Free-space optical communications (FSOC) are very sensitive to atmospheric turbulence as they induce local variations of the refractive index and alter the propagation of light. The SHAdow BAnd Ranger (SHABAR) uses solar scintillation to estimate a profile of refractive index structure constant, which can be integrated into other turbulence metrics like Fried parameter, isoplanatic angle and coherence time.
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, C2N (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.
The optical turbulence in the Earth’s atmosphere is a major limitation to free-space optical communications. It is therefore critical that we are able to model and forecast realistic atmospheric optical turbulence conditions for site selection, instrument development, instrument performance validation and network switching. Here, we present global maps of optical turbulence strength and associated parameters (Fried parameter, isoplanatic angle, coherence time and Rytov variance), from a turbulence forecasting tool. These maps can be used by the community to understand the expected performance of free-space optical systems anywhere in the world, day and night. These maps also demonstrate that optical turbulence can be modelled and visualised in the same manner as other aspects of the Earth’s weather system such as wind, rain or temperature, opening the door for more advanced turbulence forecasting functionality. We show global averages, examples of temporal sequences and more detailed analysis from some example sites.
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, C2N (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.
The optical turbulence in the Earth’s atmosphere is detrimental to ground-based optical astronomy. it is therefore critical that we are able to model and forecast realistic atmospheric optical turbulence conditions for observatory site selection, instrument development, instrument performance validation and queue scheduling, to maximise observatory scientific output.
Here, we present global maps of optical turbulence strength and associated parameters (Fried parameter, isoplanatic angle, coherence time and Rytov variance / scintillation index), from a turbulence forecasting tool.
These maps can be used by the community to understand the expected performance of optical systems anywhere in the world, day and night. These maps also demonstrate that optical turbulence can be modelled and visualised in the same manner as other aspects of the Earth’s weather system such as wind, rain or temperature, opening the door for more advanced turbulence forecasting functionality.
Many applications such as ground to satellite optical communications or astronomy require precise knowledge of cloud cover, turbulence and absorption. In the case of telecoms, this data is critical for the initial ground station sites survey; during ground station operation to inform link availability and bandwidth; and finally, to predict atmospheric conditions over different ground stations for network planning. Historically the turbulence by night time has been measured by astronomers with research class solutions installed on observatories sites. Many implementations exist using either the moon or the stars as reference target. One of them is the Differential Image Motion Monitor (DIMM) from M. Sarazin and F. Roddier with the first implementations back in the 80’s for the ESO. All these turbulence monitors have in common the integration of a small telescope in the 20 to 40cm aperture range with various aperture masks on an automatic tracking mount within a protective dome. This form factor and cost is not in line with the requirement of a more industrial utilization as expected by telecom operators or for atmospheric studies. Since 2018, Miratlas has been using a simpler implementation of the image motion monitor (NSM) with a fixed outdoor system using a single aperture aiming at Polaris. Nevertheless this single aperture system requires a very stable fixture which is not always available and doesn’t apply in southern hemisphere. Therefore, Miratlas has developed a small outdoor implementation of a legacy DIMM named the C(ompact)-DIMM. It uses two different optical assemblies and two identical synchronized cameras to fulfil the same features. The C-DIMM is small enough to be installed anywhere, is not sensitive to vibration and therefore can be installed either on a fixed mount aiming at Polaris, or on a small outdoor tracking mount to operate on any sufficiently bright star and therefore under any latitude.
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