Laser beams could soon become a viable alternative to radio waves for the transmission of large quantities of data over long distances through space. High data rates combined with little power consumption and low payload weight make laser communication terminals particularly interesting for application on-board satellites, space telescopes and scientific space probes. The first satellites equipped with laser communication terminals are already orbiting the Earth and more will follow in the coming years.
Laser-based data transmission has several advantages over conventional radio links. Due to the shorter wavelength, lasers can achieve higher data rates than radio signals for the same given aperture. Laser beams are inherently less divergent than radio signals and, therefore, require less power for data transmission. In addition, due to the higher efficiency and the low beam divergence of the laser beam, the laser link is an extremely secure point-to-point connection; a bugging device would have to be in the immediate vicinity of the receiver, or would even have to be introduced into the beam, and this would terminate the connection immediately.
These advantages are particularly useful in space applications. Lasers would be effective when large quantities of data need to be transmitted rapidly back and forth between satellites. In addition, lasers would be highly functional when data has to be transmitted across vast distances. Lasers will certainly be the communications medium of choice in the future. Among many other applications, laser communication is currently being considered for:
- Data relay services for unmanned aerial vehicles (UAV): UAV inspecting remote areas can send their observation data to a data relay satellite in geostationary (GEO) orbit via an optical link.
- Data relay services for satellites: High-speed laser communication can be used to replace an expensive network of ground stations needed to constantly receive low earth orbiting (LEO) satellites` data. The data gathered by the LEO satellites can by transmitted to a relay satellite in GEO orbit by means of laser communication. The relay satellite then transmits the data to a single ground station thus offering cost savings in operations and infrastructure.
- Inter-satellite links between GEO satellites can be used to share resources and/or route traffic around a satellite network. They are also of interest for intra-continental communications (e.g. between satellites providing services throughout Europe with satellites providing services to Western Europe being linked to satellites providing services to Eastern Europe) and inter-continental links (e.g. between satellites providing services in Europe linked to satellites providing services to the US and / or Asia Pacific rim).
- Deep space data transmissions: The amount of data being collected on exploration missions, such as those to Mars, and are increasing and will soon become limited by RF capacity. This increase may require on-board data processing and coding be introduced, with the resulting loss of access to the raw scientific data. In addition, an increase in the long data transmission times increases operation costs and severely reduces the time available for scientific tasks. By using optical links, the data rate can be dramatically increased, thereby allowing the raw scientific data to be received and resulting in the increased scientific value of future missions.
The four OPTEL terminals for near Earth telecommunication applications are:
- OPTEL 02; the short-range terminal capable of transmitting data at Gbps rates over distances of, typically, 2,500 km. This class of terminal is of interest for applications such as the short range GEO GEO crosslinks. Oerlikon Space has developed a demonstration model of this terminal under an ESA contract.
- OPTEL 25; the medium range terminal capable of transmitting data at Gbps rates over distances of, typically, 25,000 km to 45,000 km. This class of terminal is of interest for applications such as LEO GEO inter satellite links and medium range GEO GEO crosslinks. An engineering model (EM) of this terminal has been developed by Oerlikon Space under an ESA contract.
- OPTEL 80; the long-range terminal capable of transmitting data at Gbps rates over distances of 40,000 km up to 80,000 km. This class of terminal is of interest for applications such as LEO GEO inter satellite links and long range GEO -GEO crosslinks.
- OPTEL AP; the OPTEL terminal design for atmospheric communications between either high altitude (stratospheric) platforms (HAP). The communications links between HAPs are dimensioned to a data rate of 155 622 Mbps over a distance of around 400 km. Oerlikon Space has developed a demonstration model of this terminal.
The transmission unit was modified in such a way that the conditions on the 142-kilometer stretch between the islands exactly reflected those that would prevail on a 1.5 million kilometer link through space. This was achieved primarily by reducing the emission aperture of the laser to a diameter of less than half a millimeter in order to weaken the light signal.
The Oerlikon team installed the transmission unit in a container beside the Nordic Optical Telescope at an altitude of 2400 meters on Roque de los Muchachos, the highest mountain on La Palma. Because of the unusually clear air, this is an ideal location for optical experiments. The receiver terminal was situated in the Optical Ground Station (OGS) of the European Space Agency ESA on Tenerife.
