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SENTINEL-2 (S2) is a European wide-swath, high-resolution, multi-spectral imaging mission consisting of two twin satellites with a high revisit frequency that contributes to ongoing multispectral observations and enhances Copernicus services and applications such as continuous coverage of overseas and oceans with the production of global maps of phytoplankton chlorophyll and other sea surface properties that contribute to the air-sea flux of carbon dioxide and other climate relevant gases. The S2 Multispectral Instrument (MSI) features two distinct arrays of 12 detectors mounted on each focal plane covering VNIR and SWIR channels respectively, in order to cover the whole instrument field of view. The configured operation of the 12 detectors introduces bidirectional reflectance effects on the S2 satellite images, which are more distinct over water bodies due to water inherent optical properties. In this study, three methods for the elimination of the bidirectional reflectance effects are evaluated: i) The c-factor approach along with the Ross-Li model, that simulates the bidirectional reflectance distribution function (BRDF) for calculating the Nadir BRDF Adjusted Reflectance (NBAR), ii) an image-based method, which encompasses the acrosstrack reflectance variances, using for each column, the deviation between the column mean value and a polynomial trendline which is constructed to fit all the column mean values, and iii) a method which adjusts the reflectance values of each detector to those originated from the detector with the closest to nadir view zenith. Furthermore, the ACOLITE atmospheric correction software was evaluated in terms of reflectance variance elimination when S-2 images are processed. A mosaic of S2 images presenting the Ionian Sea, southwest of Greece has been used for the evaluation. Results demonstrate the effectiveness of the NBAR methodology over ocean scenes proving that physical models adequately describe bidirectional causes and effects. The other two methods smooth variances between the detectors, however, they do not eliminate the BRDF effects. ACOLITE software did not achieve to reduce reflectance variances in S-2 images.
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