Low Cost, Safer and More Efficient! How CLAS Expands the Applications of High-Precision Positioning

In recent years, demand for "high-precision positioning" using positioning satellites has grown rapidly. Technologies that can determine position to the centimeter level (cm) directly contribute to improved safety and operational efficiency in areas such as autonomous driving, drones, and ICT construction. However, conventional high-precision positioning has required expensive equipment, specialized knowledge, and communication infrastructure, creating barriers to widespread field adoption. At the center of attention is CLAS (Centimeter-Level Augmentation Service). With CLAS, which can be used at low cost and with stable accuracy, the range of applications for high-precision positioning is poised to expand dramatically. This article explains in detail how CLAS works, its characteristics, how it differs from conventional technologies, and various use cases.

What is CLAS (Centimeter-Level Augmentation Service)?

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

The main features of CLAS can be summarized as follows.

• High accuracy of a few cm: Errors are reduced to the order of several centimeters, a dramatic improvement compared with standalone GPS positioning, which has errors of about 5–10 m (16.4–32.8 ft).

• No communication required: Augmentation information is received directly from satellites, so cellular networks or radios are not necessary.

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

• Free to use: It is a public service provided by the government, so there is no usage fee (only the dedicated receiver needs to be prepared).

CLAS calculates satellite positioning error information (such as orbit errors, clock errors, and ionospheric errors) from observation data obtained from the Geospatial Information Authority of Japan’s network of continuously operating reference stations, and broadcasts that information from Michibiki satellites passing over Japan on the L6 band. With only a dedicated receiver, you can continuously receive correction information even without a communication line, enabling real-time high-precision positioning.

The service area covers the Asia–Oceania region centered on Japan, and within Japan it is available almost nationwide. It is currently operated by four Michibiki satellites, and is expected to expand to seven satellites around 2025. Increasing the number of satellites will allow augmentation signals to be received more stably for longer periods, further improving accuracy and convenience. In the future, a configuration in which positioning can be completed using Michibiki alone is also envisioned, and as CLAS develops, the usability of high-precision positioning will continue to improve.

Differences from RTK and SLAS

In addition to CLAS, other methods for achieving high-precision positioning include RTK (Real-Time Kinematic) and SLAS (Sub-Meter-Level Augmentation Service). Each has different characteristics, so it is important to understand how they differ from CLAS.

• RTK method: RTK obtains centimeter-level accuracy in real time by relative positioning with a reference station (a GNSS receiver installed on the ground). The rover receives correction data from the reference station via radio or the Internet and calculates its own position with high precision. RTK can achieve extremely high accuracy—roughly 2–3 cm (0.8–1.2 in) horizontally—but requires the installation and maintenance of reference stations and securing communication lines, which can be costly. Accuracy also degrades as distance from the base station increases, so using RTK over wide areas often requires subscription to a network RTK service that uses cellular networks (e.g., VRS), making communication infrastructure essential in many cases.

• SLAS method: SLAS, the "Sub-Meter-Level Augmentation Service," is another augmentation service provided by Michibiki, but it aims to limit errors to several tens of centimeters to around 1 m (several inches to 3.3 ft). It broadcasts SBAS-equivalent signals on the L1 band and is intended for aviation navigation support and general car navigation. It is relatively easy to use without a dedicated receiver, but its positioning accuracy is insufficient for surveying and construction applications that require centimeter-level precision.

Against this backdrop, CLAS stands out for delivering RTK-like accuracy—approaching centimeter-level accuracy (several centimeters when static, and around 10 cm (3.9 in) in motion)—without the peripheral infrastructure that RTK requires. CLAS may be slightly inferior to RTK in terms of real-time performance and precision (there can be delays of a few to several tens of seconds in correction signals due to satellite transmission), but its flexibility and low cost, with no constraints from communication environment or service area, are very attractive. By choosing RTK or CLAS according to the application, high-precision positioning can be utilized efficiently. SLAS, on the other hand, serves a very different purpose in terms of accuracy and should be distinguished from CLAS based on intended use.

Effective even in areas without communication infrastructure

One of CLAS’s greatest advantages is that it enables high-precision positioning even outside communication coverage. In mountainous areas, remote islands where cellular signals do not reach, or in disaster situations where infrastructure has been damaged and communications are cut, CLAS-compatible GNSS receivers can achieve centimeter-level positioning as long as the sky is visible. Previously, to perform high-precision positioning in such areas, one had to set up one’s own reference station or post-process positioning results (PPK or static surveying). With CLAS, real-time standalone positioning with high accuracy has become a realistic option, independent of communication environment.

