A U.S. government project to develop unmanned aerial vehicles capable of staying in flight for five years has entered its second phase.

The Defense Advanced Research Projects Agency (DARPA) is looking for designs for its Vulture program. The UAVs must be able to remain at 60-90,000 feet for years at a time, surviving the jet stream winds found at those altitudes. The designs must also be able to provide 5kw of power and carry a 1,000lb payload, allowing the Vulture II to act as a communications and sensor platform.

Three teams - Aurora Flight Sciences, Boeing Integrated Defense and Lockheed Martin — are competing for US$155 to build a small-scale demonstration model of the Vulture II high-altitude UAV.

Each design is currently relying on solar power during the day and battery power overnight.

The current phase of the Vulture project, which is expected to run until mid-2012, will end with an uninterrupted three-month flight demonstration by the three teams.

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