合成孔径雷达海面风场反演研究进展

合成孔径雷达高分辨率、全天候的观测能力使其成为大面积获取高分辨海面风场的重要手段,这有助于我们理解各种海洋现象的物理过程,尤其是在海岸带,合成孔径雷达反演风场更具优势。以合成孔径雷达风向确定方式为主线,详细阐述了国内外学者在合成孔径雷达风场反演方面的研究进展,主要包括：利用影像线性纹理特征进行风场反演、基于外部初始风向的风场反演方法以及利用影像本身所包含的其它信息进行的风场反演工作,诸如距离向入射角差异、多极化后向散射等,并对合成孔径雷达风场反演的发展给出自己的观点。     

High-resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and sea state by simple empirical models from synthetic aperture radar (SAR) data are presented and applied for use in high-resolution numerical modelling for coastal applications. The new radar satellite, TerraSAR-X (TS-X), images the surface of the sea with a high resolution up to 1 m. Therefore, not only wind information and integrated sea state parameters but also individual ocean waves with wavelengths down to 30 m are detectable. Two-dimensional information on the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of finely resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable of taking into account fine-scale effects in coastal areas. Wind and sea state information retrieved from SAR data are applied as the input for a wave numerical spectral model (wind forcing and boundary condition) running at a fine spatial horizontal resolution of 100 m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speeds and comparisons against wave height, simulated using original TS-X-derived wind data, showing the sensitivity of waves to local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight (North Sea) are shown. The TS-X satellite scenes render well-developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70 m, caused by the influence of underwater topography in coastal areas, is imaged on the radar scenes. A technique is developed for tracking wave rays depending on changes in swell wavelength and direction. We estimate the wave energy flux along wave tracks from deep water to the coastline based on SAR information: wave height and wavelength are derived from TS-X data.     

Assessment of an analytical model for sea surface wind speed retrieval from spaceborne SAR

An analytical model based on radar backscatter theory was utilized to retrieve sea surface wind speeds from C-band satellite synthetic aperture radar (SAR) data at either vertical (VV) or horizontal (HH) polarization in transmission and reception. The wind speeds were estimated from several ENVISAT Advanced SAR (ASAR) images in Hong Kong coastal waters and from Radarsat-1 SAR images along the west coast of North America. To evaluate the accuracy of the analytical model, the estimated wind speeds were compared to coincident buoy measurements, as well as winds retrieved by C-band empirical algorithms (CMOD4, CMOD_IRF2 and CMOD5). The comparison shows that the accuracy of the analytical model is comparable to that of the C-band empirical algorithms. The results indicate the capability of the analytical model for sea surface wind speed retrieval from SAR images at both VV and HH polarization.     

星载SAR在海气边界层气象学中的应用研究

SAR(Synthetic Aperture Radar,合成孔径雷达)作为一种现代高空间分辨率成像侧视雷达,对地球表面海洋所成的图像中蕴含了极为丰富的中尺度及亚中尺度海洋大气边界层的信息,因此对边界层气象学研究有着非常重要的意义。但是,使用SAR研究海气边界层这一涵盖微波遥感、气象学及海洋学等学科的科学前沿课题在国内却少有文献报道。在此背景下,首先介绍了SAR反演海洋大气边界层的研究概况,回顾了SAR反演海气边界层参数的原理和方法。然后以2002年5月7日当地时间10时53分ERS-2卫星获取的香港地区(22.097°N,E 114.300°E)SAR海洋图像为例,进行了反演风向风速的初步试验,最终获得了较高精度的风矢量。具体过程如下:先对SAR图像进行预处理,包括ADC(Analog Digital Converter,模数转换器)补偿、精确校准及斑点滤波等过程;然后利用经典的谱分析方法求得具有180°模糊度的风向,再用香港天文台气象浮标实测资料消除这一不确定性得到了真实的相对风向;紧接着利用CMOD4地球物理模式函数计算得到了海面上10 m高的风速。与气象浮标站所记录的平均风速和风向比较,两个20 km×20 km大小的试验区域求得的风向误差分别为23.71°和7.00°,平均风速误差分别为0.18 m/s和-0.12 m/s。结果表明,如果对SAR预先进行严格的预处理,结合经典的谱分析方法和CMOD4模型,即可获取高精度的风矢量。这一结果为今后海洋大气边界层的研究奠定了良好的基础。     

