Optical link gives ground controllers a perfect view

11 Jul 2007

French researchers have demonstrated a two-way laser-based communications link between an aircraft, a geostationary satellite and the ground.

LOLA

The communications link between a Mystère 20 aircraft and the Artemis telecommunications satellite delivered clear audio and video footage of the cabin and countryside below to a team on the ground, proving the operational performance of this kind of laser optical link. The demonstration was made at the Paris Air Show by Astrium, a subsidiary of the European Aeronautic Defence and Space Company (EADS).

Maintaining a robust link placed some stringent requirements on the optical system, as Gilles Planche of Astrium EADS explained to optics.org. "Our 150 mW laser source had to hold a pointing accuracy of 1 µrad, since the optical antennas were only 12 cm in diameter but separated by large distances - up to 40,000 km."

Preparing LOLA

The system also had to deal with the constant disturbances and perturbations that the airborne environment caused. "High bandwidth and robust control were essential to maintain the link," said Planche. "We used silicon carbide structures to provide high stiffness and stability, and a CMOS detector with windowing capability to allow either full matrix reading or specific area selection."

The system proved able to establish a permanent link in less than one second and achieved a reliable transmission rate of 50 Mbit/s in both directions. Sensitivity was claimed to be better than 80 photons per bit.

Modeling and simulation played a large part in developing the laser link from concept to testing in less than three years. "Atmospheric propagation modeling, especially on the return link from aircraft to satellite where perturbations are close to the antenna, was a big challenge," said Planche. "In fact, no observations were available to validate the model of this critical link before the flight."

The plane

The demonstration was part of LOLA (Liaison Optique Laser Aéroportée), a French program to develop an optical link between medium- and high-altitude unmanned aerial vehicles and ESA's Artemis geostationary satellite.