I remember reading a long time ago about multi-static radar systems, were you have multiple transmitters and receivers in different locations. IIRC the idea was

I remember reading a long time ago about multi-static radar systems, were you have multiple transmitters and receivers in different locations. IIRC the idea was that a) transmitters can be very cheap and even unmanned (so it doesn't make sense to fire a $million HARM to whack a $1000 transmitter) b) to the extent that stealth aircraft are designed to not reflect incoming waves back towards the transmitter, multi-static radar systems could detect such aircraft better than traditional systems. No idea if anything like this is actually in use anywhere, or whether it was/is just an academic curiosity.