What is the difference between RTK Float and Fix? 7 on-site checkpoints to avoid mistakes

In high-precision positioning using the RTK (Real-Time Kinematic) method, there are two solution status types: the "Float" solution and the "Fix" solution. Correctly understanding their differences and responding appropriately on-site is the key to obtaining high-precision positioning results. The Fix solution (fixed solution) is a centimeter-level (cm; inch-level) high-precision result obtained after RTK processing completes, whereas the Float solution (float solution) is an uncertain, intermediate state during processing, with accuracy remaining on the order of several tens of centimeters (cm; several tens of inches). When operating RTK positioning in the field, it is important to keep the difference between Float and Fix in mind to prevent failures caused by incorrect judgments.

In this article, we first explain what Float and Fix solutions mean in RTK positioning and how they differ. We then introduce seven checkpoints to keep in mind to ensure positioning accuracy on site and to avoid situations such as being unable to obtain a Fix solution or remaining at a Float solution with large errors. Aimed at practitioners who use RTK in surveying and construction sites, we provide a clear explanation that includes concrete countermeasures.

Table of Contents

• Difference between Float and Fix Solutions in RTK Positioning

• Checkpoint 1: Check Positioning Status (Float/Fix)

• Checkpoint 2: Ensure Satellite Visibility (Optimize Reception Environment)

• Checkpoint 3: Reference Station Coordinate Settings and Stable Installation

• Checkpoint 4: Keep the Distance to the Reference Station as Short as Possible

• Checkpoint 5: Unify GNSS Settings Between Reference and Rover Stations

• Checkpoint 6: Ensure Reliable Reception of Correction Data (Maintain Communication Environment)

• Checkpoint 7: Utilize Multi-GNSS and Multi-Frequency

• Easily Achieve RTK Positioning with LRTK

Differences between Float and Fix Solutions in RTK Positioning

With RTK positioning, the rover receives correction information from the base station and performs calculations that result in a solution displayed as either “Fix” or “Float.” A Fix solution (fixed solution) is the state in RTK processing where the unknown integers contained in the satellite signal phases (integer ambiguities) have been resolved exactly. In this state, RTK’s inherent accuracy is realized, enabling very high-precision positioning—horizontally within a few centimeters (a few in), and vertically within several centimeters to several tens of centimeters (several in to several tens of in). On the other hand, a Float solution (floating solution) refers to an intermediate solution in which corrections are being applied but the integer part has not yet been fixed. Its accuracy is significantly worse than that of a Fix solution, typically remaining on the order of ±0.5–1 m (±1.6–3.3 ft). Float solutions are common shortly after RTK positioning begins or when satellite signal reception is unstable, and as processing continues and conditions improve they will eventually converge to a Fix solution. Note that standalone positioning with no corrections applied is displayed as “Single,” and the error is several meters or more (several ft or more), much larger than when using RTK.

Because there is a large difference in positioning accuracy obtained between Float solutions and Fix solutions, the reliability of survey results that can be obtained in the field also differs greatly. For example, in tasks that require centimeter-level precision (cm level accuracy, half-inch accuracy), such as establishing land boundaries or accurately laying out the positions of structures, if positioning results obtained while still in a Float solution are used, positional offsets of several tens of centimeters (several tens of inches) can occur and may lead to serious mistakes. Therefore, in RTK surveying it is extremely important to confirm that the solution is in a Fix state before adopting the results. Only when a Fix solution is obtained can RTK’s centimeter-class accuracy (cm level accuracy, half-inch accuracy) be guaranteed, and the positioning results can be trusted with confidence. Conversely, while the solution is still Float it is a preliminary stage in which stable accuracy has not yet been achieved, so one should not hastily use the results but wait for the positioning solution to converge to Fix. Also, even after obtaining a Fix, degraded signal reception can sometimes cause it to revert to Float. In such cases, remain calm, identify the cause, and resume work only after confirming the solution has become Fix again. Distinguishing between the Fixed and Float states and responding appropriately can be said to be fundamental when handling RTK positioning.

