High-precision GNSS equipment that can measure positions accurately is indispensable at construction and surveying sites. However, in areas outside communication coverage such as mountainous regions, or when communication infrastructure is down due to large-scale disasters, conventional real-time positioning has been difficult. The CLAS-compatible receiver (LRTK) that has emerged is a reliable ally on-site, enabling centimeter-level positioning even without communication coverage. By leveraging CLAS, the cutting-edge positioning service provided by the Quasi-Zenith Satellite System "Michibiki" (QZSS), this technology allows precise positioning anywhere without base stations or network lines, and it is now poised to transform construction and surveying workflows.

This article explains in detail the benefits of CLAS-compatible receivers (especially LRTK), the technical overview of CLAS, and the differences from conventional RTK. It also introduces specific scenes where they shine without communication (mountainous areas, disaster response, infrastructure maintenance, etc.), the latest use cases in conjunction with smartphones (iPhone), and applications to AR technology and point-cloud scanning. At the end of the article we summarize the advantages of simple surveying with LRTK and answer frequently asked questions in a Q&A format.

Table of Contents

• What is a CLAS-compatible receiver?

• Conventional RTK positioning methods and challenges

• Benefits of introducing CLAS-compatible receivers

• Scenes where they excel outside coverage

• Convenience expanded by smartphone (iPhone) integration

• Applications to AR technology and point-cloud scanning

• Summary

• FAQ (Frequently Asked Questions)

What is a CLAS-compatible receiver?

CLAS (Centimeter Level Augmentation Service) is a high-precision positioning service provided by Japan’s Quasi-Zenith Satellite System "Michibiki" (QZSS). Standalone GNSS positioning such as conventional GPS inevitably yields errors on the order of meters, but by using CLAS you can measure positions with dramatically higher accuracy—errors within a few centimeters. A major feature is that high precision can be achieved without relying on base stations or Internet communication. CLAS correction information is distributed directly from the Michibiki satellites across Japan, so as long as you have a clear view of the sky, stable centimeter-level positioning is possible even in mountainous areas, remote islands, and offshore. This service is available to anyone, and by simply preparing a CLAS-compatible GNSS receiver you can begin high-precision positioning with no additional communication fees.

LRTK is a next-generation GNSS receiver that supports CLAS. Technically, it uses a positioning method called PPP-RTK and receives correction data via the CLAS satellite signals based on error information collected from networks such as the Geospatial Information Authority of Japan’s GEONET (satellite orbit and clock errors, ionospheric delays, etc.). The correction data are delivered to the receiver in real time, enabling position correction. Users do not need to be aware of the complex mechanisms; by simply powering on a CLAS-compatible receiver on site, the satellites automatically deliver the correction information, allowing users to complete centimeter-level positioning with a single standalone receiver.

Conventional RTK positioning methods and challenges

A commonly used high-precision GNSS method has been RTK positioning (Real-Time Kinematic). In RTK, a known-accurate coordinate base station (reference) and a mobile rover receive satellite signals simultaneously; by comparing their observations to correct errors, centimeter-level accuracy is achieved. Standalone positioning errors of about 5–10 m can be reduced to a few centimeters, with horizontal accuracy typically around 2–3 cm (about 5 cm vertically), which has made RTK a mainstay in surveying and construction management. The time to obtain a high-precision solution known as a fixed solution is also quick—on the order of seconds—so RTK became established as a method that provides immediate high precision.

However, conventional RTK has several challenges. The biggest is that it requires continuously receiving correction information from a base station in real time. RTK cannot function where base station radio or data communications cannot be received. Consequently, two main operational approaches have typically been used:

• Local base station method (standalone RTK): The user sets up their own GNSS receiver as a base station near the survey site and continuously transmits correction data to the rover via radio (e.g., low-power radio). This method is simple and one-to-one, but preparing and installing base station equipment is time-consuming, and coverage of the base station radio is limited (typically a few km up to about 10 km). Accuracy also degrades with distance from the base station, and beyond about 10 km errors become significant.

• Network RTK method (VRS/Ntrip): This uses a reference station network (e.g., continuously operating reference stations) operated by national or private entities and receives correction information over the Internet. The user equips the rover with a communication modem and obtains virtual reference station (VRS) corrections via an Ntrip client. This method eliminates the need to set up your own base station and largely removes accuracy degradation with distance from reference stations. However, service fees are typically charged monthly or annually, and the method cannot be used where mobile communications are out of range.

Because base station installation or communication infrastructure were indispensable, there were many situations—remote mountain areas, places out of radio range, and large-scale disasters where base station equipment or communication networks were down—where real-time centimeter-level positioning had to be abandoned. In such cases, teams often had to forgo on-site positioning and bring data back for post-processing (PPK: Post-Processed Kinematic), adding time and effort. Additionally, the cost of base station equipment and communication fees presented economic barriers to RTK adoption.

