An improved atmospheric correction method in repeat-pass InSAR measurements

ABSTRACT

As is well known, the problem of atmospheric phase delays is currently one of the major limitations to high precision repeat-pass interferometric synthetic aperture radar (InSAR) applications. Therefore, accurately mitigating atmospheric phase delays is a key but unresolved problem. In recent decades, many researchers have attempted to mitigate the atmospheric phase delays. A typical method is to calculate the atmospheric phase along radar line of sight direction using precipitable water vapour products. However, this method completely neglects the atmospheric turbulence and heterogeneity in the horizontal direction when the zenith wet delays are converted to slant wet delays, reducing the accuracy of the approach. In this article, we systematically analysed the atmospheric turbulence and heterogeneity in the horizontal direction as well as the vertical stratification of water vapour in the troposphere. From this analysis, we found that the atmospheric turbulence and heterogeneity in the horizontal direction cannot be neglected in estimating atmospheric phase delays. Moreover, the vertical stratification of water vapour and the satellite orbit azimuth can be integrated into the calculation of slant phase delays. We therefore propose an improved atmospheric correction method in repeat-pass InSAR measurements, weighted-sampling radiometer correction. The proposed method was applied to the investigation of ground subsidence in an area of Beijing, China to validate its feasibility and accuracy. The results illustrate that atmospheric phase delays and ground subsidence information can be retrieved more accurately using the proposed method than was obtained using the conventional method.     

基于加权联合导向矢量模型的InSAR干涉相位估计

基于干涉图的传统干涉相位估计方法,当由于图像配准误差而导致的干涉图质量较差时,就难以恢复出准确的真实干涉相位.本文提出了一种基于加权联合导向矢量模型的InSAR干涉相位估计方法.该方法构造最优联合观测矢量和加权联合导向矢量,同时利用相邻像素的相干信息,并采用波束形成技术,因此具有自适应图像配准和降低相位噪声的功能,因而可以在SAR图像配准精度很差(可以允许达到一个分辨单元)的条件下准确地估计相应像素间的干涉相位.仿真及实测数据的处理结果证明了此方法的有效性.     

InDeandCoE: A framework based on multi-scale feature fusion and residual learning for interferometric SAR remote sensing image denoising and coherence estimation

Synthetic aperture radar (SAR) interferometry is a high-resolution microwave remote sensing imaging method. Over the past two decades, many researchers working on remote sensing have applied this technology in various disciplines, including environmental monitoring, disaster monitoring, and elevation mapping. However, due to the existence of many influencing factors in the acquisition stage, such as atmospheric humidity and temperature, the reflected wave signals from the ground will be disturbed when received by remote sensing satellites. The presence of noise in interferograms is inevitable. Therefore, the accuracy of interferometric SAR phase denoising and coherence estimation has a decisive impact on the validity of subsequent processing results. In this paper, we pioneer the use of a nested U-net as a feature extractor for interferometric SAR phase and coherence. In addition, we build a phase filter and a coherence estimator by using the residual learning module. With the aim of determining the unique non-local similarity of InSAR images, we use non-local convolution and channel attention mechanisms to extract features in different dimensions of the interferogram. Through quantitative and qualitative experiments, the proposed method performs better in phase denoising and coherence estimation than state-of-the-art methods.     

Residual motion estimation with point targets and its application to airborne repeat-pass SAR interferometry

Due to the lack of accuracy in the navigation system, airborne repeat-pass synthetic aperture radar (SAR) interferometry (InSAR) suffers residual motion errors (RMEs). Previously, we proposed the multisquint technique with point targets (MTPT) to estimate the errors, but its implementation needs improvement and its accuracy has not been assessed by real airborne repeat-pass InSAR data. In this article, the modified MTPT is introduced first. The modification is mainly embodied in two aspects: automatic target selection and noise removing. Because the multisquint (MS) technique is also capable of estimating the RMEs in SAR interferogram and its accuracy has been verified to be high, here it is used as a comparison. In addition, from the viewpoint of MTPT, MS can be understood from another perspective. Using real X-band airborne repeat-pass SAR data, the performance of MTPT is evaluated. The experiment shows that MTPT is able to achieve high accuracy when a large number of point targets are distributed in the observed scene. The limitations of MTPT are also discussed at the end of this article.     

