Abstract. TanSat is a key satellite mission in the Chinese Earth Observation program
and is designed to measure the global atmospheric column-averaged dry-air
CO2 mole fraction by measuring the visible and near-infrared
solar-reflected spectra. The first Chinese super-high-resolution grating
spectrometer for measuring atmospheric CO2 is aboard TanSat. This
spectrometer is a suite incorporating three grating spectrometers that make
coincident measurements of reflected sunlight in the near-infrared
CO2 band near 1.61 and 2.06 µm and in the molecular oxygen
(O2) A-band at 0.76 µm. The spectral resolving power
(λ∕Δλ) values are ∼19 000, ∼12 800, and
∼12 250 in the O2 A-band, and the weak and strong absorption bands of
CO2, respectively. This paper describes the prelaunch spectral
calibration of the atmospheric carbon dioxide grating spectrometer aboard
TanSat. Several critical aspects of the spectrometer, including the spectral
resolution, spectral dispersion, and the instrument line shape function of
each channel, which are directly related to producing the Level 1 products
are evaluated in this paper. The instrument line shape function of the
spectrometer is notably symmetrical and perfectly consistent across all
channels in the three bands. The symmetry is better then 99.99 %, and the
consistency in the worst case is better then 99.97 %, 99.98 %, and 99.98 %
in the O2 A, WCO2, and SCO2 bands, respectively. The
resulting variations in the spectral calibrations and the radiometric response
errors are negligible. The spectral resolution characterizations meet the
mission requirements. The spectral dispersions have excellent consistency in
the spatial dimension of each band, and there is good linearity in the
spectral dimension of each band. The RMS errors of the fitting residuals are
0.9, 1, and 0.7 pm in the O2 A-band, the WCO2 band, and the
SCO2 band, respectively. Taken together, these results suggest that
the spectral characterizations of the spectrometer aboard TanSat meet the
mission requirements.
The modeling of the absorption lineshape for embedded molecules through a polarizable QM/MM approach