# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 4 Mission Elements
### Mission Orbit  
The selection of the MERLIN orbit (see Figure 6) is driven by the following considerations. To reduce sampling biases in the estimation of the methane sources, rather homogeneous measurement coverage in space and time is generally preferred. Of particular importance are measurements over ecosystems such as tropical and boreal wetlands, which are poorly monitored by the existing surface network. In practice, however, the measurement coverage of a Lidar will be limited by the overpass positions and frequency, and the presence of optically thick clouds. As an optimal compromise, a near-polar orbit to observe all latitude zones providing good samples of all climate zones over the continents and the oceans has been chosen. Even though the sampling of diurnal variations is not the focus of the mission, the choice of a sun-synchronous orbit at a local time of the ascending node LTAN = 06:00 or 18:00 appears preferable. At these times, the relatively stable sun-illumination condition and, therefore, stable thermal environment, helps to mitigate possible biases related to instrument frequency drifts or transmitter pointing issues. At both 06:00 and 18:00 local time, the low sun elevation angle reduces the amount of solar flux reflected from the surface to the satellite and thus limits the noise caused by the background light. Of key importance is the selection of a low orbit height of about 500 km in order to maximize the signal strengths that directly impacts measurement precision considering the limited power of the OPO. Orbit cycling of 28 days allows for overpasses within a circle of 50 km of all ground reference stations (notably TCCON) on a monthly basis which will help the long-term validation of the satellite measurements. The eclipse phase will be \(\leq\)20 min for each orbit during winter times, which in principle allows continuous operation of the instrument throughout the mission. All relevant orbit parameters are listed in Table 2.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Platform** & **MYRIADE Evolutions** \\ \hline Satellite Mass (Platform+Payload): & 430 kg \\ Satellite Power (Platform+Payload): & 500 W \\ GPS receiver: & 2 sensors \\ Star tracker: & 2 optical Heads \\ Pavload: & IPDA Lidar for Methane \\ Mass allocation: & 140 kg \\ Power allocation: & 150 W \\ Laser transmitter type: & Nd:YAG pumped OPO \\ Online frequency: & 6076.9896 cm\({}^{-1}\) \\ Offline frequency: & 6075.97 cm\({}^{-1}\) \\ Pulse energy: & 9.5 mJ \\ Pulse length: & 20 ns \\ Time lag between on/offline transmission: & 250 μs \\ Pulse-pair repetition rate: & 20 Hz \\ Receiving telescope size: & 69 cm \\ Detector type: & Avalanche Pin Diode (APD) \\ Spot size on ground & 100 m (90\% encircled energy) \\ Orbit: & Sun synchr. polar, low Earth orbit (LEO) \\ LTAN: & 06:00 h or 18:00 h \\ Height: & \(\sim\)500 km \\ Inclination: & 97.4\({}^{\circ}\) \\ Repeat cycle: & 28 days \\ Attitude control: & 3 axis stabilized \\ \hline \hline \end{tabular}
\end{table}
Table 2: Satellite and orbit parameters for MERLIN. Optical Parametric Oscillator (OPO).