Variation of the Infrared Spectral Radiance near the Limb of the Earth

Abstract

The infrared spectral radiance has been calculated near the limb of the earth, using a curved refracting atmosphere. Radiance was calculated in narrow spectral intervals for four atmospheric models representing a wide range of meteorological conditions. Comparison is made of the radiance across the limb for each model atmosphere in several narrow spectral intervals; two hypothetical filters are then convoluted with spectral radiance to show the effective radiance across the limb, as it might be viewed by a horizon sensor. The results indicate that the 15-μ carbon dioxide band and the rotational vapor band are best suited for horizon sensing and that neither is decisively superior on the basis of radiance. It is also shown that radiative flux calculated using the exact radiance values near the limb is only about 1% more precise than that calculated from an extrapolation to the limb of radiance values determined at zenith angles where the plane parallel approximation is valid.