# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 2 Mission Objectives  
The overarching scientific goal of the French-German Climate Mission MERLIN is to provide atmospheric constraints to improve our knowledge of the global methane emissions and sinks. This scientific goal is to be achieved by the development, launch and operation of an Earth observation satellite in the framework of a French-German cooperation. The mission aims to provide information on the global methane cycle at all latitudes, including the still-underexplored regions such as tropical lands and high latitudes. The measurements are consequently of relevance for climate change research, and with respect to the monitoring approach needed for the implementation of the Paris Climate Agreement signed during COP21 in December 2015. Indeed, CH\({}_{4}\) is a potent greenhouse gas, and has interesting mitigation options due to its limited lifetime of approximately nine years [77] which is shorter than that of most other anthropogenic greenhouse gases, thus offering a variety of mitigation opportunities in different sectors (e.g., [2; 78; 79]). It is therefore important to measure its atmospheric variations with time and to develop methods to estimate its sources and sinks independently from the Intended National Determined Contributions (INDCs) provided by the signatory countries. The main scientific objective of MERLIN is to deliver column-weighted dry-air mole fractions of CH\({}_{4}\), referred to as XCH\({}_{4}\) along the satellite sub-track with a targeted random error better than 27 ppb (Table 3, 50 km averaging along the satellite track, as illustrated in Figure 1) and a systematic error better than 3.7 ppb (68% interval). As a novel feature, the MERLIN mission will use pulsed narrow-bandwidth laser radiation, thus not relying on sunlight or Earth infrared emission. With a range-gated receiver for detection of the signals reflected from the Earth's surface or from optically thick clouds, the MERLIN instrument will distinguish surface from cloud or aerosol backscatter, permitting high-precision retrievals of XCH\({}_{4}\) in the presence of particle layers with low optical depth, such as thin cirrus or aerosol. In the presence of optically thick clouds, the Lidar beam can reach the surface when gaps between clouds occur due to the near-nadir view and the small Lidar ground spot. MERLIN can also provide XCH\({}_{4}\) measurements above dense stratiform clouds to be used as a reflective target instead of the surface target. As MERLIN has its own transmitter, its observations will produce data at high latitudes, in winter and at night. MERLIN XCH\({}_{4}\) will complement and enhance the existing observation systems for GHG monitoring, complementing surface-based, balloon, aircraft and passive satellite observations. In addition to XCH\({}_{4}\), the MERLIN products will also include information on secondary climate-relevant parameters, for example, cloud top heights, surface retro-reflectance and, potentially, canopy height. MERLIN XCH\({}_{4}\) will nicely complement existing measurement techniques by providing XCH\({}_{4}\) where either surface networks (tropics, high latitudes) or other satellites (high latitude winter, night, some cloudiness conditions) provide no or only sparse observations in space and time. MERLIN will deliver maps of XCH\({}_{4}\), built from XCH\({}_{4}\) measurements using an original Bayesian approach [80] that provides daily global maps of XCH\({}_{4}\) with reliable (in the statistical sense) associated uncertainties in the form of a full space-time covariance matrix. These maps will be used, together with their error statistics, to rigorously compare the retrievals with distant validation data, in the absence of a target mode for MERLIN. The main scientific goal of the MERLIN mission is to contribute to the estimations of CH\({}_{4}\) sources and sinks using atmospheric inversions in order to reduce uncertainties in the global CH\({}_{4}\) cycle.