Receive correction signals from satellites continuously! Centimeter-level positioning achieved by QZSS CLAS

In fields such as construction sites, surveying, agriculture, forestry, and disaster prevention, improving the accuracy of position information is indispensable. Traditionally, to obtain centimeter-level accuracy, GNSS positioning used a method called RTK (Real-Time Kinematic), which required the mobile unit to receive correction information from base stations or a network in addition to its own observations. However, conventional methods that depend on communication environments were limited in mountainous areas or regions outside communication coverage. At the center of attention is Japan’s Quasi-Zenith Satellite System (QZSS) offering CLAS (Centimeter Level Augmentation Service). By using correction signals continuously broadcast from the satellites, centimeter-level positioning can be achieved without relying on communication infrastructure. This article explains the overview of the Quasi-Zenith Satellite System, how CLAS works, comparisons with RTK and SLAS, field use cases, and future prospects.

What is the Quasi-Zenith Satellite System (QZSS)

The Quasi-Zenith Satellite System (QZSS) is a regional satellite positioning system operated by Japan. Nicknamed “Michibiki,” it is often introduced as the “Japanese version of GPS,” but more precisely it is a regional augmentation system designed to complement and enhance GPS (the U.S. Global Positioning System). As the name implies, quasi-zenith satellites adopt orbits that keep them near the zenith over Japan, so that at least one satellite is positioned almost directly overhead Japan throughout the day. This makes it easier to receive satellite signals even in mountainous areas or among high-rise buildings, and provides benefits in environments where GPS alone might not provide enough satellites for reliable positioning.

QZSS entered full operation with a four-satellite configuration in 2018, and by combining signals from GPS and other countries’ GNSS (such as Russia’s GLONASS and Europe’s Galileo), it has improved positioning accuracy and availability around Japan. Additional satellites were launched in 2023–2024, and operation with a seven-satellite configuration is targeted for 2025. Increasing the number of satellites will further stabilize satellite visibility, and in the future it may become possible to obtain positioning information using only QZSS satellites. Another major feature of QZSS is that, in addition to standard positioning signals, it provides Japan-specific augmentation services for higher accuracy. These are the augmentation signals for high-precision positioning described later: CLAS (Centimeter Level Augmentation Service) and SLAS (Sub-meter Level Augmentation Service).

How CLAS works and its benefits – Centimeter-level positioning via PPP-RTK

CLAS (Centimeter Level Augmentation Service, “CLAS”) is a centimeter-level positioning augmentation service provided by the Quasi-Zenith Satellite System (QZSS). By using CLAS, you can obtain position information with extremely high accuracy on the order of several centimeters. The underlying technology is a positioning method called PPP-RTK. PPP-RTK stands for Precise Point Positioning – Real-Time Kinematic, a state-of-the-art technique that combines the advantages of traditional PPP (Precise Positioning) and RTK (Real-Time Kinematic).

PPP-RTK corrects not only wide-area errors such as orbit and clock errors of GNSS satellites like GPS, but also regional errors such as ionospheric and tropospheric delays in real time. Specifically, the Geospatial Information Authority of Japan (GSI) utilizes observation data from about 1,300 electronic reference points (GNSS reference stations) deployed nationwide to estimate atmospheric error information. That correction information is continuously transmitted by the quasi-zenith satellites on the L6 band radio (L6D signal). Ground users receive this L6 signal with CLAS-compatible receivers and apply the corrections to their own GNSS observations to compute highly accurate positions. In other words, the reference station network spread across Japan shares its information via satellite so that a single receiver can achieve RTK-level accuracy.

A major advantage of CLAS is that it is not dependent on communication infrastructure and can provide uniform high accuracy over a wide area. There is no need to install your own base station or obtain correction data via the Internet; as long as you can receive signals from the quasi-zenith satellites, you can obtain uniform correction information anywhere in Japan. Also, receiving the correction information itself carries no usage fee: the service is open and provided free of charge (you must prepare compatible receiving equipment). Therefore, once a CLAS-compatible receiving environment is set up, centimeter-level positioning can be performed continuously with low running costs, providing an economic advantage.

However, using CLAS requires a compatible GNSS receiver (equipment that supports the L6 signal). At present, smartphones and typical consumer GNSS terminals cannot directly handle CLAS signals, so dedicated high-precision GNSS modules or receivers are necessary. Also, note that the initial convergence time to achieve centimeter-level positioning can be somewhat longer than with RTK. After starting reception, reaching centimeter-level accuracy while stationary may take tens of seconds to a few minutes depending on the environment and equipment performance. Once a high-precision solution is obtained, however, corrections remain stable even while moving. In situations where rapid positioning start-up is required, conventional RTK may still be superior, but CLAS is a powerful option that complements traditional methods, especially because it can provide stable high accuracy even outside communication coverage.

