering From Tree Stems Standing on an Undulating Ground Surface

In this paper, a model for prediction of radar backscatter from coniferous forests in the VHF and UHF band is proposed. The model includes the double-bounce scattering originating from vertical stems standing on an undulating ground surface and is based on a physical-optics approach. The model can be used to assess the importance of ground topography in synthetic aperture radar (SAR) imagery of forests, and it is applicable to SAR systems using horizontally transmit and receive polarization (HH). The model was validated against data from the airborne SAR systems CARABAS-II and LORA. Precision measurements of ground topography and forest characterization at a single tree level were used as model input to simulate SAR images. The simulated images were compared to radar data in the frequency bands 22–82 and 225–470 MHz, and it was found that the model could predict much of the variation in backscatter observed in images ($R^{2} = hbox {0.44}$ and 0.65 at best, for the lower and higher frequency band, respectively), which should be compared to $R^{2} approx hbox {0.1}$ if the same model, but assuming a flat ground, was used. The results thus indicate that ground topography must be considered when predicting the variations in backscatter in the SAR images studied. The model did, however, fail to predict the absolute values of the backscattered intensity. The reason for the discrepancy is believed to be the value chosen for stem dielectric constant and unmodeled effects due to wave attenuation, tilting stems, and small-scale surface roughness.