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Satellite laser link

10 years since Airbus Defence and Space achieved the first laser communication between two satellites

On 30 November 2001 the Silex (Semiconductor-laser Inter-satellite Link Experiment) system made its first real-time transmission of images acquired by SPOT 4's instruments, via Artemis, an ESA relay satellite in geostationary orbit (GEO), down to ground stations. It was the culmination of more than 15 years of experience logged by Airbus Defence and Space in space optical communications.

© CNES 2001 - Distribution Spot Image

© CNES 2001 - Distribution Spot Image

The Silex C/D programme was started in 1989 by the European Space Agency (ESA), in cooperation with the French space agency CNES under Airbus Defence and Space prime contractorship. The system comprised two optical terminals, Pastel and Opale. The Pastel terminal was installed on SPOT 4 and launched into low Earth orbit (LEO) at about 800km in March 1998. The Opale terminal was installed on the Artemis satellite, which was launched in July 2001.

After the launch of SPOT 4, Pastel was tested by an in-orbit star pointing procedure before starting operation. The complete laser link was then tested following the launch of Artemis. Once fully operational, Silex was able to transmit high-definition video images in real time at 50Mbps from the SPOT 4 satellite to the ground.

The main advantage of optical technology over conventional RF links is that it allows the use of smaller antennas, and there are no regulatory constraints. This translates into a significant reduction in mass, power and volume to be boosted into orbit, especially for high data rates, and with minimum risk of interference with other communications systems.

The Silex system was the precursor to the European Data Relay System (EDRS) which will use TESAT laser technology (1.06µ) and for which Airbus Defence and Space signed the prime contract agreement with ESA in September this year. During the last ten years, Silex Geo terminal (still operational) has also been used in the frame of the LOLA programme for F MoD to successfully demonstrate the capability to perform Geo- Airplanes/UAVs bidirectional data links above 6000 m.


Silex technology

The Silex communications system is based on the use of semiconductor laser diodes at 0.8µ with direct detection techniques and acquisition using a beacon spiral scanning and a large field of view detector .

Inter-orbit communications link features:
. data rate (LEO --> GEO): 50 Mbps (GEO--> LEO capability: 2 Mbits/s)
. 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 was only feasible because of the extremely high antenna gain possible with optical frequencies (telescope diameter = 25 cm) . This in turn entailed the use of very narrow beams, with divergence of no more than 2 arc seconds.