INS aligment
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I do better understand now! … Yeah, it is because of the ship (moving). The principle I described above can only work if the a/c is not moving. Maybe this is why the mag heading input on navalised a/c.
Part of it - the ship’s velocities are more important - but not the whole picture. I don’t care where you are operating, you have to know the initial orientation of the body during the alignment…so mag var plays a part.
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I’m curious to know if the embedded GPS can tell the direction and attitude of the A/C in addition to it’s position? I remember that compass rose is far away from any thing because any piece of metal or EMF field can cause problems. Some people even claimed dog tags could cause problems. I would be interested in knowing what technology has taken the place of physically putting the A/C in a known spot with enough accuracy to give the initial or master alignment. When we weren’t using compass rose, we were using other objects, like someone said about using an old F-16 that was used to calibrate off of we would use an LHD and the LHD’s have a lot of redundant navigation equipment.
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I can tell you that nothing has taken the place of using the compass rose, and that we still drag jets out there at scheduled intervals…
…I’m also old enough to have seen changes in mag var cause us to have to renumber local runways!
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I don’t care where you are operating, you have to know the initial orientation of the body during the alignment…so mag var plays a part.
Only if you are using mag heading to get the initial orientation of the body during alignment. Seems like the F-16 INS at least uses true heading internally.
Probably one of the folks here who have actually worked on the jet can confirm/deny it, but Im pretty sure the F-16 doesnt have a MAD in the tail. Or anywhere else.
I’m curious to know if the embedded GPS can tell the direction and attitude of the A/C in addition to it’s position?
I would be interested in knowing what technology has taken the place of physically putting the A/C in a known spot with enough accuracy to give the initial or master alignment.
Dont think GPS can tell direction and attitude, just position. Given that it works by placing you on the surface of a set of spheres of known radii, it would not seem to have any capacity for judging attitude, and its direction would just be a case of historic direction of travel.
Seeing as the squat fix consists of physically putting the A/C in a known spot with enough accuracy, then doing a FIX, it seems like not much has changed, in terms of how its achieved. The computers doing it all behind the scenes might work things out differently, but the base idea seems the same.
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You have to know mag heading in order to get true heading…that’s why mag var is so critical. You just plain have to know it…dead nuts. It’s the only thing you can measure directly.
And actually, GPS can give you a limited amount of attitude information…but it’s pretty lousy, and it’s only available if you have some 3D velocity above a threshold. So no, not initially on deck.
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A compass rose is for calibrating the standby compass, and for doing TACAN maintenance. If you have a station you can receive from that location.
The whiskey compass I referenced is the F-16 name for the backup compass. Never saw a compass rose used for TACAN maintenance. Never recall seeing one work other than the direction it pointed to.
Not all INS systems are ring laser equipped - in the older mechanical ones they may in fact get some input from a magnetic anomaly detector, which aids the alignment and nav solution by estimating local mag var. Those are becoming increasingly rare, though.
In the F-16 community the SINS doesn’t require a magnetic anomaly detector.
One of the more annoying problems with GPS aided INUs is that due to the GPS aiding during alignment the inertial unit can actually break or degrade and that can go undetected until someone operates them in an un-aided mode…by which time it’s generally a bit too late. So they need to be spot checked without using GPS at least occasionally.
Well I cannot speak for outside of the F-16 community on this one but that isn’t a factor in the F-16. The GPS provides data to the GAC just like the INS does for Kalman filter processing not directly to the INS. From wording in the 34-1-1 I would say that the Kalman filter isn’t applied until after NAV mode is entered as it specifically talks about NARF and EIAs and the Kalman filter having to be set to INS or those procedures cannot be done.
Also after each flight the INS performance data in X, Y and Z axis is noted and if outside of performance parameters is written up for maintenance to look at. So at least in the F-16 world you won’t have the INS going to complete garbage before it is caught and replaced.
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I’ll seriously bet that it either has a stored table for mag var in it (which sounds like the most likely case), or it has a MAD someplace…
No MAD it has a stored table for MagVar.
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Usually the MAD isn’t co-located with the INU…in general it’s in the top of the vertical tail, as far away from surrounding aircraft structure as it can be placed. So the Viper could have one in it’s fin cap. Dunno…but I do know you can also get mag var from GPS…when you can get it. Dunno how good it is, though.
Trust me Stevie I worked the real deal for 19 years. No MAD. Just like Dee-Jay said the system finds true heading on it’s own and per the MagVar tables it finds that.
