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Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment

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A special issue of Remote Sensing (ISSN 2072-4292). This special issue belongs to the section "Remote Sensing Image Processing".

Deadline for manuscript submissions: closed (31 December 2024) | Viewed by 23393

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Special Issue Editors

Dr. Bahjat Alhammoud

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Guest Editor

Earth Observation R&D, ARGANS Ltd., Plymouth PL68BX, UK
Interests: optics; vicarious calibration/validation methods; remote sensing; numerical modeling; oceanography

Dr. Sebastien Clerc

E-Mail Website
Guest Editor

ACRI-ST, 06904 Sophia-Antipolis, France
Interests: geophysical image processing; remote sensing; calibration; image classification; learning (artificial intelligence); image segmentation; radiometry

Dr. Steffen Dransfeld

E-Mail Website
Guest Editor

European Space Agency, ESA-ESRIN, Earth Observation Ground Segment Department, 1 00044 Frascati, Italy
Interests: calibration and validation of optical remote sensing instruments; validation of level 2 remote sensing products (LST, vegetation parameters, AOD, FRP, IWV, SDR)

Dr. Jean Christopher Lambert

E-Mail Website
Guest Editor

Royal Belgian Institute for Space Aeronomy, BIRA-IASB, 1180 Brussels, Belgium
Interests: remote sensing; atmospheric composition; satellite validation; metrology

Pierre Féménias

E-Mail Website
Guest Editor Assistant

European Space Agency, ESA-ESRIN, Earth Observation Ground Segment Department, 1 00044 Frascati, Italy
Interests: earth observation; altimetry; radiometry; calibration; validation; data quality

Special Issue Information

Dear Colleagues,

Continuous observation of the Earth by remote sensing satellites provides cost-effective acquisition of global data, which can be fed into internationally agreed key datasets, such as the Climate Data Records of Essential Climate Variables targeted by the Global Climate Observing System (GCOS) of the United Nations.

With the increasing number of spaceborne Optical, Thermal, Radar, and Microwave sensors, we are moving toward systematic and continuous global Earth observation of the most relevant processes in land, oceans, the atmosphere and over the cryosphere. Hence, the need for consistency among (past, present and future) EO remotely sensed data remains more than ever critical for building coherent time series of geophysical quantities.

Rigorous Calibration and Validation (Cal/Val) processes and protocols are required to ensure the highest quality of the data products and to document this quality, hence, building confidence in satellite data and in their use by the widest community. At the same time, new technologies emerge, new Earth observation methods appear, and new types of data become available, raising new Cal/Val challenges.

Accurate calibration and validation require the availability of Fiducial Reference Measurements (FRM), which are a suite of independent ground-based measurements with traceability to established standards and community protocols that provide independent validation datasets. For this purpose, the European Space Agency (ESA) and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) support a series of Fiducial Reference Measurements (FRM) projects targeting the validation of satellite data products of the atmosphere, land, and ocean under the auspices of the Committee on Earth Observation Satellites (CEOS).

The goal of this Special Issue is to combine and summarize recent scientific advances related to the satellite Cal/Val techniques, FRM datasets and innovations in the field, with a focus on - but not limited to - the Copernicus Sentinels.

Potential topic areas covered by Copernicus Sentinels missions but are not limited to:

remote sensing of atmospheric composition, land, ocean, snow and ice surface,
calibration and sensors’ intercomparison,
validation of geophysical data products,
innovations to products’ retrieval algorithms and Cal/Val techniques,
Fiducial Reference Measurements (FRM) for satellite data validation.