基于FY-3B微波成像仪的海面风速反演

风云三号B星微波成像仪的10.65、18.7、23.8和36.5 GHz频点对海表面粗糙度和介电特性比较敏感,能够用于海面地球物理参数的反演。为获得一种适用于全球大部分海域的海面风速反演算法,利用快速辐射传输模式和再分析大气廓线库模拟微波成像仪海面微波辐射特性,在此基础上建立了半经验反演算式,并利用浮标现场测量数据及WindSat全极化辐射计风速产品对半经验算法和经验算法分别进行了验证和对比。另外,通过分析风向对风速反演的影响,借助AT BV-TBH模型,尝试利用查找表算法对风向造成的晴空区域风速反演偏差进行初步校正。校正风向误差后,反演风速与浮标风速的均方根误差为0.9775 m/s。     

北斗反射信号海风海浪探测系统软件设计

介绍了北斗反射信号海风海浪探测系统的总体设计结构,重点阐述了软件系统的设计,其中对数据通信模块、定位信息显示模块以及有效波高、海面风速等海洋参数反演模块的设计与实现进行了详尽说明,并基于VS2008软件平台,使用C++语言对软件系统进行了实现.实际试验结果表明,北斗反射信号海风海浪探测软件系统可以长期稳定工作,并有效反演出有效波高、海面风速等海洋信息,而且具有良好的扩展性.     

基于长时间序列ASAR数据的近海风场反演

首先获取覆盖浙江省近海215景长时间序列多模式ASAR Level 1B数据,对获取的ASAR数据进行几何校正、辐射定标、极化比转化、噪声剔除后,将所有的影像统一重采样得到1km×1km分辨单元的后向散射系数影像,尝试利用CCMP风向作为SAR风速反演地球物理模式函数的风向初始场,对比目前国际上最常用的3种模式函数CMOD4、CMOD-IFR2和CMOD5的风速反演效果,将反演得到风速分别与CCMP插值风速和现场观测气象站点的风速进行对比,结果表明CCMP风向数据可作为SAR风速反演的风向初始场,其中结合CMOD4模式函数能够得到较高精度的反演风速,由于CCMP数据有效时间长、覆盖范围广、易于获取,因此联合CCMP风向有利于SAR近海风场反演的业务化应用。     

Evaluation of hurricane wind speed retrieval from cross-dual-pol SAR

Understanding and forecasting hurricanes are important components of weather prediction and climate studies. A critical concern is accuracy in hurricane wind speed estimates, especially in areas over the ocean where in situ measurements are sparse. Moreover, for very intense hurricanes, remotely sensed ocean surface winds generally lack accuracy. Recent studies on cross-polarization RADARSAT-2 synthetic aperture radar (SAR) data show very promising capability for high wind speed retrieval. The monotonic increase of cross-pol radar backscattered intensity with wind speed, and less sensitive dependence on wind direction, makes it superior to co-pol SAR measurements for operational application. Before further application of the methodology, it is important to evaluate the capability of hurricane forced wind speed retrieval from the promising, simple cross-pol SAR data. In this study, we apply a newly developed wind retrieval model to hurricanes using VH polarization dual-pol mode (VH dual-pol) RADARSAT-2 ScanSAR images. Validation of SAR wind retrievals is via surface wind analysis data from the Hurricane Research Division (HRD) of the National Oceanic and Atmospheric Administration (NOAA) and airborne stepped frequency microwave radiometer (SFMR) measurements. We found that compared to the co-polarization RADARSAT-2 SAR measurements, retrieved wind speeds from cross-polarization mode SAR have better overall accuracy and are more consistent with expected hurricane structures. Moreover, the cross-polarization model for VH dual-pol-retrieved winds does not appear to exhibit the speed ambiguity problem, which is one of the main obstacles for co-polarization retrieval of hurricane winds. SAR-retrieved winds contain detailed wind structures of the hurricane eyewall which are potentially of value for ongoing improvements in numerical models of hurricanes, air–sea interactions, and climate change. Special attention must be paid to biases caused by precipitation in order to reduce remaining errors in SAR retrieval of hurricane winds; precipitation has not been explicitly considered by current geophysical model functions.     

