How CLAS Is Changing RTK Positioning – Centimeter-Level Accuracy (half-inch accuracy) Without Base Stations

What CLAS Is: A Satellite-Transmitted High-Precision Positioning Augmentation Service

CLAS (pronounced "Sīrasu") is a centimeter-class positioning augmentation service (half-inch accuracy) provided by Japan’s Quasi-Zenith Satellite System, Michibiki (QZSS). Standalone GNSS positioning such as GPS can have errors of several meters (several ft), but by using CLAS you can measure positions with dramatically higher accuracy of a few centimeters (a few in). Its major feature is that it achieves this high accuracy without using base stations or communication lines that were traditionally required. CLAS signals are broadcast directly from Michibiki satellites and can be received anywhere in Japan (and can be used in mountainous areas or at sea as long as sky visibility is available). The service is free, and anyone with a compatible GNSS receiver can start centimeter-class positioning (half-inch accuracy).

Technically, CLAS employs a positioning method called PPP-RTK. Error information observed by the Geospatial Information Authority of Japan’s GNSS reference station network (GEONET)—such as satellite orbit and clock errors and ionospheric delays—is aggregated and distributed via Michibiki satellites so users can apply corrections to their GNSS positioning. Concretely, correction data are carried on Michibiki’s L6 band radio (L6D signal), and compatible receivers receive and decode that signal to perform real-time corrections. This mechanism allows a single receiver to achieve centimeter-level accuracy (half-inch accuracy) without the user having to deploy a base station or obtain correction data over the Internet.

Traditional RTK Positioning Methods and Their Challenges

RTK (Real-Time Kinematic) positioning, a representative method for high-precision GNSS, is a technique for obtaining real-time centimeter-level accuracy. The basic principle is that a base station with known accurate coordinates and a rover receiver simultaneously receive GNSS satellite signals and the two positions are compared to cancel out errors. This can correct errors that, in standalone positioning, would be 5-10 m (16.4-32.8 ft) down to a few centimeters. RTK typically yields high-precision results of around 2-3 cm (0.8-1.2 in) horizontally and about 5 cm (2.0 in) vertically, and the time to obtain a fixed solution (Fix) is also fast—on the order of a few seconds.

However, traditional RTK had a major drawback: it always requires correction information from a base station. In environments where base station radio or data communications do not reach, real-time RTK positioning cannot be established. For this reason, the following operational modes have been used in broad terms:

• Local base station method (standalone RTK): The user installs a GNSS receiver for the base station near the survey site and transmits correction data (e.g., in RTCM format) sequentially to the rover via radio (low-power radio or simple radio). This is a relatively simple one-to-one configuration, but preparing and installing base station equipment is time-consuming, and the usable radio range is limited (typically a few km to about 10 km). Accuracy also degrades as you move away from the base station; beyond 10 km the errors tend to become non-negligible.

• Network RTK (VRS/NTRIP method): This uses a network of reference stations operated by national or private entities and delivers correction information over the Internet. Services are provided in many areas that distribute virtual reference station (VRS) data based on networks such as the Geospatial Information Authority’s reference stations. The user equips the rover with a cellular modem and obtains correction data from the Internet via an NTRIP client. This method eliminates the need to set up a personal base station and largely removes accuracy degradation due to distance from reference stations. However, service fees are incurred on a monthly or annual basis, and it cannot be used where cellular service is unavailable.

Because either “deploying a base station” or “using communication infrastructure” was indispensable in both approaches, there were many situations—deep in the mountains, in areas with no radio coverage, or when base station equipment or communication networks were down after a major disaster—where real-time high-precision positioning had to be abandoned. In such cases, on-site positioning had to be deferred and handled later by post-processing (e.g., PPK: Post-Processed Kinematic). The cost barrier of expensive base station equipment and communication fees also hindered RTK adoption.

