The Application of the Spectral and Spatial Characteristics of the Earth´s Infrared Horizon to Horizon Scanners

The spectral and spatial characteristics of the Earth´s infrared background are presented. It is shown that the vibration band of CO2 centered at 15 ¿ provides maximum discrimination against clouds at the tropopause. An optimum spectral bandwidth is defined and the radiance values computed as a function of scan angle for different latitudes. These data are used to produce the requirements for the optics of a typical horizon scanner system. The effect of the spatial variations in the Earth´s apparent horizon on the angular accuracy is discussed for both amplitude-dependent systems and shape-dependent systems. The calculations are shown to be in agreement with experimental results. The work of other investigators is discussed in terms of the physics of the atmosphere and the assumptions made for the different calculation procedures.