Petri, CA; Galvao, LS; Lyapustin, AI (2019). MODIS BRDF effects over Brazilian tropical forests and savannahs: a comparative analysis. REMOTE SENSING LETTERS, 10(2), 95-102.

Bidirectional reflectance distribution function (BRDF) effects over Brazilian tropical forests (Amazon) and savannahs (Cerrado) were inspected for differences in high-quality pixel retrievals, view direction, view zenith angle (VZA), solar zenith angle (SZA) and relative azimuth angle (RAA). By comparing Moderate Resolution Imaging Spectroradiometer (MODIS) data, corrected and non-corrected for BRDF effects (2000-2014), we evaluated the magnitude of such effects over reflectance and vegetation indices (VIs). The VIs were the Enhanced Vegetation Index (EVI) and the Normalized Difference Vegetation Index (NDVI). From the Amazon to the Cerrado, we observed a higher frequency of high-quality pixel retrievals, a decrease in VZA, and an increase in seasonal SZA amplitude. Brightness increased in the backscattering direction and with shifts in RAA over tropical forests toward the BRDF hotspot at the end of dry season (September). Compared to the savannahs from the Cerrado or those from northern Amazon, stronger BRDF effects were observed for Amazonian tropical forests and for the EVI. BRDF changes in the dry season NDVI amplitude (NDVISept minus NDVIJune) were lower than 5% for both biomes. For the EVI amplitude, we observed changes up to 20% over the savannahs and close to 60% for most of the Amazon. However, even after BRDF correction and in spite of the observed differences close to 0.02, EVI values from June and September were still statistically different from each other.