# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 1 Introduction
Instruments based on thermal infrared (TIR) spectroscopy can retrieve XCH\({}_{4}\) at high latitudes, but are much more sensitive to mid-troposphere concentrations than to concentrations in the boundary layer where CH\({}_{4}\) emissions occur. Recently, CH\({}_{4}\) data from SCIAMACHY and GOSAT was also used to detect and quantify local emission hotspots from oil/gas production and coal mining (e.g., [44; 45]). Although it offers very low sensitivity in the boundary layer, data from Infrared Atmospheric Sounding Interferometer (IASI) flying on the European MetOp satellite series since 2006 [46] also provides statistically consistent methane emissions [42]. Passive satellite data have brought a new vision of the space-time distribution of CH\({}_{4}\) atmospheric mole fractions that complements that from the surface observations, albeit with comparatively lower precision and accuracy. The use of SCIAMACHY to infer CH\({}_{4}\) emissions in global inversions necessitates the use of accurate radiative transfer models in the retrieval of XCH\({}_{4}\), which account for scattering by cirrus and aerosol. As a result of the loss of a SCIAMACHY detector at the end of 2005, instrumental noise increased, resulting in XCH\({}_{4}\) retrieval uncertainties increasing up to 40 ppb [38; 40; 47]. Although GOSAT retrievals still present significant biases [31; 48], these data represent an important improvement both for random and residual systematic errors, the latter being now estimated to be only 4-6 ppb on individual column soundings [48]. Several passive missions aiming to measure CH\({}_{4}\) (Table 1) are scheduled to be launched in the upcoming years. TROPOMI on ESA Sentinel 5 Precursor is planned to be launched in 2017 [49], followed by JAXA-NIES-MOE GOSAT-2 in 2018 [50]. NASA plans the geostationary mission geoCARB [51] to be launched after 2020, while CNES and EUMETSAT are preparing the next generation of IASI (IASI-NG, 2021) [52] and ESA-EUMETSAT are preparing UVNS/Sentinel 5 (2021). In this context of the evolving development of passive instruments, the MEthane Lidar missioN (MERLIN) proposes an active measurement of XCH\({}_{4}\) by employing an IPDA (Integrated Path Differential Absorption) nadir-viewing Lidar instrument [53; 54; 55; 56]. As described below, this new technique has the potential to deliver global coverage during all seasons and with low systematic errors. In view of this potential benefit, the development of Lidar sensors for the measurement of CH\({}_{4}\), and also CO\({}_{2}\), has attracted increasing attention. Several experimental setups have been tested by different groups, each using slightly different approaches [57; 58; 59; 60; 61; 62; 63]. Recent efforts in the framework of NASA's ASCENDS (Active Sensing of CO\({}_{2}\) Emissions over Nights, Days, and Seasons) mission proposal have led to a significant step towards an active CO\({}_{2}\) sensor in space (NASA report on ASCENDS, and references therein [64]). For CH\({}_{4}\), reports on active airborne measurements are still sparse [65]. CHARM-F, DLR's new airborne IPDA Lidar instrument for CH\({}_{4}\) and CO\({}_{2}\), has just recently been completed and tested on the German research aircraft HALO (High Altitude and Long Range Research Aircraft) for the first time [66]. Initial results from this campaign show a high measurement precision (<0.5%) for both species, even in strongly varying geophysical conditions (mountain-like topography, surface albedo jumps, water surfaces, atmospheric aerosol load, and cloudy conditions). MERLIN is a joint French (CNES) and German (DLR) space mission, with a launch scheduled for the timeframe 2021/22 (Table 1). The development of the payload is the responsibility of the German space agency (DLR), while the platform (MYRIADE Evolutions product line) is that of the French space agency (CNES).