Correlation Analysis between L-band SAR and ForestStem Volume in Northeast China

Because Synthetic Aperture Radar(SAR)can penetrate into forest canopy and interact with the primary stem volume contents of the trees (trunk and branch),SAR data are widely used for forest stem volume estimation.This paper investigated the correlation between SAR data and forest stem volume in Xunke,Heilongjiang using the stand-wise forest inventory data in 2003 and ALOS PALSAR data for five dates in 2007.The influences of season and polarizations on the relationship between stem volume and SAR data were studied by analyzing the scatterplots;that was followed by interpretation of the mechanisms primarily based on a forest radar backscattering model-water cloud model.The results showed that the relationship between HV polarization backscatter and stem volume is better than HH polarization,and SAR data in summer dry conditions are more correlated to stem volume than the data acquired in other conditions.The interferometric coherence with 46-day temporal baseline is negatively correlated to the stem volume.The correlation coefficients from winter coherence are higher than those from summer coherence and backscatter.The study results suggest using the interferometric coherence in winter as the best choice for forest stem volume estimation with L-band SAR data.     

Comparison of annual changes in winter ERS-1 SAR images and glacier mass balance of Slakbreen,Svalbard

The Arctic glaciers are sensitive to climate change, and glacier mass balance is used as an indicator of climate change. However, few mass balance observations are available from the Arctic region. Winter ERS-1 SAR images of the Arctic glacier Slakbreen (78degreesN, 16degrees 30'E) on Svalbard were analysed to investigate a possible relation between SAR backscatter and temporal variations in glacier mass balance. A winter ERS-1 SAR image acquired in 1993 after a summer with large ablation was compared with a winter ERS-1 SAR image acquired after the following summer with low ablation. Changes in mass balance from one year to another were difficult to detect using SAR backscatter data only. Comparison of ground-penetrating radar and SAR data showed that the SAR data contain a signal of the near-surface glacier properties. SAR data were interpreted to reflect variations in accumulation and ablation integrated over several years.     

Properties of ERS-1/2 coherence in the Siberian boreal forest and implications for stem volume retrieval

Properties of multi-temporal ERS-1/2 tandem coherence in boreal forests and retrieval accuracy of forest stem volume have been investigated mostly for small, managed forest areas. The clear seasonal trends and the high accuracy of the retrieval are therefore valid for specific types of forest and question is if these findings extend to large areas with different forest types in a similar manner. Using multi-temporal ERS-1/2 coherence data and extensive sets of inventory data at stand level at seven forest compartments in Central Siberia we confirm that the trend of coherence as a function of stem volume is mainly driven by the environmental conditions at acquisition. In addition, we have now found that the variability of the coherence for a given stem volume are due to spatial variations of the environmental conditions, strong topography (slope > 10°), small stand size (< 3-4 ha) and low relative stocking (< 50%). Further deviations can be related to errors in the ground data. Stem volume retrieval behaves consistently under stable winter frozen conditions. For stands larger than 3-4 ha and relative stocking of at least 50%, a relative RMSE of 20-25% can be considered the effective retrieval error achievable in Siberian boreal forest. Combined with previous experience from managed test forests in Sweden and Finland, C-band ERS-1/2 tandem coherence observations acquired under stable winter conditions with a snow cover and an at least moderate breeze can be considered so far the most suitable spaceborne remote sensing observable for the estimation of forest stem volume in homogeneous forest stands throughout the boreal zone.     

Analysis of ERS SAR coherence images acquired over vegetated areas and urban features

Synthetic Aperture Radar (SAR) coherence images were analysed over vegetated areas and urban features. Coherence images were formed from interferometric SAR data acquired 1 day or 35 days apart by two European Remote sensing Satellites (ERS). Forested areas are discriminated very well from cultivated fields using 1-day SAR coherence data taken in the winter when the temperatures were below freezing. This is because fields under these conditions decorrelate much less than forest. Open sandy fields gave high coherence for both winter and summer acquisitions. All vegetated areas experienced a strong temporal decorrelation over a 35-day period. This is mainly due to changing wind, precipitation and temperature conditions, but could also be due to vegetation growth or man-made changes. Many urban objects were found to decorrelate slowly with time, regardless of changing weather conditions.     

