How do IRUs work do to give you location?
Comment on Commercial Flights Are Experiencing 'Unthinkable' GPS Attacks and Nobody Knows What to Do
Dettweiler42@lemmyonline.com 10 months ago
That just means you can’t use autoland in low visibility conditions. Modern IRUs (inertial reference unit) are highly accurate laser gyros that can use GPS for correction, but will throw out the data if it doesn’t make sense. Navigation won’t be affected much, and autoland (if used) will still rely on VHF guidance.
someguy3@lemmy.world 10 months ago
dhork@lemmy.world 10 months ago
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.
scarabic@lemmy.world 10 months ago
“Give me a stopwatch and a map and I’ll fly the Alps in a plane with no windows?”
This was supposed to be a wild boast by the Russian navigator in Hunt for Red October but is apparently now standard piloting procedure.
Maudfer@lemmy.world 10 months ago
They know where you took off from, and can detect your movement with precision.
Hupf@feddit.de 10 months ago
BarrelAgedBoredom@lemm.ee 10 months ago
It’s knows where it was and where it isn’t
thehatfox@lemmy.world 10 months ago
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.
Offbus@lemmy.world 10 months ago
Dettweiler42@lemmyonline.com 10 months ago
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.
redcalcium@lemmy.institute 10 months ago
But the article mentioned that “the spoofing corrupts the Inertial Reference System”. How?
assembly@lemmy.world 10 months ago
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.
Dimand@lemmy.world 10 months ago
Pretty much this, look up Kalman filters if you want details. The most likely explanation is that they are tuned to effectively trust GPS more than the internal IMU for long periods of time. Really good IMUs are very expensive and still drift but have high speed output. When it works well, GPS is cheap and doesn’t drift but with a slow update rate. The cost optimisation probably means that the IMU data is usually only trusted for a few seconds, probably 10 min at most before it takes whatever the GPS says as truth. If they lost gps signal through jamming, then they would keep navigation on the less certain IMU data, but the GPS sensor thinks all is well so they shift position.
There is probably a software upgrade to the filter that could be used to limit these attacks, but I imagine it’s an active area or research.
Dettweiler42@lemmyonline.com 10 months ago
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.
_s10e@feddit.de 10 months ago
Ignore my ignorance. Are you saying the aircrafts track where they are going by calculating their position from gyroscope data? And this is more precise than GPS?
That’s like using the accelaration sensors in your phone to navigate. Or sailing with compass and nautical maps.
Possible. Tech isn’t even that novel. But still impressive.
Dettweiler42@lemmyonline.com 10 months ago
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.
Vqhm@lemmy.world 10 months ago
If anyone is really curious about how INS works en.m.wikipedia.org/…/Inertial_navigation_system
Also this Air Force training audio REALLY clears the subject up: youtu.be/VUrMuc-ULmM
The Missile Knows Where It Is The transcription for the audio is as follows:
The missile knows where it is at all times. It knows this because it knows where it isn’t. By subtracting where it is from where it isn’t, or where it isn’t from where it is (whichever is greater), it obtains a difference, or deviation. The guidance subsystem uses deviations to generate corrective commands to drive the missile from a position where it is to a position where it isn’t, and arriving at a position where it wasn’t, it now is. Consequently, the position where it is, is now the position that it wasn’t, and it follows that the position that it was, is now the position that it isn’t. In the event that the position that it is in is not the position that it wasn’t, the system has acquired a variation, the variation being the difference between where the missile is, and where it wasn’t. If variation is considered to be a significant factor, it too may be corrected by the GEA. However, the missile must also know where it was. The missile guidance computer scenario works as follows. Because a variation has modified some of the information the missile has obtained, it is not sure just where it is. However, it is sure where it isn’t, within reason, and it knows where it was. It now subtracts where it should be from where it wasn’t, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn’t be, and where it was, it is able to obtain the deviation and its variation, which is called error."
Apollo@sh.itjust.works 10 months ago
The missile knows where it is by knowing where it is not.
peanutyam@lemmy.world 10 months ago
I’m glad I wasn’t the only one scratching my head at why was this an issue….(30 + years in aircraft maintenance just not avionics trade, airframes and engines)
ColeSloth@discuss.tchncs.de 10 months ago
Well the article says it caused at least one plane to almost fly into Iran’s restricted airspace…
Delogrand@lemmy.world 10 months ago
Modern IRUs also take input from multiple sources (GPS, Navaids) to update their drift error. With spoofed GPS, bad drift corrections are made and when the navigation solution eventually fails the IRU is just as unusable.
Dettweiler42@lemmyonline.com 10 months ago
ADIRUs will throw out bad GPS data if it disagrees with multiple IRUs, hence why there’s usually 3 on the aircraft.
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.
Rivalarrival@lemmy.today 10 months ago
Anyone with the ability to jam GPS can easily spoof VOR signals.
Dettweiler42@lemmyonline.com 10 months ago
But in this case, they’re not. Plus, the crew are going to be the ones determining if their VOR/DME makes sense or not.