Checkpoint 1: Verify positioning status (Float/Fix)

First and foremost, make it a habit to consistently check the positioning status displayed on the screens of your GNSS receiver and surveying app. On many RTK-capable devices, the current solution status is shown as text such as "FIX", "FLOAT", or "Single (single)". On the work site, always watch this status display and be sure to confirm that it shows "FIX" (fixed solution) before recording survey points or staking out positions. If the display remains "FLOAT", high-precision results have not yet been obtained, so you should avoid using the positioning results immediately. Especially for RTK beginners, there are cases where coordinates were inadvertently recorded while in a Float solution, and the large errors were discovered later. To prevent such mistakes, it is important to constantly check the status during positioning and proceed to the next task only after confirming that a FIX solution has been obtained.

If the receiver does not transition to a Fix state easily, or if it falls back to Float after having been Fix, calmly investigate the cause and take appropriate measures. Check the factors described from point 2 onward below in sequence, and try to restore the Fix state by eliminating the problems. In RTK positioning, correctly understanding the solution status is the first step in quality control.

Checkpoint 2: Securing Satellite Visibility (Optimizing the Reception Environment)

To obtain a stable Fix solution in RTK positioning, it is important to keep the GNSS satellite signal reception environment as good as possible. In locations surrounded by buildings or trees, signals from satellites are blocked and the number of satellites that can be received is greatly reduced. Also, receiving signals reflected off concrete walls or vehicles causes multipath (multiple-path errors), which can lead to unstable positioning solutions. In such degraded environments, even if correction data is being received, the necessary observation information may be insufficient and the RTK engine cannot resolve the integer ambiguities. As a result, the solution may remain a Float and never reach a Fix, or in the worst case the corrections may fail entirely and the solution may revert to a Single.

As a countermeasure, on-site we try to install and use the GNSS antenna in locations with as much open sky as possible. In urban areas surrounded by tall buildings or inside forests, moving to a spot with a clear view of the sky and re-running the positioning can significantly speed up convergence from a float solution to a fix solution. Mount the antenna as high as possible using a tripod or pole, and keep it away from nearby obstructions and reflecting surfaces. For example, placing the antenna directly on the ground or on a vehicle hood often degrades reception, so it is preferable to secure it to a stable location using a dedicated pole or a magnetic mount. Also, it can be effective to choose observation times to avoid periods when satellite visibility temporarily worsens (such as when satellite geometry is unfavorable). Furthermore, if strong radio communication equipment or high-voltage power lines are operating nearby, their electromagnetic interference may adversely affect GNSS signals. If an interference source is suspected, consider conducting positioning during times when those devices are not in use, or applying noise-mitigation measures to the antenna if possible.

Confirmation Point 3: Coordinate Setting and Stable Installation of the Reference Station

In RTK positioning, the rover (mobile station) applies corrections based on the accurate position information of the base station (reference station). Therefore, if there are problems with the base station’s configuration or installation, the rover will not obtain a fixed solution (Fix) no matter how long it waits. One typical example is an input error in the base station’s coordinates. If the base station’s latitude, longitude, or height are set incorrectly, or if a placeholder coordinate value with too large an error is used, the initial RTK computations will be significantly off. In particular, in cases where the base station coordinates are offset from the actual position by tens of meters (tens of ft) or more, the correction data will be inconsistent with the rover, and integer ambiguity convergence will be extremely slow or impossible. When installing a new base station, always prepare accurate reference coordinates. It is desirable to perform averaged positioning over a sufficient period (for example about 1 hour) to compute high-precision coordinates, or to perform calibration at known points to correct errors. Even when installing on existing public reference points, pay attention to differences between the World Geodetic System and local coordinate systems and ensure the coordinate system settings are consistent. Double-check that the entered values contain no mistakes, and correct them to the proper values as necessary.

