Double-Bounce Component in Cross-Polarimetric SAR From a New Scattering Target Decomposition

Common vegetation scattering theories assume that the synthetic aperture radar (SAR) cross-polarization (cross-pol) signal represents solely volume scattering. We found that this assumption is incorrect based on SAR phase measurements acquired over the South Florida Everglades indicating that the cross-pol radar signal often samples the water surface beneath the vegetation. Based on these new observations, we propose that the cross-pol signal consists of both double-bounce and volume scattering components. The simplest multibounce scattering mechanism that generates cross-pol signal occurs by rotated dihedrals. Thus, we use the rotated dihedral mechanism to revise some of the vegetation scattering theories and develop a four-component decomposition algorithm with single-bounce, co-pol double-bounce, cross-pol double-bounce, and volume scattering components. We tested the new decomposition in both urban and rural environments using RADARSAT-2 quad-pol data sets. The decomposition of the San Francisco area shows higher double-bounce scattering and reduced volume scattering in the urban area with respect to the common three-component decomposition. The decomposition of the rural Everglades area shows that the relation between volume and cross-pol double bounce depends on the vegetation density. Thus, we suggest that, when possible, SAR-based biomass estimate studies should use the volume scattering calculated by our decomposition rather than the cross-pol signal, which also contains a double-bounce component.