Probability density functions for multilook polarimetric signatures

Derives closed-form expressions for the probability density functions (PDF's) for copolar and cross-polar ratios and for the copolar phase difference for multilook polarimetric SAR data, in terms of elements of the covariance matrix for the backscattering process. The authors begin with the case in which scattering-matrix data are jointly Gaussian-distributed. The resulting copolar-phase PDF is formally identical to the phase PDF arising in the study of SAR interferometry, so the authors' results also apply in that setting. By direct simulation, they verify the closed-form PDF's. They show that estimation of signatures from averaged covariance matrices results in smaller biases and variances than averaging single-look signature estimates. They then generalize their derivation to certain cases in which backscattered intensities and amplitudes are K-distributed. They find in a range of circumstances that the PDF's of polarimetric signatures are unchanged from those derived in the Gaussian case. They verify this by direct simulation, and also examine a case that fails to satisfy an important assumption in their derivation. The forms of the signature distributions continue to describe data well in the latter case, but parameters in distributions fitted to (simulated) data differ from those used to generate the data. Finally, the authors examine samples of K-distributed polarimetric SAR data from Arctic sea ice and find that their theoretical distributions describe the data well with a plausible choice of parameters. This allows the authors to estimate the precision of polarimetric-signature estimates as a function of the number of SAR looks and other system parameters     

合成孔径雷达多视图像中4个Stokes参数的统计特性

该文应用散射振幅函数的Wishart联合分布,推导了合成孔径雷达(SAR)多视图像中4个Stokes参数的概率密度函数(PDF)。用视图数和4个Stokes参数的均值解析表达的PDF与现有的R.Touzi和A.Lopes(1996)的结果作了比较。同时,用机载SAR在几个典型区域上的多波段(P,L,C)成像数据验证了4个Stokes参数的理论统计分布,讨论了有效视图数、方差和分布斜度。实验数据与理论分布有相当好的吻合,还讨论了PDF在地表分类中的应用。     

星载干涉SAR中的基线问题

基线在星载干涉SAR的系统设计和数据处理中起着非常重要的作用.本文比较全面地阐述了空间基线在星载干涉SAR中的作用.特别分析了星载干涉SAR中空间基线对SAR图像相关性、地距分辨率以及干涉SAR数据处理中的距离向平地效应和多视平均处理的影响.讨论了星载干涉SAR空间基线的设计问题,推导了空间最优基线的确定公式.最后进行了计算机仿真,仿真结果表明分析正确.     

Statistics of the Stokes parameters and of the complex coherenceparameters in one-look and multilook speckle fields

Barakat [1987] derived the Stokes parameter statistics, applicable to one-look synthetic aperture radar (SAR) images, of a partially polarized wave backscattered from a Gaussian area. In this paper, the statistics of the Stokes parameters and of the phase difference are derived as a function of the mean effective phase difference and the degree of coherence for one-look and multilook SAR data. The statistics of the degree of coherence are also derived for multilook SAR data. It is shown that the estimator currently used for calculation of the degree of coherence is biased under low coherence conditions     

Reflectivity estimation for multibaseline interferometric radar imaging of layover extended sources

In recent years, there has been great interest in exploiting the advanced multibaseline operation of synthetic aperture radar interferometry (InSAR) for solving layover effects from complex orography, which can degrade both SAR and InSAR imagery of terrain radar reflectivity and height. In this work, the problem of retrieving radar reflectivity of layover areas is addressed. It is formulated as the problem of estimating a multicomponent sinusoidal signal corrupted by multiplicative complex correlated noise and additive white Gaussian noise. Application of nonparametric [e.g., Capon, amplitude and phase estimation filter (APES)], parametric [least squares, modern parametric RELAXation spectral estimator (RELAX)], and hybrid spectral estimators for amplitude estimation is investigated for a multilook scenario. In particular, the multilook extensions of RELAX and APES are applied to the interferometric problem. Performance analysis is investigated through a Cramer-Rao lower bound calculation and Monte Carlo simulation. The method of least squares, coupled with Capon's approach to spatial frequency estimation, multilook APES, and multilook RELAX turn out to provide accurate reflectivity estimates for undistorted multibaseline image formation of layover areas.     

