基于SAR后向散射的海上溢油检测研究

应用海面雷达后向散射系数检测海上溢油是目前海上溢油遥感监测的一个重要方向。本文以南海Envisat-ASAR数据为例,在分析SAR数据的基础上,应用Envisat-ASAR绝对定标计算方法,计算后向散射系数,研究应用SAR进行海上溢油遥感监测的散射特性,计算目标与海面边界后向散射系数梯度均值0σ与目标与海面后向散射系数均值差Δμ,并以两者结合作为区分海面油膜与自然现象的解译标志,从而为溢油识别提供依据。     

基于水体散射特性的SAR图像水体检测

基于SAR图像中水体后向散射系数服从Gamma分布的特点,提出了一种基于水体散射特性的自动化水体提取算法。研究重点包括:第一,利用高斯分布拟合地物后向散射系数,并结合改进的Gamma模型进行分析,确定最优阈值位置,构建优化准则,自适应迭代求解目标阈值。第二,引入含水体的目标块选择准则,剔除非目标块,对目标块运用所建模型获取阈值,取各目标块平均值作为全景图最优阈值,该过程大大缩短了运行时间,增加了阈值准确率,对SAR图像中水体的实时检测具有深刻意义。实验数据选用Radarsat-2获取的淮南地区单极化数据,实验结果表明该算法可以实现大幅图像中水体的快速、精确和自动化提取。     

星载雷达高度计沙漠散射特性及定标方法研究

为了提高星载雷达高度计海面风速产品的精度并更好地修正海况偏差的影响,有必要对高度计测量的后向散射系数进行定标。分布目标是星载雷达散射系数定标的有效手段,而沙漠的散射特性特别适于高度计的散射系数定标。检验了Jason-2卫星高度计的沙漠回波波形,在澳大利亚辛普森沙漠和我国的塔克拉玛干沙漠选取了适当的区域,收集了在这些区域长达7a的Jason-2高度计数据。基于这些数据,研究了Jason-2卫星雷达高度计在沙漠上的散射系数特性,并提出了基于沙漠的散射系数定标方法,分析了其原理、可行性和试验方案,探讨了高度计和现场定标散射计足迹匹配等关键技术,并对其定标误差进行了分析,论证了沙漠定标场可实现优于0.5dB的定标精度。该方法有助于进一步提升海洋2号(HY-2A)卫星风速和海况偏差产品的质量和应用前景。     

雷达遥感技术在水稻识别中的研究进展

合成孔径雷达技术拥有全天时、全天候、高分辨率和不受云层干扰的特点,利用SAR在我国水稻种植区进行水稻识别拥有无可比拟的优势。该文主要对雷达遥感技术在水稻识别和监测中的应用情况进行了综述。首先总结了合成孔径雷达的发展历史,介绍了目前用于水稻识别的SAR传感器的参数和特点;介绍了水稻的后向散射特性,根据水稻的后向散射模型分析了影响其后向散射系数的因素;综述了国内外利用SAR进行水稻识别的各种方法,并对未来利用SAR监测水稻进行了展望。     

具有一维带限Weierstrass分形特征实际路面电磁散射FDTD研究

一般的实际路面系统是由沥青混凝土面层、水泥碎石联结层、水泥稳定碎石基层和路基层等构成的多层介质,可以看作是具有一维带限Weierstrass分形特征的水泥混凝土路面系统。运用时域有限差分方法(FDTD)研究了这种实际的水泥混凝土路面系统电磁散射,具体计算了3层路面系统电磁散射的双站散射系数,得出了散射系数随散射角和入射波频率变化的曲线。分析了散射系数随路面分维数、入射波频率、入射角、路面材料介电常数、面层厚度、基层厚度等参数变化的规律,讨论了面层存在圆柱形空洞时,空洞内填充物变化对散射系数的影响,得到了具有一维带限Weierstrass分形特征的实际路面系统电磁散射的特性。数值计算结果表明:路面分维数、入射波频率、入射角、路面材料介电常数、面层厚度、基层厚度、面层存在的空洞对散射系数的影响是非常复杂的。     

星载微波散射计的在轨定标技术现状及展望

星载微波散射计通过测量海面不同方位向的后向散射系数反演出海面的风速和风向,为了满足测量精度要求,必须对其进行在轨定标.本文总结了现有微波散射计的在轨定标技术,包括有源定标器法、陆地自然目标法、海洋目标法和星星交叉定标法;然后分析了每种定标方法的优缺点;最后,描述了现有定标方法适用扇形波束体制散射计和笔形波束圆锥扫描体制散射计的特性.     

