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Fig. 3.3a,b. Scheme of the airborne sounding: a in the coordinates “time-altitude”, b in the
coordinates “cosine of the solar incident angle - atmospheric pressure”. Observation 14th
October 1983 above the Kara-KumDesert, the points show the altitudes of themeasurements
variation of the solar zenith angle during the detailed descent for obtaining the
altitudinal dependence of the irradiance and the application of the irradiance
values registered during the preliminary ascent and descent for correction of
the solar zenith angle variations during the detailed descent were the main
ideas of sounding. The minimal altitude 50 m was taken due to the special
demands of flight safety; the maximal altitude 5600m was taken due to the
technical abilities of the IL-14 aircraft. While flying with the optimal regime,
we succeeded in only two ascents and descents during one experiment, how-
ever, the crew gladly assisted during the observations allowing us to carry out
three ascents and descents.
The flight altitude has been changed during the sounding but the scale
of pressure has been used instead of the altitude scale during further data
processing as Fig. 3.3b demonstrates. It was connected with the following: at
altitudes higher than 500m theaircraftabsolutescaleofaltitudes was used,
i. e. the altitude registered by the altimeter related to the level 1013mbar or the
atmospheric pressure was expressed in altitude units according to the stan-
dard atmospheric model (Standards 1981). The accuracy of the instrumental
measurement of the altitude according to the absolute scale was about 50m
but it was difficult for the crew to set a concrete altitude level exactly while
working under the conditions of time shortage so the real uncertainty of the
 
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