Why RTK Sometimes Shifts: Real Causes and How to Prevent It

Table of Contents

• Cause 1: Incorrect base station coordinates or geodetic datum settings

• Cause 2: Corrections not applied, resulting in Float/Single solutions

• Cause 3: Degraded satellite environment or multipath-induced positioning errors

• Cause 4: Long baseline distance / atmospheric effects reducing accuracy

• Cause 5: Equipment handling mistakes and vertical error factors

• What is simplified surveying with LRTK

• FAQ

RTK (Real Time Kinematic) positioning is a technique that achieves much higher accuracy (centimeter-level) than conventional GPS by applying GNSS satellite correction data in real time. In recent years, the use of RTK surveying has rapidly spread in civil engineering and construction sites, becoming an important means to streamline tasks such as setting batter boards and quality control of as-built conditions. However, even with RTK’s high accuracy, you may sometimes face the problem that the reported position “shifts occasionally.” Surveyors on site sometimes report that “the RTK-measured position did not match a known point” or “the elevation was off by several centimeters,” causing trouble.

Why do such shifts occur with RTK positioning? In this article, we explain the main causes of occasional RTK shifts and the countermeasures to prevent them. By correctly understanding the true causes of errors and taking appropriate measures, you can perform RTK surveying more stably. At the end of the article, we also introduce simplified surveying using LRTK as a way to address these issues at their root and easily achieve high-precision positioning.

Cause 1: Incorrect base station coordinates or geodetic datum settings

In RTK surveying, it is fundamental to set accurate known coordinates on the base station. However, mistakes in the base station coordinate settings often cause shifts. For example, if you mistype a digit when entering the coordinates of a control point or set the wrong coordinate system or datum, the results can be significantly displaced from the true positions. When using Japan’s plane rectangular coordinate system, selecting the wrong zone number can lead to a major error where the entire positioning result shifts by tens of meters. When conducting RTK surveys overseas, differences in datums can cause a mismatch in coordinate references. If you perform positioning without matching the datum, even if RTK yields centimeter-level relative accuracy, the coordinates may not align with maps or drawings by several meters.

Attention must also be paid to differences in height reference. The height calculated by RTK receivers is usually the ellipsoidal height (height above the GPS reference ellipsoid), while the elevations used in civil engineering are orthometric heights (heights above mean sea level), i.e., based on the geoid. The difference between the two can be tens of meters depending on the region; within Japan, ellipsoidal heights are typically about +35–40 m (+114.8–131.2 ft) higher than orthometric heights. Therefore, if you compare RTK-derived heights directly with field elevations, you may find them “more off than expected.”

Countermeasures:

• Thoroughly verify the coordinate values and datum settings to be used on the base station in advance, and take care to avoid input errors or incorrect settings.

• When using known points’ coordinates, unify the datum (e.g., JGD2011 or an older datum) and perform coordinate transformation or localization as needed.

• For heights, apply geoid model corrections to convert ellipsoidal height to orthometric height to align with the reference datum.

• After positioning, compare RTK-derived coordinates with known control points and make it a habit to check for discrepancies.

• To prevent human errors during coordinate setup, use work procedures and checklists and perform mutual verification by multiple people.

Cause 2: Corrections not applied, resulting in Float/Single solutions

To obtain high-precision RTK positioning, the rover must continuously receive correction data from the base station. However, if communication failures or configuration errors prevent correction data from reaching the rover, RTK corrections will not take effect. As a result, you may measure while the solution status remains as Float or Single, causing large position errors. A Float solution means the rover is receiving corrections but integer ambiguities have not yet been resolved; accuracy is typically limited to about ±0.5–1 m (±1.6–3.3 ft). A Single solution (standalone GNSS without DGPS) can incur errors of several meters or more. If you overlook the receiver display on site and continue working while assuming “it must be fine” even though the solution is Float, you run a high risk of rework or returning to redo measurements later.

Countermeasures:

• While surveying, always check the receiver’s status display and confirm it is FIX before recording or staking points.

• If the display shows Float or Single, calmly wait for FIX or try adjusting the antenna placement or the environment to improve conditions.

• Check communication settings such as radio or NTRIP connections and whether the correction service subscription has expired. Monitor communications at both base and rover; if correction data is not arriving, quickly identify and fix the cause.

• To avoid the situation where the solution “quietly drops to Float,” make a habit of frequently checking the status screen so you do not overlook anomalies.