Dr. Bahjat Alhammoud
Dr. Sebastien Clerc
Dr. Steffen Dransfeld
Dr. Jean Christopher Lambert
Guest Editors

Pierre Féménias
Guest Editor Assistant

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

remote sensing
calibration/validation
fiducial reference measurement
copernicus sentinel missions
altimetry
radiometry

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Published Papers (13 papers)

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34 pages, 16526 KiB

Open AccessArticle

Copernicus Sentinel-3 OLCI Level-1B Radiometry Product Validation Status After Six Years in Constellation by Three Independent Expert Groups

by Bahjat Alhammoud, Camille Desjardins, Sindy Sterckx, Stefan Adriaensen, Cameron Mackenzie, Ludovic Bourg, Sebastien Clerc and Steffen Dransfeld

Remote Sens. 2025, 17(7), 1217; https://doi.org/10.3390/rs17071217 - 29 Mar 2025

Viewed by 352

Abstract

As part of the Copernicus program of the European Union (EU), the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) are currently operating the Sentinel-3 mission that consists of a constellation of two unites A and B (S3A, S3B). Each unit carries on board an Ocean and Land Colour Instrument (OLCI) that is acquiring moderate-spatial-resolution optical imagery. This article provides a description of the Level-1B radiometry product validation activities of the constellation Sentinel-3A and Sentinel-3B after six years in orbit. Several vicarious calibration methods have been applied independently by three expert groups and the results are compared over different surface target types. All methods agree on the good radiometric performance of both instruments. Although OLCI-A shows brighter Top-of-Atmosphere (TOA) radiance than OLCI-B by about 1–2%, both sensors exhibit very good stability and good image quality. The results are analyzed and discussed to propose a set of vicarious gain coefficients that could be used to align OLCI-A with OLCI-B radiometry time-series. Finally, recommendations for future missions are suggested. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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27 pages, 5777 KiB

Open AccessArticle

Fiducial Reference Measurements for Greenhouse Gases (FRM4GHG): Validation of Satellite (Sentinel-5 Precursor, OCO-2, and GOSAT) Missions Using the COllaborative Carbon Column Observing Network (COCCON)

by Mahesh Kumar Sha, Saswati Das, Matthias M. Frey, Darko Dubravica, Carlos Alberti, Bianca C. Baier, Dimitrios Balis, Alejandro Bezanilla, Thomas Blumenstock, Hartmut Boesch, Zhaonan Cai, Jia Chen, Alexandru Dandocsi, Martine De Mazière, Stefani Foka, Omaira García, Lawson David Gillespie, Konstantin Gribanov, Jochen Gross, Michel Grutter, Philip Handley, Frank Hase, Pauli Heikkinen, Neil Humpage, Nicole Jacobs, Sujong Jeong, Tomi Karppinen, Matthäus Kiel, Rigel Kivi, Bavo Langerock, Joshua Laughner, Morgan Lopez, Maria Makarova, Marios Mermigkas, Isamu Morino, Nasrin Mostafavipak, Anca Nemuc, Timothy Newberger, Hirofumi Ohyama, William Okello, Gregory Osterman, Hayoung Park, Razvan Pirloaga, David F. Pollard, Uwe Raffalski, Michel Ramonet, Eliezer Sepúlveda, William R. Simpson, Wolfgang Stremme, Colm Sweeney, Noemie Taquet, Chrysanthi Topaloglou, Qiansi Tu, Thorsten Warneke, Debra Wunch, Vyacheslav Zakharov and Minqiang Zhou Show full author list Hide full author list

Remote Sens. 2025, 17(5), 734; https://doi.org/10.3390/rs17050734 - 20 Feb 2025

Viewed by 770

Abstract

The COllaborative Carbon Column Observing Network has become a reliable source of high-quality ground-based remote sensing network data that provide column-averaged dry-air mole fractions of carbon dioxide (XCO₂), methane (XCH₄), and carbon monoxide (XCO). The fiducial reference measurements of these gases from the COCCON complement the TCCON and NDACC-IRWG data. This study shows the application of COCCON data for the validation of existing greenhouse gas satellite products. This study includes the validation of XCH₄ and XCO products from the European Copernicus Sentinel-5 Precursor (S5P) mission, XCO₂ products from the American Orbiting Carbon Observatory-2 (OCO-2) mission, and XCO₂ and XCH₄ products from the Japanese Greenhouse gases Observing SATellite (GOSAT). A total of 27 datasets contributed to this study; some of these were collected in the framework of campaign activities and covered only a short time period. In addition, several permanent stations provided long-term observations. The random uncertainties in the validation results, specifically for S5P with a lot of coincidences pairs, are found to be similar to the comparison with the TCCON. The comparison results of OCO-2 land nadir and land glint observation modes to the COCCON on a global scale, despite limited coincidences, are very promising. The stations can, therefore, expand on the coverage of the already existing ground-based reference remote sensing sites from the TCCON and the NDACC network. The COCCON data can be used for future satellite and model validation studies and carbon cycle studies. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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26 pages, 46256 KiB

