Refractive effects, turbulence, and the EOSTAR model

An infrared signal or a laser beam propagating along a horizontal near-surface path will encounter substantial perturbations. The fluxes of momentum and heat near the surface are relatively large, and these in turn cause large changes in the propagated intensity, direction, and coherence. It is important to be able to accurately model the separate effects that generate changes in a propagated beam, and it is also important to combine the different factors accurately. We will present some evidence from field experiments to demonstrate how refractivity changes on a ten-minute scale are manifested in a recorded infrared transmission signal. The EOSTAR (Electro-Optical Signal Transmission and Ranging) model is used to provide performance predictions for the experimental work. The EOSTAR model is built upon a geometrical optics approach to infrared propagation: a ray is traced through the propagation environment, and path-dependent perturbations to the signal can be determined. The primary computational tool for analysis of refractive effects in the EOSTAR model is a geometrical optics module that produces a ray-trace calculation for a given refractive environment. Based on the vertical profiles of temperature, humidity, refractive index structure parameter, and the calculated ray trajectories, EOSTAR calculates the path-integrated and spectrally-resolved transmission, background-radiation and path-radiation, as well as the scintillation and blur for a point source at any range and height position.