For example, in deep forest surveys or agricultural work in newly developed areas without ground infrastructure, CLAS can consistently provide correction information. In large-scale disasters when cellular networks are down, disaster response teams carrying CLAS-compatible equipment can immediately begin surveying and assessing conditions on-site. This is a major advance toward a “usable anywhere” era of high-precision positioning, and CLAS demonstrates its power in forestry in remote mountain regions and infrastructure management on remote islands—places where achieving accuracy was previously difficult.

Agricultural applications – autonomous tractors and precision agriculture

Agriculture is one of the fields that stands to benefit greatly from CLAS. To have tractors and combines autonomously drive accurately across vast fields, centimeter-level position accuracy is essential. Although agricultural machinery with automatic steering systems has recently become available, RTK has traditionally been a prerequisite for high-precision autonomous operation. RTK requires either setting up a base station or subscribing to a paid correction information service, imposing the burden of establishing communication environments for each farm. By equipping machines with CLAS-compatible receivers, the necessary accuracy for autonomous operation can be achieved using only satellite correction signals, without connecting to the Internet. Field tests have reported that agricultural machines could follow nearly ideal straight paths using only CLAS, supporting smart agriculture in areas with limited communication infrastructure.

High-precision positioning is also useful for pesticide- and fertilizer-spraying drones. Even when flying over fields of several hectares, CLAS enables drones to follow identical flight paths precisely each time and spray pinpoint areas as required. Standalone GNSS can have meter-level offsets, causing overlap or gaps in spraying, but using CLAS reduces such losses, saving materials and reducing environmental impact. Additionally, remote operation of multiple agricultural machines by a single person becomes safe and feasible when each machine’s position can be accurately known. CLAS makes it realistic to operate autonomous tractors and agricultural robots efficiently with a single operator. In agriculture, expectations are rising for high-precision positioning as a trump card for managing large areas with limited personnel.

Surveying applications – boundary restoration and as-built management

CLAS is bringing major changes to surveying as well. Being able to position to the centimeter level is critically important for tasks such as boundary restoration when land markers are lost, and as-built management, which involves measuring structures and terrain after construction is completed. Traditionally, these tasks required detailed surveys using total stations or RTK surveys with established control points for GNSS. With CLAS-compatible GNSS, surveyors can go to the site alone and obtain high-precision coordinates on the spot. For example, in confirming boundaries of farmland or forest, if reference coordinate values are known in advance, a single surveyor carrying a CLAS-compatible receiver can trace boundary points and verify positions accurately.

In as-built management, many points must be measured to compare the design with the actual outcome, but CLAS can greatly reduce the manpower required. In municipalities with limited personnel, CLAS is useful for position measurements in road and bridge maintenance. Simple field surveys that previously had to be outsourced to specialist surveying contractors can now be performed by in-house staff carrying CLAS equipment and a smartphone, obtaining the necessary data in a short time. This enables both cost savings and faster response, allowing quick handling of emergency inspections and post-disaster field verification. CLAS makes surveying easier and more accessible, ushering in an era where measurements can be taken anytime, anywhere.

Construction site applications – stakeout and ICT construction

In construction, progress in positioning technology greatly affects efficiency and quality. For building and road works, accurately translating design positions on drawings to on-site stakeout (such as setting stakes and batter boards) is indispensable. Traditionally, surveyors working in teams operated total stations and guided prisms to determine points. With CLAS-compatible GNSS equipment, one person can perform stakeout. If design coordinate data are preloaded into a GNSS terminal, a worker with a receiver on site can be guided to the specified positions and immediately set stakes or mark points. Because no base station is required and large areas can be covered, stakeout work can proceed continuously even on large-scale land development sites or long roadworks.

Furthermore, high-precision positioning is central to the emerging field of ICT construction (smart construction). In machine control, where excavators and bulldozers with GNSS automatically adjust blade height based on 3D design data, knowing the precise current height and position is essential. Heavy machinery compatible with CLAS can obtain stable, high-precision self-positioning even on sites with unstable communication, reducing work interruptions. CLAS can also be used for drone photogrammetry for earthwork volume measurement and construction management. Photos taken by GNSS-equipped drones yield more accurate 3D models when position accuracy is improved by CLAS. In AR-enabled construction management apps, CLAS is powerful when overlaying design data onto live camera images of the site. Accurate AR alignment without positional drift allows on-the-spot as-built inspections and decisions about additional work. In construction, adopting high-precision positioning is becoming indispensable for progressing work safely and efficiently.

