## 2 Materials and Methods  
### Description of the Remote-Controlled Lidar  
Concordia Station is a French-Italian research facility that was built in 2005, located on the Antarctic Plateau at 3233 m above sea level (\(75.1^{\circ}\)S 123.3\({}^{\circ}\)E; see Figure 1). Its position is at the Antarctic Plateau, about 1100-1200 km from the French research station at Dumont D'Urville, Australia's Casey Station (see Figure 1), and the Italian Zucchelli Station at TerraNova Bay, all of which are coastal stations, making it a unique location for the observation of polar stratospheric clouds. Concordia is operational throughout the year, with a winter staff of about 15 technicians and scientists. At a latitude of 75.1\({}^{\circ}\)S, no daylight is present from May through July, and quite spectacular views can be observed when the green laser emission illuminates the sky (see Figure 2). Figure 1: The figure shows a map of Antarctica. The position of Concordia Station with respect to other bases is indicated with a black circle. Figure 2: The lidar illuminates the polar sky at Concordia Station. Photo courtesy of [PERSON]. The lidar observatory at Concordia Station has been active since 2014. The main equipment was already deployed at McMurdo Station from 2004 to 2010 [7]. However, many components have been upgraded and added to increase the reliability and redundancy of the instruments. The substitution of the laser and the photon-counting electronics improved the data quality and extended the measurement range. The remote control of data acquisition, the setting of system parameters, and laser alignment have been implemented in the last four years. The lidar observatory uses a compact Quantel Big Sky laser (model CFR400) emitting at two wavelengths at a 10 Hz repetition rate, with a pulse energy of about 180 mJ at 532 nm and 100 mJ at 1064 nm. Two receivers are used to collect the scattered light. A [PERSON] telescope (Celestron model C14-AF XLT) with a diameter of 14 inches (355.6 mm) is used for the observation of PSCs at ranges between 10 and 30 km, while a smaller 6-inch (152.4 mm) telescope (Celestron model C6S GT XLT) is used to observe tropospheric clouds. The larger receiver is coupled to an optical box where the optical signal is separated into different components and detected by miniaturized photomultipliers (Hamamatsu models H6780-20, H5783P, and H10721P-210) for the optical wavelengths or by an avalanche photodiode (APD, EG&G, Perkin-Elmer model SPCM-AQR-14) for the infrared signal. Details of the optical system can be found in [12]. The most important lidar specifications are displayed in Table 1. The lidar has a bistatic configuration, with a distance of about 30 cm between the laser emission and the axes of both telescopes (see Figure 3). The laser emission is directed vertically with a piezo-controlled mirror with two axes of freedom, while the main telescope is static. The smaller telescope has a computer-controlled mechanical angular movement, which allows it to vary its axis with respect to the laser emission. Figure 3: The bistatic configuration is schematically shown. The laser emission is directed vertically with a piezo-controlled mirror with two axes of freedom, while the main telescope is static. The smaller telescope has a two-dimensional computer-controlled mechanical movement, which allows it to vary its axis with respect to the laser emission. The output of the optical detectors is fed into a photon-counting system (ALA Systems, model APC26 main) with ten available channels, offering some redundancy for the 7 detectors. The parameters of the photon-counting system, including dwell time, number of averages, and pretrigger, among others, can be remotely controlled and the lidar profiles are automatically stored on a local computer and transmitted to a dedicated server in Italy. A script on this server checks every 30 min if new data are available and provides a quick view of the raw data, allowing one to check if the lidar is functioning properly.