OLCI Instrument Calibration

In-flight calibration of OLCI is a fundamental component of the instrument design. All OLCI measurements are made via a calibration assembly of a similar design to MERIS that includes a mechanical rotating table. Either a direct view of the Earth (for imaging mode) or one of several calibration targets may be selected by rotating the table: a dark shutter plate (for dark current calibration), a primary Polytetrafluoroethylen (PTFE) calibration diffuser (viewed every 2 weeks for radiometric calibration), a redundant PTFE calibration diffuser (viewed every 3 months to determine degradation of the primary diffuser due to solar exposure) or an erbium doped 'pink' diffuser plate for spectral calibration. During the calibration sequence, a selected diffuser plate is moved into the instrument Field of View (FOV) and illuminated by the sun so that all five cameras can be calibrated at the same time. Characterisation of diffuser ageing is determined through on-ground processing using the two OLCI diffusers in synergy.

The OLCI calibration sequence is carried out before the terminator crossing over the southern hemisphere to maintain a stable internal instrument temperature in a similar manner to that of MERIS. Two successive orbits are required; the first for radiometric calibration and the second for spectral calibration. Each calibration sequence begins with a dark current evaluation. This sequence lasts 45 s and acquires 1 024 measurement frames that are averaged on-ground to reduce noise and used to accurately derive the signal produced under dark conditions.

The OLCI Calibration Mechanism (credit TAS-France)


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