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

Authors :
Viktor Vabson
Joel Kuusk
Ilmar Ansko
Riho Vendt
Krista Alikas
Kevin Ruddick
Ave Ansper
Mariano Bresciani
Henning Burmester
Maycira Costa
Davide D’Alimonte
Giorgio Dall’Olmo
Bahaiddin Damiri
Tilman Dinter
Claudia Giardino
Kersti Kangro
Martin Ligi
Birgot Paavel
Gavin Tilstone
Ronnie Van Dommelen
Sonja Wiegmann
Astrid Bracher
Craig Donlon
Tânia Casal
Source :
Remote Sensing, Vol 11, Iss 9, p 1129 (2019)
Publication Year :
2019
Publisher :
MDPI AG, 2019.

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.

Details

Language :
English
ISSN :
20724292
Volume :
11
Issue :
9
Database :
Directory of Open Access Journals
Journal :
Remote Sensing
Publication Type :
Academic Journal
Accession number :
edsdoj.88bc222b4344dfe8f9e377a18fccda2
Document Type :
article
Full Text :
https://doi.org/10.3390/rs11091129