This optical communications system (called SILEX for Semiconductor Intersatellite Link Experiment) will transmit picture data from SPOT-4 via the ESA ARTEMIS satellite (scheduled for launch in late 1999/early 2000) to a data processing centre near Toulouse, France. The advantage of this double-hop is that because ARTEMIS is in a higher, geostationary orbit, data can be relayed from SPOT-4 to the ground for much longer periods than with the system transmitting data direct to ground currently used by earth observation satellites.

SILEX will also support an experiment between ESA's ARTEMIS satellite and the OICETS spacecraft built by the Japanese Space Agency NASDA.

The concept of the SILEX system

SILEX provides a data relay link between a terminal on board ARTEMIS in geostationary orbit and a second terminal on the French SPOT-4 spacecraft. The data rate to be transmitted is 50 Mbps from SPOT-4 to ARTEMIS. The link is operated at optical frequencies at a wavelength of 800 to 860 nanometers. The light source is a solid state laser diode operated typically at 60 mW optical power. The applied modulation scheme is direct intensity modulation (on/off switching). As optical data detector a photodiode is used.

For acquisition and tracking purposes CCD (charge-coupled device) detectors are used. For link acquisition an optical beacon is provided on the ARTEMIS terminal which consists of 19 laser diode arrays, each having a nominal optical power of about 700 mW. The communication signal is split in the receiving terminal and used for data detection and for tracking purposes respectively. To concentrate the optical beam in the direction of the partner satellite the light is transmitted via a 25 cm aperture telescope providing a beamwidth of only 10 microrad (equivalent to 0.000057 degrees). In the SILEX configuration, where the distance between the two partner spacecraft is in the order of 40 000 km, the light-travel time is about 1/4 of a second. During this period the partner moves up to 2000 m whereas the beamwidth is only about 300 m, consequently requiring extremely tight beam pointing accuracies.

The SILEX mission

SILEX has an experimental and a pre-operational purpose. As part of the experiment the behaviour of the optical terminal, the short and long term stability of the various electrical, mechanical and optical elements and the communication link quality characteristics will be explored. Also, operational procedures will be optimised.

The pre-operational part of the mission consists of transmitting (via the optical link to the ground) image data from the SPOT-4 earth observation instrument. This instrument generates data at 50 Mbps, which can be received at a ground station in southern France in real time. The SILEX service will thus eliminate the need for an extended ground station network and extend the direct visibility of the spacecraft to the central ground station. SILEX will also be used by SPOT-4 to dump the content of the data memory collected during periods when the spacecraft is neither in the visibility zone of a ground station nor within reach of the ARTEMIS terminal.

SILEX has been developed by ESA with Matra Marconi Space (MMS) as prime contractor leading a team of European companies.