In a bistatic radar the transmitter and receiver are in different places. A passive radar goes further still: it transmits nothing itself and uses the signals of other people's transmitters, such as radio and TV.
In practicePassive radars (such as the Hensoldt TwInvis, Passive radar) see aircraft by the reflection of FM radio and TV signals. Because they transmit nothing, no RWR warns and an anti-radiation missile has nothing to home on. A stealth shape that deflects energy away from the transmitter may send it precisely towards such a receiver.
An ordinary radar is monostatic: transmitter and receiver are in the same place and usually share one antenna. In a bistatic radar they are some distance apart, separated by a baseline. With several receivers (or transmitters) it is called multistatic. Passive radar (Passive radar) is a bistatic radar with a transmitter that does not belong to the radar itself.
A bistatic radar does not measure the distance to a single point, but the sum of two distances: transmitter → target → receiver. All points with the same sum lie on an ellipse with transmitter and receiver as its foci. Only with the measured bearing from the receiver is the position on that ellipse known.
In the radar equation (Inverse square law), R⁴ is replaced by the product RT² × RR². The operational properties are what make bistatic radar interesting:
- The receiver is silent. It emits nothing and therefore cannot be found with ESM or attacked with an anti-radiation missile (Anti-radiation missiles and the operator's dilemma). Only the transmitter — which can be far behind the lines — gives itself away.
- Jamming becomes guesswork. A jammer has to reach the receiver, but does not know where it is. Its energy therefore goes in the wrong direction or is spread over a wide sector.
- Counter-stealth. Stealth shaping (Stealth) deflects energy away from the transmitter — but possibly straight towards a receiver elsewhere. At a bistatic angle close to 180° (target between transmitter and receiver, forward scatter) the RCS increases sharply, regardless of shape.
Against that stand drawbacks: transmitter and receiver must be precisely synchronised in time and frequency, coverage depends on the geometry, and the processing is more complex. A well-known bistatic system sits in every SARH weapon: the illumination radar is the transmitter, the missile's seeker the receiver (Semi-Active Radar Homing).
Radartutorial.eu: bistatic radar.