Correction information continuously receivable without communication infrastructure

One of CLAS’s biggest features is that communication infrastructure is unnecessary. In conventional network RTK, the rover needed to receive base station data or VRS (Virtual Reference Station) information via cellular or other networks. In contrast, CLAS delivers all correction data directly from satellites, so centimeter-level positioning is possible in remote work sites or areas without cellular reception as long as the sky is visible. This capability is especially valuable in disaster-response scenarios where infrastructure may be disrupted. For example, even if the communication network is down in a disaster-stricken area, positioning can continue using augmentation signals from the quasi-zenith satellites overhead, aiding swift situation assessment.

Another benefit to field operations is that there are no communication fees or service usage charges to obtain correction information. As mentioned, CLAS signals themselves are publicly available for free, so no paid distribution service contracts such as NTRIP are required. For example, operating autonomous tractors over vast farmland can continuously receive corrections without monthly communication costs, providing economic advantages. Furthermore, the same correction data format is available nationwide in Japan, eliminating the need to prepare region-specific base stations or worry about coordinate system transformations. These features make CLAS highly valuable in fields where “anywhere” and “always” high-precision positioning is required.

Comparison with other methods such as RTK and SLAS

Besides CLAS (PPP-RTK), there are other methods for achieving high-precision positioning. Representative methods include the long-established RTK positioning and SLAS (Sub-meter Level Augmentation Service), which provides sub-meter accuracy. Let’s compare their characteristics from the perspectives of accuracy, communication requirements, availability, and ease of introduction.

• Positioning accuracy: RTK achieves high accuracy on the order of about 2–3 cm horizontally. CLAS (PPP-RTK) can also achieve comparable centimeter-level accuracy, though in dynamic positioning errors may reach nearly 10 cm in some cases. SLAS (SBAS) provides accuracy on the order of several tens of centimeters to about 1 m (3.3 ft), which is inferior to RTK and CLAS.

• Communication requirements: RTK requires wireless communication with base stations or network communication via the Internet. In contrast, both CLAS and SLAS receive correction information via one-way satellite broadcast, so ground communication infrastructure is unnecessary.

• Availability and coverage: RTK is typically usable only within about 20–30 km (12.4–18.6 mi) of a base station, and network RTK requires cellular coverage. CLAS and SLAS are usable wherever satellite visibility exists across Japan’s territory (and parts of Asia and Oceania). In particular, CLAS is optimized for the sky over Japan and can be expected to provide uniform accuracy regardless of region.

• Ease of adoption: Using RTK requires expensive base station equipment, paid correction service contracts, and communication devices, and its operation requires expertise. CLAS, once a compatible receiver is available, automatically receives correction signals, making operation relatively easy (initial investment is required, but operational costs can be kept low). SLAS is often supported by standard GPS receivers and can be used without additional equipment investment, but its limited accuracy restricts its applications.

As described above, each of RTK, CLAS, and SLAS has strengths and weaknesses. RTK still has advantages in scenarios that demand the highest level of accuracy and instant positioning, but CLAS’s convenience in providing wide-area operation regardless of communication environment is revolutionary. For applications with lower accuracy requirements, SLAS may be sufficient and easier to use. By choosing the appropriate method according to field needs, efficient and cost-effective use of positioning information is possible.

Field use cases of CLAS

Because CLAS provides high accuracy without requiring communication infrastructure, it is expected to be used in many practical fields. Below are some examples.

Autonomous steering in agriculture

In smart agriculture, where tractors and combines are autonomously operated over vast farmlands, high-precision positioning that keeps path deviation within a few centimeters is essential. Traditionally, RTK with an installed base station has been mainstream, but operations could be difficult in mountain fields or farms out of radio range. With CLAS, farms outside communication coverage can continue to receive corrections from quasi-zenith satellites, enabling stable autonomous steering. Demonstration experiments have confirmed that tractors equipped with CLAS-compatible GNSS receivers can maintain straight-line travel accuracy without an Internet connection. This helps prevent overlap and unevenness in fertilization and seeding, improving production efficiency. CLAS’s high accuracy is also useful for precision spraying by agricultural drones.

Positioning and construction management at construction and surveying sites

In civil engineering and construction, centimeter-level positioning is important to carry out work according to design drawings. Traditionally, total stations and RTK-GNSS were used for tasks such as stakeout (setting out positions) and as-built verification. With CLAS-compatible GNSS equipment, surveying work can begin immediately on site without installing a base station. For example, using CLAS for machine guidance or benchmark surveys in road construction can streamline the complicated process of establishing control points. Recently, advanced cases have emerged where tablets or smartphones are combined with high-precision GNSS to display design lines on-site in AR for checking. By using accurate coordinates obtained through CLAS, site conditions can be digitally recorded and shared, enhancing quality and progress management. CLAS, which allows positioning without concern for communication environments, is useful for tunnel construction in mountainous areas and infrastructure projects in rural regions.

