# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 4 Mission Elements
### Data and Products  
The precise measurements performed by MERLIN require a careful processing, with reliable algorithms, to ensure the final performance of the mission. A Payload Data Processing Centre (PLDP) will operationally produce and distribute the official products of the mission to the users by request. The level 1 to level 3 products will be made publicly available, while access to the level 0 data will  
Figure 6: (**a**) Example of global ground track density for the 28-day repeat cycle. (**b**) Ground track density over Europe per month. The yellow spots indicate the Total Carbon Column Observing Network (TCCON) ground reference stations considered for long-term validation. be restricted to expert users. In the following the individual products and processing steps are briefly described. Level 0 (L0) data consist of raw data containing the backscattered signals for each individual laser shot pair, auxiliary data, and some geometric information (latitude, longitude) in a chronological data sequence. The ordering of the data and its cutting into individual products of one orbit is done in this L0 processing step. Level 1a (L1a) products consist of vertically resolved, pulse energy corrected Lidar data on a shot-by-shot basis. The processing steps include removal of background signals and performing minor non-linearity corrections. The vertical sampling of this product is 2 m below an altitude of about 3 km. This corresponds to the length of the range bin given by the speed of the Analog-Digital-Converter (ADC), which is 75 MHz. It is 50 m (sum of 25 range bins) between 3 and 20 km and 200 m (sum of 100 range bins) between 20 and 40 km, respectively. The vertical resolution of the Lidar signals (e.g., the length of uncorrelated vertical backscatter signals) is 30 m, which is given by the limited bandwidth of the detection electronic chain. L1a product will be complemented by the ensemble of parameters that are necessary to go from L0 data to L1a product. This complement will be gathered as the level 1p data and distributed to experts on request. Level 1b (L1b) products consist of DAOD values both on single shot-pair basis and for an averaging window (nominal 50 km to improve the signal-to-noise ratio (SNR), about 142 shot- pairs). The first processing step is to retrieve the range distance from satellite to the hard target, which is done by a sophisticated search of the maximum value using the offline measurement due to the higher SNR. An integration of the vertical signal around the hard target echo provides the signals \(P_{\textit{off,on}}\) from Equation (1) and a similar algorithm yields \(E_{\textit{off,on}}\). The DAOD is then computed for each shot-pair. The retrieved range, combined with knowledge of the altitude and attitude of the spacecraft, also serves to refine the geolocation and determine the _SSE_. Once this shot-by-shot processing is performed, an average value of DAOD for a window of 50 km is computed, combining about 142 shot-pairs [66; 90]. Note that the averaging distance might change in accordance to a changing albedo. A clear sky only averaging is also realized. Level 2 (L2) products consist of the column-weighted dry-air mole fraction of methane (XCH4), at the scale of both single shot-pairs and across the averaging window. The processing includes NWP analyses to obtain the pressure, temperature and humidity information needed to apply Equations (3) and (4) and compute the weighting function. The random error associated with XCH4 is also computed. For the averaged product, the last processing step consists of removing the bias introduced by the averaging of noisy signals [91]. Level 3 (L3) products consist of XCH4 maps. Processing incudes the visualisation of the L3 products using gap-filling and comparison with independent distant XCH4 measurements. A Kalman-filtering algorithm, as described in [80], will be applied.