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Commercial airliners intended for mass transportation have inertial reference units (IRUs); these are usually spinning laser autogyros. The plane knows where it

Commercial airliners intended for mass transportation have inertial reference units (IRUs); these are usually spinning laser autogyros. The plane knows where it is based on this; every action has an equal and opposite reaction, so banking left or pitching up will have the opposite effect on the IRU. GNSS merely augments this. If the IRUs and the GNSS receiver significantly disagree, the pilots will be warned, and the plane may start automatically ignoring the GNSS receiver. I can't speak for much simpler smaller aircraft with e.g. 2 or 4 seats. They may well have only GNSS to rely on. However, if you are at any point unsure as to where you are exactly, you can always try to grab a VOR-DME radial or the intersection of 2 VORs.

kylehotchkiss 3y ago
Opsgroup mentions this in their current and previous article, the IRU fallbacks are failing at a higher than expected rate because they can handle GPS outages but the spoofing wasn't a covered use case. Airplanes are ending up in a situation where they have zero location info and having to get it from ATC.
- KRAKRISMOTT 3y ago
  Start bringing back celestial navigation computers. Airliners have enough altitude to install those.
- aaronmdjones 3y ago
  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.
- cjrp 3y ago
  Back to VORs and NDBs then!

kylehotchkiss 3y ago

Opsgroup mentions this in their current and previous article, the IRU fallbacks are failing at a higher than expected rate because they can handle GPS outages but the spoofing wasn't a covered use case. Airplanes are ending up in a situation where they have zero location info and having to get it from ATC.

KRAKRISMOTT 3y ago
Start bringing back celestial navigation computers. Airliners have enough altitude to install those.
aaronmdjones 3y ago
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.
cjrp 3y ago
Back to VORs and NDBs then!