Benefits of introducing CLAS-compatible receivers

CLAS-compatible receivers address these conventional issues in one stroke. Using a receiver like LRTK that leverages CLAS, you do not need to provide your own base station nor receive correction data via mobile networks. Because corrections arrive directly from satellites, uniform high-precision positioning is possible across Japan—from mountainous regions to remote islands and offshore—so long as the CLAS signal from the satellites can be received. There’s no need to worry about distance from a base station or to search for radio or network coverage, and centimeter-level accuracy can be maintained consistently across wide-area surveys and tasks that involve movement. The labor of setting up base station equipment or preparing communication environments is eliminated, and the convenience of being able to start real-time positioning simply by powering on the device on arrival is a major advantage.

Moreover, real-time positioning using a CLAS-compatible receiver completes everything on site, allowing immediate use of measurement results. For example, you can check coordinate values on the spot for stake-out or quality inspections, or immediately share acquired point data to the cloud for collaboration—enabling a fast workflow. Reducing the need to return to the office for post-processing is a significant advantage in urgent disaster response situations.

Key expected benefits from introducing CLAS-compatible receivers include:

• No base station or communication lines required: Positioning is completed by bringing a standalone receiver to the site. Centimeter-level positioning is possible in mountainous or out-of-coverage areas as long as the CLAS signal from satellites can be received.

• Stable accuracy across Japan: Corrections via Michibiki provide uniform accuracy nationwide without being affected by distance from a base station. You can maintain the same accuracy across multiple distant sites simply by transporting the device.

• Obtain results in real time: Measurement and result confirmation are completed on-site, allowing immediate incorporation of survey results into construction or decisions. Real-time position acquisition speeds up on-site decision-making.

• Reduced cost and effort: No need to purchase, install base station equipment, or subscribe to communication services, which reduces expenses. The simplicity of starting positioning by powering on the device makes it easy enough for non-specialist staff to operate after short training.

Scenes where they excel outside coverage

CLAS-compatible receivers demonstrate their true value in environments where communication does not reach. In scenes where positioning would previously have had to be abandoned, LRTK and other CLAS-compatible receivers can provide centimeter accuracy:

• Construction sites in remote mountainous areas: Road construction, dam, and tunnel sites in mountainous terrain are often out of mobile phone range. Bringing a CLAS-compatible receiver to such sites allows acquisition of high-precision coordinates on-site without establishing reference points or providing radio repeaters. Even when surveyors cannot be dispatched, local workers can perform positioning, immediately record and share data, and thus improve efficiency in remote construction management.

• Disaster response and recovery work: In disaster sites such as areas affected by earthquakes or heavy rain causing landslides, rapid surveying to grasp damage is important. However, communication infrastructure may be paralyzed or base stations may be without power. With a CLAS-compatible receiver, provided satellites can be tracked, terrain surveying and coordinate recording at damaged sites can proceed. There are cases where local governments used LRTK with smartphones for emergency surveys immediately after disasters, aiding early recovery and cost reduction. A positioning system not dependent on communications is a powerful asset for disaster prevention and mitigation.

• Infrastructure maintenance: High-precision positioning is essential for inspection of roads, bridges, dams, and other infrastructure in mountainous or coastal areas. For example, monitoring points on slopes in mountains or checking subsidence at port facilities on remote islands can be easily measured with a CLAS-compatible receiver. Sites that previously required dispatching a survey team can now be measured by on-site personnel bringing a CLAS-compatible receiver and directly importing the data into GIS. Positioning technology that is not affected by communication networks contributes to greater efficiency in large-scale infrastructure management.

Convenience expanded by smartphone (iPhone) integration

CLAS-compatible receivers like LRTK become even more usable and versatile when integrated with smartphones and tablets. By connecting the receiver to a smartphone via Bluetooth or similar means through a dedicated app, the high-precision position data can be displayed in real time on the phone. You can view your position on a map on an iPhone screen and save and manage measured point coordinates, making the operation more intuitive than traditional surveying equipment.

Combining smartphone sensors and cameras enables advanced uses beyond simple coordinate measurement. For example, attaching the receiver to an iPhone or iPad with a dedicated adapter allows one-handed operation while performing positioning. Using a dedicated monopod, the smartphone screen can automatically correct height offsets (pole length), enabling accurate single-point measurements as easily as with a professional surveying pole by one person. With compact, lightweight designs weighing under 200 g, carrying and walking around the site is not burdensome. They run on batteries for long continuous positioning, and can also be used while being powered via USB from a mobile battery, allowing operation with the same familiarity as everyday smartphone use.

Applications to AR technology and point-cloud scanning

Combining CLAS-compatible receivers with smartphones makes it easy to use advanced digital technologies on site, such as AR (augmented reality) and point-cloud scanning. High-precision position and attitude information enables precise outdoor AR displays and 3D scanning that were previously difficult.

• Improving on-site work efficiency with AR: AR apps now overlay design drawings or buried-object locations on real-world views through cameras like those on an iPhone. Previously, accurate outdoor AR required placing ground markers or manual alignment. But by combining centimeter-level position information from a CLAS-compatible receiver with precise heading information from a smartphone’s gyroscope, markerless AR alignment outdoors becomes possible. For example, 3D models of design lines or buried pipes can be visualized on the ground without offset, greatly reducing excavation errors and improving as-built checks. Applications include following AR guideline overlays safely and accurately even at night.

