Unit 31 Interferometric SAR (In SAR) Introduction 干涉雷达(SAR)介绍
Radar interferometry is a technique for extracting three-dimensional information of the Earth’s surface by using the phase content of radar signal as an additional information source derived from the complex radar data. It was first used in observation of the surface of Venus and the Moon.
雷达干涉测量技术中提取三维信息的地球表面的利用雷达信号的相位内容作为额外的信息源来自复杂的雷达数据。它最初是用于观察金星和月亮的表面
Graham was the first to introduce Synthetic Aperture Radar (SAR) for topographic mapping in 1974. There are two kinds of information which are required for the production of topographic maps. Firstly, the various objects and features to be mapped must be presented in an image with sufficient resolution to be identified. Secondly, a three_dimensional measurement of position, with respect to the platform, of a sufficient number of points must be made to define the terrain surface . The three_dimensional measurement can be made by SAR interferometry with a side_looking geometry from both airborne and spaceborne SAR sensors.
格雷厄姆是第一个介绍合成孔径雷达(SAR)在1974年地形映射。有两种类型的信息所需的地形图的生产。首先,各种对象和特征映射必须以足够的分辨率的图像。其次,一个三维位置测量,对平台,足够数量的点必须定义地形表面。三维测量可以通过侧视SAR干涉测量几何机载和星载SAR传感器。
A Synthetic Aperture Radar is an active sensor transmitting and receiving microwave signals, i .e . Measuring distances between the sensor and the point on the Earth’s surface, where the signal is backscattered. The sensor emits electromagnetic radiation (EMR) and then records the strength and time delay of the returning signal to produce images of the ground. The EMR involved can be imagined as a sine wave. Conventional SAR images are made up (as a raster) of the amplitude or ‘strength’of the sine wave — shown in images as grey level intensity values .When the sine wave starts to repeat itself (phase angle > 360 degrees) , one cycle of phase has occurred . If we collect two separate images from exactly the same satellite position (same range) , but at different times with nothing in the target area changing, one would expect the two sine waves from each image to be the same and in phase with each other . 合成孔径雷达是一个活跃的传感器传送和接收微波信号,
我即。测量传感器之间的距离和地球表面上的点,背散射信号。传感器发出的电磁辐射(EMR),然后记录返回信号的强度和时间延迟产生的地面图像。涉及的EMR可以想象为一个正弦波。传统的SAR图像是由(光栅)的振幅或强度相近正弦波-所示图像灰度级强度值当正弦波开始重演(相位角> 360度),一个周期的阶段发生。如果我们收集两个单独的图片完全相同的卫星位置相同(范围),但在不同的时间与目标区域中没有改变,人会期望每个图像的两个正弦波是相同的,彼此在阶段。
In practice, the position of the satellite between two image acquisitions is never identical, and the corresponding difference in the path (distance between satellite and ground) means there is a difference in phase between the two signals — a phase shift. The physical path difference can be expressed as an integer number of wavelengths plus the fraction of one wavelength. It can also be expressed as a difference in phase angle between the two signals. SAR interferometry makes use of this phase information by subtracting the phase value in one image from that of the other, for the same point on the ground. This is, in effect, generating the interference between the two phases signals and is the basis of interferometry .For the interferometric process to work successfully, a degree of similarity, or correlation must exist in the surface properties between the two image acquisitions . In most parts of the world, particularly temperate regions, correlation between images will degrade with time due to changing / moving vegetation, differing climatic conditions — termed’ temporal decorrelation’. Correlation tends to remain good in arid, desert regions where little change occurs. An output from the processing chain is a coherence image, and this represents the correlation that exists between corresponding pixels of the two images — lighter pixels showing good correlation (e .g . arid, dry land cover ) , and darker pixels showing bad correlation (e .g . water, changing vegetation) . 在实践中,卫星的位置两个图像之间的收购不相同,和相应的不同路径(卫星和地面之间的距离)意味着不同的两个信号之间的相位相移。物理路径的不同可以表示为一个整数的波长数量+一个波长的一部分。它也可以表示为一个两个信号之间的相角差。SAR干涉测量法利用这个阶段信息阶段价值减去另一个图像的同一点在地上。实际上,这是产生两个阶段之间的干扰信号和干涉,干涉的基础过程成功地工作,一定程度的相似性,或相关性必须存在于表面性质两个图像之间的收购。在世界的大部分地区,特别是温带地区,图像之间的相关性会随着时间降低由于改变/移动的植被,不同的气候条件,称为“时间解相关”。相关倾向于保持在干旱,沙漠地区发生微小的变化。处理链的输出是一个连贯性的形象,这代表了相应的像素之间的相关性存在两个图像——轻像素显示良好的相关性(e .g。干旱、干燥的土地覆盖),和较暗的像素显示坏相关性(e .g。水、植被变化)。
The phase value or angle (and hence phase differences in an interferogram) is not known absolutely, but is given in the range 0 - 360 degrees , i .e . the phase is wrapped onto a fixed range of angle of 0 - 360 degrees . In order to compute terrain heights and generate a DEM, the interferogram fringes have to be unwrapped, i .e . the correct multiple of 360 degrees must be added to the phase difference at each pixel, the problem of solving this 2π ambiguity is called phase unwrapping . If the ground were flat, unwrapping the above interferogram would produce an image of constant grey level.
