全偏極合成孔徑雷達(Fully Polarimetric Synthetic Aperture Radar, POLSAR)具全偏極特性，其應用與發展不同於單一波段合成孔徑雷達系統。其最大優勢是採多種極化方式發射與接收，可以獲得目標物的散射矩陣，散射矩陣描述了目標物與電磁波之間複雜的交互作用。矩陣內每一個元素都與電磁波的頻率、極化狀態、物體的形狀與排列方向等因素有關，可知散射矩陣包含了雷達目標物最完整的資訊。 然而，在應用全偏極雷達資料時，有別於傳統單偏極雷達只需進行輻射校正，即可獲得目標物的散射特性。由於各個偏極之間可能會有相位、振幅的不平衡與串擾，因此全偏極合成孔徑雷達還需進行各個極化之間的校正，才能讓資料更廣泛的被應用。 極化校正通常藉由架設已知散射特性的目標物，獲得測量到的散射矩陣和真實目標物的關係，求得系統的校正矩陣，並利用此校正矩陣進行全極化雷達系統的校正。本文利用三角反射器、兩面角反射器、旋轉22.5度和45度的兩面角反射器進行實驗室陸基全極化雷達的校正與Pi-SAR系統的校正，經由此校正方法，能使振幅誤差可降至0.5dB，相位誤差可降至5度。 藉由分析旋轉22.5度的兩面角反射器隨角度的變化，我們得知旋轉22.5度的兩面角反射器其同偏極化的散射特性隨角度變化十分敏感，因此於本文不建議使用旋轉22.5度的兩面角反射器當作同偏極化的參考目標。 Fully polarimetric synthetic aperture radar (SAR) has found itself in wide applications for terrain classification, land-use, and physical parameters retrieval, among others. In so inclined, it is required that SAR data be well calibrated both in the relative amplitudes and the relative phases between the polarimetric channels. Polarimetric calibration of a radar image is to correct the polarization distortion, caused by imperfect radar system, in a measured scattering matrix. The distortion as far as polarimetry is concerned is mainly from cross talk between channels and imbalances in the channel gain. The proposed calibration method relies only on the characteristics of the calibration targets and requires no assumptions regarding scene statistics and radar characteristics. It allows the correction of polarimetric distortions of the radar system using only three different kinds of calibration target: trihedral, dihedral, the rotated 22.5 degree and 45 degree dihedral. Validation of the proposed method is made by using chamber measurements and real world airborne L-band Pi-SAR data. It is concluded that the amplitude error can be achieved to as small as 0.5dB, while the phase error is about 5 degrees. It is revealed that the rotated the 22.5 degree dihedral is very sensitive to rotation angle and easily causes unacceptable error and hence is not recommended for purpose of polarimetric calibration.