• Point-cloud scanning with a smartphone: Using LRTK and a smartphone, you can acquire point-cloud data on-site without dedicated 3D laser scanners. Scanning structures and terrain with an iPhone’s built-in LiDAR or camera, and correcting each point’s position with high-precision data from the LRTK receiver, produces point-cloud models aligned to an absolute coordinate system. Obtained point clouds can be immediately overlaid on existing topographic maps or CAD drawings for comparison with design. For example, for slope deformation measurement over 100 m, you can walk while holding up a smartphone to obtain dense point clouds that reveal surface irregularities and erosion in detail. Cross-sections can be extracted and volumes automatically calculated from the point cloud on-site, reducing reliance on specialist contractors for measurement and analysis. Sharing acquired data via the cloud enables remote experts to review results in real time.

Summary

A CLAS-compatible receiver that delivers centimeter-level positioning regardless of base stations or communication environments is truly a strong ally on site. Devices like LRTK dramatically improve efficiency and accuracy across many construction and surveying scenarios. They provide reliable real-time position information even in challenging environments such as mountainous construction sites and disaster response. Integration with smartphones enables one-person surveys, AR-based design verification, and point-cloud scanning to document as-built conditions, accelerating on-site digital transformation.

Most importantly, the advantage of simple surveying with LRTK is that even non-specialist personnel can achieve accurate positioning after short training. If tasks previously outsourced to external surveyors can be handled in-house, this brings not only cost savings but also faster turnaround. The era in which anyone can instantly acquire and share position information using a pocket-sized receiver and a smartphone is imminent, and CLAS-compatible receivers like LRTK are key enablers. As tools supporting on-site DX (digital transformation), their role is expected to grow.

FAQ (Frequently Asked Questions)

Q1. What is a CLAS-compatible receiver (LRTK)? A1. A CLAS-compatible receiver is a high-precision GNSS device that can receive the Centimeter Level Augmentation Service (CLAS) distributed by the Quasi-Zenith Satellite System "Michibiki." LRTK is one such device; it is characterized by its ability to achieve centimeter-level positioning standalone without relying on base stations or communications.

Q2. Can it really position without a communication environment? A2. Yes. CLAS-compatible receivers can receive correction signals directly from satellites even outside mobile network coverage, enabling centimeter-level positioning in mountainous or remote island sites. However, GNSS satellite signals must be receivable, so the system cannot be used indoors or inside tunnels where the sky is not visible (positioning resumes once you move to a location with satellite visibility).

Q3. What is the positioning accuracy and does initial convergence take time? A3. CLAS-compatible receivers including LRTK can achieve horizontal accuracy on the order of a few centimeters. In practical terms, this is comparable to conventional RTK and is sufficient for many surveying and construction tasks. However, it can take some time from power-up to reach centimeter-level accuracy (a fixed solution). Convergence typically takes approximately 30 seconds to 1 minute; during that time you may have a float solution with errors on the order of tens of centimeters. Once a fix is obtained, high precision remains stable.

Q4. How are they used with smartphones? Can they be used with iPhone? A4. Yes, they can be used with an iPhone. LRTK receivers connect to smartphones via Bluetooth or similar, and are operated through a dedicated app. The app allows you to view real-time coordinates, record points and names, and load drawing data. Using the phone’s map and camera display makes operation more intuitive than traditional surveying equipment. For example, your real-time position is displayed on a map, making point measurement and navigation easy.

Q5. Can anyone use them? Are special skills or qualifications required? A5. Basic operations are done through a smartphone app, so they are much easier to use than specialized surveying equipment. Some GIS or surveying knowledge is helpful, but with a brief orientation site workers can operate them effectively. Advanced calculations are handled automatically by the receiver and app, so the user only needs to follow on-screen prompts to acquire and check points. Certain official surveying tasks may still require professional qualifications, but for routine as-built checks or preliminary surveys, qualifications are not necessarily required.

Q6. What applications and sites are they suitable for? A6. In construction, they are used for as-built measurements of roads and earthworks, stake-out for foundation piling, and guidance assistance for heavy machinery. In infrastructure inspection, they are used for displacement measurement of bridges and tunnels, damage mapping at disaster sites, and in agriculture for field surveying and correction for autonomous tractors. Because they do not depend on communications, they are especially suitable for mountain construction, work on remote islands, and emergency surveys immediately after disasters—any site where consistent precision is needed anywhere, anytime.

Q7. Are there running costs or service fees? A7. CLAS itself is free to use. The Michibiki CLAS signal is provided as a public service, and once you purchase a compatible receiver there are no additional fees to receive correction information. However, you will need to bear the initial cost of the receiver and any smartphone apps. For devices like LRTK, only the purchase cost is required—there are no ongoing monthly subscriptions—so running costs can be much lower than network RTK services.