GPS RTK, or Real-Time Kinematic, is a GNSS technique used to enhance the precision of position data received from satellite-based positioning systems. RTK GNSS uses carrier-based ranging rather than code-based positioning, and relies on a single reference station to provide real-time corrections, resulting in the centimetre-level accuracy often required for applications such as surveying. GPS makes use of RTK to reduce and remove common errors.
In code-based positioning, the receiver establishes its position by correlating with four or more satellites to determine their ranges. Using these ranges, and the position of the satellite, the receiver can establish its position within a few metres. However, carrier-based ranging calculates the range by determining the number of carrier cycles between the satellite and the rover station, and multiplying this number by the carrier wavelength, resulting in more precise positions than those established by code based positioning.
GPS RTK GNSS consists of a base station, which monitors the signals coming from the satellites, one or several rover users to which the base station sends information, and a communication channel with which the base broadcasts correction data to the users in real time. RTK GNSS provides highly accurate positioning information within the vicinity of this base station.
To be able to compute an RTK solution, satellite measurements are transmitted from a fixed reference station to receivers nearby. By combining their own measurements with those of the reference station, whose exact location is well known, they are able to remove common errors (ionosphere errors, satellite clock errors etc.). Once these errors have been removed, it will use the phase measurements to determine an exact position within 2–5 centimetres.
Employing the use of GPS RTK is most suitable when trying to determine a large number of unknown points within the vicinity of the known base station, and when the coordinates are required in real time. It’s also important that the line of sight from the base station to the rover users is relatively unobstructed. GPS RTK position accuracy will decrease as the distance between the reference station and the rover receivers increases.
It is recommended to limit this distance to between 20 and 30 kilometres, as a significant amount of position accuracy will be lost when the distance is greater than 30 kilometres.