https://tech.slashdot.org/story/16/09/17/0231204/the-us-government-is-buildi... The US government plans to launch "a three and a half year initiative to develop an urban drone detection system." The Aerial Dragnet program is to use off-the-shelf commercial components and mostly established technologies and methods to create a network of floating or tethered platforms that will ultimately provide 95% efficient drone identification in urban areas up to 180 square kilometers. The call to proposers states that the total cost of the system for a city should be around $90,000, and would likely include the ability to identify the micro-Doppler signatures given off by UAVs -- and birds. Unmanned aerial systems are becoming platforms "for hostile reconnaissance, targeting, and weapon delivery," warns the government document, noting drones are hard to detect because they're small and fly slowly at low altitudes. "In future urban battlegrounds, U.S. forces will be placed at risk by small UAVs which use buildings and naturally-occurring motion of the clutter to make surveillance impractical..."