摘要
SAR测量技术用几幅影像就可以监测上万平方公里的地表形变,其监测精度可达到亚毫米级(Ferretti,2007)。相对传统的水准测量和GPS测量监测技术它具有很大的成本优势。其中,为克服时间空间去相干以及大气影响而设计提出的新方法中,PS技术是一种典型且极具应用价值的手段(Ferretti,2002)。
PS技术通过同一地区获取的多幅多时相SAR图像,先提取出经过长时间间隔仍具有较好相干性的像元构成一个成像区的小子集,也就是相位稳定像元构成的小子集,然后研究该子集内的像元的相位变化,分析得到可信度高的形变测量值,以此来监测轻微的地表运动。那些相位稳定的单个像元被称为“永久散射体”(Permanent Scatterer,PS),它们可以被看作天然的GPS测量网结点,即使周围地区的相干性不好,在这些相位稳定的像元上也能得到可靠的高程和形变测量结果。另外,如果PS的实际尺寸小于图像分辨率单元,即使干涉基线距超过平常所谓的临界基线距,在这些PS点上像对之间的相干性仍然可以得到较好的保证。借助这种技术就有可能把感兴趣区所有SAR影像都利用起来。
PS技术解决了InSAR的两个关键问题,一是时间去相关性,二是大气的影响。永久散射体的有效识别是PS技术的核心,现存的PS点选取方法大致可归为两种,即相关系数阈值法和振幅离差指数阈值法(Ferretti,2001)。相关系数法主要考虑两方面问题,一是确定窗口大小,二是确定PS识别阈值。相关系数是基于局部移动窗口来计算的,其大小对相关系数估计值具有直接的影响。Colesanti等人的研究表明(Colesanti,2002),阈值取0.3时,能较好地过滤掉雷达散射特性不稳定的分布型面状目标,如水体和植被覆盖区域。Ferretti等人的理论研究表明,在高信噪比像元上,可以用时序振幅信息来衡量相位噪声水平,以像元的振幅稳定特征来代替相位噪声水平进行PS的识别行之有效。但是对于信噪比较低的散射体,振幅离差与相位稳定性之间的简单关系被打破了,这种方法不再有效。
本文重点介绍了一种基于干涉相位空间相关性的PS技术的原理与处理过程,这种方法能够在缺少高亮度散射体的区域识别并提取出形变信息,所需数据量比通常PS技术大大减少,并且能够适用于没有先验形变信息、形变速度不稳定的地区。
另外,InSAR的一个主要误差源是电磁波信号在穿过大气时产生的相位延迟,尤其是对流层水气的影响。Zebker等人在1997年提出,在相对潮湿的云层中,20%的空间或时间变化会导致10到14厘米的形变测量修复误差。而北京地处华北平原北端,西北都是山区,山前迎风坡加剧水汽凝结。因此大气作用对InSAR监测北京地区精度的影响不可忽视。GPS同步观测可以获得高精度大气对流层中的可降水汽量,因此可以用GPS大气数据来模拟和削弱干涉图中的大气效应。但是此方法的局限性是:必须有足够多的GPS监测站,必须获得与SAR图像拍摄同步的GPS连续观测资料,目前在很多情况下还无法满足这些条件。使用MODIS数据进行大气延迟改进存在的主要问题是,由于MODIS比ASAR过境时间晚两个小时,所以数据存在时差。
通过对大气延迟理论与方法的深入分析研究,本文提出一种使用MERIS数据改进大气延迟作用的模型。用时间基线大于一天的两景MERIS_PWV数据,制成天顶延迟路径差分图。MERIS与ASAR同时搭载2002年3月1日发射升空的ESA ENVISAT卫星。实验使用MERIS降低分辨率的近红外水汽产品改进大气延迟作用,并将此方法融入到新PS技术中,以北京地区为例,进行了实例分析与结果验证。
Synthetic Aperture Radar(SAR) is a very effective technique for measuring crustal deformation.It can monitor an area over ten thousands kilometers just with a few images,and the accuracy could achieve a level of submillimeter[Ferretti,2007].No conventional detection methods could compare with InSAR technique considering the cost.Among other techniques, persistent scatterers(PS) method is a typical means which has great applicable potential to overcome temporal and spatial decorrelation and reduce atmospheric effects.
