3.2 Characterisation and Calibration

3.2.1 Calibration Modes

The calibration is performed at the orbital south pole, where the calibration diffusers are illuminated by the Sun by rotating a calibration mechanism. Two identical diffusers are available.

There are two types of calibration and associated operational scenarios.

1. Radiometric calibration is performed every two weeks using diffuser one. A monitoring of the degradation of the BRDF of diffuser one is performed every three months by deploying diffuser two and comparing the results.
2. Spectrometric calibration is performed every three months by first performing a radiometric calibration with an appropriate band settings for the spectral calibration under consideration, and deploying the erbium-doped diffuser with the same band settings on a second orbit. Two erbium spectral absorption features are used, one in the green and one in the NIR Additionally, a high-precision spectrometric calibration for the O₂A absorption bands is performed by exploiting the shape of the absorption band.