Open AccessArticle

Evaluation of In Situ FAPAR Measurement Protocols Using 3D Radiative Transfer Simulations

by Christian Lanconelli, Fabrizio Cappucci, Jennifer Susan Adams and Nadine Gobron

Remote Sens. 2024, 16(23), 4552; https://doi.org/10.3390/rs16234552 - 4 Dec 2024

Viewed by 874

Abstract

The fraction of absorbed photosynthetically active radiation (FAPAR) is one of the bio-geophysical Essential Climate Variables assessed through remote sensing observations and distributed globally by space and environmental agencies. Any reliable remote sensing product should be benchmarked against a reference, which is normally determined by means of ground-based measurements. They should generally be aggregated spatially to be compared with remote sensing products at different resolutions. In this work, the effectiveness of various in situ sampling methods proposed to assess FAPAR from flux measurements was evaluated using a three-dimensional radiative transfer framework over eight virtual vegetated landscapes, including dense forests (leaf-on and leaf-off models), open canopies, sparse vegetation, and agricultural fields with a nominal extension of 1 hectare. The reference FAPAR value was determined by summing the absorbed PAR-equivalent photons by either all canopy components, both branches and leaves, or by only the leaves. The incoming and upwelling PAR fluxes were simulated in different illumination conditions and at a high spatial resolution (50 cm). They served to replicate in situ virtual FAPAR measurements, which were carried out using either stationary sensor networks or transects. The focus was on examining the inherent advantages and drawbacks of in situ measurement protocols against GCOS requirements. Consequently, the proficiency of each sampling technique in reflecting the distribution of incident and reflected PAR fluxes—essential for calculating FAPAR—was assessed. This study aims to support activities related to the validation of remote sensing FAPAR products by assessing the potential uncertainty associated with in situ determination of the reference values. Among the sampling schemes considered in our work, the cross shaped sampling schemes showed a particular efficiency in properly representing the pixel scale FAPAR over most of the scenario considered. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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38 pages, 8761 KiB

Open AccessArticle

Fiducial Reference Measurements for Air Quality Monitoring Using Ground-Based MAX-DOAS Instruments (FRM4DOAS)

by Michel Van Roozendael, Francois Hendrick, Martina M. Friedrich, Caroline Fayt, Alkis Bais, Steffen Beirle, Tim Bösch, Monica Navarro Comas, Udo Friess, Dimitris Karagkiozidis, Karin Kreher, Alexis Merlaud, Gaia Pinardi, Ankie Piters, Cristina Prados-Roman, Olga Puentedura, Lucas Reischmann, Andreas Richter, Jan-Lukas Tirpitz, Thomas Wagner, Margarita Yela and Steffen Ziegler Show full author list Hide full author list

