Far-seeker:
GPS units with 5 m accuracy might exist right now.
I have a GPS data logger that has sub one meter resolution - it very clearly shows lane change maneuvers. It's not pure GPS - it uses a combination of very precise GPS position sensing and accelerometers - but shows that this sort of accuracy is available if you're willing to put enough effort into it. I'm not clear what the accuracy requirements are in this case, though. If the objective is to hit an altitude target, then it's probably more important to be consistent with the equipment that the organisers use to measure height, than to be accurate in absolute terms.