The Chinese medium resolution spectral imager (MERSI) carried on the Fengyun-3C (FY-3C) satellite can realize the high-precision global measurement of the cloud, aerosol, land surface, sea surface, and low-level water vapor. The MERSI is described conceptually in this paper, and the radiometric calibrations of its thermal emission bands by the on-board blackbody (BB) were given by two different algorithms. In order to evaluate the stability of the instrument’s performance, the measured irradiance after the dark current correction was compared with the theoretical value calculated from the BB temperature by Planck function. The results showed that the algorithm of linear fitting to the BB measurements averaged over sampling points was an optimal choice for the radiometric calibration, and that the calibration coefficients determined by this method could reflect the pixel difference while maintaining a relative stability between each frame. Using the perturbation method, the influences of the BB temperature, wavelength, and emissivity offsets on the calibration coefficients were analyzed and discussed through theoretical deduction and computer simulation for improving the calibration accuracy of the instrument.
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