Monitoring land deformation in Changzhou city (China) with multi-band InSAR data sets from 2006 to 2012

Over exploitation of groundwater in Changzhou city, China can cause land deformation, which in turn proves detrimental to the urban infrastructure. In this study, multi-band synthetic aperture radar (SAR) data sets (C-band Envisat ASAR, L-band ALOS PALSAR, and X-band COSMO-SkyMed) acquired from 2006 to 2012 were analysed using the synthetic aperture radar (SAR) interferometry (InSAR) time-series method to investigate the relationship between spatial–temporal distribution of land deformation and groundwater exploitation. Annual deformation rate inferred from multi-band interferograms ranges from ?58 to 24 mm year^?1. Levelling-survey data were used to validate the multi-band InSAR measurements. The results showed that these two types of measurements were generally in agreement. Correlating groundwater-table and multi-band InSAR measurements at six groundwater-well stations showed that with the rise of the water table, the land rebounded. But in some areas with larger subsidence, continual subsidence was observed even though the water table rose after the prohibition of groundwater exploitation. This may have been caused by the hysteresis effect due to the consolidation of strata (especially for the creep deformation). Our study provides scientific evidence on the management of groundwater extraction and the assessment of land-subsidence hazards.     

星载SAR干涉技术获取DEM及其精度分析

星载合成孔径雷达干涉（InSAR）技术是一种数据覆盖范围广、廉价、高效、方便的数字高程模型（DEM）获取方法,但在地面植被覆盖广、大气水汽含量高的地区其影像相干性随时间基线的增加迅速降低;同时,SAR卫星的轨道误差也影响DEM精度。利用ERS-1/2卫星串行模式SAR数据获取镇江地区DEM,分析了轨道误差对DEM精度的影响;根据干涉相位的统计特性,从理论上给出干涉相位噪声与相干系数和视数之间的关系。实验结果表明就干涉像对的卫星轨道误差和相位噪声而言,在小区域内DEM精度优于3.5 m。     

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.     

A wavelet domain detail compensation filtering technique for InSAR interferograms

The quality of interferogram filtering affects the accuracy of interferometric synthetic aperture radar (InSAR) applications. This article presents a new wavelet domain filtering method for phase noise reduction in an InSAR interferogram. The method first transforms the real and imagery parts of the original interferogram into the wavelet domain using the stationary wavelet transform (SWT). Then the coefficients for each sub-band are filtered with detail compensation. Finally, the wavelet coefficients are reconstructed in the space domain by the inverse SWT. The results show that the proposed method can suppress the speckle effectively, maintain details of the interferogram well, and greatly reduce the number of residues.     

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.     

一种星载InSAR高精度快速DEM重建方法

面对全球干涉测量的海量实测数据,在保持精度的情况下,如何提高处理速度是星载干涉合成孔径雷达(interferometric synthetic aperture radar,InSAR)数据处理必须考虑的重要问题.数字高程模型(digital elevation model,DEM)重建是InSAR数据处理中较为耗时的一个关键环节.针对星载InSAR处理中DEM快速重建的难题,从DEM重建原理出发,分析揭示了干涉相位与目标点三维坐标映射关系的两个基本特性,一是目标点的三维坐标与干涉相位的关系可以分别用多项式来进行拟合,二是SAR图像上相近像素各自对应的多项式变化不大,并从理论上对特性的成立进行了论证.基于此,提出了一种快速DEM重建方法,给出了快速算法的详细步骤及关键参数的取值方法.最后,利用德国最先进的在轨雷达卫星TerraSAR-X获取的重复轨道干涉数据进行快速DEM重建,处理结果表明在重建精度损失较小的情况下,显著提高了重建速度,验证了该方法的高效性和正确性.     