Permanent Scatterers processing over a good many of multi-temporal SAR images only takes into account pixels with high quality signal levels,over long period,in terms of amplitude and coherence values,and form a small imaging subset.Then it analyses phase change of the pixels inside the subset.At last we can get high reliable deformation values and use the results to detect minor move of the earth's surface.Such individual with stable phase is called Permanent Scatterer.PS could play the role of natural nodes of GPS surveying.In the areas where lack good correlated environment,we can also obtain reliable data of altitude and deformation. Furthermore if the actual size of the PS pixels are smaller than the resolution cell of the image, even the length of the interferential baseline beyond the so called critical length,the coherence between image pairs within the PS could be well kept.By this technique we can make use of all the SAR data within the interested area.
PS technique addresses the problems of decorrelation and atmospheric delay which has been obsessed InSAR for long.One of the kernels of PS technique is how to identify PS effectually. Existing methods for for selecting PS candidates could be roughly divided into two groups,that is,Correlation Coefficient(CC) and Amplitude Dispersion Index(ADI).CC method should take account of two key points,one is to decide the size of analyzing window,and the other is to confirm identifying threshold value.The correlation coefficient is calculated by partially moving the analyzing window,whose size has direct effect on estimating CC value.Colesanti etc. showed when the threshold value taken 0.3,most of the area tagets,as waters and areas coverd with vegetation,which behaved unsteady radar scatter properties would be filtered.Theoretic study by Ferretti and others revealed that,for high signal to noise ratio(SNR),amplitude dispersion is an accurate proxy for phase standard deviation and thus the method has a high success rate at picking bright PS.However,for low SNR scatterers,the simple relationship between amplitude dispersion and phase stability breaks down and the method is no longer effective.
In this paper,we introduced a new PS method developed by Stanford University.This method used spatial correlation of interferogram phase to find a network of stable pixels in all terrains, with or without buildings.Prior knowledge of temporal variations in the deformation rate is not required.We refer to these pixels as persistent scatterers(PS).
A major source of error for repeat-pass InSAR is the phase delay(especially the part due to water vapour) in radio signal propagation through the atmosphere.In the year 1997,Zebker and others formulated that a 20%spatial or temporal change in relative humidity was estimated to cause an error of the order of 10-14 cm in the case of deformation estimates(Zebker el al.1997). Beijing is located to the north of North China Plain,surrounded by mountainous areas in the northwest.Such topography makes vapor condensing more serious.As a result,the atmospheric effects on detectation accuracy of InSAR in Beijing can not be ignored.GPS synchronous data can provide high accurate Precipitable Water Vapour(PWV) in troposphere.Thereby we can use GPS PWV product to reduce water vapour effects on InSAR interferograms.But this method has some limitations.It requires suffice GPS stations and the observation must be synchronous with SAR images.At present such conditions would be hard to meet.Using MODIS PWV product to correct atmospheric delay bear a critical setback:because MODIS data was taken two hours later than ASAR data,so there are time difference between them.
This paper analysed the theories and method of atmospheric delay comprehensively and proposed an applicable mothod,which used MERIS PWV product to correct atmospheric delay. The Medium Resolution Imaging Spectrometer(MERIS) was launched together with the Advanced Synthetic Aperture Radar(ASAR) on the ESA ENVISAT spacecraft on 1 March 2002. We use MERIS reduced resolution near-infrared water vapour products to correct atmospheric effect and applied it to the new PS mothod.The method was tested in Beijing.
