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Comment by csense | Hacker News Reader

If you can't receive signals from the GPS satellites because some country's military is jamming them, how does having an accurate on-board clock help?

Are they somehow able to determine position via dead reckoning? How does that account for errors from wind, vibration, etc. and compounding of errors over time? (I'm pretty sure dead reckoning is not a closed-loop system)

"Miniaturize a very accurate clock" seems like a fairly straightforward engineering challenge. "I can give you clocks as precise as you need, now design me a system that can give your coordinates in thick fog without GPS or any other external radio signals" seems like a much harder one.

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Historically ships at sea could determine latitude through sun-sighting or stars, but longitude was impossible because they did not have a clock which was accurate enough

I doubt they're navigating using the sun an stars, but if the airspeed indicator is accurate, and you know you're heading, all you need is an accurate clock to determine absolute position since the last known good position.

But quartz is accurate enough for that, let alone a properly calibrated oscillator. So why is the article focused on giving planes atomic clocks?

> if the airspeed indicator is accurate, and you know you're heading, all you need is an accurate clock to determine absolute position

Airspeed gives speed relative to the air, not the ground. To compute ground speed, you also need winds aloft, which can be huge if the aircraft is operating in the jet stream.

Getting good data for winds aloft is difficult because there are a relatively small number of actual measurements, and everything else is model output with all the usual caveats.

Indeed. The article is a bit confusing on details. At the end it talks about accelerometers and gyros, but aircraft have been using laser ring gyros for decades. They now use gps because it is much more accurate.

The article does a poor job explaining, but seems to imply that they are working on replacing GPS altogether with a local system that relies on an atomic clock and quantum engineering. From what I can find, there are many approaches to this, including quantum gravimetry, quantum accelerometers, etc.

https://en.wikipedia.org/wiki/Quantum_sensor

It's not white noise jamming it's replaying with a delay, so the receiver is getting a meaningful signal.

Technically both techniques can be used for gps hacking. It also seems you can fake a gps signal altogether, because the public signal is not cryptographically signed, which surprises me (the only thing that makes sense to me is that the gps protocol doesn't have room for adding a signature, so it'd be a breaking change to the protocol).

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