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Old 09-12-2017, 12:17 PM   #11
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Originally Posted by Vatikus View Post
@Nerd1000

Yes, the clutter should be made out of - & I, however there is also blooming effect (crt brightness side effect) when you have a lot of ground return. This is what we see with those "fuzz" as you mentioned.
True, it would been nice to have it done properly.

(ground clutter of RP21: http://www.mig-21-online.de/Funkmessvisier/fmv_UEH.htm )
Very interesting website. Thanks!
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Old 09-12-2017, 03:00 PM   #12
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Just did some testing, seems that the radar is finally horizon stabilized, that the antenna tilt filter is working again and they indeed increased the clutter when you are getting ground returns from a hill or clouds. Even if the radar is horizon stabilized thought, when i point my nose to the ground (shallow dive, -10°) i get more clutter then when i'm level (i tested it for a couple of different altitude and observed the same behavior), now i think the antenna can tilt up and down more then 10°, so that would be wrong.
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Old 09-12-2017, 03:50 PM   #13
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Originally Posted by Rex854Warrior View Post
Just did some testing, seems that the radar is finally horizon stabilized, that the antenna tilt filter is working again and they indeed increased the clutter when you are getting ground returns from a hill or clouds. Even if the radar is horizon stabilized thought, when i point my nose to the ground (shallow dive, -10°) i get more clutter then when i'm level (i tested it for a couple of different altitude and observed the same behavior), now i think the antenna can tilt up and down more then 10°, so that would be wrong.
Nice info, thanks !

Some screens would be welcome to illustrate
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Old 09-12-2017, 07:25 PM   #14
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The effect breaks off at ~12k feet, wich makes sense since this is about the altitude where clutter starts to disapear alltogether. Same altitude as another Mig-21 flying in front (to test for antenna stab).



Level, the contact is in the clutter a little bit to the left.



10° dive, contact is still there, in the clutter, but i've got returns from the ground, far.



20°, contact is still here, clutter all over the place.

It all depends on how much the antenna can tilt up and down and how big the cone is, the "radar field was limited to 20° vertically" taken from wikipedia page on the RP-21, not sure if it's cone + tilt or just tilt, is it 10° up, 10° down tilt only,..... Many questions .

EDIT : Two screenshots are gone for good :/
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Old 09-13-2017, 04:49 AM   #15
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Radar is not older RP-21 (Spin Scan) but is new RP-22 (Jay Bird). The RP-21 had a scan pattern that was fixed relative to the airframe. It had up to 3 lateral bars about which the conical spinning beam traversed.

The RP-22 is different having no rotation to its pattern, 10 lines of scanning (5 interleaved with 5), and this pattern was stabilized relative to the horizon within antenna limits.

The RP-22 antenna is mechanically limited to approximately 60 degrees in traverse and 60 degrees elevation. In the normal scan mode only a portion of this solid angle is searched, roughly a 60 degree wide, 20 degrees tall wedge. Mostly this scan zone is pointed above the horizon with a small extent vertically below. The lower altitude filters would reduce sensitivity and in the lowest alt filter also shift the scan pattern totally above the horizon. The limits of antenna axis (in scan) are actually 28° from center laterally and the vertical range of 17°40'. The zone which targets may be detected in is slightly larger than this as the beam itself has some width (~3°).

The radar may track a 16m^2 target at 15km (MiG-21 being 3m^2 for comparison). Assuming an R^4 range relationship being 3/16th the size would be tracked at (3/16)^1/4 distance (65.8%, 10km). A B-52 having 100m^2 RCS would have a simple math improvement of (100/16)^1/4 power (158%).

https://www.flightglobal.com/FlightP...20-%200515.PDF (and ...516.PDF)
Spoiler:
Jay Bird
Late-model MiG-21s carry a more advanced radar than the Spin Scan fitted to earlier aircraft. The Nato reporting name Jay Bird may refer either to the operating frequency, which lies in J-band, or to the Fishbed J (MiG-21MF), which first carried the set. Jay Bird operates at three different PRFs: 1,592-1,792 pulses/sec, 2,042-2,048 and 2,716-2,724. For search operations the antenna performs a raster scan at between 2.9 and 3.6 scans/sec. Lobe switching is used when tracking targets. Unlike Western radars of similar vintage, Jay Bird does not use pulse-compression techniques to improve the resolution. US sources claim that, like all current Soviet Al radars, it cannot reliably distinguish low-altitude targets and ground clutter.


Average 3.25 scans per second means lateral bars. As each half-scan consists of 5 sweeps at different elevations it would take approximately 1.5s or 3.1s for all 10 bars. As 17.67 degrees is nearly exactly covered by 5 bars of 3 degree width it's likely to detect a target each half-scan within the scan region.