It is also important to ensure that the reference station antenna is securely and stably fixed and installed in a safe location where workers will not accidentally touch it. If the base station antenna is placed in an unstable location and moves during the operation, or falls over or tilts due to strong winds, disturbances will occur in the reference station’s own observations. Naturally, the correction information based on that cannot be trusted, and the rover’s solution will become unstable and the Fix will be lost. To prevent such situations, firmly secure the reference station antenna with a tripod or mast, and once installed make sure its position cannot move. If the antenna is accidentally moved during surveying, stop RTK positioning at that point, reset the coordinates of the reference station, and re-run the positioning. It is also desirable to choose a location around the reference station with as good satellite visibility as possible and check that there are no strong reflectors or tall buildings nearby. In addition, don’t forget to carry out hardware checks such as the connection condition of the reference station antenna cable and the power supply.

Note that even when using public reference station networks or external services, you may not be able to obtain a fix depending on the provider’s reception environment or maintenance status. In such cases there is little the user can do, but if possible consider switching to another service or waiting a while and trying to reconnect. Also, when combining your own base station data with external data, be careful about mismatches in coordinate systems (local vs. global).

Checkpoint 4: Keep the distance to the reference station as short as possible

If the distance (baseline length) between the rover and the base station is too large, the RTK solution becomes difficult to fix. The principle of RTK corrections is that the base and rover cancel common error sources, but as the distance between the two points increases, the errors included in the satellite signals received by each station become different, reducing the effectiveness of the corrections. In particular, the effects of the ionosphere and troposphere produce larger differences with distance, and over long distances the correction information from the base station alone cannot fully cancel the rover’s errors. As a result, the integer ambiguity solution becomes unstable, and it often takes a very long time to obtain a Fix solution, or the solution may remain a Float solution.

In general, RTK positioning is more stable and achieves higher accuracy when the baseline length is shorter. As a rule of thumb, it is said that for practical purposes it is desirable that a reference station be within 10 km (6.2 mi). Up to about 10-20 km (6.2-12.4 mi), most of the common errors with the reference station are cancelled out, and accuracy of a few centimeters can be obtained relatively easily. However, as the distance opens to 30 km (18.6 mi), 40 km (24.9 mi), and 50 km (31.1 mi), residual errors that cannot be fully corrected accumulate and solution convergence becomes unstable. For ultra-long baselines exceeding 50 km (31.1 mi), it will likely become difficult to obtain a Fix solution with conventional RTK methods (in such cases, it is necessary to consider using networked RTK or satellite augmentation technologies).

Therefore, you need to take measures to keep the distance to the reference station as short as possible. If you operate your own reference station, plan its location so it is not too far from the rover's work area. If the site spans a wide area, consider switching and repositioning the reference station as needed so it is placed near the center of the site each time. When using public electronic reference stations or private correction services, if multiple reference stations or a VRS option are available, configure the system to use data from the nearest reference station whenever possible. If a long distance of several tens of kilometers (several tens of miles) is unavoidable, using the high-performance multi-frequency receiver described later will increase the likelihood of obtaining a Fix solution. In any case, "shortening the baseline length" is the very foundation of maintaining RTK positioning accuracy.

Checkpoint 5: Standardize GNSS settings between the base station and the rover

If the satellite systems or signal settings used by the reference station and the rover do not match, RTK will not achieve a Fix. RTK processing requires both stations to compute from common satellite observation data, so if only one station is using GLONASS satellites or if one side is observing L1 only while the other is sending L1/L2 data, there will be insufficient shared information and the correct corrections cannot be applied. For example, if the reference station is tracking GPS+GLONASS but the rover is set to GPS only, GLONASS-derived corrections will be ignored by the rover, effectively reducing the number of usable satellites and lowering the stability of the solution. Conversely, even if the rover supports multiple constellations, if the reference station’s data contains only GPS, RTK processing will ultimately be done with GPS alone and, depending on conditions, the solution may remain a Float.

Also, when combining older equipment or devices from different manufacturers, you need to pay attention to the compatibility of correction-data formats. The common RTCM format is used for RTK corrections, but differences in version or message types can mean that the rover may be unable to interpret some correction information. In particular, if the base station outputs observation messages for additional constellations such as GLONASS or Galileo while the rover is a model that does not support them, it will not be able to process them correctly. As a result, you can encounter problems such as never achieving a Fix. In mixed environments, carefully check each other’s supported capabilities and, if there are configuration mismatches, adjust the settings on both sides.

