Towards an Urban DEM Generation with Satellite SAR Interferometry

Kurzfassung

The mapping of urban areas is a claimed task for many management applications, such as urban development monitoring, urban climate studies, and renewable energy surveys. Airborne data are widely used at this purpose for the generation of Digital Elevation Models (DEMs). LiDAR (Light Detection And Ranging) is a mature technology for obtaining DEMs in urbanized environment. Spaceborne Synthetic Aperture Radar (SAR) is on the contrary not largely exploited due essentially to the lack of suitable data. In fact, an essential prerequisite for a successful exploitation of SAR data finalized to urban reconstruction is a resolution capable to map the desired target. A second one, but not less important, is the absence of temporal decorrelation in between the two SAR acquisitions, or, in other words, an interferometric bistatic configuration. The TanDEM-X mission, started in June 2010, has as primary objective the generation of a global DEM following the high standard accuracy HRTI-3. The X-band sensors employed for the mission allow an accurate building mapping, hence accomplishing the first prerequisite. Additionally the mission constitutes the first bistatic SAR interferometer in space, letting to a faithful surface reconstruction free of atmospheric and temporal decorrelations. The acquisition mode used to achieve the HTRI-3 requirements is the stripmap one, with a final spatial DEM resolution of 12 meters. This resolution does not assure an accurate mapping of dense metropolitan areas, nevertheless semi-urban and industrial areas are well mapped as shown with a validation made using reference airborne data over Munich (Germany). The mission foresees however experimental spotlight acquisitions finalized to the generation of higher resolution DEMs following the HRTI-4 standards with a 6 meters spatial resolution. Considering this resolution, a denser urban mapping becomes then possible. The first example of urban experimental high-resolution DEM is shown over Las Vegas (USA). The processing chain adopted for the interferometric DEM generation, also called Raw DEM, is built in the Integrated TanDEM-X Processor (ITP). Although the chain is highly optimized in a global scale, some modifications are necessary for the purposes of the paper. In particular, the spectral shift filtering, the coregistration, the interferogram generation and the phase unwrapping processing steps are redesigned towards an optimal configuration for the local municipal zone to map. More in detail, the range spectral shift and the phase unwrapping stages are enabled or disabled depending on the scene configuration; the coregistration stage is configured through its patches size and density and the multi looking requested to reduce the phase noise is set to an optimal value controlled by the actual application. Examples over Munich, with different configuration parameters and applications, are provided. The limits of an interferometric SAR city modelling are also introduced and analyzed in detail. The major restriction for a correct DEM generation is intrinsic to the SAR viewing geometry with issues like layover and shadow. Their effects in the Raw DEM, basically consisting in false heights, are analyzed and the road to overcome these problems, based on the adoption of tomographic techniques, is introduced. Acquisitions with different geometries planned to solve geometric difficulties are also scheduled in the TanDEM-X mission but still not available. This paper analyzes a strategy for the generation of urban DEMs, now possible also with satellite SAR interferometry, exploiting the potentials offered by the TanDEM-X mission.