## 3 Experimental Setup  
The experimental setup thatwas used here is a terrestrial horizontal FSO link that was installed in the Piraeus port in Greece, connecting a building of the Hellenic NavalAcademy and the lighthouse of the island of Psyttaleia. The link appears in Figure 1, while the corresponding technical characteristics were adjusted to take on the values presented in Table 1.  
In order to achieve precise alignment between the FSO transceivers during the installation, the scopes of the devices were used in order to receive the maximum power at each side. The received optical signal was transformed into voltage signal that was proportional to the square root of the optical power, see Appendix A. This voltage represented the received signal strength indicator (RSSI) and it could be monitored through the software of the FSO device. The values of RSSI and the time were recorded and saved in the internal storage of the FSO head, every thirty seconds. Following the procedure that appeared in Appendix A, the RSSI values could be transformed into optical power measurements. This accurate transformation is necessary as the transmitted power cannot easily transform to the RSSI units. Taking into account the transmitted and the received optical power, the attenuation factor could be evaluated from Equation (4).  
The RSSI values strongly depend on weather and atmospheric conditions. Therefore, by installing a precise meteorological station close to the FSO transceivers, various meteorological parameters were measured, with temperature, relative humidity, and wind speed being the ones that most affect the laser beam propagation [4, 13, 14, 18, 19]. Another factor  
\begin{table}
\begin{tabular}{l c} \hline \hline \multicolumn{1}{c}{**Parameter**} & **Value** \\ \hline Distance & 2940 m \\ \hline Height & 35 m \\ \hline Bit Error Rate & Less than \(10^{-12}\) (unfaded) \\ \hline Wavelength & 850 nm (total frequency range between 830 nm and 860 nm) \\ \hline Bit Rate & Up to 155 Mbps \\ \hline Output Power & 3 laser beams with total power 150 mW \\ \hline Total Power Consumption & 22 W \\ \hline Beam Divergence & 2 mrad \\ \hline Transmitter aperture diameter & 5 cm \\ \hline Receiver aperture diameter & 20 cm \\ \hline Receiver Field of View (FOV) & 2 mrad \\ \hline Sensitivity & \(-46\) dBm \\ \hline Eye Safety Class & 1M \\ \hline \hline \end{tabular}
\end{table}
Table 1: FSO system specifications.  
Figure 1: Overview of the link.  
that affects the attenuation is the link's operational wavelength which was invariable in the experimentsin this work.  
The experimental measurements were collected continuously for more than a six-months time-period. However, in order to decrease the degrees of freedom of the specific problem that we studied and to obtain accurate results, we tried to examine cases where the influence of the sunlight radiation could be assumed to be negligible. Thus, in this work, we used the data that corresponded to time-periods between 8:00 p.m. to 5:00 a.m. For the same reason, measurements with nonzero precipitations were also excluded. Specifically, the experimental data that were used in this work are the ones that met the criteria that appeared in Table 2.