INS aligment
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Nope Stevie.
… On mine, is it done by sensing the three rotational (earth) vectors in the three dimensions and by knowing our INIT position it calculates the true heading. Then, the software apply the var (present in FMS database) to give us the mag heading (comparing to what is sensed by the mag flux sensors … Above two ° or disparency, the system is triggering an alarm.) … But this IRL is perfectly working without any mag inputs. And can provide a software calculated Mag heading based on the true heading (or can provide a Grid heading for polar navigation in a sort of “free gyro” mode)
My IRS is the TOTEM200 build for ArianeIV rocket.
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…
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Yeah. Now I’m starting to put it together. We tried to stay away from inputting the calibration coordinates and while we did have several choices, it was made clear that compass rose was the most accurate. On the ship we used the LHD’s SINS to calibrate it. The last and least accurate was doing it in flight. All those worked but we always took the birds to compass rose when we could. Especially before we deployed on ship and when we came back.
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.
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Hmm. I dont think the F-16 ones have a MAD in them. Just three RLGs and three accelerometers, and the embedded GPS receiver.
So long as you know the original position, and the original true heading, the final position and true heading can be calculated from that. I guess if it was to display mag heading, it would need to have a stored table for mag var.
The principle can work with a moving aircraft too DeeJay, just not so well. IFA is still a thing!
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Manual seems to think it still holds true. Cheers for the pointer!
It also recommends doing an OFLY fix while still on the ground for more accurate initial position, so long as the precise position the jet is parked is known - and that if GPS accuracy is HIGH, you dont need to bother.
GPS seems like a fair bit of trouble for all its perks. If you dont load the GPS almanac with known positions of the satellites, it can take up to 90 minutes for the jet to get a GPS solution.
You got it…same holds if you put in a new one with blank waypoints and it doesn’t know where to look at the sky in the first place. I’ve seen that take hours to resolve itself.
In the old days there was an INS maintenance procedure called “g-bias”…guys used to come home and do multiple overfly updates in the chocks and call that a “mini g-bias” thinking it would tighten up the mechanical INUs. Which might have worked…as long as nobody re-spotted the jet…
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Hmm. I dont think the F-16 ones have a MAD in them. Just three RLGs and three accelerometers, and the embedded GPS receiver.
So long as you know the original position, and the original true heading, the final position and true heading can be calculated from that. I guess if it was to display mag heading, it would need to have a stored table for mag var.
The principle can work with a moving aircraft too DeeJay, just not so well. IFA is still a thing!
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.
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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.