Last night, 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.

The experiment performed last night 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.

Last night’s 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.

Note to Editors:

Artemis was launched on 12 July 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 24 March
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

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