Ground-based SAR interferometry for terrain mapping: theory and sensitivity analysis

This short communication studies the problem of terrain mapping by means of the ground-based synthetic aperture radar (GB-SAR) interferometry technique. A phase-to-height relationship tailored for the GB-SAR interferometric configuration is introduced and verified by a simulated analysis. A sensitivity study is carried out aiming to optimize the use of a GB-SAR system for the terrain mapping and to derive the precision of this technique.     

Millimeter-wave Ground-based Synthetic Aperture Radar Imaging for Foreign Object Debris Detection: Experimental Studies at Short Ranges

In this paper, millimeter-wave imaging of foreign object debris (FOD)-type objects on the ground is studied with the help of ground-based synthetic aperture radar (GB-SAR) technique. To test the feasibility of detecting runway FODs with this technique, some preliminary experiments are conducted within short antenna-to-target ranges of small imaging patches. An automated stripmap GB-SAR system with stepped-frequency transmission is constructed together with a quasi-monostatic data collection operation. The imaging experiments for various braces and screws are then carried out by using 32–36?GHz and 90–95?GHz frequency bands of the millimeter-wave. Images reconstructed by a matched-filter based algorithm are analyzed to determine the proper system parameters for an efficient imaging and to comprehend the factors against a successful detection. Results demonstrate the capability of GB-SAR imaging in accurately locating these FOD-like targets under near-range operating conditions.     

Indoor C-band polarimetric interferometry observations of a mature wheat canopy

We present results from experiments carried out in the ground-based synthetic aperture radar (GB-SAR) facility at the University of Sheffield to ascertain the role of polarimetric interferometry in crop height retrieval. To this end, a mature wheat canopy, grown in outdoor conditions, was reassembled inside the GB-SAR chamber and imaged at C-band using a two-dimensional scan. This allowed fully polarimetric tomography and interferometry. Interferometry using the VV, HH, and VH polarization states shows that the HH and VH interferograms retrieve a height close to the top of the soil layer for all angles of incidence considered, whereas the height retrieved from the VV interferogram increases with angle of incidence. The use of a Pauli basis gives poor results, due to the different location of the scattering phase centers in the VV and HH channels. The use of arbitrary polarization states shows that the top of the soil can be very accurately estimated using left-circular polarization, whereas, for angles of incidence close to 45/spl deg/, a polarization state similar to VV can be used to retrieve the top of the canopy; hence crop height can be recovered as the difference of these two interferometric heights. Polarimetric coherence optimization techniques are also studied. Unconstrained coherence optimization gives very unstable results, due to the small number of available samples. Constrained optimization results in stable retrieved heights, and the retrieved polarization states agree well with the polarization synthesis results.     

地基SAR子图相干合成快速成像算法

地基合成孔径雷达（Ground Based Synthetic Aperture Radar, GB-SAR）差分干涉技术可获得亚毫米级的测量精度,在人造大型建筑物和地表形变监测领域具有广泛的应用前景。在成像处理方面,地基SAR成像几何构型具有合成孔径极短、方位波束宽、成像范围大的特点,与传统机载、星载SAR系统有很大差别,现有成像算法大都不能有效满足地基SAR实时成像处理需求。为此,本文提出一种基于子图像相干合成的地基SAR快速成像算法,能够实现地基SAR宽角度、近远场混合大场景数据的快速处理,得到伪极坐标系下成像结果。最后,利用仿真数据和实测数据对本算法进行了验证,证明了算法的有效性。      

Permanent scatterers analysis for atmospheric correction in ground-based SAR interferometry

Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, complementary or alternative to spaceborne SAR interferometry, for terrain monitoring, and for detecting structural changes in buildings. It has been noted that, in spite of the very short range, compared with the satellite configuration, in GB-SAR measurement the disturbances due to atmospheric effects cannot be neglected either. The analysis of the interferometric phases of very coherent points, called permanent scatterers (PSs), allows the evaluation of the atmospheric disturbance and the possibility of removing it. In this paper, the PS analysis is carried out both on a test site facility and on a real campaign (Citrin Valley, Italy) that provided data with a temporal baseline of about ten months.     

