5 Causes Why RTK Remains at Float｜How to Deal When It Doesn't Obtain a Fix

In recent years, RTK (Real Time Kinematic) has attracted wide attention as a GNSS-based method for real-time high-precision positioning. When RTK positioning is functioning properly, the rover receiving correction information from the base station can obtain a Fix solution (fixed solution) that determines the position to within a few centimeters (a few in). However, in the field the problem that "positioning remains as a Float solution (float solution) and does not enter Fix" often occurs. A Float solution is an intermediate stage of RTK in which the integer ambiguities of the satellite signals remain unresolved. At this stage, position errors can reach several tens of centimeters (several dozen in), and the position measurements are unstable and fluctuate as if drifting. In other words, while in a Float solution, RTK’s inherent accuracy cannot be achieved, resulting in significant practical problems.

There are several typical factors that can cause RTK to remain in a Float state and fail to obtain a Fix solution. In this article, we address five representative causes why RTK often does not reach the Fix state in the field and explain each in detail. We also introduce concrete countermeasures to take when you cannot achieve a Fix. We cover common causes such as positioning-environment problems and device configuration mistakes, so those involved in RTK surveying should find this useful. At the end of the article, we also introduce a new high-precision positioning solution, LRTK（エルアールティーケー）, that can resolve these issues all at once.

Table of Contents

• Poor satellite reception environment

• Correction information is not being received

• Errors in reference points or coordinate settings

• GNSS device misconfiguration or malfunctions

• Effects of baseline distance and atmospheric errors

• Recommendation for simplified surveying with LRTK

1. Poor satellite reception environment

In RTK positioning, stable reception of radio signals from multiple GNSS satellites is a prerequisite for obtaining a Fix solution. However, if the satellite reception environment at the observation site is poor, the number of receivable satellites may be insufficient and the quality of positioning data may deteriorate, causing the solution to fail to converge from a Float solution to a Fix solution. For example, in urban areas surrounded by tall buildings or in forests the sky can be obstructed, and the number of usable satellites tends to decrease significantly. Multipath (multiple paths), where satellite signals are reflected by concrete walls, metal surfaces, or water surfaces, is also a serious problem. When errors from reflections are mixed into the observation data, the RTK engine cannot correctly resolve integer ambiguities, making it difficult to obtain a Fix solution.

Furthermore, caution is required when there are strong sources of radio-frequency noise nearby. Directly beneath high-voltage transmission lines or close to radar facilities, GNSS signals are more likely to pick up interfering noise, which can cause positioning accuracy to degrade significantly and in some cases not improve from a float solution. It is also known that during severe weather such as heavy rain or thunderstorms, ionospheric disturbances and radio signal attenuation increase, making RTK unstable. Thus, when the satellite reception environment is under poor conditions, there is a higher risk that RTK will not reach a fixed solution and will remain at a float solution.

Countermeasures: When performing positioning, first choose a location with as wide an open sky as possible and ensure a clear view of the sky overhead. Install the base station antenna at a high location with a 360° unobstructed view, and on the rover side remove any obstructing objects near the survey point beforehand if possible. It is also prudent to avoid observations in environments where reflective objects such as the walls of large buildings or vehicles are nearby. If that is difficult, attaching a metal ground plane to the antenna to reduce ground reflections or using a high-performance choke ring antenna to suppress the effects of multipath signals is also effective.

Furthermore, recently receivers that support multi-GNSS (multiple satellite systems) and dual-frequency (multiple frequency bands) have become widespread. By utilizing these high-performance GNSS devices, you can increase the number of satellites available even at sites with somewhat poor visibility and more effectively cancel ionospheric errors, so an improved convergence rate from Float to Fix can be expected. If RTK still does not stabilize due to environmental factors, consider not forcing it and instead surveying during times with favorable satellite geometry (confirm times with good DOP values in advance using GNSS visible-satellite prediction services, etc.). If conditions cannot be met no matter what, it is also important to be flexible and complement RTK with alternative positioning methods such as total stations rather than insisting on RTK.

