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Description
Mapping features lying beneath a forest cover using SAR measurements represents a major challenge, due to the response of the overlying vegetation volume, wave attenuation caused by propagation through the forest canopy, and high-intensity scattering mechanisms occurring at the ground level, which mask ground contributions and prevent direct interpretation of conventional SAR imagery. Interferometric notching, a processing technique that exploits the spatial diversity of multi-baseline SAR acquisitions, represents an appealing solution to address these limitations, as it enables the selective suppression of specific scattering contributions by an appropriate choice of the interferometric baseline. Canopy notching suppresses the vegetation volume response, isolating the residual ground-level scattering mechanisms that would otherwise remain hidden beneath the forest cover. The resulting filtered image provides a representation of the ground reflectivity, whose polarimetric and radiometric features can then be analyzed to characterize the underlying scene.
This work investigates the application of canopy notching to the mapping of sub-canopy features in dense tropical environments, using P-band SAR data acquired by the ESA BIOMASS mission. The suppression of canopy contributions enables the identification of ground-level scattering mechanisms associated with natural and anthropogenic features, including surface water bodies, ancient settlement structures, and other landscape elements that would otherwise remain undetectable beneath the forest cover. Further characterization of the revealed ground-level features is then carried out through a polarimetric analysis, exploiting the full polarimetric diversity of the acquisitions. Classical decomposition techniques, such as the Van Zyl decomposition, along with polarimetric parameters including entropy and indicators of single and double-bounce scattering, allow discrimination between different types of ground cover.
The proposed approach is applied to fully polarimetric P-band Biomass data acquired over three tropical study sites: Manaus (Brazil), used as an urban/forest interface reference site; Amacayacu (Colombia), a partially flooded environment where ground scattering retrieval is investigated; and São Paulo (Brazil), a mixed environment featuring complex topography. Results show the ability of interferometric canopy notching to reveal sub-canopy features of both natural and anthropogenic origin, highlighting the potential of the ESA Biomass mission for below-canopy mapping in dense tropical environments.