What is Mobile RTK? Practical Guide for Construction Teams

Currently, the construction industry is increasingly adopting ICT technologies to improve productivity. There are various advanced technologies such as drone surveying and machine guidance, but one technology attracting attention is high-precision positioning using GNSS called RTK. This article explains, in an easy-to-understand way for construction teams, what mobile RTK is, how it works, how to use it on site, and the benefits of adopting it.

Table of Contents

• What is RTK?

• How RTK positioning works

• What changes with mobile RTK?

• RTK use cases on construction sites

• Benefits of introducing mobile RTK

• Points to note when using mobile RTK

• Simple surveying with LRTK

• FAQ

What is RTK?

RTK stands for Real-Time Kinematic and is a technology that enhances satellite positioning (such as GPS) in real time for high precision. Normally, standalone GPS positioning has errors on the order of several meters (several ft), but RTK reduces the error to the order of centimeters by using correction information from known reference points. This centimeter-class positioning accuracy (cm level accuracy (half-inch accuracy)) is widely used in fields that require highly accurate positioning, such as civil surveying, construction machine guidance, and precision agriculture. In recent years, RTK high-precision positioning has become an important element in initiatives like i-Construction promoted by the Ministry of Land, Infrastructure, Transport and Tourism and ICT construction, further increasing its prominence.

How RTK positioning works

RTK achieves high precision through a principle called relative positioning. Specifically, you prepare two GNSS receivers: a reference station (base station) with accurately known coordinates and a mobile receiver (rover) that measures while moving. By comparing the satellite signals received by both receivers and subtracting common error factors (satellite orbit and clock errors, atmospheric effects, etc.), the rover’s relative position can be calculated with high precision. Correction data obtained at the reference station are sent to the rover via wireless communication or the Internet using standard protocols such as Ntrip. The rover applies those corrections in real time, enabling its positioning results to be improved to centimeter-level accuracy (half-inch accuracy).

In Japan, the Geospatial Information Authority has installed roughly 1,300 electronic reference stations nationwide, and a network-type RTK (VRS method) using these stations is widespread. Users subscribe to this service and receive correction information at the on-site rover (receiver) to achieve high-precision positioning. A major advantage is that RTK positioning can be performed anywhere nationwide via mobile communications without having to set up your own dedicated base station.

What changes with mobile RTK?

Mobile RTK means performing RTK positioning using portable devices such as smartphones or tablets. Traditionally, RTK surveying required dedicated high-performance GNSS receivers, radio modems, tripods, and survey poles, and specialized surveyors usually operated the equipment. However, with recent improvements in smartphone and tablet performance and advances in GNSS technology, high-precision positioning is becoming more accessible. For example, the latest smartphones include multi-band GNSS chips that can receive multiple-frequency signals from a single satellite, improving positioning accuracy. In addition, positioning augmentation signals provided by Japan’s Quasi-Zenith Satellite System “Michibiki” (such as CLAS and sub-meter level augmentation) are becoming available on smartphones, enabling smartphones alone to achieve much higher accuracy than before (in some cases errors on the order of several tens of centimeters (several tens of inches)).

Even so, RTK corrections remain indispensable to reduce positioning errors to the centimeter level. The idea of mobile RTK emerged to address this. By attaching a small RTK-compatible device to a smartphone or receiving network-type RTK correction information through a dedicated app, a smartphone can function as a high-precision GNSS receiver on site. This has enabled site personnel to perform tasks that previously required specialized surveying teams, producing significant changes. In other words, the democratization of RTK positioning is progressing. If you have one smartphone on site, you can perform necessary high-precision surveying on the spot—this is the impact of mobile RTK.

RTK use cases on construction sites

• Control point surveying and as-built surveying: RTK is useful for tasks such as setting control points for construction and capturing the current terrain. Using network-type RTK, you can directly set control points on site in the Geospatial Information Authority’s coordinate system, which then forms the basis for subsequent surveying and construction management. It is also possible to survey wide areas quickly using RTK-enabled drones.

• Stakeout (marking) work: RTK can be used for stakeout tasks such as laying out roads and building positions on the ground according to design drawings. Traditionally, these tasks often required two-person teams using total stations, but mobile RTK enables one person to perform precise stakeout. By following guidance displayed on the smartphone screen to the designated coordinates, you can drive stakes or make markings on the spot.

• As-built management and quality checks: RTK positioning is also effective for verifying whether structures are constructed in the correct positions and dimensions per design. For example, you can measure the top elevation of a cast concrete footing with RTK and compare it with design values, or instantly capture and record coordinates of measurement points during as-built inspections. With mobile RTK, construction supervisors can quickly carry out these quality checks on site and easily retain photo-attached records.