Use in surveying and investigation during disasters

In disaster sites such as earthquakes and landslides, rapid situation assessment and response are required. However, communication is often cut off in affected areas, and conventional network RTK may be unusable. Because CLAS obtains correction information via satellite, it is a powerful tool for disaster-response surveying. For example, after a major earthquake, investigators carrying CLAS-compatible GNSS equipment can obtain accurate position data even outside communication coverage for measuring crustal deformation or surveying landslide areas. When flying drones to capture aerial images of damage, equipping them with CLAS-compatible receivers allows high-precision geotagging of images for quick and accurate map production. In disasters, time is of the essence; having CLAS-compatible equipment prepared in advance enables immediate positioning without the need to build local infrastructure, speeding up initial surveys.

Positioning in forestry and forest management

CLAS is also expected to be useful for surveying and GIS data collection within forests. In mountain forests, mobile phones often have no reception, and standalone GPS with meter-level errors was commonly used for boundary confirmation or vegetation surveys. In the future, combining lightweight CLAS-compatible receivers with tablets could bring centimeter-level accuracy to forest compartment surveys and resource management. For example, precisely recording thinning positions in broadleaf forests or determining boundaries using GNSS alone without installing survey markers would become easy. Although canopy and branches can cause signal blockage and CLAS will not perform as well as in open skies, the quasi-zenith satellites deliver correction signals from high elevation angles, supporting stable positioning even in high-altitude mountainous areas. In rugged forests where manual surveying is difficult, the value of communication-free CLAS positioning is particularly evident.

Future prospects of quasi-zenith satellite positioning

Japan’s Quasi-Zenith Satellite System and CLAS-based centimeter-level positioning are expected to evolve further. As mentioned earlier, QZSS is being expanded toward a seven-satellite configuration. Additional deployments beyond the late 2020s are also being considered, which will further improve the reliability and convenience of satellite positioning. Increasing the number of satellites could reduce the load per satellite and enable reception of correction signals from multiple satellites simultaneously. That would likely shorten the current initial convergence time and improve performance in signal-obstructed environments, leading to further enhancements in service quality.

Next is the evolution of receiver technology on the user side. As high-precision GNSS receivers and antennas become smaller and less expensive, the barriers to CLAS adoption will drop significantly. If smartphones eventually become multi-band GNSS capable and can directly process CLAS signals, an era may come when anyone can enjoy centimeter-level positioning on their handheld device. Even now, small CLAS-compatible devices that attach to smartphones and tablet-connected high-precision positioning solutions have begun to appear. In the future, CLAS-compatible GNSS will be installed on consumer drones and autonomous vehicles, and high-precision positioning will become a standard part of social infrastructure.

Furthermore, high-precision positioning including CLAS will create new value through integration with social infrastructure. For example, autonomous driving systems require highly accurate self-positioning; if QZSS provides augmentation signals from more satellites and improves reliability and responsiveness, the scope of applications for vehicle and vessel autonomous navigation support will expand. In urban smart infrastructure, high-precision GNSS data could be used to streamline inspection and monitoring of roads and railways. High-precision spatiotemporal information provided by quasi-zenith satellites could be linked with disaster monitoring networks and meteorological observation systems to contribute to real-time safety and security services. Going forward, the fusion of satellite positioning and ground infrastructure will advance integrated digital management of Japan, a “merging of sky and ground.”

Conclusion: High-precision positioning accessible to everyone

Centimeter-level positioning via CLAS, continuously receivable from satellites, is a technology that benefits not only specialists but also field personnel. Enabling high-precision GNSS use without base stations or communications has the potential to dramatically change staking work previously left to surveying professionals and to transform GPS usage in agriculture and forestry that lacked sufficient accuracy. Solutions that use smartphones, such as [LRTK Phone](https://www.lrtk.lefixea.com/lrtk-phone), have emerged; by attaching such a CLAS-compatible device to a smartphone, anyone can easily perform simple surveying or inspection tasks. LRTK is equipped with an internal battery and antenna, and with just a smartphone running the app, centimeter-level positioning can be started immediately on site. Because its antenna is capable of receiving augmentation from quasi-zenith satellites in areas without cellular coverage, it can maintain high accuracy in places where conventional RTK was difficult. Such solutions are representative examples of bringing the convenience of quasi-zenith satellites to the field.

Japan’s Quasi-Zenith Satellite System and CLAS have opened a new era of high-precision positioning. Through continued fusion of satellite and ground technologies, a society in which “accurate position information anytime, anywhere” is available in every aspect of life and industry will be realized. As QZSS services continue to evolve, consider evaluating their introduction in your field.