RTK Fixed vs Float: Meaning and "When to Trust"

In recent years, RTK positioning using GNSS (Global Navigation Satellite Systems) has attracted attention in surveying because it enables high-precision real-time positioning. RTK (Real Time Kinematic) uses two GNSS receivers—a base station and a rover—and corrects positioning errors in real time, reducing errors that would be several meters with standalone GPS to several centimeters (several in). Leveraging that high precision, RTK is increasingly used across surveying, civil engineering, agriculture, drone positioning, and more. In recent years, applications such as disaster investigation and infrastructure maintenance and inspection are also expected.

When operating RTK positioning, you will often see displays such as “Fixed” or “Float” on receivers or software. These indicate the solution state in RTK and are critically important information directly tied to positioning accuracy and reliability. This article explains in an easy-to-understand way for RTK beginners to intermediate users what “Fixed” and “Float” mean, the differences between them, and their reliability. We also cover key points to reliably obtain a Fixed solution and introduce our tool “LRTK,” which makes it easy to adopt RTK positioning.

Table of Contents

• What is RTK?

• What is a Fixed solution?

• What is a Float solution?

• Differences and reliability between Fixed and Float solutions

• Tips to obtain a Fixed solution

• Recommendations for simple surveying with LRTK

• FAQ

What is RTK?

RTK positioning is a method that uses two receivers—a base station and a rover—to correct satellite positioning errors and obtain highly accurate coordinates. First, the base station is set up at a known point with accurately known coordinates, and the rover observes the point to be measured. Both receivers receive signals from multiple GNSS satellites such as GPS and GLONASS at the same times. The base station calculates the difference between the obtained positioning data and the known accurate coordinates. This difference data (corrections) is sent to the rover in real time via radio communication or the Internet, and the rover applies those corrections to its own observations to cancel out positioning errors. Through this mechanism, RTK can reduce errors that would normally be on the order of several meters to the level of several centimeters (several in).

To perform RTK positioning, the basic configuration requires two GNSS receivers (one for the base station and one for the rover) and a communication method connecting them. Communications can use short-range radio that directly links base and rover (e.g., the 920 MHz band) or UHF radio, or Internet distribution services using cellular networks (Ntrip). Recently, network RTK (such as VRS) that uses public or private reference station networks and allows reception of corrections without installing your own base station has become widespread. In any case, the rover must be able to secure a communication line to receive correction data. If these conditions are met and the baseline length (distance from the base station) is short, RTK can achieve high-precision real-time positioning with planar accuracy around 1～2 cm (0.4～0.8 in) and vertical accuracy around 3～5 cm (1.2～2.0 in). Historically, centimeter-level positioning required specialist knowledge and expensive equipment, but RTK has made this level of precision far more accessible.

What is a Fixed solution?

A Fixed solution is the state in RTK positioning in which the unknown integer values contained in the carrier-phase measurements (integer ambiguities) have been resolved exactly. In simple terms, it is the stage when it is determined exactly “how many whole wavelengths” the carrier wave contains from the satellite to the receiver, and at this point RTK delivers its intended high-precision performance. When a Fixed solution is obtained, extremely precise positioning is possible, with planar errors of several centimeters (several in) and vertical errors ranging from several centimeters to several tens of centimeters (several in to several tens of in).

After accumulating satellite data for a while from the start of positioning, the RTK engine performs complex internal calculations and gradually narrows down candidates for the integer ambiguities. When the correct combination is found, the solution is fixed and made the definitive solution. This highest-precision state is called a “Fixed solution.” With modern multi-frequency, multi-GNSS RTK systems, it is common in good conditions to reach a Fixed solution within several tens of seconds to several minutes. Once a Fixed solution is obtained, as long as satellite tracking conditions do not significantly deteriorate, centimeter-level positioning can be continuously maintained even while moving. In the field, before recording points or staking out positions, it is important to confirm that the receiver or software display shows “FIX” (fixed).

What is a Float solution?

A Float solution refers to an intermediate RTK state in which integer ambiguities have not yet been fully resolved. Right after starting positioning or when satellite signals are unstable, RTK will start in this Float state. While in Float, the integer number of carrier wavelengths is uncertain, so positioning accuracy is not as high as for a full Fixed solution. Typically, positions obtained under a Float solution have horizontal and vertical errors on the order of several tens of centimeters to about 1 meter. Corrections from the base station improve accuracy relative to standalone positioning, but because the integer part remains “floating,” corrections are not fully effective.

