Atmospheric limitations on satellite communications at frequencies exceeding 15 GHz
Interest in use of frequencies exceeding 15 GHz for satellite communications is steadily increasing, and indeed it is planned to share frequency bands between satellite and terrestrial systems. It is advantageous to consider use of two polarizations, thereby increasing the information capacity by perhaps a factor of two. However, fundamental limitations are imposed on such systems by effects of rain: attenuation, scattering, and depolarization. The present status of measurement and theory of these effects in the 15 to 100 GHz range is discussed. From terrestrial measurements, attenuation is known to depend on polarization; both this dependence and depolarization of the waves are caused by the oblateness of the raindrops. The role that path diversity can play in improving the reliability of satellite systems is evaluated from measured data. The techniques involved in measuring at the various frequencies are discussed as time permits. The use of radiometer measurements of noise from the rain for determining the attenuation is discussed in some detail. Since the measuring equipments are required to operate continuously in order to produce reliable statistics, advantage of solid-state technology is taken where possible.