Calibration of linearly polarized polarimetric SAR data subject to Faraday rotation

A model for linearly polarized fully polarimetric backscatter measurements is used, incorporating the effects of system noise, channel amplitude, phase imbalance, crosstalk, and Faraday rotation. A step-by-step procedure is outlined for correction (or calibration) of fully polarimetric data subject to Faraday rotation, to recover the true scattering matrix. The procedure identifies steps for crosstalk removal and correction of channel imbalances that are robust in the presence of Faraday rotation. The final steps in the procedure involve a novel strategy for estimation and correction of Faraday rotation. Three approaches to estimate the (one-way) Faraday rotation angle /spl Omega/ directly from linear (quad-) polarized synthetic aperture radar (SAR) backscatter data obtained by a spaceborne SAR system are described. Each approach can initially be applied to the signature of any scatterer within the scene. Sensitivity analyses are presented that show that at least one of the measures can be used to estimate /spl Omega/ to within /spl plusmn/3/spl deg/ to 5/spl deg/, with reasonable levels of residual crosstalk, noise floor, channel amplitude, and phase imbalance. Ambiguities may be present in the estimates of /spl Omega/ of /spl plusmn/n/spl pi//2 - the impact of this is discussed, and several approaches are suggested to deal with this possibility. The approach described in this paper is relevant for future L-band spaceborne SARs and removes one key obstacle to the deployment of even longer wavelength SARs (e.g., an ultrahigh frequency or P-band SAR) in Earth orbit.     

Single-baseline polarimetric SAR interferometry

Examines the application of single-baseline polarimetric SAR interferometry to the remote sensing and measurement of structure over forested terrain. For this, a polarimetric coherent scattering model for vegetation cover suitable for the estimation of forest parameters from interferometric observables is introduced, discussed and validated. Based on this model, an inversion algorithm which allows the estimation of forest parameters such as tree height, average extinction, and underlying topography from single-baseline fully polarimetric interferometric data is addressed. The performance of the inversion algorithm is demonstrated using fully polarimetric single baseline experimental data acquired by DLR's E-SAR system at L-band     

Radar imaging of moving targets in foliage using multifrequency multiaperture polarimetric SAR

Because of the low signal-to-clutter ratio, it is a difficult problem to detect and image moving targets in foliage. In this paper, a multifrequency multiaperture polarimetric synthetic aperture radar (MFMA POLSAR) system is proposed for imaging of moving targets in foliage. The MFMA POLSAR extends the multifrequency antenna array SAR (MF-SAR) system to multiple polarizations. Full polarization is used in MFMA POLSAR to achieve an optimal polarization adaptive to the environment such that the images obtained by different apertures are of the best coherence that is used to obtain the highest accuracy of the phase estimation. It is also shown that the MFMA POLSAR cannot only accurately locate both the slow and the fast moving targets but also reveal moving targets in foliage.     

Orientation angle preserving a posteriori polarimetric SAR calibration

Fully polarimetric synthetic aperture radar (SAR) data analysis has found wide application for terrain classification, land-use, soil moisture, and ground cover classification. Critical to all analyses and applications is accurate calibration of the relative amplitudes of and phases between the various polarimetric channels. Here we propose an a posteriori method imposing only the weakest of constraints, scattering reciprocity, on the polarimetric data. Calibration parameters are self-consistently estimated from full 4/spl times/4 polarimetric covariance matrices. Whilst the complete set of calibration parameters is underdetermined, we give several reasonable heuristic methods to provide a complete calibration. Stronger constraints reduce the number of independent parameters and provide an overdetermined set of equations but at a cost - the loss of polarimetric fidelity when the underlying assumptions are violated. Without recourse to in situ calibration targets, the extent of the polarimetric distortion that results from polarimetric calibration remains unknown. We apply our new method to simulated data, anechoic chamber data and polarimetric SAR imagery. We also present comparisons with alternate calibration methods and different approximate solutions of the new technique.     