Remote Sens. 2024, 16(23), 4523; https://doi.org/10.3390/rs16234523 - 2 Dec 2024

Cited by 5 | Viewed by 1192

Abstract

The UV–Visible Working Group of the Network for the Detection of Atmospheric Composition Changes (NDACC) focuses on the monitoring of air-quality-related stratospheric and tropospheric trace gases in support of trend analysis, satellite validation and model studies. Tropospheric measurements are based on MAX-DOAS-type instruments that progressively emerged in the years 2010 onward. In the interest of improving the overall consistency of the NDACC MAX-DOAS network and facilitating its further extension to the benefit of satellite validation, the ESA initiated, in late 2016, the FRM4DOAS project, which aimed to set up the first centralised data processing system for MAX-DOAS-type instruments. Developed by a consortium of European scientists with proven expertise in measurements, data extraction algorithms and software design specialities, the system has now reached pre-operational status and has demonstrated its ability to deliver a set of quality-controlled atmospheric composition data products with a latency of one day. The processing system has been designed using a highly modular approach, making it easy to integrate new tools or processing updates. It incorporates advanced algorithms selected by community consensus for the retrieval of total ozone, lower tropospheric and stratospheric NO₂ vertical profiles and formaldehyde profiles. The ozone and NO₂ products are currently generated from a total of 22 stations and delivered daily to the NDACC rapid delivery (RD) repository, with an additional mirroring to the ESA Validation Data Centre (EVDC). Although it is still operated in a pre-operational/demonstrational mode, FRM4DOAS was already used for several validation and science studies, and it was also deployed in support of field campaigns for the validation of the TROPOMI and GEMS satellite missions. It recently went through a CEOS-FRM self-assessment process aiming at assessing the level of maturity of the service in terms of instrumentation, operations, data sampling, metrology and verification. Based on this evaluation, it falls under class C, which is a good rating but also implies that further improvements are needed to reach full compliance with FRM standards, i.e., class A. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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28 pages, 4077 KiB

Open AccessArticle

Inter-Sensor Level 1 Radiometric Comparisons Using Deep Convective Clouds

by Louis Rivoire, Sébastien Clerc, Bahjat Alhammoud, Frédéric Romand and Nicolas Lamquin

Remote Sens. 2024, 16(23), 4445; https://doi.org/10.3390/rs16234445 - 27 Nov 2024

Viewed by 748

Abstract

To evaluate the radiometric performance of top-of-atmosphere reflectance images, Deep Convective Clouds (DCCs) can be used as temporally, spatially and spectrally stable targets. The DCCs method has been developed more than 20 years ago and applied recently to Sentinel-2 and Sentinel-3 sensors. In this paper, among other developments, we built a new methodology upon those existing by using the bootstrap method and spectral band adjustment factors computed with the Hyper-Spectral Imager (HSI) from the Environmental Mapping and Analysis Program (EnMAP). This methodology is applied to the two Multi-Spectral Imager (MSI) instruments onboard Sentinel-2A and 2B, but also the two Operational Land Imager (OLI) instruments onboard Landsat 8 and 9, from visible wavelength at 442 nm to shortwave-infrared at 2200 nm, using images with a ground resolution spanning from 10 m to 60 m. The results demonstrate the good inter-calibration of MSI units A and B, which are within one percent of relative difference on average between January 2022 and June 2024 for all visible, near-infrared and shortwave-infrared bands, except for the band at 1375 nm for which saturation prevents the use of the method. Similarly, OLI and OLI-2 are found to have a relative difference on the same period lower than one percent for all 30 m resolution bands. Evaluation of the relative difference between the MSI sensors and the OLI sensors with the DCCs method gives values lower than three percent. Finally, these validation results are compared to those obtained with Pseudo-Invariant Calibration Sites (PICSs) over Libya-4: an agreement better than two percent is found between the DCCs and PICSs methods. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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35 pages, 16179 KiB

Open AccessArticle

Vegetative Index Intercalibration Between PlanetScope and Sentinel-2 Through a SkySat Classification in the Context of “Riserva San Massimo” Rice Farm in Northern Italy

by Christian Massimiliano Baldin and Vittorio Marco Casella

Remote Sens. 2024, 16(21), 3921; https://doi.org/10.3390/rs16213921 - 22 Oct 2024

