The overall goal of validation is to assess the utilization of the main MERLIN products for their intended scientific application. In practice, the MERLIN data and products will be compared to temporally and spatially coincident measurements performed by independent instrumentations from various platforms and from a variety of locations around the globe.

We work together with both, long-term and campaign-based validation activities, to ensure high data quality over the mission lifetime. For this, the full suite of existing, high-accuracy and reliable techniques, including flask sampling and remote sensing, are used.

The Total Carbon Column Observation Network (TCCON), which has been developed and already successfully deployed for validation of space-based passive greenhouse gases (GHG) sensors, is regarded as a beneficial validation infrastructure, since it consists of various stations distributed over the globe.

Of particular importance for validation are balloon-borne and airborne in situ sensors, both providing profile information on CH₄, which can be converted to total atmospheric methane column (XCH₄). In situ sensors provide the most accurate data calibrated against the WMO standard, thus promising the highest level of confidence.

Measurements from aircraft campaigns provide temporally and spatially coincident measurements almost anywhere on the globe. This is especially usefull for data-sparse regions without any information, or far away from ground-based measurements.

After launche of the MERLIN satellite validation activities are crucial. The validation of the DAOD (L1b) products will benefit from co-located flights with the airborne MERLIN demonstrator CHARM-F. This instrument will uses identical measurement frequencies and a similar viewing geometry for a direct inter-comparison of the airborne DAOD values to the MERLIN observations, which is particularly helpful for validation over orographically structured terrain and areas with changing surface reflectivity. In the case of the aircraft HALO (High Altitude and LOng range) aircraft (up to 15 km altitude), about 95% of the CH₄ column as seen by MERLIN can be captured by airborne validation. The remaining part of the profile (above the aircraft) will be taken from simulations of state-of-the-art atmospheric chemistry and transport models, as well as from co-located in situ profiles acquired by balloons. In order to fly more frequently, the installation of CHARM-F on the SAFIRE (Service des Avions Français Insrumentés pour la Recherche en Environnement) and Falcon aircraft are also considered. Synergies of validation with other space missions scheduled for the early 2020s are also foreseen (e.g., GOSAT II, Sentinel 5P, MICROCARB for CO₂ and IASI-NG for broader atmospheric composition).