The first civil high data rate optical communications system, developed by the European Space Agency (ESA), is scheduled to be launched tonight, Monday 23 March, on board the French earth observation satellite SPOT-4 by Arianespace (Ariane flight V 107).
This optical communications system (called SILEX for Semiconductor
Intersatellite Link Experiment) will transmit picture data from SPOT-4
via the ESA ARTEMIS satellite (scheduled for launch in late 1999/early
2000) to a data processing centre near Toulouse, France. The advantage
of this double-hop is that because ARTEMIS is in a higher, geostationary
orbit, data can be relayed from SPOT-4 to the ground for much longer
periods than with the system transmitting data direct to ground currently
used by earth observation satellites.
SILEX will also support an experiment between ESA’s ARTEMIS satellite
and the OICETS spacecraft built by the Japanese Space Agency NASDA.
The concept of the SILEX system
SILEX provides a data relay link between a terminal on board ARTEMIS
in geostationary orbit and a second terminal on the French SPOT-4
spacecraft. The data rate to be transmitted is 50 Mbps from SPOT-4 to
ARTEMIS. The link is operated at optical frequencies at a wavelength of
800 to 860 nanometers. The light source is a solid state laser diode
operated typically at 60 mW optical power. The applied modulation
scheme is direct intensity modulation (on/off switching). As optical data
detector a photodiode is used.
For acquisition and tracking purposes CCD (charge-coupled device)
detectors are used. For link acquisition an optical beacon is provided
on the ARTEMIS terminal which consists of 19 laser diode arrays, each
having a nominal optical power of about 700 mW. The communication
signal is split in the receiving terminal and used for data detection
and for tracking purposes respectively. To concentrate the optical beam
in the direction of the partner satellite the light is transmitted via a
25 cm aperture telescope providing a beamwidth of only 10 microrad
(equivalent to 0.000057 degrees). In the SILEX configuration, where the
distance between the two partner spacecraft is in the order of 40 000
km, the light-travel time is about 1/4 of a second. During this period the
partner moves up to 2000 m whereas the beamwidth is only about 300 m,
consequently requiring extremely tight beam pointing accuracies.
The SILEX mission
SILEX has an experimental and a pre-operational purpose. As part of the
experiment the behaviour of the optical terminal, the short and long term
stability of the various electrical, mechanical and optical elements and the
communication link quality characteristics will be explored. Also,
operational procedures will be optimised.
The pre-operational part of the mission consists of transmitting (via
the optical link to the ground) image data from the SPOT-4 earth
observation instrument. This instrument generates data at 50 Mbps,
which can be received at a ground station in southern France in real
time. The SILEX service will thus eliminate the need for an extended
ground station network and extend the direct visibility of the
spacecraft to the central ground station. SILEX will also be used by
SPOT-4 to dump the content of the data memory collected during periods
when the spacecraft is neither in the visibility zone of a ground
station nor within reach of the ARTEMIS terminal.
SILEX has been developed by ESA with Matra Marconi Space (MMS) as
prime contractor leading a team of European companies.
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