Spectrodirectional Minnaert-k retrieval using CHRIS/PROBA data

This work uncovers the spectral trajectory of the unique structurally-related information content that is embedded in the angular domain of multi-angular remote sensing data. CHRIS (Compact High Resolution Imaging Spectrometer) mounted onboard the PROBA (Project for On-board Autonomy) spacecraft is capable of sampling terrestrial reflectance anisotropy over the visible/nearinfrared region of the electromagnetic spectrum with high spatial resolution. The spectral anisotropic behavior of an Alpine coniferous forest during wintertime in relation to forest cover density was investigated using the Minnaert-k parameter of the Rahman-Pinty-Verstraete (RPV) model. Although earlier studies have demonstrated that it is possible to use Minnaert-k as a proxy for characterizing surface heterogeneity at subpixel scale, its spectral dependency has not yet been fully assessed. Minnaert-k parameter retrievals over the full spectral domain of CHRIS data revealed that a turnover from bell-shaped to bowl-shaped anisotropic reflectance patterns occurs when entering the red edge. Specifically, analysis of the underlying dynamics revealed that forest cover density controls the spectral location where this turnover takes place. Due to the bright snow background situation the 773 nm spectral band of CHRIS yielded Minnaert-k values that were best related to forest cover density. In this region, medium forest cover densities (40-70%) led to bell-shaped anisotropy patterns, while either sparse canopy covers or dense canopy covers led to bowl-shaped reflectance anisotropy patterns.