Diversity systems are foreseen for earth to satellite links operating at frequencies above 10 GHz in localities with high rain-induced attenuation. The paper discusses orbital diversity, which uses two satellites and an earth receiving site, through an analytical model for the rainfall random process. The model uses rain gauge measurements and radar-derived information on the slant structure; it gives results that are similar to experimental data and physically meaningful. A comparison is also carried out with corresponding site diversity, and the procedure for computing both attenuation statistics is given. It is shown that orbital diversity may replace site diversity as far as rain attenuation is concerned and may give the same performance up to a certain limit of geometry which depends little on attenuation. It is also shown that site diversity gain can be converted to orbital diversity gain and that the geometrical parameters (site separation and angle aperture) which yield this conversion are related by linear relationships, which are independent of frequency.