Darpa completes UAV inflight refuelling demo.

The Defense Advanced Research Projects Agency (DARPA) has completed its Autonomous Airborne Refueling Demonstration (AARD) programme, showing that unmanned aircraft can autonomously perform in-flight refueling under operational conditions.

[ILLUSTRATION OMITTED]

The AARD used precise inertial, GPS, and video measurements, combined with advanced guidance and control methods, to plug a refueling probe into the centre of a 32-inch" basket trailed behind a tanker. Flights were conducted at Edwards Air Force Base, Calif., with a NASA Dryden Flight Research Center F/A-18, configured to operate as an unmanned test bed, refueling from a 707-300 tanker. A NASA pilot was on board the F/A-18 for safety purposes.

Several control techniques were tested, and the best was 100 percent effective in 18 attempted probe-and-drogue connections. Each attempt was made in level flight across a range of turbulence conditions, the most challenging of which were characterized by up to five feet of peak-to-peak drogue motion, approaching the limits of routine manned refueling operations.

The AARD system also demonstrated the ability to make contact during turns. Although pilots routinely follow a tanker through turns while connected, they typically do not attempt to make contact in a turn. In the AARD programme, fuel was routinely transferred in turns as well as during straight and level flight. The system further demonstrated the ability to join the tanker from up to two nautical miles behind, 1,000 feet below, and 30[degrees] off heading, thus providing a ready transition from the waypoint control approach used by most unmanned aircraft to a fully autonomous refueling mode.

The performance ultimately achieved by the programme was made possible by two major enhancements to the AARD system. Improved video processing eliminated troublesome dropouts, allowing the system to conduct four times as many plug attempts per flight, while advanced control algorithms proved capable of anticipating much of the overall drogue motion. These algorithms were actually able to precisely match the drogue motion--something pilots are specifically taught to avoid. In one case, the system followed the drogue through a full three-foot cycle in the two seconds before making contact, never deviating more than four inches from the exact centre-line of the drogue.