MUlti Zone phase Unwrapping using advanced Split Band Interferometry
The current MUZUBI project is aiming at developing a novel methodology to improve the phase unwrapping in SAR interferometry (InSAR). The methodology is based on the results of a former project (Vi-X). The innovative method to be developed here will be tested on two case studies in Argentina and DRC. Results will be compared with the results from a state-of-the-art method (MSBAS) currently used in the frame of a running project (RESIST) in order to assess and quantify the benefit of the proposed methodology. In SAR interferometry (InSAR), phase unwrapping remains the bottleneck to get continuous phase measurements among separated coherent areas, which is required to compute the topographic or the relative displacement component. In addition to giving relative measurements, classical interferometry often leads to unconnected patches, each unwrapped independently. For each patch, ground truth is required to allow connecting them together and get continuous measurement across the scene. Unfortunately, in general, these ground truth data are unavailable. Most recent SAR sensors use wide band signals to achieve metric range resolution. One can also take advantage of wide band to split it into sub-bands and generate several lower-resolution images, centred on slightly different frequencies, from a single acquisition. This Split Band process, also named Multi Chromatic Analysis (MCA), corresponds to performing a spectral analysis of SAR images. Split-Band SAR interferometry (SBInSAR) is based on spectral analysis performed on each image of an InSAR pair, yielding a stack of sub-band interferograms. Scatterers keeping a spectrally coherent behaviour in each sub-band interferogram show a phase that varies linearly with the carrier frequency, the slope being proportional to the absolute optical path difference. This potentially solves the problems of phase unwrapping on a pixel-per-pixel basis. Of prime importance, it might allow connecting independent zones, estimating the absolute phase on most coherent pixels of each patch, provided that the required accuracy could be achieved. The MUZUBI project aims at filling that gap toward a fully developed Split Band-assisted phase unwrapping processor for SAR interferometry using Multi-Chromatic Analysis. A former project (Vi-X) showed the high potential of the technique to perform absolute height/displacements measurement, connecting independently unwrapped zones. However, the spectral diversity of the TanDEM-X (TDX) data appeared to be insufficient to achieve the required precision because of the bistatic focusing procedure that limits the bandwidth to 100MHz. It is thus proposed to use among others, more advanced sensed data that will be made available through a recently approved proposal submitted to the TanDEM-X Science Phase AO. These exceptional data will be acquired in both stripmap and spotlight pursuit mode, granting both wide bandwidth (spectral diversity) and high coherence. It is thus proposed to adapt existing processor to specific pursuit and spotlight TerraSAR-X (TDX) acquisition schemes and combine SBInSAR processor with classical phase unwrapping procedure in order to get absolute phase measurement on all coherent zones. The developed technique will be applied to the study and monitoring of two active volcanic zones: the Nyiragongo/Nyamulagira (RDC) and the Copahue (Argentina). In the first case, it should allow getting connected displacement measurements on separated areas around the highly vegetated volcanoes. In the second case, known to be more challenging in terms of topography, it should allow to resolve the required topographic component to then perform displacement measurements using DInSAR/MSBAS technique.
External collaborators:Dominique De Rauw - Centre Spatial de Liège
Nicolas d'Oreye - European Center for Geodynamics and Seismology, Luxembourg
Yanina Rubio - Comahue National University, Argentina
Sergey Samsonov - Canada Centre for Mapping and Earth Observation, Natural Resources Canada