Laser technology enables high-speed datalinks in space

Laser–optical data link

Satellites are generating an increasing volume of data that needs to be transmitted for use back on Earth. Until now, such data has been transmitted via radio frequency (RF). A major step forward has now been achieved by the teams responsible for TerraSAR-X – designed and built by Astrium – and the American NFIRE satellite. Together they have succeeded in setting up a laser–optical data link between the two satellites, located 5,000 kilometres apart, and maintaining an error-free connection at a transmission rate of 5.5 Gbit/s. This is the equivalent of transmitting 200,000 A4 pages of text per second or the content of 400 DVDs in a single hour.

The flawless data transmission was assured by two laser terminals supplied to the German Aerospace Centre (DLR) by Astrium’s Tesat-Spacecom subsidiary in Backnang. In addition to their excellent performance, the Tesat laser terminals demonstrated that they were more efficient than their predecessors despite their smaller size.

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Tested under extremely tough conditions

The laser terminals had to be capable of withstanding the high acceleration forces and vibrations during launch as well as the inhospitable conditions of space thereafter – such as extreme exposure to radiation and widely fluctuating temperatures. This is why the laser terminals’ subassemblies were tested under extremely tough conditions on the ground, with temperatures ranging from -35°C to 60°C, acceleration forces 1,300 times Earth’s gravity (g-force), and constant bombardment with gamma rays. Each of the modules had to be small, light and energy efficient for deployment in space. The low weight was achieved through careful choice of materials and an expensive housing. Every last bit of material that was not absolutely essential was trimmed away. During 5.5Gpbs data transmission, over a distance of 5,000 km, the laser terminals require less than 130 watts of power to operate – little more than a light bulb.

20-minute laser link

TerraSAR-X and the US satellite NFIRE fly in a low Earth orbit and pass each other several times each day. During each of these encounters, a laser link can be established between the two satellites for up to 20 minutes at a time. One of the most challenging tasks is pointing the laser beam precisely in the direction of the partner satellite, as it flies by at a speed of around 25,000 km/h and at a distance of up to 5,000 km away. Achieving the required angle is like trying to pinpoint and track a coin in the window of a passenger jet flying at very high altitude from a vantage point on the ground.

Extensive experience in laser communication technologies

Tesat-Spacecom and Astrium already have a great deal of experience in laser communication technologies. One example is the successful experiments carried out using the Astrium-built SILEX terminal installed on the Artemis telecommunications satellite, a first for Europe. The Artemis satellite transmitted data via laser from geostationary orbit, using the SILEX terminal, at a rate of 50 Mbit/s to the low-Earth orbiting Spot-4 satellite. A later generation of Silex terminals, built by Astrium, was used in experiments with the LOLA technology demonstrator in 2006. The LOLA terminal was installed on board a passenger airliner, where it also communicated with Artemis.

Tesat-Spacecom, a world-leading specialist in laser technology

In order to meet the growing demand for data transmission capability in all areas of the space industry – telecommunications, navigation, Earth observation, meteorology and human spaceflight (i.e. the ISS) – broadband inter-satellite links will be needed to set up networks of geostationary satellites and enable them to communicate with platforms in lower orbits. It is important that the technical advantages offered by laser communication and the corresponding engineering skills should be further developed and utilised. Tesat-Spacecom is one of the world’s leading specialists in this technology.

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Laser technology enables high-speed datalinks in space