Statistical analysis of multilook SAR interferograms for CFAR detection of ground moving targets

This paper examines the statistics of the phase and magnitude of multilook synthetic aperture radar (SAR) interferograms toward deployment of along-track interferometry (ATI) for slow ground moving-target indication (GMTI). While the known probability density function (pdf) of the interferogram's phase (derived under the assumption of Gaussian backscatter) is shown to agree almost perfectly for a wide variety of backscatter conditions, the corresponding magnitude's pdf tends to deviate strongly in most cases. Motivated by this discrepancy, a novel distribution is derived for the interferogram's magnitude. This pdf, called the polynomial or p-distribution, matches the real data much more accurately, particularly for heterogeneous composite terrain. For extremely heterogeneous terrain, such as urban areas, both pdfs for interferometric phase and magnitude fail and are extended. Based on these statistics, a completely automatic detection scheme with constant false-alarm rates (CFARs) for slow moving targets is proposed. All involved parameters required to determine the detection thresholds are estimated from the sample data. It is demonstrated, on the basis of experimental airborne SAR data, that this detector is capable of detecting slow moving vehicles within severe ground clutter.     

On the multilook images of moving targets by synthetic aperture radars

In the multilook processing of synthetic aperture radar (SAR) data, subapertures are synthesized at different center times so that a time-lapse exists between looks. This does not affect the imaging of stationary targets but if targets are in motion, the information content about the targets differs from look to look. The purpose of this paper is to investigate the effects of motions on the SAR multilook images of moving targets. Expressions are derived for the impulse response function from a moving point target in terms of the look number and the nature of the motion. Discussions are given only on the effects inherent to multilook processing.     

Estimation of the Equivalent Number of Looks in Polarimetric Synthetic Aperture Radar Imagery

This paper addresses estimation of the equivalent number of looks (ENL), an important parameter in statistical modeling of multilook synthetic aperture radar (SAR) images. Two new ENL estimators are discovered by looking at certain moments of the multilook polarimetric covariance matrix, which is commonly used to represent multilook polarimetric SAR (PolSAR) data, and assuming that the covariance matrix is complex Wishart distributed. First, a second-order trace moment provides a polarimetric extension of the ENL definition and also a matrix-variate version of the conventional ENL estimator. The second estimator is obtained from the log-determinant matrix moment and is also shown to be the maximum likelihood estimator under the Wishart model. It proves to have much lower variance than any other known ENL estimator, whether applied to single-polarization or PolSAR data. Moreover, this estimator is less affected by texture and thus provides more accurate results than other estimators should the assumption of Gaussian statistics for the complex scattering coefficients be violated. These are the first known estimators to use the full covariance matrix as input, rather than individual intensity channels, and therefore to utilize all the statistical information available. We finally demonstrate how an ENL estimate can be computed automatically from the empirical density of small sample estimates calculated over a whole scene. We show that this method is more robust than procedures where the estimate is calculated in a manually selected region of interest.     

Dependence of Speckle Statistics on Backscatter Cross-Section Fluctuations in Synthetic Aperture Radar Images of Rough Surfaces

A theory is described to relate the statistical properties of the fields backscattered from rough surfaces to those of speckle in synthetic hetic aperture rdar (SAR) images. The expressions are derived for the autocorrelation and cross-correlation functions of speckle intensity in both single-look and multilook images of stationary random rough surfaces in terms of the SAR system parameters and the autocorrelation function of backscatter radar cross-section fluctuations. It is shown that if the correlation scale of cross-section fluctuations is comparable with or greater than the SAR resolution, the correlation functions of speckle intensity depend on those of the cross-section fluctuations. This property, therefore, may be applied to image classification. Comparison on of the theory with computer simulation shows good agreement.     

