The correction data is calculated from changes in the atmospheric conditions and other sources of error such as satellite clock delay, using either a local reference station, or a network of reference stations. In the first 2 forms of RTK, the data is relayed to the rover using RTCM, which the rover uses to correct its own position. In the SBAS solution, the data is relayed as an augmentation or in place of the original GPS signal. When using a network of reference stations the accuracy of the position can be affected by how far away the closest reference station is. The closer the reference station, the more accurate the corrections.
A popular misconception is that the US based satellites have scrambled the accuracy of GPS, making it less accurate for civilian use, which is not the case.
High fidelity GPS solutions still require reasonable sky view to ensure the accuracy, which means that this technology is not suitable for situations where there is limited view of the sky