TerraSAR-X High Resolution Spotlight Persistent Scatterer Interferometry

The new class of meter-resolution spaceborne SAR systems (TerraSAR-X and COSMO-Skymed) offers a tremendous improvement in interferometric monitoring of urban areas and man-made infrastructure. The high resolution fits well to the inherent scale of buildings (floor height, distance of windows, etc.). In particular, Persistent Scatterer Interferometry (PSI) benefits substantially from this new data quality. Several tens or hundreds of persistent scatterers (PS) per building for the first time be exploited for geodetic analysis from space. Many small constructive elements of buildings, like edges in facades originating from notches for windows and similar structures act as stable scatterers and thus are detected as PS at a wavelength of 3.1cm. The large amount of PS on single objects establishes the opportunity to monitor e.g. structural stress or seasonal deformation of certain objects of interest. In this investigation we will show how the number of PS depends on resolution, incidence angle, and polarization. To this end the area of the Berlin inner city has been acquired by TerraSAR X in high resolution spotlight mode at 300 MHz (slant range resolution = 0.6 m, azimuth resolution = 1.1 m) using different beams (ascending and descending) leading to six stacks for PSI evaluation. The comparison will be carried out on several regions in order to show the impact of the corresponding parameters on the density of PS also for different building development, like present in a common European city. The deformation estimates of the six stacks of Berlin show a very interesting pattern at the central railway station. It turns out that a linear deformation model insufficiently describes the movement at this location and consequently a more complex model has to be introduced. The results of PS deformation estimation using a seasonal sinusoidal model comprise several very interesting patterns, which will be presented in this study. Periodic deformation in vertical as well as horizontal directions of the building parts and the rail tracks follow a pattern of thermal expansion of steel. The amplitudes are shown to be consistent with temperature records for the given area. Especially the unique constellation of six different stacks covering the same region facilitates the interpretation of deformation. Vertical and horizontal contributions can be separated, which could be validated by constructional conditions of the building and the rail tracks. Besides the deformation parameters a height update relative to the used digital elevation model is estimated, using the PSI processing software PSI-GENESIS of DLR. To this end the exact 3D positions of the PS are retrieved both in the azimuth-range-elevation system and, after geocoding, in x, y and z UTM co-ordinates. The PS clouds of the different stacks are fused after adjustment of a possible elevation error. All geolocation errors in range and azimuth can be neglected for TerarSAR-X. The precision of the localization depends amongst others on the number of acquisitions, the baseline distribution and the signal to noise ratio and can be assessed in advance for the given stack and parameters of acquisition. In this paper the expected precision will be compared to the one obtained from PSI stacking by evaluating the distribution of scatterers from rows of windows in facades of buildings which accumulate in a line. This constellation of PS could be observed at different buildings in the data and serve as a basis for this investigation