INS aligment
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This warrants some research I think! Stevie, do you know which models took magnetic anomaly input? I had thought they exclusively worked off of the earths rotation similar to how Stubbies described it.
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I think that the old CAINS (ASN-130?..or previous) that was in earlier USN inventory operated this way…I think these were common to Harriers and Hornets back in the day. At least, I recall them from my own Harrier days. That one was replaced by the CAINS II, which I believe was an RLG upgrade, but even it still used MAD input for nav.
…then think about aligning one at sea…where you’re not stationary. Not by a long shot.
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Makes sense. I had thought you were saying old F-16 ones worked that way, which confused me.
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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. .
I guess you know that, Gyro laser nav units are called IRS (Inertial Reference System) or “Strap Down”. But even older INS (Inertial Navigation System) do not require mag inputs. They all works in true heading. But yes, they need the var info (instrument or sofware database in FMC/S) to elaborate the mag heading to send to the compass. But the inertial system itself do never require the var to be able to give your true heading, attitude, speed and and position. Unless it is something VERY old (prehistoric) between an INS and something else which I do not know (?).
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I think that the old CAINS (ASN-130?..or previous) that was in earlier USN inventory operated this way…I think these were common to Harriers and Hornets back in the day. At least, I recall them from my own Harrier days. That one was replaced by the CAINS II, which I believe was an RLG upgrade, but even it still used MAD input for nav.
^ this is precisely what I do not know
Thank you Stevie!
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Does the F-16 use GPS? INS keeps track of where the A/C has moved by gyroscopes and accelerometers, but if something doesn’t tell the INS it’s correct location before it starts moving then it won’t know where it is. It will have an arbitrary reference frame.
Newer F-16s have an EGI, which is a combination GPS/INS. As I understand it, normally the specific location old F-16s are parked is given to the jet for alignment. would love to hear more on how that is done from someone in the know!
The new ones have GPS and can work it out for themselves given a minute to do it.
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In two words: EGI are permanently (builded-in) hybridised IRS.
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Good videos covering older and newer INS systems. When I first started working on the F-16 the setuo was close to the first video which came to be known as the SINS or Standard INS to differentiate it from the RLG later on.
They didn’t get much into the limitations of SINS which come about from imperfections in the gyros, gyroscopic precession, etc. or in ways they used to limited drift (gyro rotational speed and mass) but not surprising since RLG is better as far as drift and MTBF is concerned.
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Newer F-16s have an EGI, which is a combination GPS/INS. As I understand it, normally the specific location old F-16s are parked is given to the jet for alignment. would love to hear more on how that is done from someone in the know!
The new ones have GPS and can work it out for themselves given a minute to do it.
Even though the EGI is a combined unit it is still RLG and GPS just integrated. On RLG you had to enter the coordinates within the first two minutes of alignment. If this was done after that two minute period the INS would automatically start the alignment over again.
I never worked on a EGI equipped F-16 (was already on the CV-22 by that point) so I cannot say for certain if that holds true for EGI.
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All INUs require a magnetic input for alignment - because in order to resolve earth rotational velocities into their components they need to know which way the body is pointed/oriented as the earth rotates; this can be done either by sensing mag var or by having the pilot hammer in the mag var at alignment manually (used to have to do this in the Day Attack Harrier early on). And trust me - this can really screw things up if you don’t get it correct.
Thinking is that one can get this from GPS these days…unless you can’t. Which is another way aided systems can mask faults in a combined GPS/INS (like the EGI).
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Makes sense. I had thought you were saying old F-16 ones worked that way, which confused me.
I’m not certain I can say that older ones like A/Bs didn’t use the same as USN, given that the jet was originally spec’d to use as many off the shelf parts as it could. I wasn’t intending to say, but I really don’t know for sure.
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Even though the EGI is a combined unit it is still RLG and GPS just integrated. On RLG you had to enter the coordinates within the first two minutes of alignment. If this was done after that two minute period the INS would automatically start the alignment over again.
I never worked on a EGI equipped F-16 (was already on the CV-22 by that point) so I cannot say for certain if that holds true for EGI.
I’ve had a passing experience with the (an?..Embeded GPS/INS) EGI on the T-45A…at that time I was told it was actually the standby system from the F-14. We used to say EGI stood for “Evil Guidance Instrument”…
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All INUs require a magnetic input for alignment
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.
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Even though the EGI is a combined unit it is still RLG and GPS just integrated. On RLG you had to enter the coordinates within the first two minutes of alignment. If this was done after that two minute period the INS would automatically start the alignment over again.
I never worked on a EGI equipped F-16 (was already on the CV-22 by that point) so I cannot say for certain if that holds true for EGI.
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.
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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.
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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.