What Is Difference Between RTK Fix & RTK Float?

Real Time Kinematic, or RTK, refers to a data collection method used for surveying based on the Global Positioning System, or GPS. The GPS relies on a network, or constellation, of 24 satellites that transmit signal information to Earth. Depending on the number of satellites visible in the sky at any time, RTK data collection can be "fixed" or "float," with differing levels of precision.

RTK involves a stationary base station and one or more mobile GPS receivers, also known as rovers. Provided that the base station has continuous line-of-sight to each rover, it transmits GPS corrections to each in real time using radio waves. If a sufficient number of satellites are visible, RTK can provide a fixed position, within a fraction of an inch. If insufficient satellites are visible, RTK can provide only a float solution, with a precision of a few inches.

RTK uses a complicated mathematical formula or algorithm to calculate the exact number of radio wavelengths between the satellites and the base station antenna — a process known as ambiguity resolution — and yield either a fixed or float solution. In a fixed solution, the number of wavelengths is a whole number, or integer, and the algorithm is constrained to yield a whole number. A low number of visible satellites, poor satellite constellation geometry and a poor radio link between the base station and the rover may prevent a fixed solution.

In a float solution, the algorithm does not yield an acceptable fixed solution, so the ambiguity is allowed to be a decimal or floating point number. According to Tripod Data Systems, a float solution typically generates precise coordinates to between 4 and 18 inches over a known distance between two points of just over half a mile. If a float solution is the only solution available, it may be possible to reinitialize an RTK system, or simply wait, for a more precise fixed solution. However, if poor satellite visibility is to blame, a fixed solution may be unavailable.

The precision of RTK data collection depends on the distance between the base station and the rovers, so it's desirable to keep the distance between them to less than 6 miles. RTK systems are available in single and dual frequency versions; dual frequency versions are typically faster, more precise and operate over longer distances than single frequency versions, but they are correspondingly more expensive.