Large area forest stem volume mapping in the boreal zone using synergy of ERS-1/2 tandem coherence and MODIS vegetation continuous fields

ERS-1/2 tandem coherence was reported to have high potential for the mapping of boreal forest stem volume (e.g. Santoro et al., 2002, 2007a; Wagner et al., 2003; Askne & Santoro, 2005). Large-scale application of the data for forest stem volume mapping, however, is hindered by the variability of coherence with meteorological and environmental acquisition conditions. The traditional way of stem volume retrieval is based on the training of models, relating coherence to stem volume, with the aid of forest inventory data which is generally available for a few small test sites but not for large areas. In this paper a new approach is presented that allows model training using the MODIS Vegetation Continuous Fields canopy cover product (Hansen et al., 2003) without further need for ground data. A comparison of the new approach with the traditional regression-based and ground-data dependent model training is presented in this paper for a multi-seasonal ERS-1/2 tandem dataset covering several well known Central Siberian forest sites. As a test scenario for large-area application, the approach was applied to a multi-seasonal ERS-1/2 tandem dataset of 223 ERS-1 and ERS-2 image pairs covering Northeast China (~ 1.5 million km²) to map four stem volume classes (0-20, 20-50, 50-80, and > 80 m³/ha).     

Review Article SAR interferometry—issues,techniques, applications

Data from interferometric synthetic aperture radar (INSAR) can provide three-dimensional information by using the phase as an additional information source derived from the complex radar data. In this paper, the issues, techniques and applications of SAR interferometry are reviewed. After a brief historical review, the geometric implementations of SAR interferometry are described. The general processing techniques are summarized and an introduction to various phase unwrapping techniques is given. Besides the differential use of SAR interferometry, the review focuses on parameters such as baseline, decorrelation and motion compensation which have a limiting influence on the quality of the data. After evaluating the various existing and potential applications using SAR interferometric techniques such as topographic mapping, digital elevation modelling, slope measurement, change detection, classification, ocean currents, polar research, seismic events and volcanic hazards, the paper concludes with some research issues, trends and developments in SAR interferometry. The paper is illustrated by examples of SAR interferometric data and derived products from the ESA ERS-1/ERS-2 and SIR-C/X-SAR satellite mission and from single pass aircraft data. An extensive list of references and bibliography on SAR interferometry is provided at the end of this review paper.     

Land surface change detection in a desert area in Algeria using multi-temporal ERS SAR coherence images

Multi-temporal synthetic aperture radar (SAR) interferometric coherence imagery is a useful information source for the detection of random changes of the land surface. Three coherence images derived from three ERS-1 SAR images (acquired on 8 September 1992, 13 October 1992 and 28 September 1993) of a desert area in Algeria revealed several interesting phenomena, including distribution of mobile sand, erosion along river channels, variation of ephemeral lakes and 'mystery' seismic survey lines.     

On the focusing of the ocean swell images produced by a regular and by an interferometric SAR

Focusing of a monochromatic ocean wave image by either a regular or an interferomeric SAR (INSAR) is studied. The model developed simulates the images of both SAR and INSAR for an arbitrary focus setting. The effects of the various operational parameters, i.e., integration and scene coherence times, ocean wave length, direction and amplitude, etc., on the focus sensitivity of both SAR and INSAR is examined. A dimensionless focusing parameter a which depends on the platform velocity and on the azimuthal component of. the phase velocity of the imaged ocean wave is introduced. It is shown that a is the major parameter which determines the image contrast of the regular SAR output, as well as that of the magnitude and the phase components of INSAR. The focus sensitivity, however, is also a function of the imaged ocean wave parameters. The effects of the varying SAR integration time and of the scene coherence time on the contrast of the resulting image is also examined.     

