is an ECCM fix
against AM-modulated jamming in angles
It was introduced for the first time in C-25 Berkut, later - into C-75 Volkhov and C-125 Neva. In Berkut, this mode was called GSh, in Neva - GShN. I guess that the abbreviation GSh stands for the Russian Gashenie Shumov
- that is, noise reduction
, letters V and N means the implementation in Volkhov and Neva systems.
GShV is a side-lobe blanking and AM-demodulation scheme based on the comparison of the signal from the narrow-beam antenna, epsilon or beta (this is called main channel) with a reference signal coming from the wide-beam antenna (this is called auxiliary, or compensation channel).
In order to understand the main idea, you need some basic math.
- target return (no jamming) in by main receiving channel. Uaux(O)
- target return (no jamming) in auxiliary receiving channel. Ujam(O)
- AM-modulated signal, introduced by jammer.
Here, letter U
means voltage (corresponds to the signal strength in respective channel), O
- means angle (O
When tracking a jamming target, the return from the target in the main channel is Umain(O)
where the factor Ujam(O)
accounts for the parasite AM-modulation. Jamming is so strong, that Ujam(O)
might obscure target return Umain(O)
In auxiliary channel, the return is Uaux(O)
(note that you have same Ujam(O)
in the auxiliary and main channels because jammer signal is very powerful).
You get rid of parasite modulation by dividing the two signals: Umain(O)*Ujam(O)
) = Umain(O)/Uaux(O)
Now comes the trick: since Uaux(O)
comes from the wide beam antenna, its graph is much "flatter", than the Umain(O)
, so one can assume that Uaux(O)
= const (this is not very precise, but enough to understand the idea).Uaux(O)
is amplified in such a way, that the ratioUmain(O)
for any return coming from the first side lobe of narrow beam antenna (this is called the threshold, or base level).
Then the signal is passed to the filter with a cut-off at or below the base levelUmain(O)
so you get a side lobe blanking as a bonus. This process is illustrated below (the form of the curves correspond to the antenna gain diagrams in the corresponding plane).
GSh was first implemented in C-25 Berkut M2 system in 1964. After Vietnam war experiences with Dvina, in 1970 it was introduced to Volkhov (C-75V-M3) and C-125-M Neva. In different systems, they use different antennas are for the main and aux channels, also the wiring for demodulation is done differently (in Berkut - by cutting of the signal at lower threshold, in Volkhov and Neva - by using logarithmic amplifiers in the subtraction scheme).
In C-75M3, separate GShV circuits are used for azimuth and elevation angle tracking.
In azimuth (beta
is connected to P-11V
(azimuth wide beam) antenna,aux channel
is connected to P-13V
(azimuth narrow beam) antenna.
In elevation (epsilon
), main channel
is connected to P-12V
(elevation wide beam) antenna,aux channel
is connected to P-14V
(elevation narrow beam) antenna.
At first this might seem confusing, because one expects narrow beam antenna to function as main receiver and wide beam to function as aux receiver. But the notion of "wide" and "narrow" depends in what plane you are looking! For instance, in beta
-plane, P-11 beam dimensions (vertical x horizontal) are 7.5 x 1.1]
degrees, P-13 beam dimensions are 1.5 x 1.5
; since 1.1 < 1.5
, the P-13 beam is actually wider than P-11 beam in azimuth plane.
1) since GShV needs main/aux channels with wide & narrow beam antennas, in Volkhov it can be used only in LORO ('Podsvet')
2) GShV mode is not compatible with SDC
(the phase information required to filter targets from clutter is destroyed when transforming the signal).
3) GSh modes were designed to counter single jamming target; I've no idea if it works for group targets or in the background of stand-off jamming coming from from EB-66C or similar aircraft.
Sadly, I have found no pictures of the operator view of beta
indicators of a jamming target with GShV on/off.