How CLAS Is Changing RTK Positioning – Centimeter Accuracy Without Base Stations

What is CLAS: A Satellite-Communication-Based High-Accuracy Positioning Augmentation Service

CLAS (Clear-what?) is a centimeter-class positioning augmentation service provided by Japan’s Quasi-Zenith Satellite System, QZSS (known as "Michibiki"). Standalone GNSS positioning such as GPS can have errors of several meters, but by using CLAS users can measure positions with dramatically higher accuracy on the order of a few centimeters. Its major distinguishing feature is that it achieves this high accuracy without the conventional need for base stations or communication lines. CLAS signals are transmitted directly from Michibiki satellites and can be received anywhere in Japan (usable in mountainous areas or at sea as long as sky visibility is maintained). The service is free, and anyone with a compatible GNSS receiver can start centimeter-class positioning.

Technically, CLAS adopts a positioning method called "PPP-RTK." Error information observed by the Geospatial Information Authority of Japan’s permanent GNSS network (GEONET)—such as satellite orbit and clock errors and ionospheric delays—is aggregated and broadcast via Michibiki satellites, allowing users to apply corrections to their own GNSS positioning. Specifically, correction data are carried on Michibiki’s L6 band (L6D signal); compatible receivers receive and decode that signal to perform real-time corrections. With this mechanism, users can achieve centimeter-level positioning with a single receiver alone, without installing their own base station or obtaining corrections via the Internet.

Traditional RTK Positioning Methods and Their Challenges

RTK positioning (Real-Time Kinematic), a representative method for high-precision GNSS positioning, is a technique to obtain centimeter-level accuracy in real time. The basic principle is that a "base station" with known accurate coordinates and a "rover" receive GNSS satellite signals simultaneously, and by comparing both sets of positioning data, common errors are canceled out. This allows errors that would be 5–10 meters with standalone GNSS to be corrected down to a few centimeters. RTK generally yields high-precision results of about 2–3 cm horizontally (about 5 cm vertically), and time to fixed solution (Fix) is fast—typically a few seconds.

However, traditional RTK positioning has had major challenges. Chief among them is that correction information from a base station is always required. Real-time RTK cannot function in environments where base station radio or data communications are unavailable. Accordingly, the following operational modes have commonly been used:

• Local base station method (standalone RTK): The user installs a GNSS receiver for the base station near the survey site and repeatedly transmits correction data (e.g., in RTCM format) to the rover via radio (low-power radio or simple radio, etc.). This is a relatively simple one-to-one configuration, but preparing and installing base station equipment is time-consuming, and the radio coverage is limited (typically a few km to around 10 km). Accuracy also degrades with distance from the base station, and beyond about 10 km the errors tend to become significant.

• Network RTK method (VRS/Ntrip method): Users receive correction information via the Internet by using national or commercial reference station networks. For example, services that distribute Virtual Reference Station (VRS) data based on the Geospatial Information Authority’s network of reference stations are available in many regions. Users equip the rover with a cellular modem and use an Ntrip client to obtain corrections from the Internet. This eliminates the need to set up one’s own base station and largely removes accuracy degradation due to distance from reference stations. However, service fees charged monthly or annually and the limitation that the service cannot be used where cellular coverage is unavailable are drawbacks.

Because both approaches require either "base station installation" or "use of communications infrastructure," there have been many situations—such as deep mountains, radio-dark areas, or large-scale disasters where base station equipment and networks are inoperable—where real-time high-precision positioning could not be performed. In such cases, on-site measurements had to be postponed and data brought back for post-processing (PPK: Post-Processed Kinematic, etc.). High costs for base station equipment and communication fees also posed a cost barrier limiting RTK adoption.

CLAS-Driven RTK Innovation: The Advantages of Being Base-Station-Free

CLAS dramatically solves these challenges and represents an innovation in RTK positioning. With CLAS, users do not need to provide their own base station, nor do they need to obtain correction data over cellular networks. Because corrections are broadcast directly from satellites, uniform high-accuracy positioning is possible anywhere within Japan—from mountainous regions to remote islands and maritime areas. There is no need to worry about distance to a base station or searching for network coverage, and wide-area surveying or tasks that require movement can maintain centimeter-level accuracy consistently.

