Prelaunch Radiometric Calibration of the TanSat Atmospheric Carbon Dioxide Grating Spectrometer

TanSat is an important satellite in the Chinese Earth Observation Program which is designed to measure global atmospheric CO₂ concentrations from space. The first Chinese superhigh-resolution grating spectrometer for measuring atmospheric CO₂ is aboard TanSat. This spectrometer is a suite of three grating spectrometers that make coincident measurements of reflected sunlight in the near-infrared CO₂ band near 1.61 and <inline-formula> <tex-math notation="LaTeX">$2.06~\mu \text{m}$ </tex-math></inline-formula> and in the molecular oxygen A-band (O₂A) at <inline-formula> <tex-math notation="LaTeX">$0.76~\mu \text{m}$ </tex-math></inline-formula>. Their spectral resolving power (<inline-formula> <tex-math notation="LaTeX">$\lambda /\Delta \lambda$ </tex-math></inline-formula>) is ~19 000, ~12 800, and ~12 250 in the O₂A, 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 O₂A and weak CO₂ band but slightly miss the requirements in a few channels in the strong CO₂ 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%.