How satellite positioning works and how RTK reaches millimetre precision.
What is GNSS?
GNSS (Global Navigation Satellite System) is the umbrella term for satellite navigation systems: the American GPS, European Galileo, Russian GLONASS and Chinese BeiDou. A receiver computes its position from the travel time of signals from multiple satellites.
From metres to millimetres
A simple receiver achieves a precision of a few metres. That is insufficient for surveying. With phase measurement and correction data, precision is driven down to centimetres or even millimetres.
RTK and post-processing
RTK (Real-Time Kinematic) uses corrections from a reference station to reach centimetre precision in real time. For monitoring, post-processing is often applied too: long observation times and after-the-fact computation deliver the highest, millimetre precision.