Speckle filtering is often carried out to improve the interpretation of polarimetric synthetic aperture radar (PolSAR) data for terrain classification, retrieval of physical scattering properties etc. As inspired by Newton's law of universal gravitation, a force of attraction between two image pixels is formulated in polarimetric-spatial domain. It is directly proportional to polarimetric similarity but inversely proportional to spatial distance. By using the attractive force as a filtering weight, this paper presents another variant of bilateral filter, called gravitational filter, for speckle reduction in PolSAR data. The performance of the gravitational filter was examined with ALOS-2 PALSAR-2 polarimetric data. Four aspects of the filtering performance were investigated: 1) speckle reduction, 2) image feature retention, 3) polarimetric property preservation, and 4) computational efficiency. The experimental results confirmed the effectiveness of the gravitational filter in the first two aspects, which was archived at the cost of an increased computational load. Applying to the four-look ALOS-2 PALSAR-2 test data, a bias in radiometric estimation was observed and can be reduced by using only intensity components when computing the attractive force.
