Activities of the European Space Agency in the Field of Optical Space Communication

The European Space Agency ESA is developing an optical inter-orbit communication system enabling a link between a low-Earth orbiting (LEO) and a geostationary spacecraft (GEO). This so-called SILEX system allows a transmission of 50 Mbps from LEO to GEO in an experimental and pre-operational mode. The system uses GaAlAs laser diodes operating at 60 mW average power and direct detection. Both terminals are in the main development phase C/D. The terminals will be launched on board the ESA satellite ARTEMIS and the French satellite SPOT 4 with nominal launch dates in 1996 and 1995, respectively. A fully operational data relay satellite DRS 1 will be launched in 1999 to complement the European data relay infrastructure. Apart from the SILEX project the Agency has investigated advanced system concepts to optimize the terminals and to fulfil the data rate requirements of future Earth observation instruments. Results of system studies and ongoing R&D activities in the areas of high-power NdYAG lasers and coherent receivers are presented.