An iterative Goldstein SAR interferogram filter

This article presents a new modified Goldstein synthetic aperture radar (SAR) interferogram filter algorithm, named the iterative Goldstein filter. The main idea of this approach is to iteratively filter the SAR interferogram, by determining the filtering parameter alpha adaptively with respect to the pseudo-correlation value of the original and/or last filtered interferograms several times. The filter can be stopped automatically by pre-setting the threshold of mean value and the improvement of pseudo-correlation in given filter windows. Experimental results with both a simulated digital elevation model (DEM) interferogram and real SAR deformation interferogram show an improvement in the new algorithm results compared with those using the Goldstein filter, and its enhanced version, the Baran filter. In addition, from a pseudo-correlation map of the iteratively filtered interferogram, some valuable information can also be abstracted based on the signal residues.     

基于小波域HMT模型InSAR干涉图噪声滤波研究

相干斑噪声是InSAR干涉图固有的,且InSAR干涉图对相干斑噪声十分敏感,为了得到精度更高的干涉测量产品,需要对干涉图进行相干斑噪声滤波。针对InSAR干涉图中相干斑噪声的统计特性,提出了一种基于小波域HMT模型的InSAR干涉图滤波算法,对InSAR干涉图的实部和虚部分别进行处理。试验研究结果表明,该方法在有效抑制相干斑的同时,还能有效地保持相位的细节信息和条纹的边缘结构,而且大大地减少了残余点的数量,有利于提高InSAR干涉测量的精度。     

重复轨道InSAR处理中的频域滤波研究

在重复轨道InSAR(synthetic aperture radar interferom etry)处理干涉测量中,干涉图像对在方位向和距离向的频谱偏移是重要的去相干因素,严重影响干涉图的质量,必须对主辅图像对采取适当的频域滤波。本文提出了一种方位向频域滤波和距离向频域滤波的处理流程,即根据频域特征选择在精配准前后做频域滤波。它既能保证精配准的准确度,又能充分降低频谱偏移导致的去相干,从而保证干涉图的高质量。利用4组不同时间间隔和垂直基线距的ERS-1/2 SLC(single look comp lex)数据对进行实验验证,通过对频域滤波前后相干图和残差点的对比分析,验证了本处理流程的有效性。     

Using C/X-band SAR interferometry and GNSS measurements for the Assisi landslide analysis

This work presents an analysis of the applicability of synthetic aperture radar (SAR) interferometry to landslide monitoring. This analysis was carried out by using different interferometric approaches, different spaceborne SAR data (both in the C-band and in the X-band), and in situ global navigation satellite system (GNSS) measurements. In particular, we investigated both the reliability of displacement monitoring and the issues of the cross-comparison and validation of the interferometric synthetic aperture radar (InSAR) results. The work was focused on the slow-moving landslide that affects a relevant part of the urban area of the historical town of Assisi (Italy).

A C-band ENVISAT advanced synthetic aperture radar (ENVISAT ASAR) dataset acquired between 2003 and 2010 was processed by using two different interferometric techniques, to allow cross-comparison of the obtained displacement maps. Good correspondence between the results was found, and a deeper analysis of the movement field was possible. Results were further compared to a set of GNSS measurements with a 7 year overlap with SAR data. A comparison was made for each GNSS marker with the surrounding SAR scatterers, trying to take into account local topological effects, when possible.

Further, the high-resolution X-band acquired on both ascending and descending tracks by the COSMO-SkyMed (CSK) constellation was processed. The resultant displacement fields show good agreement with C-band and GNSS measurements and a sensible increase in the density of measurements.     

Detection of river/sea ice deformation using satellite interferometry: limits and potential

In this paper, we present a study consisting of the application of radar interferometry for river/sea ice monitoring in inhabited regions and on commercial waterways. The sites studied are located in Canadian regions where ice jams constitute a common winter hazard that can cause extensive socio-economic damage and impose severe restrictions on ship traffic. ERS and Radarsat images were jointly used with traditional in situ observations to detect ice break-up in order to prevent ice jams and related problems. A coherence study served to define the synthetic aperture radar interferometry (InSAR) limits for river/sea ice dynamics monitoring. Other factors that also help to define the limits of InSAR technology for this application include the frequency of image acquisition, the minimum dimension of detected ice floes and the determination of appropriate ice types. Significant phase shifts were found for small ice floes of several hundred metres with ERS-tandem images. The analysis of the interferograms showed that it is possible to detect deformations in the ice shelf and to discriminate quantitatively the horizontal and vertical components of ice movement when the interferograms are combined with traditional observations such as meteorological data, water level, water flow and ice charts. The deformation estimated on a piece of fast river ice can be interpreted as the first sign of the ice break-up. On an estuary river that is a busy seaway, a qualitative interpretation of the interferograms served to highlight the interaction of river and tidal flows affecting the ice cover. We showed, in particular, the potential of radar interferometry and its integration with other techniques to help the authorities to prevent problems related to ice jams.     