As a countermeasure, when introducing an RTK system it is essential to unify the GNSS settings of the base station and the rover. Enable all satellite systems available to both units (GPS, GLONASS, Galileo, BeiDou, QZSS, etc.) and aim to track as many common satellites as possible. Check each settings screen to make sure you haven’t disabled any system on just one side. Also ensure the frequency bands used by both units match. Dual-frequency (L1/L2) capable units will usually use both frequencies automatically, but if one unit is a single-frequency receiver you must set the other unit to an L1-only correction mode to match (or, ideally, upgrade both to dual-frequency units). Furthermore, if you are receiving corrections via NTRIP or similar, verify the types of RTCM messages being output by the base station software or the distribution service. At a minimum, it is important that the base station coordinate data and observation data for each satellite system are included. If the rover is an older model that does not support newer RTCM formats, some data may be missing and it may fail to obtain a fix. In that case change the message types on the base station to compatible ones or address the issue via a firmware update for the equipment.

If the above settings still don't lead to a Fix, it can also be effective to restart the entire system just in case. This is because, on rare occasions, software glitches can prevent settings from being applied correctly. Restarting all RTK equipment and powering them up again in the order of base station → rover can, in some cases, allow the settings to be applied properly and result in a Fix.

Checkpoint 6: Ensuring Reliable Reception of Correction Data (Establishing a Stable Communication Environment)

If correction information from the reference station does not reach the rover, it will never reach Fix. In RTK you must continuously receive correction data via real-time communication; if that communication is interrupted or misconfigured so that corrections cannot be received, the rover will be unable to escape single solution (Single). Specifically, with the NTRIP method that receives corrections over the Internet, if the mobile network is out of range or the smartphone’s tethering is turned off, corrections will not arrive and it will revert to Float or Single. Also, if even one of the NTRIP connection settings—host name, port number, mount point name, or username/password—is incorrect, the connection will not be established and correction information cannot be received. Similarly, when corrections are transmitted via short-range low-power radio or UHF radio, moving outside the radio coverage or experiencing interference will cause correction data to be lost and the RTK solution cannot be maintained.

Even if the communication link itself remains connected, it becomes difficult to maintain a Fix when correction data is intermittently missing. For example, if the reference station’s data transmission interval is extremely long (about once every 30 seconds) or packet loss on the communication link is frequent, the information the rover needs does not arrive in a timely manner and the computations cannot keep up. As a result, even if a Fix is achieved temporarily, continued data dropouts will cause it to revert to a Float solution. To consistently maintain stable high accuracy, it is essential to establish a communication environment that can receive correction information continuously and in real time.

As a countermeasure, first check the correction data reception status and communication status displayed on the device or app screen. Many RTK-capable systems display indicators such as communication latency with the base station and "Differential Age" (the delay time until corrections are applied). If these are not updating or their values are increasing significantly, communication problems are likely. If you are using NTRIP, check that the rover device (smartphone, tablet, etc.) is reliably connected to the Internet. If signal conditions are poor, take basic steps such as moving a short distance or toggling airplane mode on and off to reset the connection. In mountainous or other areas where cellular signal is unstable, it is advisable to have an alternative SIM card or another carrier's line available so you can switch to whichever has better signal.

Also, review the NTRIP connection settings once more. If even a single character is incorrect in the host (IP address), port number, mount point name, or login information, you will not be able to receive corrections. Carefully check for differences in uppercase/lowercase or any unnecessary spaces, and correct any errors found. If you are using wireless communication, ensure line of sight to the base station antenna and improve the communication environment by installing repeaters or changing channels to avoid interference. Once communication is restored, verify that the correction data reception indicators are updating normally and that the solution returns to Fix.

