Two satellites transmit data using laser light
Posted: November 23, 2001

This week, for the first time, a data link between satellites was established using a laser beam as signal carrier. On board ESA's Artemis satellite - launched last July by an Ariane 5 - is the SILEX system. This system provides an optical data transmission link with the CNES Earth observation satellite SPOT 4, which is orbiting the earth at an altitude of 832 km while Artemis is temporarily in a parking orbit at 31,000 km.

Through the laser data link, images taken by SPOT 4 can be transmitted in real time to the SPOT 4 image processing centre at Spot Image in Toulouse, France, via Artemis, thus drastically reducing the time between taking the picture and its delivery to the control centre. This is possible whenever the two satellites are in mutual visibility. Without the relay function of Artemis the images are stored on board SPOT 4's memory and dumped over the ground stations.

An artist's concept of laser link between Artemis and SPOT 4. Photo: ESA
The experiment performed consisted in establishing the link four times: in the course of four successive SPOT 4 orbits, the SILEX terminal on board Artemis activated its optical beacon to scan the area where SPOT was expected to be. When contact was made, SPOT 4 responded by sending its own laser beam to Artemis. On receiving the SPOT 4 beam, Artemis stopped scanning and the optical link was maintained for a pre-programmed period lasting from 4 to 20 minutes.

During the period in which the two satellites were "communicating", test data were transmitted from SPOT 4 to the ground via Artemis at a rate of 50,000 000 bits per second (50 Mbps). The extremely high accuracy of the data stream was confirmed at ESA's test station in Redu (Belgium) and the SPOT 4 receive station in Toulouse.

The main challenge in establishing an optical link between satellites is to point a very narrow beam with extreme accuracy to illuminate the partner spacecraft flying at a speed of 7000 m/s. Last night's experiment was performed under worst-case conditions since Artemis is not in its nominal geostationary position but in an lower parking orbit, circling the Earth every 19 hours.

This experiment was preceded by a series of tests a week earlier, during which a link was established between Artemis and ESA's optical ground station in Tenerife. Thoses tests demonstrated the correct operation of the SILEX terminal and paved the way for the following steps.

Another preparatory test will be performed over the weekend. The first experimental transmission of a SPOT 4 image is planned from the beginning of December. Before Christmas, the ion-propulsion phase is expected to start moving Artemis to its final geostationary orbit at 36,000 km. Once the spacecraft has reached that orbit, in the middle of next year, the operational phase will start and the link between the two satellites will be established at least 5 times a day.

The SILEX system consists of two terminals: one on board Artemis, the other on SPOT 4. Both terminals were designed and built by Astrium. The definition and procurement of the system were conducted in close cooperation between ESA and the French space agency, CNES.

Artemis is an advanced communication satellite built under the leadership of Alenia Spazio (Italy). The satellite is operated from a control station in Fucino (Italy) by the Italian consortium Altel (Alenia Spazio/Telespazio). The test was organised and technically supported by an ESA team at Redu (Belgium) and an Astrium team at Fucino in close cooperation with the CNES SPOT 4 operational team in Toulouse.

Artemis was launched on July 12, 2001 from Kourou by an Ariane 5 launcher. Due to a malfunction of the launcher's upper stage, the satellite was left in a low orbit. Since then the orbit has been lifted by the satellite's own means to an altitude of 31,000 km. To raise the orbit to the geostationary altitude of 36,000 km, the satellite will use its newly designed ion propulsion system. This system will provide enough acceleration using only 20 kg of xenon gas as fuel. For the orbit-raising manoeuvre, the satellite will have to be oriented in a direction which was not included in the baseline. A team of experts from ESA, Alenia and Astrium UK and Germany (also responsible for the development of the double ion-propulsion systems) are developing new control software to be uplinked to the satellite. It is planned to have the new software fully validated by mid-December and to start raising the orbit before Christmas. This will bring Artemis to its final position by summer next year. Artemis will have an operational lifetime of at least 5 years.

SPOT 4 was developed by the French Space Agency (CNES) under a partnership agreement between France, Sweden and Belgium ; it was launched on March 24, 1998. The SPOT system comprises today three satellites (SPOT 1, 2 and 4), and a worldwide network of receiving stations. Data is distributed by Spot Image located in Toulouse. SPOT 5, with enhanced capabilities (2,5 m resolution, 60 km swath and a new stereoscopic high-resolution instrument) will be launched in April 2002, thus ensuring continuity of service to the users.