Although the optical experiment was hampered by unfavorable weather conditions with unusually high cloud and strong winds during the first few days, the experts from Oerlikon Space succeeded in establishing a laser link between La Palma and Tenerife. In the course of the experiment, they achieved transmission rates of over 10 Mbit/sec. At this speed, it would take a mere two seconds to transmit the entire text of the Bible. The data rate would also be sufficient to transmit three digital television programs simultaneously.
The distance of 1.5 million kilometers that was simulated on the Canary Islands is equivalent to the distance between the Earth and Lagrange points L1 and L2. These mark specific positions in space at which it is particularly advantageous to place space telescopes.
Scientists could benefit enormously from laser communications: Equipped with laser terminals, telescopes will, in the future, be able to transmit far greater quantities of observation data to Earth than is possible today by radio. Image data from todays space telescopes are compressed on board the telescopes before they are transmitted to Earth. This compression inevitably brings information loss. Equipped with laser terminals, future telescopes could transmit raw data and thereby provide scientists with the full information gathered by their instruments.
The next step Oerlikon Space is planning is for the coming summer. This will be another test campaign on the Canary Islands to simulate a laser data link over a distance of 400 million kilometers. This is the maximum distance between the planets Earth and Mars. Similar to space telescopes, the amount of data transmitted from planetary probes to Earth could rise dramatically. This would significantly increase the scientific benefit drawn from these expensive missions.
While optical data transmission from deep space is an interesting mid-term perspective, it is currently becoming operational in the Earths orbit. The ESA Silex project was the first program to successfully demonstrate laser communications links between two satellites Artemis and Spot 4 as well as between the geostationary Artemis satellite and the ground.
In November 2001, Artemis established its first laser connection to the French Earth observation satellite Spot 4 and successfully demonstrated the feasibility of achieving the challenging accuracy requirements for pointing, acquisition and tracking that is demanded by the small divergence of laser beams. Since then, Artemis, as a routine operation, has performed optical link services, with link distances in the order of 40000 km and data rates of 50 Mbps. In December 2006, Artemis successfully demonstrated a laser communications links with an aircraft. These airborne laser links, established over a distance of 40000 km during two flights at altitudes of 6000 and 10000 meters, represented another world first.
The U.S. military satellite NFIRE is also equipped with a laser communication terminal built by the German company Tesat-Spacecom. The German radar satellite TerraSAR-X was launched last year with another Tesat terminal on board. An inter-satellite link between NFIRE and TerraSAR-X is planned for the near future.
In 2009, the German Aerospace Centre will launch TanDEM-X, a second radar satellite nearly identical with TerraSAR-X: TanDEM-X will also be equipped with a Laser Communication Terminal, jointly developed and built by Tesat-Spacecom and Oerlikon Space. The two radar satellites will circle the earth in formation and generate a highly accurate digital elevation model of the Earth. Inter-satellite communication between TerraSAR-X and TanDEM-X will be established using the laser terminals.
Laser communications terminals comparable to those on-board the TerraSAR-X and TanDEM-X satellites may also be used on the European Sentinel satellites. These four satellites will be launched starting in 2009 and they will constitute a key element in the European GMES program. GMES (Global Monitoring for Environment and Security) is a joint initiative of the European Commission and the European Space Agency. Its aim is to establish European-wide co-operation for an independent, continuously available, cost-efficient and user friendly observation capability for political decisions makers.
Laser communication offers a number of advantages when compared to radio signals but also poses new technical challenges to engineers. However, the enabling technologies needed for a laser communications link have been demonstrated over recent years and are now believed to be understood and the difficulties solved. Laser communications can now be considered as a technology thats moving from being experimental to an operational technology.
Hendrik Thielemann studied communications science at the University of Münster and graduated with a M.A. After completing practical training, he worked as newspaper editor. Since 2001, he has been working in the European Space Industry, holding various positions. Hendrik Thielemann has been the Head of Communications at Oerlikon Space in Zürich, Switzerland, since 2007.