Angular dependence of low divergence optical beams propagated in the marine atmospheric surface layer

Low divergence optical beams propagating through the atmospheric surface layer (ASL) are significantly impacted by turbulence effects. A primary challenge for free-space optical communications and directed energy systems is variability in pointing requirements. Angle-of-arrival (AoA) measurements of a low divergence optical beam that is propagated across a 16 km path across the Chesapeake Bay, taken over a 1-month period, reveal a pattern of consistent, predictable AoA under unstable meteorological conditions that transition to larger and more highly variable AoA under neutral and stable conditions. It is shown that, for a given refractive index structure parameter (Cn2), in stable conditions, the expected AoA can vary by up to 1 mrad, compared to less than 0.1 mrad variation in unstable conditions. Meteorological data on the Chesapeake Bay propagation path are input to the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM), and beams are ray-traced through the resulting simulated refractivity profiles, with account being taken for earth curvature. The resulting AoA are evaluated and show the same angular range as the observations but, while ducting conditions are predicted for highly stable conditions, the model does not explain the instability in pointing seen at the onset of stable conditions. The strong correlation between air and water temperature difference (AWTD) and stability conditions in maritime environments provides a practical use for indicating the expected accuracy of beam pointing in the field.