The European Space Agency (ESA) is using three telescope flight models from Carl Zeiss for its Satellite Interlink Experiment (SILEX) space research project. The mission to provide optical data links between ESA's SPOT 4 low-flying Earth-observation satellite and the ARTEMIS geostationary satellite testing the use of optical data transmission in space. Until now, the lack of suitable transmitting and receiving facilities in the optical range had prevented optical communication in space. Researchers believe that optical data transmission will offer higher data transmission capacity, frequency-band availability and interference protection.