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 Técommunication Laser, is one of the two optical terminals making up the space communications system, SILEX: (Semi conductor Intersatellite Link EXperiment).

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:

• SPOT 4 is in a low-earth orbit, which means that the relative velocity of the two satellites has to be taken into account,
• of the time needed for light to travel between PASTEL and OPALE,
• of dynamic perturbations generated on board the satellites (activation of mechanisms, etc.) which means that neither of the terminals is mounted on a perfectly rigid structure.

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,

• SPOT 4 scenes can only be relayed to the ground immediately after acquisition if the satellite is within range of a SPOT receiving station.
• Outside their acquisition circles, scenes have to be stored by onboard recorders or the solid-state memory; these two storage devices can only dump their data when the satellite is again within range of a receiving station.
  
  

• 
  Areas of direct image acquisition without PASTEL

With PASTEL,

• it is possible to send the ground segment SPOT imagery of any part of the Earth's surface for which the PASTEL and OPALE terminals are within range. This zone known as the"terrestrial coverage " is shown below. It is easy to see that it covers a much larger area than the acquisition circles of the SPOT system stations.
• it is also be possible to dump SPOT 4's solid-state memory. Consequently much less time is needed for memory dumping and filling.
  

Areas of direct image acquisition with PASTEL

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PASTEL: what does it consist of?

To ensure that the gimbal-mounted assembly is as light as possible, the PASTEL terminal was divided into two sub-assemblies:

• the aerial part (the part which actually moves), mounted on that side of SPOT 4 which is furthest from the Earth, and which consists of:

- 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.

• the fixed part which includes all electronic systems which do not have to be next to the mobile part.

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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.

The SILEX system is operated:

• either by CNES centres:

  - the CPR, the SPOT 4 programming centre in Toulouse,
  - the CMP, the SPOT 4 satellite control centre in Toulouse,
  - the SRIP, the PASTEL imagery receiving station, located at Aussaguel near Toulouse,

• or at European Space Agency centres:

- 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:

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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:

• Coarse pointing phase
  
  This phase involves moving PASTEL (respectively OPALE) from its standby position so that it is turned towards OPALE (respectively PASTEL) at the beginning of the acquisition phase. These pointing directions are predicted using orbit models of ARTEMIS and SPOT 4 uploaded to both-PASTEL and OPALE.

• Acquisition phase
  
  Following the coarse pointing phase, the pointing error of each of the terminals is much greater than the divergence angle of the communication beams and therefore requires that a lock-on procedure be carried out.
  The acquisition strategy adopted makes it possible to lock-on in less than 239 secs.
  
  

• Communication phase
  
  This phase is used either:
  - to transmit a specific signal generated by PASTEL to OPALE to assess link quality.
  - or to transmit SPOT 4 imagery for reception by the SRIP station in Toulouse.

• 'Return to standby' phase
  
  Following the communication session, each terminal returns to its standby position.

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    page updated on the 05-07-05