Noise power radiated by the Sun and stars, Earth and man-made noise, antenna characteristics and receiver performance all affect UHF Satellite Communications (SATCOM) operation. Stationary terminals deploy high-gain, directional antennae for SATCOM operation, while mobile terminals utilize low-gain, hemispherical-coverage antennae. As a result, the mobile terminal is plagued by low link margin, due to a limited antenna coverage profile, multipath fading, and shadowing. Aircraft communications are discussed in particular. Concern about solar noise impacting UHF SATCOM operation arises from two sources, solar transit and link margin. The noise level of a UHF receiver is ultimately limited by the receive antenna temperature. A large amount of published data is available to assist the engineer in evaluating the level of antenna noise. Just because the Sun appears as a black-body radiator of 800,000 K at UHF, it cannot be blindly inserted into a link margin algorithm just as much as a quiet sky temperature of 90 K is not always appropriate. Antenna noise temperature is linked to the type of antenna used which is further linked to the mobility of the platform. Actual antenna temperature is determined by that portion of the antenna pattern that views the warm Earth or man-made noise of a city. Spatial diversity is employed on mobile platforms to combat shadowing and multipath fading, while effectively achieving hemispherical coverage.
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