SILEX PROGRAM LASER COMMUNICATION BETWEEN ARTEMIS AND SPOT 4

The Silex (Semiconductor-laser Inter-satellite Link Experiment) system will enable real-time transmission of the images acquired by Spot 4's instruments, via Artemis, an ESA relay satellite in geostationary orbit (GEO), to ground stations.

The Silex programme was started in 1989 by the European Space Agency (ESA), in cooperation with the French space agency CNES under Astrium prime contractorship. This system comprises two optical terminals, Pastel and Opale. The Pastel terminal was installed on Spot 4 and launched into low Earth orbit (LEO) at about 800 km in March 1998. The Opale terminal is installed on the Artemis satellite, to be launched in July 2001.

Following the launch of Spot 4, Pastel has regularly been tested by an in-orbit star pointing procedure. The complete laser link will be tested in 2001, following the launch of Artemis. Once fully operational, Silex will be able to transmit high-definition video images at 50 Mbps from the Spot 4 satellite to the ground, thus significantly improving Spot 4's operational availability and programmability.

The main advantage of optical technology over conventional RF links is that it allows the use of smaller antennas and onboard systems. This translates into a significant reduction in mass and volume to be boosted into orbit, especially for high data rates, and with minimum risk of interference with other communications systems.

ADVANCED TECHNOLOGY The Silex communications system is based on the use of semiconductor laser diodes.

Inter-orbit communications link: . data rate (LEO--> GEO): 50 Mbps . average laser diode transmission power: 60 mW . laser diode transmission wavelength: 0.8 to 0.85 µm . link range: up to 45,000 km

Because of the limited power output of GaAlAs laser diodes, this type of long-range, high-rate communication link is only feasible because of the extremely high antenna gain possible with optical frequencies. This in turn means the use of very narrow beams, with divergence of no more than 2 arc seconds.

No satellite today can provide this type of directional stability, which means that beam pointing, acquisition and tracking are essential. OPERATIONAL CONCEPT The Silex system is based on the use of two terminals, requiring close coordination between the two control units in the host satellites: . optical link planning . command message programming and management . downloading of orbital data . processing and correlation of the two management telemetry signals.

The feasibility and performance of the Silex concept have been validated in extensive ground testing. CHARACTERISTICS Data rate: LEO-GEO uplink: 50 Mbps GEO-LEO downlink: 2 Mbps

Terminal: Total mass: 150 kg Moving part: 70 kg Telescope diameter: 25 cm Power: 130 W Laser diodes: 60 mW power, 0.8 to 0.86 µm Pointing accuracy: better than 1 arc second