The tight spectral calibration performance requirement calls for on-board calibration. The instrument is pointed in a specific way to get a spectral calibration reference that will consolidate the instrument calibration diagram.
This pointing is achieved by a specific position of the scan mirror, enabling the instrument to point at cold space through a Fabry-Pérot filter.
The signal uses the reflection of the hot parts of the instrument on the filter to create a comb of known spectral lines. In combination with the calibrations made on the atmospheric lines, this calibration pointing makes it possible to verify the instrument calibration throughout the mission across the entire spectral band of interest.