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You can also use the compass rose to cross-check TACAN or VOR if it is a special one (called a VOT in civvie-speak) point that can pick up a local TACAN/VOR and has a measured location in radial and/or DME from the station. But only in that case. In general when I’ve seen Naval compass swings the compass and the TACAN may be cross checked at the same time…if the equipment/facilities are available.
Naval SINS doesn’t require a MAD either…at least not the one on the jet. It’s either cabled in or RF input…depending on operations. In both cases the alignment data are produced by an off board source.
I wouldn’t be surprised if the USAF just plain didn’t know they have a problem with embedded systems masking failures because in general the GPS portions of the systems are pretty reliable…and guys use them, so there aren’t many gripes until something breaks in tandem. The GPS keeps the filters tight, and that’s the issue…for everyone. I don’t think the USN has come up with any ideas as of yet either…and probably won’t as long as GPS keep working as well as it does.
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Trust me Stevie I worked the real deal for 19 years. No MAD. Just like Dee-Jay said the system finds true heading on it’s own and per the MagVar tables it finds that.
…I’m still working with the real thing now (just not Vipers…for 30 years) and I can believe it uses tables. You can’t measure true heading directly - it must be calculated.
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Isn’t it what Stubbies means by “find” ?
Stubbies seemed to imply it went the other direction - for known true heading, find mag heading. Stevie posits instead that for known mag heading, the jet finds true heading.
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Isn’t it what Stubbies means by “find” ?
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Stubbies seemed to imply it went the other direction - for known true heading, find mag heading. Stevie posits instead that for known mag heading, the jet finds true heading.
Ah, yes.
Well, can’t bring much to the debate. Interesting, still.
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Stubbies seemed to imply it went the other direction - for known true heading, find mag heading. Stevie posits instead that for known mag heading, the jet finds true heading.
Precisely. You cannot measure true heading directly…I don’t care what you use.
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I can think of a few implicit ways of getting it actually, but I dont think they are directly used by the F-16 based on my lay knowledge of the topic, so I may have to concede that one.
If you have GPS, and you have a solid alignment, but a fuzzy initial heading value, GPS’ constant update of your position could be used to back calculate the original true heading with high accuracy, and no use of mag heading at any point.
With that said though it would require some movement and turning. Perhaps this is related to the EIA?
I can see how given mag heading and a table you could get true heading, and I can see how an EIA could generate accurate heading data without any reference to mag heading at all. Seems like either method could be used, and thinking about it, seeing as the INS initialises with the same position/heading values it had when it shut down, that would make the GPS assisted method viable.
Then again historically the old systems didn’t have GPS, so they logically must have had to use the mag heading and mag var tables to work out true heading. Cant really rule out either method for the new jets though.
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Naval SINS doesn’t require a MAD either…at least not the one on the jet. It’s either cabled in or RF input…depending on operations. In both cases the alignment data are produced by an off board source.
F-16s don’t require a MAD or any external reference to a magnetic reading. Just the true heading it gets itself during standard alignment and the tables cover most flying that you would see done with an F-16.
I wouldn’t be surprised if the USAF just plain didn’t know they have a problem with embedded systems masking failures because in general the GPS portions of the systems are pretty reliable…and guys use them, so there aren’t many gripes until something breaks in tandem. The GPS keeps the filters tight, and that’s the issue…for everyone. I don’t think the USN has come up with any ideas as of yet either…and probably won’t as long as GPS keep working as well as it does.
Not so per one of my posts above. INSM locations are checked every flight to keep the INS system honest.
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…I’m still working with the real thing now (just not Vipers…for 30 years) and I can believe it uses tables. You can’t measure true heading directly - it must be calculated.
So the only thing to be taken from this is whichever bird you are working uses a completely different INS as the one the F-16 uses only requires initial lat/long/SALT for the start of the alignment and it finds true heading by itself and does the math for magnetic heading based on current lat/long and gives a good indication for mag heading within 45 seconds of the start of alignment.
My guess is the navy INS is different because you guys aren’t sitting on a stable surface with the only movement to the INS during alignment being the earth’s rotation. You have the aircraft carriers to take into account while producing an INS alignment.
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Precisely. You cannot measure true heading directly…I don’t care what you use.
Heh well according to that then the F-16 is breaking the rules every time it spins up because it does all by itself.
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