• Machine guidance for heavy equipment: RTK is used for advanced automatic control of construction machinery such as excavators and bulldozers. GNSS receivers mounted on machines use RTK high-precision self-positioning to automatically adjust blade height and slope, assisting operators. While mobile RTK does not directly serve this application, it is important as a technology that supports high-precision position references on site.

• Photogrammetry and 3D scanning: Combining RTK with cameras or LiDAR allows you to record precise 3D information. For example, aerial photos taken by an RTK-equipped drone can produce accurate orthophotos and terrain models, and point cloud data obtained with a smartphone LiDAR scanner can be augmented with RTK position information to record ground and structure geometry with high accuracy. Such data are useful for volume calculations, construction records, and creating as-built drawings.

• Infrastructure inspection and maintenance: RTK’s precise position records are powerful in inspecting infrastructure such as roads and bridges. If you attach accurate coordinates via RTK when photographing cracks or defects at inspection points, you can easily locate the same spots during future re-inspections and track changes over time. With mobile RTK, inspectors can perform positioning on site without bringing bulky equipment, improving maintenance efficiency.

• Disaster surveys: Mobile RTK contributes to rapid damage assessment in disasters such as earthquakes and heavy rains. For example, at sites of ground subsidence or landslides, using a smartphone to measure coordinates and elevation differences of damaged areas allows you to produce accurate situation maps in a short time. The collected data can be used to plan restoration methods and identify hazardous zones. Thanks to its portability, mobile RTK demonstrates strong mobility in emergency initial surveys.

Benefits of introducing mobile RTK

• Easy access to high-precision positioning: Mobile RTK using a smartphone is significantly more convenient than traditional surveying equipment. You don’t have to carry large tripods or heavy equipment around the site; you can start surveying immediately with a device that fits in your pocket. Preparation and cleanup time are reduced, improving on-site mobility.

• Labor and personnel savings: Mobile RTK enables surveying with fewer people and is effective at sites with labor shortages. One person can measure multiple points or perform stakeout, greatly streamlining tasks that used to require two to three people. Reducing personnel can also save on labor costs.

• Faster operations: Being able to take measurements on the spot and check results immediately is another advantage. Commissioning specialized surveyors used to take days, but mobile RTK allows real-time data acquisition and sharing. This enables quick measurements and decisions aligned with construction progress.

• Cost reduction: You can reduce costs of purchasing and maintaining expensive dedicated surveying equipment or outsourcing to external survey companies. Many tasks can be handled with just a smartphone and a mobile RTK device, making it economical in terms of initial investment and operating costs.

• Data sharing and immediate use: Survey data obtained with mobile RTK are stored digitally and can be imported directly into drawing software or cloud services. If photos and point cloud data are shared via the cloud, on-site information can be instantly shared with the office team for rapid decision-making.

• Multifunctional use: Mobile RTK platforms allow you to use various functions on the same device, not just positioning. In addition to GNSS coordinate measurement, you can use camera photo records, LiDAR scanning, and AR navigation guidance, offering multifunctionality that traditional devices do not have.

• Improved safety: Because surveying can be done with a lightweight smartphone, you can reduce the risk of sending personnel into hazardous areas. For example, you can shorten surveying time beside busy roads or reduce the number of times workers must perform high-elevation inspections with lifelines, delivering safety benefits.

• Ensured accuracy and quality: Consistently performing high-precision surveying with mobile RTK prevents rework caused by positioning mistakes or measurement errors. Improved surveying accuracy directly contributes to higher construction quality and more reliable inspections. Construction based on high-precision data also stabilizes the quality of as-built management.

Points to note when using mobile RTK

• Pay attention to satellite reception conditions: RTK uses signals from GNSS satellites, so it performs best in open-sky environments. Near tall buildings or in areas with dense trees, signals can be blocked or reflected, degrading accuracy. Perform positioning in places with as clear a view of the sky as possible.

• Ensure communication coverage: When using network-type RTK, stable mobile communication (4G/5G) is a prerequisite. In mountainous areas or underground, communication may be unstable, so check coverage in advance. If necessary, consider satellite-based augmentation available offline (for example, Michibiki’s CLAS) or a simple base station mode.

• Positioning stability: RTK positioning accuracy is highest while the receiver has locked onto the satellites and obtained a fixed solution. It may take time for the solution to stabilize after starting positioning, so wait until accuracy has sufficiently converged before recording measurements. If the fixed solution is lost—such as entering a tunnel while moving—it can take time to regain high precision.

• Battery management: Prolonged GNSS positioning or 3D scanning with a smartphone quickly drains the battery. Carry a mobile battery, take charging breaks as needed, and close apps when not in use to prevent power loss. Pay special attention in cold regions, where battery performance tends to decline.