In RTK, given a sufficient number of satellites and good observation conditions, the Float solution will gradually converge to a Fixed solution. However, if satellite reception is poor or the baseline to the base station is too long, the solution may remain Float indefinitely. Even after achieving Fix, deteriorating radio conditions can cause a return to a Float solution. Many RTK-capable devices and software clearly indicate the solution state as “FLOAT” or “DGPS” so users can see it is not fixed. If the solution displays Float during fieldwork, review satellite reception conditions and the base station connection, and take measures to get to Fix as soon as possible.

Differences and reliability between Fixed and Float solutions

As noted above, Fixed and Float solutions differ significantly in the achievable positioning accuracy. While Fixed solutions typically yield horizontal errors on the order of several centimeters (several in), Float solutions can show deviations of several tens of centimeters or more. This difference greatly affects the reliability of survey results obtained on site. For example, when high precision is required for tasks such as installing boundary markers or staking out precise structural positions, using results obtained in a Float state could result in position errors of several tens of centimeters and lead to serious mistakes.

Therefore, in RTK surveying, it is extremely important to adopt results only after confirming that the solution is Fixed. Only when a Fixed solution is obtained can the centimeter-level accuracy of RTK be guaranteed and the results be trusted. Conversely, while in a Float state, the solution is still a preliminary stage and stable accuracy has not yet been achieved, so you should not rush to use those results; wait for the solution to converge to Fix. Also, even after obtaining Fix, if signal reception later deteriorates and the solution reverts to Float, calmly investigate the cause and confirm you have returned to a Fixed solution before resuming work. Judging Fixed versus Float and responding appropriately is fundamental to handling RTK positioning.

Tips to obtain a Fixed solution

To reliably achieve and stably maintain a Fixed solution in RTK, keep the following points in mind.

• Keep the baseline length as short as possible: The longer the baseline (distance between the base station, the source of correction data, and the rover), the larger the differential errors in the satellite signals received by the two stations, and the less effective the corrections become. For single-base RTK, it is generally desirable that the baseline remain within about 20 km. If your survey site is far from the base station, consider installing your own base station near the site if possible, or use a network RTK service (such as VRS) that utilizes electronic reference stations from the Geospatial Information Authority of Japan to effectively shorten the baseline.

• Ensure a clear view of the sky: RTK depends on receiving radio signals from GNSS satellites, so it is important to observe in areas with an open sky. In urban areas surrounded by tall buildings or within forests, satellite signals can be blocked, reducing the number of receivable satellites and preventing convergence from Float to Fix. Mount the antenna in as unobstructed a location as possible and avoid surveying during times when satellite availability temporarily decreases; such environmental considerations are important.

• Stabilize correction data communication: If transmission of correction data from the base to the rover is interrupted, a Fixed solution cannot be maintained even if achieved temporarily, and accuracy will degrade. For direct radio communication, ensure line-of-sight distance or install repeaters; for network RTK, check cellular radio conditions at the site. Establish a communication environment that enables stable reception of correction information. If communication is unstable and data loss is frequent, the RTK engine cannot maintain a solution and will more easily revert to Float.

• Use multi-GNSS and multi-frequency: Choose GNSS receivers that support as many satellite systems (not only GPS but GLONASS, Galileo, QZSS, etc.) and multiple frequency bands as possible. Increasing the number of observable satellites and frequency bands allows the RTK engine to resolve integer ambiguities more quickly and reliably. Older single-frequency (L1-only) equipment tends to take longer to reach a Fixed solution and has difficulty maintaining Fix over long distances; using the latest multi-frequency multi-GNSS equipment can shorten initialization time and improve stability over longer baselines.

Recommendations for simple surveying with LRTK

LRTK is our high-precision RTK positioning system developed with the concept of “making difficult RTK surveying easy for anyone.” It combines a dedicated compact GNSS receiver (the LRTK device) with a smartphone, allowing users to obtain centimeter-level coordinates with a single touch without worrying about complex settings. For example, attaching the LRTK device to the included lightweight pole (monopod) allows one person to quickly observe many points in a short time. The smartphone app automatically calculates antenna height (the height of the GNSS device above ground), so even users without specialized knowledge can record accurate ground coordinates.

Although advanced GNSS correction processing is performed internally by the LRTK system, users do not need to perform complicated procedures themselves. The LRTK device, functioning as the rover, has a built-in mobile communication module and automatically obtains necessary correction information such as electronic reference station data from the Geospatial Information Authority of Japan via the Internet. Therefore, you do not need to set up your own base station or struggle with communication settings on site; with just the LRTK device and a smartphone, you can start high-precision positioning immediately. The device itself is battery-powered and compact enough to fit in a pouch, making it highly portable and easy to handle in confined sites.

