"Ukrainians put 8,000 cellphones connected to microphones on 6-foot poles around the country to detect incoming Russian drones. They detected 84/84 and shot down 80 of them with AA guns. Cost: $500ea
--- Gen James B Hecker, Commander US Air Forces, Europe"
https://www.youtube.com/watch?v=1O0sRM-uY7E&t=109s