Properties of X-, C- and L-band repeat-pass interferometric SAR coherence in Mediterranean pine forests affected by fires

Synthetic Aperture Radar (SAR) data has been investigated to determine the relationship between burn severity and interferometric coherence at three sites affected by forest fires in a hilly Mediterranean environment. Repeat-pass SAR images were available from the TerraSAR-X, ERS-1/2, Envisat ASAR and ALOS PALSAR sensors. Coherence was related to measurements of burn severity (Composite Burn Index) and remote sensing estimates expressed by the differenced normalized burn ratio (dNBR) index. In addition, the effects of topography and weather on coherence estimates were assessed. The analysis for a given range of local incidence angle showed that the co-polarized coherence increases with the increase of burn severity at X- and C-band whereas cross-polarized coherence was practically insensitive to burn severity. Higher sensitivity to burn severity was found at L-band for both co- and cross-polarized channels. The association strength between coherence and burn severity was strongest for images acquired under stable, dry environmental conditions. When the local incidence angle is accounted for the determination coefficients increased from 0.6 to 0.9 for X- and C-band. At L-band the local incidence angle had less influence on the association strength to burn severity.     

Simulation of ship wakes image by an along-track interferometric SAR

A mathematical model that allows simulations of the image of waves in ship wakes by either regular or interferometric airborne Synthetic Aperture Radar (SAR or INSAR) is described. The three-component velocity field induced at the ocean surface by a moving ship serves as the input to the model. The simulations take into account the effect of temporal variations of the wave field in the wake on the INSAR imaging by the velocity bunching mechanism. The model also accounts for the scanning distortion of the image. The developed algorithm allows study of the visibility of the ship wave wake by a regular SAR or by INSAR for arbitrary imaging parameters, as well as for different ship sizes and ship velocity vectors relative to the platform flight track. Various patterns of ship wake images obtained by numerical simulations are presented.     

INSAR复数影像配准方法研究

文章提出了一种INSAR复数影像自动配准方法,该方法包括重叠景区概略配准、像素级粗匹配、亚像素级精匹配三个环节。为了提高匹配的效率并发现和消除误匹配,在粗匹配时采用了伪金字塔影像分频道相关和回归分析法检验相结合的算法。通过对几景ERS-1/2 SAR影像数据的试验,证明该方法具有良好的性能,对INSAR数据的规模化应用具有实际意义。文中还对常用的亚像素级匹配算法的性能和最小二乘匹配法对复数影像配准的有效性等进行了分析。     

Three-dimensional space-borne synthetic aperture radar (SAR) imaging with multiple pass processing

Three-dimensional (3D) synthetic aperture radar (SAR) imaging via multiple-pass processing is an extension of interferometric SAR imaging. It exploits more than two flight passes to achieve a desired resolution in elevation. In this paper, a novel approach is developed to reconstruct a 3D space-borne SAR image with multiple-pass processing. It involves image registration, phase correction and elevational imaging. An image model matching is developed for multiple image registration, an eigenvector method is proposed for the phase correction and the elevational imaging is conducted using a Fourier transform or a super-resolution method for enhancement of elevational resolution. 3D SAR images are obtained by processing simulated data and real data from the first European Remote Sensing satellite (ERS-1) with the proposed approaches.     

