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What are PASTEL and SILEX?
Why use a laser link?
SILEX : what is it used for?
PASTEL: what does it consist of?
SILEX: how is it implemented from the ground?
SILEX: how does it work in orbit?

What are PASTEL and SILEX?

PASTEL, which stands for PAssager SPOT de TÚcommunication Laser, is one of the two optical terminals making up the space communications system, SILEX: (Semi conductor Intersatellite Link EXperiment).

logosilx.gif (5578 octets)The SILEX system was developed for the European Space Agency by Matra Marconi Space.

It provides a laser link between the PASTEL optical terminal on SPOT 4 and the OPALE optical terminal, mounted on the geostationary satellite ARTEMIS, which has been placed in orbit in 2001.

The information carried between PASTEL and OPALE is modulated at 50 Mbits/sec and relayed to the ground (Toulouse) by the K-band payload on ARTEMIS.

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Why use a laser link?

The choice of a laser rather than a radio link is due to the potentially higher transmission rates offered by laser technology: Radio transmission is in fact limited to less than 250 Mbits/sec while, even given the current state of laser technology, it is possible to achieve rates of 10 Gbits/sec!

Laser links have another advantage, in that by their very nature, there can be no interference between optical and radio transmissions.

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SILEX: What is it used for?

The SILEX experiment has a twofold objective: validation of the laser transmission concept and operational transmission of SPOT 4 imagery to the ground.

A technological challenge

This type of optical communication, which is the first European demonstration of laser communications, is a real technological challenge.

The difficulty involves aligning and stabilizing two terminals which are on average 38 500 km away from each other, with a pointing error of less than 2 micro-radians. In other words, the beam from PASTEL (which at that distance, would have a diameter of 200 m) would have to be kept centred on OPALE with a tolerance of +/- 38m. To better grasp the difficulty involved, let us just say that it would be equivalent to hitting an orange with a rifle shot from 51 km!

This pointing is made even more difficult because:

A means of transmitting SPOT 4 imagery

During operations, PASTEL is used to send SPOT 4 imagery via an optical link. This could either be done directly or later by dumping of data from SPOT 4's onboard solid-state memory.

It should be remembered that without PASTEL,


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Areas of direct image acquisition with PASTEL

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PASTEL: what does it consist of?

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int-ptel.jpg (30250 octets)To ensure that the gimbal-mounted assembly is as light as possible, the PASTEL terminal was divided into two sub-assemblies:

- a telescope,
- an optical bench with a fine pointing system, a communication sensor and laser diodes,
- a thermal control system for precision temperature control,
- a two-axis gimbal mechanism,
- the launch locking mechanisms needed during the launch phase.

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SILEX - how is it used?

Establishing a link between two optical terminals implies perfect coordination in planning and also in programming them and acquiring information for monitoring them.

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The SILEX system is operated:

- the PMCS, the PASTEL terminal control centre in Belgium,
- the AMCS, the control centre in Italy for the ARTEMIS satellite and the OPALE optical terminal,
- the PTL, the SILEX technological assessment centre in Belgium.

In the Canary islands there is also a send/receive optical ground station (OGS) which will be used to set up a link between OPALE and the ground segment. It will be used to validate the OPALE termminal and should, moreover, yield very interesting data on atmospheric perturbation phenomena.

The tasks assigned to these different centres are summarized below:


CNES centres

ESA centres

Mission management


(five week period)


Receiving HRVIR customer requests

Resource management (PASTEL, solid-state memory, etc.)

computing PASTEL/OPALE visibility windows

Reception of SILEX user requests

Five-week SILEX mission scheduling

checking mission timing diagrams  
Programming terminals



  SPOT 4 payload telemetry programming optical, X-band, archiving, etc...   Programming PASTEL Programming OPALE  
Terminal command/control



PASTEL memory load commands

Transmitting SPOT 4 telemetry to PMCS

Generating PASTEL commands

Exploiting CMP telemetry data received

Generation and memory loading of commands for OPALE

Exploiting telemetry data received

Data reception and processing


SPOT 4 imagery data         PASTEL technological data

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SILEX - how does it work?

When programmed to do so, PASTEL and OPALE carry out the four following operational phases in order:

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    page updated on the 05-07-05