Sea ice monitoring by L-band SAR: an assessment based on literature and comparisons of JERS-1 and ERS-1 imagery

Spaceborne single-polarization C-band synthetic aperture radar (SAR) imagery is widely used to gather information about the state of the sea ice cover in the polar regions. C-band is regarded as a reasonable choice for all-season monitoring capabilities. For specific mapping tasks, however, other frequency bands can be more suitable. In the first part of this paper, the summary of a literature study dealing with the utilization of L-band SAR imagery for sea ice monitoring is presented. Investigations reveal that if deformation features such as ice ridges, rubble fields, and brash ice are to be mapped, L-band radar is superior in a number of cases. The second part of this paper addresses the comparison of JERS-1 and ERS-1 SAR images that were acquired over sea ice east of Svalbard and along the east coast of Greenland. The effects of the different frequencies, polarizations, and incidence angles of the two SAR systems are discussed. It is demonstrated that the images of both sensors complement one another in the analysis of ice conditions, resulting in a more detailed view of the sea ice cover state.     

Automated delineation of dry and melt snow zones in Antarctica using active and passive microwave observations from space

This paper presents the algorithms and analysis results for delineating snow zones using active and passive microwave satellite remote sensing data. With a high-resolution Radarsat synthetic aperture radar (SAR) image mosaic, dry snow zones, percolation zones, wet snow zones, and blue ice patches for the Antarctic continent have been successfully delineated. A competing region growing and merging algorithm is used to initially segment the SAR images into a series of homogeneous regions. Based on the backscatter characteristics and texture property, these image regions are classified into different snow zones. The higher level of knowledge about the areal size of and adjacency relationship between snow zones is incorporated into the algorithms to correct classification errors caused by the SAR image noise and relief-induced radiometric distortions. Mathematical morphology operations and a line-tracing algorithm are designed to extract a vector line representation of snow-zone boundaries. With the daily passive microwave Special Sensor Microwave/Imager (SSM/I) data, dry and melt snow zones were derived using a multiscale wavelet-transform-based method. The analysis results respectively derived from Radarsat SAR and SSM/I data were compared and correlated. The complementary nature and comparative advantages of frequently repeated passive microwave data and spatially detailed radar imagery for detecting and characterizing snow zones were demonstrated.     

Observations of variations in ERS-1 SAR image intensity associatedwith forest fires in Alaska

Investigations have shown that fires in boreal forests result in characteristic responses which are recorded on ERS-1 SAR imagery. Using one of the many fire signatures observed on ERS-1 SAR imagery, the analysis of the data revealed there is >10 dB in variation in image intensity within the burned areas on the SAR images. Possible explanations for this variation include increases in soil moisture, increased surface roughness exposed to the incoming microwave radiation, and damage to the vegetation canopy by the fire     

Determination of the age distribution of sea ice from Lagrangianobservations of ice motion

A procedure for monitoring the local age distribution of the Arctic sea ice cover is presented. The age distribution specifies the area covered by ice in different age classes. In the authors' approach, a regular array of grid points is defined initially on the first image of a long time series, and an ice tracker finds the positions of those points in all subsequent images of the series. These Lagrangian points mark the corners of a set of cells that move and deform with the ice cover. The area of each cell changes with each new image or time step. A positive change indicates that ice in a new age class was formed in the cell. A negative change is assumed to have ridged the youngest ice in the cell, reducing its area. The ice in each cell ages as it progresses through the time series. The area of multiyear ice in each cell is computed using an ice classification algorithm. Any area that is not accounted for by the young ice or multiyear ice is assigned to a category of older first-year ice. The authors thus have a fine age resolution in the young end of the age distribution, and coarse resolution for older ice. The age distribution of the young ice can be converted to a thickness distribution using a simple empirical relation between accumulated freezing-degree days and ice thickness, or using a more complicated thermodynamic model. The authors describe a general scheme for implementing this procedure for the Arctic Ocean from fall freeze-up until the onset of melt in the spring. The concept is illustrated with a time series of five ERS-1 SAR images spanning a period of 12 days. Such a scheme could be implemented with RADARSAT SAR imagery to provide basin-wide ice age and thickness information     

Extracting Sea Ice Data from Satellite SAR Imagery

With the prospect of operational satellite SAR's by the end of the decade, there is a clear need to develop automated algorithms for the extraction of geophysical data about sea ice from highresolution radar imagery. To this end, we have developed techniques for distinguishing ice from open water and for resolving the details of deformation within areas 100 km square imaged by Seasat SAR. The classification of ice and open water is based on the creation of a second band of image data consisting of the local variance of the original brightness, the first band being a local average brightness. In the space of these two variables, ice and open water are separated into two distinct clusters. The deformation is found on a 3.4-km mesh by local cross correlations of the brightness. The strategy is to find a coarse displacement field with a highly averaged image, and to proceed through a hierarchy of images with increasing resolution, improving the accuracy of the displacements at each step. Comparison with manually determined displacement shows room for improvement in regions of high deformation by using smaller areas for cross correlation. The concentration and deformation data are used together to determine localized regions of the scene where open water is produced or lost.     

