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I have noticed this many times across my company's fleet of A320's that is even after a full IRS alignment has been completed many a times there is still a drift value present between the FMGC position and the IRS position. Could any help explain as to how this happens ?

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TLDR: The FMGC position is a mix of GPS and IRS (called GPIRS), but the IRS position is aligned to the airport reference coordinate, not to the GPS position.


The difference you observe is between the FM position and the MIX IRS position. This difference is called the bias.

Position Computation

Each FMGC computes its own aircraft position (called the "FM position") from a MIX IRS position (see below), and a computed radio position or GPS position.
The FMGS selects the most accurate position, considering the estimated accuracy and integrity of each positioning equipment.
GPS/INERTIAL is the basic navigation mode provided GPS data is valid and successfully tested. Otherwise, navaids plus inertial or inertial only are used.

(Airbus A320 FCOM - Auto Flight - Flight Management)

The FM position will move towards the GPS position in flight, if GPS is available (otherwise to the radio position):

FM Position

At flight initialization, each FMGC displays an FM position that is a mixed IRS/GPS position (GPIRS).

  • At takeoff, the FM position is updated to the runway threshold position, as stored in the database, possibly corrected by the takeoff shift entered on the PERF TO page.
  • In flight, the FM position approaches the radio position, or the GPS position, at a rate that depends upon the aircraft altitude.

Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active.

FM Position

Bias

Each FMGC computes a vector from its MIX IRS position to the radio or GPIRS position. This vector is called the "bias". Each FMGC continuously updates its bias, if a radio position, or a GPIRS position is available.

(Airbus A320 FCOM - Auto Flight - Flight Management)

The MIX IRS position is determined from all three IRSs:

MIX IRS Position

Each FMGC receives a position from each of the three IRSs, and computes a mean-weighted average called the "MIX IRS" position.

MIX IRS

(Airbus A320 FCOM - Auto Flight - Flight Management)

I think your confusion comes from the fact that you think the IRS is aligned with the GPS position and therefore MIX IRS and GPIRS should agree directly after alignment. This is however not true:

Initialization

The F-PLN origin airport coordinates are extracted from the FMS database. These coordinates appear on the MCDU INITA page, and are normally used for initialization. They are the airport reference coordinates.
If a high navigation performance is desired, (i.e. for long-range flights without GPS and without radio navigation updates, or if low RNP operation is expected), the crew should adjust the airport reference coordinates to the gate coordinates, provided that this data is published or available on board.

(Airbus A320 FCTM - Normal Operations - Pre Start - ADIRS Initialization)

As you can see the IRS is aligned against whatever is in the MCDU INITA page (airport coordinate by default), not GPS. Therefore, the IRS position can deviate from the FMGC position (GPIRS), even right after alignment. This deviation will remain in flight, only the FM position will move towards GPS and the bias vector will describe the difference.

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  • $\begingroup$ I re checked this occurs in a minority of my companies fleet. Typically A320s which are over 10 years old. But in majority of the fleet the aircrafts are equipped with automatic position initialisation for the IRS. That is if GPS is available the initialisation is automatic using the GPS position. So in that case shouldn't the drift value be close to 0 or ar 0? $\endgroup$ – Jai Oct 18 '20 at 9:32
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From my understanding the FMGC position is a mix from GPS, IRS and radio navigation signals and maybe even other values, e.g. re-calibration to the runway database position as you apply takeoff thrust "on the runway". (I think I read this somewhere, not sure if it is true).

And as far as I know the IRS is aligned only with the GPS position, which could explain a discrepancy between the two values even right after an alignment.

Because the IRS on its own has a constant drift over time the FMGC computes a correction vector for each GPIRS. Maybe these drift corrections are not reset during the alignment (assumed installation error) or they are reset and take a bit of time to be re-evaluated after alignment. They could be an integrator type correction that accumulates the error of the GPS and IRS positions over time to "trim away" the IRS position error. The IRS could drift during the alignment which might only corrected for after the alignment is completed. This would take several minutes before the drift is compensated for.

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