Whether it is an aircraft that will stay aloft for its entire five-year life, or be rushed by rocket to fill a surveillance gap half the world away, the US Defense Advanced Research Projects Agency wants to push the boundaries of unmanned aviation to extremes. Exploring the outer limits of technology is DARPA's charter, and the agency has its successes and failures, but the stated goals of its latest UAV programmes are raising eyebrows. "We want to completely change the paradigm of how we think of aircraft," says Daniel Newman, manager of the Vulture programme to demonstrate an unmanned aircraft capable of staying aloft for five years. "Aviation has a perfect record - we've never left one up there. We will attempt to break that record." DARPA has set the goals for Vulture as five years on station with a 450kg (1,000lb) payload, 5kW of onboard power and sufficient loiter speed to stay on station for 99% of the time against winds encountered at 60,000-90,000ft altitude, Essentially, the Vulture is an aircraft that operates like a satellite, but is not regulated by orbital mechanics. "It could be positioned over the battle, at 65,000ft versus 260 miles," says Pulliam.

Operating as a pseudo-satellite in the stratosphere and not low Earth orbit would provide a 65dB improvement in communications capability, he says, and significantly increases onboard sensor resolution. As for the design, the Vulture will most likely draw on NASA designs but there is no definitive time table on when we might actually see a working craft.

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