The intermediate filter "may be insufficient if target is <3000m and in mountainous terrain" to the extreme low altitude filter which raises the bottom of the scanning by ~3° to above the horizon. From there the primary limitation in radar use is safety when flying beneath a low altitude target.

Last edited by Frederf; 09-13-2017 at 04:55 AM.
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Old 09-13-2017, 02:52 PM   #16
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Quote:
Originally Posted by Frederf View Post
Radar is not older RP-21 (Spin Scan) but is new RP-22 (Jay Bird). The RP-21 had a scan pattern that was fixed relative to the airframe. It had up to 3 lateral bars about which the conical spinning beam traversed.

The RP-22 is different having no rotation to its pattern, 10 lines of scanning (5 interleaved with 5), and this pattern was stabilized relative to the horizon within antenna limits.

The RP-22 antenna is mechanically limited to approximately 60 degrees in traverse and 60 degrees elevation. In the normal scan mode only a portion of this solid angle is searched, roughly a 60 degree wide, 20 degrees tall wedge. Mostly this scan zone is pointed above the horizon with a small extent vertically below. The lower altitude filters would reduce sensitivity and in the lowest alt filter also shift the scan pattern totally above the horizon. The limits of antenna axis (in scan) are actually 28° from center laterally and the vertical range of 17°40'. The zone which targets may be detected in is slightly larger than this as the beam itself has some width (~3°).

The radar may track a 16m^2 target at 15km (MiG-21 being 3m^2 for comparison). Assuming an R^4 range relationship being 3/16th the size would be tracked at (3/16)^1/4 distance (65.8%, 10km). A B-52 having 100m^2 RCS would have a simple math improvement of (100/16)^1/4 power (158%).

https://www.flightglobal.com/FlightP...20-%200515.PDF (and ...516.PDF)
Spoiler:
Jay Bird
Late-model MiG-21s carry a more advanced radar than the Spin Scan fitted to earlier aircraft. The Nato reporting name Jay Bird may refer either to the operating frequency, which lies in J-band, or to the Fishbed J (MiG-21MF), which first carried the set. Jay Bird operates at three different PRFs: 1,592-1,792 pulses/sec, 2,042-2,048 and 2,716-2,724. For search operations the antenna performs a raster scan at between 2.9 and 3.6 scans/sec. Lobe switching is used when tracking targets. Unlike Western radars of similar vintage, Jay Bird does not use pulse-compression techniques to improve the resolution. US sources claim that, like all current Soviet Al radars, it cannot reliably distinguish low-altitude targets and ground clutter.


Average 3.25 scans per second means lateral bars. As each half-scan consists of 5 sweeps at different elevations it would take approximately 1.5s or 3.1s for all 10 bars. As 17.67 degrees is nearly exactly covered by 5 bars of 3 degree width it's likely to detect a target each half-scan within the scan region.

The intermediate filter "may be insufficient if target is <3000m and in mountainous terrain" to the extreme low altitude filter which raises the bottom of the scanning by ~3° to above the horizon. From there the primary limitation in radar use is safety when flying beneath a low altitude target.
very intresting, that explains the clutter when diving then (if i understood well).
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Old 09-13-2017, 08:05 PM   #17
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so the scan pattern is one of the two attached?
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Old 09-13-2017, 09:10 PM   #18
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Sorry to burst your joy bubble, but ground clutter in the Mig continues to be as unreal as it used to be. It shouldn't be a yellow glowing stain on the scope, but a bunch of false contacts that should dinamically be affected by terrain geometry - as somebody already stated on this thread.

BST F-5 did an excellent modelling of ground clutter for the APQ-159, for example.

Regards!
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Old 09-13-2017, 09:14 PM   #19
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Quote:
Originally Posted by amalahama View Post
Sorry to burst your joy bubble, but ground clutter in the Mig continues to be as unreal as it used to be. It shouldn't be a yellow glowing stain on the scope, but a bunch of false contacts that should dinamically be affected by terrain geometry - as somebody already stated on this thread.

BST F-5 did an excellent modelling of ground clutter for the APQ-159, for example.

Regards!
It is affected by hills and moutains. i'll report on Mantis hub the clutter not being correctly represented.

And BST, while modelling the ground clutter nicely, forgot about clouds and chaffs ^^.
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Old 09-13-2017, 09:22 PM   #20
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Originally Posted by Rex854Warrior View Post
It is affected by hills and moutains. i'll report on Mantis hub the clutter not being correctly represented.

And BST, while modelling the ground clutter nicely, forgot about clouds and chaffs ^^.
Last time I checked (it was long ago, I admit), clouds representation in the Mig was pretty random, like it just read how broken the sky was and then adjust "stain density" accordingly, but randomly distributed. It didn't work well with dynamic weather. I don't know if in the meantime they've reprogrammed the whole thing to read actual cloud positions, I doubt it though.

Regards
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