INS aligment
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I don’t think the INS alignment uses the gyro suite to compute positioning. You’re at an airport; you should already know where you are. What it does is compute True North to adjust the compass.
A magnetic compass can have proximity issues with iron devices (like bombs) and other devices that generate a magnetic field (like a TGP…maybe). I’m not sure if a fluxgate compass has proximity issues with magnetic fields and not so much iron devices. Not sure if the F-16 has one.
If you where to mount a magnetic compass in your car(or a/c) there are several step must be taken because the car is make of iron.
- with the compass out of the car point the car north.
- Mount the compass in the car and note that the compass is not reading north.
- There are two adjustment screws that are used to align the compass to read north.
- iirc you have to point the car west and repeat. One adjustment screw does north and the other does west. Me thinks
Now that you got your compass reading correctly, hang 4000lbs of bombs on the car and see what it does.
You can see the adjustment screws on the a/c compass
What I’m digging for is:
after you drop your bombs, would you have to do a IN FLT ALLIN to fix the compass.? -
I don’t think the INS alignment uses the gyro suite to compute positioning. You’re at an airport; you should already know where you are. What it does is compute True North to adjust the compass.
It cannot compute it’s current position. That must be entered within 2 minutes of starting a gyrocompass alignment. This is one area where BMS fudges how the system works as you never do this in the sim. During the alignment it is sensing true heading and earth turn rate. It adjusts the compass CARD on the HSI. It does nothing what so ever to the manual backup compass.
What I’m digging for is:
after you drop your bombs, would you have to do a IN FLT ALLIN to fix the compass.?Wow. Ok not sure why you are mixing up the manual backup compass and the INS system as other than for cross reference they have NOTHING to do with each other what so ever. No data is sent to or received from the backup compass. The only electrical connection it has at all is for lighting. Performing alignments on the INS adjusts the backup compass about as much as performing a BIT check on the FCR system. That is to say, none.
There are plenty of problems with navigating in a fighter aircraft via a magnetic compass and you have just pointed to one of many. Trust me if he is down to the compass only for navigation he has bigger problems on his hands than simply the magnetic deviation caused by some iron bombs that he is most likely going to be getting rid of.
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The 8min is for the “one time” static heading sampling (1?). Where the gyros detects the rotation of the earth to compute true North (geographic). I don’t think this is that important because once the a/c is moving it can no longer detect the earth’s rotation.
Without a solid gyrocompass alignment the INS system cannot set up the platform. Without the platform set up it cannot accurately know which velocities it senses are for what axis and thus the entire house of cards falls apart quickly. The farther and longer that system ran with that flawed setup the worse it would be (as with all inertial nav systems even with good alignments) but since it didn’t have a solid foundation in the first place it would be dependent on the GPS until the system was shut down and started again.
And over time, the INS will degrade and will need support from the compass and GPS.
Regardless of how good the INS system is it will degrade over time. Living on a roundish earth that rotates absolutely guarantees that.
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My point is that if the (non-back-up) compass worked perfect, you wouldn’t need to wait around to find the direction witch the earth turns. The (non-back-up) compass with an almanac would provide true heading. I guess you agree that the INS needs a external heading correction and a corrected alligned compass would be best?
Most of us know the issues with the pilots’ backup compass type….what’s wrong with the “other” compass?
Why can’t it be used for allignment? The INS is going to have to use it sooner or later. My guess was loadout distortion. what’s yours?? -
I understood how it could discover latitude from Coriolis forces but longitude has to be supplied to the INS somehow, right? The symmetry of a sphere suggests it can’t be discovered by a set of gyros.
as mentioned, the INS does not calculate where it is by alignment.
it calculates its present displacement from its original location by measuring all accelerations applied to it.
alignment is needed to be able to accurately read those accelerations.
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The rate gyros and the accelerometers work together to determine attitude. The software is able to isolate the earths’ 1g gravity vector(one of the damnest thing I’ve ever seen). The accelerometer works like bubble level. (no drift in attitude)
A compass aligns the INS package with the earth. The rate gyros can hold heading but it is subject to drift.
The accelerometers measures the acceleration vectors(x,y,z) and with a clock computes a velocity vector.
The velocity vector with the clock computes position.
(if you level and travel a 400mph for 1 hour you traveled 400miles) -
All very useful information, thanks for that, but my original question still stands. For the JA37 the minimal alignment time is 90 seconds, full alignment is 3 minutes. It would be great if it could be implemented.
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My point is that if the (non-back-up) compass worked perfect, you wouldn’t need to wait around to find the direction witch the earth turns. The (non-back-up) compass with an almanac would provide true heading. I guess you agree that the INS needs a external heading correction and a corrected alligned compass would be best?
You are thinking about it backwards. The INS system senses true heading and adjusts the compass card on the HSI based on that. Not the compass directing the INS. Initial heading is not the problem with INS systems as to why they need external help to maintain accuracy. Earth rotation and movement of the platform across a non flat surface (the surface of the earth) is what induces these errors.
