Geoscience Reference
In-Depth Information
Therefore, together with the modern level of optical instrument making, the
features of the old measuring complex are not of great interest nowadays.
However, the methodological experience of many-years exploitation of the
measuring complex on board aircraft has not lost its actuality. Thus, we will
concentrate on it.
By the beginning of the 1980th, instruments for radiation observations were
united in a complex: an information-measuring system (Vasilyev O et al. 1987).
The measuring part of the complex was provided with the K-3 spectrometer
(Mikhailov and Voitov 1969). It was a diffractive mirror spectrometer with
a grating of 600 lines per mm as a dispersing element. The operating spec-
tral range of the instrument was 330-978 nm, registration time of the single
spectrum was 7 s and the spectral scanning was a mechanical one. The spec-
trometer had three overlapped operating ranges: the ultraviolet (UV), visual
(VD) and near infrared (NIR), with corresponding photomultipliers used as
a light receiver. The digital output signal was recorded to the magnetic tape
using an ordinary tape-recorder. It allowed the carrying out of all consequent
processing of the information on computer.
The spectral graduation, i. e. the determination of wavelengths
λ
i
,attributed
to the discrete points with numbers
i
, which are defined by the time moments
in the process of mechanical scanning, was accomplished in the laboratory by
measuring of the mercury lamp spectrum and by identification of the known
spectral lines of the atmospheric gases at the output recording. The oxygen
band at 760 nm in the spectrum of diffused radiation of the sky was also used
for the graduation in the NIR range. As the mechanical scanning led to the
displacement of the registration points, graduating values
λ
i
were determined
with a rather large uncertainty (both randomand systematic). This uncertainty
was equated to the root-mean-square (RMS) deviation of the same series of
measurements of the mercury lamp spectrum and was equal to 1 nm.
The spectral instrumental function of K-3 spectrometer
f
λ
)(lookat
Sect. 1.1) has been presented in the study by Vasilyev O et al. (1987). This
function has been obtained in the laboratory through the registration of laser
line (in visual range) and can be approximated by the triangle function with
halfwidth
(
λ
∆λ
equal to 3 nm,namely:
1−
λ
λ
i
2
−
λ
=
f
(
)
,
(3.1)
λ
∆λ
∆λ
=
where
3 nm. It is obvious, that the mentioned graduating uncertainty
influences halfwidth
∆λ
according to the conditions of the signal registration.
The accuracy of approximation
f
λ
) by the triangle function is about 1%. It
is important to mention that the measured value of the signal varies weakly
with wavelength change within the majority of the spectral range of the K-3
instrument, so it is possible not to take into account the instrumental function
and the uncertainties of the graduation for halfwidth
(
λ
∆λ
=
3 nm as has been
mentioned in Sect. 1.1. The exception is the comparatively narrow and deep
band of oxygen absorption 760 nm together with some deep Fraunhofer lines
of the solar spectrum in the UV region (Fig. 1.3). Therefore, the attention
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