ESA world first in laser satellite data transmission
The European space agency (ESA) set a world precedent on the night of 21-22 November when it established a data link between satellites using a laser beam to carry signals. The SILEX system on board ESA's Artemis satellite was used to provide an optical data transmission link with the French space agency CNES's Earth observation satellite SPOT 4 SPOT 4 is orbiting the Earth at an altitude of 832 km while Artemis is temporarily in a parking orbit at 31,000 km. Whenever the two satellites are in line of sight, images taken by SPOT 4 can be transmitted in real time via Artemis to the Spot Image processing centre in Toulouse, France, using the data laser link. Without the Artemis relay, the images are stored in SPOT 4's memory and downloaded to the ground stations in batches. During the laser link experiment, the link was established four times during the course of four successive SPOT 4 orbits, during which it was maintained for a pre-programmed period of between four and 20 minutes. When the two satellites were in communication, test data was transmitted from SPOT 4 at a rate of 50 million bits per second with a high degree of accuracy.