As for positioning accuracy, LRTK achieves levels comparable to professional GNSS surveying equipment. Under good conditions, LRTK can achieve planar accuracy of ±1～2 cm (±0.4～0.8 in) and vertical accuracy of ±3 cm (±1.2 in). It also includes a function to observe the same point multiple times and average the results; for example, measuring the same point 60 times and averaging can yield an error of under about 1 cm (under about 0.4 in) and in practice down to a few millimeters (a few mm (a few in)). Despite such precision, operation is very simple—just press a button on the smartphone app.

RTK technology is no longer limited to a small number of specialists; tools like LRTK are ushering in an era where anyone on site can benefit. If you are thinking, “I want to introduce RTK to my site” or “I want to streamline surveying operations,” please consider using LRTK. Even first-time users can master high-precision positioning after a short briefing, and LRTK will transform the way surveying is done on site.

FAQ

Q. What is RTK? A. RTK stands for “Real Time Kinematic” and is a technique that measures centimeter-level high-precision positions by correcting GNSS positioning errors in real time. One receiver is fixed as a base station and another rover receives real-time differential observations to cancel errors and achieve high-precision positioning.

Q. What is required for RTK positioning? A. The basic configuration requires a GNSS receiver for the base station, a GNSS receiver for the rover, and a means of communication between them. The base station should be set up at a point with known accurate coordinates, and corrections are transmitted to the rover via radio (short-range radio or UHF) or the Internet (Ntrip using cellular networks). Recently, network RTK using public or private reference station networks has become common, allowing reception of corrections without preparing your own base station. In any case, the rover must secure a communication environment and be able to receive correction information.

Q. How far from the base station can RTK positioning be performed? A. For typical single-base RTK, it is desirable that the distance from the base station be within about 20 km. Beyond that, differential errors due to the ionosphere and troposphere increase, making integer ambiguity resolution unstable and reducing positioning accuracy by several centimeters or more. However, using network RTK, the effect of virtual reference stations can allow similar accuracy over areas tens of kilometers away.

Q. Can RTK be used in any environment? A. RTK surveying generally needs to be conducted in locations with an open view of the sky. In urban areas surrounded by buildings or in forests, satellite signals are blocked and positioning becomes difficult. Accuracy can also degrade under strong ionospheric disturbances. If communication with the base station is cut, corrections cannot be received, so using network RTK in mountainous areas outside cellular coverage is difficult. Because RTK has environments where it is less effective, it may be necessary to adjust survey points or temporarily use other surveying methods as appropriate.

Q. What is the difference between Fixed and Float? A. In RTK positioning, when the integer biases in carrier-phase measurements are correctly resolved, you have a Fixed solution, which yields centimeter-level accuracy horizontally and vertically. If ambiguities are unresolved, the solution is Float, with accuracy roughly on the order of several tens of centimeters to around 1 meter. RTK typically starts in a Float state and converges to Fixed when enough satellites and good observation conditions are available. Maintaining a Fixed solution is necessary to reliably use RTK’s high-precision results.

Q. Can I perform RTK positioning with a smartphone? A. The GNSS in an ordinary smartphone cannot achieve RTK-level precision by itself, but if paired with an external RTK-capable GNSS receiver, centimeter-level positioning is possible with a smartphone. For example, our product LRTK Phone is an RTK positioning system that works with a smartphone: simply connect a dedicated compact GNSS device to your phone and launch the app to easily obtain high-precision position data. Recently, other vendors have also released small Bluetooth RTK receivers, and using a smartphone or tablet for RTK surveying is becoming widespread. By leveraging smartphones as field tools, the benefits of RTK-level high-precision positioning—previously available only to specialists—are becoming accessible to everyone.

Dramatically improve field surveying accuracy and efficiency with LRTK

The LRTK series delivers centimeter-level GNSS positioning for construction, civil engineering, and surveying, greatly contributing to time savings and productivity improvements. It also supports the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction initiative, making it an optimal solution for digital transformation in the construction industry.

For more details about LRTK, please see the [LRTK Official Site](https://www.lrtk.lefixea.com). For product inquiries, quotes, or consultation on deployment, please feel free to contact us via the [inquiry form](https://www.lrtk.lefixea.com/contactlrtk). Let LRTK take your site to the next stage.