System requirements for optical HAP-satellite links

Between a high-altitude platform (HAP) in the stratosphere and a satellite, high-data-rate optical links are conceivable but the challenges are still not clearly identified. We thus review the main requirements to properly establish such links. The design of a HAP-LEO link is in several aspects different from that of a HAP-GEO link. Among others, we consider the possible limitations caused by atmospheric propagation, the Sun and the environment of the HAP terminal. Signal transmission appears markedly more difficult when the optical beam must propagate near the HAP horizon or below.