## 2 Materials and Methods
### Description of the Remote-Controlled Lidar
The local computer can be accessed remotely and allows the control of the laser, the photon-counting system, the data storage, and transmission, as well as control of the laser alignment. Since 2018, the lidar system has been equipped with an automated alignment system controlled by the local computer, which can be accessed via a satellite VPN connection. This advanced configuration allows control and alignment of the complete system at Concordia Station independently of the weather conditions. The remote control procedures have been designed to control system parameters and to verify and optimize the alignment of the lidar. Initially, the laser emission is aligned almost parallel with the axis of the main telescope with a slight tilt towards the telescope. In this way, the laser illuminates the field of view of the telescope. Subsequently, the smaller telescope is adjusted to capture the signals produced by the air mass illuminated by the laser emission at low altitudes. Optimal lidar alignment is achieved when the emission of the laser remains within the field of view of the telescopes. The current configuration allows for adjustments of the laser emission with respect to the main telescope by using a piezo-controlled mirror (Newport model AG100), which can be scanned in azimuth and tangent angles with respect to the axis of the large telescope. The alignment procedure involves monitoring the 532 nm polarized (high and low) and depolarized channels for the main receiver, as depicted in Figure 4.  
A dedicated computer program in Python provides an automatic alignment procedure by scanning the two angles with respect to a starting point and acquires the lidar signal at each setting, thus creating an intensity map by integrating the amplitude of the return signal (in photon counts) between predetermined altitudes. The maximum signal of this map becomes a new starting point, and the process is repeated until the optimum signal is obtained. Alternatively, the piezo-controlled mirror can be accessed directly by the operator to find the best conditions \"manually\". Once the laser emission is aligned on the main telescope, the smaller telescope can be aligned on the laser beam by using a similar procedure. In this case, a mechanical movement of the smaller telescope allows it to optimize its field of view with respect to the laser emission. During both procedures, a so-called quality factor, derived from the integrated signals between opportune altitudes, which are different for the large telescope and smaller telescope, since the latter should \begin{table}
\begin{tabular}{l l} \hline Laser energy per pulse & 180 mJ \\ \hline Pulse repetition frequency & 10 Hz \\ \hline Laser pulse duration & 9 ns \\ \hline Laser divergence full angle & 1.5 mrad \\ \hline Laser pointing stability & 100 \(\upmu\)rad \\ \hline Main telescope diameter & 355.6 mm \\ \hline Main telescope focal length & 3910 mm \\ \hline Main telescope field of view & 4 mrad \\ \hline Small telescope diameter & 152.4 mm \\ \hline Small telescope focal length & 1500 mm \\ \hline Small telescope field of view & 2 mrad \\ \hline FWHM interference filter @ 532 nm & 2 nm \\ \hline \end{tabular}
\end{table} 
Table 1: Lidar specifications. cover the troposphere only, is calculated and can be displayed in a 2D graph as a function of the two angles with respect to the telescope. The darkest areas in Figure 5 correspond to the maximum overlap between laser emission and the field of view of the telescope. The alignment procedure for both telescopes is iterated until a satisfactory alignment has been obtained. Figure 4: Screenshot of the automatic alignment program, showing the three signals of the 532 nm channels. The vertical axis is in logarithmic scale to display the full dynamics of the signals. The horizontal axis reports the distance from the lidar in km. Figure 5: Screenshot of the automatic alignment program showing the intensity map of the received signals in the function of the two angular displacements of the piezo-controlled mirror.