I was thinking about this also. Practically speaking, I don't think you would be able to get much from civilian sources. It could be used to effectively boost t

I was thinking about this also. Practically speaking, I don't think you would be able to get much from civilian sources. It could be used to effectively boost the range of your own radar stations though. If you put a passive receiver (of the same receiving sensitivity as your radar station) at the maximum range of your radar station you would effectively increase the range of your system by 50%. But there are some caveats. The biggest is that both your emissions source and your passive receiver need line of sight to the target, making it easier for a jet to break contact at the outer range.