不同入射角下的雷达后向散射系数图像模拟

选择了新疆渭库三角洲(库车县、新和县、沙雅县)为实验区,利用Ulaby和Chapp的后向散射模型分别模拟了HV极化和HH极化的入射角为20°、24°、30°、33°、37°和40°的雷达后向散射系数图像。分析了不同入射角下图像各点后向散射系数值的散点图以及统计特征,得出由于入射角度的变化,两种极化的图像后向散射系数值变化较大,其变化趋势均符合余弦函数曲线分布,且模拟SAR图像很好的保留了地面散射信息。     

微波主被动遥感相关特性的数值模拟和实验对比

本文采用一层具有粗糙界面的连续随机介质模型，计算双站散射系数和后向散射系数，由互易性定理计算热发射率。数值模拟了植被在整个生长过程中后向散射和热发射的变化，以及它们的相关特征。     

Ku波段成像散射计后向散射系数精度分析

成像微波散射计是增加了成像功能的新型散射计系统,其后向散射系数的测量精度仍然是系统的关键指标.根据成像微波散射计的系统特点,提出了基于FFT对信号与噪声进行同时测量的数字处理系统,推导了适用于衡量成像散射计后向散射系数测量精度的归一化标准差Kp .对于不同观测条件下的Kp 进行了公式描述及数值仿真.得出不同区域和不同目标观测情况下如何配置系统参数的结论,并落实到成像散射计系统的性能分析中,对如何折衷设计性能参数并在实验过程中获取最佳试验结果进行理论指导.     

分层粗糙面与上方目标的复合电磁散射研究

针对粗糙面与目标复合电磁散射特性的应用前景,采用空间域合成法生成Alpha-Stable分布分层粗糙面,运用矩量法研究了锥形波入射时分层粗糙面与上方圆柱体的复合电磁散射特性,通过数值计算得到了复合散射系数随散射角的变化曲线,分析了稳定性系数、归一化尺度参数、互相关长度、入射波频率、粗糙面间距、圆柱体横截面半径以及粗糙面下方介质介电常数对复合散射系数的影响.结果表明,稳定性系数、归一化尺度参数、互相关长度、入射波频率、粗糙面间距对复合散射系数影响较大,而圆柱体横截面半径、粗糙面下方介质的介电常数对散射系数影响较小.     

基于QAA算法的昆承湖固有光学量反演模型

基于2010年4月在昆承湖采集的野外实验数据,建立了适用于昆承湖的QAA固有光学量反演模型。结果表明:该模型可以得到较高精度的反演结果。所有验证点在411~700nm波段范围内,反演总吸收系数和实测总吸收系数的决定系数R2都高于0.984,平均相对误差ε都低于14.5%。在440、488和532nm 3个波段处,总吸收系数反演值和实测值决定系数R2分别为0.655、0.742和0.826,平均相对误差ε分别为6.5%、3.6%和3.4%,精度较高。反演后向散射系数与和参考后向散射系数在440、488和532nm处均具有较好的相关性,且532nm处反演后向散射系数与实测总悬浮物浓度决定系数为0.624,呈正相关,反演后向散射系数具有一定的可信度。该模型能够为昆承湖固有光学量的反演提供一条有效途径。     

Estimating suspended sediment concentrations from spectral reflectance data

Abstract

A method for estimating sediment concentrations at different depths in coastal waters from measured spectral reflectance above the sea is proposed. The water backscattering coefficient is obtained by integrating a scattering cross section derived from Mie theory over a Junge type particle-size distribution. The results for turbid water, near an arid coast, with vertical mixing, shows a maximum of particle concentration in the middle layer, while the same method applied to reflectance data for ‘blue’ water, gives a relative concentration of sediment that gradually increases with depth.     