双站合成孔径雷达去取向极化分析

在分析单站合成孔径雷达去取向参数的基础上,研究了双站合成孔径雷达的非对称散射矩阵,利用矩阵代数知识得到双站去取向参数。推导了目标的去取向变换矩阵,将目标散射矢量进行矢量变换,使得研究目标具有相同的取向,目标本身的信息得以凸显。经过去取向变换,对双站模拟图像的特征参数进行分析,为进一步解读双站图像提供了初步手段。     

Knowledge-based classification of polarimetric SAR images

In preparation for the flight of the Shuttle Imaging Radar-C (SIR-C) on board the Space Shuttle in the spring of 1994, a level-1 automatic classifier was developed on the basis of polarimetric SAR images acquired by the JPL AirSAR system. The classifier uses L- and C-Band polarimetric SAR measurements of the imaged scene to classify individual pixels into one of four categories: tall vegetation (trees), short vegetation, urban, or bare surface, with the last category encompassing water surfaces, bare soil surfaces, and concrete or asphalt-covered surfaces. The classifier design uses knowledge of the nature of radar backscattering from surfaces and volumes to construct appropriate discriminators in a sequential format. The classifier, which was developed using training areas in a test site in Northern Michigan, was tested against independent test areas in the same test site and in another site imaged three months earlier. Among all cases and all categories, the classification accuracy ranged between 91% and 100%     

Calibration of a polarimetric radar using a rotatable dihedralcorner reflector

Based on the existing mathematical formalisms of radar polarimetry, it is necessary to perform accurate and diversified polarimetric measurements in the real world to thoroughly investigate signature definition, identification, and classification of radar targets. For this study the Delft Atmospheric Research Radar (DARR) is used. This ground-based polarimetric FM-CW radar operates in the S-band. The purpose of the present paper is the polarimetric calibration of the DARR. Among the passive reflectors, a rotatable dihedral corner reflector is a suitable calibration object. It enables one to measure different scattering matrices with only one reflector. One alignment must be performed and the scattering matrices are measured at the same range. By measuring several scattering matrices, the accuracy of the calibration result can be estimated. A measurement campaign with a rotatable dihedral corner reflector was therefore performed. The experimental results and the calibration procedure are presented in this paper     

Calibration of radars using polarimetric techniques

A method that uses the properties of rain medium itself to obtain accurate weather radar system gain calibration is discussed. This technique is based on the principle that the rainfall rate measured using absolute reflectivity (Z) and differential reflectivity ( Z_DR) is the same as that obtained from specific differential phase (K_DP). The measurements required for this technique are Z, Z_DR, and K _DP. The rainfall rate estimates obtained from Z and Z_DR are compared with the estimates obtained from K_DP. The scatter plot between the two rainfall estimates should lie close to a 1:1 line, and any systematic deviation from this line can be removed by appropriately adjusting the system gain. It is noted that Z_DR can be calibrated accurately because it is a differential power measurement, and K_DP is obtained from differential phase measurement, which is unaffected by system calibration. The sensitivity and accuracy of this technique are studied, and theoretical and simulation results for C-band frequencies are presented     

Calibration of bistatic polarimetric radar systems

A calibration technique for laboratory type polarimetric, bistatic instrumentation radars is presented. It describes the errors induced by the standard radiation transfer approach (I-SRT) in a way similar to that for the monostatic case. A 12-term error correction and absolute polarimetric calibration is performed with two external reference targets. Only the polarimetric bistatic reference of the first target must be theoretically determined. The scattering reference of the second target is determined by a measurement during the calibration process (single reference calibration). The simulation of a third cross-polarization measurement is performed by an antenna rotation and a remeasurement of the second target. Thus all data are gained for the determination of the error terms and measurements of unknown objects can be full polarimetrically calibrated. The procedures are shown for an adapted dihedral corner reflector. Misalignment errors are discussed     

