A significant capability of unmanned airborne vehicles (UAV's) is that they can operate tirelessly and at maximum
efficiency in comparison to their human pilot counterparts. However a major limiting factor preventing ultra-long
endurance missions is that they require landing to refuel. Development effort has been directed to allow UAV's to
automatically refuel in the air using current refueling systems and procedures. The 'hose & drogue' refueling system
was targeted as it is considered the more difficult case. Recent flight trials resulted in the first-ever fully autonomous
airborne refueling operation.
Development has gone into precision GPS-based navigation sensors to maneuver the aircraft into the station-keeping
position and onwards to dock with the refueling drogue. However in the terminal phases of docking, the accuracy of the
GPS is operating at its performance limit and also disturbance factors on the flexible hose and basket are not predictable
using an open-loop model. Hence there is significant uncertainty on the position of the refueling drogue relative to the
aircraft, and is insufficient in practical operation to achieve a successful and safe docking.
A solution is to augment the GPS based system with a vision-based sensor component through the terminal phase to
visually acquire and track the drogue in 3D space. The higher bandwidth and resolution of camera sensors gives
significantly better estimates on the state of the drogue position. Disturbances in the actual drogue position caused by
subtle aircraft maneuvers and wind gusting can be visually tracked and compensated for, providing an accurate
This paper discusses the issues involved in visually detecting a refueling drogue, selecting an optimum camera
viewpoint, and acquiring and tracking the drogue throughout a widely varying operating range and conditions.