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 2.06 mu m and in the molecular oxygen A-band (O(2)A) at 0.76 mu m. Their spectral resolving power (lambda /Delta lambda) is similar to 19 000, similar to 12800, and similar to 12250 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 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%.