I Introduction
The performance requirements imposed on the antennas dedicated to science data links of present and future space science observatories are becoming ever more demanding. The science telemetry link antenna for GAIA is a typical example. The flight dynamics of the platform, dictated by the observation strategy (type of orbit. distance from the earth, and platform self-rotation) induces unusual requirements on the antenna angular coverage. Additionally, as the amount of science data continuously acquired by the payload instruments increases, so do the requirements on antenna equivalent isotropic radiated power and transmission data rate. This aspect is worsened by the limited temporal window available for transmission (set by the network of ground stations available to the mission). usually of only 6 hours a day, during which the observations are not to be interrupted. Finally, the extremely fine pointing accuracy demanded by the payload, key to proper scientific output, excludes the selection of antennas with any moving parts, which could disturb the attitude of the platform.