TABLE OF CONTENTS In this chapter, some of the aspects of propagation to be considered at the point of installation of Free-Space Optical (FSO) links were presented (for example, atmospheric effects such as molecular and aerosol absorption, molecular and aerosol scattering, extinction, cloud attenuation, rain attenuation, refraction and scintillations). Weather phenomena bring the most important attenuation and particularly: fog, dry snow, rainstorms (generally short) and light rain (in this order of importance). The fog constitutes the most penalizing phenomenon. It occurs frequently, certain years and is slow to dissipate. Because of the intensity of dry snow, FSO equipment cannot currently be used in mountainous zones.
Currently, there is little in the form of documentation, recommendations, standards or software allowing a budget calculation of links using data related to availability and quality of service. Three approaches are to be recommended in order to obtain a model for link budget calculation following the example of radio-relay systems (UIT-R P.530-8) "Propagation data and prediction methods required for the design of terrestrial line-of-sight systems":
a more detailed analysis of the collected documents, taking as the first step Recommendation UIT-R P.530-8, in order to outline a model which would not be, initially, a very good approximation, because the validity of the suggested model will be limited by the lack of historical data,
a complete information search for the various fog densities (liquid water concentration in g/m 3) and their annual frequencies (expressed as a percentage) over a given geographical area in order...
