TanSat ACGS on-orbit spectral calibration by use of individual solar lines and entire atmospheric spectra

Abstract. The spectra measured by the Atmospheric Carbon dioxide Grating Spectrometer (ACGS) carried by the China global carbon dioxide observation satellite (TanSat) in the band of 0.76 μm, 1.61 μm and 2.06 μm can be used for the retrieval of carbon dioxide (CO₂) concentrations by fitting the observations and simulations using the optimal estimation algorithm. Accurately detecting the change of the center wavelength is highly important because of its very high spectral resolution and accuracy requirement for product retrieval. The variations of center wavelength for all three bands of ACGS have been calculated on the locations of the individual solar absorption lines by comparing the solar-viewing measurements and the high resolution solar reference spectrum. The variations with magnitudes less than 10 % of the spectral resolution for each band have been detected. The changes are probably caused by vibration and the instrument status difference between the ground and space, especially temperature variation on orbit. In addition to solar lines, the entire atmospheric spectra simulated by radiative transfer model can be used as the reference spectrum to determine the wavelength change by fitting the measured and simulated spectra. The change of wavelength determined by atmospheric spectra is closely consistent with that by solar lines. Both schemes described here can be used not only for monitoring spectral stability but also to gain spectral knowledge prior to the level-2 product processing. These minor temporal changes of wavelength on orbit should be corrected in the product retrieval.