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

In high-precision positioning using the RTK (Real-Time Kinematic) method, there are two positioning result statuses called "Float solution" and "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, high-precision solution obtained after the RTK calculation is completed, whereas the Float solution (floating solution) is an indeterminate state during the calculation, with accuracy remaining on the order of tens of centimeters. When operating RTK positioning in the field, it is important to take this difference between Float and Fix into account and prevent failures caused by incorrect judgments.

In this article, we first explain the meanings and differences between Float and Fix solutions in RTK positioning. We then introduce seven checkpoints to keep in mind to ensure positioning accuracy on site and to avoid situations such as "unable to obtain a Fix solution" or "large errors while still in a Float solution." Aimed at those who use RTK in surveying and construction sites as part of their work, we provide a clear explanation that includes concrete countermeasures.

Table of Contents

• Difference between Float and Fix solutions in RTK positioning

• Checkpoint 1: Verify positioning status (Float/Fix)

• Checkpoint 2: Ensure satellite visibility (optimize reception environment)

• Checkpoint 3: Set reference station coordinates and secure stable installation

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

• Checkpoint 5: Unify GNSS settings between the reference and rover stations

• Checkpoint 6: Ensure reliable reception of correction data (improve communication environment)

• Checkpoint 7: Use multi-GNSS and multi-frequency

• Easily achieve RTK positioning with LRTK

Differences between Float and Fix solutions in RTK positioning

In RTK positioning, after the rover receives correction information from the base station and performs calculations, the positioning solution is displayed as either "Fix" or "Float". A Fix solution (fixed solution) means that the unknown integers contained in the carrier-phase of the satellite signals (integer ambiguities) have been correctly resolved in the RTK computation. In this case the inherent accuracy of RTK is realized, allowing very high-precision positioning with horizontal errors within several centimeters (several in) and vertical errors on the order of 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 positioning solution in which correction data is being applied but the integer part has not yet been fixed. Its accuracy is much lower than that of a Fix solution, generally remaining in an error range of approximately ±0.5–1 m (±1.6–3.3 ft). Float solutions are likely shortly after starting RTK positioning or when satellite signal reception is unstable, and as computations progress and conditions improve they will eventually converge to a Fix solution.

Note that when no corrections are applied (standalone positioning) the display shows "Single" (single solution), and errors are several meters or more (several ft or more), much larger than when using RTK.

Because there is a large difference in positioning accuracy between Float solutions and Fix solutions, the reliability of survey results obtainable in the field also differs greatly. For example, in tasks that require centimeter-level accuracy (half-inch accuracy), such as establishing land boundaries or precisely laying out the positions of structures, using positioning results obtained while still in the Float solution can produce position errors of tens of centimeters (tens of inches) and could lead to serious mistakes. Therefore, in RTK surveying it is extremely important to confirm that the solution is in the Fix state before adopting the results. Only once a Fix solution has been obtained can RTK’s centimeter-level precision (half-inch accuracy) be guaranteed, and the positioning results be trusted with confidence. Conversely, while in the Float solution the accuracy is still unstable and preliminary, so you should not hastily use the results but wait for the solution to converge to Fix. Even after obtaining a Fix, reception conditions may subsequently deteriorate and the solution revert to Float. In such cases, calmly check the cause and resume work only after confirming the solution has returned to Fix. Distinguishing between the Fixed and Float states and responding appropriately can be said to be the basic principle of handling RTK positioning.

Checkpoint 1: Confirming positioning status (Float/Fix)

First and foremost, make it a strict habit to check the positioning status displayed on the screen of your GNSS receiver or surveying app. On many RTK-capable devices, the current solution status is shown as terms such as "FIX", "FLOAT", or "Single (single)". At the work site, always monitor this status display and be sure it reads "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 common mistakes where coordinates are accidentally recorded while in a Float solution and the large error is discovered later. To prevent such errors, always check the status while positioning and proceed to the next task only after confirming that a Fix solution has been obtained.

If, for some reason, it does not switch to a Fix state, or if it becomes Fix but then falls back to Float, you should calmly investigate the cause and take corrective action. Verify, in order, the factors described from point 2 onward below, and try to bring the solution back to the Fix state by eliminating the problems. In RTK positioning, correctly understanding the solution status is the first step in quality control.

