Surface Reflectance (SR)

Surface Reflectances are calculated by applying a correction for the bidirectional reflection distribution (BRD) to the SDR (Surface Directional Reflectance) data using the Ross-Li-Maignan BRD function (BRDF) model. This model computes the surface reflectances as the sum of three terms, which describe the isotropic, the volumetric and the geometric part of the reflection. The model parameters are directly derived from the surface directional reflectance time series by minimizing the merit function. This inversion technique assumes no temporal variation in the shape of the BRDF during the compositing period, but allows the change of target reflectances within the period. The duration of the compositing period is made dependent on the data availability per pixel and ranges from 30-60 days. The BRD correction is applied to the unprojected swath data. Resulting surface reflectances form the main input for subsequent land variables, e.g. vegetation variables.