salty dog 502 trailed omega air refuelling 707 tanker from a mile off .
it used optical sensors and a camera to monitor approach to 20 feet .
it then plugged its refuelling probe into hose of the omega air kc-707 .
drone is size of an f/a -18 super hornet and weighs 44,000 lb -lrb- 20,000 kg -rrb- .