Checkpoint 2: Ensuring Satellite Visibility (Optimizing Reception Conditions)

To obtain a stable Fix solution in RTK positioning, it is important to keep the GNSS satellite signal reception environment as favorable 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 error), which can make the positioning solution unstable. In such degraded environments, even if correction data is received, the necessary observation information is lacking and the RTK engine cannot resolve the integer ambiguities. As a result, you may remain at a Float solution indefinitely and never reach a Fix, or in the worst case the corrections may not work at all and you may revert to a Single solution.

As a countermeasure, on site we make an effort to install and use the GNSS antenna in locations with as open a sky as possible. In urban areas surrounded by tall buildings or in forests, moving temporarily to a spot with an open view of the sky and retrying positioning can, in some cases, greatly speed 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 reflection sources. For example, placing the antenna directly on the ground or on a car hood tends to degrade reception, so it is preferable to secure it in a stable location using a dedicated pole or a magnetic mount. Also, it is effective to choose observation times carefully, avoiding periods when satellite visibility temporarily worsens (such as when satellite geometry is poor). Furthermore, if strong radio communication equipment or high-voltage power lines are operating nearby, their radio interference may adversely affect GNSS signals. If an interference source is suspected, consider measuring at times when that equipment is not in use if possible, or take measures such as applying noise mitigation to the antenna.

Verification Point 3: Reference Station Coordinate Setting and Stable Installation

RTK positioning uses the precise position information of a reference station (base station) for a mobile station (rover) to apply corrections. Therefore, if there is a problem with the reference station’s configuration or installation, the rover will not obtain a fixed solution no matter how long it waits. One common example is an input error in the reference station’s coordinates. If the reference station’s latitude, longitude, or height is set incorrectly, or if a provisionally assigned coordinate has an error that is too large, the initial RTK calculations will be significantly off. In particular, in cases where the reference station coordinates are offset by tens of meters or more (tens of meters or more in ft), the correction data will be inconsistent with the rover, and integer ambiguity convergence will be extremely slow or impossible. When newly installing a reference station, always prepare an accurate reference coordinate. It is desirable to calculate a high-precision coordinate by performing averaged positioning over a sufficient period (for example, about 1 hour), or to perform calibration at known points to correct errors. Even when installing on existing public control points, pay attention to differences between the global geodetic system and local coordinate systems and ensure the coordinate system settings are unified. Double-check the entered values for mistakes and correct them to the proper values as necessary.

It is also important to securely and stably fix the reference station antenna and install it 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 operations, or is blown over or tilted by strong winds, the reference station’s own observations will be disturbed. Naturally, the correction information based on those observations will no longer be reliable, and the rover’s solution will become unstable and the Fix will be lost. To prevent such situations, make sure the reference station antenna is firmly fixed with a tripod or mast, and once installed ensure its position cannot move. If the antenna is accidentally moved during surveying, RTK positioning should be stopped at that point, the reference station coordinates must be re-established, and positioning must be redone. It is also desirable to choose surroundings for the reference station that provide as good a satellite view as possible, and to check for nearby strong reflectors or tall buildings. In addition, don’t forget to inspect hardware aspects such as the reference station antenna cable connections and power supply.

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

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

If the distance between the rover and the base station (baseline length) is too great, it becomes difficult for RTK solutions to fix. The principle of RTK corrections is that the base station and the rover cancel out common error sources, but as the distance between the two points increases the errors contained in the satellite signals received by each diverge, reducing the effectiveness of the corrections. In particular, the effects of the ionosphere and the troposphere produce increasingly large differences with distance, and over long baselines the base station’s correction information alone cannot fully cancel the rover’s errors. As a result, integer ambiguity resolution becomes unstable, and it can take a very long time to obtain a Fix solution, or the solution may remain a Float solution indefinitely.

