Design Considerations for Optical Inter-HAP Links

Abstract

A perspective for future high data-rate communication systems are quasi-geostationary high altitude platform (HAP) systems, where communications platforms are placed in the stratosphere at altitudes between 20 and 30 km and are kept stationary. Situated over congested urban areas they will be capable of handling high data rate traffic like a geostationary satellite. Due to the specific altitudes of these platforms, communications cross links between either two platforms are not as severely affected by Earth´s atmosphere, as near-ground links are. Hence this scenario is very well-suited for optical inter-platform communications. Nevertheless such optical links suffer from various atmospheric influences that we will discuss here. In particular this paper addresses intensity fluctuations due to atmospheric turbulence, atmospheric attenuation due to absorption and scattering, and background radiation. Platform vibrations and according mispointing and –tracking losses are regarded as well. It turns out that possible links distances are very much limited by vibrations, scintillations and background light. The terminal design is generally a trade-off between those effects. From an extensive investigation of all influence factors, hints for the design of optical cross links in HAP systems will be given in terms of suitable transmitter divergence angles and receiver field of view. Link budget calculations clearly prove the feasibility of optical cross links. However, they as well show the limitations to optical communications in this particular application.