On 30 November, the first-ever transmission of an image by laser link from
one satellite to another took place (*). The system, called SILEX,
consists of the Opale terminal on Artemis and the Pastel terminal on the
SPOT 4 satellite. It was designed in close cooperation between the
European Space Agency (ESA), the French space agency (CNES) and
manufacturer Astrium with over 20 European contractors involved. The
terminals exchange high-definition imagery data at 50 megabits per second.
Artemis subsequently beams the data at its leisure to the receiving
station operated by Spot Image at Toulouse, using a conventional 20 GHz
radio link.
In the past few weeks ESA, CNES and experts from Astrium have performed a
battery of tests to validate the performance of a laser link established
between ESA’s Artemis spacecraft, operating at an altitude of 31 000 km,
and the CNES SPOT 4 satellite orbiting at 832 km.
The SILEX terminals on both satellites demonstrated their robustness,
successfully carrying out all the required operations. Acquisition was
systematically obtained within the specified periods and the optical link
was in all cases maintained throughout the pre-programmed duration.
Communication performance was assessed by measuring the bit error rate –
excellent characteristics were demonstrated, with the bit error rate
consistently in the range 10 -9 to 10 -10 .
Once it had been confirmed that the optical link could be safely
established and maintained and that communication quality was virtually
perfect, the parties went on yesterday to transmit images from SPOT 4 via
Artemis to the Spot Image control centre in Toulouse. The images had been
recorded shortly before the transmission and stored in the satellite’s
data memory. Transmission was as successful as all previous tests and
picture quality was perfect.
This successful result was achieved not only thanks to the performance of
the satellite hardware but equally to the very effective cooperation
between all the operations control centres involved – the Spot 4 control
centre and the Spot Image data reception and processing facility in
Toulouse, the ESA-Artemis mission control facility in Redu/Belgium and the
Artemis operations control centre in Fucino/Italy, run by the Italian
consortium Altel (Alenia Spazio/Telespazio).
The main industrial contractors for development of the SILEX system are
Astrium in France, Germany and the United Kingdom, Bosch and Zeiss in
Germany, supported by many subcontractors. The Artemis main contractor is
Alenia Spazio, Italy.
The main advantage offered by the optical data relay system is enhanced
availability. When operated with Spot 4 the link can be maintained, if so
desired, for more than 50 % of the orbit. This increases contact time
dramatically and shortens the time between when images are recorded and
when they are available at the customer’s premises.
The potential of the new technology extends beyond Earth observation; it
promises to revolutionise sat-to-sat communications for constellations in
low-earth orbit, geostationary satellites and deep-space exploration
probes.
(*) This picture of Lanzarote in the Canaries is available in ftp format
at the following site :
ftp://ftp.spotimage.fr/incoming/PASTEL
Copyright CNES 2001/Distribution Spot Image
Note to Editors :
Artemis was launched on 12 July 2001 from Kourou on an Ariane 5 launcher.
Due to a malfunction of the launcher’s upper stage, the satellite was left
in a lower than intended orbit. Since then the orbit has been lifted using
the satellite’s own systems to an altitude of 31 000 km. To move on to its
permanent home in geostationary orbit, 36 000 km above the Earth, the
satellite will use its newly designed ion propulsion system. This system
will provide the required acceleration using only 20 kg of xenon gas as
fuel. To carry out the orbit-raise manoeuvre, the satellite will have to
be oriented in a direction that was not included in the baseline. A team
of experts from ESA, Alenia and Astrium UK and Germany (which also
developed the dual ion-propulsion systems) are having therefore to develop
new control software that will then be uplinked to the satellite. It is
planned to have the new software fully validated by mid-December and start
raising the orbit before Christmas. This should bring Artemis to its final
position by summer next year. The satellite will have an operational
lifetime of at least 5 years.
SPOT 4 was developed by CNES under a partnership agreement between France,
Sweden and Belgium; it was launched on 24 March 1998. The Spot system
today comprises three satellites (SPOT 1, 2 and 4), and a worldwide
network of receiving stations. Data are distributed by Toulouse-based Spot
Image. SPOT 5, with enhanced capabilities (2.5 m resolution, a 60 km swath
and a new stereoscopic high-resolution instrument), will be launched in
April 2002, thus ensuring continuity of service to users.
For further information, please contact :
Gotthard Oppenhäuser
ESA/ESTEC
Artemis Project Manager
Tel : +31(0)71.565.3168
or Press Offices at :
ESA
Franco Bonacina
Tel: +33(0)1.53.69.7155
Fax : +33(0)1.53.69.7690
CNES
Sandra Laly
Tel: +33(0)1.44.76.7732
ASTRIUM
Rèmi Roland
Tel: +33(0)1.34.88.34.24
Spot Image :
Anne-Marie Bernard
Tel: +33(0)5.62.10.40.10
