Laboratory Spectral Calibration of the TanSat Atmospheric Carbon Dioxide Grating Spectrometer

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 5 near 1.61 and 2.06 micrometers and in the molecular oxygen (O2) A band at 0.76 micrometers. Their spectral resolving power (λ/Δλ) are ~19000, ~12800 and ~12250 in O2 A-band, WCO2 and SCO2 band 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, that are directly related to producing the Level 1 products were evaluated in this paper. The instrument line shape function of the spectrometer is notably symmetric and perfectly consistent across all channels in three bands.The variations resulting in spectral calibrations and 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. Taken together, these results suggest that the spectral characterizations of the spectrometer aboard TanSat meet the mission requirements.