Abstract:
Diurnal variations observed on satellite beacon experiments were often ascribed to changes in satellite antenna beam pointing due to the satellite not being in a truly geostationary orbit, or to variations in pointing due to diurnal heating effects on the satellite antenna. However, experiments in Papua New Guinea which used co-located radiometer and satellite beacon receivers, detected diurnal and seasonal variations in the received satellite beacon signal level during clear sky that suggested the atmosphere played a role in the signal variation. These effects were termed as atmospheric tides, and could significantly affect the fade margin calculations in low margin systems. Additional evidence supporting the existence of atmospheric tides were found in several time series satellite propagation data collected from sites in North America and in Brazil. This showed that the atmospheric tides phenomenon also affects communications satellite links to temperate regions as well. Factors that contribute to the existence of atmospheric tides as well as procedures that can help limit these effects in measuring clear-sky mean levels were proposed. The result of this research presents evidence that a 24 hour diurnal variations seen is due to atmospheric effects, and not due to satellite payload or orbital instabilities. It is recommended that the seasonal and diurnal variation of the mean clear-sky level is considered in determining link budget for low margin systems, and the sinusoidal nature of the variation augment the function of radiometers in evaluating the true rain fade level.