Measurement of topography using polarimetric SAR images

A processing technique for polarimetric synthetic aperture radar (SAR) data has been developed which produces profiles of terrain slopes and elevations in the azimuthal (or along-track) direction. This technique estimates the average shift in orientation angle of copolarization backscatter caused by azimuthal tilts of the scattering plane. Using P-band data, tests of this technique have been made for an area in the Black Forest near Villingen/Schwenningen in Baden-Wurttemberg, Germany. The radar measured slope and derived elevation profiles have low rms errors and high correlation values when compared with a stereo-photograph digital-elevation map (DEM) for the area. This algorithm is capable of adaptively making transitions from the forested areas to nearby regions with open-terrain. Subsequent tests of the algorithm have been conducted using polarimetric SAR L-band data for a mountainous, nonforested, region in the Mojave Desert (Ft. Irwin, CA) where an accurate DEM also was available. Complete elevation and slope mapping of the terrain in two dimensions using this technique is possible when azimuthal elevation profiles are produced throughout the range extent of the SAR image     

Calibration of WindSat polarimetric channels with a vicarious cold reference

Absolute calibration of WindSat's third and fourth Stokes brightness temperatures (T/sub 3/ and T/sub 4/) is needed at the tenth of Kelvin level in order to adequately resolve their dependence on wind direction. Previous aircraft based fully polarimetric microwave radiometers have generally relied on "circle flights", during which a single area of the ocean is observed at all azimuth angles, to estimate residual biases in the calibration of its polarimetric channels. WindSat, the first spaceborne fully polarimetric microwave radiometer, operates in low Earth orbit and thus cannot execute this traditional calibration technique. A new method is presented to estimate the residual biases that are present in WindSat's T/sub 3/ and T/sub 4/ estimates. The method uses a vicarious cold reference brightness temperature applied to measurements made by WindSat at /spl plusmn/45/spl deg/ slant linear (T/sub P/ and T/sub M/) and left- and right-hand circular (T/sub L/ and T/sub R/) polarization. WindSat derives the third and fourth Stokes brightness temperatures by the differences T/sub P/-T/sub M/ and T/sub L/-T/sub R/, respectively. The method is demonstrated by applying it to the 10.7-GHz WindSat observations. Calibration biases of 0.2-0.6 K are determined with a precision of 0.04 K.     

极化SAR遥感中森林特征的提取

提出了极化散射矩阵总功率、极化熵、相似性参数的组合表达式。通过特征值分析可得该组合表达式的最优系数。该表达式对森林地区的特征敏感,所以可用来检测森林地区。使用中国天山地区的极化SAR数据,验证了所提方法是有效的。     

Statistical analysis of multi-look polarimetric SAR imagery

With a multiplicative speckle model, this paper shows the multi-look polarimetric synthetic aperture radar (SAR) data obeys a generalized K-distribution. To validate this distribution model, the multi-look intensity K-distribution is particularly tested. The relationship between the heterogeneity coefficient of the scene and the proper statistical model is experimentally established. In addition, based on the results of the statistical analysis, an adaptive classification scheme is presented, and the improved classification shows the importance of the statistical analysis.     

Scattering-model-based speckle filtering of polarimetric SAR data

A new concept in polarimetric synthetic aperture radar (POLSAR) speckle filtering that preserves the dominant scattering mechanism of each pixel is proposed in this paper. The basic principle is to select pixels of the same scattering characteristics to be included in the filtering process. To achieve this, the algorithm first applies the Freeman and Durden decomposition to separate pixels into three dominant scattering categories: surface, double bounce, and volume, and then unsupervised classification is applied. Speckle filtering is performed using the classification map as a mask. A single-look or multilook pixel centered in a 9 /spl times/ 9 window is filtered by including only pixels in the same and two neighboring classes from the same scattering category. This filter is effective in speckle reduction, while perfectly preserving strong point target signatures, and retains edges, linear, and curved features in the POLSAR data. The effect of speckle filtering on scattering characteristics, such as entropy, anisotropy, and alpha angle, will be discussed.     

