Blind whitening of correlated speckle to enforce despeckling of single-look high-resolution SAR images

Abstract

During the last three decades, several methods have been developed for the despeckling of synthetic aperture radar (SAR) imagery. While some of them are totally empirical, the majority of those relying on signal and noise models has been derived under the assumption of a fully–developed speckle model, in which the multiplicative fading term is supposed to be a white process. Unfortunately, the frequency response of the SAR system may introduce a statistical correlation, which decreases the capability of reducing speckle for filters that assume a white speckle model. In this work, an unsupervised method for whitening single–look complex (SLC) images produced by very–high resolution (VHR) SAR systems is proposed. By using the statistical properties of the SLC image and some likely assumptions, estimation of the frequency response of the SAR system is performed. After that, a decorrelation stage is applied to the complex image in order to yield uncorrelated speckle in the intensity/amplitude component. Strong scatterers are automatically detected and left unprocessed. After the whitening step, the complex image is detected and the resulting intensity/amplitude may be despeckled. Experimental results have been carried out both on optical images corrupted by synthetic correlated complex speckle and on true SLC images acquired by the COSMO-SkyMed SAR satellite constellation. Both advanced and classical despeckling filters achieve significantly better performance when they are preceded by the proposed whitening step. On homogeneous areas the equivalent number of looks (ENL) increases by four–five times. The sharpness of edges and strong textures is negligibly diminished by the whitening step; this effect is even less noticeable after the despeckling step has been performed. The radiometric characteristics of images are preserved by the whitening process to a large extent.