Timo Balz

Building-damage detection using post-seismic high-resolution SAR satellite data

Radar satellite imagery was valuable in supporting extensive rescue operations after the Wenchuan Earthquake on 12 May 2008 due to its ability to operate independently of weather conditions, day and night. However, it is a challenging task to identify damaged or destroyed buildings using synthetic aperture radar (SAR) data. The standard procedure for identifying damaged buildings is to use change detection by comparing post-seismic to pre-seismic images, but almost no archived high-resolution SAR images were available of the rather remote area damaged by the Wenchuan Earthquake. Building-damage assessment using only post-event SAR images was therefore necessary to assess the areas of damage. In this paper, theoretical assumptions about the appearance of collapsed buildings in high-resolution SAR images were drawn and verified with visual feature interpretations of real SAR images from the area.

Hybrid GPU-Based Single- and Double-Bounce SAR Simulation

In this paper, a new hybrid graphics-processing-unit (GPU)-based real-time synthetic aperture radar (SAR) simulation system is presented. Previous real-time SAR simulators only supported single-bounce simulation in real time. The new hybrid system uses the rasterization approach for real-time single-bounce simulation and a new image-based GPU ray-tracing approach for monostatic SAR double-bounce simulation. This approach provides fast simulation results even while simulating complex and extended scenes. The simulation results are compared to a high-resolution airborne SAR image, and the limitations of the approach are discussed.

Landslide monitoring with high-resolution SAR data in the Three Gorges region

Employing the well-known D-InSAR technique, we investigated landslide monitoring in the Three Gorges region using TerraSAR-X data. The experiment demonstrates that using both the amplitude and differential phase allows us to identify the precise location, deformation and time range of occurrence of certain landslides. To overcome the atmospheric effect on D-InSAR results, a time-series analysis was also carried out. The observed nonlinear relationship between the deformation and water level suggests that reservoir water level fluctuation is one of the major causes of landslides, which is significant in terms of issuing landslide warnings. In addition, the comparison of TerraSAR-X and C-band ASAR data results indicates that TerraSAR-X data provide far more reasonable deformation measurements because of their high temporal and spatial resolutions.

Characterization of Landslide Deformations in Three Gorges Area Using Multiple InSAR Data Stacks

In the areas with steep topography and vulnerable geological condition, landslide deformation monitoring is an important task for risk assessment and management. Differential Synthetic-Aperture Radar interferometry (D-InSAR) and Persistent Scatterer Interferometry (PS-InSAR) are two advanced SAR Interferometry techniques for detection, analysis and monitoring of slow moving landslides. The techniques can be used to identify wide displacement areas and measure displacement rates over long time series with millimeter-level accuracy. In this paper, multiple SAR datasets of Advanced Land Observing Satellite (ALOS) Phased Array L-band Synthetic Aperture Radar (PALSAR) and Environmental Satellite (ENVISAT) C-band Advanced Synthetic Aperture Radar (ASAR) are used for landslide monitoring with both D-InSAR and PS-InSAR techniques in Badong at the Three Gorges area in China. Two areas of significant deformation along the southern riverbank of Yangtze River in Badong are identified by joint analyses of PS-InSAR results from different data stacks. Furthermore, both qualitative and quantitative evaluations of the PS-InSAR results are carried out together with preliminary correlation analysis between the time series deformation of a PS point in high risk location and the temporal variation of water level in the Three Gorges Reservoir.

Error Analysis of Bistatic SAR Imaging and Stereoscopy Bistatic SAR

The flexible geometry configuration of the bistatic synthetic aperture radar (SAR) has many advantages. However, it causes serious measurement error in the bistatic SAR system, which degrades the quality of the SAR images and the precision of the digital elevation model (DEM) obtained using stereoscopy bistatic SAR. In this paper, the influence of the scene height estimation error, trigger delay, transmitter position measurement error, receiver position measurement error, and transmission line length measurement error are analyzed. These analyses are very useful in bistatic SAR system design. The scene height estimation error, trigger delay, transmitter position measurement error, and synchronization receiver position measurement error affect both the quality of the images and the precision of the DEM obtained by stereoscopy bistatic SAR slightly. The echo receiver position measurement error and transmission line length measurement error affect the quality of the imaging only slightly, but seriously affect the precision of the DEM obtained by stereoscopy bistatic SAR. Luckily, their measurement precision can be quite satisfactory. Simulations and real bistatic experimental results verify the proposed theoretical analysis.

