MIG29 and SU27 RDR Ant Stabilisation ??

[IvanK]

Doing some testing to try and determine the exact Antenna scan coverage in elevation on the MIG29 and SU27 has led me to believe that the Antenna stabilisation is based on Nose position not Earth referenced to the horizon or the lower beam is clipped or internally (from a coding point of view) the radar axis is not coincident with the beam centreline in elevation

 

Here is the test I performed.

 

MIG29/SU27 in level flight at 4500m 2 deg pitch attitude about 2 alpha as well. Antenna elev set to zero degrees.

 

2 Il76 Targets head on one at 8500m the other at 500metres.

So each target is 4000m separated from the fighter.

 

If the antenna stabilisation is Earth referenced then it follows that as both targets reduce range then they should disappear outside the scan at the same range. They don't ! The High target goes off scope at 35Km the low target at 50Km.

 

Doing some quick Maths gives a rough difference of 2 degrees ... the same as the pitch attitude.

 

I repeated the test with the Ant set 2 deg down (AH-2, Expected target range 60km). the test was better but the lower target still drifted of the scope at a greater range than the higher target though at a lesser difference than previously. This tends to make me think that its more likely the Antenna axis and beam axis (in pitch) are different.

[Breakshot]

Have you tried doing this test inverted? Im pretty sure the radar is gyroscopically stabilized to the horizon, as it should be... try the same thing while flying inverted and u will see it immediately... ;)

 

As for the lower target disappearing first there can be other factors at play such as ground clutter, thus its necessary to keep the correct azimuth settings and expected target range to "focus on" the target (usually u want to lead the target by 10km), otherwise u risk loosing lower target more easily... the high target on the other hand is silhoetted against the sky so there is more room for detection error...

 

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Btw one thing i noticed which makes no sense, IMO... and it looks to me like a bug! Its regarding EOS...

 

EOS is also stabilized by the horizon just like radar, which is fine.... However when you go inverted you are still limited in your downward scan ability, its as if you were still level (which is due to nose of AC blocking FOV for EOS).

 

But this is wrong because in inverted position you SHOULD be able to scan down below as EOS FOV is not obstruted anymore! Oh and funny thing is if you actually aquire lock first from level flight in EOS, and then go inverted in a climb, you CAN maintain track through the EOS looking downward in this fashion... the problem is that you somehow cant actually scan there when u are passive BVR searching mode!! this is obviosly due to down scan parameter being "locked" to just a few notches... even in inverted flight

 

This needs a fix!

[IvanK]

"Have you tried doing this test inverted? Im pretty sure the radar is gyroscopically stabilized to the horizon, as it should be... try the same thing while flying inverted and u will see it immediately... ;)"

 

I am talking about Pitch stabilisation not roll stabilisation :)

 

This was done over water to minimise any clutter issues. Both Contacts appeared at the same time. Look down into clutter was not an issue. When the lo tgt disappeared (a simple click down on a subsequent test showed the low tgt still there again confirming lookdown clutter was not an issue.

 

No requirement in the test to vary Ant elev. The specific purpose of the test was to determine exactly when the contacts flew out of the scan to determine the angular limits of the beam. Varying elevation would invalid the test.

 

The fact remains that with 0 antenna elevation 2 contacts with equal (but opposite) Altitude seperation fly out of the scan at significantly different ranges .... this should NOT happen. It leads to one conclusion the radar beam is NOT symmetrical in elevation.... why ?

 

The radar may be Roll stabilised Earth vertical but in pitch either the beam is asymmetrical or its not exactly aligned with the horizon ... something is broken.

Edited April 21, 2010 by IvanK