基于曲面投影的毫米波InSAR数据成像方法

与其它波段相比,毫米波系统具有体积小、重量轻、分辨率高等优点,成为近几年InSAR技术的研究热点。但因其波长短,毫米波InSAR对平台运动轨迹测量精度要求更高,非理想运动情况下传统成像方法数据处理及干涉相位提取困难,另外传统方法基于平面投影成像,在地形陡变时干涉相位缠绕和目标几何畸变较严重。为了解决传统方法在毫米波InSAR成像的以上不足,该文提出了一种基于曲面投影的毫米波InSAR成像方法,将不同通道回波数据投影到相同地形高程曲面上进行成像及干涉相位提取,并推导了曲面下地形高程与干涉相位的关系。仿真和实测数据结果验证了该文方法的有效性,结果显示该方法在平台非理想运动下较传统算法获得更好的InSAR成像和干涉相位质量,且减小了地形高程起伏引起的几何畸变及干涉相位缠绕,更有利于毫米波InSAR图像地形特征描述及高程提取。      

A test statistic in the complex Wishart distribution and its application to change detection in polarimetric SAR data

When working with multilook fully polarimetric synthetic aperture radar (SAR) data, an appropriate way of representing the backscattered signal consists of the so-called covariance matrix. For each pixel, this is a 3/spl times/3 Hermitian positive definite matrix that follows a complex Wishart distribution. Based on this distribution, a test statistic for equality of two such matrices and an associated asymptotic probability for obtaining a smaller value of the test statistic are derived and applied successfully to change detection in polarimetric SAR data. In a case study, EMISAR L-band data from April 17, 1998 and May 20, 1998 covering agricultural fields near Foulum, Denmark are used. Multilook full covariance matrix data, azimuthal symmetric data, covariance matrix diagonal-only data, and horizontal-horizontal (HH), vertical-vertical (VV), or horizontal-vertical (HV) data alone can be used. If applied to HH, VV, or HV data alone, the derived test statistic reduces to the well-known gamma likelihood-ratio test statistic. The derived test statistic and the associated significance value can be applied as a line or edge detector in fully polarimetric SAR data also.     

分布式卫星的环绕对InSAR测高精度的影响

分布式卫星SAR系统是最近提出的一种新的SAR系统,该文结合干涉SAR(InSAR)技术,研究了该系统中InSAR成像所面临的问题,分析和推到了系统环绕引起的干涉相位、基线等参数的变化,以及对成像高度精度的影响,给出了相应的数学模型及关系曲线。分析结果表明随着环绕角度的增加,测高误差增大,必须对相位和基线进行合理修正。     

InSAR for estimation of changes in snow water equivalent of drysnow

This paper describes the theoretical relation between interferometric phase and changes in snow water equivalent (SWE) and show results from experiments using ERS-½ tandem data. The main scattering contribution from a dry snow cover is from the snow-ground interface. However, the radar wave will be refracted in the snow. Thus, only small changes in the snow properties between two interferometric synthetic aperture radar (SAR) images will change the interferometric phase. This phase change is shown to introduce a significantly increase in the digital elevation model (DEM) height error, although no effects are observed on the degree of coherence. The phase change is also shown to affect the differential interferometric results and may wrongly be interpreted as range displacement. The presented theory and results implies that light snowfall and/or small changes in snow properties between interferometric SAR (InSAR) image acquisitions, may introduce significant height errors in DEM derived from glaciers, ice sheets, or bare ground, even in the case of high degree of coherence. Thus, meteorological observations in addition to degree of coherence must be considered when generating DEM in areas covered with snow or where snow fall is likely to have occurred     

一种InSAR建筑物图像仿真及高程反演方法

城市建筑区域叠掩、阴影严重,图像理解困难且干涉相位变化复杂紊乱,一直是InSAR处理的困难区域。SAR图像仿真能为图像理解和处理方法研究提供数据支撑,然而现有建筑区域SAR图像仿真方法大多无法获得具有相干性的干涉SAR图像对。该文提出了一种面向建筑区域的干涉SAR复图像对仿真方法,能够获得建筑的复数图像对、干涉相位图以及叠掩成分数目等信息,为城区干涉SAR处理及信息提取研究提供仿真数据支撑。同时,基于仿真中对相位变化规律的分析,提出叠掩区相位解缠时的基准确定方法,解决传统解缠方法面临的叠掩区域干涉相位不连续问题,进而反演建筑高程信息。最后,通过建模仿真结果与实际SAR图像和干涉相位的对比,验证了仿真方法的正确性,并对仿真及实际干涉相位进行解缠和高程反演处理,验证了该文高程反演方法的有效性。      

