Speckle

appearing in synthetic aperture radar (SAR) images as granular noise

is generated by the coherent processing of radar signals. It severely affects the image qualities. To alleviate deleterious effects of speckle

various ways have been devised to suppression it. An ideal algorithm should smooth the speckle without blurring edges and fine detail. But most algorithms cannot satisfy these two demands very good.This paper investigates the problem of speckle suppression comprehensively and thoroughly. It first analyzes the principle of speckle forming

then describes the general approaches of speckle reduction filtering

discusses several well known speckle reduction filters

compares the algorithms qualitatively. To compare speckle smoothing algorithm quantitatively

a set of performance criteria is established. In the end

conclusions are made and the possible future work is explained.