 It served as the basis for DARPA's Autonomous High-Altitude Refueling program which, in 2012, demonstrated the potential for fully autonomous aerial refueling of unmanned air vehicles.The final test involved modified RQ-4 drones flying in close formation less than 100 ft from a tanker, close enough for refueling to take place.Appropriate flight control systems (FCS) include sensors to detect the position of the tanker and its refueling drogue along with rules to control the client aircraft.