Like many drones before it, Taranis is designed for persistent surveillance (flying over an area for hours, with cameras and sensors pointed down). Another goal of Taranis is to demonstrate the ability of a drone to deter adversaries in the sky. Drones like the Predator and Reaper, used by the United States over Afghanistan and elsewhere, are slow, propeller-driven light bombers. They fly in skies without hostile aircraft--an effective strategy for decade-long wars of counterinsurgency, but not so much for wars in which drones may encounter enemy aircraft. Besides stealth construction, it's unclear how the Taranis would deter enemy aircraft. The UK and BAE Systems aren't giving out all their secrets.