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

Monica Robustelli,Olivier Morgan,Nadine Gobron,Jennifer Adams,Luke A. Brown,Christian Lanconelli,Fabrizio Cappucci,Jadunandan Dash,Mirko Marioni

doi:10.1016/j.rse.2021.112850

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 Photosynthetically Active Radiation (FAPAR) and its associated rectified channels. These products are estimated using OLCI spectral measurements 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 FAPAR OLCI values was based on the availability of data acquired over several years by Sentinel-3A (S3A) and Sentinel-3B (S3B). The evaluation exercise consisted of several stages: first, an overall comparison of the two S3 platform products was carried out during the tandem phase; second, comparison with an FAPAR climatology derived from the Medium Resolution Imaging Spectrometer (MERIS) provided information on the seasonality of various types of land cover. Then, direct comparisons were made with the same type of FAPAR products retrieved from two sensors, the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Sentinel-2 (S2) Multispectral Instrument (MSI), and with several ground-based estimates. In addition, an analysis of the efficiency of the retrieval 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 of 0.95 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.

Highlights

Remote sensing products contribute to the continuous monitoring of the Earth’s surface
The results indicated that the con sistency 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 of 0.95 and 0.93 (RMSD = 0.07 and 0.09) with Joint Research Centre (JRC) Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) S2 and JRC FAPAR Moderate Resolution Imaging Spectroradiometer (MODIS), respectively
The results indicated that the impact of top-of-atmosphere radiance uncertainties on the opera tional Medium Resolution Imaging Spectrometer (MERIS) FAPAR products accuracy was expected to be at about 5–10% whereas agreement with the ground-based estimates of the Fraction of Intercepted PAR (FIPAR) over different canopy types was achieved within ±0.1

Summary

Introduction

Remote sensing products contribute to the continuous monitoring of the Earth’s surface. Within the European Union’s Copernicus program, the Sentinel missions were launched with the specific objective of monitoring Essential Climate Variables (ECV) identified by the Global Climate Observing System (GCOS) (GCOS, 2016) These variables include the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR), which is provided as a Level 2 land core product by the Sentinel-3 (S3) Ocean and Land Colour Instrument (OLCI) sensors at 300 m and 1.2 km, for each daily acquisition. The rectified reflectances correspond to the amplitude parameter of the BRF entering the Rahman, Pinty, Verstraete (RPV) parametric model (Rahman et al, 1993) They are virtual (in that they are not directly measurable in the field) spectral reflectances in which the atmospheric and angular effects are supressed. The rectification is done in such a way as to minimise the difference between the rectified bands and the spectral reflectances that would have been measured at the top of the canopy under identical geometrical conditions but in the absence of the atmosphere (Gobron, 2011)

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Results

Discussion

Conclusion