Monostatic vs. bistatic geometry
In a conventional radar, transmitter and receiver share one antenna. The target must reflect energy directly back to the source. Stealth shapes are precisely engineered to avoid this: flat facets, blended curves and absorbent coatings all work against the monostatic return. In a bistatic system the receiver is elsewhere. The reflection angle that stealth optimised against is no longer the one that matters.
Forward scatter
At bistatic angles near 180° — the transmitter behind the target, receiver in front — a phenomenon called forward scatter dominates. Even a stealth aircraft blocks a measurable portion of the signal. The radar cross section in forward scatter can be 1,000× larger than the monostatic RCS. Every TV tower and cell base station becomes a potential illuminator.
Passive bistatic radar
Why build a transmitter at all? Use existing broadcast signals — FM radio, DAB, DVB-T, cell towers — as illuminators. A network of cheap receivers listens for echoes. This is completely covert: no emission, no frequency allocation, no giant radar signature revealing your location. Research systems in the UK, Poland and China have tracked aircraft and ships this way.
Challenges
Synchronising transmitter and receiver in time and frequency is hard. The baseline geometry changes constantly as platforms move. Multipath from terrain and buildings corrupts the signal. And the noise floor is higher when you don't control the transmitter power. But for detecting stealth, these trade-offs are worth it.