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Twinkle, twinkle little satellite

Satellite laser link set to improve communications. By Eric Russell.

The first optical data link between two satellites in space has just been tried and tested successfully. On 21 November, the French Earth observation satellite SPOT 4 passed images to the European Space Agency’s Artemis satellite over the SILEX – Semiconductor Intersatellite Link Experiment –laser beam data transmission system. This enabled the images to be sent in real time to the image processing centre at Spot Image in Toulouse, even though SPOT 4 was out of sight of the centre.
Previously, images had to be stored on board SPOT 4 – sometimes for up to 20 hours – until a ground station was sighted. But Artemis, at an altitude of 31,000km, can see the satellite, which is in low Earth orbit at 832km, for nearly half of its orbit and for much longer than an Earth station. And because Artemis – the Advanced Relay and Technology Mission Satellite – will eventually be in a geostationary orbit, it will be in continuous contact with Earth.
Carrying three payloads plus a number of experiments, Artemis has been developed to test and operate new telecommunications techniques. This system opens the way to a satellite network through which data collecting satellites can transmit information to Earth continuously, instead of waiting for an Earth station to appear. Such a network could also be used to carry communications to parts of the Earth that are not busy enough to merit their own dedicated satellite. And, with the further development of all optical switching and amplifying systems on satellites, bandwidth will also increase substantially.
Bernard Cabrieres, head of remote sensing satellite operations at the French space agency CNES, says that Artemis will increase dramatically the global capacity of Earth observation activity in terms of the volume of data. It will also make significant improvements to the speed at which data can reach Earth in times of environmental disaster or other crises.
The choice of laser, rather than a radio, link is due to the higher transmission rates offered by laser technology. Whilst radio transmission is limited to less than 250Mbits/s, laser technology can achieve up to 10Gbits/s.
The SPOT 4 image data rate is 50Mbit/s but SILEX will support about five times that rate. 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. It takes about 30 minutes to clear the satellite’s data memory.
Gotthard Oppenhauser, Artemis project manager, says the optical high gain antennae are also much smaller than rf equivalents, so the transmitting power is of the order of milliwatts instead of watts. This means systems are lighter, smaller and less power hungry, in turn allowing satellites to stay aloft longer and need less launch power.

Pointing accuracy
The narrow beam makes the system almost interference free, which helps when coordinating with other systems, and avoids interception by unwanted listeners. But the accuracy of the pointing system has to be extreme – of the order of one microrad or 0.0001o. This is because the two satellites are on average 38,500km apart. The task has been likened to trying to hit an orange with a rifle bullet from 50km.
That in itself is hard enough, but the degree of difficulty is compounded by the fact that SPOT 4 is moving at 7km/s, so the relative velocity of the two satellites has to be taken into account. And it takes time for the light to travel between the two.
A further complication is the dynamic perturbations generated on board the satellites: through activation of mechanisms, for example. This means neither of the optical terminals is mounted on a perfectly rigid structure. And once a satellite starts to pass behind the Earth, the optical link suffers losses as it passes through the Earth’s atmosphere.
The lasers operate at 850nm with a power of 60mW for communication purposes. For sighting and establishing the transmission link, a beacon of 19 lasers – each of 1W – is used.
When communications are to be established, each end of the SILEX link is pointed at the other in the coarse pointing phase. Pointing directions are predicted using orbit models of Artemis and SPOT 4 and uploaded to both satellites.
Following this phase, the pointing error of each of the terminals is much greater than the divergence angle of the communication beams and therefore requires a lock on procedure to be carried out.
The beacon on the OPALE – Optical PAyload for intersatellite Link Experiment – terminal on Artemis is switched on and scans the area where SPOT 4 is expected to be.
When the PASTEL – PAssager SPOT de Télecommunication – terminal on SPOT 4 is illuminated by the beacon beam, it switches on its communication laser, and enters a closed loop tracking routine within 90ms. The beacon is switched off 20s after communications have been established.

The optical terminals are calibrated and tested by pointing them at very bright stars – such as Arcturus, Betelgeuse and Sirius. The inter satellite test was preceded by tests between Artemis and ESA’s optical ground station in Tenerife.
The link up itself was performed under worst case conditions since Artemis is not in its nominal geostationary position but in a lower, parking orbit where it circles the Earth every 19 hours.
Due to a malfunction in the upper stage of the Ariane 5 launcher, Artemis was put into a degraded orbit at 31,000km. For conventional telecommunication spacecraft, this would be considered a graveyard orbit as they would be unable to reach geostationary orbit. But the combination of chemical propellants and two ion propulsion systems on board will provide the extra boost that Artemis needs to raise itself into operational orbit.
When it reaches its geostationary orbit some 36,000km above the Earth, Artemis will provide Europe with mobile communication and navigation services for a planned period of 10 years.
It will also provide data relay capacity and satellite to satellite communications and will be used as a relay station between the International Space Station and Earth.

Artemis weighed 3100kg at launch. Its solar arrays span 25m tip to tip and deliver 2.5kW of electrical power. It is positioned on the equator over central Africa at 21.5o East and will be a key element of Europe’s EGNOS satellite navigation system, broadcasting gps like navigation signals.
EGNOS – the European Global Navigation System Overlay – will be completed in 2003 and enter operations in 2004. It is the precursor to Galileo, the future European global navigation satellite system promoted by ESA and the EU.
Artemis is now being moved to its final geostationary orbit at 36,000km under ion propulsion. Once there, in the middle of 2002, the operational phase will begin and the link between it and SPOT 4 will be established at least five times a day.
The advanced communication satellite – built under the leadership of Alenia Spazio of Italy – is operated from a control station in Fucino by the Italian consortium Altel.
The optical link test was organised and technically supported by an ESA team at Redu and an Astrium team at Fucino, in close cooperation with the French CNES SPOT 4 operational team in Toulouse. SILEX was developed for ESA by Matra Marconi Space.

Vanessa Knivett

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