[beerandt]

Rtk already is 'relative' error- it requires one or more base stations (with either known absolute location or assumed one for relative positioning).

But survey grade gnss is a web of rabbit holes, if you want to get into it.

And there are ways to get sub mm accuracy both relative and absolute, but idk of one that would be quick enough for the required reaction time of dynamic landing via 'catching'.

But multi-centimeter (4-5) that's really easily doable is probably good enough for other systems to take over from.