Enhanced and new Mapping and Monitoring Possibilities Opened by the Multidimensional SAR Tomography Framework
Lombardini, Fabrizio1; Viviani, Federico1; Dini, Francesco2; Cai, Francesco2
1University of Pisa / CNIT-RaSS, ITALY; 2University of Pisa, ITALY
Interest is continuing to grow in techniques of coherent combination of complex (amplitude and phase) SAR data, going beyond the mature phase-only Interferometry , for extraction of rich information on the observed scenes. Among these, 3D SAR Tomography [2,3] (Tomo-SAR) is an emerged experimental multibaseline (MB) mode for full 3D imaging through elevation beamforming (i.e. spatial/baseline spectral estimation). Tomo-SAR resolves multiple scatterers in height in the same SAR range-azimuth cell, allowing analysis of complex scenes (e.g. for estimation of forest height/biomass, derivation of sub-canopy topography and ice thickness, and of heights/reflectivities in layover in natural or urban areas) . However, especially for spaceborne sensors, observed scenes are typically non-stationary, as temporal decorrelation and deformation motions can occur during the repeat-pass MB acquisition. This can cause height blurring in Tomo-SAR imaging . Most importantly, although Tomo-SAR can separate multiple scatterers, it has no measuring sensitivity to their deformation motions that can be of great interest. On the other hand, Differential Interferometry (D-InSAR) senses deformations but degrades with multiple scattering.
In this context, a novel coherent data combination mode termed Differential SAR Tomography (Diff-Tomo) has been originated , integrating D-InSAR and Tomo-SAR concepts to allow "opening" the SAR cell in complex non-stationary scenes - University of Pisa (UniPi) patent. Exploiting space-time spectral analysis, it allows joint resolution of multiple elevation and deformation velocity components of the scatterers mapped in a SAR cell (4D i.e. 3D+velocity imaging), with application e.g. to subsidence monitoring of dense layover urban areas [6,7] and infrastructures. Diff-Tomo enables also "Volumetric Differential Interferometry" capabilities, in which continuous profiling may be possible of velocity vs. height . Even more generally, Diff-Tomo can identify scattering components distributions in the domain of spatial (height) and temporal frequency of harmonics in which a signal from a scatterer with temporal perturbations (decorrelation) can be decomposed, avoiding their misinterpretation in tomographic processing [4,8], and allowing development of new 3D sensing of dynamical processes .
2. DIFF-TOMO FUNCTIONALITIES
In this work, an overview of UniPi and CNIT/RaSS Nat. Laboratory research and recent advances are presented of these Multidimensional Tomography techniques of coherent data combination for the analysis of complex layover or volumetric scatterers and non-stationary deformating or decorrelating scenarios.In particular, a first theme concerns Diff-Tomo application over urban areas to estimate heights and deformation velocities of multiple scatterers with new generation high resolution COSMO-SkyMed data over Naples (Italy). This Diff-Tomo processing is with height superresolution and light-burden [7,9], includes automated detection of single and multiple layover scatterers , and is single-look to maintain the full horizontal data resolution with no urban detail loss .
The possibility of Diff-Tomo to handle non uniform deformation motions, in particular from seasonal thermal dilations, is also addressed and reported ("5D" processing).
A second theme regards the application of Diff-Tomo based techniques to repeat-pass MB data of forest scenarios, characterized by the presence of the temporal decorrelating volumetric scatterer, in layover with the ground scatterer.
The Diff-Tomo enabled experimental functionalities for vegetated scenarios are 3D Tomography robust to temporal decorrelation [4,8], and separation in the height dimension of different temporal coherence levels , that are mixed (undiscriminated) in the classical (overall) coherence analysis. Both these unique functionalities exploit the Diff-Tomo identifiability of temporal harmonics from decorrelation. Robust Tomography may overcome a possible hindrance to operational development of spaceborne forest tomography. Temporal coherence separation allows new phenomenological investigations to characterize the effect of decorrelation in Pol-InSAR and Tomo-SAR applications, and possibly new dynamic 3D forest features.
In particular related experiments  are expanded with MB P-band E-SAR data over the boreal forest of Remningstorp.
Sample results of Diff-Tomo processing for urban and forest applications are reported here. Fig. 1 reports a typical Diff-Tomo "frame" [5,7] for a cell of Naples' San Paolo Stadium COSMO-SkyMed data (CSK data provided by ASI), with joint height and deformation velocity resolution of a double (layover) scatterer. Some extensive Diff-Tomo results are shown in Fig. 2 showing a full 3D tomographic height reconstruction of the whole San Paolo Stadium, including resolved layover zones. Fig. 3 reports a temporal coherence separation  along a range cut of the forest E-SAR data. The green and blue markers denote the canopy and ground scatterer coherence time estimates, respectively.
The described methodologies may be applied in particular to the candidate mission BIOMASS, to the missions COSMO-SkyMed, TanDEM-X, RADARSAT-2, the future Sentinel-1, SAOCOM, and the possible TanDEM-L, DESDynI.
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