Preliminary Evaluation of Angular Reflectance Downscaling Using FSDAF Spatiotemporal Fusion Model and MODIS BRDF Data

As the resolution of remote sensing quantitative inversion for albedo, clumping index, canopy height, and other biophysical parameter data is gradually refined, there is an increasing demand for higher spatial resolution bidirectional reflectance distribution function (BRDF) and angular reflectance data. In this article, we applied and evaluated flexible spatiotemporal data fusion to downscale angular reflectance using low-resolution BRDF data (MCD43A1) and high-resolution Sentinel-2 data as inputs. We selected Sentinel-2 orbital overlap regions with various anisotropic flat index (AFX) types and overpassing times as the study area as to obtain angular reflectance data under different anisotropic and solar-viewing geometry conditions. We found that the fusion method can downscale the angular reflectance with high accuracy (root mean square error < 0.04, bias < 0.02). The downscaling accuracy was higher in the cross-principle plane than in the principle plane, whereas there was no significant variation in accuracy under different AFX conditions. Owing to the lack of angular surface reflectance at larger viewing angles, validation was performed only within a view zenith angle of 10.3°. Further validation using satellites with larger viewing angles, such as Gaofen-6 or unmanned aerial vehicles, can be used to evaluate the accuracy of this method. This article presents a strategy for downscaling angular information, preliminary validation results, and satellite-based evidence of scale effects between the 500–10 m scales of the BRDF.