Coherence loss then occurs often in the presence of vegetation or surface water. We note that the active scree slope where there is little vegetation remains the most coherent part. Displacements with high values of gradient, such as those associated with the landslide, lead also to incoherence since phase variation across a pixel exceeds one cycle. Orbit cycles of a few days as well as Tandem configuration allows the user to overcome this limitation of ERS data. Furthermore, it provides only the gradient of displacements, and hence relative displacements can only be evaluated by remote sensing.
Reference points are necessary to determine absolute displacements.
This investigation demonstrates the capability of SAR interferometry to monitor surface displacements at the scale required for landslide monitoring. The constraints of this kind of study are totally different from the ones associated with earthquakes, due to small spatial extension and often the high topography encountered. SAR interferometry demonstrated its capability for studying the deformation over small areas. We were able to construct several interferograms on which the landslide is clearly evidenced. These interferograms show an organized fringe system for an elapsed time as large as 12 days, allowing us to construct a steady-state model of surface displacements valid for the whole period of observation.