A customer asked us why Alpha has lesser spec of 1cm + 1ppm than another brand’s 0.7cm + 1ppm horizontal and 1.5cm + 1ppm vertical accuracy.
Listing 1cm + 1ppm horizontal accuracy, we actually measured below numbers:
The numbers look better than the above other brand's, yet we still choose to use the typical 1cm + 1ppm horizontal accuracy and tell customers twice the horizontal accuracy, 2cm + 2ppm, for vertical accuracy if they ask, to avoid over-promise. Listing a smaller number that appears to be superior, a customer new to RTK unfamiliar with accuracy spec definitions might later have surprises. 1cm + 1ppm for horizontal and 2cm + 2ppm for vertical will give less surprises.
The noise-like high frequency variation riding on a low frequency variation in below altitude plot is due to the changing ionospheric/atmospheric conditions; such won't show up in laboratory's superior-looking simulator test results and derived numbers!
A nice introduction on GNSS Accuracy Specifications versus Reality by Joe Sass for reference.