Large-scale mapping of boreal forest in SIBERIA using ERS tandem coherence and JERS backscatter data

Siberia's boreal forests represent an economically and ecologically precious resource, a significant part of which is not monitored on a regular basis. Synthetic aperture radars (SARs), with their sensitivity to forest biomass, offer mapping capabilities that could provide valuable up-to-date information, for example about fire damage or logging activity. The European Commission SIBERIA project had the aim of mapping an area of approximately 1 million km² in Siberia using SAR data from two satellite sources: the tandem mission of the European Remote Sensing Satellites ERS-1/2 and the Japanese Earth Resource Satellite JERS-1. Mosaics of ERS tandem interferometric coherence and JERS backscattering coefficient show the wealth of information contained in these data but they also show large differences in radar response between neighbouring images. To create one homogeneous forest map, adaptive methods which are able to account for brightness changes due to environmental effects were required. In this paper an adaptive empirical model to determine growing stock volume classes using the ERS tandem coherence and the JERS backscatter data is described. For growing stock volume classes up to 80 m³/ha, accuracies of over 80% are achieved for over a hundred ERS frames at a spatial resolution of 50 m.     

Interferometric SAR imagery of a monochromatic ocean wave in the presence of the real aperture radar modulation

Abstract

The model which allows one to simulate the Synthetic Aperture Radar (SAR) and the along track Interferometric SAR (INSAR) imagery of a monochromatic ocean wave is presented. The model accounts for the effects of the real aperture radar (RAR) modulation of the radar backscatter cross-section. The comparison of the SAR and the INSAR imagery of the wave system under consideration is carried out and the performance of the two imaging systems is studied under various operational conditions. The results of the study indicate that the interferometric SAR is less sensitive to the RAR modulation as compared lo the regular SAR. This fact further augments the assertion that INSAR has considerable advantages over SAR in its potential of providing quantitative information about the ocean wave lengths, directions and amplitudes     

Seasonal variation of coherence in SAR interferograms in Kiruna,Northern Sweden

This article presents the results of a study conducted to quantify the seasonal variation of coherence in synthetic aperture radar (SAR) interferograms in Kiruna, Northern Sweden. In SAR interferometry (InSAR), coherence is an important concept that provides a good indication of the phase stability of the scatterers. Therefore, in this study, the degree of coherence is used as a parameter to identify the seasonal variation interferograms. For this study, 34 Radarsat-2 ultra-fine beam mode (U6D) images of the Kiruna area (67°51?N, 20°14?E) and the improved digital elevation model (DEM) created by merging the Radarsat-2 DEM and ASTER DEM were used to produce 561 differential interferograms. The interferograms were arranged in three different ways for the analysis, with the first including common master interferograms (with the summer master image), the second including the sequential interferograms that have the shortest temporal baseline, and the third accounting for all possible combinations of the interferograms (full network of interferograms). Following this step, the variation of coherence for forest areas, urban areas, and flat waste rock areas was studied. This study shows that interferograms generated for the Kiruna region exhibit seasonal variations in coherence due to the ground snow layer in winter. Furthermore, when there is water on the ground due to the melting of the snow layer (in the spring) or due to rains in autumn, the coherence is reduced considerably. Another significant feature is that there is a significant change in summer-to-summer coherence for some regions even over the course of a few years. Based on this study, it is clear that the winter Radarsat-2 U6D beam mode images are not suitable for differential interferometric SAR (DInSAR) deformation measurements in flat waste rock regions in Kiruna. It is expected that even with winter images, PSInSAR or CTM techniques will be able to provide better deformation measurements, but, in this study, those techniques were not assessed. The next step will be to study the seasonal variations in coherence in natural or man-made targets/persistent scatterers using persistent scatter InSAR (PSInSAR) or coherence target monitoring (CTM) techniques.     

干涉雷达永久散射体识别方法的对比分析

基于永久散射体的干涉测量技术可以克服卫星合成孔径雷达应用于区域地表形变探测所面临的时空失相关和大气影响.永久散射体的有效识别是干涉系统中的一个关键环节.本文对永久散射体的两种识别方法, 即相关系数阈值法和振幅离差指数阈值法进行了理论上的分析和实验上的对比.以上海市局部地区作为研究对象, 采用26幅欧洲空间局的ERS-1/2卫星C波段SAR影像为源数据, 使用两种识别方法分别进行了永久散射体的提取, 结果表明, 振幅离差指数方法具有更为稳健的抗噪能力.     

