What is GNSS RTK?

"Real Time Kinematic (RTK) is a satellite positioning technique based on the use of carrier phase measurements from signals transmitted by GPS, GLONASS, or Galileo systems. A reference station provides real-time corrections that allow centimeter-level accuracy to be achieved.
In practice, RTK systems use a fixed receiver (a base station whose position is precisely known) and a number of mobile receivers. The base station compares the position calculated from the GNSS signals with its actual position, then retransmits the corrections to the mobile receivers. This allows the mobile units to compute their relative position from centimeter to millimeter-level accuracy, although their absolute position is only as accurate as the position of the base station. The typical nominal accuracy for these systems is 1 cm horizontally and 2 cm vertically.
Although this limits the usefulness of RTK for general navigation, it is perfectly suited for applications such as monitoring or precision guidance. In this case, the base station is located at a known point that serves as the reference, and mobile units can produce highly accurate maps by taking measurements relative to this point. RTK is also used in autopilot systems, public works, precision agriculture, and other similar applications.
Operational reliability and achievable accuracy depend on the density and capabilities of the reference station network."