# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 6 Conclusions  
The overall goal of the French-German Climate Mission MERLIN is to reduce uncertainties on our knowledge of the global methane budget. For this purpose, the scientific objective is to deliver observations of atmospheric methane from space for all latitudes using a satellite based on an innovative Lidar instrument. The programmatic goal is to develop, launch and operate this satellite in French-German cooperation to provide relevant information about the global CH\({}_{4}\) cycle. Since atmospheric CH\({}_{4}\) is the second-most important anthropogenic greenhouse gas, MERLIN will give insight into the nature and variety of its sources which will offer interesting opportunities in the context of climate change--also the context of the implementation of the Paris Agreement signed during COP21 in December 2015. MERLIN will deliver column-weighted dry-air mole fractions of CH\({}_{4}\), referred to as XCH\({}_{4}\), along the satellite sub-track (level 2 product) with arandom error better than 27 ppb (50 km averaging along the satellite track) and a (1-sigma) systematic error of better than 3.7 ppb. The mission serves as a scientific and technological demonstrator for a trace gas Lidar in space that will use the differential absorption measurement principle. The low-biased data will be highly useful to the scientific community, especially for atmospheric inversions estimating CH\({}_{4}\) emissions from XCH\({}_{4}\) products. In particular, the mission will provide complementary information to the ground-based network and to passive satellite sensors that are limited either by insufficient sunlight (NIR-sensors) or lack of sensitivity to CH\({}_{4}\) molecules in the planetary boundary layer (PBL) (IR-sensors). The combined use of different data sets (including in situ observations) in a single model parameter optimization framework offers the perspective of applying the concept of carbon cycle data assimilation (e.g., [93]) to CH\({}_{4}\). Such a combined use of different data sets in a consistent manner will certainly be challenging, but potential inconsistencies will provide directions for improvements. The methodology proposed for the MERLIN observations is well demonstrated by ground-based and airborne measurements. It involves the measurement of the DAOD using Lidar signals at two distinct frequencies in the vicinity of a CH\({}_{4}\) absorption line originating either from ground or cloud top as the target. The measured DAOD values are converted to XCH\({}_{4}\) by means of auxiliary  
Figure 7: Uncertainty reduction on methane emission (%, y-axis) for three latitudinal bands (x-axis) for MERLIN (orange) using the scenarios developed to represent MERLIN random and systematic errors. Sahara pixels, with very low methane flux and high albedo, have been removed from the 30\({}^{\circ}\)N-30\({}^{\circ}\)S barplot. meteorological data from NWP and spectroscopic information. To observe all latitude zones for year-round observations, a sun-synchronous, near-polar orbit was selected. The MERLIN space segment is comprised of a mini-class satellite carrying only one scientific instrument, which is the IPDA Lidar system operating in near-nadir-viewing configuration. A Nd:YAG pumped OPO will serve as the MERLIN transmitter operating in the 1.64 \(\upmu\)m spectral domain. The platform benefits from the French MYRIADE Evolutions Program, which is an enhancement of the original MYRIADE concept of small satellite series. The development of the payload also benefits from CHARM-F, the airborne MERLIN demonstrator and from European space laser development efforts at ESA and DLR (ALADIN, ATLID, FULAS). MERLIN provides three levels of products, which will be made publicly available. Level 1 processing comprises vertically resolved Lidar data up to a height of 40 km at three different vertical sampling rates, as well as the generation of DAOD products on different horizontal averaging intervals (baseline is 50 km).