Good morning,
I would like to know how the true north is updated in the IRS during flight.
Thanks,
Lotetu
As far as I know it isn't! The Inertial Nav needs to be aligned on the ground and is not updated until you land. The alignment procedure aligns the IRS/INS to TN using the Coriolis effect (and this alignment is made faster by inputting the PP...Latitude being the important parameter). There will always be a drift during flight but the laser IN has made this a lot less than earlier designs...the drift now is due mainly to noise rather than mechanical friction. When you land and arrive at the gate, the IRS should give you the errors when you put in a fix with your current position.
When using attitude mode, I believe the frame of reference is not to TN,,,but wherever the gyro aligns and although the pitch, roll and heading information is okay in the short term will be much more likely to drift (because there is no compensation for Coriolis). I imagine that since the only other source of information available in these axes is heading, then updating this will reduce the errors in the other two.
Checking out the attitude mode when stationary may not give you the sort of drift errors you are looking for because you will have no accelerations. Without these, the errors will not be anywhere near the same.
I'm not sure what formulae you are after (and how advanced)...ones for Coriolis or ones for drift rate depending on system used?