Stable Coherent Area in SAR interferometry

Synthetic aperture radar images have been a tool of interest in many land cover studies for their night and day, all weather conditions operability. The classic InSAR technique is often limited by temporal and geometrical decorrelation, especially in the context of long time series analysis. Indeed, one of the major drawback of radar images is that some surfaces such as vegetated areas have a varying backscattering coefficient over time. While two images are acquired in multipass interferometric mode, this leads to a partial or sometimes complete loss of correlation of the data. This paper deals with coherence measurements based on 83 ERS images of a large area which is particularly remarkable due to the temporal stability of its interferometric coherence. This area is located on the Serre-Poncon dam, France. Longitudinal observations based on seven years frame ERS interferograms show an expected complete loss of coherence of the scene over vegetated and mountainous areas, excepted on the Serre-Poncon dam for which coherence degree remains high despite the long time span between these acquisitions. An experimental qualitative analysis is lead on the measured coherence of this Stable Coherent Area (SCA). As classic coherence estimators are biased on small averaging windows, a specific method is thus proposed to improve the results, by measuring the global coherence of the dam. This requires a compensation of the topographic and orbital phase components.