Boost Safety and Efficiency at Low Cost! CLAS Expands the Range 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 (half-inch accuracy)—such as for autonomous driving, drones, and ICT construction (information-based construction)—directly contribute to improved safety and operational efficiency. However, conventional high-precision positioning required expensive equipment, specialized knowledge, and the establishment of communication environments, creating barriers to its widespread deployment in actual field settings. One solution attracting attention is CLAS (Centimeter-Level Positioning Augmentation Service). CLAS, which can be used at low cost and with stable accuracy, is poised to dramatically expand the range of high-precision positioning applications. In this article, we will explain in detail how CLAS works, its characteristics, how it differs from conventional technologies, and various use cases across different fields.

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

CLAS (Shirasu) 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 the name suggests, by receiving augmentation signals from satellites it can correct GNSS positioning errors to within a few centimeters.

The main features of CLAS can be summarized as follows.

High accuracy of several centimeters (a few in): Errors can be reduced to several centimeters (a few in), providing a dramatic improvement in accuracy over conventional GPS standalone positioning (errors of 5–10 m (16.4–32.8 ft)).

No communication required: Since augmentation information is received directly from satellites, mobile networks or radios are not necessary.

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

Available free of charge: A public service provided by the government with no usage fees (only preparation of a dedicated receiver is required).

This CLAS calculates satellite positioning error information (such as orbit errors, clock errors, and ionospheric errors) based on observation data obtained from the Geospatial Information Authority of Japan's network of reference stations, and broadcasts it via L6-band radio signals from the "Michibiki" satellites passing over Japan. As long as a dedicated receiver is provided, correction information can always be received even without a communication line, enabling real-time high-precision positioning.

The service coverage area is the Asia–Oceania region centered on Japan, and within Japan it is available in almost the entire country. It is currently operated by four "Michibiki" satellites, and is scheduled to move to a seven-satellite configuration by around 2025. With an increased number of satellites, augmentation signals can be received more stably for longer periods, and further improvements in accuracy and convenience are expected. 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

Other methods for achieving high-precision positioning, in addition to CLAS, include RTK (Real-Time Kinematic) and SLAS (Sub-meter-class Positioning Augmentation Service). Since each has different characteristics, be sure to understand how they differ from CLAS.

• RTK method: RTK is a technique that achieves centimeter-level accuracy (half-inch accuracy) in real time by relative positioning with a base station (a reference GNSS receiver) installed on the ground. The mobile unit (rover) receives correction data from the base station via radio or the internet and calculates its own position with high precision. RTK can provide very high accuracy—about 2–3 cm (0.8–1.2 in) horizontally—but there are costs for installing and maintaining the base station and securing communication links. Also, because accuracy degrades as the distance from the base station increases, using RTK over wide areas often requires a subscription to a networked RTK service 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 another augmentation service provided by "Michibiki"; it aims to limit errors to several tens of centimeters to about 1 m (several tens of inches to 3.3 ft). It transmits signals on the L1 band equivalent to aviation navigation support (SBAS), and is a service for aircraft and general car navigation systems. A dedicated receiver is unnecessary and it is relatively easy to use, but the positioning accuracy is insufficient for surveying and construction applications that require accuracy of a few centimeters (a few inches).

Against this backdrop, CLAS’s major strength is that it can achieve accuracy approaching RTK without requiring as much ancillary equipment as RTK (horizontal accuracy of several centimeters (a few inches) when stationary, and around 10 cm (around 3.9 in) even when moving). Compared with RTK, it is slightly inferior in real-time performance and accuracy (because corrections are sent via satellite, there may be correction signal delays of several to tens of seconds), but its flexibility without constraints on communication environment or service area and its low cost are very attractive. By using RTK and CLAS according to the application, high-precision positioning can be utilized efficiently. On the other hand, SLAS is a service with a purpose that differs greatly from CLAS in terms of accuracy, so it needs to be clearly distinguished and considered according to the intended use.

Delivers powerful performance even in areas without established communications infrastructure

One of CLAS’s greatest advantages is that it enables high-precision positioning even outside communication coverage. Even in mountainous areas or remote islands where mobile phone signals do not reach, or in situations where infrastructure has been damaged and communications are cut off, a CLAS-enabled GNSS receiver can achieve centimeter-level positioning as long as it has a clear view of the sky. Traditionally, to perform high-precision positioning in such areas, you had to either set up your own reference station or post-process the positioning results (PPK or static surveying). With the advent of CLAS, high precision can now be obtained in real time by standalone positioning, making positioning that is not dependent on the communications environment a practical option.

