Fernando Camacho
Validation and Conformity Testing of Sentinel-3 Green Instantaneous FAPAR and Canopy Chlorophyll Content Products
Camacho, Fernando; Martínez-Sánchez, Enrique; Brown, Luke A.; Morris, Harry; Morrone, Rosalinda; Williams, Owen; Dash, Jadunandan; Origo, Niall; Sánchez-Zapero, Jorge; Boccia, Valentina
Authors
Enrique Martínez-Sánchez
Dr Luke Brown L.A.Brown4@salford.ac.uk
Lecturer
Harry Morris
Rosalinda Morrone
Owen Williams
Jadunandan Dash
Niall Origo
Jorge Sánchez-Zapero
Valentina Boccia
Abstract
This article presents validation and conformity testing of the Sentinel-3 Ocean Land Colour Instrument (OLCI) green instantaneous fraction of absorbed photosynthetically active radiation (FAPAR) and OLCI terrestrial chlorophyll index (OTCI) canopy chlorophyll content (CCC) products with fiducial reference measurements (FRM) collected in 2018 and 2021 over two sites (Las Tiesas—Barrax, Spain, and Wytham Woods, UK) in the context of the European Space Agency (ESA) Fiducial Reference Measurement for Vegetation (FRM4Veg) initiative. Following metrological principles, an end-to-end uncertainty evaluation framework developed in the project is used to account for the uncertainty of reference data based on a two-stage validation approach. The process involves quantifying uncertainties at the elementary sampling unit (ESU) level and incorporating these uncertainties in the upscaling procedures using orthogonal distance regression (ODR) between FRM and vegetation indices derived from Sentinel-2 data. Uncertainties in the Sentinel-2 data are also accounted for. FRM-based high spatial resolution reference maps and their uncertainties were aggregated to OLCI’s native spatial resolution using its apparent point spread function (PSF). The Sentinel-3 mission requirements, which give an uncertainty of 5% (goal) and 10% (threshold), were considered for conformity testing. GIFAPAR validation results revealed correlations > 0.95, RMSD ~0.1, and a slight negative bias (~−0.06) for both sites. This bias could be partly explained by the differences in the FAPAR definitions between the satellite product and the FRM-based reference. For the OTCI-based CCC, leave-one-out cross-validation demonstrated correlations > 0.8 and RMSDcv ~0.28 g·m−2. Despite the encouraging validation results, conclusive conformity with the strict mission requirements was low, with most cases providing inconclusive results (driven by large uncertainties in the satellite products as well as by the uncertainties in the upscaling approach). It is recommended that mission requirements for bio-geophysical products are reviewed, at least at the threshold level. It is also suggested that the large uncertainties associated with the two-stage validation approach may be avoided by directly comparing with spatially representative FRM.
Journal Article Type | Article |
---|---|
Acceptance Date | Jul 19, 2024 |
Online Publication Date | Jul 23, 2024 |
Publication Date | Jul 23, 2024 |
Deposit Date | Jul 25, 2024 |
Publicly Available Date | Jul 30, 2024 |
Journal | Remote Sensing |
Electronic ISSN | 2072-4292 |
Publisher | MDPI |
Peer Reviewed | Peer Reviewed |
Volume | 16 |
Issue | 15 |
Article Number | 2698 |
Pages | 2698 |
DOI | https://doi.org/10.3390/rs16152698 |
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Publisher Licence URL
http://creativecommons.org/licenses/by/4.0/
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