IDL并行计算技术在SAR风场反演中应用

SAR影像反演海面风场需要对反演模型进行迭代求解,难以满足近实时预报的需求。利用IDL并行计算技术在多核计算机实现SAR风场反演。基于IDL程序设计特点,分析IDL桥并行计算程序设计方法,根据SAR风场反演流程,提出基于IDL桥对象构建的风场反演并行计算算法,验证了其实用性。结果表明,基于IDL桥对象构建的并行计算算法提高了计算效率,为其他数据快速处理提供了解决方案。     

基于Contourlet旋转不变特征的遥感图像检索

除了具有纹理属性外,遥感图像中还包含大量的结构性边缘特征,如何有效捕捉这些特征中的信息进行检索,成为提高遥感图像检索效率的关键。文章依据Contourlet在离散域的多尺度几何分析的功能,提出一种利用Contourlet子带能量分布特性提取原始图像在多个尺度下的边缘方向信息进行检索的方法。针对Contourlet变换捕获不同特征方向能力的差别,文章采用正交补偿法加以改正,并通过傅里叶算子的处理,实现基于结构性边缘特征旋转不变的图像检索。实验结果表明,该检索算法对含有丰富规则边缘特征的遥感图像更为有效。     

Sea state in the Baltic Sea from space-borne high-resolution synthetic aperture radar imagery

In this work, remote sensing synthetic aperture radar (SAR) data from X-band TerraSAR-X and TanDEM-X (TS-X and TD-X) satellites have been used to adopt the algorithms for estimating sea state parameters in the specific condition of the Baltic Sea with archipelago islands and where short steep sea state dominates. Since the moving targets can be defocused and shifted in SAR images, sea state consisting of short windsea waves with strong local orbital velocities and wave breaking needs additional effort for accurate estimation of the total significant wave height that consists of swell and windsea parts. The XWAVE_C algorithm, developed for the North Sea, where the long swell waves coming from the Atlantic Ocean are present during storms, was further enhanced for the short steep windsea which dominates under ordinary storm conditions in the Baltics. For the empirical XWAVE_C model function, based on the spectral analysis of subscenes as well as on local wind information, an additional term was incorporated for assessment the minimal windsea significant wave height by applying JONSWAP wave spectra. A term to compensate spectral distortions triggered by windsea waves moving in SAR flight direction has also been introduced. In total, 95 TS-X/TD-X StripMap scenes between 2012 and 2017 were acquired in Eastern Baltic Sea, processed and analysed. The wave height results from SAR images were compared with collocated in situ data from 11 available buoys. The analysed data include both high and low windsea conditions. The comparison of SAR-derived wave heights with measured wave heights shows high agreement with a correlation coefficient r of 0.88. The wind speed, estimated from SAR images, was compared to measurements from 14 collocated in situ stations, yielding a high agreement with an r value of 0.90. This article is focused on the algorithm developments; however, it is also the first study of sea state retrieval in the Baltic Sea using high-resolution satellite-based techniques. The results show the local variability in the wave fields connected to atmospheric features. The observed local wave height can increase by 1–2 m in kilometre-size cells that are accompanied by wind gusts. The developed algorithms are installed in the German Aerospace Center’s (DLR) ground station Neustrelitz and can also be used in near-real-time.     

基于纹理和高斯密度特征的图像检索算法

直接从DCT域中提取图像的特征是提高图像的检索效率的方法.直接从压缩域中提取图像的高斯密度,即计算图像在8个方向上的分段累加值,形成一个8*4的二维向量,再结合图像的纹理特征来进行图像检索.为了验证算法的可行性,建立了10000幅图像的图像库.实验结果表明,该方法能够准确地检索出目标图像,有效地提高了图像检索的精度和速度.     

基于多尺度密集网络的肺结节图像检索算法

现有基于内容的医学图像检索（CBMIR）算法存在特征提取的不足，导致图像的语义信息表达不完善、图像检索性能较差，为此提出一种多尺度密集网络算法以提高检索精度。首先，将512×512的肺结节图像降维到64×64，同时加入密集模块以解决提取的低层特征和高层语义特征之间的差距；其次，由于网络的不同层提取的肺结节图像信息不同，为了提高检索精度和效率，采用多尺度方法结合图像的全局特征和结节局部特征生成检索哈希码。实验结果分析表明，与自适应比特位的检索（ABR）算法相比，提出的算法在64位哈希码编码长度下的肺结节图像检索查准率可以达到91.17%，提高了3.5个百分点；检索一张肺切片需要平均时间为48 μs。所提算法的检索结果在表达图像丰富的语义特征和检索效率方面，优于其他对比的网络结构，适用于为医生临床辅助诊断提供依据、帮助患者有效治疗。     