Cause 3: Degraded satellite environment or multipath-induced positioning errors

RTK positioning uses signals from GNSS satellites, so if the satellite reception environment is poor, accuracy will be affected. Tall buildings or forests nearby can block the sky and reduce the number of satellites tracked, degrading the positioning geometry. Reflections from concrete walls or metal surfaces cause multipath, introducing bias and noise into measurements. For example, in urban canyons, under elevated structures, or beneath trees, even if correction data is being received, satellite signals themselves can be unstable, causing RTK solutions to struggle to achieve FIX or to produce unstable accuracy.

Countermeasures:

• Place the antenna where the sky is widely open to secure satellite visibility. Mount base station antennas on rooftops or poles in high positions; ideally, there should be no obstructions within 360°.

• When observing with a rover, choose locations with open sky as much as possible. Under trees or in building shadows where few satellites are visible, accuracy drops significantly.

• Avoid measuring near strong reflectors such as metal structures, large vehicles, or mirror-like water surfaces. If unavoidable, remove or move those reflectors before measurement, or attach a ground plane (reflector shield) to the antenna to mitigate multipath.

• Use a GNSS receiver that supports multi-GNSS (GPS, GLONASS, Galileo, QZSS, etc.). Tracking multiple constellations helps maintain accuracy even in environments where satellite numbers are limited.

Cause 4: Long baseline distance / atmospheric effects reducing accuracy

In RTK, accuracy tends to degrade as the baseline distance between base and rover increases. This is because atmospheric errors (ionospheric and tropospheric delays) that cannot be fully canceled by relative positioning increase with distance. Even high-performance RTK-GNSS equipment typically has specifications around ±(8 mm + 1 ppm). Here, 8 mm (0.31 in) represents a fixed component and 1 ppm represents about 1 mm (0.04 in) per 1 km (0.62 mi) of baseline. Thus, theoretically, being 10 km (6.2 mi) from the base station yields about an additional 1 cm (0.4 in) of error, and 20 km (12.4 mi) yields about 2 cm (0.8 in). Tropospheric differences in particular tend to produce vertical bias errors, so vertical accuracy is known to deteriorate more easily in long-range RTK.

Countermeasures:

• Keep the distance between base and rover as short as possible. If you set up your own base, place it near the center of the site.

• If shortening the baseline is difficult, consider using network RTK services such as the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations or VRS to effectively reduce baseline length.

• Use dual-frequency receivers (L1/L2, etc.). Dual-frequency observations reduce ionospheric error effects and make it easier to obtain FIX even over longer distances.

• If accuracy is unstable due to baseline length despite sufficient satellite numbers, try restarting the receiver or switching to supplementary measures (e.g., using CLAS from QZSS).

Cause 5: Equipment handling mistakes and vertical error factors

Although RTK surveying uses advanced equipment for precise positioning, human errors or small operational mistakes can cause unexpected shifts. For example, even if you think the rover pole (staff) is vertical, a slight tilt will offset the measured point by the corresponding amount. A 2 m (6.6 ft) pole tilted by 1° results in a horizontal displacement of about 3 cm (1.2 in). Ideally, the pole should be precisely vertical using a bubble level, but achieving perfect verticality on site can be difficult. Similarly, errors in entering the antenna height (distance from pole tip to antenna reference point) will produce errors in the height component of the positioning result. Also, poor calibration of the receiver or IMU sensors can prevent tilt compensation functions from working correctly and become sources of error.

In addition, note that RTK generally has poorer vertical (height) accuracy than horizontal accuracy. Because the geometry for height is inherently weaker—satellites are mainly above the receiver—small error factors can produce vertical shifts on the order of several centimeters. In fact, the Geospatial Information Authority of Japan’s public surveying manual specifies RTK permissible errors as “within 15 mm horizontally and within 50 mm vertically,” indicating a looser standard for height than for horizontal. In other words, vertical shifts of a few centimeters are within what RTK typically allows. Nevertheless, a 5 cm (2.0 in) height error on site can affect batter boards and as-built measurements and cannot be ignored.

Countermeasures:

• Check verticality carefully with the pole’s bubble level and do not measure while the pole is tilted. Even a slight tilt of the pole can lead to several centimeters of error.

• Use RTK receivers with IMU-based tilt compensation so you can obtain accurate coordinates even if the pole is somewhat tilted. This is helpful in confined sites.

• Verify antenna height settings for each site to avoid input errors, and perform regular calibration of receivers and sensors.

• Always perform check measurements during surveying. Measure a known control point with RTK; if errors are large, immediately investigate and resolve the cause before proceeding with primary measurements.

What is simplified surveying with LRTK

So far, we have reviewed the main causes of occasional RTK shifts and their countermeasures. Nevertheless, some sites may still face challenges such as “narrow locations where the antenna cannot be moved sufficiently,” “unavoidably long baselines,” or “lack of confidence in complex settings.” In such cases, our company’s simplified surveying solution using LRTK can be useful.

