Potentials of Volumetric Differential Interferometry and Robust Tomography
(1) University of Pisa, Via Caruso 16, 56122 Pisa, Italy
Multipass differential interferometry and multibaseline 3D SAR tomography are two advanced operation modes of SAR interferometry, the former operational, the latter experimental. Recently, a novel interferometric mode crossing the differential SAR interferometry and SAR tomography concepts, termed differential SAR tomography, has been proposed. This is a coherent complex data fusion technique based on two-dimensional space (baseline)-time spectral analysis, fully exploiting the information content of multipass multibaseline data. To reject leakage from the sparse baseline-time sampling and to enhance resolution, modern spectral estimation techniques play an important role in this method. The resulting joint elevation-deformation velocity resolution of multiple scattering components in a same range-azimuth cell has been demonstrated by processing ERS-1/2 data over an urban area with layover concentrated scatterers, even in relative motion. In this framework, both existing SAR data archives, experimented multi-antenna airborne systems, and future multistatic satellite clusters may be efficiently exploited by use of differential tomography for 3D imaging and monitoring of complex surface scattering scenarios.
In this paper, another challenging potential is investigated of differential SAR tomography. The possibility is introduced of joint and continuous scattering profiling in the two-dimensional elevation-deformation velocity domain of distributed volumetric scatterers in motion, including the case of non-rigid motion, i.e. of reconstructing elevation-velocity joint reflectivity distributions. A simulated analysis is reported showing that good results can be obtained under ideal calibration conditions, provided that proper leakage-suppressing two-dimensional spectral estimation techniques are applied and the baseline-acquisition time pattern of the multipass multibaseline data is reasonably rich. This novel result may pave the way to the possible development of a “volumetric differential interferometry” technique, where both non-blurred elevation scattering profiles, and elevation velocity profiles may be reconstructed of moving volumetric scatterers, which constitute new interferometric products. In particular, this challenging potential might be useful for deepening current SAR-based investigations of glaciers internal structure and glacier flows, which are important issues in the global warming problems and studies as indicated by the Intergovernmental Panel on Climate Change (IPCC).
In particular, first obtained results concerns a simple case study of a homogeneously lossy volumetric scatterer with a non-rigid (layered) motion over a bottom surface, which is representative of a glacier flow scenario as seen with a C-band or lower carrier frequency and frequent revisit time sensor. Volume thickness is 7 elevation Rayleigh resolution units, i.e. around 50 m for typical ERS-1/2 multibaseline parameters, the number of coherently processed looks is 16 and SNR is 15 dB. Two different acquisition patterns with 10 passes have been considered, one classical with a single track per pass, and one advanced with 3 simultaneous tracks per pass (satellite cluster). Despite some ambiguity, solvable by considering the imaging geometry and known direction of flow, a clear elevation-velocity “signature” of the simulated glacier flow is detected even by the first quite poor acquisition pattern. Unambiguous estimates of elevation-velocity joint reflectivity distribution are furnished by the cluster pattern with satisfactory accuracy, for the assumed parameters. These preliminary simulated experiments are encouraging about the potential of differential SAR tomography for subsurface glacier velocity profiling and buried scatterers investigations, coupling the velocity dimension to 3D reflectivity maps. In the full paper, other simulated analyses of the new concept will be reported, to sample its performance and limits, in particular for what concerns scenarios of temporal decorrelating multiple distributed surface scatterers (layover in natural areas), moving scatterers buried under a temporal decorrelating volume (subsurface water tables, scatterers below forest). Also, side issues that should be attacked for the method to be efficient with real data are discussed, such as problems of range migration effects in elevation for significantly extended volumetric scatterers.
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Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,