Abstract
Measurements of solar radiation reflected from Earth’s surface are the basis for calculating albedo, vegetation indices, and other terrestrial attributes. However, the “bi-directional” geometry of illumination and viewing (i.e., the Bi-directional Reflectance Distribution Function (BRDF)) impacts reflectance and all variables derived or estimated based on these data. The recently released MODIS BRDF/Albedo Model Parameters (MCD43A1 Collection 6) dataset enables retrieval of directional reflectance at arbitrary solar and viewing angles, potentially increasing precision and comparability of data collected under different illumination and observation geometries. We quantified the ability of MCD43A1 Collection 6 for retrieving directional reflectance and compared the daily Collection 6 retrievals to those of MCD43A1 Collection 5, which are retrieved on an eight-day basis. Correcting MODIS-based estimates of surface reflectance from the illumination and viewing geometry of the Terra satellite (MOD09GA) to that of the MODIS Aqua (MYD09GA) overpass, as well as MCD43A4 Collection 6 and Landsat-5 TM images show that the BRDF correction of MCD43A1 Collection 6 results in greater consistency among datasets, with higher R2 (0.63–0.955), regression slopes closer to unity (0.718–0.955), lower root mean squared difference (RMSD) (0.422–3.142), and lower mean absolute error (MAE) (0.282–1.735) compared to the Collection 5 data. Smaller levels of noise (observed as high-frequency variability within the time series) in MCD43A1 Collection 6 in comparison to Collection 5 corroborates the improvement of BRDF parameters time series. These results corroborates that the daily MCD43A1 Collection 6 product represents the anisotropy of surface features and results in more precise directional reflectance derivation at any solar and viewing geometry than did the previous Collection 5.
Highlights
Reflection of solar radiation from complex surfaces varies with the directions of incoming and outgoing energy
This paper evaluates the quality of MCD43A1 Collection 6 by using it to generate bi-directional reflectance distribution function (BRDF)-corrected MODIS directional reflectance and compares the resulting estimates to nearly coincident MODIS and Landsat Thematic Mapper (TM) observations compared to Collection 5
We normalized the MODIS surface reflectance (MOD09) bands to the observation geometry derived from reference data (e.g., MYD09GA, Landsat-5, or MCD43A4) using the c-factor approach [23], by multiplying the observed reflectance (ρ) by the ratio between normalized reflectance based on the MCD43A1 parameters and reflectance modeled using observed viewing, illuminating and azimuthal angles
Summary
Reflection of solar radiation from complex surfaces varies with the directions of incoming and outgoing energy. Collection 6 of the MCD43A1 dataset, being distributed, is retrieved daily and represents the best-fit BRDF based on a 16-day observation period [8]. Collection 5 of the MODIS MCD43A1 dataset has been validated using ground measurements and images from the Enhanced Thematic Mapper Plus (ETM+) sensor aboard Landsat-7 [9]. Román et al validated satellite retrievals from global land-surface albedo and reflectance anisotropy products against tower-based measurements and Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo [11,12]. Wang et al assessed the MODIS standard Bidirectional Reflectance Distribution Function (BRDF)/Albedo product over tundra, grassland, agriculture, and forest surface types using ground-based albedo measurements [13,14]. High-frequency noise in time-series directional of directional reflectance based on MCD431A1 Collection 6 and Collection 5 were compared to assess the improvement of MCD431A1 Collection 6 to represent land surface phenology
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