Context and objectivesTo 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
Expected scientific results
Baseline combination has been implemented successfully. Some aspects of the technique were badly evaluated especially the fact that only unwrapped phases can be averaged. But, we successfully overcome the encountered difficulties and developed a method to unwrapped interferograms jointly and iteratively. This phase unwrapping procedure allowed us to unwrapped much efficiently than classically. A specific tool was implemented to evaluate directly the phase noise in an interferogram and/or in an unwrapped phase surface. This tool allows to evaluate beforehand the height and phase accuracy we can expect from an InSAR processing. Baseline combination revealed to have two aspects. The first one consists in using the technique to lower the fringe rate as far as possible in order to put atmospheric artefacts in evidence. Useful information can be obtained this way to evaluate and locate much more accurately the importance of such artefacts. The second aspect of baseline combination consists in using it by averaging unwrapped phase to lower the phase standard deviation of the combined phase surface. Such combination allows to obtain more accurate DEMs than those obtained using a single interferogram. Phase standard deviation lowering appears to be more important where using « low quality » intereferograms than when combining good ones. Even when combining low quality phase surfaces with good one, one can expect phase noise reduction. Apart from noise reduction, baseline combination offers also the advantage of averaging the artefacts that might be present in each combined one. Finally, the combined InSAR DEM appears to be accurate enough to put in evidence some events like high buildings. Therefore, we consider that the objectives of the present study were successfully achieved.