引文
Afraimovich E . L , Terekhov A l , UdodovM Y , et al. Refraction distortions of transionospheric radio signals caused by changes in a regular ionosphere and by traveling ionospheric disturbances, J. Atm. and Terr. Phys., 1992, 5 4:1013 — 1020
Albert, P., R. Bennartz, and J. Fischer, Remote Sensing of Atmospheric Water Vapor from Backscattered Sunlight in Cloudy Atmospheres, Journal of Atmospheric and Oceanic Technology, 18 (6), 865-874, 2001.
Arrigoni, M., C. Colesanti, A. Ferretti, D. Perissin, C. Prati, and F. Rocca, Identification of the location phase screen of ERS-ENVISAT permanent scatterers, European Space Agency, (Special Publication) ESA SP, (550), 181-186, 2004.
ASAR product handbook, European Space Agency, 2002.
Benedicte fruneau, Furancesco Sarti. Detection of ground subsidence in the city of Paris using radar interferomitry: isolation of deformation from atomospheric artifacts using correlation[J]. Geophysical Research Letters, 2000, 27(24): 3981-3984.
Bevis, M., S. Businger, S. Chiswell, T. Herring, R. Anthes, C. Rocken, and R. Ware, GPS meteorology: mapping zenith wet delays onto precipitable water, Journal of Applied Meteorology, 33, 379-386, 1994.
Charles Meade; David T. Sandwell, Synthetic Aperture Radar for Geodesy, Science, New Series, Vol. 273, No. 5279. (Aug. 30, 1996), pp. 1181-1182.
Colesanti C, Locatelli R, Novali F. Ground deformation monitoring exploiting SAR permanent scatterers [J] . IEEE, 2002 0 - 7803 - 7536 - X: 1219-1221 .
Emardson T R , Simons M , Webb H F. Neut ral Atmospheric Delay in Interferometric Synthetic Aperture Radar Applications: Statistical Description and Mitigation [J]. Journal of Geophysical Research , 2003 ,108 (B5) : 2 231
Emardson T R, Elgered G, Johansson J M. et al. Three Months of Continuous Monitoring of Atmospheric Water Vapor with a Network of GPS Receivers. J. Geophys. Res., 1998, 103 (D2): 1 807-1 820.
Ferretti A, Prati C, Rocca F. Non-linear subsidence rate estimation using permanent scatterers in differential SAR interferometry [J] . IEEE Transactions on Geoscience and Remote Sensing, 2002, 38 (5): 2202-2212.
Ferretti A, Prati C, Rocca F. Permanent scatterers in SAR interferometer[J]. IEEE Transactions on Geoscience and Remote Sensing, 2001, 39 (1): 8—19.
Graham L C. Synthetic Interferometer radar for topographic mapping [J]. Proceeding of IEEE, 1974, 62: 763-768
Hanssen R F. Radar Interferomet ry: Data Interpretation and Error Analysis [C]. Kluwer Academic , Dordrecht .Boston , 2001.
Hooper. Persistent Scatterer Radar Interferometry for Crustal Deformation Studies and Modeling of Volcanic Deformation.
Kim Y J, Zyl J V. In: Proceedings of IEEE 2000 Aerospace Conference, New York: Wily-IEEE Press, 2000, 3:231
Klees, R., and D. Massonnet, Deformation measurements using SAR interferometry: potential and limitations, Geologie En Mijnbouw, 77 (2), 161-176, 1999.
Li, Z., J.-P. Muller, P. Cross, and E. J. Fielding, Interferometric synthetic aperture radar (InSAR) atmospheric correction: GPS, Moderate Resolution Imaging
Spectroradiometer(MODIS), and InSAR integration, journal of Geophysical Research, 110 (B3), B03410, doi:10.1029/2004JB003446, 2005.
Li, Z., J. -P. Muller, P. Cross, P. Albert, T. Hewison, R. Watson, J. Fischer, and R. Bennartz, Validation of MERIS near IR water vapour retrievals using MWR and GPS measurements, in MERIS user workshop, ESA ESRIN, Frascati, Italy, 10-13 November, 2003.