Checkpoint 7: Utilizing Multi-GNSS and Multi-Frequency

In recent years, advances in GNSS positioning technology have led to an increase in high-performance receivers that can utilize multiple satellite systems (multi-GNSS) and multiple frequency bands. By using such latest equipment, it is possible to greatly shorten the time to obtain an RTK Fix solution and improve positioning stability. As the number of observable satellites increases and the diversity of radio frequency bands expands, RTK engines can resolve unknown integer ambiguities more quickly and reliably. For example, older receivers that only used GPS and a single frequency (L1 band only) tended to take several minutes or more to reach a Fix, and it was often difficult to maintain a Fix over long baselines. However, with the latest multi-GNSS, multi-frequency-capable receivers, it is not uncommon to reach a Fix in as little as a few tens of seconds once conditions are favorable.

To reliably achieve high accuracy with RTK positioning, we recommend choosing a GNSS receiver that supports multi‑GNSS and dual‑frequency or higher whenever possible. If both the base and rover can track multiple satellites, as mentioned in point 5 above, there will be greater redundancy in common observation data, so if some satellites become unusable along the way they can be more easily replaced by others. Also, observing on two or more frequencies significantly improves ionospheric delay correction, making it easier to maintain a Fix solution even for long‑baseline positioning. Utilizing multi‑GNSS and multi‑frequency capabilities is a powerful tool for ensuring accuracy in today’s RTK operations. If you are currently in a mainly single‑frequency configuration and are experiencing long times to acquire a Fix, consider upgrading equipment or reviewing settings.

Easily achieve RTK positioning with LRTK

Finally, to address the RTK operational challenges described above and to provide a method by which anyone can easily perform centimeter-level positioning, we introduce our high-precision RTK system "LRTK". LRTK (pronounced "El-Ar-Tee-Kay") consists of a compact GNSS receiver device (LRTK terminal) that can be attached to an iPhone and a dedicated smartphone app, and was developed with the concept of "making complex RTK surveying easy for anyone to perform." A key feature is that users can obtain highly accurate position coordinates with a one-touch operation without having to worry about complicated settings, so even those using RTK positioning in the field for the first time can handle it without confusion.

For example, you can attach the LRTK terminal to the included lightweight pole (monopod), set it to a height above ground of about 1.5 m (4.9 ft), and complete the recording of measurement points simply by pressing a button on the smartphone app. Because the app also automates the antenna height correction calculation (antenna height being the height at which the GNSS terminal is set from the ground), you no longer need to pay attention to vertical corrections that previously required manual work or separate input. By carrying the pole and repeatedly pressing the button while moving alone, you can measure a large number of points in a short time, dramatically improving surveying efficiency.

Advanced GNSS correction processing is performed in real time within the LRTK system, but users do not need to worry about the communication method or reference station data format themselves. The LRTK terminal, which functions as a mobile station, has a built-in mobile communication module and automatically retrieves correction information for the nearest reference station data and virtual reference stations (VRS) from the Geospatial Information Authority of Japan’s network of electronic reference stations via the Internet. There is no need to set up your own base station on site or manually enter NTRIP information, and the ease of being able to start high-precision positioning immediately with just the LRTK terminal and a smartphone is a major advantage. The device itself is compact enough to fit in the palm of your hand, has a built-in battery, and can be carried easily in the supplied dedicated pouch. Even on cramped sites or tasks that involve a lot of walking, handling the equipment is never a problem.

The positioning accuracy of concern has been achieved at a level comparable to that of conventional large, professional GNSS surveying instruments. Using LRTK, under favorable conditions planar positions can be determined with errors of about ±1–2 cm (±0.4–0.8 in), and heights can be within about ±3 cm (±1.2 in). It also includes a function to observe the same point multiple times and average the results; for example, if the same point is measured 60 times and the mean is calculated, the error can be reduced to less than about 1 cm (less than about 0.4 in), even achieving astonishing accuracy down to a few millimeters (a few hundredths of an inch). Despite such accuracy, operation is as simple as pressing a button on a smartphone.

RTK technology is no longer the preserve of a few specialists; with tools like LRTK, an era is dawning in which everyone on site can benefit. If you are thinking, "I'd like to introduce RTK at my site" or "I want to make surveying and stakeout work more efficient," please consider trying LRTK. Even first-time users can, after a short lecture, quickly master high-precision positioning, and LRTK will transform the way surveying is done on site. LRTK will革新 your site's positioning work and become a powerful partner in delivering smart, fail-safe surveying.