# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 4 Mission Elements
### Space Segment  
The space segment of MERLIN is comprised of a mini-class satellite carrying only one scientific instrument, an IPDA Lidar system operating in a near-nadir-viewing configuration (Figure 3). To share the philosophy of a joint development between France and Germany, the satellite uses a modular approach with a clear separation between the payload provided by Germany (DLR) and the platform module from France (CNES). Figure 4 illustrates the assembled Lidar instrument with its various subsystems attached. All optical units (laser head, telescope, star tracker) are mounted on a common stiff optical bench with an isostatic interface to the spacecraft panel. Other subsystems, such as the Instrument Control Unit (ICU), Frequency Reference Unit (FRU), and Laser Electronics Unit (LEU), will be hard-mounted on a secondary structure, which also contains a thermal interface to a radiator not shown in this figure. The online and offline pulses in the 1.64 \(\upmu\)m spectral domain are generated by the laser head (Figure 4b) consisting of an OPO that is pumped by a pulsed single-frequency Nd:YAG laser at 1.064 \(\upmu\)m. By means of injection seeding using a continuous wave (cw) single-frequency laser system at 1.64 \(\upmu\)m, the OPO provides itself spectrally narrow pulses which can be tuned to the desired frequency position within the absorption profile of the selected molecular transition. The spectral properties of the transmitted OPO pulses (frequency, spectral profile) are controlled by means of the FRU, indicated as the green box in Figure 4a. To ensure narrow-band and spectrally stable operation of the OPO, its cavity must be matched to the frequency of the seed lasers. A CH\({}_{4}\)-gas cell in combination with an interferometric standard serves as a frequency-calibrated wavemeter. This enables spectral control of both the seed lasers and transmitted pulses on a shot-by-shot basis. As outlined in Section 3, the IPDA Lidar method is not free from \"internal\" radiometric calibration needs and requires the  
Figure 2: Conceptual drawing illustrating on the definition of Surface Scattering Elevation (SSE) and local surface pressure \(p_{surf}\) according to the description in Section 3.  
knowledge of the ratio of outgoing online and offline pulse energies at similar accuracy as requested for the measurement of the Lidar signals. In order to account for ageing of the detector, this measurement will be performed by the same Lidar detector. For this purpose, an integrating sphere is used to attenuate the calibrating laser beam to a power level that is comparable to the received Lidar signals. Initial breadboard studies revealed that this concept for the relative pulse energy measurement in principle works well, but at the expense of introducing a further error source. The integrating sphere gives rise to a speckle pattern, which is not stationary within the relevant timing sequence (~7 s) due to small temperature drifts. Investigations show that this kind of error source can sufficiently be suppressed through the implementation of a synthetic high frequency speckle generator. Such device can be regarded as an additional white noise source and therefore does not increase systematic errors in the IPDA Lidar measurement [89]. Further optical elements, which will be traversed by the outgoing beam, include the Active Pointing Control (APC) and the transmitter telescope to ensure that the outgoing OPO pulses are centred on the telescope receiver footprint of about 200 m in diameter. For the latter, an off-axis Cassegrain-like telescope (with a diamater of about 69 cm) in bi-static configuration has been selected. This configuration satisfies both the scientific requirements of a large signal collection area and possible launch constraints for a piggyback satellite, which calls for a rather compact instrument in the x-direction (see Figures 3 and 4a).