Most of us know the issues with the pilots’ backup compass type….what’s wrong with the “other” compass?
Why can’t it be used for allignment? The INS is going to have to use it sooner or later. My guess was loadout distortion. what’s yours??Ok to be as plain as possible the backup/whiskey compass is not used in ANY fashion by the INS system. It is never referenced at any stage of the INS system. It is purely a backup instrument for the pilot in case both INS and GPS go down. It suffers from magnetic variation via bombs, electrical systems in the aircraft, sudden changes in aircraft velocity and heading, blah blah. There are a million things that affect it’s accuracy. It is understood to have these limitations and is only there for rough back up reference or in emergency circumstances.
The only external information that the INS needs for alignment is it’s current location in latitude/longitude coordinates entered at the start. It figures out it’s own true heading all by itself during the gyrocompass alignment.
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A compass aligns the INS package with the earth.
Wrong. The system is aligned via the lat/long coordinates entered by the operator, the sensed gravity vector direction, and the sensed earth rate. It determines heading this way, not by a compass.
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Wrong. The system is aligned via the lat/long coordinates entered by the operator, the sensed gravity vector direction, and the sensed earth rate. It determines heading this way, not by a compass.
You believe there is way to senses the earth’s rotation while in flight.
You believe the only compass in the a/c is the backup compass. BTW, I know backup compass is not intergraded into any system in the a/c. use a little intuition.
You don’t believe there is algorithm in the INS software that correct for the “known” spinning of the earth spinning under the a/c.
You believe that the clock in the INS is perfect.
You believe that accelerometers are perfect.
You believe that light ring rate gyros are perfect.
You believe that Analog to Digital converters are perfect.
ok, enuff said…… -
All very useful information, thanks for that, but my original question still stands. For the JA37 the minimal alignment time is 90 seconds, full alignment is 3 minutes. It would be great if it could be implemented.
Post #2 was good. I’ve done ramp starts and rolled out as soons as every thing was turned on. It worked in single player. If you need more bells and wissels you need a Dev.
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Wrong. The system is aligned via the lat/long coordinates entered by the operator, the sensed gravity vector direction, and the sensed earth rate. It determines heading this way, not by a compass.
Stubbies is right. There is no compass entry for INS/IRS alignment.
INS/IRS is giving a true heading.
Mag heading is given by software computation based on location, true heading and known Mag variation table.
Software mag heading is compared with mag heading coming from “heading central” (do not remember English name).
Bkp compass is only a backup working without electric power. It can have from 1 to 5° deviation due to electromagnetic perturbation (electronic devices and a/c body). It must be calibrated periodically on ground by technical staff. -
The system is aligned via the lat/long coordinates entered by the operator.
Or taken directly from GPS device (if GPS is good and pilot is validating this position)
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Or taken directly from GPS device (if GPS is good and pilot is validating this position)
All is correct but one note to remember, INS alignment uses manual or GPS latitude entry only for alignment. Longitude is a comparison of the manual or GPS longitude coordinates entry with last know Longitude stored in the memory.
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All is correct but one note to remember, INS alignment uses manual or GPS latitude entry only for alignment. Longitude is a comparison of the manual or GPS longitude coordinates entry with last know Longitude stored in the memory.
100%
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It is possible for an INS to discover its latitude on its own. I know this to be true. The motion around the Earth’s axis is unique for every latitude (N S pair). Heading and local vertical are self-discovered during attitude alignment. The combination of accelerations suggest a loop path the vehicle takes around the Earth. With knowledge of the craft’s azimuth and local vertical it can be combined with the path the craft is taking around the Earth’s axis to find latitude uniquely.
I would guess that the F-16 does not derive latitude for navigation purposes but does check the calculated against the entered as large discrepancies will prove to be unusable in flight.
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Stubbies is right. There is no compass entry for INS/IRS alignment.
INS/IRS is giving a true heading.
Mag heading is given by software computation based on location, true heading and known Mag variation table.I was thinking it’s like “witch came first the chicken or the egg?”
If you takeoff without allignment there is no heading tape?
Does INS heading drift? Note that when heading NW, in a roll the INS heading will be handed off to all three gyros.
Mag heading must be in an ICP page because I didn’t see a switch for it. -
Mag heading is given by software computation based on location, true heading and known Mag variation table.
.My 1984 D1 says the HSI compass card is the “INS magnetic heading”. I didn’t find that it was computed from True North. And the HSI compass card is INS servo driven just like the ADI.
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You can only base very little when comparing BMS with 1984 F-16. The only similarities is that it is still an F-16 and…. yeah all the system are different. Engine, avionics, hell even the structure is different.
This one is better but still only a reference, nothing more and not 100% -
page 1-163
The face of the HSI is a compass card driven by the INS so that aircraft magnetic heading is always read at the upper
lubber line.I do not understands much (certainly not this), but I do not think this says anything about INS using magnetic heading, I will keep looking.