Prediction of rain attenuation at low-availabilities using models and data of widespread and convective rains

The design of low-availability satellite communication systems operating at the frequencies above 10 GHz requires precise prediction of rain attenuation for annual outage time percentages greater than about 0.1%. The Leitao-Watson models (see IEE Proc. F, Commun. Radar Signal Process., vol.133, no.4, p.429-40, 1986) for widespread rain and convective rain are used for the calculation of mean cumulative distributions of rain attenuation. The effect of the melting layer, which exists above the widespread rain, is included in the calculations by using the model proposed by W. Zhang et al. (see Proceedings 20th European Microwave Conference, Stuttgart, Germany, p.1235-40, 1991). The mean distributions of 20 and 30 GHz rain attenuation are predicted for Jokioinen, Finland, in three cases: (1) rainfall consists of two rain types, and the melting layer exists above the widespread rain; (2) rain is merely of the widespread type having the melting layer above it; and (3) rain is only of the convective type without the melting layer. For Graz, Austria, the cumulative distributions of cases 2 and 3 are given. The results of each case are compared mutually and with atmospheric attenuation measured by radiometers