ADIRUs will throw out bad GPS data if it disagrees with multiple IRUs, hence why there's usually 3 on the aircraft. That being said, if the GPS is close enough to the three, then correction will still be applied.

If they're using the older IRUs, the drift is corrected via redundancy and not GPS. Usually pilots will report drift based on their final IRU coordinates compared against GPS. Even then, they should still be checking their course with VOR.

Imagine you can't see or hear anything but you can read a compass, and you have an internal map of your house and neighborhood. You also know how long your steps are with some amount of accuracy. You would probably be able to get out of your house and maybe to the corner store, but the inaccuracies in your compass and distance estimation would add up over time, and on a long walk you might overshoot the sidewalk and walk down the middle of a busy street by mistake.

They know where you took off from, and can detect your movement with precision.

It's knows where it was and where it isn't

They use gyroscopes and accelerometers to measure the aircrafts movement from the starting position at takeoff. That can then be used to plot the course the aircraft has taken to show the current location.

https://en.m.wikipedia.org/wiki/Dead_reckoning

First, they have to align on the ground. You initialize them with your current known position (usually by GPS or your known airport/gate spot). Then, you wait for them to synchronize with the Earth's rotation. If you're far north, like in Alaska, this could take half an hour. If you're close to the equator, it could take 5 minutes. Once they're ready, from that point, any movement you make, it will know where you are and where you've been.

If you spin up a gyro and begin moving around, it will maintain it's starting position. You can use this deflection to calculate direction. If you know how fast you are going and for how long, you'll have your position.

Mechanical gyros drift. It's the nature of a world with friction. Newer IRUs use laser gyros, so the only real drift they have comes from extremely minute rounding errors.

Yeah I have the same question. Based upon a comment above, it looks like the independent gyro system is updated for drift based upon the spoofed GPS data and thus causes issues. If the IRS is not updated at all then drift becomes a bigger issue but if it’s updated regularly with valid GPS data then it’s a good thing. So the challenge is to only update the gyro drift with valid GPS data which I am guessing is hard to determine.

If it's a smaller plane (such as a CRJ / ERJ) with only one IRU, it will not be able to determine if GPS is valid or not, so the drift correction gets spoiled.

Large commercial aircraft are using 3 IRUs, with newer aircraft using ADIRUs. If GPS does not agree with the three IRUs, the GPS data is thrown out. If the GPS is within tolerance, correction is applied. You could build up very small errors over a long distance, but you should still be pretty close to the airfield when you get there.

Yes. Most of commercial navigation systems rely on the IRUs as a primary source of position data, and they'll usually have 3 of them. VHF is used by the crew to confirm that the aircraft is on track by referencing VOR stations, though these are slowly being phased out due to GPS.

That being said, a single traditional IRU can have up to 2km of drift over a 2 hr flight (at which point it's removed from service and replaced). When used in combination with two other IRUs, the error is dramatically reduced. Traditional IRUs are gyroscopically mechanical in nature and do not talk to GPS.

Now, that being said, the new standard is called an ADIRU (ADvanced IRU), which ties in with GPS and features laser gyros. They're extremely accurate and have essentially zero drift, plus multiple redundant components within each unit.

The missile knows where it is by knowing where it is not.