Bloodhound 2 – Non-Coherent Jamming

A little while ago Pete Murray asked whether the Type 86 was vulnerable to non-coherent CW jamming. I reported in the Work Day blog for 22nd July that he tried it out on the simulator and it appears that the radar can be jammed by an non-coherent jamming signal at the skin echo frequency. As promised here is that more detailed report.If the jamming signal perfectly overlaps the skin echo, the radar will not acquire the jamming signal (as it fails the coherency check) and it cannot ‘see’ the skin echo.

The radar was able to acquire on a side band if the target was set-up to generate them.

Radar Display Non Coherent Jammer wm.jpg

Pete concludes that intelligent, autonomous jammers of the mid-eighties would have been able to jam the Type 86 radar using fairly simple non-coherent jamming techniques.

The Swiss modified their T87 radars to remove this vulnerability; it’s surprising the UK didn’t do likewise with the T86.

Here’s the theory:

  • Each type 86 had a unique ID modulation frequency. When a target is first detected, a coherency check is carried out on the received modulation. If the check fails, because the modulation frequency is wrong or absent, the target is rejected.
  • If a target is able to re-transmit a boosted copy of the skin echo without the ID modulation, the radar will not see (AGC out) the true return that carries the ID modulation.
  • If the jamming signal differs from the skin echo by any amount, the radar will see the skin echo.
  • Deviation of the ID FM widens with decreasing range, which might help the radar see the skin echo as range decreases.

The technology to analyse the radar signal and transmit a perfectly tailored jamming signal probably wasn’t available until the mid 80’s.

Pete thinks there must have been some sort of work around that allowed non-coherent jammers to be tracked, but he has not been able to find it. Otherwise, it seems odd to build this type of jamming into the RAF simulator.

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