Impact of ground profile on scintillation index for high-altitude optical wireless links

Optical wireless communications is an emerging technology for various types of high-bandwidth wireless communications. Several application scenarios involving links between aircraft, high altitude platforms, unmanned aerial vehicles and others have been proposed over the years. For simplicity of the propagation models and mathematical tractability often horizontal links with homogenous propagation characteristics along the path are used for calculations of atmospheric effects. This simplification assumes a flat earth model, where the ground profile underneath the optical link is ignored. For long range links also a curved earth model is used, but again the shape of the ground below the link is ignored. For modeling atmospheric turbulence, it is quite common to use the Hufnagel-Valley model. This model is based on the height above ground and therefore it is sensitive to the ground profile. The impact of the flat earth or curved earth assumptions on the scintillation of the propagating beam is discussed within this work. It is shown that the ground profile has the strongest impact on links at lower altitudes, while for higher altitude links a curved earth model is sufficient. The flat earth model can be best used at altitudes of around 10000 m above mean sea level.