Radically new approach for large antenna in space: the 'RF prism' concept

摘要

RF prism is a kind of space-borne antenna lens with an illuminator that is not at the end of a boom but on a separate satellite, usually located on the same orbit. This lens deviates the waves towards or away from the Earth; it also needs to amplify and sometimes to change the frequencies. Provided that the illuminating axis and the Earth-sighting axis have incidences on the antenna plane that meet a specific relation, the antenna flatness requirement is relaxed by a factor of 10 at least. Prism therefore eases the deployment of very large antennas in space (20 or 50 m). In the option where the antenna deformation is known, which can be achieved by measuring the illuminating signals, and corrected by phase shifters on the antenna, the relaxation applies to the accuracy of the deformation knowledge and the deformation is no longer a constraint. The antenna panels do not need to be fully joined together and locked in the same plane, which enables two-axis deployment and very compact folding of the antenna. As the flatness constraint intervenes above all on an interpanel scale, the coupling between sides of the prism is typically on a single point per panel with a small rear radiator. The illuminator satellite is kept small since it is not constrained by the illumination function but just by the central payload function moved away from the main satellite. A cluster of illuminators is reproduced transparently into an equivalent cluster of beams, with the cluster apertures modified according to the ratio between the frequencies on illumination side and Earth side. With a high illuminating frequency, the illuminating cluster can be obtained within a single satellite with booms of 1 to 3 m. The illuminators can therefore ensure a close beam pattern while the multi-beam function of the antenna is used to multiply this pattern to provide a broad mosaicing. The internal complexity of the antenna is therefore reduced by a factor equal to the number of illuminators. The concept can be implemented with current technology although it would also benefit from the permanent progress made in active antennas (reduction in mass and thickness).