Persistent Scatterers from TerraSAR-X Data Stacks: Localization Precision and Deformation Regimes in Urban Areas
Gernhardt, Stefan¹; Auer, Stefan¹; Bamler, Richard^2
1Chair of Remote Sensing Technology, TUM, GERMANY; ²Chair of Remote Sensing Technology, TUM & Remote Sensing Technology Institute, DLR, GERMANY

Persistent scatterer interferometry (PSI) [1,2,3] is a well established method for the estimation of surface deformation using stacks of SAR data acquired over several years. Since the availability of meter resolution radar data sufficient persistent scatterers (PS) are available for each building in order to monitor single objects [4]. However, the precision of the estimated positions of the PS has not been analyzed in detail for natural PS, yet. In addition, stacks of meter resolution SAR data reveal spatially limited deformation regimes of different origins that cannot be identified in medium resolution data.

Due to the limited spatial resolution of previous generation SAR satellites like ERS or ENVISAT, a precise assignment of PS to structural elements of real world objects in urban areas was difficult. Thus, a detailed evaluation on the accuracy of PS localization could not be carried out using this resolution class of SAR satellites. A differentiation between individual structural elements on building facades is only possible at by far higher resolutions, e.g. using TerraSAR-X data captured in high resolution spotlight mode. For typical stacks of TerraSAR-X data the precision of elevation estimates is in the order of one meter [5]. This facilitates the analysis of localization precision of natural PS in urban areas. The investigation at hand shows the procedure and results of a localization analysis of natural PS in Munich, Germany.

The investigation is based on a comparison between ideal PS positions obtained from 3D SAR simulation. Therefore, several building facades have been reconstructed by a photogrammetrical evaluation of optical images. The selection of facades for this investigation is based on the presence of a regularity of structural elements, i.e. regular arrangements of balconies and/or windows. In consequence the occurring PS patterns are assumed to be related to these facade regularities and, thus, facilitate an analysis of the relative location accuracy. From the photogrammetric evaluation precise 3D models of the building fronts are available that can be processed by RaySAR, a SAR simulator developed at TUM [6]. One of the outcomes of the simulation are the 3D coordinates of phase centers related to dominant signals apparent at the building. These positions are compared to the PS locations available from the PSI analysis of real TerraSAR-X data stacks. As a result, the accuracy of natural PS positions is derived, whereas the relative and absolute localization precision is analyzed.

Besides, deformation velocities are available from the PSI results. Typically, seasonal and linear deformation is estimated from the SAR data in urban areas. Periodic movements are usually present throughout the city, as all steel constructions are affected by thermal dilation. The amplitudes of seasonal deformation are important information for, e.g., the analysis of building stress. Nevertheless, linear deformation might be of major interest for security issues. As the precise localization of PS is assured and many of these points are available for single buildings, linear deformations of specific types can be observed at spatially limited areas in cities. In this investigation several isolated deformation patterns are presented including the analysis of potential origins of the estimated linear motion. Interestingly, most of the discovered subsidence or uplift is caused by ''ordinary'' construction works at building locations. However, the capability to monitor at least one of the presented kinds of motion from space is quite astonishing.

References:

[1] Ferretti, A., Prati, C. & Rocca, F. Permanent Scatterers in SAR Interferometry, IEEE Transactions on Geoscience and Remote Sensing, 2001, 39, 8-20

[2] Adam, N., Kampes, B. & Eineder, M. Developement of a Scientific Permanent Scatterer System: Modifications for Mixed ERS/ENVISAT Time Series, Proceedings of Envisat & ERS Symposium, 2004, on CD-ROM

[3] Hooper, A., Segall, P. & Zebker, H. Persistent Scatterer Interferometric Synthetic Aperture Radar for Crustal Deformation Analysis, with Application to Volcan Alcedo, Galapagos, Journal of Geophysical Research, 2007, 112, 21 pp.

[4] Gernhardt, S. & Bamler, R. Deformation monitoring of single buildings using meter-resolution SAR data in PSI ISPRS Journal of Photogrammetry and Remote Sensing, 2012, 73, 68-79

[5] Bamler, R., Eineder, M., Adam, N., Zhu, X. & Gernhardt, S. Interferometric Potential of High Resolution Spaceborne SAR, Photogrammetrie Fernerkundung Geoinformation, 2009, 5, 407-419

[6] Auer, S., Hinz, S. & Bamler, R. Ray-Tracing Simulation Techniques for Understanding High-Resolution SAR Images, IEEE Transactions on Geoscience and Remote Sensing, 2010, 48, 1445-1456