Cited by 1 | Viewed by 3403

Abstract

Rice farming in Italy accounts for about 50% of the EU’s rice area and production. Precision agriculture has entered the scene to enhance sustainability, cut pollution, and ensure food security. Various studies have used remote sensing tools like satellites and drones for multispectral imaging. While Sentinel-2 is highly regarded for precision agriculture, it falls short for specific applications, like at the “Riserva San Massimo” (Gropello Cairoli, Lombardia, Northern Italy) rice farm, where irregularly shaped crops need higher resolution and frequent revisits to deal with cloud cover. A prior study that compared Sentinel-2 and the higher-resolution PlanetScope constellation for vegetative indices found a seasonal miscalibration in the Normalized Difference Vegetation Index (NDVI) and in the Normalized Difference Red Edge Index (NDRE). Dr. Agr. G.N. Rognoni, a seasoned agronomist working with this farm, stresses the importance of studying the radiometric intercalibration between the PlanetScope and Sentinel-2 vegetative indices to leverage the knowledge gained from Sentinel-2 for him to apply variable rate application (VRA). A high-resolution SkySat image, taken almost simultaneously with a pair of Sentinel-2 and PlanetScope images, offered a chance to examine if the irregular distribution of vegetation and barren land within rice fields might be a factor in the observed miscalibration. Using an unsupervised pixel-based image classification technique on SkySat imagery, it is feasible to split rice into two subclasses and intercalibrate them separately. The results indicated that combining histograms and agronomists’ expertise could confirm SkySat classification. Moreover, the uneven spatial distribution of rice does not affect the seasonal miscalibration object of past studies, which can be adjusted using the methods described here, even with images taken four days apart: the first method emphasizes accuracy using linear regression, histogram shifting, and histogram matching; whereas the second method is faster and utilizes only histogram matching. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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39 pages, 13148 KiB

Open AccessArticle

Fiducial Reference Measurement for Greenhouse Gases (FRM4GHG)

by Mahesh Kumar Sha, Martine De Mazière, Justus Notholt, Thomas Blumenstock, Pieter Bogaert, Pepijn Cardoen, Huilin Chen, Filip Desmet, Omaira García, David W. T. Griffith, Frank Hase, Pauli Heikkinen, Benedikt Herkommer, Christian Hermans, Nicholas Jones, Rigel Kivi, Nicolas Kumps, Bavo Langerock, Neil A. Macleod, Jamal Makkor, Winfried Markert, Christof Petri, Qiansi Tu, Corinne Vigouroux, Damien Weidmann and Minqiang Zhou Show full author list Hide full author list

Remote Sens. 2024, 16(18), 3525; https://doi.org/10.3390/rs16183525 - 23 Sep 2024

Cited by 2 | Viewed by 1494

Abstract

The Total Carbon Column Observing Network (TCCON) and the Infrared Working Group of the Network for the Detection of Atmospheric Composition Change (NDACC-IRWG) are two ground-based networks that provide the retrieved concentrations of up to 30 atmospheric trace gases, using solar absorption spectrometry. Both networks provide reference measurements for the validation of satellites and models. TCCON concentrates on long-lived greenhouse gases (GHGs) for carbon cycle studies and validation. The number of sites is limited, and the geographical coverage is uneven, covering mainly Europe and the USA. A better distribution of stations is desired to improve the representativeness of the data for various atmospheric conditions and surface conditions and to cover a large latitudinal distribution. The two successive Fiducial Reference Measurements for Greenhouse Gases European Space Agency projects (FRM4GHG and FRM4GHG2) aim at the assessment of several low-cost portable instruments for precise measurements of GHGs to complement the existing ground-based sites. Several types of low spectral resolution Fourier transform infrared (FTIR) spectrometers manufactured by Bruker, namely an EM27/SUN, a Vertex70, a fiber-coupled IRCube, and a Laser Heterodyne spectro-Radiometer (LHR) developed by UK Rutherford Appleton Laboratory are the participating instruments to achieve the Fiducial Reference Measurements (FRMs) status. Intensive side-by-side measurements were performed using all four instruments next to the Bruker IFS 125HR high spectral resolution FTIR, performing measurements in the NIR (TCCON configuration) and MIR (NDACC configuration) spectral range. The remote sensing measurements were complemented by AirCore launches, which provided in situ vertical profiles of target gases traceable to the World Meteorological Organization (WMO) reference scale. The results of the intercomparisons are shown and discussed. Except for the EM27/SUN, all other instruments, including the reference TCCON spectrometer, needed modifications during the campaign period. The EM27/SUN and the Vertex70 provided stable and precise measurements of the target gases during the campaign with quantified small biases. As part of the FRM4GHG project, one EM27/SUN is now used as a travel standard for the verification of column-integrated GHG measurements. The extension of the Vertex70 to the MIR provides the opportunity to retrieve additional concentrations of N₂O, CH₄, HCHO, and OCS. These MIR data products are comparable to the retrieval results from the high-resolution IFS 125HR spectrometer as operated by the NDACC. Our studies show the potential for such types of spectrometers to be used as a travel standard for the MIR species. An enclosure system with a compact solar tracker and meteorological station has been developed to house the low spectral resolution portable FTIR systems for performing solar absorption measurements. This helps the spectrometers to be mobile and enables autonomous operation, which will help to complement the TCCON and NDACC networks by extending the observational capabilities at new sites for the observation of GHGs and additional air quality gases. The development of the retrieval software allows comparable processing of the Vertex70 type of spectra as the EM27/SUN ones, therefore bringing them under the umbrella of the COllaborative Carbon Column Observing Network (COCCON). A self-assessment following the CEOS-FRM Maturity Matrix shows that the COCCON is able to provide GHG data products of FRM quality and can be used for either short-term campaigns or long-term measurements to complement the high-resolution FTIR networks. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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35 pages, 14791 KiB

