Polarization Orientation Angle and Polarimetric SAR Scattering Characteristics of Steep Terrain

Polarization orientation angle (POA) is an important parameter of polarimetric radar scattering from slopes in mountainous region. It is known that surface tilted in azimuth direction and buildings not aligned in the along-track direction induce polarization orientation shifts. Earlier research has established orientation angle as a function of radar imaging geometry and surface slopes, and that POA estimation can be derived from polarimetric radar data using circular polarization. Besides these, polarimetric scattering from steep slopes and its relation to POA remain not well understood. In this paper, we address these issues by adopting a tilted surface model based on Bragg scattering. We have found that, as the azimuthal slope increases, vertical bar VV vertical bar decreases at a faster rate than vertical bar HH vertical bar, they become equal, when POA is +/- 45 degrees, and vertical bar HH vertical bar > vertical bar VV vertical bar afterward. In other words, the Pauli component, vertical bar HH-VV vertical bar reduced to zero at POA = +/- 45 degrees, and the typical Bragg scattering characteristics of vertical bar VV vertical bar > vertical bar HH vertical bar does not apply when steep slope is present inducing vertical bar POA vertical bar > 45 degrees. Furthermore, the cross-pol vertical bar HV vertical bar does not always increase with azimuth slope but also reaches a maximum then decreases to zero. In addition, we investigate the effect of soil moisture on polarimetric SAR (PolSAR) scattering characteristics of steep terrain and the effect of vegetation over surface on POA estimation. The latter is demonstrated with NASA/JPL TOPSAR L-band PolSAR data and C-band InSAR data. Another significance of this paper is that it provides a direct and rigorous derivation of POA equations. The earlier version was derived from a different concept.