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

Journal Title

Journal ISSN

Volume Title


Remote Sensing


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.



ocean color radiometers, radiometric calibration, field intercomparison measurement, agreement between sensors, measurement uncertainty


Vabson, V., Kuusk, J., Ansko, I., Vendt, R., Alikas, K., Ruddick, K.,… Casal, T. (2019). Field Intercomparison of Radiometers Used for Satellite Validation in the 400–900 nm Range. Remote Sensing, 11(9), 1129.