PolSAR Ship Characterization and Robust Detection at Different Grazing Angles With Polarimetric Roll-Invariant Features

Polarimetric synthetic aperture radar (PolSAR) plays an important role in remote sensing. As a valuable application, PolSAR ship detection receives great attention and obtains fruitful achievements recently. Since ship targets' scattering responses are highly sensitive to the radar grazing angles, robust PolSAR ship detection still faces challenges especially the very low target to clutter ratio (TCR) phenomenon at large grazing angles. How to find stable polarimetric features for robust ship detection at different grazing angles becomes the key scientific problem. This work dedicates to this issue and the main idea is to explore the potentials of polarimetric roll-invariant features which are relatively independent to radar looking directions. The main contributions are threefold. First, quantitative investigations are conducted to disclose the variation law of ship targets and sea clutters' polarimetric scattering mechanisms in terms of different grazing angles with electromagnetic computation data and PolSAR datasets. Second, the performances of polarimetric roll-invariant features are significantly examined with the TCR and the absolute difference (AD) indexes. Five optimal polarimetric roll-invariant features with stable TCRs over the wide grazing angle range are founded, which can clearly enhance the contrast between ship targets and sea clutters at large grazing angles. Finally, robust PolSAR ship detection approaches are established with the selected polarimetric roll-invariant features. Comparison studies with Gaofen-3 and Radarsat-2 PolSAR datasets of different grazing angles are carried out. Compared with traditional polarimetric features, the experimental results demonstrate that the selected polarimetric roll-invariant features exhibit superior detection performances in terms of both detection accuracy and detection robustness.