## 4 Discussion  
Current systems [9; 75] used for atmospheric monitoring of IACTs use a single-line elastic LIDAR with 25 \(\upmu\)J power (250 Hz PRF) or a system of two elastic lines with \(O(100)\) mJ laser power and 10 Hz PRF. Both systems rely on a 60 cm primary mirror with no Raman line detection capabilities, and both versions achieve maximum ranges beyond 20 km; however, only the LIDAR of Fruck et al. [9] is steerable. While the version based on two elastic lines of Bregeon et al. [75] achieves measurements of Angstrom exponents with a resolution of \(\sim\)0.3 for periods affected by biomass burning, neither of the two systems has been capable of retrieving LIDAR ratios on their own. These limitations have led to excess fluctuations in the reconstruction of gamma-ray fluxes from the Crab Nebula (used as reference standard candle) in excess of 10% for LIDAR-corrected data (see Figure 5 of Devin et al. [76]), incompatible with the CTAO requirements. The system developed by Fruck et al. [9], evaluated on Crab Nebula data [13], achieves 8% systematic uncertainty for the VAOD of the ground layer, still beyond the requirements of CTAO and limited by the missing wavelength dependence of the aerosol extinction and the lack of precise measurements of the LIDAR ratio. The considerably worse performance of the MAGIC LIDAR on clouds is due to the LIDAR's pointing offset with respect to the telescopes and will be solved for CTAO by the introduction of an accompanying stellar photometer [7; 15]. In addition to LIDARs fully incorporated in IACT data analysis, the INFN Raman LIDAR (IRL) has been taking data at the ORM since 2018 for site characterization purposes. This system operates with a single laser line of 355 nm only (6 mJ pulse energy, 100 Hz PRF), without steering capabilities [77]. It has proven to retrieve LIDAR ratios above \(\gtrsim\)1 km height [78] but no Angstrom exponents. Perhaps the solution of a LIDAR system that comes closest to the BRL needs and is used for aerosol and cloud monitoring for a major astroparticle physics observatory is the LIDAR system of the Pierre Auger Observatory (PAO) [66]. Interestingly, the PAO LIDAR was built upon a recycled steerable mirror system [79], such as the pBRL, consisting of three 80 cm parabolic mirrors with parallel optical axes and a large FoV (\(f/\#=0.5\)). Together with a 100 \(\upmu\)J pulse energy, 333 Hz PRF laser operating at 351 nm and only one elastic line, the PAO LIDAR covers ranges from 750 m to 25 km. Additional Raman capabilities had been added through a separate Raman LIDAR (operating at 355 nm, 10 mJ pulse energy,  
Figure 28: Logarithms of range-corrected signals \(\mathcal{S}\) in the two elastic channels at 355 nm (pink) and 532 nm (green) and one Raman channel at 387 nm (blue) for the vertically pointing LIDAR. The profile has been taken in 50 s (\(N_{sh}=500\) shots, at 10 Hz repetition rate). 20 Hz PRF and using a single 50 cm mirror), which reaches \(\sim\)7 km height. The performance of the pBRL comes close to that of the two PAO LIDARs combined, with the benefit of comprising both elastic and Raman lines in one single system. We briefly mention that the ORM had also been characterized by another system capable of Raman line detection at 607 nm [17]. Nonetheless, the signal-to-noise ratios were too low in the Raman channel for a useful application of the Raman inversion to the data taken for the site characterization of the ORM (see Section 2.1 of [17]). Uncertainties on the elastic line VAODs were provided, but are published with three decimals, an unrealistically good accuracy. A detailed and systematic comparison of quasi-simultaneous data taken by the photometer of Ebr et al. [7], the AERONET sun photometer data [80] taken at the ORM, the MAGIC LIDAR [9], and the IRL [77] is beyond the scope of this paper, but is foreseen for a future publication. The pBRL described in this work and in Ballester et al.