Estimation of optical attenuation in reduced visibility conditions in different environments across free space optics link

Low-visibility events detected in the city of Milan (Italy) and in the mountainous site of Mileovka (Czech Republic) during several months are analysed to compare optical attenuation measurements collected by a laser link and by a visibility sensor. General guidelines for data processing and automatic event identification are given. In Milan, the visibility sensor is a valuable support to estimate link attenuation during moderate radiation fogs or when it is raining, provided data are averaged in time. In Mileovka, the propagation conditions are challenging and the effect of the inhomogeneous distribution of fog (cloud) limit the usefulness of visibility measurements. In the case of snowfalls, that can produce high-attenuation values, the agreement between the two measurements is poor.