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.
Highlights
The FRM4SOC project aimed to support the consistency of the ground-based validation measurements for “ocean color (OC)”, or water reflectance, with the SI units, and contribute to higher quality and accuracy of Sentinel-2 Multispectral Instrument (MSI) and Sentinel-3 Ocean and Land Color Instrument (OLCI) products
During the relatively short time needed for LCE-2 measurements, this uncertainty component normally is not contributing to the variability between radiometric sensors freshly calibrated at the same laboratory using the same calibration standards
The difference in cosine response is the main source of differences between different sensor groups revealed during the field experiment
Summary
The FRM4SOC project aimed to support the consistency of the ground-based validation measurements for “ocean color (OC)”, or water reflectance, with the SI units, and contribute to higher quality and accuracy of Sentinel-2 Multispectral Instrument (MSI) and Sentinel-3 Ocean and Land Color Instrument (OLCI) products. Intercomparison of data produced by a number of independent radiometric sensors measuring simultaneously the same object allows assessment of the consistency of different results and their estimated uncertainties depending on the type of the sensor, the spectral composition, intensity and angular variability of the measured radiation, environmental temperature, and the particular method used for collecting and handling the measurement data [2,3]. This information can serve for further elaboration of uncertainty estimation. TThhiiss sstutuddyyaimaismtso etvoaleuvaatelutahteeefftheectivefefneecstisvoefnSeIs-straocfeaSbIl-etrraacdeiaobmleetrriacdciaolmibreatrtiiocncfaolribcroantsioisntenfocyr coofnOsiCstefinecldy mofeaOsuCrefmieeldntsm, eparseuserenmtseLnCtsE, -p2rdesaetnatps roLcCeEss-2ingdarteasupltrso,caenssdindgisrceussusletss,teacnhdniqduisecsusasneds ptercohcneidquureess afonrdipmrporcoevdiunrgestrfaocreiambiplirtoyvoinf gOtCraficeeladbimliteyasoufrOemCefnietlsd. measurements
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