从BRDF到BPDF:遥感反演基础模型的演进初探

From BRDF to BPDF:a premilinary study on evolution of the basic remote sensingquantitative inversion model

光的本质是横波电磁波矢量,以双向反射分布函数BRDF (bidirectional reflectance distribution function)为基础模型的标量遥感体系只利用植被反射光的整体强度信息,无法进一步区分反射辐射蕴含的叶表、叶内和冠层结构信息;在标量遥感基础上考虑垂直于电磁波传播方向的二维偏振矢量特性(强度+方向)就能深化为矢量遥感体系,从而有望精确刻画反射光中蕴含的多种信息,提升植被参数反演精度.刻画偏振反射空间分布的双向偏振分布函数BPDF目前存在精度低、泛化性差等问题,因此进一步探索矢量遥感基础理论,构建通用性强的植被矢量遥感基础模型具有迫切的现实需求.本文旨在利用光子与植被元素的相互作用构建通用性较强的植被BPDF物理模型.首先基于光子–植被元素相互作用的光谱不变原理,提出基于方向逃逸概率的植被BPDF物理模型基础形式,并基于植被单次反射的辐射传输理论推导模型解析表达;随后通过考虑叶片散射随干物质含量的变化,提出光谱不变原理优化表达并推导模型通用表达式;最后分别利用三维矢量辐射传输模型和多尺度实测数据实现模型的正演直接验证和间接验证.结果表明,本文构建的BPDF物理模型的解析表达和通用表达在不同植被场景下,对偏振反射率的正演均方根误差可达0.001以内,与矢量辐射传输模型正演结果在半球空间内具有很强一致性,在多尺度实测数据也简洁验证了模型关系的稳定性,模型R2普遍高于0.9.相比于现存植被偏振反射模型,本文构建的模型兼具物理机理、简洁形式、可接受的参数化方案、较高精度和较强泛化能力,对浓密植被具备通用性,为在BRDF一维标量遥感的基础上进一步考虑BPDF二维偏振特性从而形成矢量遥感体系这一转变过程提供了理论基础和探索性方案.

The essence of light is shear wave electromagnetic wave vector.The scalar remote sensing system based on the bidirectional reflectance distribution function(BRDF)model only uses the overall intensity information of refected light from vegetation.The structural information of leaf surface,leaf,and canopy contained in reflected radiation cannot be distinguished.On the basis of scalar remote sensing,considering the two-dimensional polarization vector characteristics(intensity and direction)perpendicular to the propagation direction of electromagnetic waves,it can be deepened into a vector remote sensing system,which is expected to accurately describe a variety of information contained in the reflected light and improve the inversion accuracy of vegetation parameters.The bidirectional polarization distribution function(BPDF),which describes the spatial distribution of polarization refection,has the problems of low accuracy and poor generalization at present.Therefore,it is urgent to further explore the basic theory of vector remote sensing and construct the basic model of vegetation vector remote sensing with strong universality.In this paper,we aim to use the interaction between photons and vegetation elements to construct a more general physical model of vegetation BPDF.Firstly,based on the spectral invariant model of photon-vegetation elements interaction,the basic form of vegetation BPDF physical model based on directional escape probability is proposed,and the analytical expression of the model is derived based on the radiation transfer theory of vegetation single reflection.Then,by considering the variation of leaf scattering with dry matter content,the spectral invariant model was optimized and the general expression of the model was derived.Finally,the 3D vector radiative transfer model and multi-scale measured data were used to realize the direct and indirect verification of the model.The results show that the root mean square error of the forward polarization reflectance of the analytical expression and the general expression of the BPDF physical model constructed in this work is within 0.001 under different vegetation scenes,which is consistent with the forward results of the vector radiation transfer model in the hemisphere space.The stability of the model relationship is also briefly verified by the multi-scale measured data.The model R2 is generally higher than 0.9.Compared with the existing vegetation polarization refection model,the model constructed in this paper has a physical mechanism,simple form,acceptable parameterization scheme,high accuracy,and strong generalization ability,and is universal for dense vegetation.It provides a theoretical basis and an exploratory scheme for the transformation process of further considering the two-dimensional polarization characteristics of BPDF to form a vector remote sensing system based on BRDF one-dimensional scalar remote sensing.