What is high-precision GPS positioning "LRTK"? Explaining the new technology that supports construction DX

In recent years, the construction industry has been accelerating its building DX (digital transformation) using digital technologies. One of the core technologies is GPS positioning with far higher accuracy than before. Ordinary car navigation systems and smartphone GPS have errors on the order of several meters (several ft), but it has become possible to improve this to an accuracy of a few centimeters (a few in), and surveying and construction management methods are beginning to change significantly. This article first organizes the basics of GPS and its use and limitations in the construction field, and explains the mechanisms and needs for high-precision positioning. It then focuses on the high-precision GPS terminal attracting attention as a new technology, LRTK, and introduces the benefits and case studies that support construction DX, as well as challenges and countermeasures when introducing it.

What is GPS? Its use and limitations in the construction field

GPS (Global Positioning System) is a system that measures one’s current position using artificial satellites. Originally the name of the satellite positioning system operated by the United States, in everyday use the term “GPS” is often used collectively to include other countries’ satellite systems such as Russia’s GLONASS and Japan’s “Michibiki” (quasi-zenith satellite). A GPS receiver (positioning terminal) receives radio signals from multiple satellites overhead and calculates latitude, longitude, and altitude on Earth by distance calculation. The GPS function installed in smartphones and car navigation systems determines position based on this principle.

In the construction field, GPS has been used in a variety of applications. For example, in surveying and as-built management at large development sites, terrain data can be acquired by GPS positioning instead of having people walk around to measure. GPS is also used for machine guidance of heavy equipment such as bulldozers and excavators, enabling advanced construction such as automatically grading the ground while maintaining designed surface elevations. Recently, it has also become common to add GPS position information to photos taken by drones (unmanned aerial vehicles) to create 3D terrain models. Compared with traditional labor-intensive methods, GPS-based measurement can cover wide areas in a short time and contributes to productivity improvements.

However, there are also limitations to conventional GPS use. First, standalone GPS accuracy is generally said to be about 5–10 m (16.4–32.8 ft), which is insufficient for the precision required on construction sites. For tasks such as staking out building positions (layout) and establishing control points, errors of a few centimeters (a few in) directly affect quality, so optical surveying instruments like total stations have traditionally been used instead of GPS. Second, radio signals from GPS satellites are easily blocked in building shadows or mountainous areas, making positioning unstable in places surrounded by tall buildings in urban areas, inside tunnels, or in forests. In locations with poor communication environments or areas where infrastructure has been destroyed immediately after a disaster, reliance on GPS alone can be problematic. High-precision positioning technologies were developed to compensate for these limitations and enable more accurate and stable position measurement.

How high-precision positioning works (RTK, Michibiki, correction information, etc.)

To raise GPS positioning accuracy to the level usable on construction sites, several technical methods exist. The representative method is the high-precision positioning technique called RTK (Real-Time Kinematic). In RTK, a reference station (base station) is set up at a known and accurate coordinate, separate from the receiver that moves around on site (the rover). The base station calculates in real time the difference (error information) between its precise position and the position received by GPS, and sends correction data to the rover. By applying this correction information, the rover can cancel out many error factors and determine its position with centimeter-level accuracy. In other words, the system achieves a dramatic accuracy improvement by having “another GPS” tell the rover the errors.

Besides RTK, there are satellite augmentation systems that improve GPS accuracy. In Japan, one example is the CLAS (Centimeter-Level Augmentation Service) provided by the quasi-zenith satellite system Michibiki. With a receiver that supports CLAS, it is possible to receive correction information directly broadcast from the Michibiki satellites and obtain RTK-like accuracy of a few centimeters using a single GPS receiver. This is a groundbreaking Japan-specific service that enables high-precision positioning as long as the satellites are visible, even in mountainous areas where terrestrial communications are unavailable. Previously, obtaining centimeter accuracy required installing dedicated base stations or subscribing to a correction network, but with the advent of Michibiki, high-precision positioning has become easily available simply by equipping a compatible receiver.