In general, RTK positioning becomes more stable and more accurate as the baseline length becomes shorter. As a guideline, it is said that in practice it is desirable for the reference station to 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 canceled out, and centimeter-level accuracy can be obtained relatively easily. However, as the distance widens to 30 km (18.6 mi), 40 km (24.9 mi), 50 km (31.1 mi), residual errors that cannot be fully corrected accumulate and the solution convergence becomes unstable. For ultra-long baselines exceeding 50 km (31.1 mi), obtaining a Fix solution with the normal RTK method itself becomes difficult (in such cases, consideration should be given to using network 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 installation so that it is not too far from the rover’s work area. If the site covers a wide area, consider relocating the reference station as needed and repositioning it near the center of the site. Even when using public electronic reference stations or commercial correction services, if multiple reference stations or a VRS method are available, try configuring the system to use data from the nearest reference station whenever possible. If you inevitably face long distances on the order of tens of km (tens of mi), using the high-performance multi-frequency receivers described later can increase the likelihood of obtaining a Fix solution. In any case, “shortening the baseline length” is the fundamental principle for maintaining RTK positioning accuracy.

Checkpoint 5: Standardize GNSS settings between base and rover stations

If the satellite systems or signal settings used by the base 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 one station is observing L1 only while the other is sending L1/L2 data, there will be insufficient shared information and corrections cannot be applied correctly. For example, if the base station is tracking GPS+GLONASS but the rover is configured for GPS only, GLONASS-derived correction information will be ignored by the rover, effectively reducing the number of usable satellites and lowering solution stability. Conversely, even if the rover supports multiple satellite systems, if the base station data contain only GPS, RTK processing will ultimately be GPS-only and, depending on conditions, may remain at a float solution.

Also, when combining older equipment or devices from different manufacturers, you need to pay attention to correction-data format compatibility. RTK corrections use the common RTCM format, but differences in versions or message types can cause the rover to be unable to interpret some correction information. In particular, if the base station outputs observation messages for additional constellations such as GLONASS or Galileo but the rover model does not support them, it will be unable to process those messages correctly. As a result, you may encounter a situation where a fix never becomes established. In mixed environments, carefully check each other’s compatibility, 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 and rover stations. Enable all satellite systems available to both stations (GPS, GLONASS, Galileo, BeiDou, QZSS, etc.) and aim to track as many common satellites as possible. Check each station’s settings screen to ensure no satellite system is disabled on one side only. Also ensure the frequency bands used by both stations match. If both units support dual-frequency (L1/L2) they will normally use both frequencies automatically, but if one side is a single-frequency receiver, the other side must be set to an L1-only correction mode to match it (ideally, upgrade both to dual-frequency units). Furthermore, if you are receiving corrections via NTRIP connection or similar, verify the types of RTCM messages output by the base station software or distribution service. At minimum, it is important that 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 prevent obtaining a fix. In that case, change the base station to compatible message types or address the issue via a firmware update of the equipment.

If, after reviewing the above settings, you still do not achieve a Fix, it can be effective to reboot the entire system just in case. This is because, on rare occasions, software issues can prevent settings from being applied correctly. Restart all RTK equipment once and power them up again in the order base station → rover; in some cases the settings will be applied correctly and a Fix will be achieved.

Checkpoint 6: Ensuring Reliable Reception of Correction Data (Improving the Communication Environment)

If correction information from the base station does not reach the rover, it will never achieve a Fix. In RTK the rover must continuously receive correction data via real-time communications, but if that communication is interrupted or reception fails due to a configuration error, the rover cannot escape standalone positioning (Single solution). Specifically, with the NTRIP method that receives corrections via the Internet, if the mobile network is out of coverage or the smartphone’s tethering is turned off, corrections will not arrive and the solution will revert to Float or Single. Also, if any 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 corrections cannot be received. Similarly, when corrections are transmitted via low-power short-range radio or UHF radio, moving out of the radio’s coverage area or the occurrence of interference will cause correction data to be lost and the RTK solution cannot be maintained.

Furthermore, even if the communication link itself is connected, it becomes difficult to maintain a Fix state 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 line is frequent, the mobile station will not receive the required information in a timely manner and the computations cannot keep up. As a result, even if a Fix is achieved temporarily, continued data loss 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 in real time.

As a countermeasure, first check the correction-data reception status and communication status displayed on your device or app screen. Many RTK-capable systems show indicators such as communication latency with the base station and "Differential Age" (the delay until corrections are applied). If these are not being updated or the values have increased significantly, a communication problem is suspected.

