Lasers link orbiting satellites

17 Jun 2002

The European Space Agency has made the world's first optical data link between two orbiting satellites.

The European Space Agency (ESA) yesterday established the world's first optical data link between two orbiting satellites using a laser beam as a signal carrier.

Data from the French Space Agency's Earth observation satellite SPOT-4 was transmitted to the Artemis satellite at 50 Mbit/s with a bit error rate less than 10^-9.

The SPOT-4 is the first satellite to carry the semiconductor laser inter-satellite link experiment (SILEX) communications terminal. These were developed by Astrium in France. Michel Fruit, optical design and engineering manager at Astrium, said: "The terminals used a 60 mW GaAs-based laser diode operating at 800-850 nm which allows for high-frequency direct modulation."

Laser-based communication has several advantages over conventional radio-frequency-based links. Radio-frequency bandwidth availability has become very scarce and optical communication provides an improved data-rate-to-power ratio and lower interference.

Gotthard Oppenhauser, Artemis project manager at ESA said: "The advanced communication satellite Artemis was specifically designed to explore the use of laser beams for high-bandwidth transmission of image data between satellites over a link distance of up to 40 000 km. It took us more than a decade to demonstrate the concept in space because of launch delays."

The main challenge in establishing an optical link between satellites is to point a very narrow beam with extreme accuracy at a spacecraft which is flying at a relative speed of about 7 km/s.

Fruit said: "The beam divergence of the laser leads to an illumination area the size of a football-field after travelling over 30 000 km. The time needed for the light to reach the other satellite requires aiming ahead of target. The SILEX terminals comprise a combination of coarse and fine pointing as well as an acquisition and tracking CCD sensors system. This exhibits an accuracy of 1 micro-rad, or 30 m at the operating distance in the experiment."

"Artemis incorporates several significant technological advances that will not only impact services and facilities on earth, but will also influence and enhance future satellite missions," said Oppenhauser.

Artemis comprises mobile, data relay, and navigation payloads. The latter forms the first orbital element in the European Geostationary Navigation Overlay System (EGNOS) - a joint programme between ESA, Eurocontrol and the European Commission to create a future European global satellite navigation system.