Polarimetric SAR is an advanced imaging radar system; it plays an important role in radar remote sensing. With a polarimetric SAR, we can obtain much more information than conventional SAR systems (i.e., single polarized SAR systems) [1]. Up to now, various airborne and spaceborne polarimetric SAR systems have been developed, such as AirSAR, PI-SAR-1/2, E/F-SAR, CV-580 SAR, SIR-C/X-SAR, ALOS-PALSAR1/2, Radarsat-2, TerraSAR-X, and TanDEM-X. They have measured a huge mass of fully polarimetric data. Now polarimetric SAR has many applications in many fields, including agriculture (crop classification, soil moisture extraction, and crop assessment), oceanography (surface currents and wind field retrieval), forestry (forest monitoring, classification, and tree height estimation), disaster monitoring (oil spill detection, disaster assessment), and military (ship detection, target recognition/classification).