# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 6 Conclusions
Level 2 processing yields XCH4 and level 3 products will consist of corresponding maps using an original Kalman-filtering algorithm. The full suite of the existing, high accuracy and reliable techniques, including flask sampling and passive remote sensing (e.g., TCCON), will be used for validation of the MERLIN products as long as the mission is operating. Of particular importance for campaign-based validation are balloon-borne and airborne in situ sensors, as well as airborne remote sensors such as CHARM-F, the DLR's IPDA Lidar instrument operated onboard the German research aircraft HALO. The latter serves as an airborne demonstrator for MERLIN, as it uses identical transmitter frequencies and viewing geometry. To prepare for the validation exercise, a pre-launch campaign will take place, possibly involving both German (HALO) and French (SAFIRE) aircraft. A first analysis of an end-to-end error budget revealed that the following CH4 surface uncertainty reduction can be achieved by the MERLIN observations for the different regions: 59% for the continental tropics (without the Sahara dessert); 84% for continental mid-latitudes, and; 53% for continental high latitudes. This performance relies to nominal random errors, modelled with a dependency on the surface reflectivity and aerosol load (as a reference case, of 24.3 ppb on XCH4, dominated by the DAOD related error with an impact of 23.1% on XCH4). The situation is quite different for the calculated systematic error budget of 3.1 ppb where several contributors show similar errors. However, the largest impact of 2.3 ppb arises from assumed detector non-linearity. Uncertainty reductions obtained here for the aggregated boreal regions (+53% on average) indicates that MERLIN observations will further constrain flux estimates at high northern latitudes, contrary to most passive missions. A successful MERLIN mission will provide the first low-bias, global, spaceborne IPDA Lidar data for an anthropogenic greenhouse gas, allowing for the significant improvement of our knowledge of the global CH4 budget. The MERLIN mission will open the path for future active remote sensing missions dedicated to the monitoring of greenhouse gases from space. We are grateful to CNES and DLR who selected MERLIN in 2010 as a future climate mission based on a dedicated Lidar instrument for the measurement of atmospheric methane for the first time. Special thanks are given to the members of MERLIN project teams from French CNES ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON]), from French LMD ([PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON]), from French groupe de spectrometrie moleculaire et atmospherique (GSMA) ([PERSON], [PERSON], [PERSON]), from French small and medium-sized enterprises (SMEs): NOVELTS ([PERSON]), CAPGEMINI ([PERSON]), MAGELLIUM ([PERSON], [PERSON]), SYERGE ([PERSON]), from DLR Raumfahrtmanagement ([PERSON], [PERSON], [PERSON], and [PERSON], [PERSON]), DLR Institut fur Physik der Atmosphare ([PERSON], [PERSON], [PERSON]), DLR Institut fur Methodik der Fernerkundung ([PERSON], [PERSON], [PERSON]), Deutsches Fernerkundungsdatenzentrum of DLR ([PERSON], [PERSON], [PERSON]), and Airbus Defence and Space in France and Germany with their supplies, who all strongly support the mission development during the subsequent phases. Error budget calculations have been possible thanks to CNES, CEA (LSCE) and national (GENCI TGCC) computational means. MERLIN is supported by the Federal Ministry of Economy and Energy on the basis of a decision by the German Bundestag and the French Ministere de l'Enseignement superieur, de la Recherche et de l'Innovation. **Author Contributions:** The authors are all members of the MERLIN Science Advisory Group and therefore responsible for the content of the manuscript. Their particular contribution to the manuscript is given as follows: [PERSON]*: Drafting Sections 1, 3, 4 and 6, Figures 1, 2 and 5, input references, drafting acknowledgement.