The rapid growth of satellite services using higher frequency bands such as the Ka-band has highlighted a need for estimating the combined effect of different propagation impairments. Many projected Ka-band services will use very small terminals and, for some, rain effects may only form a relatively small part of the total propagation link margin. It is therefore necessary to identify and predict the overall impact of every significant attenuating effect along any given path. A procedure for predicting the combined effect of rain attenuation and several other propagation impairments along earth-satellite paths is presented. Where accurate model exist for some phenomena, these have been incorporated into the prediction procedure. New models were developed, however, for rain attenuation, cloud attenuation, and low-angle fading to provide more overall accuracy, particularly at very low elevation angles (<10°). In the absence of a detailed knowledge of the occurrence probabilities of different impairments, an empirical approach is taken in estimating their combined effects. An evaluation of the procedure is made using slant-path attenuation data that have been collected with simultaneous beacon and radiometer measurements which allow a near complete account of different impairments. Results indicate that the rain attenuation element of the model provides the best average accuracy globally between 10 and 30 GHz and that the combined procedure gives prediction accuracies comparable to uncertainties associated with the year-to-year variability of path attenuation. Index Terms-Microwave radio propagation meteorological factors, Satellite communication. I. INTRODUCTION Rapid growth in new satellite services incorporating Very Small Aperture Terminals (VSAT) and Ultra Small Aperture Terminals (USAT) is expected in the coming years. Small size terminals allow for widespread use of satellite services in small business and domestic applications. Some of the services that can benefit from small

SUMMARYA simple, low-cost propagation terminal for measuring path attenuation and site diversity, using satellite transmissions at 12 GHz, is described. The novelty of this system is the use of a standard domestic satellite low-noise block (LNB) front-end connected directly to a spectrum analyser in place of an expensive beacon receiver. A description of the equipment, an analysis of relevant system parameters, and a discussion of the environmental conditions are presented. The method used to obtain a clear-sky baseline reference and a description of the channel signal processing are given. Analysis of the rainfall rate and attenuation exceedances statistics has led to the proposal of a new model to derive the e!ective rain-cell diameters and e!ective rain-column heights in tropical areas for attenuation predictions in satellite communication link budget calculations. Site-diversity statistics are also examined. Though the main site is only 6.5 km inland from the second coastal site, there are marked di!erences in their rain-cell characteristics. The signi"cance of volume saturation of rain cells on modelling, the operating margins of the measuring system, and the relevance of the site-diversity data to enhancing the understanding of the proposed volume saturation model are discussed.

SUMMARYTropospheric propagation impairments that a!ect earth}space communication signals increase in severity with the increase of frequency. Introduction of satellite services using higher frequency bands such as Kaand V-band requires the characterization of propagation factors that are normally considered negligible at lower frequency bands. Cloud attenuation is considered one such factor. Clouds are present during a large fraction of an average year and cloud attenuation, together with gaseous absorption, will determine the system performance under non-rainy conditions. A cloud attenuation model that has global applicability is presented in this paper. The model is based on average properties of four cloud types and their occurrence probabilities. The occurrence probabilities are derived from long-term observations of cloud cover carried out by several thousand meteorological stations throughout the world. Predictions made with the model are compared with cloud attenuation data gathered using Ka-band beacon signals and radiometers.

The move to provide direct to home (DTH) multi‐media services from satellites requires a number of new insights to be gleaned from the propagation effects that occur along satellite‐to‐ground paths. Annual statistics will not be sufficient to describe the likely performance of the link, nor will they provide information on the likely customer acceptance of the DTH service. What are required are the likely number of outages due to rain attenuation in a given period, their duration, the time between the outages, and the diurnal characteristics of rain attenuation along the link. This paper presents results obtained in three, two‐year, Ku‐band experiments in tropical Africa on diurnal attenuation effects, as well as on rain and attenuation event duration and inter‐event duration. The implications of the results are discussed. Copyright © 2000 John Wiley & Sons, Ltd.