## 3 Results
Based on the obtained measurement results, the atmospheric attenuation of radiation with a wavelength of 1.5 \(\upmu\)m and 10 \(\upmu\)m was determined. The results obtained were compared with the analytical calculations carried out using the MathCAD software presented in the previous paper [38]. These calculations are based mainly on using the formula combining the transmission coefficient with the propagation length and the extinction coefficient [39] and using the relationship between absolute humidity and water sedimentation indicator [40]. The performed calculations showed that the currently used FSO systems with lasers generating radiation of about 800-900 nm and 1.5 \(\upmu\)m were sensitive to bad weather conditions, particularly fog. For the visibility of 1 km, which corresponds to light fog, the link operating with a wavelength of 10 \(\upmu\)m was characterized by the best range parameters. Therefore, the radiation with a wavelength of 10 \(\upmu\)m, therefore, has better transmission properties in the case of limited visibility (e.g., light rain and fog) than near-infrared waves. For visibility below 600 m (e.g., heavy rain), the obtained  
Figure 11: The laboratory stands for testing the FSO link outside the Military University of Technology building. Figure 10: The scheme of the measurement of the FSO link in the real conditions. attenuation, taking into account the scattering effect, is comparable for the three analyzed wavelengths. This means that in difficult weather conditions, for significantly reduced visibility, all systems will experience problems with ensuring good communication quality, and there may even be problems with establishing a connection. On the other hand, for the visibility of 1 km (fog), the attenuation coefficient of the atmosphere takes the values of 13.43 dB, 10.11 dB, and 3.98 dB for the wavelengths of 880 nm, 1.5 \(\upmu\)m, and 10 \(\upmu\)m, respectively. Also, radiation with a wavelength of 10 \(\upmu\)m is less sensitive to turbulence in the atmosphere compared to short-wave radiation. In the case of strong turbulence, for the distance between the transmitter and the receiver of 1 km, the radiation's attenuation with a length of 880 nm and 1550 nm takes similar values, i.e., about 0.9 dB. However, radiation with a wavelength of 10 \(\upmu\)m will be attenuated by 0.75 dB. The ambient temperature is one of the critical factors influencing the quality of the link's operation, the thermal radiation from buildings and the environment produces a significant signal that must be removed if possible. Such links should not be placed near air conditioners or pipes with hot liquids, as well as when the facade of the building is made of metal, as its heating may create additional turbulence. It turned out that, in the case of good visibility (3000 m), radiation with a wavelength of 10 \(\upmu\)m was characterized by attenuation similar to radiation with a wavelength of 1.5 \(\upmu\)m. As visibility decreases, the attenuation for both wavelengths increased. On the other hand, for the visibility of 800 m and a slight haze, the attenuation of radiation in the far-infrared range was approximately 10 dB/km, and near-infrared approximately15 dB/km. The obtained analytical results were comparable to the experimental data as presented in Figure 12.