Analysis of Ground-Based SAR Data With Diverse Temporal Baselines

In this paper, the algorithms developed for satellite synthetic aperture radar (SAR) interferometry were adapted to the ground-based SAR (GB-SAR) configuration and used for detecting the displacements of an alpine landslide which have occurred over many years. Indeed GB-SAR interferometry is based on the same principles as satellite SAR techniques but benefits from the GB-SAR's versatility and capability of gathering many images per day. In monitoring applications of landslides moving only few centimeters per year, as the case here reported, the GB-SAR sensor is installed at repeated intervals several months apart over the observation period. Although the revisiting time is very similar to the satellite one, for each survey, lasting two or three days, more than ten images are available. They are analyzed separately and in combination with images from other surveys for coherent pixel selection. Interferograms are formed by cross-combining images from different surveys. Finally, the evolution of the deformation across the surveys is retrieved in a least square sense without any assumptions on its regularity. The used GB-SAR technique is described in detail in this paper, and the results obtained with regard to a landslide in the Italian Alps that has been monitored over a period of about three years are discussed.     

一种多部地基SAR联合观测时的图形配准方法

地基SAR（Synthetic Aperture Radar,合成孔径雷达）在形变监测领取得到了广泛应用,但仅能获取沿雷达视线方向的一维形变信息。为实现三维形变信息的测量,可以采用多部地基SAR从不同的观测角度对同一区域进行联合观测,但不同雷达的图像特征差别较大,基于常规的多项式拟合的变换方法无法实现配准。因此,本文提出了一种新的适合多部地基SAR联合观测下的几何配准方案。该方案中包含两个关键问题:三维坐标系变换和同名投影点关联,首先基于多个控制点将各部雷达与场景高程信息统一到同一坐标系下,其次基于雷达成像的投影原理实现不同雷达图像中同名投影点的关联。基于两部雷达获取的实测数据,初步验证了该方案的可行性。      

地基SAR形变监测误差分析与实验

地基SAR(Ground Based Synthetic Aperture Radar,GB-SAR)是进行区域性、全天时、全天候、定点连续形变监测的重要手段。该文对中国科学院电子学研究所研制的地基SAR系统 ASTRO (Advanced Scannable Two-dimensional Rail Observation system)的形变监测性能进行分析,结合ASTRO系统成像几何给出了其形变监测模型和精度分析模型,着重分析了影响形变监测精度的主要误差源。最后以ASTRO系统为实验平台开展了形变监测实验与分析,验证了ASTRO系统用于形变监测的能力。     

地基差分干涉雷达发展现状及应用实例

地基差分干涉雷达在形变监测领域已经得到了广泛的应用。该文首先概述了地基差分干涉雷达的主要类型,分为地基实孔径雷达和地基合成孔径雷达两类,并选择代表性系统介绍了工作原理及重要参数。然后以地基合成孔径雷达为例,介绍了现阶段差分干涉处理中的重要技术,包括差分干涉、PS点选择、大气相位补偿等。最后以3个应用实例,展现了地基差分干涉雷达,在露天开采边坡监测、山体滑坡监测和桥梁振动测量方面的应用。      

Tree height influence on ERS interferometric phase in boreal forest

The European Remote Sensing 1/2 (ERS-1/2) "tandem" coherence has been shown to provide estimates of stem volume in boreal forest in agreement with in situ data. Tree height estimation from ERS interferometric phase represents a further step in the investigations concerning the retrieval of biophysical parameters using repeat-pass synthetic aperture radar (SAR) interferometry. At two test sites located in Sweden and Finland, sets of respectively nine and eight ERS "tandem" interferograms were available. Images acquired under stable winter weather conditions and during nighttime were found to be less affected by atmospheric artifacts. Reduction of atmospheric artifacts in interferograms was performed with a phase screen estimated over a dense grid of open areas. Nonetheless, at each test site, only a limited set of pairs was useful for tree height investigations. Under stable winter conditions, the interferometric tree height obtained from an inversion of the differential interferometric phase at stand level was found to be much lower than the true tree height. Spread and uncertainty in the interferometric tree height measurements were caused by phase noise and residual atmospheric artifacts. Using the semiempirical interferometric water cloud model (IWCM), the modeled interferometric tree height was generally in reasonable agreement with the measurements, showing the need of a phase term in interferometric modeling of forests. The inversion of the IWCM for tree height retrieval showed the strong effect of phase noise and atmospheric artifacts on the estimates. Hence, tree height retrieval from ERS repeat-pass SAR interferometry seems to have limited forestry applications. The results also indicate under what conditions the forest influence is small on digital elevation models derived from repeat-pass interferometry.     

