OLCI is equipped with on-board calibration hardware based on sun diffusers. There are three sun diffusers: two "white" diffusers dedicated to radiometric calibration and one with spectral reflectance features dedicated to spectral calibration.
Radiometric calibration: using the nominal "white" diffuser and including dark signal calibration (with shutter on) on a regular basis, typically every 2 to 4 weeks. The second radiometric "white" diffuser is used in the same way but much less frequently, typically every 3 months, allowing monitoring of their relative evolution.
Spectral calibration: successively using the nominal "white" and spectral diffusers with specific band sets focused at the highest possible spectral resolution, on the spectral features of the spectral diffuser. Comparison of their respective responses in almost identical illumination conditions allows accurate location of the spectral diffuser features onto the CCD and derivation of spectral calibration information. These modes shall be operated with a typical frequency of 3 months.
These two products are not available to users. These are restricted to NRT (less than 3 hours after measurement by the satellite).