The biggest advantage of CLAS is that it enables real-time positioning to be completed entirely on-site. For example, traditional remote RTK surveying required preparing base station equipment and verifying communications in advance. With CLAS-compatible equipment, a user can arrive on site, power on, and the satellites will deliver correction data—drastically reducing setup time. Because the correction service has no usage fee, it also leads to reduced running costs. For municipalities and companies that use high-precision positioning regularly, the ability to operate with zero communication fees is significant.

Furthermore, CLAS contributes to improved resilience in emergencies. Because it does not rely on communications infrastructure, positioning can continue even if cellular networks go down. In recent large earthquakes in rural areas, CLAS-compatible compact RTK receivers were used to survey and document disaster sites while local base stations and lines were inoperative. High-precision positioning obtained directly from satellites enabled immediate 3D data generation of damage conditions, which could be quickly shared among stakeholders and greatly aided recovery efforts. Thus, CLAS’s base-station-free positioning achieves “high accuracy anytime, anywhere”, making it notable from a disaster prevention and mitigation perspective.

Using CLAS with LRTK Devices and Advanced On-Site Use Cases

To use CLAS you need a compatible GNSS receiver, and recently compact high-precision GNSS devices that pair with smartphones have emerged. One such product line is the “LRTK” series, developed by a venture originating from Tokyo Institute of Technology. LRTK is designed as an ultra-compact RTK-GNSS receiver that attaches to a smartphone; the antenna, receiver, battery, and communications module are integrated into an all-in-one device. Weighing only a few hundred grams and small enough to fit in a pocket, it offers performance comparable to conventional stationary surveying instruments.

Key features of LRTK include multi-GNSS and multi-frequency support. It can use multiple satellite constellations—GPS, GLONASS, Galileo, BeiDou, and Japan’s QZSS (Michibiki). In particular, because it can receive the L6 band distributed by CLAS, CLAS positioning can be achieved with just a smartphone plus an LRTK unit. This allows a device to obtain a Fix even outside cellular coverage. Multi-frequency support increases the number of observable satellites and improves Fix stability. Even in urban areas affected by partial satellite blockage or multipath from buildings, other satellites can compensate, making positioning less likely to drop out than before. For example, in sites where sky visibility is partially blocked, LRTK devices demonstrate high Fix retention capability (tunnels or fully indoor environments remain challenging, but positioning often continues under tree cover or under elevated structures).

LRTK also offers convenience through smartphone integration. Using the phone’s screen and an app as the interface makes intuitive surveying possible without learning complex dedicated hardware. Measured point data and 3D models can be shared to the cloud on-site, and AR features can visualize results there and then—these are examples of advanced workflows. For instance, through an iPhone or Android camera, design lines or buried utilities can be AR-displayed over the ground to guide placement. AR-based positioning, which previously required marker placement or manual calibration, can now be achieved outdoors at scale without markers thanks to high-precision GNSS. Combining an LRTK with a smartphone enables AR navigation that reflects the user’s position and orientation in real time, greatly streamlining tasks such as visualizing buried infrastructure or accurately locating structures. LRTK and smartphone combinations make it possible to dramatically improve efficiency in precise placement tasks.

As such, CLAS-compatible compact RTK devices have a significant impact as “portable surveying instruments that one person can carry and use immediately.” Tasks that once required two or more experienced surveyors can increasingly be performed by a single person in a short time using a smartphone plus LRTK. On actual sites using LRTK, point cloud scans are shared to the cloud immediately and used to check as-built conditions on that data—examples of smart construction are already emerging. The robust yet lightweight devices are easy to bring to high or hazardous locations, contributing to improved safety. As core tools of digital site management using the latest technologies, smartphone-connected LRTK devices maximize the benefits of CLAS.

