The Autonomous Airborne Refueling Demonstration used precise inertial, GPS, and video measurements, combined with advanced guidance and control methods, to plug a refueling probe into the center 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 program, 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.

In recent flights, automatic sequencing reflected improved confidence in the system, compared to last year's flight where pilot consent was required at specified points in the refueling maneuver.

While NASA test pilot Dick Ewers characterized some of the less mature versions of AARD software as "flying like a second lieutenant," he found the final configuration demonstrated this year "better than a skilled pilot."

Ewers explained, "Skilled pilots can actually save some tricky, last second movement the basket has a habit of making, but in so doing they set themselves up for a basket strike, ripping off the basket from the hose, or sometimes breaking the probe or parts of the airplane."

The exceptional performance ultimately achieved by the program 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 centerline of the drogue, all while traveling at 250 miles per hour, 18,000 feet above the Tehachapi Mountains.

Autonomous in-flight refueling is a critical enabler for affordable persistent unmanned strike systems. It offers to revolutionize unmanned air operations and enhance the reliability, safety and range of operating conditions for in-flight refueling of manned aircraft. Sierra Nevada Corp. led the AARD team. The 707-300 tanker was operated by Omega Air Refueling Services.

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