2. Correction information is not being received

With RTK positioning, centimeter-level accuracy is achieved only when error correction data transmitted from a base station (reference point) is continuously received in real time. Therefore, in situations where communication of the correction information is interrupted, no matter how long you wait you cannot obtain a fixed (Fix) solution. While correction data is not being received, RTK does not function, the positioning solution reverts to standalone (single) positioning or code differential (DGPS) level accuracy, and the display remains a float solution.

Possible causes for losing reception of correction information include problems with the communication environment or equipment settings. In RTK using a private radio, if the radio settings on the base station and the rover (transmission frequency or group ID) do not match, or if there are obstructions between the base station and the mobile station that block the radio signal, the correction data will be interrupted. When using specified low-power radios or UHF radios, the effective radio range is limited to several kilometers (several thousand ft), so on wide-area sites the rover can end up outside the communication coverage. Also, in radio-shielded environments such as inside tunnels or in the shadow of buildings, the base station’s signal cannot reach, causing communications to be cut and corrections to stop. On the other hand, for network RTK that distributes corrections via the Internet (such as the NTRIP method), it is also possible that the survey site is outside mobile phone coverage so mobile communication is unstable, or that a failure occurs on the base station service side and data distribution stops.

Countermeasure: First, thoroughly check the communication settings between the base station and the rover in advance. When using radio, inspect at both stations that transmit and receive frequencies and channels match correctly, and check that antenna connections are not loose or broken. Mount the base station antenna as high as possible to secure line-of-sight distance, and, if necessary, install relay antennas to expand the coverage area. When using a network connection, prepare SIM cards from multiple mobile carriers on site and select the line with the best signal, or use external high-gain antennas or a mobile Wi-Fi router to improve reception. Also note that during long surveys communications are often interrupted by battery depletion of both the base station and the rover. Fully charge both devices beforehand and regularly check battery levels during surveying. Always monitor the reception status of correction data on the rover’s screen, and check indicators such as the Age of Differential (delay time of the differential data) and the number of received messages for reassurance. If data loss occurs, try reconfiguring the connection or reset the base station/receiver and reconnect to attempt recovery.

3. Errors in reference points and coordinate settings

RTK surveying requires, as a basic prerequisite, entering the exact coordinate values of the reference points to be used into the equipment and correctly specifying the geodetic datum and coordinate system. If these are mistaken, even if RTK has a Fix the calculated coordinate values will not match reality and large position errors will occur. For example, if the known-point coordinates entered into the base station are mistyped by a single digit, or if the zone number (system number) is selected incorrectly in Japan’s plane rectangular coordinate system, the positioning results can be off from the actual position by several tens of meters (several tens of ft). In fact, on-site you hear failure stories such as “I mistyped a reference-point value by one digit and later went pale when the results didn’t match.” Errors in coordinate settings are by no means a laughing matter.

Also, special care is required at sites that use geodetic or surveying coordinate systems different from those in Japan. Japan’s current official geodetic system is the World Geodetic System (JGD2011 and JGD2022), but when using known points whose coordinates are given in the older Tokyo geodetic system or when surveying in a proprietary local coordinate system, the resulting coordinates will not match the site reference values unless an appropriate coordinate transformation is applied. In the vertical direction as well, if the geoid height correction is omitted, the obtained heights can differ significantly from the site’s leveling datum.

Precautions: Before starting surveying, always confirm the coordinate values of the reference points to be used and the adopted geodetic datum and coordinate system. Refer to the contract drawings and known-point tables, and set the correct reference system (e.g., a specific system of JGD2011 or the coordinates for a particular prefecture/zone) on the equipment. When setting up a base station, it is important to double-check that the entered known-point coordinates are correct. If possible, observe one on-site known point before work begins to verify that the coordinates obtained match the equipment settings. If measuring in a local, proprietary coordinate system, you need to perform site calibration (on-site adjustment) using multiple known points to align GNSS positioning coordinates with the local reference. If you realize there was a settings mistake, it may be possible to recover by applying the correct parameters to the recorded data afterward and performing a coordinate transformation. However, the transformation work requires effort and advanced knowledge. Therefore, it is essential to carefully verify settings from the outset and prevent mistakes.