Open AccessArticle

Earth Observation Multi-Spectral Image Fusion with Transformers for Sentinel-2 and Sentinel-3 Using Synthetic Training Data

by Pierre-Laurent Cristille, Emmanuel Bernhard, Nick L. J. Cox, Jeronimo Bernard-Salas and Antoine Mangin

Remote Sens. 2024, 16(16), 3107; https://doi.org/10.3390/rs16163107 - 22 Aug 2024

Viewed by 1444

Abstract

With the increasing number of ongoing space missions for Earth Observation (EO), there is a need to enhance data products by combining observations from various remote sensing instruments. We introduce a new Transformer-based approach for data fusion, achieving up to a 10- to-30-fold increase in the spatial resolution of our hyperspectral data. We trained the network on a synthetic set of Sentinel-2 (S2) and Sentinel-3 (S3) images, simulated from the hyperspectral mission EnMAP (30 m resolution), leading to a fused product of 21 bands at a 30 m ground resolution. The performances were calculated by fusing original S2 (12 bands, 10, 20, and 60 m resolutions) and S3 (21 bands, 300 m resolution) images. To go beyond EnMap’s ground resolution, the network was also trained using a generic set of non-EO images from the CAVE dataset. However, we found that training the network on contextually relevant data is crucial. The EO-trained network significantly outperformed the non-EO-trained one. Finally, we observed that the original network, trained at 30 m ground resolution, performed well when fed images at 10 m ground resolution, likely due to the flexibility of Transformer-based networks. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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27 pages, 14277 KiB

Open AccessArticle

Validation and Conformity Testing of Sentinel-3 Green Instantaneous FAPAR and Canopy Chlorophyll Content Products

by Fernando Camacho, Enrique Martínez-Sánchez, Luke A. Brown, Harry Morris, Rosalinda Morrone, Owen Williams, Jadunandan Dash, Niall Origo, Jorge Sánchez-Zapero and Valentina Boccia

Remote Sens. 2024, 16(15), 2698; https://doi.org/10.3390/rs16152698 - 23 Jul 2024

Cited by 1 | Viewed by 1185

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. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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29 pages, 2729 KiB

