This article gives some details about a DARPA prototype observation platfrom that would operate at 65,000 feet for up to 10 years. Not sure of what part of the spectrum it would operate in, but radar is mentioned, and it seems that it will be using large antenna structures. Sounds technically cool – solar panels used to recharge the hydrogen fuel cells that power the craft, which is filled (and buoyed) by helium.
The Air Force has signed an agreement with DARPA to develop a demonstration dirigible by 2014. The prototype will be a third as long as the planned surveillance craft — known as ISIS, for Integrated Sensor Is the Structure, because the radar system will be built into the structure of the ship.
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