摘要
随机粗糙面的电磁散射理论和计算近年来发展很快，它在雷达探测、遥测遥感，海上营救及通信等领域有着广泛应用。研究方法主要有近似方法和数值方法。近似方法主要是基尔霍夫近似和微扰法近似，这种方法公式简单，计算速度快，但精度较低；数值方法主要是矩量法、时域有限差分法及各种快速算法，其精度高，仿真结果能更准确地反映实际情况。
本文比较了描述海面的几种常见的海谱，给出了求解海谱参数的方法。研究了海面模拟的统计模型和分形模型，并利用Monte Carlo方法模拟了高斯谱海面和P-M谱海面；利用带限Weierstrass分形函数建立不同分维的粗糙海面。采用时域有限差分方法和改进的单轴各向异性完全匹配层，计算了海面上雷达目标的电磁散射，为海面雷达目标的散射特性分析提供了参考。
在边界条件设置上，本文舍弃了传统的Mur吸收边界和Berenger提出的完全匹配层(PML)吸收边界条件，采用一种改进的单轴各向异性材料完全匹配层(UPML)，大大提高了匹配层的吸波性能。通过对电场、电位移矢量、磁场、磁通密度矢量进行归一化，引入D-B迭代替代传统的E-H迭代方法，并利用Z变换技术，在保证计算精度的基础上简化了计算。文中利用该方法分析了波源的辐射和无限长导体方柱的散射并与传统矩量法(MOM)比较，验证了改进吸收边界的效果。给出改进方法在二维、三维散射中的应用。数值结果验证了改进算法的有效性。
最后利用FDTD方法分析了海平面的散射，并与采用高斯谱和P-M谱函数模拟的随机粗糙海面及利用带限Weierstrass分形函数模拟的海面的电磁散射进行比较，同时分析了入射角对海面雷达散射截面的影响、不同均方根高度对计算结果的影响。由于随机粗糙面与目标的复合散射特性在军事和民用上的广泛应用，文中采用海面与海面上方无限长金属方柱进行复合散射模拟，并分析了目标的存在对海面后向散射的影响。
Researches on electromagnetic scattering of rough surfaces have developed quickly in recent years. It has abroad application in radar detect, remote sensing, salvage, communication and so on. Method of researches includes approximate methods and numerical method. Approximate methods such as Kirchhoff approximate, small perturbation method(SPM) is simple and quick but low precision. Numerical methods such as MOM, FDTD is precise and the simulation is more exact.
In this paper, some familiar sea-spectrum are introduced, method of calculate sea-spectrum parameter is also given. Then the methods for modeling sea surfaces are outlined, including models and fractal models. According to this paper, One-dimensional rough sea surfaces are created by the method of Monte Carlo technology and Weierstrass-Mandelbort fractal function. In the end, scattering of targets above the sea is calculated using FDTD and UPML, which gives a suggestion to scattering characteristic of targets on ocean like surfaces.
As for absorbing boundary condition, UPML is used instead of Mur absorbing boundary and PML. And the computing of the FDTD is simplified by the Z transformation of digital signal processing. D-B method is uses instead of E-H method, which is more suitable for dealing with dispersive material. Based on this method, radiation of wave source and scattering of infinite conductor square cylinder is analyzed, compared with MOM. In addition, the application of improved absorbing boundary condition using in 1D and 2D is given. The numerical experiments have verify this method.
In the end, electromagnetic scattering of sea level is analyzed compared to rough surfaces caused by Gauss spectrum P-M spectrum and band-limited Weierstrass fraction function. We also pointed out the influence of incident wave angle and the root-mean-square high of the sea. Due to the application of compound scattering of rough surface and the goals above in military and civil affairs, the compound scattering between sea surfaces and targets above sea surfaces is studied. According to the result, it is found that the existence of the goals have great influence to back electromagnetic scattering.
引文
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