沿海地区不同空间尺度的典型地物相干性研究

基于雷达干涉相干原理,利用6景时间跨度为4年的ENVISAT ASAR雷达影像,分析了不同空间尺度（分辨率分别为20 m和80 m）的苏中沿海地区典型地物的相干特征。研究结果表明：①低分辨率图像的地物相干性总体上要低于高分辨率图像,但前者的相干系数对比度要优于后者;②对于面积较小的地物,空间尺度的变化对相干性的影响显著,而对于面积较大的地物,则影响较小。该研究成果可用于提高土地利用类型分类的精度以及有效地提取长时间地表形变反演中所需的高相干点信息。     

Decorrelation of SAR data by urban damages caused by the 1995 Hyogoken-nanbu earthquake

Large part of urban area of around Kobe city were damaged by the 1995 Hyogoken-nanbu earthquake. For detecting the damaged area, the authors computed correlation coefficients from single-look complex ERS-1/SAR data. Two types of correlations, intensity correlation and coherence were computed. Decorrelation was found in the damaged urban built-up area in the data pair of which interval included the earthquake occurrence. The similarity of the decorrelation between the intensity correlation and the coherence indicated that major factor of the decorrelation is closely related to interferometric processes, which results in the change of speckle patterns in the single-look intensity images. A normalized difference was calculated from the correlation coefficients between the data pair including the earthquake occurrence and the pair before the earthquake. The distribution patterns of the pixels for which normalized difference was higher than a threshold showed good correspondence with the result of the ground survey. The result of this study indicates the possibility of detecting urban disasters using the decorrelation of SAR data.     

Mapping damage during the Bam (Iran) earthquake using interferometric coherence

We use changes in the interferometric coherence to map earthquake damage that occurred in the city of Bam during the Bam earthquake on 26 December 2003. The approach presented here defines a coherence change index that can be interpreted quantitatively in terms of damage during a catastrophic event like a major earthquake. Using five differential interferometric synthetic aperture radar images, we compute maps of interferometric coherence. Three coherence change images are computed from these based on different interferograms. These three damage assessments yield very similar results despite the range of times and interferometric baselines spanned. This coherence‐based damage assessment also agrees closely with independent damage maps derived from other types of imagery. Using existing or planned synthetic aperture radar (SAR) satellites, coherence‐based damage assessments can be obtained within days after a catastrophic event, provided the necessary reference images are prepared ahead of time. As SAR sensors can operate independently of weather conditions and daylight, this may present a reliable and robust means of damage assessment.     

A new phase unwrapping method based on region recognition and region expansion

Phase unwrapping is a key step in retrieving digital elevation models (DEMs) from across-track interferometric synthetic aperture radar (InSAR) data. The coherence of synthetic aperture radar (SAR) data set is an effective indicator for the quality of phase unwrapping. However, the coherence of different regions usually distributes unevenly in SAR images monitoring heterogeneous areas. Errors in low-coherence areas are prone to pollute the whole image. In order to mitigate propagation error, a new phase unwrapping algorithm based on region recognition and region expansion is proposed. In the region recognition step, optical images are incorporated to recognize low-coherence regions by virtue of supervised classification technique. Low-coherence regions and the ones that are not of interest for the application are then discarded. In the region expansion step, stable pixels of high coherence are selected as growing seeds, and then phase unwrapping grows from high-quality regions to low-quality ones guided by coherence information and weighted numbers of neighbouring unwrapped pixels. The ambiguity number of a wrapped pixel is estimated from its neighbouring pixels under the criteria of pixel distance and phase gradient. Iterative examination continues until the whole image is unwrapped. Experiments on PALSAR and ASAR data demonstrate its validity and advantages over other classical methods.