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.