Every measurement, regardless of technique and tools, contains errors. Data obtained from Global Positioning Systems is NO different. RTK is an acronym used to describe a process called Real Time Kinematics Surveying; it is a process by which GPS signals are augmented thru a process called DGPS but in Real Time.
RTK is implemented by utilizing a second receiver on a known point to correct GPS data; the base station calculates the difference from the GPS solution and compares it to the accepted positional value. A correctional value is then calculated and transmitted via a radio modem to one or more GPS rover units.
A base station broadcasts correction values and doesn’t, typically, have two-way communication with the individual rover units. Thus many receivers may utilize a single base station.
The distance between the base station and the rover units is ultimately limited to the GPS constellation; both the base station and the rover must simultaneously observe at least four of the same satellites. The distance between rover and base has some other limiting factors but often times these can be safely ignored by grading contractors.
This communication is often performed via non-licensed low power uhf/vhf radio modems; however some high power systems are available with licenses from the FCC. Although not always practical for construction uses, some surveying practices use a cell phone link thru paid services to obtain the RTK corrections.