Investigation of Sentinel-2 Bidirectional Reflectance Hot-Spot Sensing Conditions

Directional reflectance effects, often described by the bidirectional reflectance distribution function (BRDF), occur in Sentinel-2 multispectral instrument reflectance. The bidirectional hot-spot is a special case of the BRDF used to describe the increased backscatter reflectance that occurs over most surfaces when the solar and viewing directions coincide. A global year of Sentinel-2A metadata extracted using the Committee on Earth Observation Satellite Visualization Environment (COVE) tool and an established astronomical model were used to quantify the range of solar geometry and scattering angles expected in Sentinel-2A data. The established astronomical model was adapted to be Sentinel-2A specific and was parameterized as a function of the sensor acquisition date and nadir latitude. Solar zenith angles varied from 15.335° to 91.454°, and the scattering angles varied from 84.714° to 173.967°. To confirm the global COVE results, the scattering angles of a sample of Sentinel-2A data were examined and differed by less than 0.17° with respect to the COVE data. Given that hot-spots are only apparent when the scattering angle is close to 180°, we conclude that hot-spot will not occur in Sentinel-2A data. Equations and software to predict the scattering angle at the Sentinel-2A swath edge as a function of acquisition date and nadir latitude are provided so users may obtain data over a range of scattering angles in support of their BRDF studies.