Furthermore, in Europe and the US, GNSS correction services (methods called SBAS or PPP) are advancing high-precision positioning over wide areas. For example, satellite augmentation services like WAAS or EGNOS in the West reduce errors to within a few meters (a few ft), and more advanced PPP (Precise Point Positioning) techniques distribute global satellite error information over the Internet for rovers to apply and achieve higher accuracy. However, these approaches often take time to converge (until accuracy stabilizes) or are offered as paid services, so on construction sites that require immediate centimeter-level accuracy, RTK methods and augmentation signals from systems like Michibiki are still highly valued.

In summary, the key points of high-precision positioning are “correction information” and “multi-source (multi-GNSS) observations.” High-performance antennas that can receive multiple satellite frequencies utilize satellites other than GPS, and by applying corrections from base stations or satellites, error factors (such as atmospheric disturbances and clock errors) are canceled out. As a result, position measurements that were once limited to several meters can reach accuracy levels suitable for surveying instruments.

Required positioning accuracy and needs on construction sites

The required positioning accuracy on construction sites varies depending on the application, but tasks that demand particularly high accuracy require errors within a few centimeters (a few in). For example, the stakeout work that sets out the positions of buildings and structures cannot tolerate a 5 cm (2.0 in) deviation, and checking reference elevations in road construction also requires accuracy on the order of a few centimeters (a few in) to avoid affecting the finished surface. In earthworks, small surveying errors in calculating fill and cut volumes can lead to large differences in soil quantities, so highly accurate terrain data is required. Traditionally, such precise measurements have been performed with optical surveying instruments (transits or total stations). However, optical surveying instruments require skilled operators and multiple personnel to measure linear distances and angles manually, making them inefficient for covering wide areas.

On the other hand, under initiatives promoted by the Ministry of Land, Infrastructure, Transport and Tourism such as *i-Construction* and *ICT-enabled construction*, the demand for DX (business transformation through digitization) in the construction industry is increasing. The initiative aims to increase productivity by utilizing 3D data consistently from design through construction and inspection. Achieving this requires accurately acquiring position and shape data on site and being able to immediately share and use it as digital data. In other words, there is growing demand for tools that can measure “quickly,” “easily,” and “accurately.” Also, with the severe labor shortage caused by an aging and declining population, it is necessary for site technicians themselves to be able to perform tasks that used to be left to surveyors or specialized operators, often working alone. To meet this demand, expectations for easy-to-use, compact, and highly accurate positioning devices are very high.

Technical background and introduction benefits of LRTK (device configuration, centimeter-level accuracy, single-person surveying, etc.)

One of the new technologies meeting these needs is the high-precision GPS positioning terminal "LRTK." LRTK is a Japan-developed positioning device based on RTK technology, sized to fit in a pocket and attachable to a smartphone or tablet. Technically, LRTK leverages the nationwide network of electronic reference stations (the Geospatial Information Authority of Japan’s GPS reference stations) and the augmentation signals from the Japanese GPS system Michibiki, enabling centimeter-level positioning without installing additional base stations. In other words, without the dedicated base stations or expensive correction services that used to be necessary, LRTK terminals and a smartphone make it possible to start high-precision positioning on site immediately. This is a technological breakthrough that significantly lowers the barrier to high-precision positioning.

The LRTK terminal configuration is simple: a small RTK-GNSS receiver that attaches to a dedicated smartphone case. The receiver body weighs only a few hundred grams and includes a built-in battery, allowing it to be carried together with the smartphone (optionally, it can be mounted on a monopod or pole). It connects to the smartphone wirelessly or via a dedicated connector, and positioning operations are performed through a dedicated app. This setup packs GNSS surveying functionality that used to require a backpack or tripod into a size that can be held in one hand.

Main benefits of introducing LRTK:

• Centimeter-level accuracy (half-inch accuracy): By supporting RTK and Michibiki’s CLAS signals, it achieves positional errors within a few centimeters (within a few in). Because this accuracy can be obtained immediately without setting up control points or repeated measurements, it can be used directly for precise surveying and as-built management.

