# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 4 Mission Elements
### Space Segment
The collected signals pass an optical filter for background light rejection and are focused on a low-noise, single-pixel detector with a relative large sensitive area of about 200 \(\upmu\)m (circular diameter). For MERLIN, a commercially available Avalanche Photo Diode (APD) that does not require extra cooling was chosen as the detector. To fulfil the demanding measurement performance, an intensive evaluation and critical selection program in connection to breadboard activities has been initiated and will continue over the development phases of the instrument. The goal is to select the best-qualified device on the market. Figure 3: MERLIN satellite with the IPDA Lidar instrument from DLR attached on the MYRIADE evolution platform from CNES. Copyright Airbus Defence and Space. With respect to operations, the instrument will collect data without interruptions all over the orbit. The targeted pulse-pair sampling rate is 20 Hz, while the pulse energy for each OPO pulse amounts to 9.5 mJ. The overall sizing of these major instrument parameters was driven by the needs to fulfil the observational requirements and to fit platform resources and overall mission costs. The proposed transmitter concept is expected guarantee stable instrument operation throughout the envisaged mission lifetime of 3 years. With respect to timing, the round-trip time of the emitted pulses and received echoes from the Earth's surface is about 3.4 ms for a 500 km orbit height. Since only one detector unit is used, the online and offline pulses are not transmitted simultaneously. A temporal separation of about 250 us will be applied to avoid possible signal ambiguities due to interfering Lidar signals from polar stratospheric clouds (PSC) or strong aerosol layers in the stratosphere after a big volcanic eruption. As a result of this small time delay, the online and offline ground spots will be separated by about 2 m, which is regarded as negligible compared to the spot diameter of about 100 m (90% of encircled energy). The maximum sampling distance between two shot-pairs of the MERLIN measurements amounts to about 350 m, which is given by the ground spot velocity of about 7 km/s and the pulse-pair sampling rate of 20 Hz. Single shot- pairs are then averaged during the on-ground processing on 50 km windows (about 142 shot- pairs). The near infrared spectral region around 1.6 microns was preselected to minimize the sensitivity to atmospheric temperature and aerosol loading and cross-sensitivities to other trace gases (mainly CO2 and H2O). Four candidate line multiplets and associated pairs of online/offline frequencies were identified and considered. The final choice is the R6 multiplet at a frequency of 6077 cm\({}^{-1}\) (corresponding to 1.645 um in vacuum), which promises higher measurement sensitivity to CH4 molecules residing in the planetary boundary layer (PBL). In reference to the A-SCOPE study,  
Figure 4: (**a**) IPDA-Lidar instrument with all subsystems integrated, (**b**) assembled laser head (will be mounted upside down on the optical bench). Copyright Airbus Defence and Space. the decision was based on calculation of a scaling factor for each candidate line that expresses the fractional amount of absorption taking place in the PBL (e.g., up to 850 hPa [86]. It turned out that the preferred R6 multiplet yields a scaling factor that is 25% to 35% larger with respect to the other candidate lines, and about 23% with respect to an instrument with a constant weighting function (as illustrated in Figure 5). A further important aspect for implementation of the envisaged payload is the need to minimize systematic errors that may arise from \"small\" laser frequency instabilities or unknown Doppler shifts caused by instrument/satellite pointing errors. These mainly spectral-based error sources can seriously impact the systematic error budget of IPDA Lidar measurements, as analyzed to some depth in the framework of the A-SCOPE report for CO\({}_{2}\)[86].