Real Time Kinematic (RTK) increases the accuracy of GNSS/GPS systems to centimeter-level. RTK allows PX4 to be used in applications like precision surveying, where pinpoint accuracy is essential.
PX4 supports a number of RTK GPS devices. It requires QGroundControl running on a laptop/PC and a vehicle with a WiFi or Telemetry radio link to the laptop.
Information about supported devices and setup can be found in the section: Peripheral Hardware.