Understanding the MiG-21 radar orientation. - ED Forums
 


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Old 03-10-2018, 01:57 AM   #1
JLX
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Default Understanding the MiG-21 radar orientation.

I'm trying to better understand how the physical orientation of the radar works in the MiG-21.

I'm clear on what the manual states:
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, searching for any targets at a maximum of 30 km distance.

When [the compensation mode switch is] in the middle position, the radar will try to erase the lower side-lobes, thus cleaning the image; in the top position it will tilt its antenna upwards by about 1.5°
From this, I gather the orientation and position of the radar dish is fixed relative to the plane. The only exception being the option to tilt it up 1.5° in the dorsal direction.

In all the examples/tutorials I've seen/read, the plane is always in a strictly horizontal orientation. What I want to understand is how and what the radar displays when it is NOT horizontal.

I constantly read how you should be below your target to detect them due to the shallow -1.5° scan angle. Makes sense if you're flying perfectly level, but why not just enter a shallow dive and look down?

Alternatively, why not just roll 90° so that that ±30° of left/right scan becomes ±30° of vertical scan? Assuming you'd rolled 90° to the right, that would mean that targets on the radar to the "left" would actually be above you and vice versa.

Even trying to account for the fact that the radar display takes a second or two to update, my brief tests have left me unclear as to how it operates when not in level flight.

Often I fly with the radar off. Then, switch it on and fly a level circle to scan the surrounding area for any missed contacts. An aerial "Crazy Ivan" if you will.

However, this often doesn't work as expected which leaves me to think I do not sufficiently understand how the radar works and/or am interpreting it incorrectly.

Can anyone shed any light on this for me?
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Old 03-10-2018, 06:16 AM   #2
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The Sapphire fire guidance radar is stabilized to the horizon within it's gimbal limits. This applies both the pitch and roll axes. you can see this when entering a bank angle superior to 30-45 degrees. The radar will suddenly start displaying tiled ground clutter. Same applies with pitch angle, where slight movements in the pitch axes don't effect the picture, but anything outside the gimbal limits does. You can see this when entering a dive and pulling out. As you pull out the radar picture normalizes once you reach the gimbal limits of the antenna.
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Old 03-10-2018, 04:14 PM   #3
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The electromagnetic pulses emitted by this radar technology is reflected by the ground so it clutters the radar display. It becomes impossible to discriminate a target from this clutter. That's why the radar scans no lower than -1.5° below horizon, and it is more convenient to stabilize it rather than making it fixed to the aircraft orientation.
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Old 03-10-2018, 08:28 PM   #4
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RP-21 and RP-22 are both "Sapphire". RP-21 is in older MiG-21 which is fixed to airframe. RP-22 is in this module which is horizon stabilized.

RP-22 antenna can be moved roughly: +-30° azimuth +-30° pitch and this is relative to a rolling base plate which can move +-70° (80?) roll. So if airplane is heading 010, pitch +10, roll 45 clockwise then antenna can point north on horizon by twisting base plate 45 counter-clockwise, pitch -10, azimuth -10.

"Field of regard" of radar is a zone smaller than physical limits of how antenna can point. If airplane pitch is 0 then radar antenna points in range -1.5 to +17.5 relative to airframe but if pitch is +9 then antenna points from -10.5 to +8.5. This is the same direction relative to horizon.

Radar moves as best as it can to stay centered to the airplane's heading and the same place relative to horizon (few degrees below, most degrees above).
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Old 03-11-2018, 10:37 AM   #5
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Quote:
Originally Posted by Bogey Jammer View Post
The electromagnetic pulses emitted by this radar technology is reflected by the ground so it clutters the radar display. It becomes impossible to discriminate a target from this clutter. That's why the radar scans no lower than -1.5° below horizon, and it is more convenient to stabilize it rather than making it fixed to the aircraft orientation.
That is actually a limitation of the nosecone, more than a radar feature.
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Old 03-12-2018, 11:07 PM   #6
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Man I love this community. What excellent and detailed info! Thanks to all.

With the info provided in mind, what I'm seeing makes much more sense. I couldn't figure out why the ground clutter didn't move up when I moved my nose down for instance.

So, to check my surroundings (without GCI/AWACS) rather than doing a tight continuous circle as I have been doing, I guess I'd be better off doing a series of say 45° turns and levelling between each for a couple seconds to allow the RADAR to get a sweep or two in before moving on to the next.

Thanks again guys. This is my first stab at air-to-air and finding targets (never mind killing them) is proving difficult! However, it is a lot of fun to learn and the 21 is a blast to fly! :-)

Time to get back at it. Cheers.
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Old 03-13-2018, 06:15 PM   #7
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Quote:
Originally Posted by BadHabit View Post
That is actually a limitation of the nosecone, more than a radar feature.
Is it specific to the MiG-21 only ? I thought that all non-doppler airborne radars were affected…
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Old 03-13-2018, 08:38 PM   #8
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Quote:
Originally Posted by JLX View Post
This is my first stab at air-to-air and finding targets (never mind killing them) is proving difficult!
Keep in mind, the MIG-21 is not an air superiority fighter. It's designed to be guided to the target area via GCI and the radar used in the terminal phase of the intercept to find the target (especially at night/bad weather). It will always be hard to find targets with that radar by simple random search.
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Old 03-15-2018, 05:09 AM   #9
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Quote:
Originally Posted by JLX View Post
Man I love this community. What excellent and detailed info! Thanks to all.

With the info provided in mind, what I'm seeing makes much more sense. I couldn't figure out why the ground clutter didn't move up when I moved my nose down for instance.

So, to check my surroundings (without GCI/AWACS) rather than doing a tight continuous circle as I have been doing, I guess I'd be better off doing a series of say 45° turns and levelling between each for a couple seconds to allow the RADAR to get a sweep or two in before moving on to the next.

Thanks again guys. This is my first stab at air-to-air and finding targets (never mind killing them) is proving difficult! However, it is a lot of fun to learn and the 21 is a blast to fly! :-)

Time to get back at it. Cheers.
AFAIK the SOP was to turn the radar on and wait for 2 seconds, this is the time it takes to scan the full envelope. Any further use for scanning would be discouraged, since it has limited coolant. The radar would be kept on in the terminal and fire guidance phase of the intercept, determined by the GCI.
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Old 03-15-2018, 02:04 PM   #10
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Quote:
Originally Posted by Bogey Jammer View Post
Is it specific to the MiG-21 only ? I thought that all non-doppler airborne radars were affected…
Ground clutter is on thing. Yes it is limited as well as F-5 etc but i am talking about the limitation of the antenna to look down at all farther than the 1.5 dgr. The cone is blocking that movement. AFAIK
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