Explaining the Difference Between RTK Float and Fix in 3 Minutes | Accuracy, Causes, and How to Distinguish

In RTK positioning there are two solution states called the "Float solution (floating solution)" and the "Fix solution (fixed solution)". Can you correctly explain the difference between these two? This difference is an important point that directly affects RTK positioning accuracy and reliability. In this article, we clearly explain the differences between Float and Fix solutions in RTK — focusing on differences in accuracy, causes of occurrence, and how to tell them apart — in about three minutes.

Table of Contents

• What is RTK?

• What is a Fixed solution (fixed solution)?

• What is a Float solution (float solution)?

• Accuracy differences between Float and Fix solutions

• Causes of a Float solution

• How to distinguish between Float and Fix solutions

• Easily achieve high-precision positioning with LRTK

What is RTK?

RTK (short for Real Time Kinematic) is a method for performing high-precision positioning in real time using GNSS (Global Navigation Satellite Systems). While standalone GPS positioning typically incurs errors of several meters (several ft), RTK uses two GNSS receivers—a reference station (base station) and a rover—and the base station calculates the positioning errors and transmits them to the rover as correction information, which can cancel those errors down to the centimeter level (cm (in)). By installing the reference station at a known, precise coordinate point and applying corrections to the satellite signals received by the rover at the observation point, RTK enables the acquisition of highly accurate position coordinates in real time. This method cancels common error sources such as satellite orbit errors, clock errors, and atmospheric delays, typically reducing positioning offsets that were several meters (several ft) to a few centimeters (a few in). Note that high-precision positioning using carrier-phase as in RTK is markedly more accurate than conventional differential GPS (DGPS) based on code positioning (DGPS errors are at best on the order of several tens of centimeters (several tens of in)), and the fact that centimeter-level positioning can be achieved is revolutionary.

To use RTK positioning, a base station and a rover and communication means connecting them (radio or the Internet) are required, but under the right conditions extremely precise positioning within a few centimeters is possible [for example, when the distance to the base station is short and the satellite visibility is good, horizontal errors are about 1-2 cm (0.4-0.8 in), and vertical errors are on the order of several centimeters (a few in)]. Because such centimeter-level accuracy (cm level accuracy (half-inch accuracy)) can be achieved, RTK is being used increasingly in a wide range of fields where minimizing error is important, such as civil surveying, guidance of construction machinery, agriculture, autonomous driving, and drone positioning. In recent years, networked RTK (such as the VRS method), which leverages public and private base station networks so that correction information can be received without installing one’s own base station, has also become widespread. Specifically, methods such as VRS (Virtual Reference Station), which set a virtual reference point near the rover via Internet distribution using mobile communication networks (Ntrip), are being offered in many places. This makes centimeter-class positioning possible without preparing a dedicated base station, and RTK is becoming an even more accessible technology.

What is a Fixed solution (fixed solution)?

Fixed solution (fixed solution) refers to the state in RTK positioning where the unknown integer-valued biases contained in the carrier-phase of satellite signals (the so-called integer ambiguities) have been correctly resolved. Put simply, it is the stage at which the integer number of wavelengths included in the radio path from the satellite to the receiver (an integer multiple of the wavelength) can be precisely identified, and at this point RTK’s inherent highest accuracy is achieved. When a Fixed solution is obtained, extremely precise positioning is possible: horizontal positions are accurate to within a few cm (a few in), and vertical errors are on the order of a few cm to a dozen or so cm (a few in to several in).

With RTK, immediately after positioning begins the integer bias has not yet been fully resolved and the solution is unstable, but with current multi-GNSS-capable RTK systems it is common under good conditions to reach a Fixed solution within tens of seconds to a few minutes. By accumulating satellite data for a certain period and continuing computations, the correct combination will eventually be found. Once the ambiguities are resolved to integers, that solution is confirmed as a "Fix" (fixed), and thereafter high-precision positioning can be maintained unless there is a major radio outage. In many GNSS receivers and surveying apps, when RTK reaches a fixed solution the status display changes to "FIXED" or "FIX", indicating that centimeter-level accuracy (half-inch accuracy) has been achieved. In RTK operations, obtaining this Fix state is one of the objectives.

Note that older single-frequency (L1-band only) GNSS receivers tend to take longer to achieve a fixed solution and may have difficulty maintaining a fix over long distances, but using the latest multi-frequency, multi-GNSS-capable equipment can shorten initialization time and improve stability over long distances.

What is a Float solution (floating solution)?

