article: Flying gas stations that can refuel robotic military aircraft have come a step closer. The first ever mid-air refuelling of an unmanned aircraft was today successfully achieved by the Navy's X-47B drone. The Salty Dog 502 vehicle is one of two Unmanned Carrier Air Vehicle demonstrators (UCAS-D) on the X-47B program. Scroll down for video . The first mid-air refuelling of an unmanned aircraft was today successfully achieved by the Navy's X-47B drone. Earlier today, the aircraft plugged its in-flight refuelling (IFR) probe into the hose of an Omega Air KC-707 tanker off the coast of Maryland . Earlier today, it plugged its in-flight refuelling (IFR) probe into the hose of an Omega Air KC-707 tanker off the coast of Maryland. During the test, it trailed the Omega Air Refuelling 707 tanker from a mile off before using optical sensors and a video camera to monitor its approach to within 20 feet (six metres). Depending on the weather, the aerial refuelling demonstrations are due to last throughout the weekend. The drone is about the size of an F/A-18 Super Hornet, weighs in at 44,000 lb (20,000kg) and has a 62ft (20m) wingspan. Salty Dog 502 is one of two Unmanned Carrier Air Vehicle demonstrators (UCAS-D) on the X-47B program . The goal of each of the refuelling demonstrations is to transfer 3,000lbs of fuel in five minutes. But this is the final test of the historic X-47B, which will now be donated to museums despite, according to Foxtrot Alpha, only having 20 per cent of their flight hours used up. Refuelling in flight is the last hurdle for the Northrop Grumman-built jet, claims to Captain Beau Duarte, program manager of the Navy's Unmanned Carrier Aviation. With a wingspan of 130 feet, the Triton is wider than commercial airliners such as the Boeing 737 . An airliner-sized Navy surveillance drone made its first transcontinental flight in September, flying 11 hours across the US. The cross-country test was the first for the MQ-4C Triton, which will play a role in the Navy's planned Broad Area Maritime Surveillance program. The drone can provide blanket surveillance for the Navy as part of the branch's Broad Area Maritime Surveillance program. Triton uses 'radar, infrared sensors and advanced cameras to provide full-motion video and photographs' to Navy personnel. Northrop Grumman says the unmanned aircraft also has a capacity to fly up to 28 hours at a time and surveil 1million square miles in a single mission. With a wingspan of 130 feet, the Triton is wider than commercial airliners such as the Boeing 737. Mid-air refuelling is difficult enough for human pilots, but the ability for drones to do it could eman they only ever have to land for equipment updates. The Navy envisions the X-47B's successors, the UCLASS drones, as making 24-7 patrols around aircraft carriers. 'We will have met all of the program's mission objectives and will [take] those lessons learned, fold them into the UCLASS performance specification,' Duarte said. The X-47B drone is about the size of an F/A-18 Super Hornet, weighs in at 44,000 lb (20,000kg) and has a 62ft (20m) wingspan .
highlights: 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 44,000lb (20,000kg)
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   Earlier today, the aircraft plugged its in - flight refuelling (IFR) probe into the hose of an Omega Air KC - 707 tanker off the coast of Maryland . Earlier today, it plugged its in - flight refuelling (IFR) probe into the hose of an Omega Air KC - 707 tanker off the coast of Maryland 
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