FIRST EVER LASER COMMUNICATION IN THE WORLD BETWEEN SATELLITES SPOT 4 AND ARTEMIS

On November 20th at 16 h 57 UT, an optical communication between European satellites was established for the first time ever.

The observation satellite SPOT 4 in low Earth orbit at 800 km has performed a real time transmission towards the ground through the relay of the telecommunication satellite ARTEMIS positioned in a parking orbit at 31.000 km.

The SILEX (Semiconducor Intersatellite Link Experiment) system of optical high speed (50 Mbits/s) data transmission by laser link has demonstrated its performance (less than 1 bit error in a billion). Four links were established of which two lasted 20 minutes. This connection requires first an acquisition phase through a laser beacon sent by ARTEMIS and received by SPOT 4 before tracking. Pointing precision is superior than one microradian - an exceptional result given that the two satellites were moving at relative speeds of 7 kilometers per second and separated by more than 30 000 km, necessitating pointing ahead of the target. These data links established between satellites at the first attempt are confirmation of a first successful test between Artemis and an optical ground station placed in the Canaries.

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 working with more than twenty European subcontractors. This system comprises two optical terminals, Pastel and Opale. The Pastel terminal was installed on Spot 4 in March 1998, and the Opale terminal on the Artemis satellite launched in July 2001.

Silex will be able to transmit in real time high-definition video images at 50 Mbps from the Spot 4 satellite to the image processing center (SPOT Image), thus significantly improving Spot 4's operational availability and programmability augmenting visibility percentage to 40% of orbit time and optimising recording function.

The main advantage of optical technology over conventional RF links is that it allows the use of smaller antennas and power. 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 (LEO-->GEO : 2 Mbps capacity on OPALE) 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 mission telemetry signals.

The feasibility and performance of the Silex concept have been validated in extensive ground testing before the current in orbit validation.

CHARACTERISTICS

Terminal:

• Total mass: 150 kg Moving part: 70 kg Telescope diameter: 25 cm Power: 130 W Pointing accuracy: better than 1 arc secon