一种针对多通道GMTI的SAR复图像精确配准方法

提出了一种针对多通道GMTI的SAR复图像精确配准算法。首先,系统分析了配准误差对干涉相位的影响,推导出了对应配准误差的干涉相位Cramer-Rao界,并利用Monte Carlo仿真数据对配准误差的影响进行了量化。在此基础上,通过对待配准图像进行二维精确插值处理,生成模板图像库和相应的相关系数库,以适合多通道GMTI的复图像的相位相关为准则,找出模板图像库和相关系数库中与参考图像具有最大相位相关值的图像作为配准图像,从而实现多通道SAR复图像之间的精确配准。对基于所提算法的实测三通道SAR复图像进行慢动目标检测实验,结果表明所提算法能够提供多通道GMTI所需要的配准精度,证明了该算法的有效性和实用性。      

地基SAR干涉相位滤波优化方法

地基SAR对边坡进行形变监测过程中,为了获得较高的图像质量和进行相位解缠的需要,往往要对干涉相位进行滤波优化,以提高干涉相位图的信噪比,减少干涉相位图中的残差点,从而有助于更好地进行相位解缠,提高形变监测的精度与可行性。本文针对地基SAR干涉相位图的空变等特性,对地基SAR干涉相位滤波算法进行了研究,着重介绍了四种经典、高效的干涉相位滤波算法,并进行了地基SAR边坡形变监测试验,使用实测干涉数据进行了四种滤波处理,从定量及定性的角度对每种滤波算法进行了比较与评估。最终给出了最优滤波算法,为地基SAR进行干涉相位实时滤波优化及后续数据处理提供了基础。      

Adaptive removal of azimuth ambiguities in SAR images

We introduce an innovative algorithm that is capable of suppression of strong azimuth ambiguities in single-look complex (SLC) synthetic aperture radar images, to be used both for detected products and for interferometric surveys. The algorithm exploits a band-pass filter to select that portion of the azimuth spectrum that is less influenced by aliasing, the one that corresponds to the s in the replicated azimuth antenna pattern. The selectivity of this filter adapts locally depending on the estimated ambiguity intensity: the filter is more selective in the presence of strong ambiguities and becomes all-pass in absence of ambiguities. The design of such filter frames in the Wiener approach with two different normalization options, depending on the use of the SLC image, either for getting multilook averaged detected products or for interferometric applications. Preliminary results achieved by processing ENVISAT Advanced Synthetic Aperture Radar sensor data are provided.     

基于联合像素干涉统计特性的三通道动目标检测方法

本文提出了一种基于联合像素的三通道地面运动目标检测（GMTI）方法。首先分析联合像素处理后的两多视SAR图像于涉统计特性,建立幅相联合的概率统计模型,分析虚警和漏警,考虑到最小可检测速度内存在速度模糊,采用能量重心法解模糊,最后将干涉幅度和解模糊后的干涉相位联合检测动目标。实测数据处理结果验证了该方法的有效性。     

Restoration of polarimetric SAR images using simulated annealing

Filtering synthetic aperture radar (SAR) images ideally results in better estimates of the parameters characterizing the distributed targets in the images while preserving the structures of the nondistributed targets. However, these objectives are normally conflicting, often leading to a filtering approach favoring one of the objectives. An algorithm for estimating the radar cross-section (RCS) for intensity SAR images has previously been proposed in the literature based on Markov random fields and the stochastic optimization method simulated annealing. A new version of the algorithm is presented applicable to multilook polarimetric SAR images, resulting in an estimate of the mean covariance matrix rather than the RCS. Small windows are applied in the filtering, and due to the iterative nature of the approach, reasonable estimates of the polarimetric quantities characterizing the distributed targets are obtained while at the same time preserving most of the structures in the image. The algorithm is evaluated using multilook polarimetric L-band data from the Danish airborne EMISAR system, and the impact of the algorithm on the unsupervised H-α classification is demonstrated