基于SAR实测数据的舰船成像研究

该文基于机载SAR实测数据,对舰船目标成像进行了研究。SAR成像是针对静止目标,舰船目标表现在SAR图像上是散焦的。基于舰船目标的非合作性,可采用ISAR的方法对其进行成像处理,以得到高分辨的舰船图像。该文介绍了如何从SAR实测数据中提取出舰船目标的回波数据,并讨论了对舰船成像比较有效的ISAR运动补偿方法。最后给出了多艘典型舰船的清晰成像结果。该文提供了机载雷达SAR模式工作时对舰船成像的有效处理方法。     

在振动环境下相位噪声对星机BiSAR的影响

振荡器的相位噪声是影响BiSAR获得高分辨率图像的重要因素之一。振荡器是一种对振动极为敏感的器件。因此,在振动环境下,定量分析相位噪声对BiSAR的影响是十分必要的。文中基于两个独立非同分布的振荡器,机载SAR在静态条件和随机振动环境下,推导了星载一机栽混合BiSAR（SA-BiSAR）积分旁瓣比的计算公式。仿真结果表明,在静态条件下,SA—BiSAR仍然具有很低的相噪谱;机载SAR在振动环境下,对于L波段,可以满足成像的基本要求;对于X波段,需采取必要的隔振措施提高系统的抗振性能来满足成像的条件。     

合成孔径雷达图像斑点噪声抑制与滤波

斑点噪声的存在,严重阻碍了合成孔径雷达（SAR）图像的应用。根据斑点噪声的形成机理,分析并比较了抑制SAR斑点噪声的传统滤波算法及统计滤波算法的原理,利用ERS-2的SAR图像数据比较了这几种算法对SAR图像斑点噪声的滤除效果,根据噪声滤波效果评价参数得出Gamma滤波抑制斑点噪声的综合性能最好。     

Comparison of Multifrequency Band Radars for Crop Classification

A first, systematic experiment with airborne SAR systems working in X-, C-, and L-band took place during the European Convair SAR 580 experiment in 1981. One major topic of the use of SAR images is to analyse their potential for agricultural applications including crop identification. This paper describes results from the analysis of images taken over two German test sites. These data prove that differences of the backscattering behavior of crops are mainly related to the dielectric geometry of the plants. The paper will give examples. Furthermore, the data analysis undertaken on the base of SAR 580 data will be described.     

Analyzing lead information from SAR images

Leads are relatively linear features in the sea ice cover, which are composed of open water or new, thin ice. Because of their composition, leads impact the ocean/air heat exchange. Automated analysis of leads from sea ice imagery may provide a means of gathering important information about the sea ice cover and its climatic influence. This paper describes: (1) a method for extracting and analyzing leads from ERS-1 synthetic aperture radar (SAR) images classified by ice type and (2) the results of using this method on images of the Beaufort Sea. The methodology consists of identifying potential lead features in the image and measuring their characteristics both before and after using a thinning or skeletonization technique on the features. The measurements obtained using this method include lead area, average width, number of leads in an area, amount of branching, and linearity of the lead. These measurements were analyzed with respect to the time of year and the latitude of the images. Results indicate that the measurements produced by the methodology are consistent with lead measurement distributions that others have found. The results of the study suggest that the methodology is appropriate to study lead characteristics on a large scale     

一种多视极化SAR广义K-分布的研究

本文提出一种极化合成孔径雷达(SAR)广义K-分布模型用于描述经多视处理的、高分辨率的全极化SAR成像数据的统计特性,其特点是各极化通道的纹理为部分相关的gamma分布.利用纹理与相干斑乘性模型,首先导出了部分相关纹理的联合概率密度函数和多视广义极化SARK-分布概率密度函数;然后给出了基于联合矩的分布参数估计方法;最后利用真实SAR数据进行了模型的试验.     

基于小波变换和马尔可夫随机场的极化SAR图像自动分类

本文提出一种对极化合成孔径雷达(SAR)图像进行自动多分辨率分类的方法。首先利用多视极化白化滤波(MPWF)抑制极化SAR图像的相干斑,得到反映地物辐射特征的纹理SAR图像,然后利用小波变换(WT)提取不同分辨率的纹理信息,在最低分辨率级利用Akaik信息准则(AIC)自动估计图像中的纹理类数,进而在各个分辨率级利用马尔可夫随机场(MRF)模型表征各像素间的空间关联信息,并分别利用最大似然(ML)方法和循环条件模式(ICM)进行自动的模型参数估计和最大后验概率(MAP)分类,最后应用NASA/JPL机载L波段极化SAR数据验证了本文所提分类方法的有效性和优越性。     

