[michaelt]

RTK GPS isn't hard in theory, only in practice.

To maintain a high-precision fix, you have to maintain particularly good tracking (a 'phase lock') of 5 or 6 satellites. That can be demanding - you practically need line of sight to the satellites.

That might be practical if you're flying drones outside in free space - but if you're flying in a stadium with a lot of seating? Or dodging in and around trees with lots of foliage? By all means test it out, but don't imagine it'll be easy :)

I've also heard second-hand reports that the PLL signal tracking built into many GPS modules isn't really tuned for >4g acceleration because 99.9% of GPS modules end up in cars and phones. With the consequence that trying to track acrobatic flight leads to more lost signals than you'd expect, based on the excellent view of the sky. I haven't seen that firsthand, though - it's just what I've been told.