Regarding this topic I would like to know what you think about the potential use of a bi-static benefit of SARH guided missiles against stealth. I posted this a while back on keypub but the reactions were not of much use:
There is a seemingly unusual notion which the Russians have with their SAMs and AAMs which results in the use of SARH/SAGG missiles, although ARH seekers are available.
I will concentrate on SAMs here because the developments there make the case more clear.
Russians have developed various active radar seekers for their SAMs, especially the BUK series and their very long range S-300 with 200km+ range missiles would be candidates for ARH seekers.
However, they stick to SARH seekers even for their new century designs, the S-400 and BUK-M2/-M3, where others like the French, Japanese went for ARH seekers (Aster, etc). One exception would be the naval S-400 variant with ARH being developed but here the anti-cruise missile over-horizon effect should be the reason.
Either their ARH perform bad, or the illumination power of their engagement radars offer still more RF energy on target or there are other benefits for a SARH seekers.
Stealth designs rely on deflection rather than absorption as the key physical effect for their VLO performance. Systems like the e.g S-300/-400 work in a bi-static mode with their SAGG/TVM guidance, means that the RF signal receiver is at a different position and at a different angle than the attacking illumination radar. The deflected RF energy may would not reach the emitting radar, but a bi-static SARH receiving seeker in the SAM that can attack from angle below the VLO target or above it and perform angle search pattern would have better chances to catch deflected RF energy, enabling a lock on the stealth target. This method would decrease the biggest problem with missile radar seekers, namely that they have to work in X-band, for which stealth designs are optimized and no effects like aircraft feature size can be used for improved performance (VHF-band).
One key element for this method to work, is a blind illumination capability of a portion of airspace (without actual track) based on coordinates from a radar system that actually detects the target. There is much debate about this; Russians say that VHF band radars can see smaller stealth designs such as the F-35 like any other radar. Some pro US stealth friends might say the F-35 has the same pea size in VHF-band as in X-band. We can take something in between, but a highly advanced IADS like the Russian one has assets such as the "Container" OTH system for early warning and coarse location, as well es high power static line of sight systems with very large apertures such as the Voronezh and Resonaz systems, with systems like the mobile Nebo-M acting at lower tiers. There is more, such as passive means, EO, IR, multi-static "trap" radars networks.
There must be rough target coordinated available for such a bi-static SARH engagement, with a accuracy as provided by multi-band systems such as the Nebo-M or single VHF-band Nebo-SVU, otherwise the RF energy of the illumination radar wont be sufficiently accurately concentrated on target (airspace portion).
The S-300/400 SAMs have robust missile up- and down-links, hence they would work with course updates via VHF-band target coordinates until terminal phase where the SARH seeker would try to catch illumination RF energy deflected to different directions by the stealth design.
The addition of satellite positioning systems to the engagement radars of S-300PMU2 and S-400 could be a hint for the use of this method.
Hence I have the feeling that this described bi-static method directly benefits from the main function of stealth designs, RF-energy deflection, and could be the main reason why even the S-400 is still old fashioned SARH and possibly why the R-27 soldiers on.
Counter measures like a change of aspect or direction of the VLO asset would not change to overall situation. First there is the question whether it would be worth the risk to change direction after the attack has been detected by sensors and the most optimized face (front) has been directed to the threat emitter.
Even a all aspect VLO asset would still be optimized to deflect waves away from the bore sight of the threat radar. If we assume that the biggest portion of the radar waves are deflected in a bore sight range of 15° to 90° in all directions, a limited degree portion can be determined from the bore sight of the threat radar where the SARH seeker has to search for deflected RF energy. However I'm no expect on RF wave behavior hence this deflection angle band could differ.
More so; if the VLO asset changes aspect relative to the bore sight of the threat illumination radar to change the direction of deflected radar waves, to decrease chances for the bi-static SARH seeker to pick up RF energy, this would force it to expose a less optimized face to the bore sight of the threat radar. So if the SARH seekers lock on the target is broken, the redundant SAGG/TVM system could switch back to command guidance mode as it now has got a own track of the VLO asset.
You may be thinking about a sudden maneuver that would suddenly break the lock of the SARH seeker. But any change would just reduce the amount of deflected RF energy e.g if the VLO asset exposes its least optimized face to the threat emitter, so that more RF energy is sent back to the radar and hence less is deflected to the bis-static positioned SARH seeker.
Advanced SARH missiles with missile up- and down-links + advanced autopilots only make use of the SARH seeker in terminal phase, not like the HAWK or S-200 which had to catch the RF energy at great distances while on the launcher. Hence they need much less of reflected or deflected RF energy. The deflection angles of VLO assets are confined and there should be enough RF energy deflected to that angle band to allow for terminal phase SARH guidance.
Back at Keypub, there was the arument about the penalty on kinematics a bi-static position maneuver would require.
Finally here is a graphics of the effect described in a very adverse form in favour for the stealth assets: