Palsar-2 Compact Assessment and Calibration

ALOS2-PALSAR2, which was launched on the 24th of May 2014, is equipped with an experimental Compact mode (CP) ([1], [2]), that can collect CP images at various incidence angles (H2-1 to H2-9, from 18° to 44°). 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 technology does not permit the generation of a perfectly circular polarization [3]. The transmitted polarization is non circular, and the non circularity varies with incidence angle. Since all the calibration methods developed for CP assume that the transmitted polarization is perfectly circular, there is immediate requirement for the development of a new CP calibration model that includes the transmitted polarization non circularity. In this study, a new CP calibration model, which explicitly includes the transmitted polarization non circularity, in addition to conventional radar transmit and receive distortion matrices, channel imbalances and Faraday rotation contamination, is introduced. The new CP model is adapted to ALOS2-PALSAR2 and used as the basis of a new method for assessment of CP non circularity using Amazonian rainforests, at low Faraday rotation. ALOS2-CP data collected at four different beams (H26 to H29), with incidence angle varying between 30° and 44°, permit the first ever demonstration of non circularity of ALOS2 CP transmitted polarization. It is shown that the transmitted polarization axis ratio of ALOS2-CP increases with incidence angle, and can reach up to 2.5 dB at the most raising incidence angle (44°) of the beam H29.