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Primarily because commercial airliners do not use GPS.

Don't they? AFAIK they are used for navigation, they just can't transmit back - but that's set to be implemented in the coming years. Would love to hear more on that.

All critical systems have used inertial navigation for decades. The systems may accept GPS corrections within the error margins of inertial navigation but nothing more.

Inertial navigation used to have larger margins of error than GPS. Since GPS was not trustworthy and inertial is, they accept GPS fine-tuning to the trusted inertial system. If the inertial system and GPS system disagree, GPS is ignored.

State-of-the-art inertial systems are now more precise than GPS.

What do you mean by "can't transmit back"?

I think he may mean that airliners transmit their GPS coordinates (which I've heard an ordinary citizen can pick up cheaply) but can't receive commands that way.

The exact opposite actually. Apparently airliners, like our mobile phones, can only receive GPS signals to track their own position, but they can't report that back to the control towers - position is calculated from ground radars, which only reach up to 200 miles from shore. That's why the location of AF447 couldn't be pinpointed quickly after the accident, for example, there are only estimates for location while out at sea.

Do you know why the pilots of that plane didn't use GPS to determine their altitude and speed? I read that it's suspected they slammed into the sea because they didn't know their altitude and speed, due to frozen-over measuring devices.

Apparently the entire issue came down to a junior pilot pulling back on the control stick, causing the airplane to stall and fall. It seems commercial airlines usually operate in a mode where it is impossible to stall -- pulling back all the way just ascends as fast as the plane can. But, the plane reverted to another mode of operation and the pilot did not understand this. Additionally, the Airbus control system has no physical feedback between the two pilot inputs and just merges the data. So if one pilot is pulling back, the other has no way of knowing. From the transcription, it seems that the pilots were confused as to why they were falling, until they realised the junior pilot was stalling the plane, and by then it was too late to fix.

Thanks for that description. Amazing that Airbus hadn't addressed this already. I've read that the initial problem that confused the pilots was the lack of air speed (and maybe altitude) data, due to frozen pitons. So I still wonder why the planes couldn't use GPS for that, at least as backup.

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