> like inertial

One method simply measures how long it takes light to go through a coil, and if the coil is rotating it will take slightly longer or shorter than if it isn't. Three coils perpendicular (orthogonal?) to each other and you know how you're moving. Run the long term results through a filter and you can determine your latitude as well.

(Technically it's two beams in each fiber going opposite directions and they compare the phasing).

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

https://en.wikipedia.org/wiki/Fibre-optic_gyroscope