Semi-active conformal array for ESA's GAIA mission

GAIA is one of the cornerstone space science missions of the European Space Agency (ESA). It is scheduled for launch in 2012, and will improve the accuracy in the knowledge of the position, velocity, and magnitude of stars provided by its predecessor, Hipparcos. The dynamics of the GAIA spacecraft in orbit, together with the high volume of scientific data to downlink to Earth, impose very demanding requirements on the design of the antenna for the science telemetry link. Additionally, the antenna must not make use of any sort of moving parts, in order to avoid any perturbation to the attitude of the platform that would degrade the scientific output. The selected baseline concept for the GAIA science and telemetry link antenna is a conformal phased array, with radiating elements located on the surface of a truncated cone. This antenna makes use of a semi-active excitation scheme, so that a smooth steering of the beam is guaranteed, avoiding any risk of losing the receiver lock on the ground station while transmitting. Feeding the subarrays through Butler matrices allows for the amplifiers to work at a constant operating point, where they are most efficient; it also insures a graceful performance degradation, should any of the amplifiers fail. The work reported on in this paper is a result of various contracts awarded by ESA to Alcatel Space, within the framework of the Agency's technological research program. These contracts have provided the framework for the design, manufacturing and testing of the antenna, in order to guarantee that all the required technology will be timely available for the antenna flight model. A partial elegant breadboard (EBB) of the antenna has been manufactured, allowing representative radiation tests. The predicted performance of the complete antenna, based on the partial EBB measurements, is fully compliant to specifications. Additionally, some environmental tests have also been carried out to verify the performance of the antenna in the predicted space flight environment.

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.