A Comparison Study of Polarization Orientation Angle Estimation for Rough Terrain Surface

Estimation of polarization orientation angle (POA) is of general interest in polarimetric SAR (PolSAR) image processing because it is useful to remove the orientation-induced polarimetric distortion but also to infer physical target parameters. Under the assumption of scatter reflection symmetry, POA can be readily estimated from a circular polarization basis formulation or equivalently from zeroing the Pauli-basis correlation term T-23. This solution does not account for a possible presence of helical components; alternatively, we developed a general formulation for POA estimation using Touzi parameterization that explicitly contains a helical angle. When applied to PolSAR imagery of rough terrain surface, those two estimators generally give different results. Based on the POA derived from coregistered digital elevation model data, we find the former estimator can underestimate the POA for rough terrain surface. In this article, we further investigate the underlying cause. Although these two POA estimators differ in whether assuming zero helicity or not, the inherent helicity on the rough surface scattering is negligible; instead, the difference in the estimated POA is mainly attributed to the degree of mixing of a secondary scattering mechanism. This mixing impact can be significant even for rough terrain with only light vegetation cover, suggesting a practical need for inclusion of alternative estimators.