## 1. Introduction

[2] Modern satellite communication systems employ frequencies above 10GHz, thus allowing for higher antenna directivities and larger communication capacities. On the other hand, in this band of frequencies, the propagation is adversely effected by rain, which is a basic limiting factor concerning the outage performance of radio link. In heavy rain climatic regions, the multiple site diversity has been introduced as an adaptive method to reduce the large rain fade margins. As a result of the spatial inhomogeneity in the precipitation medium the use of two (or more) interconnected Earth terminals properly spaced can significantly decrease the outage time or, equivalently, the required fade margins.

[3] Furthermore, the consideration of the interference effects is of current importance for the reliable design of a modern communication system. Because of the expected orbital and frequency congestion, we will adopt here as dominant source of interference the aggravation of the signal leakage from an adjacent Earth-space path operating at the same frequency due to differential rain attenuation. When examining the predictive analysis of the above statistical variable, we should point out the following: First, *Rogers et al.* [1982] have proposed a semiempirical model for the differential rain attenuation statistics at the 1% conditional probability level. Later, *Kanellopoulos et al.* [1993] have presented a more general predictive methodology, based on a model of convective raincells and the lognormal assumption for the point rainfall rate statistics. Some recent experimental results [*Matricciani and Mauri*, 1996; *Matricciani*, 1997] have shown the importance of the problem and the necessity of the interference analysis on the Earth-space systems. In a next step, the analysis has been extended to include double site interfered systems [*Kanellopoulos and Ventouras*, 1996]. As noted previously, this is an important subject for Earth-space stations located in regions characterized by heavy rain climatic conditions, where the use of the double site diversity scheme seems to be inevitable. One of the fundamental assumptions of the above analysis concerns Crane's simplified consideration for the vertical variation of the rainfall structure [*Crane*, 1978, 1980]. According to this consideration, a uniform rain structure from the ground up to a constant mean seasonal rain height H is being assumed. But this is a simplification and the more realistic model for the determination of the rain height has been experimentally predicted to take into account the correlation between surface rain rate and rain rate height H. For this reason, *Stutzman and Dishman* [1982] have presented a more realistic model for the effective rain height, consisting of using the constant level for low rainrates and adding a rainrate dependent term for higher rainrates. This model has been employed by the same authors [*Stutzman and Dishman*, 1982] for the prediction of the slant path rain attenuation statistics. As a following step, a motivation exists for the proper modification of the differential rain attenuation predictive analysis to take into account the more realistic model description of the rain height. Most recently, *Kanellopoulos and Margetis* [1997] have proposed a modified method, valid for the single-site interfered systems. The more complicated, but inevitable for heavy rain climatic regions, double site diversity is examined here. Also, there is a preference here for using the *Stutzman and Dishman* [1982] formulation for the rain height, instead of the corresponding one suggested by the *International Radio Consultative Committee* (*CCIR*) [1990], because the former is more consistent with the proposed calculation procedure for determining the differential rain attenuation statistics. Because of the complicated nature of the expressions that are derived, some simple pocket calculator regression-derived formulas, appropriate for use from the system designer, have been developed and concern both existing and modified results. Numerical results presented at the end indicate the influence of the more realistic assumptions for the rain height for various availabilities and frequencies.