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Comment by bestnameever | Hacker News Reader

I was under the impression that subs mostly use dead reckoning when submerged.

Good inertial navigation units are really good. I had a professor who liked to tell the story of having one on the top floor of the engineering building and if you left it on all day you could watch your altitude go up and down as the building warmed up with the sun and cooled down at night.

I think any unit that couldn't do that would be useless as it wouldn't have the accuracy required - it would drift within a day otherwise.

More interesting would be to understand how much its position drifts over a period of days or weeks.

Some level of drift is acceptable depending on the mission. Aircraft and missiles only fly for a few hours at most, and may be able to get a satellite or stellar fix at least intermittently.

dead reckoning has to account for currents etc, which is something you could use AI to improve as per the paper. It's actually a _good_ application of AI?

Subs use very expensive and very impressive inertial guidance systems, which can go up-to a week without correction. But that doesn't scale down to the modern world of small autonomous systems.

The currents would accelerate the dead reckoning unit just like any other force, including from the impellers.

Not arguing that small autonomous systems could use something smaller and cheaper.

The dead reckoning itself doesn't detect the ocean currents; these are separate estimates that need to be integrated to determine actual position https://en.wikipedia.org/wiki/Submarine_navigation#Deep_wate...

Huh, I don't understand why. :-D Someone please explain. If I tugged the sub, wouldn't the IMU detect that? Why is a current doing the tugging any different.

now I'm guessing, but perhaps it's like if you walk up the aisle of a moving train, how fast are you travelling? And how fast does it feel like you're travelling?

Sure, I would get confused by that too, but I thought the point of the IMU is that it has its own frame of reference. Elsewhere in these comments was someone mentioning an IMU which could detect a building growing in size when warmer. The IMU doesn't "know" there's water outside or how the water is moving. It knows how much force it's been pushed around by.

Aha! IMU <> "dead reckoning"? In the wikipedia article they are listed separately. Dead reckoning is meaning tracking things like the revolutions of a propeller and a compass to compute position, whereas an IMU is three accelerometers, three gyros and three magnetometers? https://en.wikipedia.org/wiki/Inertial_measurement_unit

Haha oops. :-)

many INS packages also include a DVL (Doppler velocity log) which measures speed over ground if you’re in range of the bottom.

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