ASTRONOMICAL REFRACTION

Astronomical observations near the horizon have historically been made for naval navigation and for determining the time of sunset. Recent a pplications include near-horizon daytime scene simulation to support i nfrared search and track and infrared seeker studies, The ray path of the setting or rising Sun can be highly distorted near the horizon, an d it is seldom rectilinear. An accurate model of atmospheric refractio n must include the observer's altitude as well as range, frequency, an d atmospheric pressure and temperature. At infrared through ultraviole t frequencies, refractivity depends strongly on the vertical temperatu re profile, and an explicit relationship between refractivity and temp erature can be obtained. This article describes a refraction model we developed that closely reproduces the profile of the setting Sun.