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 μm and the weak and strong absorption bands of carbon dioxide (WCO2 and SCO2 ) ranging from 1.594 to 1.624 μm and from 2.042 to 2.082 μm, respectively. The spectral resolving power (λ/λ) 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.