A technical guide to precision geolocation — RTK and PPP-RTK correction services that take positioning from meter-level to centimeter-level accuracy, multi-constellation receivers, and the dead-reckoning fallback that has to hold up under a bridge, in a tunnel, or under jamming.
The largest sources of GNSS error — satellite clock drift, orbital uncertainty, ionospheric and tropospheric delay — are shared by every receiver in a region at any given moment, so they can be measured once by a reference station and subtracted out for every nearby user. RTK does this locally over a radio or cellular link; PPP-RTK networks do it at continental scale over the internet, converging to centimeter accuracy in seconds instead of the tens of minutes classic PPP required.