A Float solution (floating solution) refers to an intermediate-stage solution in RTK positioning in which integer biases have not yet been resolved. Although correction data are being received and applied, the number of carrier-wave cycles remains undetermined, so accuracy is not fully stable. This Float solution is typically obtained immediately after positioning begins or when satellite signal conditions are insufficient. Accuracy improves over standalone positioning, but it still typically remains in the error range of approximately ±0.5–1 m (±1.6–3.3 ft). For reference, standalone positioning without corrections can reach errors of ± several meters (± several ft) or more. Compared with centimeter-level (cm level, half-inch accuracy) Fix solutions, larger deviations are still possible — the so-called "decimeter-level" accuracy (about 3.9 in), and it is not as reliable as a fixed solution; therefore, Float solutions are insufficient for precise surveying or machine control. For this reason, when using RTK it is necessary to converge from a Float solution to a Fix solution as quickly as possible. However, for applications that can tolerate errors on the order of several tens of centimeters (tens of cm, roughly several inches to over a foot), such as autonomous driving in agriculture or general navigation, Float solutions can be sufficiently practical. By the way, the term "Float" originates from the fact that, unlike the integer-fixed solution (Fix), the solution is still computed as real (floating-point) values.

Accuracy Differences Between Float and Fix Solutions

As mentioned above, there is a large gap in positioning accuracy between Fix and Float solutions. With a Fix solution, horizontal errors are confined to within a few centimeters (a few cm; a few in), whereas with a Float solution errors of several tens of centimeters (several tens of cm; a few dozen in) to about 1 m (3.3 ft) can occur. This difference greatly affects the reliability of positioning results obtained on site. For example, in tasks that require high precision—such as establishing property boundaries or precisely laying out structures—using positioning results while still in a Float solution can produce offsets of several tens of centimeters (several tens of cm; a few dozen in) and lead to serious mistakes. In fact, there have been reports of surveys being carried out while RTK remained in a Float state, with large errors discovered later that required remeasurement.

On the other hand, in situations where accuracy requirements are not particularly strict, it may be more efficient to proceed using the Float solution obtained in a short time. However, even in such cases, you should be aware that the results obtained may contain relatively large errors. Basically, to obtain accurate results with RTK, a safe strategy is to wait until it switches from Float to Fix before using the positioning results. For positioning that requires high precision, it can be said that results should only be adopted after confirming that the solution is in the Fixed state.

Therefore, when performing RTK positioning, it is extremely important to confirm that the solution is in the Fixed state before adopting the results. Only when a Fixed solution is obtained can the centimeter-level accuracy of RTK be guaranteed, allowing the positioning results to be used with confidence. Conversely, while in a Float solution the accuracy is still unstable and preliminary, so you should be patient and wait for the positioning engine to converge to a Fixed solution. Even after obtaining a Fixed solution, it can revert to a Float solution due to various factors; in such cases remain calm, identify the cause, and wait for the solution to become Fixed again before resuming work. Being able to distinguish between Float and Fixed states and respond appropriately is the basic principle for operating RTK correctly.

Causes of Obtaining a Floating-Point Solution

Normally, if the sky is open and a sufficient number of satellites are visible, RTK will enter a Fix state within tens of seconds to a few minutes after initialization. If it has not reached Fix even after more than five minutes, it is likely that some factor is preventing convergence of the solution. When RTK positioning remains in a Float solution and does not fix, the typical causes to consider are as follows.

• Poor satellite reception environment: If the sky view is narrow or there are buildings or trees around, satellite signals can be blocked and the number of satellites that can be received becomes insufficient. Also, if reflected radio waves from walls or the ground (multipath) are picked up, errors increase and the RTK engine may not be able to converge the integer bias, remaining in a Float solution.

• Errors in base station installation or coordinates: If the correction information that the rover receives itself contains errors, it may not be possible to reach a correct Fixed solution. If the base station is not installed at an accurately known point or the base station’s initial coordinate setting is incorrect, the solution can be unstable and fail to become a Fix.

• Base station distance too long: If you are too far from the base station providing correction information, the error sources in the satellite signals received by the base and the rover will differ significantly and the effectiveness of the corrections will be reduced. In general, for RTK using a single base station, a distance (baseline length) to the base station of within about 20 km (12.4 mi) is desirable, and if it is too far it becomes difficult to converge from a Float solution to a Fix. Note that high-performance receivers may be able to achieve a Fix even when more than 70 km (43.5 mi) away, but the farther the distance, the greater the solution error and instability; for example, there are reports that a baseline of 100 km (62.1 mi) can produce about a 10 cm (3.9 in) positional offset.