• Handling devices: Be careful to avoid drops and water exposure when using smartphones on site. Use waterproof/dustproof cases or attach straps to reduce the risk of damage. For high-precision positioning, it is preferable to keep the smartphone as still as possible during measurement. Fixing the phone to a monopod or a simple tripod provides more stable positioning than handheld measurements.

• Surveying standards and coordinate systems: Confirm that coordinates obtained with mobile RTK match the design coordinate system used on site. Public surveys may use project-specific coordinate systems such as plane rectangular coordinates. If necessary, compare coordinates measured at known points and apply offset corrections or geodetic transformations to align acquired data with the site’s reference system.

Simple surveying with LRTK

Finally, as a concrete solution example of mobile RTK, we introduce LRTK. LRTK is a mobile RTK system developed by a startup from Tokyo Institute of Technology and consists of a small device attached to an iPhone and a dedicated app. By using it, you can complete tasks from centimeter-level positioning (cm level accuracy (half-inch accuracy)) to 3D scanning and stakeout guidance with just an iPhone, dramatically simplifying on-site surveying.

For example, an iPhone equipped with LRTK becomes an RTK-GNSS receiver, receiving network-type RTK and Michibiki’s CLAS signals to perform real-time high-precision positioning. The acquired position information is automatically attached to photos and LiDAR scan data, so all photos taken and point clouds generated remain as records with accurate coordinates. Moreover, AR features can navigate users to specified coordinates. By following arrows and guide displays on the smartphone screen, tasks that previously required surveying expertise—such as stakeout and point observations—can be performed intuitively by a single person.

Data recorded with LRTK can be uploaded and managed in the cloud, allowing you to confirm shooting locations on a map or overlay them with CAD drawings and BIM models. Because site-collected information can be immediately shared among the whole team, reporting and consultation time is reduced. In this way, LRTK realizes simple surveying that can be used without worrying about advanced positioning technology, and it is already being used in civil engineering sites and disaster surveys. If you are considering introducing mobile RTK, solutions like LRTK demonstrate that an era in which anyone can easily perform high-precision surveying has become a reality.

FAQ

Q: What is the difference between RTK and normal GPS positioning? A: Normal GPS (GNSS) positioning calculates position by receiving satellite signals alone, resulting in errors on the order of several meters (several ft). RTK positioning, on the other hand, uses correction information from a reference point to cancel out errors and pinpoint positions with very high accuracy on the order of centimeters. Simply put, RTK dramatically improves GPS positioning accuracy by using “an additional reference.”

Q: What equipment and environment are required for RTK positioning? A: The basics are a high-precision GNSS receiver (rover) and a reference station (base) with known coordinates. You also need a communication method (radio or Internet connection) to deliver the base station’s correction information to the rover. Today, by subscribing to network-type RTK services that use the Geospatial Information Authority’s electronic reference stations, you can obtain correction information without preparing your own base station. In that case, a field RTK-compatible receiver and a communication terminal are sufficient to achieve high-precision positioning on site.

Q: Is high-precision positioning like RTK possible with a smartphone? A: Yes. Modern smartphones have high-performance GNSS chips, and by combining them with a mechanism to receive correction information, smartphones can achieve RTK-equivalent positioning. For example, if you attach an RTK-compatible device to a smartphone and use a dedicated app to ingest network corrections, the smartphone becomes a device capable of centimeter-level positioning. In practice, there are cases of on-site high-precision positioning using an iPhone combined with an LRTK device.

Q: How accurate is RTK positioning? A: Generally, horizontal position errors are on the order of 2–3 cm (0.8–1.2 in), and vertical errors are on the order of a few centimeters (a few inches). However, this is accuracy achieved in open conditions with a stable fixed solution. Accuracy can degrade depending on the surrounding environment or satellite geometry, and in some cases deviations of several tens of centimeters (several tens of inches) may occur. Conversely, by remaining stationary and averaging data for several minutes, sub-centimeter accuracy can sometimes be obtained—1 cm or less (0.4 in or less).

Q: In what situations can RTK be used? A: RTK is widely used wherever high positioning accuracy is required. Examples include establishing control points for civil engineering, stakeout (positioning), as-built surveying, and machine guidance for heavy equipment during construction phases. RTK is also applied to drone-based surveying, bridge and road maintenance inspections, and disaster damage surveys. It is not an exaggeration to say that RTK is involved in almost all positioning tasks that demand precision.

Q: Can RTK be used where GNSS signals cannot reach? A: Unfortunately, RTK positioning cannot be performed in environments where GNSS signals cannot be received, such as inside tunnels or buildings. Other surveying instruments like total stations must be used in such locations. However, it is possible to indirectly utilize RTK results by transferring an RTK-positioned control point indoors (for example, by establishing a layout line), enabling the use of RTK outcomes inside buildings.