Evaluation of Sentinel-3A and Sentinel-3B ocean land colour instrument green instantaneous fraction of absorbed photosynthetically active radiation

Abstract

This article presents the evaluation of the Copernicus Sentinel-3 Ocean Land Colour Instrument (OLCI) operational terrestrial products corresponding to the green instantaneous Fraction of Absorbed PhotosyntheticallyActive Radiation (FAPAR) and its associated rectified channels. These products are estimated using OLCI spectralmeasurements acquired at the top of the atmosphere by a physically-based approach and are available operationally at full (300 m) and reduced (1.2 km) spatial resolution daily. The evaluation of the quality of the FAPAROLCI values was based on the availability of data acquired over several years by Sentinel-3A (S3A) and Sentinel3B (S3B). The evaluation exercise consisted of several stages: first, an overall comparison of the two S3 platformproducts was carried out during the tandem phase; second, comparison with an FAPAR climatology derived fromthe Medium Resolution Imaging Spectrometer (MERIS) provided information on the seasonality of various typesof land cover. Then, direct comparisons were made with the same type of FAPAR products retrieved from twosensors, the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sentinel-2 (S2) MultispectralInstrument (MSI), and with several ground-based estimates. In addition, an analysis of the efficiency of theretrieval algorithm with 3D radiative transfer simulations was performed. The results indicated that the consistency between daily and monthly S3A and S3B on a global scale was very good during the tandem phase(RMSD = 0.01 and a correlation R2 of 0.99 with a bias of 0.003); we found an agreement with a correlation of0.95 and 0.93 (RMSD = 0.07 and 0.09) with JRC FAPAR S2 and JRC FAPAR MODIS, respectively. Compatibility with the ground-based data was between 0.056 and 0.24 in term of RMSD depending on the type of vegetation with an overall R2 of 0.89. Immler diagrams demonstrate that their variances were lower than the total uncertainties. The quality assurance using 3D radiative transfer model has shown that the apparent performance of the algorithm depends strongly on the type of in-situ measurement and canopy type.

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ORCID for Luke Brown: orcid.org/0000-0003-4807-9056

ORCID for Jadunandan Dash: orcid.org/0000-0002-5444-2109

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