Laser eye in the sky. (News & Notes).

Last November 30 marked a milestone for a group of European space agencies and manufacturers. The Silex communication system made its first laser link transmission of an image from one satellite to another. Credit for the achievement was due to a high degree of international cooperation as well

The Silex system consists of the European Space Agency's Opale terminal on the Artemis satellite, which operates at an altitude of 19,270 miles, and the Pastel terminal on the French Space Agency's Spot 4 satellite, which orbits 517 miles above the Earth's surface. The laser system was designed by both space agencies, the French manufacturer Astrium, and more than 20 European contractors.

The satellite-borne terminals exchanged data for high-definition images of Earth taken by Spot 4 at 50 megabits per second. The Artemis spacecraft then beamed the data to a receiving station operated by Spot Image in Toulouse in southwestern France over a conventional 20-gigahertz radio link. Spot Image sells digital imagery of the Earth to governments, universities, and private companies for use in oil exploration, agricultural and meteorological studies, municipal construction, and other activities.

The Artemis mission was controlled by a French Space Agency facility in Redu, Belgium, while the satellite's operational control center in Fucino, Italy, was run by the Italian consortium Altel, or Aliena Spazio/Telespazio.

The data relay system allows the Spot satellite more and longer opportunities to transmit imagery, rather than just when it is in the "zone of visibility" of a direct receiving station. This increased contact time between spacecraft and ground stations shortens the time between recording the images and making them available to the customer. The optically based communication system could also benefit satellite-to-satellite communication for spacecraft in low Earth orbit, for geo-stationary satellites, and for deep-space probes.

Spot 4 has been in use since May 1998, producing commercial images for Spot Image Corp. in Reston, Va., and Spot Image S.A. in Toulouse, France. Spot 4 typically uses a conventional radio link to transmit images from the satellite to receiving stations. The Silex link is still being tested.

According to Bernard Cabrieres, director of satellite and teledetection promotion at the French Space Agency, the two satellites will be functioning together by July 2002.