If you are using NTRIP, check that the rover-side device (smartphone, tablet, etc.) is reliably connected to the Internet. If the signal is poor, perform basic actions such as moving a short distance or toggling airplane mode on and off to reset the connection. In mountainous or other areas with unstable mobile signals, it is advisable to have an alternative SIM card or another carrier’s line available so you can switch to the one with better reception.

Also, review the various 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 you find. If you are using wireless communication, ensure line of sight to the base station antenna and improve the communication environment by measures such as installing repeaters or changing channels to avoid interference. When communication is restored, check that the correction data reception indicators are updating normally and confirm whether 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. Using these latest devices can greatly shorten the time to obtain an RTK fix and improve positioning stability. With more observable satellites and a greater variety of signal frequency bands, RTK engines can resolve unknown integer ambiguities more quickly and reliably. For example, older receivers that used GPS only and a single frequency (L1 only) tended to take several minutes or more to reach a fix and had difficulty maintaining a fix over long distances. 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 at least dual-frequency whenever possible. If both the base and rover can track multiple satellites, as mentioned in point 5 above, there will be redundancy in the common observation data, making it easier to compensate with other satellites if some become unusable along the way. Also, if observations can be made on two or more frequencies, the ionospheric delay correction effect is greatly enhanced, making it easier to maintain a Fix solution even for long-distance positioning. Utilizing multi-GNSS and multi-frequency capabilities can be considered a powerful tool for ensuring accuracy in today’s RTK operations. If you are currently using a primarily single-frequency setup and it takes a long time to obtain a Fix, consider upgrading your equipment or reviewing your settings.

Easily achieve RTK positioning with LRTK

Finally, to solve the RTK operational issues described above and provide a simple way for anyone to achieve centimeter-level positioning, we introduce our high-precision RTK system “LRTK.” LRTK (El-Are-Tee-Kay) consists of a compact GNSS receiver device (LRTK unit) that can be attached to an iPhone and a dedicated smartphone app, and was developed with the concept of “making difficult RTK surveying easy for anyone to perform.” Its key feature is that users can obtain high-precision position coordinates with one-touch operation without needing to worry about complex settings, so even those using RTK positioning on site for the first time can use it without confusion.

For example, attach the LRTK unit to the included lightweight pole (monopod), set it to a ground height of about 1.5 m (4.9 ft), and simply press the button on the smartphone app to complete the recording of the survey point. Because antenna height (the height of the GNSS receiver above the ground) correction calculations are automated in the app, you no longer need to worry about vertical corrections that previously required manual work or separate input. By carrying the pole and walking while repeatedly operating the button alone, you can observe many points in a short time, greatly improving surveying efficiency.

The LRTK system performs advanced GNSS correction processing in real time, but users do not need to worry about communication methods or reference station data formats. The LRTK terminal, which functions as a rover, has a built-in mobile communication module and automatically acquires correction information for the nearest reference station and virtual reference stations (VRS) from the Geospatial Information Authority of Japan’s network of electronic reference points via the Internet. There is no need to set up your own base station on site or manually enter NTRIP information, and the convenience 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, includes an internal battery, and can be easily carried in the supplied dedicated pouch. Even in cramped sites or tasks that involve a lot of walking, you will have no trouble handling the equipment.

Even the positioning accuracy that people worry about reaches a level comparable to conventional large professional GNSS surveying instruments. Using LRTK, under good conditions horizontal positions can be accurate to about ±1–2 cm (±0.4–0.8 in), and vertical errors can be contained to 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 average is calculated, the error can be reduced to below about 1 cm (below about 0.4 in), even down to a few millimeters (a few hundredths of an inch), achieving astonishing precision. Despite that level of accuracy, operation is as simple as pressing a button on a smartphone.

RTK technology is no longer just for a handful of specialists; tools like LRTK are ushering in an era where everyone on site can benefit. If you’re thinking, "I’d like to introduce RTK at my site" or "I want to make surveying and layout work more efficient," please consider using LRTK. Even first-time users can master high-precision positioning after a short training session, and LRTK will transform on-site surveying practices. LRTK will revolutionize positioning work at your site and become a powerful partner that enables error-free, smart surveying.