Beam trajectory studies of diurnal refraction effects for a near-ground path involving numerical weather prediction and time-lapse imaging measurements

Abstract

The performance of free space optical applications depends on accurate estimation of the trajectory of optical beams through the atmosphere. In situations where a signal at the wavelength of interest is not available at the target, the propagation path of an optical beam may be predicted based on the refractive index gradient profile of the atmosphere, typically using standard models such as the 1976 US Standard Atmosphere. However, the actual refractive conditions evolve with time and the formation of features, such as inverse temperature layers and ducts, can introduce strong refractive index gradients. We present ray tracing studies involving modeling and measurements of the effects of near-ground atmospheric refraction on near-horizontal beam propagation during daytime along a desert path at the Jornada Experimental Range in Las Cruces, NM. The amplitude of the diurnal deviation of the ray trajectory of a 1550-nm source is observed in simulation results where the refractivity profile was generated from numerical weather prediction. Visible time-lapse camera measurements of diurnal differential image effects (compression/stretch) are also compared with results predicted by numerical weather modeling. Additionally, a duct-like refractivity profile occurring in the morning at the site and whose parameters are estimated from time-lapse imagery, is imposed on the US standard atmosphere and the resulting differential trajectory effects are demonstrated.