We develop and study two approaches for the prediction of optical refraction effects in the lower atmosphere. Refraction can cause apparent displacement or distortion of targets when viewed by imaging systems or produce steering when propagating laser beams. Low-cost, time-lapse camera systems were deployed at two locations in New Mexico to measure image displacements of mountain ridge targets due to atmospheric refraction as a function of time. Measurements for selected days were compared with image displacement predictions provided by (1) a ray-tracing evaluation of numerical weather prediction data and (2) a machine learning algorithm with measured meteorological values as inputs. The model approaches are described and the target displacement prediction results for both were found to be consistent with the field imagery in overall amplitude and phase. However, short time variations in the experimental results were not captured by the predictions where sampling limitations and uncaptured localized events were factors.
The performance of free-space optical applications can be improved using beams of different wavelengths for the auxiliary actions of pointing/tracking or turbulence correction. Chromatic dispersion owing to the atmosphere is an issue for multiwavelength systems, and the dispersion of electromagnetic signals is typically predicted based on refractive conditions from standard atmospheric models. However, for long near-horizontal paths near the Earth’s surface, substantial refractive index gradients that are associated with features such as inverse temperature layers and ducts can be encountered. These features can significantly alter the ray trajectory, the chromatic divergence, and the angle of arrival of directional beams relative to standard atmosphere predictions. A ray tracing approach was implemented to examine the chromatic divergence and angle of arrival of the rays through various practical and extreme atmospheric conditions involving a temperature inversion layer. Over a distance of 150 km along the ground, a brief encounter with the layer can cause pairs of rays with wavelengths 532 and 1550 nm to diverge up to 4.5 times greater than their standard atmosphere predictions. For a single wavelength, a linear increase of angle of arrival with initial launch angle was found for the standard atmosphere, but this trend was significantly altered in the presence of an inversion layer. Extreme refractive conditions with a large inversion layer were simulated to produce optical ducting over long distances. Chromatic separation of rays as large as 280 m was observed when only one of the two wavelengths remained in the duct.
The performance of certain free space optical applications such as laser communication, LIDAR, target designation and astronomical observations may be improved by using beams of different wavelengths for the auxiliary actions of pointing/tracking or turbulence correction. Thus, wavelength dispersion in the atmosphere is a topic of concern for such applications. The chromatic effects of refraction in the atmosphere are generally well-understood and are a function of temperature, pressure, humidity and altitude, as well as the refractive index gradients. In applications such as astronomical observations, chromatic effects are typically predicted based on standard atmospheric models. However, for long horizontal or near-horizontal paths near the Earth’s surface, significant refractive index gradients can be encountered that are associated with features such as inverse temperature layers and ducts. In this study, we explore the wavelength dependence of optical propagation through these temporary and reoccurring refractive index profiles. A ray tracing approach is implemented and the chromatic divergence of the rays through an inverse temperature layer is studied and compared with the behavior expected for the standard atmosphere.