4. GNSS equipment misconfiguration or malfunction

System or equipment misconfiguration or malfunction can also cause RTK not to achieve a Fix. A common example is incorrect mode settings for the base station and the rover. If a receiver that should be operating as the base station is kept in rover mode, or conversely if a rover (mobile station) is set to fixed mode (base-station mode), correction data will not be generated or applied correctly and positioning will not be established. Also, when using network RTK services, selecting the wrong mount point (virtual reference point data) distributed by the provider can cause reception of an incompatible data format and prevent obtaining a solution. For example, be careful of configuration mistakes such as specifying correction data for multi-frequency when the receiver is single-frequency.

Do not overlook GNSS antenna or cable faults as device-specific troubles. If an antenna cable connector is loose or a long cable is about to fail, you may not be able to receive satellite signals or correction data properly in the first place, and RTK will not achieve a Fix. Also, when controlling positioning on site using software on a tablet or notebook PC, be careful about incorrect app settings and freezes. If settings such as the output coordinate system are wrong, as mentioned above, they can cause a shift in results, and the app may be forcibly closed so that correction reception stops. In particular, with GNSS receivers that are used connected to a smartphone or tablet via Bluetooth, there have been cases where the app stopped in the background and the solution had unknowingly reverted to a Float.

Measures: To prevent equipment troubles, thorough pre-checks before entering the site are essential. Check the connection status of the GNSS receiver unit, antenna, and radio equipment, and perform operational tests including whether the power turns on and whether the battery level is sufficient. When working at a new site or with newly introduced equipment, use simulations to verify that positioning software and app settings (geodetic datum and projected coordinate system, antenna height, communication ports, etc.) are correctly configured. Update receiver firmware and applications provided by the manufacturer to the latest stable versions, and check bug-fix information in advance for peace of mind. If you notice any abnormal equipment behavior during positioning, calmly suspend work, reboot the equipment, or verify operation at another known point to isolate the cause as early as possible. Also, as basic measures, be sure to confirm with the surveying pole's bubble vial that the antenna is vertical and avoid entering the wrong antenna height; enforce practices to prevent human error.

Note that if you begin moving while the solution is still a Float immediately after RTK positioning starts, you may find it difficult to transition to a Fix solution afterward. To obtain a stable fixed solution, remain stationary for several tens of seconds after power-on or reboot, and be sure to acquire the first Fix before moving the survey point or conducting observations. Once you have a Fix, you can maintain centimeter-level accuracy (half-inch accuracy) while moving, provided satellite reception does not deteriorate dramatically; however, if you lose the Fix, it is safer to stop again and wait for the solution to reconverge.

5. Effects of Baseline Distance and Atmospheric Errors

As a constraint specific to RTK positioning, you must also consider the distance between the base station and the rover (baseline length) and atmospheric errors such as those caused by the ionosphere and troposphere. In general, as the baseline distance increases, the error sources in the satellite signals received at the two stations differ more significantly, increasing residuals that cannot be canceled by corrections, so obtaining a fixed solution becomes more difficult. When a high-precision GNSS receiver's specifications list "horizontal accuracy: 8 mm (0.31 in) ± 1 ppm", this means an error increase of approximately 1 mm (0.04 in) per 1 km (3280.8 ft). Therefore, if the rover is tens of kilometers (tens of thousands of ft) away from the base station, the required observational precision to resolve integer ambiguities cannot be ensured, and the solution becomes unstable. In particular, receivers that support only single-frequency L1 cannot sufficiently cancel ionospheric errors, so on sites with long baselines it can take a very long time to go from a Float solution to a Fix solution, or in the worst case it may never fix.

