Imagine a large number of crowdsourced software defined radio receivers with FPGA's and extremely accurate femto-second timing calibration connected to the internet spaced apart geographically and a distributed supercomputer to do all the realtime calculations. No single country or army would have this detector but everyone could use it for defense. Now imagine the resolution would be accurate enough for detecting planes, drones, birds, people, ground vehicles, ships, fish, insects, wind, tree leaves. We are close, we just need a cheap planetary deployment (like we had with SETI@home) and write better software. I imagine in 10 years we all have a detector, like we all have a smartphone or a router with a firewall. A passive radar in every building for spotting drones above our house and garden.