SAR Simulation of Urban Areas: Techniques and Applications

The simulation of synthetic aperture radar (SAR) data is a widely used technique in radar remote sensing. Using simulations, data from sensors which are still under development can be synthesized. This provides data for developing image interpretation algorithms before the real sensor is launched. Simulations can further create simulated images from precisely defined scenes. They can deliver simulated images of any object of interest from various orbits, at a wide range of angles, using different wavelengths.

A New Region Growing-Based Method for Road Network Extraction and Its Application on Different Resolution SAR Images

Road network extraction plays an irreplaceable role in the applications of synthetic aperture radar (SAR) images. In this paper, we propose a new method based on the region growing to quickly extract the road network, which is suitable for different resolution SAR images. First, a weighted ratio line detector (W-RLD) is proposed to extract road features. Then, an automatic road seeds extraction method, which merges the ratio and direction information, is utilized to improve the quality of the extracted road seeds. Finally, the region growing concept is adopted to construct the road network, and a fast parameter selection procedure is presented for adaptively adjusting growing parameters. In experiments, four kinds of SAR images are used to assess the performance of the proposed method, including Envisat ASAR (30 m), HJ-1-C (5 m), TerraSAR-X (3 m), and airborne C-band data (0.5 m). Both visual and quantitative evaluation results show the adaptability and efficiency of the proposed approach.

3D SAR Simulation of Urban Areas Based on Detailed Building Models

This paper confirms the need for detailed 3D models for the simulation of high-resolution SAR images in order to support Persistent Scatterer Interferometry (PSI) focused on single urban objects. Using a building model enhanced by facade grammar, multiple reflections at building facades are analyzed using ray tracing techniques and scatterers are localized in azimuth, range, and elevation. In a case study, salient signatures of a TerraSAR-X image are analyzed based on simulated SAR reflectivity maps. Phase centers of trihedral reflections are mapped onto the building model and the physical correspondence of scatterers to building features is investigated. Surfaces contributing to salient scatterers are identified at the building model. Eventually, the use of SAR simulation to support PSI is shown from two aspects: (a) for providing a-priori information about building layover, and (b) for extending knowledge about the nature of dominant scatterers.

Improved Real-Time SAR Simulation in Urban Areas

Within the paper the applicability of modern graphic cards for SAR simulation is demonstrated. By these means SAR simulation, which is frequently used as a key tool for the analysis and interpretation of SAR scenes, is feasible in real-time even in complex urban environments. This is realised by the implementation of SAR geometry within standard graphics hardware, which offers 3D hardware acceleration and programmable graphics processing units (GPU).

Using real-time SAR simulation to assist pattern recognition applications in urban areas

In this paper, the impact of current developments in urban modeling and graphics programming to the potential of SAR simulation and interpretation in urban areas is presented. As it will be demonstrated, SAR simulation is feasible in real-time even for complex urban environments by applying modern graphics cards. For this purpose, the SAR geometry is implemented using programmable graphics processing units (GPUs), which are available as standard graphics hardware for 3D acceleration. Originally, the geometric models of urban areas, which provide the database for SAR simulation, were captured to generate realistic visualizations of virtual scenes. As will be demonstrated, such area covering databases can be modified to meet specific requirements of SAR simulations at different scales. Finally, the applicability of our SAR simulation as made available from off-the-shelf hardware components for the analysis and interpretation of SAR scenes by pattern recognition will be demonstrated.