RESUMEN
Fiducial reference measurements are in-situ data traceable to metrology standards, with associated uncertainties. This paper presents the methodology used to derive the uncertainty budget for underway, above-water measurements from the Seabird Hyperspectral Surface Acquisition System deployed on an Atlantic Meridional Transect in 2018. The average uncertainty of remote sensing reflectance for clear sky days was â¼ 6% at wavelengths < 490 nm and â¼ 12% at wavelengths > 550 nm. The environmental variability such as sun position, wind speed and skylight distribution caused the greatest uncertainty. The different components of the uncertainty budget are critically assessed to indicate how the measurement procedure could be improved through reducing the principal uncertainty sources.
RESUMEN
Stray light perturbations are unwanted distortions of the measured spectrum due to the nonideal performance of optical radiometers. Because of this, stray light characterization and correction is essential when accurate radiometric measurements are a necessity. In agreement with such a need, this study focused on stray light correction of hyperspectral radiometers widely applied for above-water measurements to determine the remote-sensing reflectance (RRS). Stray light of sample radiometers was experimentally characterized and a correction algorithm was developed and applied to field measurements performed in the Mediterranean Sea. Results indicate that mean stray light corrections are appreciable, with values generally varying from -1% to +1% in the 400-700 nm spectral region for downward irradiance and sky radiance, and from -1% to +4% for total radiance from the sea. Mean corrections for data products such as RRS exhibit values that depend on water type varying between -0.5% and +1% in the blue-green spectral region, with peaks up to 9% in the red in eutrophic waters. The possibility of using one common stray light correction matrix for the analyzed class of radiometers was also investigated. Results centered on RRS support such a feasibility at the expense of an increment of the uncertainty typically well below 0.5% in the blue-green and up to 1% in the red, assuming sensors are based on spectrographs from the same production batch.
RESUMEN
Wavelength-dependent attenuation of ground-level ultraviolet (UV) dose by different cloud and aerosol situations at the Tartu Observatory site (58°15' N, 26°28' E, 70 m a.s.l) is under scrutiny. The spectra at wavelengths ranging below 400 nm have been recorded by the simple Avantes, Inc. array spectrometer AvaSpec-256 in 2004-2009. The spectral information was supported by the conventional broadband solar irradiance and by the necessary meteorological data. The average cloud modification factor (CMF) on overcast days from May to August has been quite low, 0.36 in UVA and 0.35 in UVB. In the UVA range, the reduction of the daily dose with increasing noon solar zenith angle (SZA) from 35-50° to 65-80° in overcast days has been about 20% more than in clear days, while in the UVB range it was 45% larger. No clear difference in the influence of SZA on CMF between low level (St, Ns) and medium level (As, Ac) overcast cloudiness has been found. The aerosol attenuation during large aerosol optical depth (AOD) episode has been comparable with that of medium level clouds with the wavelength dependency in the UVA range different from that of clouds.
