Field Intercomparison of Radiometers Used for Satellite Validation in the 400–900 nm Range

Vabson, Viktor; Kuusk, Joel; Ansko, Ilmar; Vendt, Riho; Alikas, Krista; Ruddick, Kevin; Ansper, Ave; Bresciani, Mariano; Burmester, Henning; Costa, Maycira; D’Alimonte, Davide; Dall’Olmo, Giorgio; Damiri, Bahaiddin; Dinter, Tilman; Giardino, Claudia; Kangro, Kersti; Ligi, Martin; Paavel, Birgot; Tilstone, Gavin H.; Dommelen, Ronnie Van; Wiegmann, Sonja; Bracher, Astrid; Donlon, Craig; Casal, Tânia

doi:10.3390/rs11091129

Cited by 24 publications

(71 citation statements)

References 11 publications

(32 reference statements)

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“…The integration time used when capturing the measured spectra can lead to non-linearity effects in the results. The maximum value of non-linearity effects was determined in the indoor calibration for the two radiance instruments, but not for the irradiance instrument [20]. As the principal aim of this study is to outline the method and only secondary provide results, the radiance instrument corrections are used for the irradiance instrument.…”

Section: Non-linearity Correction (C Lin )mentioning

confidence: 99%

“…The stray light characterisation does have an uncertainty associated with it, but this is unknown. A value of 5% is chosen which accommodates for the true uncertainty and is negligible in comparison to the assigned uncertainty of other variables [20], meaning that the quantity will have little or no effect on the further results.…”

Section: Stray Light Correction (C Stray )mentioning

confidence: 99%

“…The same instruments were taken out of the laboratory to outdoor conditions where they were compared again at two different locations. The first comparison was in Estonia, for measurements of a lake, where the environmental conditions were unfortunately not optimal for above-water radiometry measurements [20]. Nevertheless, this did show the importance of instrument characterisation and the effects due to changeable measurement conditions.…”

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confidence: 99%

“…The instruments selected for this analysis are RAMSES Hyperspectral Radiometers that belong to the University of Tartu. These instruments, in addition to radiometric calibration, had the most optical characterisation tests performed [20]. The characterisation test results were made available for further analysis and allowed correction coefficients to be applied to the data to allow for differences between ideal and non-ideal instrument performance, such as cosine response of a diffuser or detector spectral stray light.…”

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confidence: 99%

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Example of Monte Carlo Method Uncertainty Evaluation for Above-Water Ocean Colour Radiometry

et al. 2020

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We describe a method to evaluate an uncertainly budget for the in situ Ocean Colour Radiometric measurements. A Monte Carlo approach is chosen to propagate the measurement uncertainty inputs through the measurements model. The measurement model is designed to address instrument characteristics and uncertainty associated with them. We present the results for a particular example when the radiometers were fully characterised and then use the same data to show a case when such characterisation is missing. This, depending on the measurement and the wavelength, can increase the uncertainty value significantly; for example, the downwelling irradiance at 442.5 nm with fully characterised instruments can reach uncertainty values of 1%, but for the instruments without such characterisation, that value could increase to almost 7%. The uncertainty values presented in this paper are not final, as some of the environmental contributors were not fully evaluated. The main conclusion of this work are the significance of thoughtful instrument characterisation and correction for the most significant uncertainty contributions in order to achieve a lower measurements uncertainty value.Remote Sens. 2020, 12, 780 2 of 28 of hyperspectral sensors in situ, some aspects of this work need to be repeated. Particularly since significant differences (up to 25%) are found when compared to theoretical modelling and conventional multispectral instruments [15]. It seems that the well known and well characterised multispectral instruments that are described in the NASA protocols are not utilised as much nowadays, as the world focuses on hyperspectal instruments, which provide greater spectral information. However, generally, they are less well characterised but require a wider array of characterisation tests (e.g., spectral stray light). Similarly with the new generation of Ocean Colour sensors like the Sentinel 3 series [16] and planned PACE [17] mission there is a need for even better quality measurements. The old Ocean Colour accuracy requirements, set at 5%, are now being pushed to achieve a level of 3%. In order to achieve this very ambitious target we need to better understand the individual uncertainty components that effect ocean colour measurements as well as their contribution to the final products.The Fiducial Reference Measurements for Satellite Ocean Colour (FRM4SOC) project, with funding from ESA, aimed to provide support for evaluating and improving the state of the art in satellite ocean colour validation through a series of comparisons under the auspices of the Committee on Earth Observation Satellites (CEOS). The project makes a fundamental contribution to the European system for monitoring the Earth (Copernicus) by ensuring high quality ground-based Fiducial Reference Measurements (FRM) for ocean colour radiometry for use in validation of ocean colour products from missions like Sentinel-3 Ocean Colour and Land Imager (OLCI) [16] and Sentinel-2 Multi Spectral Imager (MSI) [18]. The main aim of FRM4SOC is to establish and mainta...