SAR图像处理的最新研究与应用

系统地阐述了SAR图像的特点、研究难点、处理过程、国内外最新研究方法及应用。结合国内外SAR图像最新研究动态对SAR图像消噪、纹理分割、线性特征的提取、多目标的识别、分数维方法的应用等几个热点问题作了论述。具体地论述了利用SAR图像进行目标探测与识别、目标变化评估;在民用上利用SAR图像进行矿藏资源的探测、洪涝灾害的趋势分析,并介绍了SAR信号处理算法在医学等领域取得的显著成绩。     

基于组合欧拉向量与边缘方向直方图的图像检索方法

在图像数据库中,如何有效检索和查询图像是一个重要的研究内容.文中提出一种结合组合欧拉向量与边缘方向直方图( EOH)的图像检索方法.首先,从边缘图像中提取组合欧拉向量特征进行图像检索(EEXO算法),其次,为更好地区分不同形状但欧拉特征相近的图像,将EEXO算法与EOH算法相结合提出EEXOEOH图像检索算法.实验结果表明,EEXOEOH算法与其它4种算法相比,具有较好的检索效率.     

Estimation of Sea Surface Velocities from SAR Images Using the Doppler Shift

The sea surface velocities field plays an important role in seawater exchange and substance transportation.In this paper,the Wide Swath Mode(WSM) data derived from the ENVISAT Advanced Synthetic Aperture Radar(ASAR) are used to retrieve the high-resolution sea surface velocities field.based on the theoretical model of SAR Doppler shift,the errors caused by the relative motion between Earth and satellite are removed.Then we use the C Band Doppler Frequency Model(CDOP) and the Bragg scattering model to remove the errors caused by the sea surface wind and Bragg scattering,respectively.The data used to verify the accuracy of the retrieval results are AVISO(Archiving Validation and Interpretation of Satellite Oceanographic) velocities and the GLD(Global Drifting Buoy) velocities.Our method is applied both in the coastal area with land cover(Agulhas) and the open sea(Kuroshio) without any land.Results show that in Agulhas,the velocity ranges from -1.8 m/s to 1.8 m/s,and their directions agree very well.In the Kuroshio,the ASAR current can clearly reveal the flow path and direction of the Kuroshio,and it matches well with the AVISO current.The comprehensive results shows,the Root Mean Square Error(RMSE) of ASAR and AVISO is 0.17 m/s,and the RMSE of ASAR and GLD is 0.11 m/s.This implies that the methods used here not only simplified the processes but also has high accuracy to retrieve sea surface velocities both in the coastal area and in the open sea.     

多特征融合的复杂环境海洋涡旋识别

目的 海洋涡旋识别研究已成为物理海洋领域的研究热点之一。传统的海洋涡旋识别算法多是基于物理参数法、流场几何特征法以及两种方法的混合,通过设置统一的阈值对海洋涡旋进行识别。在特定环境的海域中,传统算法的识别效果较好。但引起海洋涡旋生成的环境复杂多变且具有快速运动的特性,导致其具有快速连续变化的特点,依赖专家经验进行单一阈值设置的方法存在显著的主观性和不确定性,通常会导致海洋涡旋的漏判和错判,缺乏通用性。针对以上问题,为构建具有泛化能力的海洋涡旋自动识别方法,兼顾考虑海洋涡旋的全局和局部特征,提出一种复杂环境下海洋涡旋多特征融合识别方法。方法 首先通过数据预处理对数据集进行扩充;其次提取海洋涡旋的纹理特征（GLCM）、傅里叶描述子（FD）和Harris角点特征;接着将提取的GLCM特征进行均值化,并通过主成分分析法（PCA）对FD描述子和Harris角点特征进行降维;然后将处理后的特征向量进行串行融合;最后通过分类器完成对海洋涡旋的识别。结果 实验结果表明,提出的多特征融合方法的识别率高于单一特征方法的识别率,采用DT分类算法的识别精度最高,达86.904 5%;本文方法中采用PCA降维能有效提高识别精度,FD特征经PCA降维后,识别精度从83.906 0%提高到86.904 5%,Harris特征经PCA降维后,识别精度从84.009 7%提高到85.354 7%;且对于形态多样的海洋涡旋具有良好的鲁棒性。结论 本文融合了基于SAR影像海洋涡旋的3种特征信息,包括GLCM、FD和Harris角点特征,有效克服了阈值选取以及单一特征的不足,提高了基于SAR影像海洋涡旋的识别率,本文方法适用于复杂环境下的海洋涡旋识别。     

