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Improved atmospheric characterization for free-space link analysis using numerical weather prediction

The atmosphere distorts and degrades Radio Frequency (RF) and Free-Space Optical (FSO) communications signals. Clouds, precipitation, turbulence, and inhomogeneities in atmospheric temperature and moisture all have the potential to disrupt communications through the atmosphere. However, there are strategies that can be employed to mitigate atmospheric impacts on communications networks such as the Free-space Optical Experimental Network Experiment (FOENEX). These strategies require an accurate characterization of the atmosphere through which the communications links travel. Atmospheric measurements provided by local instrumentation are valuable for link characterization, but provide an incomplete picture of the atmosphere. During the FOENEX demonstrations, these in situ measurements were supplemented with Numerical Weather Prediction (NWP) simulations, which provided weather forecasts for experiment planning, as well as time-varying, three-dimensional characterizations of the atmosphere. Forecast decision aids, derived from NWP ensemble forecasts, were provided several times daily to support experiment planning. Additionally, the Weather Research and Forecasting (WRF) NWP model was used to simulate the atmospheric conditions over the FOENEX test regions during the flights, and provide the high-resolution horizontal and vertical structure of the temperature, winds, moisture, and turbulence for the domain. WRF results were dictated by model inputs that included daily weather conditions, terrain, and land usage. The standard WRF model was modified to calculate the refractive index structure function, Cn2, directly from the standard NWP model parameters. This has proven to be a valuable tool for link characterization, since WRF can identify thin relatively layers of optical turbulence that are not represented by standard empirical Cn2 profiles.