Some explanation about the ADIRS please?
 

Some information about the ADIRS, how exactly it knows a/c position once aligned, and what GPIRS is?

Seems the more I read, the more confused I get :}

I believe the simple answer is that it is an triple redundant IRU with a GPS source to supply initial and updated position calculations based off of the IRU's ability to calculate change in position. The AD part basically replaces the brains of the CADC utilizing pressure sensitive modules as p&s digital inputs.

Assuming that by ADIRS you imply Airbus...
During alignment, the IR is told where it is (Lat and Long), either by the pilot, or by GPS. The IR checks this for reasonableness using last known position and earth rotation rate.
Once aligned, the IR measures acceleration and integrates that into velocity, which allows it to calculate position.
Each ADIRS sends a calculated position to the FMS.
Each GPS also sends a calculated position to the FMS.
The FMS uses the most accurate for navigation.

Z.Khalid
 

check your PM

its not just Airbus, its Honeywell, so this may ref other AC as well...
An ADIRS consists of up to three fault tolerant ADIRUs, an associated Control and Display Unit (CDU) and remotely mounted Air Data Modules (ADMs).
Each ADIRU comprises an Air Data Reference (ADR) and an Inertial Reference (IR) component.

Now GPIRS may be different, as there are some IR units that are directly coupled with GPS to update directly, instead of by the Pilot/GPS.

The FMS will use GPS until the GPS degrades past the filter limits, then will use the IRS/INS. There is a drift value associated with the IRS, typically 5nm/hour, without update.