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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Automating Aerial Refuelling for UAVs

The approach to the drogue.
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OCTEC Ltd. (Bracknell, U.K.)
Sierra Nevada Corporation (Sparks, Nev.)
NASA Dryden (Edwards, Calif.)
Radstone (Towcester, U.K. and in Billerica, Mass.)
JAI (Copenhagen, Denmark)

The Challenge

Unmanned airborne vehicles (UAV's) can operate tirelessly in comparison to their human pilot counterparts; however, a limiting factor preventing ultra-long endurance missions is the requirement to refuel. Development has been directed towards automatic refuelling using current hose-and-drogue systems. Navigation based differential-GPS techniques allow an aircraft to approach in precise formation with a refuelling tanker, but the high degree of hose flexibility and uncertainty in the actual drogue basket position requires additional sensing.

The Solution

Development has gone into precision GPS-based navigation sensors to maneuver an aircraft into a standard station-keeping position off the tanker, and then progress onwards to dock with the refuelling drogue. The GPS based solution is augmented 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, and removes any bias offsets in the GPS estimates.

Camera viewpoint positions were simulated beforehand to maximise the likelihood of tracking against background clutter. Significant work was also done to ensure accurate range measurements could be provided from a monocular sensor, with increasing tolerance requirements at closer proximity. Alignment of the sensor on the aircraft is determined semi-automatically once fixed on the aircraft, with verification routines implemented to check for subsequent errors. Recognition algorithms are also implemented to verify the drogue in the video, before reporting track measurements are valid, during any event the drogue enters or leaves the field of view. Control of the system is fully automatic once initiated.

The Tools Used

  • OCTEC Ltd. ADEPT 60 VME Image processing board
  • Sierra Nevada Corporation DGPS and flight control systems
  • NASA Dryden F18A flight test aircraft
  • Radstone ruggedized airborne VME chassis
  • JAI remote head camera, model CV-M536

The Difference it Made

The ADEPT 60 Video Tracker visually acquired the drogue when instructed to do so at the pre-contact position and confirmed its validity. For the duration of the process, the track outputs subsequently compensated for disturbance factors in airspeed, tanker attitude and aerodynamic effects on the flexible hose and basket system. Without the precision measurements it supplied, attempting to dock with the drogue basket would be impossible and extremely dangerous. A successful recent flight trial resulted in the first-ever fully autonomous airborne refuelling operation, and was dubbed 'a significant milestone' by the Defense Advanced Research Projects Agency (DARPA).



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