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

Abstract

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.