SAR Polarimetric Phase Differences in Wetlands: Information and Mis-Information

Abstract

We have previously reported anomalous polarimetric decomposition results from SAR observations of wetlands. This is caused by the abrupt change in the phase difference between the HH and VV backscatter that occurs around the Brewster angle of the emergent vegetation. We have now developed and implemented a model for backscattering from wetlands that features a cylinder emergent from a water plane. The model was used in conjunction with an extensive set of RADARSAT-2 polarimetric observations of wetlands to provide further insights into the backscattering process. We are able to show how the abrupt Brewster transition in HH-VV phase difference varies with cylinder diameter and gravimetric moisture. We find that coherent cross-pol backscatter can result from cylindrical stems being tilted. In swamps with extensive tree mortality but primarily vertical trunks, the CPD can be used to monitor the drying of the trees and thus their fire hazard. These insights may be used to identify drying trees, indicating thawing permafrost, a potentially important climate change application in the near future. We recommend that applications researchers and users choose radar wavelengths that are considerably shorter, or longer, than the diameters of the cylinders producing the dominant double-bounce backscatter to avoid resonance effects.