What is RTK? Fundamentals of Real-Time Kinematic Positioning

RTK (Real-Time Kinematic) is a technology that utilizes satellite positioning systems (GNSS) to achieve high-precision positioning in real time. By correcting positioning errors based on data from a reference station (base station) with a known location, a moving receiver (rover) can determine its own position with centimeter-level accuracy. In surveying and civil engineering sites, RTK's immediate and high-precision positioning is essential for ensuring work efficiency and quality. In recent years, its affordability and adoption have increased, expanding its use across various fields. This article clearly explains the basics of RTK from the ground up: its definition and fundamental principles, differences from standard GPS positioning, benefits for civil surveying, and specific use cases.

Definition and Basic Principles of RTK

RTK is a type of positioning method called “relative positioning,” which determines position by simultaneously using at least two GNSS receivers (a rover and a base station). Unlike the conventional method of positioning using a single receiver, it works by canceling out common errors through relative positioning between two points.

Main Process of RTK Positioning

• Base Station A base station, positioned at a known, precise coordinate location, receives signals from multiple GNSS satellites. It calculates, in real time, the difference (error) between its measured position and its actual known coordinates.

• Correction Data Transmission The base station transmits the calculated error information (correction data) to the rover via radio or communication lines.

• Mobile Station The mobile station (receiver on the moving object) applies the received correction data to its own positioning results, calculating highly accurate position coordinates.

True to its name, Real-Time Kinematic, these corrections occur simultaneously with positioning. Positioning results are obtained instantly on-site, eliminating the need to wait for post-processing. This allows immediate confirmation of position coordinates and progression of surveying work.

RTK can be considered an evolution of traditional Differential GPS (DGPS). It enables even more precise ranging by specifically utilizing the carrier phase (phase shift of the wave) of GNSS signals. Since the carrier wavelength is only tens of centimeters long, analyzing this phase difference allows positioning to be pursued with an error level of just a few centimeters.

Differences from Conventional GPS – Sources of Error and RTK Correction

Stand-alone GPS positioning (using a single receiver) typically produces errors ranging from several meters to tens of meters. In contrast, RTK positioning reduces errors to within a few centimeters. The reason RTK achieves such high precision is that it cancels out various error factors affecting standard GPS positioning through relative positioning with a base station.

Main Error Factors in GPS Positioning

• Satellite Orbit Error Errors due to slight deviations in satellite orbital positions

• Satellite Clock Error Discrepancies between the atomic clocks on satellites and the receiver's clock

• Ionospheric and Tropospheric Delay Errors caused by signal propagation delays in the ionosphere and troposphere

• Multipath Error Errors arising from signal reflections and interference caused by buildings and terrain

• Receiver Noise Errors caused by internal electronic noise within the receiver or environmental electromagnetic interference

In standalone positioning, all these error factors accumulate, inevitably limiting positioning accuracy. For example, the several-meter discrepancy in a smartphone GPS's displayed location on a map results from these errors.

With RTK, since both the base station and rover receive signals from the same satellites, common error components like satellite orbit errors, clock errors, and ionospheric/tropospheric delays can be nearly canceled out. By applying the error information obtained at the base station, the rover can achieve positioning closer to the “true position” by subtracting these errors. However, it cannot completely eliminate locally varying errors, such as multipath errors caused by signal reflections or noise specific to each receiver.

Nevertheless, since RTK corrects most major error factors, it achieves positioning accuracy that is orders of magnitude higher.

Traditionally, high-precision positioning required static surveying or long-term averaging measurements. The advent of RTK made it possible to achieve centimeter-level accuracy instantly, even while moving. This technological innovation has enabled tasks previously difficult with GPS accuracy to be replaced and streamlined using GNSS-based automatic positioning.

Benefits and Use Cases of RTK

The benefits RTK delivers can be broadly categorized into two points: “improved positioning accuracy” and “enhanced work efficiency.” Improved accuracy allows tasks requiring precise manual positioning, which previously relied on human labor, to be replaced by machine positioning. Increased efficiency enables large-scale surveying and construction management to be carried out in shorter timeframes and with reduced manpower. Here, we introduce specific use cases, primarily focusing on the surveying and construction industries.

