U.S. military researchers are surveying industry for technologies to provide persistent wide-area surveillance of unmanned aircraft operating not more than 1,000 feet above the ground in large cities. There is no good way to track unmanned aerial vehicles (UAVs) in the sky. Drones, especially commercial or hobbyist drones, are small enough to appear like birds on radar and there isn't yet a system requiring them to broadcast their location to traffic control. The Defense Advanced Research Projects Agency (DARPA) wants to change that. The DARPA Aerial Dragnet project seeks to map all drones in the sky, especially unknown and hostile drones in war zones. NASA, which has a project for tracking drones, wants a way for drones to comply with each other and laws in friendly skies.