The origin and global distribution of second order variability in satellite ocean color and its potential applications to algorithm development

Empirical algorithms based on first order relationships between ocean color and the chlorophyll concentration ([Chl]; mg m^? 3) are widely used, but cannot explain the statistical dispersion or “anomalies” around the mean trends. We use an empirical approach that removes the first order effects of [Chl] from satellite ocean color, thus allowing us to quantify the impact on the ocean color signal of optical anomalies that vary independently of the global mean trends with remotely sensed [Chl]. We then present statistical and modeling analyses to interpret the observed anomalies in terms of their optical sources (i.e. absorption and backscattering coefficients). We identify two main sources of second order variability for a given [Chl]: 1) the amount of non-algal absorption, especially due to colored dissolved organic matter; and 2) the amplitude of the backscattering coefficient of particles. The global distribution of the anomalies displays significant regional and seasonal trends, providing important information for characterizing the marine optical environment and for inferring biogeochemical influences. We subsequently use our empirically determined anomalies to estimate the backscattering coefficient of particles and the combined absorption coefficient for colored detrital and dissolved materials. This purely empirical approach provides an independent assessment of second order optical variability for comparison with existing methods that are generally based on semi-analytical models.     

Backscattering properties of marine phytoplankton Prorocentrum micans

Backscattering coefficients of Prorocentrum micans were determined in the laboratory using the Hydroscat-6 instrument. Ancillary parameters measured included absorption and scattering coefficients, chlorophyll-a concentration, cell size, and cell concentration. Spectral variability was found in the case of the backscattering coefficients, and the maximum backscattering coefficient was obtained at 488 nm, which deviated from the theory because of pigment absorption. We were unable to detect the effect of chlorophyll fluorescence on the shape of the backscattering coefficient spectra at 700 nm. However, the particulate backscattering coefficient was found to be well correlated with chlorophyll-a concentration at the six chosen wavelengths. At the same time, the backscattering ratio was accurately arranged in the range 0.012 to 0.019 at 442 nm and 0.013 to 0.021 at 620 nm. A positive correlation between particle density and backscattering ratio was established with a nonlinear regression model and the correlation coefficient was as high as 0.96 at 442 nm, this providing a good foundation for improving the accuracy of identifying the red tide alga, P. micans, for water colour remote sensing.     

Assessment of apparent and inherent optical properties derived from SeaWiFS with field data

The knowledge of the absorption and scattering characteristics of the ocean supports diverse applications for studying biological and physical processes of marine ecosystems. The determination of the related inherent optical properties from remote sensing is addressed for a site located in the northern Adriatic Sea using coincident SeaWiFS images and field measurements. The proposed methodology first combines regional algorithms to determine the spectra of irradiance reflectance and diffuse attenuation coefficient from the normalized water leaving radiance by accounting for the bidirectional structure of the light field. These spectra are then used as inputs to an inverse model that yields the absorption, scattering, and backscattering coefficients of seawater (considered without the contribution of pure water). The uncertainties associated with the different steps in the sequence of calculations are quantified and discussed. Specifically, the analysis of 48 match-ups comparing in situ irradiance reflectance with that derived from remote sensing water leaving radiance shows mean absolute differences below 20% between 490 and 555 nm and approximately 30% at 443 and 670 nm. Lower discrepancies are obtained if more stringent criteria for the selection of match-ups are implemented (12-15% and 20-22%, respectively). The results obtained with 42 match-ups show a reasonable agreement for the absorption coefficient from 412 to 490 nm (approximately 35%), the scattering coefficient from 443 to 555 nm (approximately 30%), and the attenuation coefficient in the spectral range 412-555 nm (approximately 30%). Based on 17 match-ups, the comparison for the backscattering coefficient gives mean absolute differences in the range of 31-53%. The comparison between field and derived scattering properties suggests that the particulate scattering phase function inherent to the inverse model is not appropriate for the site considered. Finally, in the framework of the selected inversion scheme, uncertainties associated with the satellite derived irradiance reflectance in the blue and the determination of K_d at 490 nm through an empirical band ratio algorithm appear as the elements where improvements would be particularly needed to derive high quality spectra of inherent optical properties.     

后向散射系数的贝叶斯估计

后向散射系数是合成孔径雷达图像中重要的物理参数.由于合成孔径雷达测量系统的噪声干扰和其他不确定因素影响使得测量数据往往不够精确,这就需要对测量数据进行合理估计.为了对后向散射系数做出准确合理的估计,文章将后向散射系数的先验知识考虑进去,给出了后向散射系数的三种贝叶斯估计算法.贝叶斯估计的关键是概率密度模型的选取.例中选用贝塔(Beta)分布作为先验概率密度模型,伽玛(Gamma)分布作为条件概率密度模型得到了合理的估计结果,并与最大似然估计(ML)算法进行了比较,比较结果表明在对后向散射系数的估计中,贝叶斯估计算法要明显优于最大似然估计算法.     