RTK Applications in Surveying

RTK demonstrates its power in land surveying and topographic mapping. Tasks that traditionally required surveyors to meticulously set up and operate optical instruments like total stations (TS) can now be covered quickly using RTK-GNSS receivers. For example, when surveying the topography of a large development site, using a base station and rover station setup allows personnel to walk around and instantly capture elevation and position data at each point, significantly reducing manpower and effort.

Coordinates obtained via RTK positioning are recorded in a global coordinate system based on known points (such as the World Geodetic System in Japan), ensuring smooth integration with subsequent drawing creation and alignment with design coordinates.

RTK is also utilized for control point surveying. Using network RTK (GNSS) surveying, which receives correction data in real time from the Geospatial Information Authority of Japan's electronic control points, allows new control points to be established with centimeter-level accuracy even at distant sites. This streamlines the creation of surveying networks at construction sites and reduces the need for lengthy traverse surveys traditionally required.

Recently, RTK-GNSS has also been integrated into drone (UAV) photogrammetry, enabling the creation of accurate terrain models from aerial photos without requiring numerous ground control points.

There are reports of RTK-equipped UAVs being introduced to survey vast construction sites in short timeframes. By performing high-precision position corrections in real-time from the air, they reduced the need for ground-based control point setup.

RTK Applications in Civil Engineering Construction and Machinery

RTK also plays a vital role in construction management for civil engineering projects. Under the Ministry of Land, Infrastructure, Transport and Tourism's ICT-based construction (smart construction) initiative, machine guidance and machine control systems are gaining traction. These systems equip construction machinery like bulldozers and shovels with GNSS receivers to perform tasks automatically or semi-automatically.

For example, attaching RTK-GNSS to a bulldozer's blade enables automatic height control to match the design grade. Similarly, mounting GNSS on a backhoe allows excavation along ground lines without prior staking out (using stakes or string lines).

Applications also include mounting it on a road roller to manage the number of compaction passes for embankments. Utilizing high-precision positioning information enables accurate and safe construction without relying on the operator's skill level.

RTK-GNSS surveyors are also used for as-built surveys (checking the shape of completed earthworks). Tasks that previously required manual measurement of numerous points can now be recorded over an area simply by walking with a rover receiver.

These capabilities allow for both improved overall construction productivity and quality assurance, making RTK an indispensable technology for next-generation construction sites.

Furthermore, in the infrastructure sector, there is a growing trend to utilize RTK for the maintenance and management of railways and roads. For example, railway companies require high-precision positioning for track inspection and equipment management. They are experimenting with RTK positioning using measurement trolleys equipped with GNSS and laser scanners to precisely measure distortions in rails and sleepers.

Although there are limitations, such as the inability to use RTK in environments where satellite signals cannot reach, like tunnels or under elevated structures, it is expected that high-precision positioning, including RTK, will play an increasingly broader role in the future. This is due to the use of augmentation signals from Japan's Quasi-Zenith Satellite “MICHIBIKI” and the integration with IMUs (Inertial Measurement Units).

Benefits of Implementation in Civil Engineering Sites

Introducing RTK high-precision GPS terminals to civil engineering and construction sites delivers numerous advantages over traditional surveying and construction management methods.

• Improved Surveying Accuracy and Immediacy By acquiring position coordinates with centimeter-level accuracy, the effort required for setting up control points and leveling is significantly reduced. High-precision positioning, previously performed by survey teams using total stations or expensive GNSS equipment, can now be executed instantly by anyone, enabling on-site as-built management and verification against design values. For example, using LRTK improves the accuracy of smartphone GPS—which traditionally had errors of several meters—to approximately ±2cm, allowing immediate access to survey results on-site.

• Improved Work Efficiency and Productivity LRTK is compact, lightweight, and highly portable, allowing each field worker to carry their own dedicated surveying device. This enables “zero waiting time” for surveying whenever needed, allowing multiple people to conduct surveying tasks simultaneously. For example, moving from a situation where one GNSS device per team caused queues, to each worker carrying an LRTK and conducting measurements autonomously, dramatically boosts overall site productivity. Furthermore, since the device fits in a pocket, carrying it around the site is effortless, allowing easy access to measurement points in high or confined spaces.

