Evaluation of the impact on low-margin systems of changes in mean clear-sky level

Abstract

The rapid growth of VSATs (Very Small Aperture Terminals) using frequencies above 10GHz has highlighted the need for a precise estimation of propagation impairments that affect link quality and availability. This is particularly important for VSAT systems as they have minimal fade margins. An accurate knowledge of the mean signal level under clear-air conditions is needed to set the fade margin for adverse (i.e. rain) conditions. Conventionally, attenuation and depolarization were the key parameters used, together with data on fade duration and intervals, site diversity and fade rates. However, experiments in Papua New Guinea showed evidence of diurnal, seasonal, and annual variations in the received satellite beacon signal level during clear sky. These effects were termed as atmospheric tides, and could significantly affect fade margin calculations. This paper reports on the finding of additional evidence supporting the existence of atmospheric tides on communications satellite links to temperate regions through an investigation of time series propagation data held in the National Aeronautics and Space Administration (NASA) data bank. Changes in the mean clear sky level seen in these data are attributed to changes in the atmospheric parameters.