Inflight Performance of the TanSat Atmospheric Carbon Dioxide Grating Spectrometer

TanSat was successfully launched on December 22, 2016, and has been acquiring global measurements of CO2 and O-2 spectral bands in reflected sunlight since early February 2017. The atmospheric carbon dioxide grating spectrometer (ACGS) is a spaceborne three-band grating hyperspectral spectrometer suite onboard TanSat. The ACGS is designed to measure high-spectral-resolution, coboresighted spectra of reflected sunlight within the molecular oxygen (O-2) A-band range from 0.758 to 0.778 mu m and the weak and strong absorption bands of carbon dioxide (WCO2 and SCO2) ranging from 1.594 to 1.624 mu m and from 2.042 to 2.082 mu m, respectively. The spectral resolving power (lambda/Delta lambda) of the ACGS is similar to 19 000, similar to 12 800 and similar to 12 250 in the O-2 A-band, WCO2 band and SCO2 band, respectively. The inflight radiometric calibration accuracy is better than 5%, which satisfies the required specification. The wavelength calibration accuracy of the O-2 A-band is similar to 0.19 pm, that of the WCO2 band is similar to 0.27 pm, and that of the SCO2 band is similar to 4.75 pm, all of which meet the 0.05 full-width at half- maximum (FWHM) requirement. The spectroscopic performance of the ACGS exceeds the mission requirements by a margin. The ACGS has noise levels that are comparable to or smaller than those observed during prelaunch testing, and the noise has remained stable in the three bands during inflight operations. The signal-to-noise ratio (SNR) levels of the three bands meet the specified requirements. As expected, the ACGS radiometric performance in the O-2 A, WCO2, and SCO2 bands was fairly good during its first 17 months inflight.