基于上下文分析的无监督分层迭代算法用于SAR图像分割

基于聚类的分割算法能够有效地分析目标特征在特征域的分布结构,进而准确判断目标的所属类别,但难以利用图像的空间和边缘信息,而基于区域增长的分割算法能够在空间域利用多种图像信息计算目标之间的相似性,但缺乏对特征结构本身的深层挖掘,容易出现欠分割或过分割的结果. 本文结合这两种算法各自的优势,针对合成孔径雷达（Synthetic aperture radar,SAR）图像的特点,提出了一种基于上下文分析的无监督分层迭代算法. 该算法使用过分割区域作为操作单元,以提高分割速度,降低SAR图像相干斑噪声的影响. 在合并过分割区域时,该算法采用了分层迭代的策略:首先,设计了一种改进的模糊C均值聚类算法,对过分割区域的外观特征进行聚类分析,获得其类别标记,该类别标记包含了特征的分布结构信息. 然后,利用多种SAR图像特征对同类区域的空域上下文进行分析,使用区域迭代增长算法对全局范围内的相似区域进行合并,直到不存在满足合并条件的过分割区域对为止,再重新执行聚类算法. 这两种子算法分层交替迭代,扬长避短,实现了一种有效的方法来组织和利用多种信息对SAR图像进行分割. 对模拟和真实SAR图像的实验表明,本文提出的算法能够在区域一致性和细节保留之间做到很好的平衡,准确地分割出各类目标区域,对相干斑噪声具有很强的鲁棒性.     

A novel approach for wide‐swath and high‐resolution SAR image generation from distributed small spaceborne SAR systems

In this paper we investigate the possibility to produce high‐resolution synthetic aperture radar (SAR) images of wide areas from the data collected by the distributed small spaceborne SAR systems. In particular, we focus on the data processing techniques for overcoming the Doppler ambiguity effects characterizing ground echoes and achieving high‐resolution SAR images of wide areas. A novel approach is proposed, the key idea of which is to retrieve the unambiguous azimuth wide (full) spectrum signal from the received ground echoes by using a space–time adaptive processing (STAP) algorithm to overcome the Doppler aliasing effect induced by the lower pulse repetition frequency (PRF); following this operation, conventional SAR data processing tools can be applied to fully focus the SAR images. The STAP‐based approach has the advantages of simplicity, robustness and high efficiency. We also discuss the array configuration requirements, and give a platform velocity/PRF criterion for array configurations.     

Radar imaging of Kelvin arms of ship wakes

The wave pattern generated by a moving ship is formed by two dominant features: the turbulent wake and a 'V'-shaped pattern trailing the ship, consisting of the two Kelvin arms. In this paper we investigate the radar imaging mechanism of Kelvin arms, which are formed by the cusp waves. A composite surface model for the radar backscattering at the ocean surface is used. The radar signatures of Kelvin arms can be attributed to tilt and hydrodynamic modulation of Bragg waves by the cusp waves. The proposed model allows the computation of the normalized radar backscattering cross-section (NRCS) as a function of radar frequency, polarization, incidence angle, wind speed and direction, and wavelength, direction, and slope of the cusp waves. By using this imaging model, radar signatures of cusp waves are calculated for several spaceborne Synthetic Aperture Radars (SARs): (1) the SEASAT L-band HH-polarized SAR, (2) the ERS-1/-2 VV-polarized SAR, (3) the RADARSAT C-band HH-polarized SAR, and (4) the X-, C- and L-band multipolarization SARs of the Space Radar Laboratory flown on the space shuttle during the SIRC/X-SAR mission in 1994. The results of the simulations are compared with SEASAT and SIR-C/X-SAR imagery of ship wake patterns. It is shown that the dependence of the observed radar signatures of Kelvin arms on radar look direction is consistent with the proposed imaging theory and that the measured relative mean NRCS values induced by Kelvin arms can be fairly well reproduced by the proposed model. The simulations indicate that ship wake signatures should be more clearly visible on SEASAT L-band SAR than on ERS-1/-2 or RADARSAT C-band SAR images. The radar signatures of Kelvin arms are strongest at low wind speeds and are not very sensitive to wind direction.