• Single-person surveying: With an LRTK terminal attached to a smartphone, a single worker can perform everything from recording survey points to stakeout. Tasks that previously required an assistant can now be handled by one person following instructions on the smartphone screen, making operations efficient even on sites with labor shortages.

• Portability and ease of use: The small, lightweight body fits in a pocket and can be carried around the site at all times. It can be taken out and used immediately when needed, and it is not location-dependent. Initial setup is minimal, and operation is intuitive with app buttons on the smartphone.

• Versatile expandability: LRTK supports a wide range of uses beyond simple positioning, such as acquiring point cloud data and integrating with AR features as described later. One device can handle various measurement tasks, reducing the need to prepare multiple instruments.

A particularly notable technical feature is that LRTK supports triple-frequency GNSS reception. By receiving multiple frequency bands from GPS, GLONASS, Galileo, and Michibiki, it can remove ionospheric errors and rapidly resolve integer ambiguities for fast RTK fixes. Additionally, when out of communication range, it can receive CLAS corrections directly from Michibiki, and when networked, it can use nationwide reference station data; the design flexibly switches correction sources according to conditions. This enables stable high accuracy from mountainous to urban areas.

Utilizing positioning data and transforming business processes (3D scanning, AR, volume calculation, etc.)

Position information obtained with high-precision GPS is not just used for measuring points; it can drive various process innovations in construction operations. Devices like LRTK are characterized by their ability to share and utilize positioning data in real time through integration with smartphones and the cloud. Specific use cases include:

• 3D scanning (point cloud surveying): By combining a smartphone camera or external sensors, surrounding structures and terrain can be photographed and acquired as high-precision point cloud data with position information. Traditionally, laser scanners or drone photogrammetry required GCP (ground control point) surveys to correct meter-level errors, but using high-precision GPS and a smartphone greatly reduces that effort. Acquired point cloud data can be stored in the cloud and viewed and analyzed immediately by remote offices.

• Design data verification with AR: Using the current position obtained by LRTK as a reference, design drawings or 3D models can be displayed in AR (augmented reality) on a smartphone or tablet. For example, the location of underground pipelines can be shown on site via AR, or BIM models from the design stage can be overlaid on the real space to verify construction points. High-precision alignment reduces the gap between digital data and reality, enabling intuitive and accurate on-site verification.

• Automated volume and distance calculations: With positioning data, distance, area, and volume measurements can be completed with a single tap. For example, soil volume can be automatically calculated from multiple points measured by LRTK, or separation between two points can be instantly computed from their coordinates on the smartphone. This allows as-built calculations and inspection tasks that used to be performed back at the office with CAD software to be completed on site, speeding up report creation.

By immediately sharing on-site positioning data to the cloud and performing analysis and visualization, the entire business process can be transformed. For example, surveying results that used to be recorded in field notebooks and digitized in the office can now be plotted on a cloud map and shared with stakeholders the moment they are measured using LRTK. Construction managers can check on-site measurement results in real time from the office and issue instructions. Also, in periodic infrastructure inspections, accumulating photos with high-precision position information makes it possible to accurately track changes over time. Thus, the introduction of high-precision GPS positioning supports not only operational efficiency improvements but also the core of construction DX, such as on-site and office data integration and faster decision-making.

Changes in the field seen from leading examples (municipalities and private sector)

The introduction of high-precision GPS and LRTK has already produced benefits in several advanced use cases. For example, one municipality used LRTK for disaster response. In earthquake-affected areas where communication infrastructure can be disrupted, LRTK terminals were useful because they can position using Michibiki’s augmentation signals even outside of Internet coverage. When investigators took photos of damage with LRTK-equipped smartphones, the photos recorded position coordinates and shooting direction with centimeter-level accuracy (half-inch accuracy). Uploading these to the cloud and comparing them with past data enabled immediate, accurate assessment of ground subsidence and the locations of collapsed structures before and after the quake. This is a good example of how small terminals contributed to rapid information sharing on disaster sites where bringing in large equipment was impractical.

