A locally adaptive noise filtering approach for phase-unwrapping improvement

In order to obtain a more accurate topographic model a noise filtering step must be performed before the unwrapping of phases. We propose an iterative process that involves a filter in the spatial domain and a multi-resolution phase unwrapping method. The approach is based on the generation of an approximate phase model, which is iteratively refined. In the aim of preserving fine details in the interferogram that are directly related to the ground topography, an edge-preserving smoothing method has been applied. The local spatial content of the phase image is exploited in order to determine for each pixel the best matching shape of the filtering mask, searching in a set of different neighbourhood systems. Then, a non-linear adaptive filtering function based on the local estimation of noise and signal standard deviation, is adopted The results obtained by processing several noisy simulated and real interferometrical images with the described method show clearly how the combination of a detail preserving adaptive filter with a noise robust phase unwrapping approach appears as a good means for reducing the influence of distributed noise and low coherence areas on the determination of a digital ground elevation model.