In the solar aureole technique, two different calibrations are needed:
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The solar calibration constant (F0):
it is the signal corresponding to the solar irradiance incident at
the top of the atmosphere. All the instruments in ESR and SKYNET are calibrated
using an in situ procedure called Improved Langley (Campanelli et al 2004; Campanelli et al. 2007), a modified version of the standard Langley plot technique.
In the Improved Langley, F0 is retrieved by fitting the natural logarithm of the direct solar irradiance versus
the product of the relative optical air mass and the total extinction optical thickness (retrieved by the inversion)
instead of the air mass alone, as done with the standard Langley plot. This in situ calibration
procedure allows the operators to track and evaluate the calibration status on a continuous basis, keeping the revision of the
instrument mostly for maintenance reasons and offering the advantage of reducing the number of instrument expeditions. In this way, the
data gaps incurred by the periodical shipments are considerably reduced, and the likelihood of instrumental damages attributable to
transport also decreases.
ESR calibrates its instruments on a monthly base.
In order to observe the requirements from WMO, ESR and SKYNET instruments are periodically calibrated at the WRCC in
Davos and at the MAUNA LOA laboratory, respectively. Some ESR instruments attended the
QUATRAM campaign for in 2017-2019 treacebility studies
and validation of the Improved Langley technique.
The Solid view angle:
it corresponds to the field of view of the instrument. Its value can be approached by the geometric solid
viewing angle of the telescope. However, several factors contribute to this value: color aberration of the lens,
diffraction at the edges, misalignment of the optical axis, and surface nonuniformity of filters and sensor.
As a consequence, an in situ method is made available for determining the actual solid view angle (only
for PREDE sun-sky radiometers) from optical data. This in situ method consists on performing a scanning of the
irradiance field around the Sun, centered at the origin of a local system of rectangular coordinates
(Boi et al. 1999).
ESR mostly assumes this value as privided by the company using the above method.
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M. Hashimoto, T. Nakajima, O.Dubovik, M. Campanelli, H. Che, P. Khatri, T. Takamura, G. Pandithurai, Development of a new data-processing method for SKYNET sky radiometer observations, Atmos. Meas. Tech., 5 (11), pp. 2723-2737, 2012.
Kudo, R., Nishizawa, T., and Aoyagi, T.: Vertical profiles of aerosol optical properties and the solar heating rate estimated by combining sky radiometer and lidar measurements, Atmos. Meas. Tech., 9, 3223–3243, https://doi.org/10.5194/amt-9-3223-2016, 2016.
Kudo, R., 2019.2.13, Developement of aerosol and cloud retrieval methods using the sky radiometer, 5th International SKYNET workshop, New Delhi, India
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V. Estellés, M. Campanelli, T.J. Smyth, M.P. Utrillas and J.A. Martinez-Lozano, Comparison of AERONET and SKYRAD4.2 inversion products retrieved from a Cimel CE318 sunphotometer, Atmos. Meas. Tech., 5, 1-11, 2012.
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