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Comment by gmrple | Hacker News Reader

Gravity map matching is an approach that can provide absolute references, the general problem there is that gravity maps are not good enough.

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Only when you're on the ground in the first place. Otherwise, you have a circular problem: past a certain point your IMU needs a gravity map to remain accurate, else the drift from variations in gravity is larger than the drift in your sensor. And you need to know where you are and how you're moving to measure gravity.

They also vary over time or something, right? So even a more precise measurement device, to get better gravity maps, doesn’t solve the problem… maybe? This is all based on half-remembered other comments.

So I would like to learn more about gravity map matching. I've seen offhand references to it in context of placing an ICBM on target (can't find ref now) but that's it. I imagine it is, (or was for long enough to make info sparse) a classified piece of know-how.

For sufficiently accurate IMUs, like the ones found in ICBMs, you need a gravity map because otherwise the variations in gravity cause your position estimate to drift

That's helpful; so presumably for the same reason that gyrocompasses function (self-aligning vis-a-vis the local gravity vector).

A part of me was subconsciously entertaining that a subsystem of the IMU / INS was actually being used to provide gravimetric navigation (producing a coarse global position update de novo rather than correct for an error-prone open-loop one from an a priori generated gravity map). a la

https://ieeexplore.ieee.org/document/9059124

https://www.researchgate.net/publication/343278497_Analysis_...

There's also the tides, and variations in the tides with 2nd and 3rd order effects on gravity. Absolute references based on variable references is ugly!

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