Flight tests to demonstrate automated aerial refuelling have shown an unmanned aircraft can be manoeuvred safely and positioned accurately behind a tanker using a GPS-based relative navigation system.

The flights, using Calspan's variable-stability Learjet 25 as a surrogate UAV, completed Phase 1 testing under the US Air Force Research Laboratory's automated aerial refuelling (AAR) programme.

The next phase will involve actual refuelling-boom contacts and potentially fuel transfers and will require a receptacle-equipped receiver aircraft to act as the surrogate UAV. "The air force wants to do a wet hook-up," says David Riley, Boeing Phantom Works' AAR programme manager.

The system developed by Boeing uses a GPS/INS on the tanker. This sends tanker state information, including velocities and accelerations, via the TTNT high-speed datalink to the receiver aircraft. "On the receiver aircraft, the system does a lot of math to calculate the relative position of the centre of the refuelling envelope on the tanker and the centre of the refuelling receptacle on the receiver," he says.

Riley says the AAR concept of operations calls for the unmanned aircraft to maintain a stable position relative to the tanker, even during turns and turbulence, while the refuelling operator steers the boom into the receiver's receptacle.

For the tests, the AAR system drove the Learjet's variable-stability flight control system. The pilot did not control the aircraft, but was ready to take over in the event of a problem.

The system manoeuvred the Learjet between seven air-refuelling positions behind the Boeing KC-135R tanker, from observation through pre-contact to contact and break away. The boom was not lowered, Riley says, because the Learjet has only a simplex control system "and we could not stand a failure".

The tests "showed the technical feasibility of using a GPS-based system to safely manoeuvre close to the tanker and to stabilise the receiver to receive fuel," he says.

AFRL is expected to seek bids for Phase 2 of the AAR programme between February and April next year. "The goal will be to show stability with the boom attached, to make sure the unmanned aircraft does not fight the boom," Riley says. The receiver surrogate is likely to be a fighter as there are no receptacle-equipped UAVs.

Source: Flight International