• Cost Savings & Lower Adoption Barriers While traditional high-precision GNSS devices and survey instruments were expensive and required specialized knowledge, LRTK is offered at a relatively affordable price point. Designed to work with smartphones, it eliminates the need for multiple dedicated devices, allowing users to leverage their existing smartphones. The software features an intuitive app, allowing even non-surveying professionals to master it with minimal training, making adoption accessible for small to medium-sized civil engineering firms. Furthermore, subscription plans including cloud services are available, enabling a low-cost initial start.

• Promoting Digitization and Information Sharing LRTK integrates with a dedicated cloud service, instantly plotting acquired point coordinate data and photos onto cloud-based maps for sharing. Field measurement data uploads with a single tap from the smartphone, allowing office staff and partner companies to instantly view and download it via web browser. This eliminates cumbersome traditional tasks like handwriting records on paper drawings and bringing them back, enabling real-time information sharing and backup. For example, during infrastructure inspections, photographing a bridge crack location saves precise coordinates and azimuth data to the cloud on the spot. This allows all stakeholders to later pinpoint the exact location accurately. This significantly contributes to streamlining maintenance and report creation.

• Strength in Harsh Environments and Disasters LRTK proves invaluable even in locations without cellular coverage, such as dam construction sites in mountainous areas or around mountain tunnels on highways. As mentioned earlier, LRTK terminals support Michibiki's CLAS, enabling them to obtain correction data via satellite even outside smartphone coverage areas. This allows for centimeter-level positioning even in offline environments. Indeed, during the 2023 Noto Peninsula earthquake, LRTK played a vital role in disaster-stricken areas where mobile networks failed, contributing to the high-precision recording and sharing of damage assessments with location data. Disaster response often involves situations where large equipment cannot be brought in. In such cases, a single pocket-sized LRTK unit can significantly aid in the rapid assessment and sharing of on-site conditions. This ability to perform positioning independent of communication infrastructure provides significant peace of mind for those responsible for infrastructure inspections and disaster prevention.

As described above, introducing LRTK brings benefits in terms of “accuracy,” “efficiency,” “digitalization,” and “reliability.” It is also expected to be a tool that dramatically improves the efficiency of data collection from the field in promoting DX (Digital Transformation) in civil engineering sites and in i-Construction initiatives.

Frequently Asked Questions (FAQ)

What is the difference between RTK and regular GPS?

What is required to perform RTK positioning?

What level of accuracy can RTK achieve?

How far apart can RTK positioning be used?

Is RTK used in fields other than surveying and civil engineering?

Can RTK be used with a smartphone?

Are RTK-compatible devices expensive?

Introduction to LRTK and Requesting Materials

To utilize high-precision positioning in the field, introducing user-friendly RTK-compatible equipment is key. This is where our digital positioning technology, “LRTK,” comes into focus.

LRTK is a solution combining an RTK-GNSS receiver with a dedicated app and cloud service, designed to enable even first-time RTK users to easily start centimeter-level positioning.

For example, using the compact LRTK Phone device, which simply attaches to a smartphone, allows you to enhance the position information obtained by your phone directly to cm accuracy.

Simply attach the palm-sized receiver—which incorporates a dedicated antenna and battery—to your smartphone and launch the app. This allows you to record high-precision coordinates on-site with the ease of a map app. It also makes it easy to record coordinate points simultaneously while taking photos. A major advantage is that anyone can easily utilize RTK positioning without traditional surveying equipment.

For more demanding field applications, the stationary LRTK Pro series is also available, featuring dustproof, waterproof construction and extended battery life. This rugged terminal integrates an antenna, GNSS receiver, radio, and battery. It enables standalone positioning even in remote mountainous areas with poor internet connectivity by utilizing Japan's Quasi-Zenith Satellite System (QZSS) augmentation signal (CLAS). It also features tilt compensation that corrects for the tip position even when the pole is tilted, proving highly effective for surveying in obstacle-filled environments.

Additionally, a unique product has emerged: the LRTK Helmet, which integrates an antenna and receiver into the helmet. Workers can perform surveying hands-free simply by walking while wearing the helmet, dramatically improving on-site safety and efficiency.

Dramatically Improve Site Survey Accuracy and Work Efficiency with LRTK

The LRTK series achieves high-precision GNSS positioning for construction, civil engineering, and surveying, enabling reduced work time and significant productivity gains. It also supports the Ministry of Land, Infrastructure, Transport and Tourism's i-Construction initiative, making it an optimal solution for advancing digitalization in the construction industry.