基于半分析模型的石头口门水库总悬浮物浓度反演研究

总悬浮物（TSM）是水质遥感反演的重要参数之一,以遥感技术为手段监测总悬浮物浓度成为未来发展的趋势。利用2008年6月13日在石头口门水库采集的高光谱数据和实验室分析数据,计算得到水体总悬浮物的后向散射系数,并经相关分析,选取675 nm处的后向散射系数建立总悬浮物浓度的反演模型,决定系数为0.8327。与基于遥感反射率的经验方法比较,半分析方法具有一定的物理意义,反演精度更高,且在悬浮泥沙含量较高的区域效果更佳。     

Remote sensing of phytoplankton pigments: A comparison of empirical and theoretical approaches

Algorithms that have been used on a routine basis for remote sensing of the phytoplankton pigment, chlorophyll- a, from ocean colour data from satellite sensors such as the CZCS (Coastal Zone Color Scanner), SeaWiFS (Sea Viewing Wide Field-of-View Sensor) and OCTS (Ocean Colour and Temperature Scanner) are all of an empirical nature. However, there exist theoretical models that allow ocean colour to be expressed as a function of the inherent optical properties of seawater, such as the absorption coefficient and the backscattering coefficient. These properties can in turn be expressed as functions of chlorophyll- a, at least for the so-called Case 1 waters in which phytoplankton may be considered to be the single, independent variable responsible for most of the variations in the marine optical properties. Here, we use such a theoretical approach to model variations in ocean colour as a function of chlorophyll- a concentration, and compare the results with some empirical models in routine use. The parameters of phytoplankton absorption necessary for the implementation of the ocean colour model are derived from our database of over 700 observations of phytoplankton absorption spectra and concurrent measurements of phytoplankton pigments by HPLC (High Performance Liquid Chromatography) techniques. Since there are reports in the literature that significant differences exist in the performance of the algorithms in polar regions compared with lower latitudes, the model is first implemented using observations made at latitudes less than 50. It is then applied to the Labrador Sea, a high-latitude environment. Our results show that there are indeed differences in the performance of the algorithm at high latitudes, and that these differences may be attributed to changes in the optical characteristics of phytoplankton that accompany changes in the taxonomic composition of their assemblages. The sensitivities of the model to assumptions made regarding absorption by coloured dissolved organic matter (or yellow substances) and backscattering by particles are examined. The importance of Raman scattering on ocean colour and its influence on the algorithms are also investigated.     

山地高原地区TM影像水体信息提取方法比较-以香格里拉县部分地区为例

采用遥感数据对地物进行识别是获取地物信息的一种重要手段。在云南省香格里拉县湿地研究中,以Land-sat ETM+影像作数据源,分别采用了阈值法、差值法和阈值法的结合运用、多波段谱间关系法和阈值法的结合运用3种方法对湿地进行识别。结果表明:采用阈值法难以将灰度值接近的山体阴影和水体区分开来,差值法和阈值法的结合、多波段谱间关系法和阈值法的结合运用提取到的水体信息精度较高,多波段谱间关系法对于消除阴影的效果最好。     

Mapping spatio-temporal water level variations over the central Congo River using PALSAR ScanSAR and Envisat altimetry data

Previous studies using synthetic aperture radar (SAR) backscattering coefficients have been used to distinguish vegetation types, to monitor flood conditions, and to assess soil moisture variations over the wetlands. Here, we attempted to estimate spatio-temporal water level variations over the central Congo mainstem covered with aquatic plants using the backscattering coefficients from the Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) Scanning SAR (ScanSAR) images and water levels from Envisat altimetry data. First, permanent open water, forest, macrophytes, and herbaceous plants have been classified over the central Congo Basin based on statistics of the backscattering coefficient values. Second, we generated multi-temporal water level maps over part of the Congo mainstem based on the relationship between Envisat altimetry-derived river-level changes and PALSAR ScanSAR backscattering coefficient variations. Finally, the water level maps were validated with Ice, Cloud and land Elevation Satellite (ICESat) altimetry-derived water levels. We obtained overall root mean square difference (RMSD) of 67.27 cm at 100-m scale resolution of PALSAR ScanSAR. Our study shows that we can obtain reasonable estimates of water levels of the rivers covered with seasonally floating or emergent macrophytes from backscattering coefficients. Furthermore, it is expected that the generated water level maps can be used as a ‘true’ data set to perform pre-launch study of the Surface Water Ocean Topography (SWOT) mission to be launched in 2021.