Mason, N., P. Hughes, P. McMullan, R. Reynolds, L. Simmonds, and J. Twidell, Introduction to environmental physics: planet earth, life and climate, Tylor& Francis, London, 2001.
Massonnet D , Feigl K L. Discrimination of Geophysical Phenomena in Satellite Radar Interferograms[J]. Geophysical Research Letters , 1995 , 22(12) :1 53721 540
Massonnet D, Rossi M, Garmona C, et al. The displacement field of the Landers earthquake mapped by radar interferometry [J]. Nature, 1993, 361:138-1-12.
Massonnet, D., and K. L. Feigl, Radar interferometry and its application to changes in the Earth's surface, Reviews of Geophysics, 36(4), 441-500, 1998.
Massonnet, D., K. Feigl, M. Rossi, and F. Adragna, Radar interferometric mapping of deformation in the year after the Landers earthquake,Nature,369,227-230,1994.
Niell,A.,COSTER,A.,SOLHEIM,F.,MENDES,V.,TOOR,P.,LANGLEY,R.and UPHAM,C.,2001,Comparison of measurements of atmospheric wet delay by radiosonde,water vapor radiometer,GPS,and VLBI.Journal of Atmospheric and Oceanic Technology,18,pp.830-850.
Saastamoinen J.Atmospheric Correction for the Troposphere and St ratosphere in Radio Ranging of Satellites[C].Geophysics Monograph 15,the 3~(rd) Int.Symp.Use of Artificial Satellites for Geodesy,A GU,Washington D C,1972Science,9,803-807,1974.
Solheim,F.S.,J.Vivekanandan,R.H.Ware,and C.Rocken,Propagation delays induced in GPS signals by dry air,water vapor,hydrometeors and other particulates,Journal of Geophysical Research,104,9663-9670,1999.
Thayer,G.D.,An improved equation for the radio refractive index of air,Radio
Wadge,G.,and B.Parsons,Definition of the interferometric requirements in support of the TerraSAR-L phase 0 study,Final Report for TSR.PP.OOOO3.EU.ASTR,pp.42,EVINSAR Science Team,2003.
Williams S,Bock Y,Fang P.Integrated Satellite Interferometry:Tropospheric Noise,GPS Estimates and Implications for Interferomet ric Synthetic Aperture Radar Product s[J].Journal of Geophysical Research,1998,103(B11):27 051227 068
Williams S,Bock Y,Pang P.Integrated Satellite Interferometry:Tropospheric Noise,GPS Estimates and Implications for Interferometric Synthetic Aperture Radar Products,(J).Geophys Res,1998,103(Bll):27051-27067.
Zeberk H A,Rosen P A,Goldstein R M,et al.On the derivation of coseismic displacement fields using differential radar interferometry:The Landers earthquake[J].Journal of Geophysical Research,1994,99(19):617-634.
郭华东.中国雷达图象分析[M].北京:科学出版社,1999.244.
纪玉杰,北京城郊的地面沉降成因浅析,北京地质,1996.
廖明生,林晖,雷达干涉测量-原理与信号处理基础,2003,第1版.测绘出版社
刘国祥,木晓利,陈永奇,等,使用卫星雷达差分干涉技术侧量香港赤腊角机场沉降场,2001,Vol.46.No.14-1 224-1228
刘永坦,雷达成像技术,哈尔滨。哈尔滨工业大学出版社,199:9.
邵芸等.多极化雷达图像的地质应用[A].郭华东;.机载雷达应用试验研究[C].北京:中国科学技术出版社,1992.93-102.
王超,张红,刘智,星载合成孔径雷达干涉侧量,2002,第1版,科学出版社
张永志,罗凌燕,刘瑞春等,三轨法D-InSAR观测确定区域的垂直变形,地震研究,2006.
赵常洲,龚固培,王晖,地面沉降成因与危害,西部探矿工程,2006