CLAS Positioning Accuracy, Initial Convergence Time, and Technical Considerations

CLAS positioning accuracy is nominally on the order of a few centimeters. Published data show that horizontal position accuracy at rest is within 6 cm with 95% probability (vertical within 12 cm). While moving, horizontal errors remain around 10 cm, which is a clear distinction from meter-class GNSS. Compared with RTK using local reference stations, CLAS can sometimes show slightly larger errors (where short-baseline RTK might achieve 2–3 cm, CLAS might be around 5–6 cm). Therefore, for tasks requiring millimeter-level precision caution is necessary, but for general surveying and construction management CLAS is acceptable. Because CLAS does not suffer from baseline-length-dependent error growth, it can be advantageous for work covering wide areas where it may provide more stable accuracy.

A technical point to note is initial convergence (TTFF) time. With CLAS, there is a brief period before a centimeter-level Fix solution is obtained after powering the receiver and starting positioning. Typically convergence takes on the order of tens of seconds to about a minute; during that time the solution may remain at float accuracy (decimeter-level). Whereas conventional RTK might achieve a fixed solution in a few seconds, CLAS may require waiting around one minute in some cases. Once convergence is achieved, centimeter-level accuracy remains stable. Operationally, avoid taking precise measurements immediately after startup—allow about one minute of warm-up before starting critical measurements.

Also noted is the issue of real-time latency. Correction data transmitted via satellite experience a few seconds of transmission and processing delay, so the current CLAS system is not suited to applications that require instantaneous response—such as autonomous driving of fast-moving vehicles. Although positioning itself is real time, update intervals and processing delays can introduce slight offsets in vehicle position estimation. However, for surveying on foot, machine guidance, or agricultural automation—applications with relatively low movement speeds—this latency is practically negligible. Since CLAS is mainly aimed at on-site measurement and slower-moving platforms, latency is not a problem in most cases.

Finally, pay attention to reception conditions and compatible equipment. Using CLAS requires stable reception of Michibiki’s L6 signal. CLAS cannot be used in environments with severely obstructed sky visibility (tunnels or deep indoor spaces). In forests with dense canopy or urban canyons, satellite visibility can be partially lost, making it harder to obtain a Fix (this issue is common to conventional RTK as well, but with CLAS it is especially important since positioning is standalone). If satellite reception is impossible at the site, it is often also difficult with other surveying instruments like total stations, so it is advisable to check how open the sky is at the site in advance. Of course, CLAS-compatible receivers and antennas are required. If your GNSS equipment is old, confirm whether a firmware update can enable decoding of CLAS signals (L6D). Commercial surveying instruments and high-precision GNSS chips are progressively gaining CLAS support, and compatible small modules and smartphone devices are already appearing. Selection options are expected to continue expanding. Note that CLAS service coverage is basically limited to Japan. Because QZSS is designed to cover Japan and its surrounding area, the same operation is not possible overseas (abroad you must rely on each country’s SBAS or local RTK services). While Japanese CLAS operational know-how may spread to other countries in the future, as of 2026 Japan is one of the world’s leaders in deploying this high-precision positioning service.

Use Cases from Mountainous and Disaster Sites to Farmlands and Cities – CLAS Applications

CLAS and the new generation of positioning devices that leverage it are opening up applications across many fields. Below are typical use-case scenarios.

Solo high-precision surveying in mountainous areas

Even in mountainous areas without cellular coverage, CLAS-compatible GNSS can provide standalone centimeter positioning. For example, surveying forest roads or dam construction sites previously required installing a base station nearby or physically moving to an area with coverage. With CLAS, that effort is unnecessary, and a surveyor can enter remote mountains alone and perform high-precision measurements on-site. Risks associated with organizing surveying teams in difficult terrain are reduced, enabling personnel reductions and improved safety. Measurements are often possible on sloped or wooded sites by choosing relatively open locations, making CLAS effective for forest surveys and civil engineering work.