In private construction sites, adoption of devices like LRTK is changing workflows. One civil engineering contractor had survey teams of one to two people move from site to site, causing delays while waiting for surveying. When site supervisors and foremen each began carrying an LRTK unit, an operation model of “measure yourself when you want to measure” became possible. For example, checking the as-built condition of one’s area first thing in the morning and immediately sharing it to the cloud for quality managers to review became routine, accelerating daily PDCA cycles. In another case, during the construction of a solar power plant, LRTK was combined with drone surveying to supplement point cloud data that drones could not capture due to tree shadows, enabling completion of a precise terrain model without additional ground surveys. Such examples show that new workflows combining high-precision positioning plus alpha are emerging, dramatically improving site productivity and data accuracy.

Municipalities are also applying the technology to maintenance management. When city and town staff conduct road and bridge inspections, they sometimes keep ledgers of photos with position information obtained by LRTK and use them for repair planning. By enabling staff to perform surveying themselves instead of outsourcing to specialists, these municipalities reported not only cost reductions but also improvements in staff DX skills. These cases demonstrate the significant changes that high-precision GPS positioning technology brings to the field.

Challenges and countermeasures when introducing (out of coverage, IT literacy, integration with existing equipment)

While high-precision GPS positioning is convenient, several challenges should be considered when introducing it. The challenges and countermeasures are organized below.

• Use outside communication coverage: RTK positioning that receives correction information via the Internet cannot be used in environments without mobile signals, such as mountainous areas or underground. A countermeasure is to utilize systems that obtain correction information via satellite, such as Michibiki’s CLAS. Devices like LRTK that support satellite augmentation can continue high-precision positioning even outside communication coverage. In some cases, temporarily setting up a simple local base station on site to broadcast corrections via radio may also be considered.

• IT literacy of site staff: New positioning devices that use smartphones and the cloud may present a hurdle for older workers or people not comfortable with devices. To address this, intuitive and simple app designs and training/support during introduction are important. The LRTK app, for example, is designed with a user-friendly UI that allows positioning and saving with a single button, making it easier to learn on site. Starting with pilot introductions at a few sites to increase the number of users and having those users teach others is also effective.

• Integration with existing equipment and systems: If data from a new positioning device cannot be used with CAD software or construction management systems already in use, it defeats the purpose. Ensuring data compatibility is a countermeasure. Specifically, providing output functions that export positioning results in CSV format or in standard coordinate systems used by the Geospatial Information Authority of Japan (such as plane rectangular coordinates or geoid heights) enables easy integration with existing drawings and other surveying instruments’ data. In fact, LRTK’s cloud service supports export in SIMA and CSV formats, facilitating smooth import into third-party CAD software and BIM platforms. For advanced integration such as matching machine control data for heavy equipment, future API provision or custom development is expected to advance support.

Finally: LRTK as a new option for simple surveying

In promoting DX in the construction industry, on-site high-precision GPS positioning is becoming an indispensable element. Advances in technologies like RTK and satellite augmentation have turned centimeter-level positioning, once only handled by experts, into an everyday tool accessible to anyone. A symbol of this trend is the solution introduced in this article, LRTK. Lightweight and easy to use when attached to a smartphone, LRTK offers a new option as a simple surveying instrument for “one person, one device.” This changes site work that used to rely heavily on specialist surveying departments, enabling construction managers and technicians to collect data and make decisions more quickly.

Of course, introduction comes with challenges such as site familiarity and integration with existing workflows, but these issues are gradually being resolved. Above all, voices from the field saying “I want to try it” and “it was useful” testify to the value of high-precision GPS positioning. As accuracy and functionality continue to improve and prices become more affordable, it may not be long before we see the commonplace sight of “craftsmen carrying GPS terminals in their hip pouches.” As a new technology supporting construction DX, simple surveying using high-precision GPS terminals and cloud utilization is likely to become a standard on future sites. Why not consider introducing such tools as the next step at your site?