Copernicus Sentinel-3 calibration and validation (cal/val) activities are essential to the quality of the mission. Data quality will be assessed through determination of the radiometric, spatial, spectral and geometric fidelity of the satellite sensor and the accuracy of geophysical products. The calibration and validation of the Sentinel-3 instruments took place in three phases:

• Pre-launch phase instrument characterisation and on-ground calibration.
• A commissioning phase (E1) that lasted 6 months for Sentinel-3A, where all instrument operation aspects have been verified and in-orbit calibration and validation activities have started. For Sentinel-3B, this phase was reduced to 3 months but a tandem phase with Sentinel-3A of ca. 3 months allowed to perform cross-comparisons between the two units. After a successful IOCR (In-Orbit Commissioning Review), the agencies gave the authorization for the data dissemination to end-users.
• An exploitation phase (E2) has then started after the IOCR with the routine implementation of the calibration and validation activities for geophysical data products.

Dedicated Copernicus Sentinel-3 calibration tasks include:

• full pre-launch characterisation and calibration of all instruments
• full in-flight calibration and (re)characterisation of all instruments
• comprehensive verification of Level-1 data processors (tuning of all relevant processing parameters, regeneration of all Level-1 auxiliary products)
• preparing and advising on necessary Level-1 processor updates
• routine calibration monitoring and assessment after the end of the commissioning phase.

Dedicated Copernicus Sentinel-3 validation tasks include:

• commissioning phase Level-2 algorithm verification for all Level-2 'baseline' products
• Level-2 algorithm validation starting during the commissioning phase and continued throughout phase E2
• quantification of Level-1 and Level-2 product error estimates
• long term monitoring for consistency and constant quality of geophysical products
• advice on re-processing campaigns.

In addition, the cal/val component of the Sentinel-3 mission (now included in the Optical Mission Performance Cluster) includes maintenance and evolution of Level-2 processors, the generation of all auxiliary data sets needed for Level-1 and Level-2 processing. Sentinel-3 cal/val plan is reviewed and updated on a regular basis as required by the mission.