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Section: Non-linearity Correction (C Lin )mentioning

confidence: 99%

Section: Stray Light Correction (C Stray )mentioning

confidence: 99%

mentioning

confidence: 99%

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confidence: 99%

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Example of Monte Carlo Method Uncertainty Evaluation for Above-Water Ocean Colour Radiometry

et al. 2020

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“…The outdoor comparisons were conducted as a direct intercomparison of the downwelling irradiance Ed, the downwelling sky radiance Ld, and the total upwelling water radiance Lu from a diving platform on the end of the 50 m pier at the southern shore of Lake Kääriku in Estonia, as shown in Figure 10. The physical and optical characteristics of the part of the lake measured were characteristically eutrophic, well known to the pilot, and are detailed in [44] and [46]. The outdoor measurements were performed in 5-min casts.…”

Section: Outdoor Intercomparison Of Measurementsmentioning

confidence: 99%

Fiducial Reference Measurements for Satellite Ocean Colour (FRM4SOC)

et al. 2020

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Earth observation data can help us understand and address some of the grand challenges and threats facing us today as a species and as a planet, for example climate change and its impacts and sustainable use of the Earth’s resources. However, in order to have confidence in earth observation data, measurements made at the surface of the Earth, with the intention of providing verification or validation of satellite-mounted sensor measurements, should be trustworthy and at least of the same high quality as those taken with the satellite sensors themselves. Metrology tells us that in order to be trustworthy, measurements should include an unbroken chain of SI-traceable calibrations and comparisons and full uncertainty budgets for each of the in situ sensors. Until now, this has not been the case for most satellite validation measurements. Therefore, within this context, the European Space Agency (ESA) funded a series of Fiducial Reference Measurements (FRM) projects targeting the validation of satellite data products of the atmosphere, land, and ocean, and setting the framework, standards, and protocols for future satellite validation efforts. The FRM4SOC project was structured to provide this support for evaluating and improving the state of the art in ocean colour radiometry (OCR) and satellite ocean colour validation through a series of comparisons under the auspices of the Committee on Earth Observation Satellites (CEOS). This followed the recommendations from the International Ocean Colour Coordinating Group’s white paper and supports the CEOS ocean colour virtual constellation. The main objective was to establish and maintain SI traceable ground-based FRM for satellite ocean colour and thus make a fundamental contribution to the European system for monitoring the Earth (Copernicus). This paper outlines the FRM4SOC project structure, objectives and methodology and highlights the main results and achievements of the project: (1) An international SI-traceable comparison of irradiance and radiance sources used for OCR calibration that set measurement, calibration and uncertainty estimation protocols and indicated good agreement between the participating calibration laboratories from around the world; (2) An international SI-traceable laboratory and outdoor comparison of radiometers used for satellite ocean colour validation that set OCR calibration and comparison protocols; (3) A major review and update to the protocols for taking irradiance and radiance field measurements for satellite ocean colour validation, with particular focus on aspects of data acquisition and processing that must be considered in the estimation of measurement uncertainty and guidelines for good practice; (4) A technical comparison of the main radiometers used globally for satellite ocean colour validation bringing radiometer manufacturers together around the same table for the first time to discuss instrument characterisation and its documentation, as needed for measurement uncertainty estimation; (5) Two major international side-by-side field intercomparisons of multiple ocean colour radiometers, one on the Atlantic Meridional Transect (AMT) oceanographic cruise, and the other on the Acqua Alta oceanographic tower in the Gulf of Venice; (6) Impact and promotion of FRM within the ocean colour community, including a scientific road map for the FRM-based future of satellite ocean colour validation and vicarious calibration (based on the findings of the FRM4SOC project, the consensus from two major international FRM4SOC workshops and previous literature, including the IOCCG white paper on in situ ocean colour radiometry).

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Performance of OLCI Sentinel-3A satellite in the Northeast Pacific coastal waters

Giannini

Hunt

Jacoby

et al. 2021

Remote Sensing of Environment

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Field Intercomparison of Radiometers Used for Satellite Validation in the 400–900 nm Range

Cited by 24 publications

References 11 publications

Example of Monte Carlo Method Uncertainty Evaluation for Above-Water Ocean Colour Radiometry

Example of Monte Carlo Method Uncertainty Evaluation for Above-Water Ocean Colour Radiometry

Fiducial Reference Measurements for Satellite Ocean Colour (FRM4SOC)

Performance of OLCI Sentinel-3A satellite in the Northeast Pacific coastal waters

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