The drone that can fly for years on solar power

While most of the exotic aircraft on display at this year's Farnborough Airshow still need a pilot, unmanned vehicles have become more and more common in recent years.

Flying robots have some key advantages over conventional aircraft - as the BBC's Theo Leggett found out from Paul Stevens, chief engineer of Airbus's high altitude pseudo-satellite (HAPS) Zephyr - a drone designed to fly at altitudes of up to 20km for years at a time on solar power alone.