Using ambient emitters (eg, television stations, radio stations, etc) as the source in bistatic radar is a technique literally as old as radar. The first British experiments used a BBC broadcast station as the source. During the war the German parasit system used the British Chain-Home radars to track British bombers - the Germans figured that no matter how much the British jammed and windowed them, they'd never turn off Chain-Home. They were right.
And yes, it defeats stealth. First because the receiver isn't where the emitter is, and so the stealth shaping is no longer redirecting the energy away from the receiver. Second because most of those sources are lower frequency than the X-band ST radars stealth is designed around. The lower frequencies "see" the whole aircraft, so the shaping is ineffective even with a monostatic system, and of course the skin is less absorptive at those freqs too. The downside is that lower frequencies give you less localization, but they get you close enough that you can switch to other techniques.
Ambient sonar is also in widespread use. Lots of ambient sources in the ocean - or even out of it. Anything supersonic in the sky dumps a lot of energy into the water over a big area. The must have been so sad when Concorde was grounded. It was even on a regular schedule...
All supposedly secret too, but it doesn't take a rocket scientist. You can get the math off wiki.
