Radar is supposed to be fixed to the aircraft axis, but behaves as if its fixed to the horizon.

[Kobymaru]

According to the MiG-21 DCS manual and any other source I could find, the radar of the MiG-21 is supposed to be fixed.

Quote

The Sapphire’s antennae can’t be manually moved up-down or left-right like in modern fighters. Instead, it will scan ±30° in azimuth, and -1.5° and +17° in elevation,

If the radar can not be moved, then these limits should be relative to the aircraft axis and not the horizon. Because why should the radar care about the horizon if its fixed and pointed along the axis? This is not the case in DCS.

You can easily verify this yourself by flying behind a target and slightly below, then pull your nose up so that it points way above the target, and observe how the radar contact does not disappear until you have climbed significantly above the target.

In this screenshot, I am about 150 below the target before I pull up the nose. You can observe that the target is way below the small aiming cross which is -26.5 mrad = -1.5° which should signify the lower limit of the radar. Specifically, we're at around -110 mrad = 6.3° ! We can happily aim at targets 6.3° below our centerline as long as we're below it.

The same situation but reversed happens when you fly above, and then point your nose downwards. Here, I pointed my nose at the target and kept it ABOVE the centerline, which should make the target appear on the radar. It does not appear, until my altitude is almost the same as the targets altitude.

Assuming that the radar is fixed to the horizon, then if you are 5km behind the target, this means that the lowest the target can be is sin(1.5°)*5000m =  130m below your altitude and the highest the target can be is sin(17°)*5000m = 1460m above your altitude. You can verify that this is approximately the altitude difference at which the contact starts disappearing, completely regardless of your aircraft attitude.

An even more fun way to verify this is flying inverted. Here, the target is CLEARLY far below -1.5°. It is also to the LEFT, relative to my current (inverted) attitude, but to the RIGHT relative to a level upright attitude. The radar shows the target on the right.

Hopefully the radar will get some attention for the rework.

Here's a track file to make bignewy and nineline feel better:mig21-horizon-radar.trk

[æck]

What's meant by the DCS manual is that the radar can't be manually slewed around by the pilot like in later jets. It's still gyro stabilized to the horizon within a range of ±70° in roll and +30° to -25° in pitch.

[Kobymaru]

22 hours ago, æck said:

What's meant by the DCS manual is that the radar can't be manually slewed around by the pilot like in later jets. It's still gyro stabilized to the horizon within a range of ±70° in roll and +30° to -25° in pitch.

Thanks for your input! I tried to do some digging and you're right that it is stabilized in roll with ±70° in roll.

However, I could not confirm the pitch stabilization. Do you have a source for this information?

[æck]

8 minutes ago, Kobymaru said:

Thanks for your input! I tried to do some digging and you're right that it is stabilized in roll with ±70° in roll.

However, I could not confirm the pitch stabilization. Do you have a source for this information?

i'll have to be a bit careful here regarding forum rules about documents newer than 1980, so here goes:

The source is the manual "Samolot MiG-21bis Instrukcja Uzytkowania" as published by Muzeum Lotnictwa Polskiego in their digital archive. The front page indicates that the document is declassified.

https://www.muzeumlotnictwa.pl/index.php/digitalizacja/katalog/1363

[Kobymaru]

59 minutes ago, æck said:

The source is the manual "Samolot MiG-21bis Instrukcja Uzytkowania" as published by Muzeum Lotnictwa Polskiego in their digital archive. The front page indicates that the document is declassified.

https://www.muzeumlotnictwa.pl/index.php/digitalizacja/katalog/1363

Good find! This helped me find the corresponding section in the english MiG-21 manual:

Looks like this is specific to the context of the Low Altitude mode. Is it in the same context in the polish version?

 

The reason why I thought it is stabilized only in the roll axis is this:

https://www.mig-21-online.de/mig-21/funkmessvisier/fmv-beschreibung/

 

Quote

Das FMV gewährleistet:
– das Absuchen der vorderen Halbsphäre des Flugzeuges bis zu einer Entfernung von 20 km im Seitenwinkelbereich von +/- 30° und im Neigungswinkelbereich von +/- 12° bezüglich einer Achse, die um 3° gegenüber der Flugzeuglängsachse geneigt ist, dabei ist die Antenne bis zu Schräglagen des Flugzeuges von +/- 70° stabilisiert.

translation

Quote

The FMV ensures
- the scanning of the front hemisphere of the aircraft up to a distance of 20 km in the lateral angle range of +/- 30° and in the elevation angle range of +/- 12° with respect to an axis that is inclined by 3° relative to the longitudinal axis of the aircraft, whereby the antenna is stabilized up to a bank angle of the aircraft of +/- 70°.

 

A similar section is from this document (page 17,18) https://archive.org/details/RP-21_Sapfir_radar_technical_manual/RP-21M -MA technical description/

Bit strange that they emphasize roll stabilization but make no mention of pitch stabilization.

Either way, doesn't seem like it 100% clear which way it's supposed to work.

[PawlaczGMD]

Even though you were wrong, I've learned that the radar is horizon-stabilized - interesting!

[Kobymaru]

8 hours ago, PawlaczGMD said:

Even though you were wrong, I've learned that the radar is horizon-stabilized - interesting!

It seems to be horizon stabilized in low-altitude mode, but in my interpretation not in regular mode.

[zerO_crash]

Not sure what you have a problem with understanding, or inventing new workings for?!

 

Going off Russian manuals, those translations are more or less 1-1. There is nothing to be surprised about that the manuals have low levels of "ease of use", "accessibility" and "structured formatting". Thse concepts are relatively modern for manuals all around, and are often still lagging behind on subjects which are complex in nature. If you'd read any original older manual, you'd see that it is common to have to skim through the document, as information will be spread across the document. That is also why pilots had to make notes during classes, otherwise it would be a job worth of a lawyer to collect everything in convenient places.

 

Everything you need to know, is precisely stated in the manual: the radar is horizon-stabilized, unless you operated it in "beam mode". With the given references for pitch and roll tollerancies, if you exceed them, you lose the target. Plain and simple. 
 

As to the "low altitude"-mode; that applies a filter to the radar which blocks out ground return. A different, but similar filter, is the one concerning filtering out cloud returns. They are simply filters, no more, no less.

 

That's all there is to it.