快速区域互相关InSAR图像配准方法

在分析复图像干涉相位对快速互相关算法配准精度影响的基础上,提出了一种快速区域互相关InSAR图像配准方法。该方法对图像的幅度谱进行区域相关操作,获得干涉相位空间角频率的粗估计,并对主图像进行干涉相位补偿。该算法通过上述步骤消除干涉相位变化对算法的影响实现了高精度配准。在仿真实验和实测实验中,通过与传统的快速区域互相关算法及最大频谱法进行对比,验证了本文算法的稳健性与有效性。     

Post-emplacement lava subsidence and the accuracy of ERS InSAR digital elevation models of volcanoes

Repeat-pass synthetic aperture radar interferometry (InSAR) using data acquired by the ERS platforms is an attractive method for acquiring topographic data of volcanoes. Caution is advised, however, when using this technique in regions covered by young, thick lava flows. In this study, the magnitude of post-emplacement subsidence associated with the 1991-93 lava flow at Mount Etna, Sicily, was measured using differential radar interferometric techniques, and it was found that the rates of subsidence are large enough to contribute a significant component to the measured phase shift, even in ERS data acquired on consecutive orbits. It demonstrates the detrimental effect that such phase shifts have on the accuracy of digital elevation models derived by repeat-pass radar interferometry.     

SAR影像配准中控制点粗差剔除方法研究

复数影像配准是雷达干涉测量中的关键步骤之一,而配准中选取的控制点往往存在粗差,影响配准精度。分析了SAR影像配准时控制点粗差产生的原因,提出了方差因子检验和Baarda数据探测法剔除控制点粗差的方法,建立了基于可靠控制点计算影像纠正的几何变换模型。利用ENVISAT卫星ASAR图像对所提方法进行了验证,结果表明,影像配准的精度达到了0.1像素,粗差剔除后SAR影像配准精度有明显提高。     

雷达干涉条纹图滤波方法研究

在合成孔径雷达干涉测量中,为滤除干涉图像中存在的各种噪声,提高相位解缠结果及生成的数字高程模型的精度.从分析干涉条纹图的特点入手,对干涉条纹图的圆周期中值滤波法进行了研究,提出一种基于高斯噪声估计的自适应圆周期中值滤波方法,给出了算法设计及详细的实验数据并与其它滤波方法进行比较.试验结果表明,该方法可以较好地消除干涉图像中存在的噪声,而且保留更多的图像边缘、细节信息.     

Land deformation monitoring using coherent target-neighbourhood networking method combined with polarimetric information: a case study of Shanghai,China

In this article, an advanced approach for land deformation monitoring using synthetic aperture radar (SAR) interferometry combined with polarimetric information is presented. The linear and nonlinear components of the deformation, the error of the digital elevation model (DEM) and the atmospheric artefacts can be achieved by a coherent target (CT)-neighbourhood networking approach. In order to detect recent land deformation in Shanghai, China, 12 ENVISAT advanced synthetic aperture radar (ASAR) alternating polarization images acquired from January 2006 to August 2008 are employed for deformation analysis. Over a 2.5-year period, two deformation velocity fields from HH and VV modes over Shanghai are derived using the CT-neighbourhood networking SAR interferometry (InSAR), then integrated into a final deformation map by a fusion scheme. It is found that the annual subsidence rates in the study area range from??20 to 10 mm year^?1 and the average subsidence rate in the downtown area reaches??7.5 mm year^?1, which is consistent with the local government statistics published in 2007.