Open AccessArticle

Towards Operational Fiducial Reference Measurement (FRM) Data for the Calibration and Validation of the Sentinel-3 Surface Topography Mission over Inland Waters, Sea Ice, and Land Ice

by Elodie Da Silva, Emma R. Woolliams, Nicolas Picot, Jean-Christophe Poisson, Henriette Skourup, Geir Moholdt, Sara Fleury, Sajedeh Behnia, Vincent Favier, Laurent Arnaud, Jérémie Aublanc, Valentin Fouqueau, Nicolas Taburet, Julien Renou, Hervé Yesou, Angelica Tarpanelli, Stefania Camici, Renée Mie Fredensborg Hansen, Karina Nielsen, Frédéric Vivier, François Boy, Roger Fjørtoft, Mathilde Cancet, Ramiro Ferrari, Ghislain Picard, Mohammad J. Tourian, Nicolaas Sneeuw, Eric Munesa, Michel Calzas, Adrien Paris, Emmanuel Le Meur, Antoine Rabatel, Guillaume Valladeau, Pascal Bonnefond, Sylvie Labroue, Ole Andersen, Mahmoud El Hajj, Filomena Catapano and Pierre Féménias Show full author list Hide full author list

Remote Sens. 2023, 15(19), 4826; https://doi.org/10.3390/rs15194826 - 5 Oct 2023

Cited by 2 | Viewed by 2351

Abstract

The Copernicus Sentinel-3 Surface Topography Mission (STM) Land Altimetry provides valuable surface elevation information over inland waters, sea ice, and land ice, thanks to its synthetic aperture radar (SAR) altimeter and its orbit that covers high-latitude polar regions. To ensure that these measurements are reliable and to maximise the return on investment, adequate validation of the geophysical retrieval methods, processing algorithms, and corrections must be performed using independent observations. The EU-ESA project St3TART (started July 2021) aims to generalise the concept of Fiducial Reference Measurements (FRMs) for the Copernicus Sentinel-3 STM. This work has gathered existing data, made new observations during field campaigns, and ensured that these observations meet the criteria of FRM standards so that they can be used to validate Sentinel-3 STM Land Altimetry products operationally. A roadmap for the operational provision of the FRM, including the definition, consolidation, and identification of the most relevant and cost-effective methods and protocols to be maintained, supported, or implemented, has been developed. The roadmap includes guidelines for SI traceability, definitions of FRM measurement procedures, processing methods, and uncertainty budget estimations. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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19 pages, 2801 KiB

Open AccessArticle

An Improved Altimeter in-Orbit Range Noise-Level Estimation Approach Based on Along-Track Differential Method

by Xiaonan Liu, Weiya Kong, Hanwei Sun, Yongsheng Xu and Yaobing Lu

Remote Sens. 2022, 14(24), 6250; https://doi.org/10.3390/rs14246250 - 9 Dec 2022

Cited by 1 | Viewed by 1426

Abstract

Satellite radar altimeters are advanced remote sensing devices that play an important role in observing the global marine environment. Accurately estimating the noise level of altimeter in-orbit ranging data is crucial for evaluating the payload performance, analyzing sea conditions, and monitoring data quality. In this study, we propose an approach based on the differential processing of along-track odd–even data sequences for altimeter in-orbit range noise-level estimation. Using the long-term along-track data sequence can notably improve the issue in the existing method in that the noise level is underestimated owing to the utilization of a relatively short data segment. On the basis of an analysis of the influence of low-frequency components on noise-level estimation, the mathematical formulas of the above differential method were deduced, and the efficacy of the approach in assessing the noise level of altimeter in-orbit data was demonstrated by simulation experiments. This method was used to estimate the noise levels of the 20 Hz datasets of Jason-3 and Sentinel-6, and the idea of the time-domain difference was extended to the frequency domain. The statistical results showed that the 20 Hz noise levels at the significant wave height (SWH) = 2 m were 7.41 cm (Jason-3 low-resolution (LR) mode), 6.66 cm (Sentinel-6 LR mode), and 3.13 cm (Sentinel-6 high-resolution (HR) mode). The power spectrum density analysis further verified its accuracy. By reprocessing the 20 Hz data of Sentinel-6 into 10, 5, and 1 Hz, the effectiveness of the along-track odd–even differential method to directly evaluate the noise level of 1 Hz data was explored, and the impact of ocean signals such as swells on noise-level estimation in synthetic aperture mode was discussed. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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35 pages, 2366 KiB

