The highest precision GPS receivers require dual-frequency reception and perform "carrier differential GPS" - any time people are talking about GPS with 2cm acc

The highest precision GPS receivers require dual-frequency reception and perform "carrier differential GPS" - any time people are talking about GPS with 2cm accuracy, that's what they're talking about. The GPS signal has a digital signal (with a wavelength of ~300m) modulated with a sine wave (with a wavelength of ~20cm) - you can measure the 20cm signal much more precisely than the 300m signal, but it's 'ambiguous' because it repeats every 20cm and you don't know how many waves of the sine have passed. If you can receive two frequencies, giving you two slightly different sine waves, you can multiply them together and get a sine wave with a longer period, which (together with some clever maths) lets you bridge the gap between the unambiguous-but-imprecise digital signal and the precise-but-ambiguous sine wave. You also need a fixed base station at a known location - or access to a commercial network of such stations.