Innovation in RTK Positioning with CLAS: Benefits of Being Base-Station-Free

CLAS dramatically solves these problems. With CLAS, users do not need to provide their own base station nor obtain correction data via cellular networks. Because correction information is transmitted directly from satellites, uniform high-precision positioning is possible anywhere in Japan—from mountain valleys to remote islands and the open sea—so long as sky visibility is available. There is no need to worry about distance from base stations or to hunt for communication coverage, and centimeter-level accuracy (half-inch accuracy) can be consistently maintained over wide-area surveys and mobile operations.

The biggest advantage of CLAS is that it enables on-site completion of real-time positioning. For example, when conducting RTK surveys in remote areas, it used to be necessary to set up base station equipment in advance or confirm communication environments; with CLAS-compatible equipment, you can simply arrive on site, power on the device, and the satellites will supply correction data, greatly reducing preparation time. Because there is no subscription fee for correction services, it also reduces running costs. For municipalities and companies that regularly use high-precision positioning, being able to operate without communication costs is highly significant.

CLAS also contributes to increased resilience in emergencies. Because it does not rely on communication infrastructure, positioning can continue even if cellular networks are down. For instance, following recent large earthquakes in rural areas where local base stations and lines were disrupted, survey and recording of disaster sites were performed using compact CLAS-compatible RTK receivers. High-precision position data obtained directly from satellites enabled rapid 3D documentation of damage and quick sharing among stakeholders, significantly aiding recovery activities. In this way, CLAS’s base-station-free positioning realizes “high precision anytime, anywhere”, and is attracting attention from a disaster prevention and mitigation perspective.

Using CLAS with LRTK Devices and Advanced Field Use Cases

To use CLAS you need a compatible GNSS receiver, but recently there have been high-precision GNSS devices that pair easily with smartphones. One example is the LRTK series developed by a startup originating from Tokyo Institute of Technology. LRTK is an ultra-compact RTK-GNSS receiver designed to attach to a smartphone; it integrates antenna, receiver, battery, and communication module into an all-in-one device. It weighs only a few hundred grams and fits in a pocket, yet its performance rivals that of traditional tripod-mounted surveying instruments.

A key feature of LRTK is multi-GNSS and multi-frequency support. It can use not only GPS but also GLONASS, Galileo, BeiDou, and others simultaneously, and supports Japan’s QZSS (Michibiki). Especially since it can receive the L6 band distributed by CLAS, CLAS positioning can be achieved with just a smartphone plus LRTK. This enables obtaining a Fix solution even outside cellular coverage. Multi-frequency support increases the number of observable satellites and improves Fix stability. Even in urban areas where some satellites are blocked by buildings or where multipath (reflections) occurs, other satellites can fill in, making positioning less likely to be lost than before. For example, in sites where sky visibility is partially obstructed, LRTK devices have shown strong Fix retention capabilities (tunnels and fully indoor environments remain challenging, but positioning can continue under tree cover or beneath overpasses in many cases).

LRTK also stands out for the convenience of smartphone integration. Using a phone screen and app as the interface allows intuitive surveying work without learning complex dedicated equipment controls. Advanced uses such as sharing measured point data or 3D models to the cloud on site, or visualizing them with AR, are possible. For example, through an iPhone or Android camera, design lines or buried utilities can be overlaid on the ground with AR guidance. AR positioning that previously required markers and manual calibration can now be implemented outdoors over wide areas thanks to high-precision GNSS. With an LRTK and a smartphone, AR navigation that reflects your real-time position and orientation can be used to visualize buried infrastructure or accurately place structures, greatly improving efficiency.

Compact CLAS-compatible RTK devices therefore have a significant impact as “survey instruments you can carry and use alone”. Tasks that once required two or more experienced surveyors can increasingly be handled by a single person quickly using a smartphone plus LRTK. On sites using LRTK, cases of immediate cloud sharing of point-cloud scan data and on-the-fly verification of as-built conditions are already appearing as examples of smart construction. The robust yet lightweight devices are easy to bring to high or hazardous locations, reducing burden and improving safety. As a core of digital site management leveraging cutting-edge technology, smartphone-integrated LRTK maximizes the benefits of CLAS.