Resolution enhancement of spaceborne scatterometer data

A method for generating enhanced resolution radar images of the Earth's surface using spaceborne scatterometry is presented. The technique is based on an image reconstruction technique that takes advantage of the spatial overlap in scatterometer measurements made at different times to provide enhanced imaging resolution. The reconstruction algorithm is described, and the technique is demonstrated using both simulated and actual Seasat-A Scatterometer (SASS) measurements. The technique can also be used with ERS-1 scatterometer data. The SASS-derived images, which have approximately 4-km resolution, illustrate the resolution enhancement capability of the technique, which permits utilization of both historic and contemporary scatterometer data for medium-scale monitoring of vegetation and polar ice. The tradeoff between imaging noise and resolution inherent in the technique is discussed     

Complex SAR imagery and speckle filtering for wave imaging

A method is described to predict the wavenumber dependence of the speckle component in spectra of synthetic aperture radar intensity images. Filtering of this component is an important step in recovering waveheight spectra for synthetic-aperture radar (SAR) images of the ocean, and an effective means of doing so is required for the 'wave mode' of the European satellite ERS-1. The method uses the correlation function of the corresponding complex images and has been tested using a variety of airborne and spaceborne imagery obtained over both land and sea. Examples are shown of both successful and unsuccessful applications of the method.<>     

Forest Biophysical Parameter Estimation Using L- and P-Band Polarimetric SAR Data

L- and P-band airborne polarimetric synthetic aperture radar (SAR) data acquired by the RAMSES system over different height maritime pine (Pinus Pinaster Ait.) stands of the Nezer forest (Landes, France) have been evaluated for forest biophysical parameter estimation. A pseudolinear correlation has been brought to evidence at P-band between polarimetric anisotropy and mean tree height, which is also linked to other biophysical parameters in the Nezer forest, meaning that SAR polarimetry constitutes a promising tool for forest parameter retrieval at low frequency. The spatial conditions have been evaluated through the quantification of the impact of signal-to-noise ratio diminution and resolution degradation on the forest height inversion. It has been shown that the inversion accuracy remains acceptable for N Esigma₀, representing the noise level of the SAR image, which is lower than -15 dB, and for spatial resolution increasing up to 15 m.     

An algorithm for interferometric SAR data processing

In this paper, an algorithm of generating INSAR unwrapped phase image from SAR single-look complex images is presented. Besides the general processing technique, this article focuses on the methods of flat-earth phase removal, phase noise reduction and phase unwrapping. The availability is tested by the results of processing ERS-1/2 SAR images.     

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180° ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and σ ₀ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11° to the right of the surface wind direction and can be used to estimate the surface wind direction to within ±19° and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately ±1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991     

干涉合成孔径雷达数据处理实现方法

本文提出了干涉合成孔径雷达(INSAR)从SAR单视复图像到非模糊相位图的一种实用数据处理方法。详细讨论了其中的去平地相位、相位图噪声抑制、相位展开等的实现方法。并用ERS-1/2的真实SAR图像数据验证该方法的有效性。     

Change detection for thematic mapping by means of airbornemultitemporal polarimetric SAR imagery

The paper addresses the detection of changes in multitemporal polarimetric radar images, focusing on small objects and narrow linear features. The images were acquired at C- and L-band by the airborne EMISAR system. It is found that the radar intensities are better suited for change detection than the correlation coefficient and the phase difference between the co-polarized channels. In the case of linear features, there is no obvious difference between the C- and L-bands , and slight variations of the flight tracks are acceptable at look angles larger than 35 degrees. Theoretical detection thresholds are evaluated from the statistical distribution of the intensity ratio due to speckle. For the linear features and for urban environments, the observed thresholds are larger than the theoretical predictions. This is interpreted as an effect of radar intensity variations on length scales smaller than the spatial image resolution. The signature of urban areas is very sensitive to deviations between the flight tracks, and the sensitivity is larger at C-band than at L-band. On the other hand, the intensity contrast between buildings and the urban background is smaller at L-band and larger at C-band. For change detection, thresholds may have to be chosen separately for each object class because the intensity ratios of different object classes vary differently as a function of time     

基于CxImage库的SAR图像Speckle噪声抑制

合成孔径雷达图像的相干斑噪声抑制是SAR信息处理中的一个重要环节。经典的Speckle噪声抑制通常作为一个处理模块集成在软件中,但现有软件代码封装无法二次开发。针对文中提出了一种基于开源图像库CxImage的空间域自适应Speckle噪声抑制算法的应用与集成。将CxImage图像库链接入MFC应用程序框架中,利用其图像管理、维护、处理功能对SAR图像进行维护管理,集成多种经典的空间域自适应滤波方法,并以ERS-2卫星的PRI SAR数据为例,进行Speckle噪声滤波处理,选取适当的滤波效果评价参数,对滤波结果进行比较,最终得出各滤波算法应用于SAR图像滤波的优劣。