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Preflight testing of AIRS determined that the shapes of the detector spectral response functions (SRFs) do not vary under different instrument conditions. This reduces in-flight spectral calibration to the determination of detector spectral centroids. A spectrometer grating model has been developed to calculate detector centroids. Only two parameters of this model need to be determined in orbit. An algorithm is presented for determining these two parameters in orbit by correlating observed upwelling radiance spectra with modeled spectra. The method of selecting spectral regions against which to correlate is detailed. The in-orbit spectral calibration algorithm was tested on one day of simulated global AIRS radiances, showing that the uncertainty in the frequencies of the SRF centroids is between 0.006 Δν and 0.01 Δν, compared to the spectral calibration requirement of 0.01 Δν, where Δν is the SRF full width at half maximum. The simulation also indicates that the stability of the spectral calibration can be monitored at the 0.001-Δν level on a daily basis.