Concept · RTK Fundamentals
Float and Fix are the two RTK solution types. They look similar on a status screen — both show a position with corrections applied — but the accuracy difference between them is enormous. Fix is centimetres. Float is decimetres to metres. Never collect survey data at Float.
On this page
To understand Float and Fix you need to understand one concept: carrier phase ambiguity. It is the reason RTK can achieve centimetre accuracy at all — and the reason Float cannot.
GNSS receivers measure position in two ways. The simpler method is pseudorange — measuring the travel time of a satellite signal to estimate distance. Pseudorange gives accuracy of 1–3 metres. The more precise method is carrier phase — measuring the phase of the satellite's radio wave at the receiver antenna. The carrier wave has a wavelength of about 19 cm (for GPS L1). By tracking how many whole wavelengths fit between the satellite and the receiver, and precisely measuring the fractional part, the receiver can measure distance to millimetre precision.
The problem: the receiver knows the fractional part of the carrier phase precisely, but it does not know how many whole wavelengths there are between it and the satellite. This unknown integer number is called the carrier phase ambiguity — or simply the integer ambiguity.
Resolving the integer ambiguity is what RTK does. Using corrections from the reference station — which has its own precisely known position — the rover can cross-check its carrier phase measurements against the reference and mathematically determine the correct integer values. When the receiver is confident it has the right integers for all tracked satellites, it declares RTK Fixed.
Float means the receiver is still working on this. It has an estimate of the integers — good enough to give sub-metre accuracy — but not yet confident enough to fix them to specific integers. Float accuracy depends on how good the estimate is: anywhere from 10 cm to 1 m, with occasional larger jumps.
When you connect to an NTRIP service and power up in the field, your receiver moves through several solution types before reaching Fix.
| Application | Single | Float | Fix |
|---|---|---|---|
| Rough navigation Finding a plot or general location |
Good enough | Good enough | Good enough |
| Drone mapping — no GCPs Direct georeferencing from drone RTK |
No | Marginal | Required |
| GCP collection for drone mapping | No | No | Required |
| Precision agriculture — auto-steer Row guidance to 2–5 cm |
No | Marginal | Preferred |
| Survey — topographic General surface mapping |
No | No | Required |
| Survey — cadastral / legal Property boundaries |
No | No | Required + verification |
| Stakeout to 1 cm | No | No | Required |
| Machine control — earthworks | No | Sometimes | Required |
Float is a transitional state. In good conditions it lasts 10–30 seconds. In challenging conditions it can persist indefinitely. Here is what helps most:
The most dangerous scenario in RTK is a false Fix — the receiver declares Fixed but has resolved the ambiguities to the wrong integers. The position looks centimetre-precise and stable, but it is wrong by one or more carrier wavelengths (19 cm per L1 cycle).
False Fix produces errors of exactly one or more multiples of the carrier wavelength — 19 cm, 38 cm, 57 cm and so on. It is more common at long baselines, in high multipath environments and during solar storms when ionospheric noise is high.
How to detect a false Fix
Always verify on a known control point at the start of any precision survey. Set up over a point with published coordinates and check your measured position against the known values. A discrepancy of exactly ~19 cm, ~38 cm or ~57 cm in any direction is a strong indicator of false Fix. Disconnect, move to open sky, reconnect and re-initialise before continuing.
Fix quality indicator — ratio
Many receivers and field software applications report a Fix quality ratio alongside the solution type. A ratio above 3.0 indicates high confidence in the ambiguity resolution. A ratio between 1.5 and 3.0 means the Fix is tentative — treat measurements with extra caution and verify on known points. A ratio below 1.5 may indicate the receiver should not have declared Fix at all.