Disaster investigation applications – 3D modeling of damaged areas

When disasters such as earthquakes or landslides occur, it is essential to quickly and accurately grasp the situation on the ground. CLAS-enabled positioning is highly effective for such emergency field surveys. For example, when creating 3D models of affected areas from drone imagery immediately after a disaster, high-precision position information is crucial. With CLAS, a drone’s capture positions and ground reference points can be recorded to centimeter-level (cm) precision, enabling rapid generation of highly accurate 3D maps and orthophotos. This makes it possible to grasp the precise scale of collapses and the extent of damage in three dimensions, facilitating smooth planning of recovery measures and identification of hazardous zones.

Another major safety advantage is that surveying teams can acquire measurements remotely without entering dangerous disaster sites. By mounting a CLAS receiver on mobile devices carried by helicopters or robots operating at the site, positioning data can be obtained without human entry into unstable rubble. Additionally, since communications infrastructure may be paralyzed during disasters, CLAS’s ability to operate without communications ensures positional information can be acquired under any conditions. There are reported cases where large-scale disaster area surveys were completed relying solely on CLAS without depending on cellular communications. Using CLAS in disaster response directly contributes to saving lives and speeding initial response, and its importance will continue to grow.

Forestry applications – resource management and boundary identification

Forestry and large-scale forest management are also promising areas for CLAS application. Forest resource surveys require accurate knowledge of tree locations and forest boundaries, yet GNSS augmentation communications often do not reach mountainous areas. With CLAS-compatible GNSS, correction signals from satellites can be received even within forests, enabling real-time boundary surveys. For planning forest road layout or selecting thinning zones, it is necessary to confirm pre-set boundary divisions on site. Using CLAS, survey teams can obtain coordinates of boundary points without long, time-consuming traverses in the mountains.

Forestry management also requires periodic resource monitoring, but labor shortages and rugged terrain are major obstacles. Forestry personnel carrying a small GNSS receiver and a tablet can record positioning data while moving through the mountains to efficiently update current-resource maps. Area measurements for logging zones and boundary confirmations can minimize errors with CLAS’s high precision, helping prevent unnecessary logging or boundary encroachment. In addition, CLAS-enabled detailed terrain surveys contribute to post-wildfire assessments and forest restoration planning after landslides. Deploying high-precision positioning in the field is becoming essential to promote digital transformation (DX) in forestry.

One-person operation enabled by smartphone integration and device miniaturization

Alongside CLAS’s practical implementation, GNSS receiver miniaturization and light-weighting have progressed. Centimeter-class GNSS equipment used to require large antennas and stationary receivers. Today, however, integrated antenna receivers weighing only a few hundred grams and devices that dock with smartphones have emerged. These new-generation GNSS receivers use smartphones or tablets as the display and control terminal, eliminating the need to carry a dedicated controller and enabling intuitive operation. On site, it is becoming common to mount the receiver on a telescopic pole or monopod, connect to a smartphone via Bluetooth, and have one person carry out point measurements.

Achieving one-person surveying helps alleviate labor shortages in construction and surveying industries. If tasks that used to require two to three people can be completed by a single skilled person, this significantly reduces personnel costs and improves operational efficiency. In practice, solutions combining smartphone cameras or LiDAR with high-precision GNSS are being developed so that anyone can perform 3D surveys. Combined with CLAS’s ability to operate anywhere without communications, a future where simply walking with a GNSS receiver completes the necessary data collection is becoming a reality. Going forward, positioning and measurement workflows that can be completed with only a smartphone and a small GNSS receiver, without special infrastructure or large-scale equipment, are likely to become the new standard.

Conclusion – A new era of high-precision positioning opened by CLAS

With the advent of CLAS, high-precision positioning has become more accessible and easier to use than ever. The combination of ease of use without dependence on communication infrastructure and accuracy on the order of several centimeters enhances both the safety and efficiency of operations across agriculture, surveying, construction, disaster prevention, forestry, and many other fields. Tasks that formerly relied on manpower and experience can now be automated and streamlined based on accurate data, reducing mistakes and accidents.

Amid this trend, LRTK has emerged as a solution that makes centimeter-level positioning even more convenient. LRTK is an ultra-compact GNSS receiver compatible with CLAS that, when used with a smartphone, enables single-person cm-level positioning even in areas without communications. LRTK is already introduced on the Cabinet Office’s Quasi-Zenith Satellite System official site as a CLAS-compatible product, underscoring its utility. Because high-precision positioning can be started immediately with just a smartphone, adoption is progressing across simple surveying and rapid situational assessment in emergencies.

With CLAS and the spread of compatible devices enabling low cost, improved safety, and greater efficiency, the range of applications for high-precision positioning will continue to expand. Why not try applying next-generation positioning technology at your site?