Never a group to pass over an interesting idea, no matter how costly or otherwise impractical, DARPA have been thinking about ways to keep a whole city and its environs under watch, 24/7. They rather like the idea of a huge robotic surveillance-panopticon blimp floating high above the target town – the trick will be making the entire hull of the airship work as a phased-array antenna. I’m not going to hold my breath.