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InSAR baseline combination for topographical phase reference generation

Research project T4/DD/38 (Research action T4)

Persons :

Description :

SAR interferometry (InSAR) can generate digital elevation models (DEMs) by measuring the interferometric phase of two coregistered images of the same region, acquired on two consecutive satellite passes. If the contribution of the terrain topography can be subtracted out in some way, the interferogram shall only reflect changes that occurred between the acquisition. This Differential SAR interferometry (DInSAR) technique provides the possibility of detecting and measuring terrain changes at a centimetre scale. Spectacular results have been obtained along these lines in the fields of tectonics, volcanology, terrain subsidence, and post-studies of seismic events.

The removal of the topographical phase while keeping the "displacement- phase can be done by using either an external DEM or another interferometric pair that plays the role of a topographical reference. In the first case, one must be sure to effectively measure displacements in the differential interferogram and not local errors of the DEM. In the second case, the topographical phase has to be free of artefacts.

It is proposed to use the potentialities of the European Space Agency (ESA) SAR archives to implement a method based on baseline combination to produce reliable topographical phases. Combining several interferograms of a given area, each having a given ambiguity altitude, allows one to generate a combined interferogram with a good signal-to-noise ratio (SNR) and a much shorter equivalent ambiguity altitude that would be impossible to obtain with a single SAR pair due to baseline decorrelation. Assessment of the method shall be performed by comparing the resulting high-resolution DEM with existing ones. Two test sites over Belgium, covering the Brussels and Liège areas, are proposed. In both cases, high-resolution reference DEMs are available with an altitude resolution < 1 m.

The Project shall be carried out by a network consisting of Centre Spatial de Liège (CSL), a research centre of the University of Liège acting as Project co-ordinator, and the Departments of Electrical Engineering and Geophysics of the Stanford University (California). It was originally submitted and accepted in the framework of the ESA Third ERS Announcement of Opportunity.

Intensive work is being done to reach the knowledge threshold for transferring the InSAR/DInSAR know-how to industry (software commercialisation) and users (provision of services). Under the National TELSAT-3 Remote Sensing Programme, CSL has developed an InSAR/DInSAR processor and set up collaborations with a wide spectrum of users to demonstrate the applicability of the technique to various disciplines. This processor was used in a recent study financed by the Data User Programme (DUP) of the European Space Agency (ESA), to carry out a quality assessment of InSAR topographic mapping. The experience gained as a result of this project lets us feel that the above-mentioned knowledge threshold is being reached for this particular application.

The present proposal constitutes a first step in the same direction for DInSAR, by addressing one particular but determining feasibility aspect of the technique: the need of a reliable topographic reference, without which small terrain changes cannot be measured.

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