Have come across a passive radar detection system called Kolchuga manufactured by Ukraine.
First...There is no such thing as a 'passive' radar system. Radar detection is a two-part process: transmit and receive. Absent either and there is no 'radar'. Whether either part is under the control of a single operator is a different issue and that is where the misunderstanding and label 'passive radar' came from.
Apparently it can detect stealth aircraft by triangulation of emissions of the radar equipment of the aircraft itself and of the disturbance produced in the atmosphere by hot gasses of the engine of the flying object. Iran claims to have purchased this system.
There is another system called VERA developed by the Czechs that performs similar function. Such a system would be immune to jamming as being 'passive' it does not emit any radar beam. However, since none has actually been used in a combat environment, it is difficult to prove or disprove the producers claims
Both the Kolchuga and the VERA have been discussed here before, or at least the basic principles of them. Both systems are 'bi-static' radar system, which is where the transmitter and receiver are physically and geographically distinct from each other.
Here...
If the transmitter is the typical radar system, which is 'mono-static' meaning one antenna does the job of transmit and receive, the amount of echo power that it will pick up will be at least an order of magnitude less than the amount of echo power that is reflected
AWAY from it.
But in a 'bi-static' radar system, the transmitter does exactly just that -- transmit -- while the many receiver stations are the ones who will pick up the majority of echo power. Due to the laws of physics, Receiver A will receive less echo power than Receiver B, but if there is another receiver that is directly behind the target, we have what is called 'back-scatter' radar, which is category of 'bi-static' radar systems, and will offer the greatest odds, not certainty, of detecting 'stealth' aircrafts.
The Kolchuga, VERA, and the US Silent Sentry systems uses 'illuminators of opportunity', meaning the 'transmit' part of radar detection is not under their control. Those 'illuminators of opportunity' are television, radio, and cellular signals that are plentiful in dense population areas. This is the source of confusion and misunderstanding that gave the lay public the wrong label 'passive radar'. I do not care what the popular press and commentators may say. I only care that technically speaking, there is no such thing as a 'passive radar' system. They should be called 'passive' sensors because that is exactly what they do: they pick up echoes from aircrafts that deflects those TV, radio, and cellular signals.
The major problem of these passive sensor systems are obvious: structural. We need geographically distinct transmitters and receivers throughout the area of intended coverage.
In the above example, Transmitters 1 and 2 could be those TV, radio, and cellular sources of opportunity.
If the system depends on 'illuminators of opportunity' such as TV, radio, and cellular transmitters, then an obvious vulnerability exist: electricity. Desert Storm is instructive in that the US attacked electrical power stations and this tactic will at least produce serious passive sensory coverage gaps because the receivers are deprived of those TV, radio, and cellular signals that they depended upon.