Each weekday, dozens of U.S. government aircraft take to the skies and slowly circle over American cities. Piloted by agents of the FBI and the Department of Homeland Security (DHS), the planes are fitted with high-resolution video cameras,often working with “augmented reality” software that can superimpose onto the video images everything from street and business names to the owners of individual homes. At least a few planes have carried devices that can track the cell phones of people below. Most of the aircraft are small, flying a mile or so above ground, and many use exhaust mufflers to mute their engines — making them hard to detect by the people they’re spying on.