Context and objectives
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 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 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 project is aiming at developing a novel methodology to improve the phase unwrapping in SAR interferometry (InSAR) and get an absolute phase map wherever possible by applying MCA on pursuit and spotlight TERRASAR-X data. The method will be tested on 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.
Results will be compared with the results from a state-of-the-art method (MSBAS) in order to assess and quantify the benefit of the proposed methodology.
Expected scientific results
• Development of an integrated absolute phase unwrapping processing method. It is expected to develop SBInSAR and to integrate it in existing processing chain to allow getting continuous phase measurement, resolving phase ambiguities between isolated or independent areas;
• A resolved displacement map of the Nyiragongo volcano;
• A validated high- resolution topographic map of the Copahue area.
Expected products and services
• Method allowing to solve or ease the phase unwrapping process, extending the measurement area.
• Derived products will be:
- A better lava lake levelling of the Nyiaragongo volcano to improve its monitoring.
- An extended DEM of the Copahue volcano
• Scientific users working on processing side and in need of a solution for unwrapping problem or using InSAR results in their studies and models.
• Public or state department, like OVG in RDC or ONEMI in Chile, in charge of volcanic or seismic monitoring, and LGA in their water management models.
|Project leader(s):||CSL - Centre Spatial de Liège|