MERLIN Level 0-1 Processing and Calibration Concept

After water vapor and carbon dioxide, methane ($CH_4$)is the most abundant greenhouse gas in the Earth atmosphere. The new generation space borne Lidar mission MERLIN (Methane Remote Sensing Lidar Mission) will make very sensitive measurements of the $CH_4$ distribution with unprecedented quality, with an accuracy <2%. After its launch, MERLIN will track down sources and sinks of methane on a global scale. MERLIN is an active space-borne instrument using a IPDA (Integrated Path Differential Absorption) LIDAR that will measure the atmospheric column content of methane. The difference in atmospheric transmission between a laser emission with a wavelength placed around the center of a CH 4 absorption line (λ_on ≈1.6μm) and a reference wavelength, λ_off , slightly shifted from λ on by a few tenths of a nm. The Level 0-1 processing converts the raw signal coming from the satellite (essentially electronic counts as a function of time) to the differential absorption optical depth (DAOD). The main steps in the Level 0-1 processing are - the removal of instrument effects and the calculation of the noise - the calculation of the backscatter signal - the calculation of the range - the calculation of the DAOD Additionally, quality parameters for the measurements and cloud flags will be retrieved. Technically, the processor is based on the ramework provided by GCAPS (Generic Calibration and Processing system), which is developed at DLR-IMF. We will give an overview of the processing steps, the mission requirements and the first design of the processor.