The Basics of RTK Positioning and How Real-Time Correction Works

RTK stands for "Real-Time Kinematic" and is a type of relative positioning system. It uses two GNSS receivers— a base station (fixed station) placed at a known point and a rover (moving receiver) installed at the location to be positioned. These two receivers simultaneously receive signals from at least four satellites and exchange data in real-time to correct errors and determine a highly accurate position.

The base station calculates the error by comparing its known position with the satellite signals it receives, and then sends the correction data to the rover in real-time via radio or other means. The rover uses the received correction data and the satellite signals it receives to compute the measurement error (path difference) in real-time and determine its position with centimeter-level precision.

In this way, RTK positioning corrects errors (such as satellite orbit errors, clock errors, and ionospheric/tropospheric delay errors) that cannot be corrected by standalone GPS positioning through a relative comparison with the base station, achieving dramatically higher accuracy.

The accuracy of RTK positioning depends significantly on the distance between the base station and the rover (baseline length). If the two stations are close, atmospheric signal delay errors will be nearly identical and can cancel each other out, but the further apart they are, the less the errors can be canceled and some residual errors will remain.

Therefore, in typical RTK setups, the base station is ideally placed near the work area (within a few kilometers), and correction information is transmitted via UHF-specific low-power radio or long-range radios. When properly implemented, horizontal positioning can achieve an accuracy of about 1–2 cm, providing positioning much more precise than traditional meter-level GPS.

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How Network RTK (VRS) Works

The limitation of "having to place a base station on-site" in RTK positioning has been overcome by a method called Network RTK. This system utilizes a network of multiple base stations, generating correction data as if a base station were located near the user, based on the user's approximate position. The most common approach is the VRS (Virtual Reference Station) method.

In Network RTK, the user's (rover’s) approximate location is used to integrate and analyze data from multiple electronic reference points (fixed base stations) located nearby, through a server. The server assumes a virtual base station located near the user and simulates the satellite signals that would have been received at that location to create the correction data.

The correction data from the virtual reference point is then transmitted to the rover via a communication link (mainly through internet communication using the NTRIP method), allowing the rover to perform RTK calculations as if there were a base station right next to it.

With the VRS method, RTK positioning gains further convenience and scalability. Since there is no need to set up a base station on-site, surveying can be done with just one receiver (rover), significantly reducing the preparation time before starting work. Additionally, since the virtual reference point is always set close to the positioning site, there is virtually no degradation in accuracy due to baseline length, enabling consistent, high-precision positioning over large areas.

In Japan, the Geospatial Information Authority has provided a correction data distribution service (GNSS Continuous Observation System) using the network of about 1,300 electronic reference points nationwide. By using this service, users can obtain real-time coordinates in the global geodetic system (Japan Geodetic System 2011) without needing to set up a base station on-site. Private companies have also introduced paid services utilizing mobile networks. For example, SoftBank has established over 3,300 of its own reference points across the country and offers a network-based RTK service.

Thanks to these network-based RTK systems, users can achieve stable, centimeter-level positioning accuracy anywhere in Japan as long as they are within the communication coverage area.

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Comparison of RTK and VRS – Differences in Accuracy, Cost, and Ease of Implementation

The main differences between the RTK method (traditional method using a standalone base station) and network-based RTK (VRS method) are summarized below. By understanding their respective characteristics, you can choose the most suitable method for your site’s needs.

Overall, the positioning accuracy of both methods is comparable, with both achieving centimeter-level precision. However, in terms of operational costs and ease of use, the VRS method is superior, making network-based surveying increasingly popular in recent years.

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Use Cases in Construction and Surveying (Construction Management, Staking, As-Built Measurements, Infrastructure Maintenance)
RTK and network-based RTK (VRS) are widely used in current construction and surveying sites. Below, we introduce some typical use cases and the benefits of their implementation.

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• Application in Construction Management: RTK positioning is playing a significant role in construction site reference point measurement and as-built management. For example, in road construction, when marking the location of structures based on the coordinates from design drawings (staking), RTK allows for quick and precise measurement of positioning and height after construction. Additionally, RTK-compatible GNSS receivers allow for real-time corrections of the machine's blade position in machine guidance and machine control (MG/MC), enabling operators to perform precise work even on their own. In this way, RTK greatly improves efficiency and precision, contributing to construction management.

• Use in Pile Driving and Foundation Work: RTK positioning is also utilized in foundation piling, such as for columns and bridge piers. Previously, surveyors would use string lines or total stations to determine the pile positions and direct heavy machinery operators. With GNSS mounted on the machinery, the operator can now verify the pile position directly on the monitor and make adjustments while working. Network RTK allows for stable precision over a wide area, so even for large bridge construction or land development projects, there's no need to reposition the base station as the rover moves. This allows for real-time detection of misalignments or tilt, reducing rework and ensuring accurate construction. Additionally, RTK surveying the installation positions of foundations in advance helps streamline the process of checking the design and identifying construction errors early.

