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 O₂ 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 (O₂) A-band range from 0.758 to 0.778 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and the weak and strong absorption bands of carbon dioxide (WCO₂ and SCO₂) ranging from 1.594 to 1.624 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> and from 2.042 to 2.082 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, respectively. The spectral resolving power (<inline-formula> <tex-math notation="LaTeX">$\lambda /\Delta \lambda $ </tex-math></inline-formula>) of the ACGS is ~19 000, ~12 800 and ~12 250 in the O₂ 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 O₂ A-band is ~0.19 pm, that of the WCO₂ band is ~0.27 pm, and that of the SCO₂ band is ~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 O₂ A, WCO₂, and SCO₂ bands was fairly good during its first 17 months inflight.