Advanced IMUs are still export controlled and the state-of-the-art is classified. The US military considers this a cornerstone technology and has invested heavily in R&D over the years. The IMUs that are widely available commercially have improved significantly over the years but so have the military versions.

> Maybe this has changed or is ineffective now that smartphone/quadcopter IMUs have caught up.

They did not caught up. There are two kind of IMUs: one where you have to account for the rotation of the Earth during signal processing and one where there is no point because it will be lost in the noise anyway. The smartphone/quadcoptee IMUs are the second kind. The first kind is still export controlled.

consumer-grade IMUs are still well below the performance of even much older military-grade IMUs (which tend to be impressive feats of precision engineering with pricetags to match, but also physically much larger). You'll still find anything that's useful for working out position over any time period is export-controlled (dual-use or stricter).

That's super interesting to learn, thanks for letting me know! I'm a software guy but I love learning about sensors, it's always amazing.