Automatic Building Reconstruction with Satellite Images and Digital Maps

This paper introduces an automated method for building height recovery through the integration of high‐resolution satellite images and digital vector maps. A cross‐correlation matching method along the vertical line locus on the Ikonos images was deployed to recover building heights. The rational function models composed of rational polynomial coefficients were utilized to create a stereopair of the epipolar resampled Ikonos images. Building footprints from the digital maps were used for locating the vertical guideline along the building edges. The digital terrain model (DTM) was generated from the contour layer in the digital maps. The terrain height derived from the DTM at each foot of the buildings was used as the starting location for image matching. At a preset incremental value of height along the vertical guidelines derived from vertical line loci, an evaluation process that is based on the cross‐correlation matching of the images was carried out to test if the top of the building has reached where maximum correlation occurs. The accuracy of the reconstructed buildings was evaluated by the comparison with manually digitized 3D building data derived from aerial photographs.     

基于有向体模型的植被参数反演及室内宽带实验研究

该文对具有极化特征的植被提出一种基于有向体(OV)模型的参数反演算法,并尝试在微波暗室内构建全极化干涉合成孔径雷达半实物宽带测量系统,对模型及算法进行实验验证。根据OV模型的植被特征,以同极化之差所代表的二面角散射机理作为地形相位点的判别依据,结合计算平均值和标准差的方法反演出植被高度。接下来,在环境可控的微波暗室内构建了实验系统,采用两个高度已知的三面角对干涉基线距进行校准,提高了参数反演精度。实验结果表明:在场景的地体幅度比较大的情况下,植被反演高度与实际高度的误差仅为0.03 m,全频带内的高度反演误差不超过0.2 m,说明了反演算法的有效性。     

ICESat-2/ATLAS数据反演林下地形精度验证

针对星载激光雷达数据反演林下数字地面模型（Digital Terrain Model,DTM）存在困难的问题,研究了冰云陆地高程卫星-2 （Ice, Cloud, and land Elevation Satellite,ICESat-2）/先进地形激光高度计系统(Advanced Opographic Laser Altimeter System,ATLAS）的强弱光束数据反演林下地形的精度,并探究了冠层高度及植被覆盖率对于ICESat-2/ATLAS反演林下DTM精度的影响。研究结果表明:强波束反演林下DTM的精度为R2=1,RMSE=0.74 m,弱波束反演林下DTM的精度为R2=1,RMSE=0.76 m,强波束相对弱波束表现出更优的反演精度,但是,强光束与弱光束的光子云数据均可为反演林下DTM提供科学数据。从研究区植被的整体情况来看,随冠层高度及植被覆盖率的增加,不同激光类型数据均出现误差逐步增加的情况。     

A Numerical Study of the Retrieval of Sea Surface Height Profiles From Low Grazing Angle Radar Data

A numerical study of the retrieval of sea surface height profiles from low grazing angle radar observations is described. The study is based on a numerical method for electromagnetic scattering from 1-D rough sea profiles, combined with the “improved linear representation” of Creamer for simulating weakly nonlinear sea surface hydrodynamics. Numerical computations are performed for frequencies from 2975 to 3025 MHz so that simulated radar pulse returns are achieved. The geometry utilized models a radar with an antenna height of 14 m, observing the sea surface at ranges from 520 to 1720 m. The low grazing angles of this configuration produce significant shadowing of the sea surface, and standard analytical theories of sea scattering are not directly applicable. Three approaches for retrieving sea height profile information are compared. The first method uses a statistical relationship between the surface height and the computed radar cross sections versus range (an incoherent measurement). A second method uses the phase difference between scattering measurements in two vertically separated antennas (“vertical interferometry) in the retrieval. The final technique retrieves height profiles from variations in the apparent Doppler frequency (coherent measurements) versus range and requires that time-stepped simulations be performed. The relative advantages and disadvantages of each of the three approaches are examined and discussed.      

基于栅格投影的地基合成孔径雷达三维地形匹配俯仰角模糊处理方法

近距离对高大建筑物进行监测时，基于栅格投影的地基合成孔径雷达(ground-based synthetic aperture radar, GB-SAR)三维地形匹配过程中易出现俯仰角模糊现象，影响投影结果的解译. 针对此问题，文中提出一种俯仰角模糊现象的处理方法. 首先推导了任意GB-SAR观测几何条件下的图像距离-方位坐标系与东北天坐标系的转换关系，基于栅格投影的方法实现了GB-SAR三维地形匹配；然后对出现的俯仰角模糊现象进行了分析，讨论了其产生原因，并提出了相应的处理方法. 实测结果表明，投影结果中虚假影像去除效果明显，说明本方法对俯仰角模糊现象具有良好的处理效果，有利于对GB-SAR三维地形匹配结果进行解译及分析.     

