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which the observations were accomplished for. The specific features of sec-
ondary processing are defined by concrete methodologies of the observations
and result production. These points will be considered further. Now we will
continue discussing the common features of the observations and estimate the
uncertainties of the results, extremely important for further interpretation.
TherandomerrorofthemeasurementswiththeK-3instrumentwases-
timatedinthelaboratoryoverthespectraseriesregisteredfromthelamp
SI-8andwasequalto5%ofthestandarddeviationintheUV,andto1%in
the VD and NIR regions (this uncertainty is not to be taken for a systematic
uncertainty of the calibration, considered above). However, the real random
uncertainty of the airborne observations was significantly higher because flight
conditions influence the measurement results too. Certainly, the accuracy of
the instrument on board was increasingly worse than the accuracy of the same
instrument in the laboratory. The deviation of the receiving surface of the opal
glasses from the horizontal plane, the deviations of the instrument optical axis
from the fixed direction during the radiance registration, the unevenness of the
ground surface illumination and heterogeneity were the additional “airborne”
factors worsening the observation accuracy.
The aircraft lengthwise axis was disposed not horizontally but at a certain
angle even during the constant flight altitude. This angle is called apitch and
it defines the slope of the light conductor. For compensation of the influence
of the pitch, the light conductor was put at a specially chosen angle with
averticaldirectionbuttheinfluencecouldn'tbeexcludedcompletelybecause
the pitch depended on the aircraft charging changing during the flight due
to fuel depletion. As has been mentioned above direct radiation is the main
partofthesolardownwellingirradianceinaclearsky.Thepitchinfluenceon
the accounting of direct radiation in the measured irradiance was obviously
owing to the deviation of the angle of the solar beam incoming to the light
conductor glass from the solar incident angle. As follows from the elementary
geometrical consideration, this deviation would be maximum for the flight
azimuth 0 and 180 , and minimum for 90 and 270 .Thus,theobservations
were mostly accomplished for the flight azimuth 90 and 270 .
It is easy to estimate the systematic uncertainty of the downwelling irradi-
ance caused by the pitch, fixing the atmospheric model and using Beer's law
(1.42). We would like to mention that this uncertainty is higher if the Sun is
lower and the upper atmosphere is optically thinner, i. e. if the flight altitude is
higher and the wavelength is longer (1.25) 1 .Itisnotcomplicatedtoestimate
this uncertainty experimentally conducting the observations at different az-
imuth angles. Both estimations have given similar results: the uncertainty is
less than 1% for the observations under azimuth angles 90 and 270 and in-
creases up to 5% in the UV and VD and up to 10% in the NIR spectral regions.
Thesevaluesincreaseupto10and15%forasolarzenithangleexceeding60 .
Aircraft flight conditions called “ bumps ” appear owing to atmospheric tur-
bulence. All parameters of the flight - altitude, pitch angle, roll angle(theangle
λ
1 Remember that the optical thickness ofmolecular scattering varies inverselywith
4 as per Rayleigh
law
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