Wavelength selection criteria and link availability due to cloud coverage statistics and attenuation affecting satellite, aerial, and downlink scenarios

The choice of wavelength is essential for the variety of different communication scenarios in the field of free space optics (FSO). Possibilities are Satellite and HAP (High Altitude Platform) Downlinks, HAP-HAP links, HAP-Satellite links and all kinds of links involving aeronautical vehicles. This paper addresses the influence of the wavelength dependent attenuation of clouds, the atmospheric transmission in the NIR and MIR and a statistical analysis of cloud coverage data for an estimation of link availability. Regarding the calculation of atmospheric transmission the free available simulation tools libRadtran and GENLN2 have been used. To identify advantageous wavelengths to increase link availability, cloud attenuation is determined by Mie scattering calculations of particle size distributions of various cloud types. Here the MIR wavelength interval between 10 μm and 12 μm has been found to give the lowest attenuation in clouds. However in most cases clouds will block the optical link. For that matter a statistical analysis of satellite based data from the European Cloud Climatology (ECC) is done to reveal favorable places with high availability in Europe. The improvement of link availability when a concept of ground station diversity is applied has also been investigated. An availability of almost 99 % is reached with four hypothetical stations in southern Europe. Further the difference between availability values of single years decreases with multiple stations.