Challenges for optical turbulence characterization and prediction at optical communication sites

editorial reviewedModelling of atmospheric optical turbulence has been of interest in astronomy for several decades, e.g. for site characterization and flexible scheduling. Nowadays, it is also considered for free-space optical communications, namely to conduct site selection and to design future optical communication systems. In this work, a general approach relying on numerical weather prediction simulations in order to perform optical turbulence prediction is presented. The approach makes use of the Weather Research and Forecasting model and raises several challenges. The latter, such as the choice of the C_n^2 models or the required temporal and spatial resolutions, are first discussed with regards to the literature. Then, optical turbulence prediction is conducted for the site of Redu, Belgium, illustrating the different challenges. These predictions are also compared with seeing measurements from a differential image motion monitor. The presented approach offers realistic seeing values that, however, do not follow rapid variations of the measured seeing. Origins of the discrepancies between measurements and predictions are to be found in the modelling of the boundary layer and motivate the use of a C_n^2 model relying on the turbulent kinetic energy. Further simulations and measurement campaigns at other optical communication sites are encouraged in order to refine some model parameters and compare statistically the prediction results.SALT

Tracking of Interaction Points for Improved Dynamic Ray Tracing

Ray tracing is a powerful tool to obtain deterministic and dynamic descriptions of communication channels. However, performing ray tracing simulations at each discrete time instant is computationally expensive. Instead, a new approach is proposed to extrapolate results obtained from a single ray tracing simulation. It relies on the geometric tracking of interaction points (i.e. reflection or diffraction points), enabling analytical or numerical predictions of the evolution of any ray identified during an initial ray tracing simulation. The performance of this new approach is studied on several canonical vehicle-to-vehicle configurations, as part of a statistical study. Focus is also given to the time horizon during which dynamic ray tracing is possible, related to the lifetime of the main rays. This time horizon can be directly estimated based on the knowledge of the geometry and its evolution. It is found to be the main parameter influencing the accuracy and the computational gain of the presented approach

Computation of Optical Refractive Index Structure Parameter from its Statistical Definition Using Radiosonde Data

Knowledge of the optical refractive index structure parameter $C_n^2$ is of interest for Free Space Optics (FSO) and ground-based optical astronomy, as it depicts the strength of the expected scintillation on the received optical waves. Focus is given here to models using meteorological quantities coming from radiosonde measurements as inputs to estimate the $C_n^2$ profile in the atmosphere. A model relying on the $C_n^2$ statistical definition is presented and applied to recent high-density radiosonde profiles at Trappes (France) and Hilo, HI (USA). It is also compared to thermosonde measurements coming from the T-REX campaign. This model enables to obtain site-specific average profiles and to identify isolated turbulent layers using only pressure and temperature measurements, paving the way for optical site selection. It offers similar performance when compared to a Tatarskii-based model inspired by the literature

Modelling of Scintillation at Radio and Optical Frequencies from Radiosonde Observations

The future of Earth-to-space communications relies on the use of higher radio-frequencies and optical frequencies. For such frequencies, tropospheric turbulence starts to have deleterious effects, leading to important phase variations and amplitude scintillation. Those effects can be described thanks to the knowledge of vertical profiles of the refractive index structure parameter Cn2. In this work, two approaches relying on radiosonde measurements are presented to obtain radio-frequency and optical Cn2. Theoretical developments highlighting the contribution of humidity to radio-frequency scintillation are presented. This contribution is also illustrated with high resolution radiosonde data at Trappes (France) and Hilo (HI, USA). Obtained Cn2 profiles are in agreement with the literature and will be validated with measurements at millimeter waves in the future

Daytime forecast of optical turbulence for optical communications

In recent years, forecast of optical turbulence has been developed for astronomical sites. It enables flexible scheduling of observations during the night, and improves the control of adaptive optics systems. Nowadays, there is also a need for optical turbulence forecast during daytime at locations considered for future satellite-to-ground optical communications, for example in cities. In this work, a numerical approach to forecast refractive index structure parameter (Cn2) profiles for optical communication sites is presented. It relies on the Weather Research and Forecasting software, with the boundary layer considered separately. Indeed, it is shown that during daytime and at non-astronomical sites, the boundary layer has a great impact on seeing conditions. This motivates the use of hybrid Cn2 profiles, separating the free atmosphere from the surface layer. This approach is applied to Tenerife, Spain, where the European Space Agency has already built an optical ground station. Measurements of integrated parameters (seeing) are available at this location, and are compared with the forecast integrated parameters. The current limitations of the model can thereby be highlighted

Challenges for optical turbulence characterization and prediction at optical communication sites

Modelling of atmospheric optical turbulence has been of interest in astronomy for several decades, e.g., for site characterization and flexible scheduling. Nowadays, it is also considered for free-space optical communications, namely to conduct site selection and to design future optical communication systems. In this work, a general approach relying on numerical weather prediction simulations in order to perform optical turbulence prediction is presented. The approach makes use of the Weather Research and Forecasting model and raises several challenges. The latter, such as the choice of the Cn2 models or the required temporal and spatial resolutions, are first discussed with regard to the literature. Then, optical turbulence prediction is conducted for the site of Redu, Belgium, illustrating the different challenges. These predictions are also compared with seeing measurements from a differential image motion monitor. The presented approach offers realistic seeing values that, however, do not follow rapid variations of the measured seeing. The origins of the discrepancies between measurements and predictions are to be found in the modelling of the boundary layer and motivate the use of a Cn2 model relying on the turbulent kinetic energy. Further simulations and measurement campaigns at other optical communication sites are encouraged in order to refine some model parameters and compare statistically the prediction results

Continuous daytime and nighttime forecast of atmospheric optical turbulence from numerical weather prediction models

Future satellite-to-ground optical communication systems will benefit from accurate forecasts of atmospheric optical turbulence; namely for site selection, for the routing and the operation of optical links, and for the design of optical communication terminals. This work presents a numerical approach based on the Weather Research and Forecasting software that enables continuous forecast of the refractive index structure parameter, 2, vertical profiles. Two different 2 models are presented and compared. One is based on monitoring the turbulent kinetic energy, while the other is a hybrid model using the Tatarskii equation to depict the free atmosphere region, and the Monin-Obukhov similarity theory for describing the boundary layer. The validity of both models is assessed by using thermosonde measurements from the Terrain-induced Rotor Experiment campaign, and from day and night measurements of the coherence length collected during a six-day campaign at Paranal observatory by a Shack-Hartmann Image Motion Monitor. The novelty of this work is the ability of the presented approach to continuously predict optical turbulence both during daytime and nighttime, and its validation with measurements in day and night conditions