Airbus A320 GPS and navigation display VOR

[AF330]

Thanks a lot, so do they get "mixed" inside the FMGS? Does it get mixed with the IRS MIX position? Or each position given by the 3 IRS? And when we align IRS, it takes 6 minutes!! Why and what does it do exactly? Because we are not moving, so how does he figure out our position?

[TURIN]

Ok I'll have a go.

The IRS takes as long as it needs to 'align'. Typically between 7 & 15 mins. That is, to settle down and become stable in it's present attitude and velocity. (Don't forget, the planet is rotating and the whole assembly needs to compensate for that).
So, when the internal magic of the IRS has done it's stuff and calculated it's attitude and stable condition, only then can the IRS be programmed to it's current position.
This is either done manually by the operator (pilot/engineer) by typing in to the scratch pad of the MCDU the Lat/Lon position.
EG 51.477.5 N , 00.461.4 W

Or by using a pre programmed position such as a particular gate at a particular airfield.
EG. EGLL gate 58

Alternatively, the IRS can be programmed with it's last known position OR it can use GPS (if fitted) to update the position in real time. As GPS becomes trusted more and more, this seems to be the favoured option.

Well, that is the way I understand it as a mere A&C.

[TopBunk]

AF330

We applaud your quest for knowledge. We have answered your original questions patiently. The thread is now drifting like a Schuler Loop - google that one - it is related!

Time to do some of your own research.

I'm out of here

[FE Hoppy]

The IRS can figure out our latitude without any external input. It cannot figure out the longitude. This we have to give it. We in fact give it both.



The earth is revolving. The IRS is sensitive to all acceleration so it can feel the acceleration around the earths axis. This is of course the axis between true north and south. So by measuring it's speed through space it can work out it's distance from the axis or rotation, i.e. it's latitude on the surface of the earth.


If it is sitting on the equator the speed through space would be fast and therefore the forces it feels strong.
If its sitting very close to the north or south pole the speed through space would be slow and the forces weak. Too close and it's not sensitive to align at all.

The alignment phase is really only finding out where the North south axis is. We then tell it what the longitude is.

When it knows lat and long and is aligned any new accelerations are converted to velocity of the aircraft and the position updated.

IRS 101

[TURIN]

I forgot about that bit FE H.

That's why I remain B1-1 (restricted).

[AF330]

Yes you all are right, just would like to confirm this and let's finish the thread.
A big thanks to MD83FO, Fe Hoppy, Top Bunk and Turin!

Just want to confirm this: 3 IRS send their signals to the FMGS with the position of the aircraft. The GPS does the same thing. The FMGS calculates the IRS MIX position. Then the GPS position and the IRS MIX position get "mixed" inside the FMGS and it calculates an hybrid position. Everything right?

Thanks a lot.

[MD83FO]

that sounds about right and you're welcomed AF

[AF330]

Thanks,

After some researches, I read all the posts back, and just had one more doubt:

So TopBunk had said:

"The aircraft has two independent GPS receivers. Each GPS receiver is integrated in a modular avionics unit called MMR (Multi Mode Receiver) (GPS 1 receiver in MMR1, GPS2 receiver in MMR2).
The MMR processes the data received and transfers them to the ADIRUs, which then perform a GP-IRS hybrid position calculation."

But you confirmed MD83FO that the GPS and IRS MIX position was mixed inside the FMGS. But TopBunk said that the hybrid GP-IRS position was calculated in the ADIRUs.

So:

1) Where does the IRS MIX position get calculated? In the FMGS or the ADIRUs?

2) Where does the hybrid GPIRS position get calculated? In the FMGS or the ADIRUs?

Thanks,