Huge quantities of data are collected by interplanetary probes, Earth-orbiting satellites or flying platforms during science, exploration, and reconnaissance missions. Due to the ever increasing resolution of the deployed instruments (optical and infrared cameras, radars, etc.), data volumes are increasing to a point where they can no longer be transported by current means. One option is to process and compress them on board. This, however, limits the possibility to re-process the data for additional purposes at a later time. The alternative is to develop new communication systems. Consequently, the institute works on such developments, in particular on optical systems that are optimized in terms of robustness and compactness.
Accurate pointing is key to optical communications
All free-space optical communication systems use tightly collimated laser beams. They must remain accurately pointed at the receiver. After test campaigns with a stratospheric balloon, the institute demonstrated the first optical connection between a propeller aircraft and a ground station in 2008. Corresponding links are now established in a routine manner over distances of up to 120 kilometres, with a data rate of 1.25 Gbit/s.
The above experiments were all worldwide firsts. The same applies to the quantum key exchange experiment between a DLR DO-228 aircraft and the optical ground station in Oberpfaffenhofen, which was carried out in cooperation with the Ludwig Maximilians University of Munich (LMU).
The first terminal for data transfer from a jet aircraft to the ground was developed in cooperation with the spin-off ViaLight Communications on behalf of Airbus. The test flight was conducted on a Tornado at Mach 0.7 in 2013. The tests immediately achieved the nominal data rate of 1.25 Gbit/s – 100 times faster than with conventional radio systems. Aircraft vibrations and atmospheric turbulence were the greatest challenges. The latter were continuously measured and used for modelling the propagation through the atmosphere in order to further optimize future systems.
OSIRIS transmits large data volumes from Low Earth Orbits
Previous work by the institute has contributed to the development of the Laser Communication Terminals (LCT) by TESAT, which are deployed on the ESA Sentinel satellites, for example. These LCTs are used in the European Data Relay Satellite (EDRS), which is the best performing data relay system ever built so far. Small satellites and microsatellites, however, require smaller, less expensive, and lighter terminals. As a consequence, the institute has developed an alternative approach. In this approach, the terminal transmits the data directly to the ground and is thus bound to communicate under line-of-sight conditions. In a number of scenarios, this disadvantage is more than compensated by the small size, low weight, and low cost of the terminal.
OSIRIS V2 is the first such terminal to fly. It is deployed on the BIROS satellite and to be launched in early summer 2016. OSIRIS V2 uses the attitude control system of the satellite to align itself with the ground station. It weighs 1.65 kilograms, and is capable of transmitting one gigabit per second from a Low Earth Orbit. It was built entirely from commercial off-theshelf components. The next generation of OSIRIS terminals is already under development. It will be equipped with a pointing assembly, which will allow its use on the International Space Station. Due to miniaturization, so-called Cubesats are becoming increasingly performant and interesting for scientific missions,
Earth observation, and component testing. This creates another opportunity for the use and testing of optical communication systems in space. Consequently, the institute has developed a first conceptual design for such a system.
Commercialization of freespace optical communications
Commercial interest in free-space optical data transfer (FSO) has increased substantially in recent years. With the experience acquired in the development of optical terminals for balloons and aircraft, a group of employees from the institute founded ViaLight Communications GmbH (VLC) in 2009. VLC develops and sells optical communication products.