Atmospheric refraction and its impact on free-space optic communication systems

Atmospheric refraction bends optical beams and lets objects appear in positions they not really are. This phenomenon is very important in astronomy. However, astronomical refraction formulas can only be used if the light source is at a very far distance from the observer. Then, atmospheric refraction mainly depends on meteorological conditions on ground. In case of optical free-space communications, the distances are comparatively short and well-known formulas for astronomical refraction are no longer sufficient. An exact knowledge of the structure of the whole atmosphere is required to assess refraction in this case. Due to the complexity of the atmosphere, analytical solutions are not possible. Hence, a numerical simulation model based on spherical symmetry, atmospheric shell modeling and standard atmosphere models was used instead. Three different categories of refraction were examined: 1) the observer is situated on ground and the object is at an altitude of at least 25 km, 2) both object and observer are below 25 km and the link path is mainly vertical and 3) both object and observer are in the stratosphere and ray paths are mainly horizontal. The results presented in this work are useful for applications like laser beam pointing and satellite tracking, UAV (Unmanned Aerial Vehicle) and HAP (High Altitude Platform) downlinks or long-haul cross-links through the atmosphere, e.g. HAP-HAP or UAV-satellite.