Space-borne high resolution SAR tomography: Experiments in urban environment using TS-X data

Synthetic Aperture Radar (SAR) tomography aims at retrieving the 3-D reflectivity from multi-pass SAR data. It is essentially a spectrum estimation problem. As a consequence, complex values of a specific range cell in a SAR image stack as a function of baseline are closely related to the Fourier transform of the reflectivity function in the elevation direction. The new generation of SAR satellites, like TerraSAR-X, allow for the first time the building up of high-resolution SAR data stacks on a regular basis. TerraSAR-X in its high resolution spotlight mode provides data with 0.6 m slant range resolution. It has already been shown that persistent scatterer interferometry (PSI) benefits enormously from this new quality of data. The data stacks used for PSI in urban areas can also be used to derive tomographic information. This paper presents the first demonstration of space-borne high resolution tomographic reconstruction of multiple scatterers in a resolution cell situation in urban areas. Different spectrum estimation strategies such as the Singular Value Decomposition (SVD) and Nonlinear Least Squares estimation (NLS) are evaluated and compared using both simulated data and TerraSAR-X spotlight data over Las Vegas with special consideration of the difficulties caused by sparse and irregular samples. The nuisance of ill-conditioning is investigated and regularization tools are utilized to overcome this problem. For validation, the spectrum estimation results with TerraSAR-X data are compared to the plausible ground truth. In a second step of processing model selection criteria such as the Bayesian Information Criterion (BIC), Akaike information criterion (AIC) and Minimum Description Length criterion (MDL) are implemented on the spectral estimates to determine the number of scatterers inside a resolution cell. The probability of correctly detecting the number of scatterers and the accuracy of the corresponding elevation estimates are evaluated from simulated data. Additionally, model selection results with TerraSAR-X data are analyzed. Finally, SAR tomography, as a straightforward extension to PSI, is integrated into DLRs PSI-GENESIS processor to support deformation estimation and solve the ambiguity due to multiple scatterers inside a resolution cell. First processing results using TerraSAR-X data are presented that confirm the capability of space-borne high resolution SAR tomography for resolving multiple targets within the same azimuth-range cell and to map the 3-D scattering properties of the illuminated scene.