For example, even for surveying work in deep mountain forests or agricultural work in newly developed areas where ground infrastructure is undeveloped, CLAS can reliably receive correction information. Also, even if the cellular network is down during a large-scale disaster, if disaster response teams carry CLAS-compatible devices they can immediately begin assessing conditions and conducting surveys in the affected area. This is a major advance that opens up an era of “usable anywhere” high-precision positioning, and CLAS’s power is being demonstrated at sites where ensuring accuracy was previously difficult, such as forestry in remote mountainous areas and infrastructure management on remote islands.

Applications in Agriculture – Autonomous Tractors and Precision Agriculture

Agriculture is one of the sectors that stands to gain greatly from CLAS. To enable tractors and combines to drive autonomously with precision across vast fields, centimeter-level positioning accuracy (half-inch accuracy) is indispensable. In recent years, agricultural machinery equipped with automatic steering systems has appeared, but conventional high-precision autonomous driving has been premised on the use of RTK. With RTK, you need to either install your own base station or subscribe to a paid correction information service, and the effort and cost of establishing communication environments for each farm have been challenges. In that regard, if a receiver that supports CLAS is installed, it can secure the accuracy required for autonomous driving using only correction signals from satellites, without an Internet connection. Field trials have also reported that agricultural machines could follow nearly ideal straight paths using only CLAS, supporting smart agriculture in regions with constrained communication environments.

High-precision positioning is also useful for pesticide- and fertilizer-spraying drones. Even when flying over fields spanning several hectares, with CLAS they can follow the exact same trajectory every time and spray with pinpoint accuracy at the required locations. GNSS alone can have errors of several meters, causing overlapping applications and uneven coverage, but using CLAS reduces such losses, leading to savings in materials and reduced environmental impact. In addition, remote operation that monitors and controls multiple agricultural machines by a single person can also be carried out safely if each machine’s position can be determined accurately. Thanks to CLAS, the era when autonomous tractors and agricultural robots can be operated efficiently by one operator is becoming increasingly realistic. In the agricultural sector, expectations for high-precision positioning technology are rising as a trump card for managing vast tracts of land with a small workforce.

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

In the field of surveying, CLAS is also bringing about major changes. In boundary restoration when land boundary markers have been lost, and in as-built management measuring structures and terrain after construction completion, being able to position with centimeter-level accuracy (half-inch accuracy) is extremely important. Traditionally, these tasks required detailed surveys using a total station, or RTK surveys with established control points for GNSS. However, by using a CLAS-compatible GNSS, a surveyor can go to the site alone and instantly obtain high-precision coordinates on the spot. For example, when confirming boundaries of farmland or forest, if the reference coordinate values are known in advance, a single surveyor carrying a CLAS-compatible receiver can simply follow the boundary points to verify positions accurately.

In as-built management, many points must be measured to reconcile the design drawings with the as-built condition, but with CLAS the workload of surveying teams can be greatly reduced. Even in municipal infrastructure management where personnel are limited, CLAS is effective for positional measurements in road and bridge maintenance. Simple field surveys that were previously outsourced to specialist surveying contractors can be carried out by staff themselves using CLAS-compatible devices and a smartphone, allowing them to collect the required data in a short time. This achieves cost reduction and improved responsiveness, enabling rapid action for emergency inspections and on-site verification during disasters. CLAS makes surveying easier and more accessible, and an era is emerging in which measurements can be taken "anytime, anywhere" across a variety of sites.

Use at construction sites – setting out and ICT-based construction

In the construction sector, advances in positioning technology have a major impact on construction efficiency and quality. In building and road construction, accurately transferring the design positions from drawings to the field—positioning (stake driving, batter board installation, etc.)—is essential. Traditionally, surveyors commonly operated a total station with several people, guiding a prism to determine points. By using CLAS-compatible GNSS equipment, one person can set points. If the design coordinate data are preloaded into the GNSS terminal, a worker carrying the receiver on site can be guided to the instructed positions and perform stake driving or marking on the spot. Because no base station is required and a wide area can be covered, even on large-scale land development sites or long-distance road construction projects, positioning tasks can be carried out sequentially while moving.

High-precision positioning is also essential for ICT construction (smart construction), which has attracted attention in recent years. In "machine control," where hydraulic excavators and bulldozers equipped with GNSS automatically adjust blade height based on 3D design data, accurate knowledge of current height and position is required. With CLAS-compatible heavy machinery, high-precision self-positioning can be obtained stably even on sites with unstable communication environments, reducing work interruptions. CLAS can also be used for earthwork volume measurement and construction management using drone aerial photography. Photos captured by GNSS-equipped drones benefit from CLAS-improved positioning accuracy, which enhances the accuracy of 3D model generation. CLAS also proves powerful in construction-management apps that use AR, when overlaying design data onto live site camera footage. Accurate, alignment-error-free AR displays enable precise on-the-spot decisions for as-built inspections and additional work. At construction sites, the adoption of high-precision positioning is becoming indispensable for progressing construction safely and efficiently.

