Foreword to the Special Issue on Compact Polarimetric SAR

The papers in this special section examine compact polarimetric (CP) synthetic aperture radar (SAR). SAR has been widely used for earth remote sensing for more than 30 years. Nowadays, considering the need for a compromise among swath coverage, hardware requirements, and scattering information of measured scenes, compact polarimetry (CP) offers a good option for many polarimetric SAR applications. As a special dual-polarization (DP) configuration, CP SAR transmits signals in a special polarization, such as a 45° linear polarization or a circular polarization, and receives echoes simultaneously in two orthogonal polarizations, such as horizontal and vertical polarizations or right and left circular polarizations. Such systems can provide a greater amount of polarization information than the standard DP linear systems while covering twice the swath width of conventional quad-polarization (QP) systems and maintaining the benefits that QP systems cannot provide. As a result, CP SAR has been considered as a suitable option of spaceborne radar with the advent of the most recent generation of spacecraft, such as RISAT-1, ALOS-2, and the future RADARSAT constellation. This significantly increases the interest of the remote sensing community in the SAR domain and poses new scientific and technological problems involving radar system, data interpretation, performance assessment, and practical applications.