相位值或角(因此在干涉图相位差异)绝对是未知的,但范围在0 - 360度,我即。阶段包装到一个固定的角度范围0 - 360度。为了计算地形高度和生成DEM,干涉图的边缘必须打开,我生产。正确的360度必须添加到多个相位差在每个像素,解决这一问题的2π模棱两可称为相位展开。如果地面是平的,打开上面的干涉图会产生恒定的灰度级的图像。在世界的大部分地区,特别是温带地区,图像之间的相关性会随着时间降低由于改变/移动的植被,不同的气候条件,称为“时间解相关”。
The interferometric data processing scheme includes in general ( 1) registration of the complex images, (2 ) the formation of the interferograms, ( 3) the phase unwrapping, and ( 4) the digital elevation model reconstruction . 干涉数据处理方案一般包括(1)登记的复杂图像,(2)干涉图的形成,相位(3),(4)数字高程模型重建。
The basic use of SAR interferometry is to estimate topographic height .However, advancement on this technique can very usefully be applied to map surface displacements such as those associated with earthquakes, landslip or subsidence. Known as differential interferometry, the method uses SAR images of different dates that might span a surface displacement event. A first interferogram is created representing topography before the event and then a second interferogram created representing topography after the event. By subtracting one interferogram from the other, fringes that relate to common topography cancel each other out , so that remaining fringes should only represent a difference in topography, i .e . a displacement .
SAR干涉测量法是估计的基本使用地形高度,发展这种技术可以非常有效地适用于地图上表面位移等与地震、塌方或沉降。被称为微分干涉法,该方法使用的SAR图像不同的日期可能跨越一个表面位移事件。在创建第一个干涉图代表地形之前创建的事件,然后第二个干涉图代表地形。从其他通过减去一个干涉图,边缘与常见的地形相互抵消,所以剩下的边缘应该只代表不同的地形,我,一个位移 Outlook
Current research on SAR interferometry mainly focuses on the potentials of SAR interferometry by investigating the limiting factors of this technique. Ten years ago, the main difficulties were in the theoretical aspects, and scientists were waiting for access to suitable data sets in order to demonstrate the potential of SAR interferometry . 目前SAR干涉测量法的研究主要集中在SAR干涉测量的潜力调查这种技术的限制因素。十年前,理论方面的主要困难是,科学家们正在等待获得合适的数据集,以展示SAR干涉测量的潜力。
Now, most of the theoretical aspects are reasonably well understood. At present, the main issues concern the operational constraints such as data availability, commercial software, manpower, and automation, adequacy of INSAR accuracy with regards to user, requirements and future system specifications. As mentioned before, one current research issue is the investigation of the influence of the atmosphere, which is assumed to have a significant influence on the quality of SAR interferometric data. The refraction can affect pixel misregistration and artefacts in the phase difference. The main problem during the data processing is the phase unwrapping. 现在,大多数的理论方面相当清楚。目前,主要问题的担忧等操作约束数据可用性,商业软件,人力,自动化,充足率INSAR的准确性对于用户需求和未来的系统规范。如前所述,当前研究的问题之一是大气的影响的调查,这是假定有显著影响SAR干涉数据的质量,折射会影响像素错误配准和文物相位差。主要的问题在数据处理阶段展开。
This part is still a problematical task and needs to be further investigated to reach an operational status. This is also one of the main reasons why commercial software packages for SAR interferometric data are still under development and not yet commercially available .Another major objective of current research is to produce high precision DEMs in an operational way. This is aimed at using SAR interferometric data from the ERS-1 / ERS-2 tandem mission. Reviewing the papers on SAR interferometry, it is clear that the basic techniques of producing interferograms, phase unwrapping, production of digital elevation models, etc . are now well studied and understood .Many organizations are now at the stage of making the software systems operational , and these should be available on the commercial market shortly . In terms of applications, many are reaching the stage of operational use, and other new applications on the use of SAR interferometry are being developed. The situation in a year or two will show a rapid increase in developments of SAR interferometry resulting from the increasing availability of ERS-1/ ERS-2 tandem mode data as well as operational airborne systems and other SAR satellite systems 这部分仍然是一个有疑问的任务,需要进一步调查达成运营状态。这也是商业软件包的主要原因之一,为SAR干涉数据仍在发展和没有商用.Another当前研究的主要目标是生产高精度民主党在一个操作方式。这是旨在利用SAR干涉数据从ERS-1 / ERS-2串联任务。回顾论文SAR干涉测量法,很明显,产生干涉图的基本技术,展开阶段,生产数字高程模型,等。现在现在学习和理解。许多组织都在软件系统操作的阶段,这些应该可以在商业市场。在应用程序方面,许多人到达阶段的操作使用,和其他新应用程序使用SAR干涉测量正在研制。一年或两年的情况将显示一个快速增长的发展导致SAR干涉测量ERS-1 / ERS-2串联模式数据的增加可用性以及操作机载SAR系统和其他卫星系统