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Optical Data Links are a very promising Technology for downlinking data from stratospheric platforms to ground or to satellite relais, as well as interconnecting these towards a network: The optical signal does not interfere with any radio communication on ground or from the HAP user links itself, and the free-space loss of such laser links is minimal compared to its RF counterparts. Still, the communication is very secure by physical nature, and even allows the application of QKD principles based on quantum effects. The impact of the atmospheric channel needs to be evaluated and, if necessary, counteracted. Dependent on the link scenario, the atmosphere can influence the optical signal by its index-of-refraction trubulence, leading to fading-like effects. Furthermore clouds can in certain link geometries cause outages which can be compensated by site- and link-diversity. DLR-IKN has evaluated the atmospheric impact both analytically and by experiments, and according mitigation techniques (like physical diversity, Forward Error Correction /FEC, as well as adaptive data repetition techniques) were developed such as high speed modems with FEC and interleaving, as well as ARQ techniques, and also physical diversity and receiver sensitivity slope techniques. German Aerospace Center's Institute of Communications and Navigation sees a great chance for the application of optical links with HAPS in certain application scenarios - either of commercial or institutional nature - and fosters cooperation based on our technology developments. |
