TanSat is an important satellite in the Chinese Earth Observation Program which is designed to measure global atmospheric CO 2 concentrations from space. The first Chinese superhigh-resolution grating spectrometer for measuring atmospheric CO 2 is aboard TanSat. This spectrometer is a suite of three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO 2 band near 1.61 and 2.06 μm and in the molecular oxygen A-band (O 2 A) at 0.76 μm. Their spectral resolving power (λ/λ) is ∼19 000, ∼12 800, and ∼12 250 in the O 2 A, weak absorption band of molecular carbon dioxide band, and strong absorption of carbon dioxide band, respectively. This paper describes the laboratory radiometric calibration of the spectrometer suite, which consists of measurements of the dark current response, gain coefficients, and signal-to-noise ratio (SNR). The SNRs of each channel meet the mission requirements for the O 2 A and weak CO 2 band but slightly miss the requirements in a few channels in the strong CO 2 band. The gain coefficients of the three bands have a negligible random error component and achieve very good stability. Most of the R-squared of gain coefficients model consist of five numbers of nine (e.g., 0.99999) after the decimal point, suggesting that the instrument has significant response linearity. The radiometric calibration results meet the requirements of an absolute calibration uncertainty of less than 5%.

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 CO 2 mole fraction by measuring the visible and near-infrared solar-reflected spectra.The first Chinese super-high-resolution grating spectrometer for measuring atmospheric CO 2 is aboard TanSat. This spectrometer is a suite incorporating three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO 2 band near 1.61 and 2.06 micrometers and in the molecular oxygen (O 2 ) A band at 0.76 micrometers. Their 5 spectral resolving power (λ/∆λ) are ∼19000, ∼12800 and ∼12250 in O 2 A-band, WCO 2 and SCO 2 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. 10The 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.

TanSat is a key satellite mission in the Chinese Earth Observation program and is designed to measure the global atmospheric column-averaged dry-air CO 2 mole fraction by measuring the visible and near-infrared solarreflected spectra. The first Chinese super-high-resolution grating spectrometer for measuring atmospheric CO 2 is aboard TanSat. This spectrometer is a suite incorporating three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO 2 band near 1.61 and 2.06 µm and in the molecular oxygen (O 2 ) Aband at 0.76 µm. The spectral resolving power (λ/ λ) values are ∼ 19 000, ∼ 12 800, and ∼ 12 250 in the O 2 A-band, and the weak and strong absorption bands of CO 2 , 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 O 2 A, WCO 2 , and SCO 2 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 O 2 A-band, the WCO 2 band, and the SCO 2 band, respectively. Taken together, these results suggest that the spectral characterizations of the spectrometer aboard TanSat meet the mission requirements.