On-site recording and recovery support immediately after disasters

Immediately after large-scale disasters, infrastructure is often disrupted, but CLAS excels for initial on-site surveys in such conditions. For example, at earthquake or landslide sites where communications are down, CLAS-compatible GNSS can still record damaged locations sequentially. Attaching an LRTK device to a smartphone and walking the site to point-scan collapsed buildings or ground fissures at centimeter accuracy enables instantaneous generation of 3D models. Sharing that data with headquarters via satellite communications or mobile base stations allows remote specialists to grasp damage details. Disaster response is a race against time, and the ability to “measure and share immediately” with CLAS+LRTK dramatically speeds up situation assessment and recovery planning.

Boundary checks for farmland and smart agriculture

CLAS is also useful for surveying and agricultural support across large farmlands. For field layout and boundary verification, meter-level errors are unacceptable, but CLAS-capable GNSS enables accurate boundary line measurements that are hard to obtain with tape measures or survey pegs. Rural areas often lack nearby base stations or reliable communications, so high-precision positioning used to be difficult there. If CLAS lets individual farmers affordably use high-precision GNSS, they may be able to perform field area measurements or boundary checks without paying professional surveyors. CLAS is also well-suited for smart farm machinery such as autonomous tractors. Since correction data are obtained without communications, stable autonomous operation is expected even on terraced fields or vast plains. Trials by agricultural equipment manufacturers show CLAS-enabled autonomous tillage can achieve nearly error-free straight-line travel and area cultivation, presenting a promising solution for addressing labor shortages and improving efficiency.

Urban infrastructure inspection, maintenance, and AR applications

In urban areas, high-precision positioning data support regular inspection and maintenance of roads, bridges, and water/sewer infrastructure. CLAS-compatible positioning devices can accurately capture the locations of road fixtures (manholes, signs, etc.) into GIS, and be used for displacement and subsidence monitoring. While GNSS in cities was previously unstable due to building shadowing, the combination of multi-GNSS and CLAS has improved accuracy and stability to a practically usable level. Integration with AR (augmented reality) on smartphones and tablets is also gaining traction in urban infrastructure management. For example, a pre-prepared 3D model of buried pipes can be overlaid transparently on the real road view through a phone camera. This requires precise knowledge of the phone’s position and orientation, and CLAS-compatible LRTK devices solve that requirement. Being able to visualize drawing information on-site without misalignment helps prevent excavation errors and boosts construction efficiency. Similarly, comparing as-built drawings with the field during inspections or guiding night-time work with accurate AR lines are among the many urban applications where CLAS plus high-precision AR is expanding.

Conclusion: Toward a New Era of Centimeter-Level Positioning

The CLAS service of the Quasi-Zenith Satellite System is significantly changing the shape of RTK positioning. By removing traditional barriers such as base station setup and communication contracts, it is opening a new era of “centimeter positioning anytime, anywhere by anyone with a receiver.” The arrival of easy-to-use devices like smartphone-compatible LRTK means not only surveyors but also construction technicians, agricultural workers, and municipal staff can begin using high-precision positioning as a tool. Michibiki is scheduled to be strengthened to a seven-satellite configuration (around 2026), and CLAS signals are expected to become more stable and robust. Along with this, applications using CLAS—such as drone aerial surveying, autonomous mobile robots, and consumer-facing location services—will likely expand further.

This trend, which could be called the democratization of centimeter-level positioning, has the potential to dramatically raise on-site productivity and creativity. For instance, i-Construction initiatives in the construction industry are standardizing ICT and high-precision GNSS-based workflows, and the spread of CLAS-compatible equipment will be a strong enabler. When accurate positioning is available to whoever needs it, whenever needed, workflows will change substantially. A future where anyone on site can instantly obtain and share position data and use it intuitively in AR is within reach.

Now, CLAS has made the benefit of “centimeter accuracy without base stations” a reality. As the availability of high-precision positioning expands, the capabilities of fields such as surveying, construction, disaster response, and agriculture will be raised. Together with new tools like LRTK, an era where anyone can easily perform centimeter-level surveys has arrived. Take this opportunity to explore cutting-edge CLAS applications and experience the next evolution of on-site operations.