The gridded trihedral: a new polarimetric SAR calibration reflector

The characteristics of the gridded trihedral used to calibrate polarimetric synthetic-aperture radar (SAR) systems are considered. The gridded trihedral is a normal trihedral with one of the conducting faces replaced with a grid of closely spaced (relative to a wavelength) parallel conductors over a layer of microwave absorber. This grid changes the incident wave's polarization and results in a calibration target with a significant cross-polarized reflection. The advantages of the gridded trihedral are that it has a broad backscatter beamwidth (unlike the dihedral), and that it is passive and simple to construct (unlike the active radar calibrator, or ARC). The performance characteristics of the gridded trihedral reflector are reviewed, using basic theoretical models and calibrated P-3 SAR imagery     

超高分辨率PolSAR图像去噪对极化特征的影响

随着超高分辨率极化合成孔径雷达(PolSAR)图像数据的出现,有必要对PolSAR图像的去噪过程对于极化分解结果的影响进行评测。相干斑去噪过程的实施将对观测场景中的PolSAR图像原有极化属性产生不同程度的影响,这将会导致分解结果出现偏差。以非相干分解模型为评估对象,以喷气推进实验室(JPL)提供的开源机载超高分辨率PolSAR系统数据为实验样本,提出了一种利用绝对相对误差的定量评测方法。评测结果显示：对于超高分辨率图像,相干斑噪声的影响明显变小,分解窗口可相应的缩小为7×7的尺寸。研究结果对于PolSAR图像的后期遥感应用中的参数选择具有重要参考意义。     

基于梯度的极化SAR图像超像素分割

超像素分割在图像分割领域以其优异的性能表现被广泛应用,准确性和高效性是评价分割性能的重要指标.简单线性迭代聚类（simple linear iterative clustering,SLIC）方法在光学图像上表现出了优异的性能,在极化合成孔径雷达（synthetic aperture radar,SAR）图像中也被广泛应用,然而SLIC方法中的初始化步骤不能准确地定位类中心,需要多次的迭代纠正误差.改进的分水岭方法（spatial constrained watershed,SCoW）是一种基于梯度阈值区分的简单且高效的分割方法,但是不能直接用于极化SAR图像.本文受SCoW的启发,提出一种对SLIC进行预处理的分割方法,通过横虚警（constant false alarm rate,CFAR）边缘检测器计算得到极化SAR图像的梯度信息,并将梯度信息用于初始化分割.基于两幅实测极化SAR图像,将本文提出方法与其他三种方法对比.实验表明本文方法可以减少整个算法的迭代次数,得到更加符合图像信息、贴合图像边界的分割结果.     

CFAR edge detector for polarimetric SAR images

Finding the edges between different regions in an image is one of the fundamental steps of image analysis, and several edge detectors suitable for the special statistics of synthetic aperture radar (SAR) intensity images have previously been developed. In this paper, a new edge detector for polarimetric SAR images is presented using a newly developed test statistic in the complex Wishart distribution to test for equality of covariance matrices. The new edge detector can be applied to a wide range of SAR data from single-channel intensity data to multifrequency and/or multitemporal polarimetric SAR data. By simply changing the parameters characterizing the test statistic according to the applied SAR data, constant false-alarm rate detection is always obtained. An adaptive filtering scheme is presented, and the distributions of the detector are verified using simulated polarimetric SAR images. Using SAR data from the Danish airborne polarimetric SAR, EMISAR, it is demonstrated that superior edge detection results are obtained using polarimetric and/or multifrequency data compared to using only intensity data.     

偏振傅里叶变换红外光谱仪定标研究

利用偏振傅里叶变换红外（FTIR）光谱仪测量目标红外偏振特性,需要对偏振FTIR光谱仪进行定标,计算出红外辐射目标的绝对光谱能量分布。利用黑体进行偏振FTIR光谱仪定标时,发现不同偏振检测方向的仪器响应有一定偏差,即仪器存在偏振敏感性。通过对仪器偏振特性的定标检测,得到仪器偏振响应函数和偏置特性曲线,应用定标结果,将会消除红外偏振仪器自身的偏振敏感性对目标偏振特性的影响,为提高目标的红外偏振探测精度打下基础。     

SAR内定标技术和内定标精度分析

分析了SAR内定标技术与内定标精度分析.讨论了SAR集中收发式的内定标技术,推导内定标状态下系统总功率和总功率变化量的表达式、系统增益和系统增益变化量的表达式;SAR内定标信号的处理;SAR内定标精度分析;最后完成对某地区SAR图像数据的灰度校正.