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Assessment of PALSAR-2 Compact Calibration

ALOS-2-PALSAR is equipped with an experimental Compact mode (CP), which can collect CP images at various incidence angles. PALSAR2-CP transmits a circular polarization and measures the received signal at the horizontal and vertical antenna polarization. Recently, it was brought out that the actual SAR technology does not permit the generation of a perfectly circular polarization and this may significantly affect CP information [1]. Since all the calibration methods assume that the transmitted polarization is perfectly circular, there is an immediate need for a new CP calibration model that is more suitable to actual CP SARs. In this study, a new CP calibration model that takes into account the non circularity of transmitted polarization, in addition to conventional SAR transmitter and receiver distortion matrices, is introduced. The CP calibration model is used as the basis for the development of an efficient method that permits the measurement of transmitted polarization axis ratio, and its variations with incidence angle. The method is validated using PALSAR2-CP data collected at various incidence angles on the Amazonian forests. It is shown that the transmitted polarization axis ratio varies with PALSAR2-CP beam, and can reach up to 1.1 dB for the CP mode (H2-8) (at about 37° incidence angle).