Estimation of Cloud-induced Optical Attenuation over Near-Earth and Deep-space FSO Communication Systems

Free Space Optics (FSO), unlike RF systems, offers higher data rates, enabling the next generation of optical feeder links for near-Earth and deep-space communications. Ground-space optical feeder links are subject to atmospheric optical attenuation or even complete blocking by clouds. Having weighted severe multiple scattering against single scattering medium approach in terms of atmospheric clouds, a complete approximation model for the cloud-induced optical attenuation is derived. Some models based on measurement campaigns and complex Monte Carlo simulations exist, but misconceptions and lack of practical approximations limit their usability. In particular, the influence of clouds over transmitted optical beam is theoretically evaluated based on optical thickness parameter that allows distinction and provides limit between single and multiple scattering medium. The coefficient describing the scattered optical power due to clouds is expressed in terms of modified gamma Particle Size Distributions (PSD) and Mie theory. Apart from the type of atmospheric clouds, both groundspace FSO link distances consisting of near-Earth satellites and deep-space single-photon communication scenarios are taken into account. A special focus is placed on Cirrus clouds and their occurrence probability, which influence even FSO transceivers located at high-altitudes over the temperature inversion point of the troposphere.