热力非均匀下垫面对流边界层流场结构特征研究

利用对流水槽成功模拟热力非均匀下垫面对流边界层的发生发展.仿照数值模拟中常用的马赛克方法在对流水槽的底部铺了一些导热率缓慢的材料(橡胶薄板),使下垫面的加热存在差异,进而产生非均匀加热.利用温度廓线和准直光系统共同决定边界层顶部的位置和对流速度尺度.采用PTV测量技术获得高精度的二维流场,看到流场具有复杂的空间结构和尺度特征.相对于均匀下垫面来说,底部的非均匀加热使得混合层湍流的组织性得到加强,具有稳定的环流结构.非均匀下垫面对流边界层的方差随高度的变化曲线与均匀下垫面的特征明显不同.为了分析非均匀下垫面对流边界层的环流特征和水平输送对湍流变化的贡献,计算了湍流动能的湍流输送.计算结果表明,加热开始不久,由于不同下垫面的的温差较大,水平输送也较大;而当一段时间后,温差变小,水平输送也变小了.由此可以看出非均匀下垫面对流边界层的水平输送依赖于下垫面的非均匀强度.     

A unified solution for digital terrain model and orthoimagegeneration from SPOT stereopairs

An original scheme to automatically generate digital terrain models (DTMs) and orthoimages from SPOT stereopairs in a unified way is presented. In addition to modeling the time-dependent orientation parameters, an algorithm is developed to generate the epipolar stereomate incorporating an initial DTM. The refining of the DTM is then accomplished by performing least-squares template matching along each conjugate epipolar line pair, space intersection, and grid value interpolation. Recursively, the epipolar stereomate is regenerated according to the refined DTM. The procedure is repeated until the disparities between the two stereomate images are small enough. The DTM generated illustrates the apparent surface. It is more correct to use this apparent DTM than the one depicting the ground surface for the geometric correction for remotely sensed data. Both images of the final epipolar stereomate are actually orthoimages because they are geometrically identical and georeferenced. Experimental results indicate that the orthoimages reach an accuracy up to 2/3 pixel     

An Analytic Method for Radiometric Correction of Satellite Multispectral Scanner Data

The problem of radiometric correction of multispectral scanner data is posed as the problem of determining an intrinsic reflectance factor characteristic of the surface material being imaged and invariant to topography, position of the sun, atmosphere, and position of the viewer. A scene radiance equation for remote sensing is derived based on an idealized physical model of image formation. The scene radiance equation is more complex for rugged terrain than for flat terrain since it must model slope-, aspect-, and elevation-dependent effects. Scene radiance is determined by the bidirectional reflectance distribution function (BRDF) of the surface material and the distribution of light sources. The sun is treated as a collimated source and the sky is treated as a uniform hemispherical source. The atmosphere is treated as an optically thin, horizontally uniform layer. The limits of this approach are reviewed using results obtained with Landsat MSS images and a digital terrain model (DTM) of a test site near St. Mary Lake, British Columbia, Canada. New results, based on regression analysis, are described for the St. Mary Lake site. Previous work is extended to take advantage of explicit forest cover data and to consider numeric models of sky radiance. The calculation of sky irradiance now takes occlusion by adjacent terrain into account. The results for St. Mary Lake suggest that the cosine of the incident solar angle and elevation are the two most important correction terms. Skylight and inter-reflection from adjacent terrain, however, also are significant.     

Novel multimedia retrieval technique: progressive query (why wait?)

A novel multimedia retrieval technique, called progressive query (PQ) is presented. PQ is designed to bring an effective solution, especially when querying large-scale multimedia databases. In addition, PQ produces intermediate query retrieval results during the execution of the query. The series of intermediate query results finally converges to the full-scale search retrieval in a faster way and with no minimum system requirements. Experimental progressive query retrieval results show that intermediate retrieval results may be satisfactory and further query processing time may be avoided.     

A method for multidirectional TEM observation of a specific site at atomic resolution

A new technique has been developed for the three-dimensional structure characterisation of a specific site at atomic resolution. In this technique, a focused ion beam (FIB) system is used to extract a specimen from a desired site as well as to fabricate the electron transparent specimen. A specimen holder with a specimen stage rotation mechanism has also been developed for use with both an FIB system and a high-resolution transmission electron microscope (TEM). The specimen holder allows both the FIB milling of a specimen and its observation in TEM without remounting the specimen from the specimen holder. A specimen for the three-dimensional TEM observation is extracted using the FIB micro-sampling technique and shaped into a pillar to mount on a tip of a needle stub enabling a multidirectional observation. The technique was applied to the multidirectional observation of the crystal structure of an Si single crystal at atomic resolution. The crystal lattice fringes of the two Si(111) planes with distances of 0.31 nm as well as the lattice fringes of the Si(200) with distances of 0.19 nm were clearly observed.