Publication Date:
2019-05-28
Description:
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 di�erent 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 di�erent, both spectrally and spatially. Major di�erences were
found for radiance sensors measuring a sunlit water target at viewing zenith angle of 139� because of
the di�erent fields of view. Major di�erences were found for irradiance sensors because of imperfect cosine response of di�users. 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 insu�cient
for outdoor, field measurements, mainly due to the di�erent angular variability of illumination.
More stringent specifications and individual testing of radiometers for all relevant systematic e�ects
(temperature, nonlinearity, spectral stray light, etc.) are needed to reduce biases between instruments
and better quantify measurement uncertainties.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf