## 2 Methods
### Pre-Process Analysis  
As introduced above, the backscattered signal is transported from the focal plane via a liquid light guide (LLG) to an optical bench and collected by PMTs (see Figure 1). The PMT signal is read with commercial Licel modules. The LUTR type TR20-12 bit delivers averaged signals from an analog (AN) and a photon-counting (PC) detection chain. Two newer versions of TR40-16 bit that were purchased also provide an average of the squared analog (AN2) signal. Due to the presence of the two readout modes and a very wide range of return power (see Figure 3), special care must be taken to maintain precision and accuracy throughout the entire range. This requires careful signal preparation, frequent sanity checks, and the use of robust statistics. We describe the procedures applied to guarantee this performance. These include several novel statistical approaches, which have not been discussed in the literature to our knowledge so far. In the following, we use the term _line_ for one of the four wavelengths of interest and _channel_ for an LUTR readout channel. We have therefore two (AN,PC) or three (plus AN2) channels per line for a given LUTR module.  
The open-access BRL Pre-Processing (LPP) software [41] converts raw data files produced by the LUTR into processed data products, primarily aerosol extinction profiles for different photon wavelengths. The LPP is divided into two components: the front-end component is in charge of providing a Graphical User Interface (GUI), allowing the expert user to perform debugging of the software and graphical analyses; the back-end component is in charge of not only implementing the functionalities directly required by the front-end but also implementing communication and interaction with the different external components involved in the complete task execution process, such as a database. The functionalities of the LPP include the following:  
Figure 5: Required time to reach a \(SNR_{\text{goal}}\geq 10\) for the 387 nm Raman line under different conditions for the pBRL with a laser power of 70 mJ at 355 nm (**left**) and an alternative solution based on a laser with lower power of only 10 mJ, but a higher repetition rate of 200 Hz (**right**). The requirement for observation times less than one minute is marked as a horizontal dashed line.  
* **LPP analysis:** The actual core of the program.
* **Graphical User Interface (GUI):** Implementation of a user-friendly interface with a finalized design using front-end technologies.
* **Logging and configuration:** A comprehensive logging system with database entries and the generation of log files. Configuration options through YAML files, including auto-update capabilities and GUI configurability.
* **Testing:** Execution of Continuous Integration (CI) tests covering GUI, HTTP interactions, database connections, and specific functionalities. In addition, ample possibilities are provided for manual testing on diverse datasets for all LPP analysis steps.
* **Molecular Density Profile (MDP):** A downloader component for molecular density profiles from the European Centre for Medium-Range Weather Forecasts (ECWMF) or the Global Forecast System (GFS) at a given location on Earth and time.
* **Licel to FITS converter:** An automatic file format converter from the Licel raw data format [42] to the FITS file format [43], used as a standard for CTAO.