IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 8, NO. 4, JULY 2011 661 Tomographic Imaging and Monitoring of Buildings With Very High Resolution SAR Data D. Reale, G. Fornaro, Senior Member, IEEE, A. Pauciullo, X. Zhu, Student Member, IEEE, and R. Bamler, Fellow, IEEE Abstract—Layover is frequent in imaging and monitoring with synthetic aperture radar (SAR) areas characterized by a high density of scatterers with steep topography, e.g., in urban environ- ment. Using medium-resolution SAR data tomographic techniques has been proven to be capable of separating multiple scatterers interfering (in layover) in the same pixel. With the advent of the new generation of high-resolution sensors, the layover effect on buildings becomes more evident. In this letter, we exploit the po- tential of the 4-D imaging applied to a set of TerraSAR-X spotlight acquisitions. Results show that the combination of high-resolution data and advanced coherent processing techniques can lead to impressive reconstruction and monitoring capabilities of the whole 3-D structure of buildings. Index Terms—Differential interferometric synthetic aperture radar (DInSAR), multidimensional SAR processing, SAR tomog- raphy, synthetic aperture radar (SAR), TerraSAR-X (TSX), 4-D SAR imaging. I. I NTRODUCTION I NTERFEROMETRIC synthetic aperture radar (SAR) (InSAR) and differential InSAR (DInSAR), particularly multitemporal DInSAR, have been proven to be effective for accurate scatterer localization and monitoring of displacements [1], [2]. The high accuracy and spatial density of the mea- surements make these techniques cost effective compared to classical geodetic techniques, typically used in environmental risk monitoring. The increase of the spatial resolution provides a tangible improvement in the monitoring capabilities: Most of the in- ternational space agencies have hence hugely invested in the launch of large bandwidth spaceborne SAR systems. The hard- ware improvement must be complemented by the development of processing techniques that are able to extract the highest possible information content from the data. In this sense, SAR Manuscript received October 14, 2010; accepted November 25, 2010. Date of publication January 23, 2011; date of current version June 24, 2011. This work was supported in part by the European Union Integrated System for Trans- port Infrastructures Surveillance and Monitoring by Electromagnetic Sensing (ISTIMES) project and in part by the International Graduate School of Science and Engineering, Technische Universitaet Muenchen, Munich, Germany. D. Reale, G. Fornaro, and A. Pauciullo are with the Institute for the Elec- tromagnetic Sensing of the Environment (IREA), National Research Council (CNR), 80124 Napoli, Italy (e-mail: reale.d@irea.cnr.it; fornaro.g@irea.cnr.it; pauciullo.a@irea.cnr.it). X. Zhu is with the Technische Universität München, Lehrstuhl für Methodik der Fernerkundung, 80333 Munich, Germany (e-mail: xiaoxiang.zhu@bv. tum.de). R. Bamler is with the Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), 82234 Oberpfaffenhofen, Germany, and also with the Technische Universität München, Lehrstuhl für Methodik der Fernerkundung, 80333 Munich, Germany (e-mail: richard.bamler@dlr.de). Digital Object Identifier 10.1109/LGRS.2010.2098845 tomography, also known as multidimensional (3-D and 4-D) imaging SAR (MDI-SAR), is recognized as a powerful tech- nique that extends interferometry. DInSAR and persistent scatterer interferometry (PSI) assume the presence of only a single (dominant) scattering center in each pixel. However, SAR images of complex scenarios are affected by the interference between the responses of scat- terers located at different elevations (slant heights). Standard multipass interferometric techniques “look” for the matching between the received signal and the “multipass signature” of a scatterer: The interference of responses may hence lead to misdetection of persistent scatterers and to height, velocity, and time-series measurement inaccuracies. The layover effect causes interference between the responses of different scatterers. Layover is particularly critical in urban areas which are characterized by a high density of scatterers distributed on vertical structures. As briefly explained next, MDI-SAR allows the overcoming of the single scatterer assumption and has opened a new sce- nario in the 3-D target reconstruction and monitoring with SAR systems [3], [4]. On medium-resolution systems, MDI-SAR imaging has already been proven to be effective in separating and monitoring scatterers in layover [5], [6]. The new generation of high-resolution SAR sensors, such as TerraSAR-X (TSX) and the COSMO-SkyMed constellation, allows the systematic acquisition of data with spatial resolution reaching metric/submetric values. The preliminary analysis of these images in dense urban areas has indicated that the reso- lution improvement brings layover of vertical structures to be more pronounced. On high-resolution SAR data, the interfer- ence between scatterers on the ground and on buildings is more frequent, and it is distributed on more pixels than on data ac- quired by medium-resolution satellites (e.g., European Remote Sensing (ERS) satellite or ENVISAT): The tomographic ap- proach is a tool that allows mitigating this problem [7]. More- over, the higher the resolution, the higher are the expectations for 3-D reconstruction on vertical structures. In this letter, we investigate the application of SAR tomogra- phy to a real data set of TSX spotlight images over the city of Las Vegas, NV. The characteristics of this data set allow clear demonstration of the potential and the advantages offered by the SAR tomography technique. II. LAYOVER AND TOMOGRAPHY The imaging mechanism of radar is measuring the distances (range) of the scatterers from the sensor. If two scatterers are 1545-598X/$26.00 © 2011 IEEE