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Old 07-12-2019, 07:47 AM   #1
QuiGon
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Default Deviation of computed magnetic variation over time

After a long ~1.5-2h combat flight as a RIO on the BlueFlag server yesterday (it was actually two flights with rearming inbetween without realigning the INS) I ran into a problem which I don't know how to handle. On our final flight back to base I noticed the MV acronym on the TID, hence I checked the computed magnetic variation and it showed a variation of 13°. We flew in western Caucasus with a local magnetic variation of 6.6°.

Now I have two questions about this:

1) What are the technical reasons that caused this huge deviation of the computed magnetic variation?
We started our flights in Gudauta, headed north over the mountains, struck some ground targets on the northern side of the mountains, did some A-A combat and returned back to Gudauta. We did that 2 times without respawning and did not realign the INS during rearming between the 2 flights. I noticed the MV on the return leg of the 2nd flight. We had no battle damage.

2) How to handle such a deviation of the computed magnetic variation?
I would have liked to somehow fix it and tell the INS that the magnetic variation should be around 6.6° and not 13°. Is that possible?
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Last edited by QuiGon; 07-12-2019 at 07:49 AM.
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Old 07-12-2019, 07:56 AM   #2
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No answer, just guessing:
The computed magnetic variation is just calculated. As the INS drifts over the time, maybe this is then also affected?
It would have been really interesting to know if it would have made a difference if you would have done a Tacan navfix?
I gues it was pre-aligned? As far as I know, this also leads to a faster drift.
How much have the radar targets and the datalink targets been seperated?
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Old 07-12-2019, 08:11 AM   #3
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Quote:
Originally Posted by viper2097 View Post
It would have been really interesting to know if it would have made a difference if you would have done a Tacan navfix?
I actually did a navfix after I noticed the magnetic deviation, to test if that might fix it. I performed a visual navfix though, because a TACAN navfix would have been pretty unprecise because of the deviation of the computed magnetic variation. The navfix fixed the drift as it should, but it did not do anything about the deviation of the computed magnetic variation.

Quote:
Originally Posted by viper2097 View Post
I gues it was pre-aligned? As far as I know, this also leads to a faster drift.
We have indeed performaed a pre-stored alignment on startup, but it's not clear yet if INS drift has anything to do at all with the computed magnetic variation deviation.

Quote:
Originally Posted by viper2097 View Post
How much have the radar targets and the datalink targets been seperated?
Haven't really paid attention to that as it was quite a busy flight (1 Sparrow kill, 2 Sidewinder kills, 1 guns kill, multiple A-G kills).
Edit: The INS drift was not that bad though, because when we performed the visual navfix the INS waypoint was not that far away from the real position of the waypoint (maybe ~2 miles difference as a rough guess).
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Old 07-12-2019, 08:22 AM   #4
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Assuming that all INS-related devices were OK, the first thing to check would be the COMPass panel (pilot's right side console) http://www.heatblur.se/F-14Manual/co...-control-panel



Fly straight, level, constants speed at low AOA and check the display marked as 1 - SYNC IND. If the needle is shifted from the centre, the AHRS has drifted/aligned to a wrong magnetic heading and caused that MV error. Press and hold that HDG PUSH button (5) until the needle moves back to the centre. If the needle doesn't move, you're probably turning/climbing/diving/accelerating/decelerating - just check your speed and attitude and try again.
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Old 07-12-2019, 08:26 AM   #5
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Interesting, thanks! This is the kind of detailed simulation I absolutly love about Heatblur modules

I was under the impression, that this panel is only relevant to the backup nav modes, which is why I haven't paid much attention to it so far, because I wanted to do that when learning how to use the backup modes which I did not had the time to do yet. I wasn't even aware that this panel has a display that shows the current sync status! I guess I will have to take a closer look at that panel now (and also backup modes while I'm at it)
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Old 07-12-2019, 10:34 AM   #6
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Just and FYI, the reason for the COMP panel being important for all navmodes is that the WCS compares the INS and compass to calculate the magvar.

At least that's my understanding.
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Old 07-12-2019, 12:11 PM   #7
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Quote:
Originally Posted by QuiGon View Post
I actually did a navfix after I noticed the magnetic deviation, to test if that might fix it. I performed a visual navfix though, because a TACAN navfix would have been pretty unprecise because of the deviation of the computed magnetic variation. The navfix fixed the drift as it should, but it did not do anything about the deviation of the computed magnetic variation.
I think you should be ok to use TACAN for a navfix. TACAN transmits bearing and distance information IIRC, so won't be thrown off by mag drift.
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Old 07-12-2019, 12:18 PM   #8
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Quote:
Originally Posted by Flamin_Squirrel View Post
I think you should be ok to use TACAN for a navfix. TACAN transmits bearing and distance information IIRC, so won't be thrown off by mag drift.
Incorrect, it will be. From the manual:
Quote:
Note: During a TACAN FIX, the MAG VAR must be the same as the TACAN station magnetic variation, or the update will be in error. Given a TACAN station with a range of 100 NM from ownship, a 1°MAG VAR error introduces a 1.74nm error into the ownship’s TACAN update.
See also here: https://forums.eagle.ru/showthread.p...14#post3976314
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Old 07-12-2019, 01:19 PM   #9
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I run quite a lot of tests to figure out what was wrong with the TACAN yesterday. In my case, active pause interatio aside, looks it was something like a drift + range issue.
Also, I forgot to do it but it's important in my video test, remember to set the elevation of the TACAN station WP before doing the fix. They confirmed yesterday that the range is slant and elevation is taken into account. If you don't set it, the Delta will be higher and we totally want to avoid that.
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Old 07-12-2019, 02:39 PM   #10
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Quote:
Originally Posted by QuiGon View Post
Incorrect, it will be. From the manual:
See also here: https://forums.eagle.ru/showthread.p...14#post3976314
Interesting, thanks.
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