LRTK consists of a compact, high-performance RTK-GNSS receiver and a smartphone app, developed to make on-site high-precision positioning easier and more reliable. Several features that distinguish it from conventional equipment address the RTK error factors described above both in hardware and software.

• Multi-GNSS and dual-frequency support for stable positioning: LRTK receivers use multiple constellations such as GPS, GLONASS, Galileo, and QZSS, and support dual-frequency positioning on L1/L2. This enables stable tracking of a sufficient number of satellites even in urban areas and improves ionospheric error removal, helping to prevent satellite shortage and baseline-related accuracy degradation and to obtain FIX solutions more quickly.

• Support for CLAS satellite augmentation signals: Some models can directly receive centimeter-class augmentation services (CLAS) from Japan’s QZSS, enabling centimeter-level positioning even at sites where you cannot install an RTK base. This maintains high accuracy in mountainous or radio-obstructed areas and reduces reliance on radio communications or concerns about long baseline lengths.

• Easy operation via smartphone integration: A dedicated smartphone app allows intuitive management of base and rover settings and connection status. You can check satellite reception and correction data status on the screen at a glance, so even if FIX does not occur you can quickly identify the cause. Complex NTRIP settings are preconfigured for major correction services so you can connect by selecting from a list. LRTK’s user-friendly design allows personnel without specialized knowledge to operate correctly.

• Cloud integration and tilt compensation: Positioning data can be synchronized to the cloud in real time so results can be checked and shared before returning to the office. LRTK receivers also include built-in tilt sensors that automatically correct for a tilted pole to obtain accurate coordinates. This lets you measure points in narrow spaces where you cannot keep the pole perfectly vertical and improves operational efficiency.

In this way, LRTK is a next-generation simplified surveying solution that balances RTK accuracy and usability. It is designed to greatly reduce common on-site troubles such as “positions not matching” or “cannot achieve FIX,” allowing anyone to easily obtain stable, high-precision positioning. If you are struggling with RTK shifts or settings in your current workflow, consider switching to LRTK. It can be a reliable partner even under complex conditions.

FAQ

Q1. What accuracy can be expected with RTK FIX? How large are errors with Float or Single? A. Generally, when an RTK FIX solution is obtained, both horizontal and vertical errors are typically within ± a few centimeters. In contrast, a Float solution can result in errors on the order of ± several tens of centimeters, and a Single (no corrections) solution can be off by ± several meters or more. For example, the target accuracy of the LRTK system is: within 10 m for Single, within 1 m for Float, and within 2 cm for FIX. For tasks requiring precise positioning, Float or Single is insufficient; always obtain FIX before adopting measurements.

Q2. How many satellites are minimally required to obtain an RTK FIX? A. It depends on the algorithm, but generally it is said that both base and rover need to simultaneously track 5 or more satellites. With GPS only, 4 satellites allow only standalone positioning and are insufficient for RTK FIX. Modern multi-GNSS receivers can receive signals from over 10 satellites simultaneously, so cases where FIX cannot be obtained due to satellite count have decreased. However, satellite geometry is also important; if satellites are clustered in one direction in the sky, high numbers alone may not yield good accuracy. Choosing times when satellites are well distributed can help achieve FIX more quickly.

Q3. How long does it usually take from starting RTK to obtaining FIX? A. Under good conditions, it often takes about 30 seconds to 2 minutes to reach FIX after starting RTK. If the sky is open and signals are stable, FIX can occur within one minute of receiver startup. In poor environments or with setup difficulties, you may remain in Float for over 5 minutes. Once FIX is obtained, it generally remains as long as communications are maintained. If FIX does not occur after 5 minutes, review the possible causes discussed in this article and check settings and the environment; sometimes restarting the receiver or changing the measurement location will help achieve FIX more quickly.

Q4. Do weather or time of day affect RTK accuracy? A. Weather itself, such as rain or clouds, has little effect on GNSS signals, so there is not a large difference in the ease of obtaining FIX between fine and poor weather. However, heavy rain can cause greater signal attenuation and may lengthen the time to obtain FIX. Also, thunderstorms warrant suspending surveying for safety. The time of day affects the configuration of available satellites, so RTK accuracy and ease of achieving FIX can vary by time. Choosing times when satellites are well balanced in the sky is advantageous for faster FIX. Additionally, periods of high solar activity increase ionospheric disturbances and can adversely affect GNSS; in extreme cases such as geomagnetic storms associated with auroras, RTK solutions may become unstable. However, for routine surveying work, this is rarely a major concern.