CLAS Positioning Accuracy, Initial Convergence Time, and Technical Notes

CLAS positioning accuracy is nominally an error of a few centimeters (a few in). Published data indicate that static horizontal position accuracy is within 6 cm (2.4 in) with 95% probability (vertical within 12 cm (4.7 in)). While moving, horizontal errors remain around 10 cm (3.9 in), clearly distinguishing CLAS from traditional meter-level GNSS. However, compared to RTK using a local reference station, CLAS can sometimes be slightly less precise (short-baseline RTK may achieve 2-3 cm (0.8-1.2 in), while CLAS may be around 5-6 cm (2.0-2.4 in), for example). Therefore, jobs that require millimeter-level precision should be approached with caution, but for general surveying and construction management CLAS is adequate. Because CLAS does not suffer from baseline-length-related error growth, it can offer more stable accuracy over wide-area work.

A technical point to note is the initial convergence time (TTFF). With CLAS, it takes a short time after starting the receiver to obtain a centimeter-level Fix. Typically it converges in several tens of seconds to about 1 minute, and during that period solutions may remain at float accuracy (decimeter-level). There may be cases where conventional RTK would produce a fixed solution within a few seconds but CLAS requires about a minute; be aware of this. Once converged, however, it maintains stable centimeter-level accuracy. In practice, rather than performing strict measurements immediately after powering on, it is advisable to warm up for about 1 minute before beginning main measurements.

Real-time aspects (latency) are also noted. Satellite-delivered correction information experiences transmission and processing delays on the order of a few seconds, so current CLAS is not suited to applications requiring instantaneous response, such as high-speed vehicle autonomous driving. While positioning itself is real-time, update intervals and processing delays can introduce slight offsets in vehicle position estimates. However, for surveying on foot, machine guidance, agricultural machinery automation, and other applications with modest movement speeds, this latency is practically negligible. Since CLAS is mainly intended for site measurements and relatively slow-moving platforms, it is adequate for most cases.

Finally, note the reception environment and compatible equipment. Using CLAS requires stable reception of Michibiki’s L6 signal. CLAS cannot be used in environments where sky visibility is severely obstructed (tunnels or deep indoor locations). In dense forest or urban canyons where satellite visibility is partially lost, obtaining a Fix can be difficult (this is also true of conventional RTK, but requires special caution with CLAS because it is a standalone correction service). In environments where satellites cannot be acquired, other surveying instruments such as total stations also often struggle, so it is wise to check in advance how open the sky is at the site. Of course, a CLAS-compatible receiver and antenna are required. If your GNSS equipment is old, check 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 compact modules and smartphone-oriented devices are appearing. The selection of options is expected to increase going forward. Note that the CLAS service area is essentially limited to Japan. The Quasi-Zenith Satellites cover Japan and nearby areas, so similar operation is not available abroad (overseas use requires local SBAS or local RTK services). While Japan’s CLAS operational know-how may spread internationally in the future, as of 2026 Japan is among the world leaders in deploying this high-precision positioning service.

From Mountainous Areas and Disaster Sites to Farmland and Cities – CLAS Use Cases

CLAS and next-generation positioning devices that leverage it are opening up applications across a variety of fields. Below are typical use-case scenarios.

Solo High-Precision Surveying in Mountainous Areas

Even in mountains without cellular coverage, CLAS-capable GNSS can provide standalone centimeter-level positioning (half-inch accuracy). For surveys of logging roads or planned dam construction sites, previously it was necessary to install a base station nearby or to walk to an area with coverage. CLAS eliminates such burdens, allowing a surveyor to enter remote mountains alone and take high-precision measurements on site. This reduces the risks of organizing survey parties in difficult terrain and contributes to reduced personnel and improved safety. On slopes or within forests, positioning is feasible if you choose relatively open spots, so CLAS is powerful for forest surveys and civil engineering surveys.

