THE EFFECT OF MEDIAN FILTERING ON SYNTHETIC-APERTURE RADAR IMAGES

Synthetic aperture radar (SAR) images are subject to intrinsic 'noise' , called speckle, over and above any spatial variability due to variat ions in the properties of the scene. Many noise-reduction techniques h ave been employed to reduce the effects of this phenomenon. In this no te we review the statistical effects of one of the simplest such techn iques, the median filter. This filter can be performed almost as rapid ly as the mean (box average) filter but has significantly better edge- preserving properties. It is, however, unsuited to images containing s ignificant point- or small-target features. Use of the median filter c an introduce significant biases into the data, for example a 25 per ce nt reduction in an intensity image after 3 by 3 median filtering. This note presents calculations of the size of these biases for the case o f homogeneous target areas, fully-developed speckle, and statistically independent looks in multi-look images.