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

1
Tartu Observatory, University of Tartu, 61602 Tõravere, Estonia
2
Royal Belgian Institute of Natural Sciences, 1000 Brussels, Belgium
3
National Research Council of Italy, 21020 Ispra, Italy
4
Helmholtz-Zentrum Geesthacht, Institute for Coastal Research, 21502 Geesthacht, Germany
5
Geography Department at the University of Victoria, Victoria, BC V8P 5C2, Canada
6
Center for Marine and Environmental Research CIMA, University of Algarve, 8005-139 Faro, Portugal
7
Plymouth Marine Laboratory, Plymouth PL1 3DH, UK
8
National Centre for Earth Observation, Plymouth PL1 3DH, UK
9
Cimel Electronique S.A.S, 75011 Paris, France
10
Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, D-27570 Bremerhaven, Germany
11
Estonian Marine Institute, University of Tartu, 12618 Tallinn, Estonia
12
Satlantic, Sea Bird Scientific, Bellevue, WA 98005, USA
13
European Space Agency, 2201 AZ Noordwijk, The Netherlands
*
Author to whom correspondence should be addressed.
Remote Sens. 2019, 11(9), 1129; https://doi.org/10.3390/rs11091129
Received: 26 March 2019 / Revised: 24 April 2019 / Accepted: 8 May 2019 / Published: 11 May 2019
(This article belongs to the Special Issue Fiducial Reference Measurements for Satellite Ocean Colour)
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Abstract

An intercomparison of radiance and irradiance ocean color radiometers (the second laboratory comparison exercise—LCE-2) was organized within the frame of the European Space Agency funded project Fiducial Reference Measurements for Satellite Ocean Color (FRM4SOC) May 8–13, 2017 at Tartu Observatory, Estonia. LCE-2 consisted of three sub-tasks: (1) SI-traceable radiometric calibration of all the participating radiance and irradiance radiometers at the Tartu Observatory just before the comparisons; (2) indoor, laboratory intercomparison using stable radiance and irradiance sources in a controlled environment; (3) outdoor, field intercomparison of natural radiation sources over a natural water surface. The aim of the experiment was to provide a link in the chain of traceability from field measurements of water reflectance to the uniform SI-traceable calibration, and after calibration to verify whether different instruments measuring the same object provide results consistent within the expected uncertainty limits. This paper describes the third phase of LCE-2: The results of the field experiment. The calibration of radiometers and laboratory comparison experiment are presented in a related paper of the same journal issue. Compared to the laboratory comparison, the field intercomparison has demonstrated substantially larger variability between freshly calibrated sensors, because the targets and environmental conditions during radiometric calibration were different, both spectrally and spatially. Major differences were found for radiance sensors measuring a sunlit water target at viewing zenith angle of 139° because of the different fields of view. Major differences were found for irradiance sensors because of imperfect cosine response of diffusers. Variability between individual radiometers did depend significantly also on the type of the sensor and on the specific measurement target. Uniform SI traceable radiometric calibration ensuring fairly good consistency for indoor, laboratory measurements is insufficient for outdoor, field measurements, mainly due to the different angular variability of illumination. More stringent specifications and individual testing of radiometers for all relevant systematic effects (temperature, nonlinearity, spectral stray light, etc.) are needed to reduce biases between instruments and better quantify measurement uncertainties. View Full-Text
Keywords: ocean color radiometers; radiometric calibration; field intercomparison measurement; agreement between sensors; measurement uncertainty ocean color radiometers; radiometric calibration; field intercomparison measurement; agreement between sensors; measurement uncertainty
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Vabson, V.; Kuusk, J.; Ansko, I.; Vendt, R.; Alikas, K.; Ruddick, K.; Ansper, A.; Bresciani, M.; Burmester, H.; Costa, M.; D’Alimonte, D.; Dall’Olmo, G.; Damiri, B.; Dinter, T.; Giardino, C.; Kangro, K.; Ligi, M.; Paavel, B.; Tilstone, G.; Van Dommelen, R.; Wiegmann, S.; Bracher, A.; Donlon, C.; Casal, T. Field Intercomparison of Radiometers Used for Satellite Validation in the 400–900 nm Range. Remote Sens. 2019, 11, 1129.

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