Also, days when ionospheric disturbances caused by solar flares occur, and time periods with poor satellite geometry, are factors that can lead to a decrease in RTK fix rate. Even at a flat site with no surrounding obstructions, if the configuration of satellites visible overhead happens to be biased and the PDOP value is high, RTK initialization may take longer than usual. Conversely, if a multi‑GNSS capable receiver can consistently receive 10 or more satellites simultaneously, the likelihood of obtaining a stable fixed solution increases even if some atmospheric errors remain.

Countermeasure: It is essential to keep the distance between the base station and the rover as short as possible. If you can install your own base station, place it near the work area; if that is difficult, make use of the Geospatial Information Authority of Japan’s network of Continuously Operating Reference Stations or network reference station services such as VRS to effectively shorten the baseline length. Also, before starting surveying, check the active GNSS satellites and their geometry, and plan to avoid time periods with few satellites and poor geometry. For example, the Geospatial Information Authority of Japan’s "GNSS Satellite Visibility Prediction Service" lets you check in advance the number of satellites and DOP values for any location and time. Use these tools to perform RTK positioning at the most favorable times possible. Furthermore, on days of high solar activity when ionospheric disturbances are large, it may sometimes be necessary to refrain from real-time RTK positioning. Instead of forcing continuation, consider recording only the observation data on site and conducting precise processing later using methods such as PPK (Post-Processing Kinematic) or PPP (Precise Point Positioning).

Guide to Simple Surveying with LRTK

So far, we have reviewed the typical causes that can leave RTK stuck in a float state and the countermeasures for them. To reliably achieve high-precision RTK positioning, there are many on-site points to watch—from securing a good satellite visibility environment to checking the finer settings of the equipment—and it requires expertise and experience. A solution that has attracted attention for solving these challenges all at once and enabling anyone to easily achieve centimeter-level positioning is our LRTK (エルアールティーケー).

LRTK is a new high-precision positioning system that combines a smartphone with a dedicated compact GNSS device, developed with the concept of "making difficult RTK surveying easy for anyone." It uses a receiver terminal that can be attached to smartphones such as the iPhone and leverages multiple satellite positioning technologies and cloud services to eliminate the complexities that have traditionally accompanied RTK operations. For example, tasks that used to be necessary—such as installing base stations, securing communication lines, and adjusting coordinate systems—are handled automatically with LRTK; on site you simply set up the terminal and press a button on the smartphone app, and high-precision positioning on the order of millimeters (mm; ≈0.04 in) begins automatically. In actual operation it also utilizes the sub-meter-class augmentation signals provided by Japan’s Quasi-Zenith Satellite System, Michibiki (QZSS), enabling stable positioning even in mountainous areas where mobile phone coverage is unavailable.

High accuracy and stability comparable to conventional professional equipment are also major features of LRTK. Under favorable conditions it can achieve an accuracy of about ±1–2 cm (±0.4–0.8 in) in horizontal position and approximately ±3 cm (±1.2 in) in height, enabling highly repeatable positioning even when working alone with a monopod. Furthermore, by observing the same point multiple times and taking an average, errors can be reduced to a few millimeters (a few tenths of an inch) if measurements are repeated over a short period and averaged. Despite this level of accuracy, operation is extremely simple—anyone can use it by simply tapping the positioning start button on the dedicated app screen. The acquired positioning data are automatically saved to the cloud, so you can be reassured that the results will not be lost even if the device experiences trouble on site.

By introducing LRTK in this way, you can be freed from common RTK surveying problems such as "never getting a Fix" and "settings are difficult and accuracy cannot be achieved." LRTK lowers the barriers to RTK surveying with cutting-edge technology and dramatically improves on-site efficiency and accuracy, and it is expected to be used across a wide range of fields, from construction and civil engineering to surveying and inspection work. If you are interested, please check LRTK's official information. We also accept requests for materials and inquiries about the product, so we would appreciate your consideration of LRTK as a next-generation high-precision positioning tool.