Why can RTK GNSS achieve so much better accuracy than standalone GNSS receivers?

GNSS receivers measure how long it takes for a signal to travel from a satellite to the receiver. Transmitted signals travel through the ionosphere and atmosphere and are slowed down and perturbed on the way. For example, travel time on a cloudy day and in clear sky conditions would be different. That is why it is difficult for a standalone receiver to precisely determine its position. 

RTK(Real Time Kinematic) technology solves this issue by using correction services. In the RTK method, a user receiver gets correction data from a single base station or from a correction service from Internet. It then uses this data to eliminate most of the GNSS errors. RTK is based on the principle that the base station and the user receiver are located close together (maximum 40 km or 25 miles apart) and therefore “see” the same errors. For example, since the ionospheric delays are similar for both the user and the reference station, they can be cancelled out of the solution, allowing higher accuracy.