Use in Disaster Investigations – 3D Modeling of Affected Areas

During disasters such as earthquakes and landslides, it is essential to quickly and accurately assess the situation in affected areas. CLAS-compatible positioning proves valuable for on-site surveys in such emergencies. For example, when aerially photographing an affected area with drones immediately after a disaster and creating a 3D model, high-precision location information is important. By using CLAS, the drone’s capture positions and ground reference points can be recorded at the centimeter level (cm level accuracy, half-inch accuracy), enabling the rapid generation of highly accurate 3D maps and orthophotos. This allows the precise scale and extent of collapse sites to be understood three-dimensionally, smoothing the planning of recovery efforts and the identification of hazardous zones.

Another major safety advantage is that it enables measurements to be taken remotely without surveying teams having to enter the disaster site directly. If a mobile device equipped with a CLAS receiver is mounted on a helicopter or a robot and operated around the site, positioning data can be acquired without people entering dangerous rubble. Furthermore, communication infrastructure may be paralyzed during disasters, but because CLAS operates without needing communications, position information can be obtained under any circumstances. In fact, there have been reported cases in which surveys of large-scale disaster areas were completed using only CLAS without relying on mobile communications. The use of CLAS in disaster response will become increasingly important going forward, as it directly contributes to protecting lives and speeding up initial response.

Applications in forestry – resource management and boundary identification

The management of vast forests and forestry operations is also a field where the use of CLAS is expected. In forest resource surveys, it is necessary to accurately determine tree positions and forest boundaries, but in mountainous areas communications for GNSS corrections often cannot reach. With CLAS-compatible GNSS, it is possible to receive correction signals from satellites even within forests and perform real-time boundary surveying. When planning forest roads or selecting thinning areas, it is necessary to confirm on-site the boundaries of pre-established plots. By using CLAS, survey teams can obtain and plot the coordinates of boundary points on their own without spending long hours conducting traverse surveys in the mountains.

Forest management also requires regular resource monitoring, but labor shortages and rugged terrain can be obstacles. If forestry personnel carrying a compact GNSS receiver and a tablet record positioning data while moving through mountainous areas, they can efficiently update up-to-date maps of forest resources. Area measurements of logging sites and boundary checks can also be kept to a minimum of error with CLAS’s high precision, preventing incidents such as unnecessary logging or boundary encroachment. Furthermore, detailed topographic surveying by CLAS contributes to evaluating post-wildfire sites and to planning forest recovery after landslides. Deploying such high-precision positioning in the field has become indispensable for promoting DX (digital transformation) in forestry.

Potential for Single-Operator Operation through Smartphone Integration and Device Miniaturization

With the practical implementation of CLAS, GNSS receivers have also become smaller and lighter. GNSS equipment that once delivered centimeter-level positioning primarily relied on large antennas and stationary receivers. However, today antenna-integrated receivers weighing only a few hundred grams and devices that can dock with smartphones have emerged. 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 person perform point measurements.

One-person surveying can also help alleviate labor shortages in the construction and surveying industries. On-site work that used to require 2-3 people could be completed by a single skilled operator, greatly contributing to reduced labor costs and improved work efficiency. In fact, solutions have been developed that combine a smartphone’s camera or LiDAR with high-precision GNSS to allow anyone to perform 3D surveying. Combined with CLAS’s ability to operate anywhere without communications, a future in which necessary data collection is completed simply by carrying a positioning device and walking is becoming a reality. In the coming era, a positioning and measurement workflow that can be completed with only a smartphone and a small GNSS receiver will likely 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 the convenience of not relying on communications infrastructure and accuracy within a few centimeters improves the safety and efficiency of operations across every field, including agriculture, surveying, construction, disaster prevention, and forestry. Tasks that once relied on human labor and experience can now be automated and streamlined based on precise data, leading to reductions in mistakes and accidents.

Against this backdrop, a solution called LRTK has emerged to make centimeter-level positioning (half-inch accuracy) even more accessible. LRTK is an ultra-compact GNSS receiver compatible with CLAS that, when used in combination with a smartphone, enables a single user to achieve centimeter-level positioning (half-inch accuracy) even in locations without communication coverage. In fact, LRTK is introduced on the official website of the Cabinet Office’s Quasi-Zenith Satellite System as a CLAS-compatible product, and its usefulness is attracting attention. Because it is so easy to start high-precision positioning with just a smartphone, adoption is progressing across a wide range of tasks, such as simple surveying and situational assessment in emergencies.

Lower costs and improved safety and efficiency achieved by CLAS, together with the spread of compatible equipment, will continue to expand the range of applications for high-precision positioning. Why not try using next-generation positioning technology at your site as well?