Low Cost, Improved Safety and Efficiency! CLAS Expands the Scope of High-Precision Positioning Applications

In recent years, demand for "high-precision positioning" using positioning satellites has been rapidly increasing. Technologies that can determine positions to the centimeter level—such as for autonomous driving, drones, and ICT construction (information-based construction)—directly contribute to improved safety and increased work efficiency. However, conventional high-precision positioning required expensive equipment, specialized knowledge, and the establishment of communication infrastructure, creating barriers to widespread adoption at actual sites. Attracting attention in this context is CLAS (Centimeter-Level Positioning Augmentation Service). By offering low-cost use with stable accuracy, CLAS is poised to dramatically expand the range of applications for high-precision positioning. This article provides a detailed explanation of how CLAS works, its features, how it differs from conventional technologies, and examples of its use across various fields.

What is CLAS (Centimeter-Level Positioning Augmentation Service)?

CLAS (Cirrus) is a centimeter-level high-precision positioning service provided by the Quasi-Zenith Satellite System "Michibiki" ([QZSS official site](https://qzss.go.jp/overview/services/sv06_clas.html)). As its name implies, by receiving augmentation signals from the satellites, it can correct GNSS positioning errors to within a few centimeters.

The main features of CLAS can be summarized as follows.

• Centimeter-level high precision: Errors are reduced to the order of a few centimeters, representing a dramatic improvement in accuracy compared to conventional standalone GPS positioning (errors of 5–10 m).

• No communication required: Because correction information is received directly from satellites, mobile networks or radios are not required.

• Wide-area service: A satellite-delivered service covering all of Japan with few geographic restrictions (usable even in mountainous areas and remote islands).

• Free to use: A public service provided by the government with no usage fees (only the preparation of a dedicated receiver is necessary).

In CLAS, observation data obtained from the Geospatial Information Authority of Japan (GSI)’s network of Continuously Operating Reference Stations are used to calculate satellite positioning error information (such as orbital errors, clock errors, and ionospheric errors), which is then broadcast via L6-band signals from the "Michibiki" satellites passing over Japan. With only a dedicated receiver, you can continuously receive correction information even without a communications line, enabling high-precision real-time positioning.

The service area covers the Asia–Oceania region centered on Japan, and within Japan it is available in virtually the entire country. It is currently operated by four "Michibiki" satellites, and is expected to expand to a seven-satellite configuration around 2025. Increasing the number of satellites will allow augmentation signals to be received more stably for longer periods, which is expected to further improve accuracy and convenience. In the future, a system in which positioning can be completed using only "Michibiki" is also being considered, and with the development of CLAS the usability of high-precision positioning will continue to improve.

Differences from RTK and SLAS

In addition to CLAS, methods for achieving high-precision positioning include RTK (Real-Time Kinematic) and SLAS (Sub-meter-level Positioning Augmentation Service). Because each has different characteristics, it's important to note how they differ from CLAS.

• RTK method: RTK is a technique that obtains centimeter-level accuracy in real time through relative positioning with a base station installed on the ground (a reference GNSS receiver). The mobile unit (rover) receives correction data from the base station via radio or the Internet and computes its position with high precision. While RTK can achieve very high accuracy—about 2–3 cm horizontally—it incurs costs for installing and maintaining the base station and securing communication lines. Also, because accuracy degrades as the distance from the base station increases, using RTK over wide areas often requires subscribing to networked RTK that uses the cellular network (e.g., VRS), making communication infrastructure essential.

• SLAS方式: SLAS is called the "sub-meter level positioning augmentation service" and is likewise an augmentation service provided by "Michibiki"; this one aims to limit errors to on the order of several tens of centimeters to about 1 m. It transmits signals on the L1 band equivalent to aviation navigation support (SBAS), and is a service for aircraft and general in-car navigation systems. A dedicated receiver is not required and it can be used relatively easily, but the positioning accuracy is insufficient for surveying and construction applications that require centimeter-level accuracy.

Against this backdrop, CLAS’s major strength is that it can achieve accuracy approaching RTK—horizontal accuracy of a few centimeters when stationary and around 10 cm even when moving—without requiring the peripheral equipment that RTK needs. It is slightly inferior to RTK in terms of real-time performance and accuracy (since it is satellite-based, correction signal delays of several to a dozen seconds can occur), but its flexibility—unconstrained by communication environments or service areas—and low cost are highly attractive. By using RTK and CLAS selectively according to the application, high-precision positioning can be utilized efficiently. On the other hand, because SLAS serves a purpose that differs significantly from CLAS in terms of accuracy, it should be clearly distinguished and considered according to the intended use.

Effective Even in Areas Without Communication Infrastructure

One of CLAS's biggest advantages is that it enables high-precision positioning even outside communication coverage. Even in areas where mobile phone signals do not reach—such as mountainous regions or remote islands—or in situations where infrastructure has been damaged and communications have been severed, a CLAS-compatible GNSS receiver can achieve centimeter-level positioning as long as it has a clear view of the sky. Traditionally, performing high-precision positioning in such areas required either establishing your own reference station or post-processing positioning results (PPK or static surveying). With the advent of CLAS, it has become possible to obtain high precision in real time with standalone positioning, making positioning that is not dependent on communication environments a practical option.

