An autonomous and passive daytime air-turbulence monitor for FSOC link optimization (Invited Paper)

In the advent of Free-Space Optical Communications, laser links between feeder stations and satellites are foreseen to surpass radiofrequency (RF) in terms on low-latency, high bandwidth and reliable security thanks to a narrow angle of emission and possibility en encrypt communications with quantum keys. Because Earth’s atmosphere is subject to wind and temperature gradients, turbulence skews laser beams over divergent and random paths, eventually corrected with Adaptive Optics (AO). Demonstrators have shown need for gateway selection optimization based on turbulence monitoring, to reduce onboard telescope re-pointing manoeuvrers, link-power budget, data errors correction and handover times. To assess day-time atmosphere optical quality, Miratlas has developed an autonomous and passive daytime turbulence monitor based on sunlight scintillation. This so-called SHAdow BAnd Ranger (SHABAR) was designed with remote-site prospective analysis requirements as well as operational feeder station monitor. Design of this small footprint, durable and low-cost device is presented, along a preliminary result campaign obtained at various locations. Power-spectral densities and autocorrelations of sunlight scintillation showed a clear effect of lower atmosphere wind gusts, while higher-layers also produced low-frequency scintillation. Air refractive-index structure parameter, C²_N (h) obtained from such scintillation measurements is presented. A Machine-Learning algorithm, fed by numerous environmental sensors embedded in our Integrated Sky Monitor (ISM) is foreseen to offer short-time turbulence prediction. Further experimental campaigns at reference site with calibrated instruments is expected later this year for commissioning of our turbulence profiler.