Scintillation loss in optical low earth orbit data downlinks with avalanche photodiode receivers

Future Earth Observation satellite data downlinks via directed laser beams are a promising technique to overcome the data repatriation bottleneck in high resolution sensor missions. A challenge for this technology are received power scintillations induced by atmospheric index-of-refraction turbulence (IRT). The strength of these signal variations depends on the IRT height profile, receiver aperture size, wavelength, and link elevation. We analyze measured optical satellite downlink power vectors to verify a model for the dynamic power scintillation loss. Due to their increased sensitivity compared with PIN-receivers, Avalanche Photo Diodes (APD) are usually used within data receivers on ground. This allows bulk receivers to avoid the demanding coupling onto single-mode detectors. As their sensitivity run is defined by the influence of thermal-, and signal-dependent shot-noise, a new formulation for this dependency is evaluated, and the according modified scintillation loss is quantified. This allows appropriate dimensioning of link budgets and the necessary error control mechanisms to enable reliable high-speed data transfer from low earth orbit satellites to the ground.