A long-distance laser communication link between two Earth-orbiting satellites has been established for the first time. The technique allows fast moving, low Earth orbit satellites to relay information to the ground almost instantaneously via a geostationary satellite fixed in a much higher orbit.
The experimental laser used sent data at five megabits per second, allowing still images to be sent. But the latest laser systems in development have much greater capacity and could be used to relay communications or video.
鈥淭he next generation of commercial satellites will use these links,鈥� says Gotthard Oppenh盲user, who is project manager for the European Space Agency (ESA) satellite Artemis. 鈥淲e have to look to the next generation of laser. These can be one quarter of the size and have a data rate of five gigabit per second.鈥�
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The laser link was established between ESA鈥檚 Artemis and the French-built observation satellite SPOT 4. The connection was used to relay images of the Earth taken by SPOT 4. Four connections varying in duration between four and 20 minutes were established.
A US military project to develop inter-satellite laser communications existed in the late 1990s, though the project鈥檚 current status is unclear. However, it aimed to permit links over just 2500 km, far too short a distance to reach from low Earth orbit to geostationary orbit.
Relay race
Retrieving data from surveillance satellites such as SPOT 4 can currently take hours. The satellites have to store information until their orbit brings them into sight of a base station on Earth and then send data through a radio link.
The new technique will improve the speed and quality of many satellite communications, says Oppenh盲user: 鈥淏y using data relay, you can greatly increase contact time with the Earth.鈥�
Lasers are also more compact, more secure and require less power than radio transmitters and receivers. But they have to operate with extraordinary precision. Artemis is located in a fixed, geostationary orbit 31,000 kilometres above the Earth, while SPOT 4 is travelling at 7000 metres per second at just 832 kilometres altitude. The laser link between the two is just a few metres wide.
To establish an accurate link, Artemis scanned a 300 km area with a broad, 30-km-wide laser beacon to locate SPOT 4. The beam was then narrowed to its finest width, allowing large amounts of data to be sent quickly. The satellites used an experimental laser system called SILEX, developed by ESA and the French space agency CNES.