[michaelt]

In the specific case of a docking-type manoeuvre presumably you only need the highest accuracy when you're getting very close to the target.

No reason you couldn't use RTK GPS for <10cm accuracy for most of the flight, then in the last few meters of landing switch over to to high-precision, short-range tracking - like optically tracking a marker on the grabbing arm.

For other specific cases - like bridge monitoring - there are reports of 2–3 mm precision [1]. Of course, bridge monitoring has quite distinctive requirements; a 5Hz vibration component and a 0.0001 Hz thermal expansion component. So there's a lot of potential to average over lots of readings to reduce noise.

[1] https://www.sciencedirect.com/science/article/abs/pii/S02632...