• GNSS setting mismatch: If the satellite systems (GPS, GLONASS, Galileo, QZSS (Michibiki), etc.) or frequency bands used by the base station and the rover do not match, the rover will lack the necessary observation data and cannot resolve integer ambiguities. (For example, if the base station is broadcasting corrections using GPS and GLONASS but the rover is configured to use only GPS satellites, required observation data will be missing.) Using the same satellite and frequency settings as the base station is a prerequisite for obtaining a Fix.

• No correction data received / communication troubles: If RTK correction data from the base station is not arriving, no matter how good the observation environment is you cannot progress beyond a Float solution. If wireless or Internet communication is unstable and correction data is missing or interrupted, the RTK engine cannot maintain the solution and will remain floating without reaching a Fix.

• Other factors: Fix solutions may also be unobtainable due to GNSS equipment or software setting errors or malfunctions. For example, if the rover mode is not set to RTK (differential positioning), if the communication format between the base and rover does not match, or if there are equipment troubles such as a broken antenna cable, you may not be able to progress beyond a Float solution.

If you recognize any of the causes above, taking appropriate measures—such as improving on-site reception conditions or reviewing device settings—can increase the likelihood of resolving the issue.

How to distinguish Float solutions from Fix solutions

So how can you tell in the field whether your positioning solution is Float or Fix? The most reliable method is to check the status display of the GNSS receiver or the application you are using. Many devices and software explicitly indicate the RTK solution status as "FIX" or "FLOAT" (if no corrections are applied it may display "Single" or "DGPS", etc.). If the screen shows "FIXED" or "fixed", it's a Fix solution; if it shows "FLOAT" or "floating", it's still a Float solution. Depending on the model, the state may also be indicated by an indicator color (e.g., green for Fix, yellow for Float) or by numerical quality metrics, but basically you can easily distinguish the state from the display. (By the way, inside the RTK engine a solution is judged as Fix when a value called the "AR ratio" exceeds a certain threshold, but for typical users checking the device's status display is sufficient.)

Also, you can roughly distinguish them by looking at the estimated positioning accuracy (position accuracy display). For example, if the current accuracy shown by an RTK receiver or app is at a level such as horizontal ±0.02 m (±0.07 ft), it can be considered to be in the Fix state, but if it shows a larger error such as ±0.5 m (±1.6 ft), it is likely in the Float state. Furthermore, with a fixed solution, the measured positions while stationary remain within a few centimeters (a few inches), whereas with a float solution the position tends to drift by tens of centimeters (tens of inches). In any case, for positioning that requires high precision, always confirm that the status is Fix before starting work, and if it falls back to Float during the operation, check the cause and wait for it to return to Fix before resuming. Being able to correctly read the device display and accuracy information to distinguish Float/Fix is a practical basic skill.

Easily achieve high-precision positioning with LRTK

In recent years, devices have emerged that enable the use of RTK positioning without specialized knowledge or complicated setup. A representative example is the compact GNSS RTK terminal "LRTK" that can be attached to and used with an iPhone. LRTK is a system that works with a smartphone to enable one-touch centimeter-level positioning (half-inch-level positioning), and it automatically performs high-precision correction processing internally. By attaching the dedicated receiver to the iPhone, users can start high-precision positioning on site immediately without having to set up base stations or worry about complex communication settings.

For example, by attaching an LRTK unit to the supplied lightweight pole (monopod) and surveying, a single person can efficiently observe many points in a short time. Also, because the smartphone app automatically performs antenna height (the height at which the unit is installed) correction calculations, accurate ground surface coordinates can be obtained without specialized knowledge. Furthermore, the LRTK unit itself has a built‑in battery and is small and lightweight enough to fit in the palm of your hand, making it highly portable and easy to handle even in confined sites.

Specifically, the LRTK terminal automatically retrieves correction information, such as the Geospatial Information Authority of Japan's electronic reference point data, over the Internet via its built-in communication module, and by simply pressing the "Start positioning" button in the smartphone app you can obtain an RTK Fix solution. Under good conditions, high-precision positioning is possible, with horizontal positioning errors within approximately ±1–2 cm (±0.4–0.8 in) and vertical errors within approximately ±3 cm (±1.2 in), allowing you to easily achieve accuracy comparable to conventional professional GNSS equipment. RTK technology is no longer limited to a subset of specialists; tools like LRTK are making it possible for anyone to benefit. If you are thinking "I want to introduce RTK into my own work" or "I want to improve positioning accuracy to increase work efficiency," why not consider utilizing such easy-to-use RTK solutions?

Correctly understand the differences between RTK float and fix solutions, and be sure to use them to achieve high-precision positioning in the field.