This seems like a bug. The IRUs are aligned and fixed to a position before the aircraft is even pushed back from the gate (you cannot align and fix them while

This seems like a bug. The IRUs are aligned and fixed to a position before the aircraft is even pushed back from the gate (you cannot align and fix them while moving). What conventionally happens is that you tell the FMS what airport you're at, and it knows the coordinates for that airport. You also tell it what gate you're at, and it usually knows the coordinates for that gate. It will also show you what it thinks your current GNSS position is, and then you pick one of these as your IRU starting point -- usually the gate, but sometimes the GNSS receiver position. One would hope that the pilots would visually validate that the GNSS receiver position is very close to the airport position before entering that into the IRU. If, in the air, you have 2 or 3 IRUs all in agreement with each other, and telling you that you're at position X within an uncertainty of, say, 1 nautical mile, and you also have a GNSS receiver telling you that you're at position Y, and Y is greater than, say, 10 nautical miles away from X, then the GNSS receiver is obviously wrong and should be ignored. IRUs are not perfect, but they are certainly accurate enough to get you to where you need to be, unaided. I find it bizarre that the aircraft systems would ignore perfectly functional IRUs that are in agreement with each other, or, even worse, use (potentially hostile) GNSS data to "correct" them. In the absence of this "correction", the IRUs are completely tamper proof; there is simply no way, without being on the aircraft in question, to get them to say anything different.