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SAAFNAV
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IRS Alignment Question

Mon Feb 27, 2012 3:46 pm

Hi Guys,

I've got a question that nobody can answer me at work.

I'm familiar with the basic operations of an IRS/INS and Ring Laser Gyros etc...

But as I have it, you need a original reference point for alignment.

On our system (Totem/Thales 3000), you start the alignment by selecting the GPS position or the airport position, and from there the system does it magic with smoke, mirrors and witchcraft.
The aircraft doesn't have any Flux Valves, Magnetic Sensing Units etc that feeds into the IRS, so how does it figure out the original heading?

We even swing the standby compass to the IRS, but what checks the IRS??

Regards,
Erich
ex L-382G Loadmaster, ex C-130B Navigator, Möchtegern Flugzeugführer
 
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747classic
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RE: IRS Alignment Question

Mon Feb 27, 2012 4:28 pm

A declination (variation) data base is probably included. When the IRS system knows it's actual position, the variation at that position is drawn from the database. Now the initial (magnetic) heading can be computed.
Operating a twin over the ocean, you're always one engine failure from a total emergency.
 
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SAAFNAV
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RE: IRS Alignment Question

Mon Feb 27, 2012 4:33 pm

OK, but how does it know what way the nose is pointed?

Erich
ex L-382G Loadmaster, ex C-130B Navigator, Möchtegern Flugzeugführer
 
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glen
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RE: IRS Alignment Question

Mon Feb 27, 2012 5:56 pm

Quoting SAAFNAV (Reply 2):
OK, but how does it know what way the nose is pointed?

By measuring the rotation around the three axis during the alignment process.
For example an aircraft standing on the equator. When the nose is pointing exactly north or south the IRS will measure a rotation around the roll axis as the aircraft is turning together with the earth during alignment. The other axis will be displaced, but there is no rotation around them. By the direction of rotation the system will determine if the nose is pointing north or south.
With the nose pointing east or west, the aircraft will turn around its pitch axis together with the earth.

The combination of the rotation around all three axis defines the true heading the aircraft is pointing to and the latitude of the aircraft as well (rate of rotation is depending on latitude).
The initial position is absolutely necesseray to define the longitude, as the IRS can not find this value by itself.

That is also the reason why alignment time increases at high latitudes as the rotation around the roll and pitch axis gets slower and slower the farther north or south you are. (At the pole you have only a rotation around your yaw axis, and the IRS would not be able to find out the heading of the aircraft.)
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Lemmy
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RE: IRS Alignment Question

Mon Feb 27, 2012 6:32 pm

The best explanation I heard involved the old systems that mounted the gyros on a gimballed platform. During normal operation, this platform stays parallel to the ground. But during alignment, the platform remains in position relative to space. So, as the earth turns, the platform tilts to maintain position. If the nose of the airplane is pointing north, the left side of the platform dips down since, relative to space, the airplane is now starting to tip to the right. From there, it's just a matter of calculating the platform's axis of rotation.

Question: If the IRS system determines only the axis of rotation, how does it know which was is north and which way is south?
I am a patient boy ...
 
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glen
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RE: IRS Alignment Question

Mon Feb 27, 2012 7:10 pm

Quoting Lemmy (Reply 4):
Question: If the IRS system determines only the axis of rotation, how does it know which was is north and which way is south?

You covered your question already in in your explanation. It does not only determine the axis of rotation but also the direction of rotation. In your example if the aircraft tips to the left isof right, it is pointing south...

(I covered this already in my previous post)

[Edited 2012-02-27 11:12:35]
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
B747FE
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RE: IRS Alignment Question

Tue Feb 28, 2012 4:07 am

Quoting SAAFNAV (Thread starter):
SAAFNAV

Basically, during alignment the internal navigation computer goes through a series of sub-modes in which among other things it levels the platform to the local vertical and a ''gyro compass'' calibration. As glen so clearly stated in his post, it's this true north alignment capability that is directly related to the horizontal earth rate component measured by the horizontal axis integrating gyros.
Once the relationship platform/true north is established, magnetic heading can be computed and refined using synthetic magnetic variation.
In the case of polar operations the inertial system will revert back to true heading and digital magnetic outputs will no longer be computed. (Above 70N or 60S)


Quoting SAAFNAV (Thread starter):
what checks the IRS??

The navigator?  

Regards,
B747FE.
"Flying is more than a sport and more than a job; flying is pure passion and desire, which fill a lifetime"
 
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SAAFNAV
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RE: IRS Alignment Question

Tue Feb 28, 2012 4:27 am

Thanks for the explanation guys.

There is some serious math involved in that little box!!

Haha, yes, in the olden days the Nav did check the IRS with sunshots, but we don't get taught that anymore.

Regards,
Erich
ex L-382G Loadmaster, ex C-130B Navigator, Möchtegern Flugzeugführer

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