In Europe there is this radio beacon tracking device listening for emergency services radio signals (Tetra/C2000), called Target Blue[1]. This is not illegal, except apparently for in Belgium.
This is a great idea. I’ve been playing around with a cheap little SDR and seeing all these Tetra signal around. Kinda disappointing there isn’t really anything to do with them since it’s all encrypted. It’s kinda fun to see the meta data and whatnot…but this gives me some ideas!