For example, whether surveying deep in dense forests or carrying out agricultural work in newly developed areas where ground infrastructure is undeveloped, CLAS can receive correction information stably. Also, even if the cellular network is down during a large-scale disaster, if disaster response teams carry CLAS-compatible equipment they can immediately begin assessing the situation and conducting surveys in the affected area. This is a major advance that opens up an era of high-precision positioning that can be used "anywhere," and CLAS demonstrates its power even at sites where ensuring accuracy was previously difficult, such as forestry in remote mountain regions and infrastructure management on outlying islands.

Applications in Agriculture – Autonomous Tractors and Precision Agriculture

Agriculture is one of the sectors that stands to benefit greatly from CLAS. To operate tractors and combines autonomously and accurately across vast fields, centimeter-level positioning accuracy is essential. In recent years agricultural machines equipped with automatic steering systems have appeared, but high-precision autonomous operation has traditionally relied on RTK. With RTK, you must either install your own base station or subscribe to a paid correction information service, making the effort and cost of establishing communication infrastructure for each farm a challenge. In that respect, if a CLAS-compatible receiver is installed, even without connecting to the Internet the correction signals from satellites alone can provide the accuracy required for autonomous operation. Demonstration experiments have reported that agricultural machines were able to follow almost ideal straight paths using only CLAS, supporting smart agriculture in regions with limited communication environments.

High-precision positioning is also useful for autonomous pesticide and fertilizer spraying drones. Even when flying over fields spanning several hectares, CLAS enables them to fly the exact same trajectory each time and spray pinpointedly where needed. GNSS alone can be off by several meters, which caused problems like overlapping sprays and uneven coverage, but using CLAS reduces such losses, saving materials and reducing environmental impact. In addition, remote operation that monitors and controls multiple agricultural machines by a single person can be carried out safely if each machine’s position can be determined accurately. Thanks to CLAS, the era in which autonomous tractors and agricultural robots can be operated efficiently by a single operator is becoming a reality. In agriculture, expectations for high-precision positioning technology are rising as a trump card for managing vast land with a small workforce.

Applications in the Surveying Field – Boundary Restoration and As-Built Management

CLAS is bringing major changes to the world of surveying. In boundary restoration, when land boundary markers are lost, and in as-built management, where structures and terrain are measured after construction is completed, being able to position at centimeter-level accuracy is extremely important. Traditionally, these tasks required detailed surveys using total stations, or, for GNSS, RTK surveying with established reference points. However, by using CLAS-compatible GNSS, a person can go to the site alone and immediately obtain high-precision coordinates on the spot. For example, when confirming the boundaries of farmland or forest, as long as the reference coordinate values are known in advance, a single surveyor carrying a CLAS-compatible receiver can simply trace the boundary points and accurately confirm positions.

Even in as-built management, many points must be measured to compare the design drawings with the as-built conditions, but CLAS can greatly reduce the surveying team's workload. CLAS also proves useful for position measurement in the maintenance of roads and bridges within municipal infrastructure management, where personnel are limited. Even simple on-site condition measurements that were once outsourced to specialized surveying contractors can be performed by the responsible staff themselves using CLAS-compatible equipment and a smartphone, allowing them to obtain the necessary data in a short time. This enables cost reduction and improved responsiveness, making it possible to respond quickly to emergency inspections and on-site verification during disasters. CLAS makes surveying more accessible and familiar, and an era is approaching in which measurements can be taken "anytime, anywhere" at various sites.

Applications on construction sites – setting out and ICT-based construction

In the construction sector, advances in positioning technology greatly influence construction efficiency and quality. In building and road construction, accurately transferring design positions from drawings to the field—known as setting out (staking/pile driving and installation of batter boards, etc.)—is indispensable. Traditionally, surveyors would operate a total station with multiple people, guiding a prism to determine points. By using CLAS-compatible GNSS equipment, one person can set points. If design coordinate data are preloaded into the GNSS terminal, a worker carrying the receiver on site can be guided to the specified position and carry out staking or marking on the spot. Because no base station is required and a wide area can be covered, positioning work can be carried out continuously while moving, even on large-scale development sites or long-distance roadworks.

In the increasingly prominent field of (smart construction), high-precision positioning is also essential. In "machine control," where GNSS-equipped hydraulic excavators and bulldozers automatically adjust blade height based on 3D design data, accurate knowledge of current height and position is required. With CLAS-compatible heavy machinery, stable, high-precision self-positioning can be obtained even at sites with unstable communication environments, reducing work interruptions. CLAS can also be used for earthwork volume measurement and construction management via drone aerial photography. Photos taken by GNSS-equipped drones achieve improved positional accuracy with CLAS, which in turn increases accuracy when generating 3D models. CLAS also shows its strengths in construction management apps that use AR when overlaying design data onto live site camera footage. Accurate AR displays without positional drift enable on-the-spot, precise acceptance inspections and decisions on additional work. On construction sites, the introduction of high-precision positioning is becoming indispensable for carrying out work safely and efficiently.

