C波段极化合成孔径雷达海面后向散射特性及风场反演研究

Coastal wind is strongly influenced by topology and the discontinuity between land and sea surface. Wind assessment from remote sensing in coastal regions thus remains challenging. Space-borne scatterometer does not provide any information about the coastal wind field, as the coarse spatial resolution hampers the radar backscattering. Synthetic aperture radar (SAR) with a high spatial resolution and all-weather observation ability has become one of the most important ways for ocean surface wind observation, especially in the coastal area. Conventional wind field retrievals from SAR, however, always need wind direction as initial information, such as from numerical weather prediction models (NWP) which may not match the time when SAR image is acquired. The polarimetric observations of SAR make the independent wind retrieval from SAR images become reality. In this project, the relationship between C-band polarimetric SAR backscattering and ocean wind field will be quantitatively analysed and a new method of using co-polarization backscattering coefficients from polarimetric SAR observations up to polarimetric correlation backscattering coefficients which are acquired from the conjugate product of co-polarization backscatter, and cross-polarization backscatter is proposed to obtain the coastal wind field. Co-polarization backscattering coefficients and polarimetric correlation backscattering coefficients are obtained form single-look complex (SLC) data, and the maximum likelihood estimation is used to gain the initial results followed by the coarse spatial filtering and fine spatial filtering. In comparison to the previous methods, the proposed research will be based on the SAR data itself to obtain the wind vectors and does not need external wind directional information. High spatial resolution and high accuracy are the most important features of this project since the full use of polarimetric observations which contained more information about the objects. This project is an useful attempt to the work of independent SAR wind retrieval. The preliminary experimental results show that it is feasible to employ the co-polarimetric backscattering coefficients and the polarimetric correlation backscattering coefficients for coastal wind field retrieval.

准确获取风向信息是合成孔径雷达(SAR)风场反演的难点，极化SAR 包含目标物更加丰富的信息使独立SAR风场反演成为可能，为此必须深入研究极化SAR海面后向散射随风场的变化特性。定量化地分析同极化后向散射系数、交叉极化后向散射系数以及极化相关后向散射系数在不同风速、风向、入射角下的变化特征是本项目研究的重点，并以此为基础开展C波段极化SAR风场反演算法研究，提出利用C波段极化SAR观测数据中同极化后向散射系数及由同极化与交叉极化后向散射信号所获得的相关极化后向散射系数协同进行海面风场反演的方法。本项目从C波段极化SAR单视复数据获取同极化和极化相关后向散射系数，利用最大似然估计以及空间滤波等方法获得海面风矢量,与以往SAR风场反演方法相比，本项目完全由C波段极化SAR数据本身获得风矢量，不依赖于外部风向信息，最终反演结果与微波散射计、浮标观测以及数值预报模式风场进行对比分析和真实性检验。

全极化SAR数据的应用使基于SAR影像的独立风场反演成为可能，由风场引起的粗糙海表对雷达入射电磁波的后向散射具有对称性，通常同极化与交叉极化后向散射系数关于相对风向呈偶对称（一般交叉极化后向散射系数要小两个数量级），而由同极化与交叉极化后向散射信号作相关运算获得的极化相关后向散射系数关于相对风向呈奇对称。利用这一特性，可以从多极化或全极化SAR数据中直接获得风场信息，而不依赖于外部初始风向。为此必须深入研究极化SAR海面后向散射随风场的变化特性。定量化地分析同极化后向散射系数、交叉极化后向散射系数以及极化相关后向散射系数在不同风速、风向、入射角下的变化特征是本项目研究的重点，并以此为基础开展C波段极化SAR风场反演算法研究，利用C波段极化SAR观测数据中同极化后向散射系数及由同极化与交叉极化后向散射信号所获得的相关极化后向散射系数协同进行海面风场反演的方法。本项目从Radarsat-2全极化单视复数据获取同极化和极化相关后向散射系数，利用最大似然估计以及空间滤波等方法获得海面风矢量,与以往SAR风场反演方法相比，本项目完全由C波段极化SAR数据本身获得风矢量，不依赖于外部风向信息，最终反演结果与微波散射计、浮标观测以及数值预报模式风场进行对比分析和真实性检验。