In-flight spectral calibration of HIRS using AIRS observations

Accurate spectral calibration is essential for atmospheric profile retrievals to be made from infrared sounder data. It is known that there are uncertainties in the spectral response functions of the HIRS (High Resolution Infrared Radiation Sounder) instruments on NOAA’s polar-orbiting environmental satellites in the prelaunch measurements. Also, possible spectral shifts in orbit may have occurred. In this study, we describe plans and methods to perform spectral calibration of the broad-band HIRS channels with the overlapping hyperspectral AIRS channels, using observations that are simultaneously taken by both instruments at nadir. The potential and limitations of this approach are evaluated. Results based on data from AQUA/AIRS and NOAA-16 & -17/HIRS are presented.