When on 28 November 2014 researchers at ESA’s European Space Operations Centre (ESOC) in Darmstadt, Germany began receiving images relayed in nearly real-time from the Sentinel-1A Earth Observation satellite, it marked an important milestone in satellite communications history. The Sentinel 1A, travelling in Low Earth Orbit (LEO), was transmitting images not directly to Earth but via a laser link with the Alphasat satellite situated much higher in Geostationary Orbit (GEO). A laser-based connection was established between two satellites moving in very high speed in space, and the data was relayed to a ground station via radio link. Inter-orbit links have been demonstrated before, but never at such high data rates. This pioneering breakthrough was made possible because when Alphasat was launched in July 2013, it included a technology demonstration payload contributed by DLR in the form of a laser terminal.
Timely data transmission from space to ground is a vital part of all missions. Despite this, our increasing reliance on information gleaned from space means we are demanding more of it, sooner; the current generation of low orbit-to-ground station radio links, using Ka-band frequencies, are are unable to keep with the increasing demand. For this reason, the Sentinel-1A – Alphasat laser link represents an important step forward, and paves the way for the deployment of the European Data Relay System, which is designed to facilitate the near real-time transmissions from Europe’s growing fleet of Earth Observation satellites and other spacecraft which generate huge volumes of valuable data.
With the support of ESA, Europe has taken the lead in optical communications, with European space researchers being among the first to demonstrate the feasibility and performance of the technology. Through the ARTES Programme, ESA has supported a series of successful R&D activities related to optical communications, and it will continue to promote the development of this key space technology as it matures.
What is optical communications and how will it be used?
Optical communications, also called laser communications, is the use of extremely narrow beams of light generated by lasers to transmit information. The optical wavelengths used in space so far are in the range from 0.8μm to 1.6μm, which is equivalent to a frequency of 187 to 353THz.
Up until now, space communication networks have been based on RF bandwidth, a scarce natural resource the use of which requires complex international coordination by the ITU. The successful demonstration of high data rate optical communications in space has opened a new spectral domain which provides vast bandwidth resources without the constraints imposed by international regulatory bodies.
Optical communications is of particular interest for inter-satellite communications. For example, a laser terminal on a GEO satellite is able to acquire and track laser signals from its counterpart in a lower orbit at distances up to 50,000 km with relative velocities up to 27,000 km/h. Once locked on, the full-duplex data link between the two laser terminals continues until the low-orbit satellite disappears from sight.
Data relay techniques significantly increase the efficiency of data transmissions from spacecraft to ground, and as such have great potential for the near real-time transmissions from satellites which produce vast amounts of data.
For this reason, European Copernicus Earth Observation satellites, the first of which, Sentinel-1A was launched in 2014, with a second, Sentinel-2A, due to be launched in 2015, will make extensive use of optical communications for relaying their data to ground. This will be done via laser links with the forthcoming European Data Relay System. EDRS-A – the first node of the new system – is slated be launched this year, and the Sentinel-1A – EDRS-A laser link will be commissioned. It is intended to be used for the near-real time downlink of Sentinel-1A and -2A data as a regular operational service. Additional Sentinels – 1C/D and 2 C/D – will follow in coming years. In 2017, a second node of the data relay system – EDRS-C – will be launched and become part of the operational system.
The advantages of optical communications
The frequencies used by optical communication are an attractive alternative to the highly-regulated and heavily-saturated radio frequency spectrum
Extremely high transmission speeds: Taking advantage of the huge bandwidth possible in optical frequencies, current optical technologies offer data transmission speeds up to 1.8Gbit/s, scalable up to 7.2 Gbit/s. In the future, this could increase by an order of magnitude.
Security: Due to theunique point-to-point contact and the very narrow width of the optical beam (approximately 300m at a distance of 45,000km), it is virtually impossible to intercept optical communications, making them not susceptible to jamming or interference.
Lower mass: Optical communications equipment should be lower in mass than comparable RF components.
One of the main the challenges, however, facing the widespread adoption of optical communications is the need to establish networks of optical ground stations in reliable locations, both in terms of political situations as well as weather; with regard to the latter, laser links from space to ground are susceptible to atmospheric conditions.
Conclusion
There are not many technologies in which Europe is a leader, but inter-satellite optical communications is one. To maintain this lead, ESA is committed to ensuring that the European space industry is able to take full advantage of the tremendous potential this technology will offer as it continues to mature.