Open AccessReview

Activities to Promote the Moon as an Absolute Calibration Reference

by Zhenhua Jing, Xiuqing Hu, Yang Wang, Ronghua Wu, Lin Chen, Lu Zhang, Yu Huang, Shuang Wang, Shuang Li and Peng Zhang

Remote Sens. 2023, 15(9), 2431; https://doi.org/10.3390/rs15092431 - 5 May 2023

Cited by 4 | Viewed by 4498

Abstract

The accuracy and consistency of Earth observation (EO) instrument radiometric calibration is a fundamental prerequisite for achieving accurate results and delivering reliable predictions. Frequent calibration and validation (Cal/Val) activities are needed during the instrument’s lifetime, and this procedure is often extended to historical archives. Numerous satellites in orbit and proposed future missions have incorporated lunar observation into their vicarious calibration components over recent years, facilitated by the extreme long-term photometric stability of the Moon. Since the birth of the first lunar calibration reference model, lunar-dependent calibration techniques have developed rapidly, and the application and refinement of the lunar radiometric model have become a welcome research focus in the calibration community. Within the context of the development of lunar observation activities and calibration systems globally, we provide a comprehensive review of the activities and results spawned by treating the Moon as a reference for instrument response and categorize them against the understanding of lunar radiometric reference. In general, this appears to be a process of moving from data to instruments, then back into data, working towards a stated goal. Here we highlight lunar radiometric models developed by different institutions or agencies over the last two decades while reporting on the known limitations of these solutions, with unresolved challenges remaining and multiple lunar observation plans and concepts attempting to address them from various perspectives, presenting a temporal development. We also observe that the methods seeking uncertainty reduction at this stage are rather homogeneous, lacking the combination of approaches or results from lunar surface studies conducted by many spacecraft missions, and joint deep learning methods to extract information. The factors that influence the accuracy of the measurement irradiance may be regulated when practical models arrive. As a central element in lunar calibration, the development of an absolute radiometric datum helps to better understand the Earth system. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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15 pages, 4199 KiB

Open AccessTechnical Note

An Evaluation of Sentinel-3 SYN VGT Products in Comparison to the SPOT/VEGETATION and PROBA-V Archives

by Carolien Toté, Else Swinnen and Claire Henocq

Remote Sens. 2024, 16(20), 3822; https://doi.org/10.3390/rs16203822 - 14 Oct 2024

Viewed by 798

Abstract

Sentinel-3 synergy (SYN) VEGETATION (VGT) products were designed to provide continuity to the SPOT/VEGETATION (SPOT VGT) base products archive. Since the PROBA-V mission acted as a gap filler between SPOT VGT and Sentinel-3, and in principle, a continuous series of data products from the combined data archives of SPOT VGT (1998–2014), PROBA-V (2013–2020) and Sentinel-3 SYN VGT (from 2018 onwards) are available to users, the consistency of Sentinel-3 SYN VGT with both the latest SPOT VGT (VGT-C3) and PROBA-V (PV-C2) archives is highly relevant. In past years, important changes have been implemented in the SYN VGT processing baseline. The archive of SYN VGT products is therefore intrinsically inconsistent, leading to different consistency levels with SPOT VGT and PROBA-V throughout the years. A spatio-temporal intercomparison of the combined time series of VGT-C3, PV-C2 and Sentinel-3 SYN VGT 10-day NDVI composite products with an external reference from LSA-SAF, and an intercomparison of Sentinel-3 SYN V10 products with a climatology of VGT-C3 resp. PV-C2 for three distinct periods with different levels of product quality have shown that the subsequent processing baseline updates have indeed resulted in better-quality products. It is therefore essential to reprocess the entire Sentinel-3 SYN VGT archive; a uniform data record of standard SPOT VGT, PROBA-V and Sentinel-3 SYN VGT products, spanning over 25 years, would provide valuable input for a wide range of applications. Full article

(This article belongs to the Special Issue Copernicus Sentinels Missions Calibration, Validation, FRM and Innovation Approaches in Satellite-Data Quality Assessment)

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