基于分形结构的植被高阶相干散射模型研究

High-order Coherent Scattering Model for Vegetation with Fractal Structures

电磁波低频入射情况下,植被中散射体独立不相关的假设无效,此时应考虑散射体之间的相干效应以及近场互作用。该文提出一种基于分形结构的植被高阶相干散射模型,该模型利用分形理论生成近乎真实植被的3维几何结构,根据每个散射体的空间位置信息考虑了相干效应,应用互易定理计算了相邻散射体间高阶互作用,结合非相干的分层模型中后向散射机制划分方式,给出了各项散射机制的表达式。与机载合成孔径雷达实验数据对比,验证了模型的准确性。在针叶林仿真参数下,分析了各项散射机制对总散射效应的贡献与入射频率、角度、植被结构的关系,结果表明,低频入射条件下,稀疏植被散射模型可进一步简化从而应用于参数反演中。

At low frequency, the assumption of independent scattering of the scatterers in vegetation medium is no longer valid. The coherent effect and near field interactions should be considered. In this paper, a high-order coherent scattering model for vegetation with fractal structure is presented. The fractal theory is employed to generate a realistic 3-D spatial structure of vegetation. The near field interaction between scatterers is formulated using an efficient algorithm based on the reciprocity theorem. For the coherent effect, every scatterer with a deterministic location is taken into account. The main scattering mechanisms are defined in the way of layered vegetation model, allowing better understanding of microwave interaction with trunk-crown structure. Good agreements are obtained from the comparisons of the theoretical predictions with the multifrequency and multipolarization measurement results of boreal forest. Through an extensive ground truth, theoretical analysis of the contribution of the scattering mechanisms for various frequencies, incident angles and vegetation structures is carried out. It is found that under specified conditions the vegetation scattering model can be simplified according to the main contribution scattering mechanism which can be applied to the inversion issue.