Stefan Gernhardt

Potential of very high resolution SAR for persistent scatterer interferometry in urban areas

Persistent scatterer interferometry (PSI) has matured to an established method for detection of large-scale and small-scale deformation phenomena in urban areas and of man-made infrastructure. Deformation regimes originating from oil, gas, or water extraction, storage of gas underground, CO2 sequestration, loading of dams and dykes, and mining activities are prominent examples for investigations that have been carried out successfully applying the PSI technique to ERS or ENVISAT data stacks. Due to the sparse spatial distribution of persistent scatterers (PSs) and the moderate resolution of the mentioned satellites it has usually not been possible to track down the source of scattering in detail. The use of PSs was also opportunistic, that is, the PSI monitoring of individual buildings or specific parts of infrastructure was not always been guaranteed. This situation changes dramatically if PSI is applied to very high resolution data obtained from modern radar satellites like TerraSAR-X or COSMO-Skymed. For example, TerraSAR-X is able to deliver images with a resolution of up to 1 m in azimuth and 0.6 m in slant range when operated in the so-called high-resolution spotlight mode with 300 MHz bandwidth. This kind of data provide much more details of single objects and at the same time much higher PS densities. Compared to ERS or ENVISAT type data, the PS density can increase by factors of 100–200 on the same area. This is caused by the physical fact that at higher resolution and shorter wavelength, small scatterers (e.g., trihedrals) show higher signal-to-clutter ratio. Often, several tens to more than a hundred PSs can be found on a single large building facade or roof. Thus, individual buildings and infrastructure can be monitored in terms of structural stress and seasonal deformation, which is also supported by the short revisiting time of only 11 days of TerraSAR-X. Besides on resolution, the density of these points also depends on additional acquisition parameters like incidence angle or polarization and also on the three-dimensional (3D) structure of the given scene, as regions of layover areas and shadow are closely related to the geometrical configuration. The dependence of PS density on these parameters is discussed in this article utilizing results of PSI analysis of different high-resolution TerraSAR-X spotlight data stacks. In addition to this increase of resolution, TerraSAR-X products benefit from a high absolute slant range georeferencing accuracy of 0.5–1 m, which supports a very precise absolute 3D localization of the PSs on the same order. Hence, the physical nature of a PS can be investigated in more detail helping the understanding of the scattering source of the man-made infrastructure. Results of 3D localization and deformation assessment on several test sites are presented in this article.

Interferometric Potential of High Resolution Spaceborne SAR

The new class of high resolution spaceborne SAR systems, like TerraSAR-X and COSMO-Skymed opens new possibilities for SAR interferometry. The 1m resolution is particularly helpful when 2D, 2.5D, 3D, or 4D (space-time) imaging of buildings and urban infrastructure is required, where the non-interferometric interpretation of SAR imagery is difficult. Structure and defor-mation of individual buildings can be mapped, rather than only coarse deformation patterns of areas. The paper demonstrates several new developments in high resolution SAR interferometry using Ter-raSAR-X as an example. Of particular interest is the very high resolution spotlight mode, which requires some care in interferometric processing. Results from interferometry, Persistent Scatterer Interferometry (PSI), and tomographic SAR in urban environment are presented. The high resolution of TerraSAR-X also supports accurate speckle and feature tracking. An example of glacier monitoring is shown and discussed.

Geometrical Fusion of Multitrack PS Point Clouds

Recent radar satellites like TerraSAR-X and COSMO-SkyMed deliver very high resolution synthetic aperture radar images at a spatial resolution of less than 1 m. Persistent scatterer (PS) positions obtained from stacks of high-resolution spotlight data show very much details of buildings and other structures in 3-D due to the enormous amount of PS obtainable from data of this resolution class. As soon as more than one stack covering the same area is available, a combination of the results is eligible. However, geocoded PSs cannot be simply united due to residual offsets in their absolute positions which stem from unknown absolute height values of the different reference points chosen when processing the individual stacks independently. In this letter, two different methods for a geometrical fusion of geocoded PSs from stacks acquired at different aspect and incidence angles are presented. The algorithms are applied to PS interferometry results of both urban and nonurban areas.

Ghost Persistent Scatterers Related to Multiple Signal Reflections

Persistent scatterer interferometry using stacks of very high resolution synthetic aperture radar (SAR) data reveals that single or even patterns of scatterers representing building structures may wrongly be localized below the ground level. In this letter, a case study on a test building model is presented using 3-D SAR simulation methods in order to explain the underlying localization problem. The case study indicates that Ghost-PSs are likely to be related to reflection levels that are higher than three. Moreover, the temporal stability of the amplitude of fivefold bounce signals is confirmed for a SAR data stack.