Use in disaster investigations – 3D modeling of disaster-affected areas

During disasters such as earthquakes and landslides, it is essential to quickly and accurately assess the condition of affected areas. CLAS-enabled positioning proves valuable in such emergency field surveys. For example, when a disaster area is aerially photographed by drone immediately after an event and a 3D model is created, high-precision location information is crucial. By using CLAS, the drone’s capture positions and ground reference points can be recorded to centimeter-level accuracy, enabling the rapid generation of highly accurate 3D maps and orthophotos. This makes it possible to grasp the exact scale of collapse sites and the extent of damage in three dimensions, facilitating smoother restoration planning and the identification of hazardous zones.

Another major safety advantage is that it enables remote measurements without surveying teams having to enter disaster sites directly. By mounting a mobile device equipped with a CLAS receiver on a helicopter or robot and operating it around the site, positioning data can be obtained without people entering dangerous rubble. Furthermore, communications infrastructure may be paralyzed during disasters, but because CLAS operates without requiring communications, it can provide location information under any circumstances. In fact, there have been reports of large-scale disaster site surveys completed using only CLAS without relying on cellular communications. The use of CLAS in disaster response will become increasingly important going forward, as it directly contributes to ensuring human safety and speeding up initial response.

Use in Forestry – Resource Management and Boundary Identification

The management of vast forests and forestry operations is another field where CLAS is expected to be utilized. In surveys of forest resources, it is necessary to accurately determine tree position information and forest boundary lines, but in mountainous areas GNSS correction communications often do not reach. With CLAS-enabled GNSS, it is possible to receive correction signals from satellites even within forests and perform real-time boundary surveying. When planning forest road construction or selecting thinning zones, it is necessary to verify on site the boundaries of pre-established plots. By using CLAS, survey teams can obtain and plot boundary point coordinates on their own without spending long hours conducting traverse surveys in the mountains.

In forest management, regular resource monitoring is required, but labor shortages and rugged terrain create obstacles. If forestry personnel carrying compact GNSS receivers and tablet devices record positioning data while moving through mountainous terrain, they can efficiently update current maps of forest resources. Area measurements of logging sites and boundary verification can also keep errors to a minimum with the high accuracy of CLAS, preventing accidents such as unnecessary logging or boundary encroachment. Furthermore, detailed topographic surveys by CLAS contribute to assessing post-wildfire sites and to planning forest restoration after landslides. Deploying such high-precision positioning in the field has become indispensable for promoting DX (digital transformation) in forestry.

Possibility of Single-Person Operation through Smartphone Integration and Device Miniaturization

With the practical implementation of CLAS, GNSS receivers have also become smaller and lighter. Whereas GNSS equipment capable of centimeter-level positioning once relied mainly on large antennas and stationary receivers, there are now integrated-antenna receivers weighing only a few hundred grams and devices that can dock with smartphones. By using a smartphone or tablet as the display and control terminal, these new-generation GNSS receivers enable intuitive operation without the need to carry a dedicated controller. On site, it is becoming common to mount the receiver on a telescopic pole or monopod, connect it to a smartphone via Bluetooth, and have a single operator perform point measurements.

One-person surveying will contribute to alleviating labor shortages in the construction and surveying industries. Tasks that used to require two to three people on site can be completed by a single skilled operator, greatly reducing labor costs and improving work efficiency. In fact, solutions that combine a smartphone camera or LiDAR with high-precision GNSS so that anyone can perform 3D surveying have been developed. Together with CLAS’s ability to be used anywhere without communications, a future in which simply carrying positioning equipment while walking completes the necessary data collection is becoming a reality. In the coming era, positioning and measurement workflows that can be completed without relying on special infrastructure or large-scale equipment—using only a smartphone and a compact GNSS receiver—will become the new standard.

Summary – CLAS Opens a New Era of High-Precision Positioning

With the advent of CLAS, high-precision positioning has become a more accessible and user-friendly technology than ever before. The combination of convenience that does not rely on communication infrastructure and centimeter-level accuracy enhances operational safety and efficiency across agriculture, surveying, construction, disaster prevention, forestry, and other fields. Tasks that once depended on manual labor and experience can now be automated and streamlined based on accurate data, leading to reductions in mistakes and accidents.

As a solution to make centimeter-level positioning even more accessible, LRTK has emerged. LRTK is an ultra-compact GNSS receiver compatible with CLAS, and when used in combination with a smartphone it enables single-person centimeter-level positioning even in locations without a communication network. In fact, LRTK is featured as a CLAS-compatible product on the Cabinet Office’s Quasi-Zenith Satellite System official website, drawing attention to its usefulness. The convenience of being able to start high-precision positioning immediately with just a smartphone has led to its adoption across a wide range of tasks, from simple surveying to situational awareness in emergencies.

Thanks to CLAS, which achieves low cost and improved safety and efficiency, and the spread of compatible devices, the range of applications for high-precision positioning will continue to expand. Why not take advantage of next-generation positioning technology at your site as well?