Forecasting of Optical Turbulence in Support of Realtime Optical Communication Systems

Optical turbulence (OT) acts to distort light in the atmosphere, degrading imagery from large astronomical and imaging telescopes and possibly reducing data quality of free space optical communication (FSOC) links. Some of the degradation due to optical turbulence can be corrected by adaptive optics. However, the severity of optical turbulence, and thus the amount of correction required, is largely dependent upon the turbulence at the location of interest. In addition, clouds, precipitation, and inhomogeneities in atmospheric temperature and moisture all have the potential to disrupt imaging and communications through the atmosphere. However, there are strategies that can be employed to mitigate the atmospheric impacts. These strategies require an accurate characterization of the atmosphere through which the communications links travel. To date these strategies have been to climatological characterize OT and its properties. Recently efforts have been developed to employ a realtime forecasting system which provides planners useful information for maintaining links and link budgets.

The strength of OT is characterized by the refractive index structure function Cn2, which in turn is used to calculate atmospheric seeing parameters. Atmospheric measurements provided by local instrumentation are valuable for link characterization, but provide an incomplete picture of the atmosphere. While attempts have been made to characterize Cn2 using empirical models, Cn2 can be calculated more directly from Numerical Weather Prediction (NWP) simulations using pressure, temperature, thermal stability, vertical wind shear, turbulent Prandtl number, and turbulence kinetic energy (TKE). During realtime FSOC demonstrations, in situ measurements are supplemented with NWP simulations, which provide near realtime characterizations and forecasts of the Cn2, the Fried Coherence Length (ro), and time-varying, three-dimensional characterizations of the atmosphere. The three dimensional Weather Research and Forecasting (WRF) model is used to produce characterizations and forecasts of OT. These forecasts are used for planning FSOC experiments in the 3-24 hour range. The WRF model is configured to run at up to 300 meters horizontal resolution over a 250km by 250km horizontal domain. The vertical resolution varies from 25 meters in the boundary layer to 500 meters in the stratosphere with over 130 vertical levels. The model top is 20 km in altitude. The model is run up to twice per day and generates forecasts out to 27 hours. The WRF model has proven to be a valuable tool for link characterization and forecasting, since it can identify thin relatively layers of optical turbulence that are not represented by standard empirically derived Cn2 profiles. Results show that WRF simulations can accurately predict upcoming turbulence events that may degrade system performance. Demonstrations of these forecasts will be shown at the conference.