On-Site Recording and Recovery Support Immediately After Disasters

Right after a major disaster, infrastructure is often disrupted, but CLAS can be a great asset for initial investigations. For example, at earthquake or landslide sites where communications are down, CLAS-capable GNSS can be used to record locations of damage. With an LRTK device attached to a smartphone, walking around and point-cloud scanning collapsed buildings or ground fissures at centimeter accuracy enables immediate generation of on-site 3D models. Sharing that data with headquarters via satellite communication or mobile command posts allows remote experts to assess damage in detail. In disaster response, time is critical, and the CLAS + LRTK combination that enables “measure and immediately share” greatly accelerates situation assessment and restoration planning.

Farmland Boundary Checks and Smart Agriculture

CLAS is useful for surveying and agricultural support over large farmland. For field subdivision and boundary confirmation, meter-level position errors are insufficient, but CLAS-capable GNSS enables accurate boundary line measurements that are difficult with tape measures or survey stakes. Rural areas often lack nearby base stations or robust communication, so high-precision positioning was previously hard to access. If CLAS makes affordable high-precision GNSS available to individual farmers, they may be able to measure field areas or confirm plots themselves without hiring surveying specialists. CLAS is also well suited to smart agricultural machines like autonomous tractors. Because correction information is obtained without communications, stable autonomous driving can be expected even on terraced fields or vast fields. Trials by agricultural machinery manufacturers using CLAS have demonstrated nearly error-free straight-line drives and precise plot tillage, showing promise as a solution to labor shortages and for improving work efficiency.

Urban Infrastructure Inspection, Maintenance, and AR Applications

In urban environments, high-precision positioning data are valuable for routine inspection and maintenance of roads, bridges, and water/sewer systems. Using CLAS-capable positioning devices, the exact positions of roadside assets (manholes, signs, etc.) can be imported into GIS, and displacements or subsidence can be monitored. GNSS positioning in cities was traditionally unstable due to building obstructions, but the combination of multi-GNSS and CLAS improves accuracy and stability enough to obtain practically usable data in urban settings. Integration with AR (augmented reality) on smartphones and tablets is also attracting attention for urban infrastructure management. For example, an AR app can preload 3D models of buried pipes and, when the site operator points a phone camera, overlay the pipe locations and depths on the road surface. Realizing this requires accurate knowledge of the phone’s position and orientation, and CLAS-compatible LRTK devices solve that problem. Because you can visualize drawings on site without offsets, this helps prevent excavation mistakes and improves construction efficiency. Similarly, comparing as-built drawings with on-site conditions during inspections, or providing precise guidance during night work with AR lines, are among the urban applications where CLAS + high-precision AR is expanding.

Conclusion: Toward a New Era of Centimeter-Class Positioning (half-inch accuracy)

Japan’s Quasi-Zenith Satellite System CLAS is transforming RTK positioning. By removing traditional barriers such as base-station installation and communication contracts, it is ushering in a new era of “anyone with a receiver can get centimeter-level positioning (half-inch accuracy) anywhere, instantly.” The emergence of easy-to-use devices like smartphone-combined LRTK means that not only surveyors but also construction technicians, agricultural workers, and municipal employees are beginning to use high-precision positioning as a routine tool. Michibiki is set to be strengthened to a seven-satellite constellation (scheduled for operation around 2026), and CLAS signals are expected to become even more stable and comprehensive. As that happens, applications such as drone-based aerial surveys, autonomous mobile robots, and even consumer-oriented location services will expand the scope of CLAS use.

This movement—what can be called the democratization of centimeter-class positioning (half-inch accuracy)—has the potential to dramatically raise productivity and creativity on worksites. For example, the construction industry’s push for i-Construction, which standardizes ICT and high-precision GNSS in construction, will be further accelerated by the spread of CLAS-compatible devices. When anyone can measure precise positions as needed without expensive specialized surveying equipment, workflows will change substantially. A future in which everyone on site can instantly acquire and share position data and intuitively use it with AR is right around the corner.

CLAS has made the benefit of “centimeter accuracy without base stations” a reality. As the reach of this high-precision positioning widens, the capability of field operations in surveying, construction, disaster management, agriculture, and other areas will be elevated. Together with new tools like LRTK, an era in which anyone can perform simple centimeter-level surveys has arrived. Take this opportunity to explore cutting-edge CLAS applications and experience the next evolution of fieldwork.