A harmonised approach to Calibration and Validation for upcoming Sentinel missions: updates for CO2M and Sentinel-5

As part of the Copernicus Programme of the European Commission, the European Space Agency (ESA) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) are expanding the Copernicus Space Component to include additional measurements of atmospheric composition. To support the evaluation of greenhouse gas emission reductions decided during the COP21 meeting in Paris in 2015, new measurements are required with improved accuracy. Measurements from space-borne instruments are a key component of this effort – requiring improvements in both spatial and spectral resolution.

The Copernicus missions are Europe’s primary contribution to this effort. The CO2M mission – a constellation of initially 2 platforms – is due to launch at the end of 2026 and will be available to contribute to the global stocktake in 2028 and those beyond. Data from the three instruments on board each platform – CO2I/NO2I, MAP, and CLIM – will be combined to act as a single “hyper-instrument”. NO₂ is often co-emitted with CO₂ & CH₄, so the spectrometer (NO2I) will be used to detect near-surface NO₂ plumes; the impact of aerosol pollution on the XCO₂ columns will be calculated using the Multi-Angle Polarimeter (MAP); and cloud contamination will be observed and removed via data from the Cloud Imager (CLIM). These data will be used within the CO₂ and CH₄ retrievals from the CO2I spectrometer to improve the retrievals and allow for a precision of 0.7 ppm and an error of 0.5 ppm for XCO₂, and an uncertainty of ~10 ppb for CH₄.

Sentinel-5 will fly on the EUMETSAT Polar System – Second Generation (EPS-SG) platform, due for launch in 2025. S-5 will be a push-broom hyperspectral UVN spectrometer, with a 7.5 km² spatial resolution and global daily coverage via a 2670 km wide swath. It will monitor various trace gases, including CH₄.

Comprehensive calibration and validation analysis during the satellite’s commissioning will be undertaken to ensure they meet the specified requirements, and ongoing monitoring will ensure the instruments continue to comply with the challenging requirements. External data from ground-, airborne-, and satellite-based instruments will be required, covering the whole globe and multiple parts of the EM spectrum. Calibration and Validation techniques are being developed across CO2M and S-5, as well as Sentinel-4 which is also due for launch in 2025. This inter-mission collaboration has been undertaken to reduce duplication of effort and ensure lessons are learned from the commissioning of each mission.

Here, we give an update on the current state of planning for the Calibration and Validation operations, and identify gaps in the current provision of external networks, ground-based in particular, so that alternative and additional data sources can be procured.