• As-Built Measurements and Inspections: RTK is used for measuring as-built conditions after structure construction. For tasks such as checking the elevation and slope of road pavement or terrain grading, RTK-GNSS receivers enable operators to collect data across large areas in a short amount of time. For example, when inspecting pavement thickness, the operator can simply walk along the completed surface with the RTK receiver to capture elevation data at specified intervals, instantly checking the differences from the design height. With network RTK, the obtained coordinates are in absolute global positioning system coordinates, making it easy to create as-built management charts and integrate the data into GIS systems. Survey results can be instantly shared with the office via the cloud, speeding up decision-making and streamlining report creation. Overall, with the introduction of RTK, as-built measurements have evolved into a new phase with real-time and digital precision.

• Infrastructure Maintenance and Inspections: High-precision positioning technology is also proving effective in the field of infrastructure maintenance. In the inspection of railways and roads, accurately recording the location of anomalies is crucial. With RTK-enabled tablets or smartphones, cracks and deformations in bridges and tunnels can be measured, and the coordinates of the anomaly can be instantly recorded and linked with photos and inspection records. For example, instead of recording as "from bridge pier No. X, 3 meters," RTK allows the precise location data, including latitude, longitude, and height, to be recorded, making it easier to compare data and plan repairs in the future. Additionally, regular surveys of subsidence on roadbeds or tracks allow for the quantitative monitoring of long-term changes, aiding in preventive maintenance. Recently, RTK is also being used in drone-based aerial surveying and mobile mapping, supporting the DX (digital transformation) of infrastructure inspections. Real-time corrected precise positioning is becoming a strong ally in ensuring safe and reliable operations in infrastructure maintenance.

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The Evolution of RTK Positioning with LRTK – The Strengths of Compact, Wireless, and Smartphone Integration

As mentioned, RTK and VRS technology have greatly transformed the surveying and construction sites, but traditional RTK equipment was often expensive, bulky, and required specialized knowledge. Enter LRTK, a solution developed by the startup company Lefixea. LRTK was designed with the concept of a "pocket-sized RTK surveying device that anyone can use on-site" and is a next-generation RTK-GNSS device that enables easy centimeter-level positioning by integrating with smartphones.

The ultra-compact device, weighing only 125g and measuring 13mm in thickness, integrates the antenna, GNSS receiver, battery, and wireless communication into a single unit. By simply attaching this device to a smartphone, high-precision positioning can begin immediately.

Communication is wireless via Bluetooth or Wi-Fi, enabling correction data reception and positioning data transmission to the cloud through the smartphone, eliminating the hassle of cable connections.

The strength of LRTK is not just its miniaturization and simplification. The high-end model, LRTK Pro2, supports Japan’s Quasi-Zenith Satellite System (Michibiki), which provides centimeter-level correction services (CLAS). This allows for high-precision positioning even in areas such as mountainous regions where there is no internet connection, using satellite correction signals.

Additionally, it features an inclination correction function, allowing accurate positioning even when the antenna at the end of the pole is tilted. This groundbreaking function makes it possible to measure even in situations where the pole must be tilted to avoid obstacles. Moreover, the LRTK devices are designed to be robust for tough field conditions, ensuring dustproof, waterproof, and shock-resistant features, making them reliable for use in rain or dirt.

Meanwhile, the LRTK Phone model is designed for ease of use and portability, enabling construction managers and workers to carry it daily and perform measurements on-site whenever needed.

With the combination of these LRTK devices and dedicated apps, the era has arrived where anyone on-site can perform tasks such as surveying and measurement that were previously outsourced to specialized surveying teams using their own smartphones.

In fact, the introduction of LRTK has had a significant impact on the field. According to one article, the pocket-sized LRTK Phone has sparked a "quiet boom" among construction managers and workers on-site. The reason is that "by simply attaching an ultra-compact RTK receiver to an iPhone or iPad, users can perform centimeter-level positioning, point cloud measurements, staking, AR visualization, and instantly share data via the cloud. And since it is extremely affordable, having one per person would significantly improve on-site productivity."

RTK, which was once an expensive specialized tool, is now becoming an affordable and accessible tool, marking an evolution in the democratization of RTK positioning. The cloud integration feature allows for immediate sharing and verification of data between the site and the office, making it modern and accelerating the DX of surveying and inspection work by tagging photos and notes to positioning data.

LRTK brings the benefits of RTK to users who previously felt challenged by carrying and operating surveying equipment, and is poised to introduce a new work style in the construction and surveying industries.