# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 2 Mission Objectives
Indeed, MERLIN should provide insights for the different scientific communities working on CH\({}_{4}\) sources and sinks including activities around land-surface modelling, emission inventories, and atmospheric chemistry. With a revisit time of 28 days and the ability to measure at all seasons, MERLIN will help constrain surface sources and sinks from seasonal (e.g., changes in gas leaks from winter to summer) to annual scale (e.g., evolution of regional sources during the three years of the mission). The inverse modelling used to convert the atmospheric columns into source estimates is not formally part of the MERLIN mission scope handled by space agencies. Uncertainties in the estimates of CH\({}_{4}\) sources and sinks from XCH\({}_{4}\) products suggest that several modelling groups should develop their own approaches, as was done for previous GHG missions. Ensemble atmospheric inversions haveproven to be more robust than individual studies to represent the remaining uncertainties in the global methane cycle (e.g., [27]) Hence, by providing global and low-biased XCH4 data, MERLIN will help reducing uncertainties both in the global and regional methane cycle (Section 5). One further objective of the MERLIN mission is to deliver XCH4 retrievals to operational centers such as the Copernicus Atmosphere Monitoring Service [81] at the European Centre for Medium range Weather Forecasts (ECMWF), which assimilates satellite GHG observations to initialize its GHG forecasts [82]. Following this objective, the MERLIN mission plans to deliver these products within two days of acquisition, as it is currently implemented for the GOSAT data used within CAMS. MERLIN will also be a scientific and technological demonstrator. On the scientific side, if XCH4 has the expected performances in terms of systematic errors, MERLIN will provide the first very low-biased space-based GHG data. Such low-biased data will be highly useful to the scientific community, especially for atmospheric inversions. This will in turn facilitate the evolution of verification systems to test the GHG emissions reported at the country level, that is, independent data complementary to the country declarations to the UNFCCC based on inventories. Until now, only country-based declarations from inventories have been used to officially report GHG emissions. In the case of CH4, inventories are associated with large errors because those emissions are often fugitive, related more to processes controlling emissions than to a measurable feedstock or pool. Atmospheric inversions can provide a complementary approach. This is especially true for changes (inter-annual variations, trends, etc.) in regional to global emissions, which can generally be more safely inferred than total annual emissions because (at least) some systematic errors vanish when  
Figure 1: Schematic diagram of a two-frequency Integrated Path Differential Absorption (IPDA) Lidar measurement. It makes use of the small fraction of emitted laser radiation which is reflected by a “hard” target (i.e., the Earth’s surface or a cloud top) towards the instrument’s receiver, and subsequently converted into electrical signals by a photodetector. Both signals from online and offline pulses are digitized and corrected for the energy of the emitted laser pulses which are monitored within the instrument. Many such pulse-pairs are accumulated along the instrument’s ground track to increase the measurement precision. The ground reflectivity and beam attenuation from contributors other than the trace gas are typically constant at the very narrow spectral scale of a molecular absorption line, as shown using dashed lines in the right-hand corner. looking at the difference between two time periods (e.g., [16; 83]). The success of MERLIN would open the possibility to develop, jointly with surface observations and passive missions providing CH\({}_{4}\) imagery, inversion frameworks for the monitoring of regional CH\({}_{4}\) emission changes with time.