Tomo-GENESIS: DLR's tomographic SAR processing system

Recently, several new algorithms for tomographic SAR inversion have been developed by DLR. These algorithms have been integrated into an efficient work flow: a system called Tomo-GENESIS. It consists of three main steps: pre-processing including APS estimation, tomographic processing and point cloud fusion. Final results over the test sites Las Vegas and Berlin are exemplified. 3D reconstructions of buildings are presented including their seasonal and linear deformations at resolution and coverage levels not possible so far.

Linear Signatures in Urban SAR Images—Partly Misinterpreted?

Corner lines, line signatures related to building facades in synthetic aperture radar images, are commonly related to signal double reflections, either specular or diffuse. However, the scene properties, i.e., building orientation, building shape, and surface roughness, often do not correspond to this expectation. This letter presents a 3-D representative simulation case study, based on ray tracing and a detailed facade model, in order to analyze the origin of corner lines at facades. The simulation results indicate that the intensity of corner lines may be dominated by signal triple reflections of different types.

Investigations on the nature of Persistent Scatterers based on simulation methods

Using long-time coherent scatterers in SAR data, called Persistent Scatterers (PSs), the deformation signal of objects can be measured from space. However, the nature of the corresponding image signatures is not known in detail. Methods focused on the simulation of highly detailed object models help to understand the origin of PSs. In this paper, simulated SAR data are geometrically linked to PSs for one test site. Thereafter, single PSs are selected and identified in the simulated 3D model. The case study indicates that a high number of PSs lack of geometrical correspondence to building features. Moreover, it is shown that the often observed occurance of PSs located below ground level is not due to processing errors but is a systematic effect caused by fivefold reflection.

Towards TerraSAR-X Street View: Creating city point cloud from multi-aspect data stacks

This article presents a recent updates of urban monitoring using TerraSAR-X data. We demonstrate a Google Street-View-like point cloud of the city of Berlin. To make such a point cloud, synthetic aperture radar (SAR) tomography is employed to process TerraSAR-X data stacks of different view angles around Berlin, obtaining 3D point cloud subsets from each of these data stacks. The density of one of these point clouds is around 1 million points / km2. These point cloud subsets are then fused to create a complete seamless one. Fuse tens of millions of points is a challenging task. Therefore, in this article, particular focus is put on the point cloud fusion. We introduce a feature-based unsupervised technique for point clouds fusion by detecting and matching building contour end points and aligning flat roofs in the two point clouds. The same idea can also be exploited as a general way to evaluate the fusion accuracy of different fusion techniques.

Advanced Displacement Estimation for PSI using High Resolution SAR Data

The Persistent Scatterer (PS) technique is a well known method for estimating object and/or surface deformation from space with high accuracy and high spatial coverage and sampling by analyzing estimation frameworks generated from stacks of interferograms [1]. It is especially suitable for urban areas which, in general, show a high density of PS to detect movements with a wide area deformation pattern as well as changes within small scales, e.g. subsidence of isolated buildings. The new high resolution class satellites like TerraSAR-X, Radarsat-2 or Cosmo-SkyMed put forth new potentials and challenges as the density of PS increases dramatically. As several PS can be found on one single object like a building or a bridge, a advanced motion estimation for this group of points can be introduced, accounting for the coupled movement. In this paper we show how the estimation can be carried out restricting the movement to certain models we employ in order to increase the accuracy of the displacement for the whole group of PS. The results obtained for the proposed estimation technique are based on simulated data to thoroughly investigate the potentials of this approach. They are the basis for real datasets available from TerraSAR-X.

Structural deformation and non-seasonal motion of single buildings in urban areas revealed by PSI

Over the past years, several results of persistent scatterer interferometry (PSI) deformation analyses based on meter resolution synthetic aperture radar (SAR) data stacks have been shown, revealing thermal dilation effects of modern steel constructions. Besides, there exists linear motion even in urban areas of European cities where one would not expect such phenomenon to occur, at least at a first thought. A long term study shows that there are many local areas in Berlin and Munich affected by subsidence or uplift which, fortunately, do not pose a threat on human life. Nevertheless, these effects are very interesting to be analyzed in details, especially as they can be monitored by a PSI analysis, i.e., from space during a period of several years. The examples shown in this paper demonstrate the great potential of space borne high resolution radar sensors for urban monitoring tasks. In addition, a surprising kind of nonlinear - albeit non-destructive - deformation is revealed which is only known from experiments and experience, but has not been measured at a building in situ, yet.