Table of Contents

• What are the Quasi-Zenith Satellite System "Michibiki" and CLAS?

• Basic Architecture and Benefits of CLAS-Compatible Receivers

• Practical Positioning Accuracy and How to Use Them

• Differences and Comparison with Conventional RTK (Network-type, UHF-type)

• Conditions and Points to Note for Using CLAS

• Features and Adoption Benefits of LRTK Smartphone-Type CLAS-Compatible Receivers

• Use Cases of LRTK CLAS in Surveying Fields

• FAQ

What are the Quasi-Zenith Satellite System "Michibiki" and CLAS?

First, let’s cover Japan’s Quasi-Zenith Satellite System "Michibiki" (QZSS). Michibiki is a satellite positioning system introduced in Japan to complement and augment GPS. It is designed so that with four or more satellites, at least one satellite is always at a high elevation angle (near the zenith) over Japan, making it easier to acquire satellites even in mountainous areas or city canyons with tall buildings. As a result, positioning errors that were on the order of about 10 meters with GPS alone can be greatly reduced to 1 meter to a few centimeters by using Michibiki.

A notable new technology among these is the Centimeter Level Augmentation Service (CLAS). CLAS is a service that creates correction data based on positioning error information obtained from the Geospatial Information Authority of Japan’s Continuously Operating Reference Station network (GEONET) and distributes it via the Michibiki satellites. In simple terms, it can be described as “satellite-based RTK correction information that can be received anywhere in Japan.” When a user’s receiver receives the CLAS signal (QZSS L6-band radio), it applies those corrections to positioning from multiple GNSS satellites such as GPS and GLONASS and computes high-precision positions with errors of a few centimeters in real time.

The biggest feature of CLAS is that it enables centimeter-level positioning without surveyors having to prepare a base station themselves. With conventional RTK positioning, users needed to set up a GNSS base station on a known point and send correction information via radio to rovers, or obtain correction data from a reference station network (e.g., VRS) over a mobile internet connection. In contrast, CLAS virtually uses the national Continuously Operating Reference Stations as reference stations, and the correction information is delivered directly from satellites, so users do not need to provide base station equipment or communication lines. Because it does not depend on communication infrastructure, it can be operated in mountain areas or at sea where mobile networks do not reach, and another major advantage is that receiving the correction signal itself is free (the satellite distribution service is provided with public funds). CLAS realizes, in effect, “RTK that works nationwide without a base station.”

Basic Architecture and Benefits of CLAS-Compatible Receivers

To benefit from CLAS, you need a compatible GNSS receiver. The key point is that ordinary standalone GNSS units or smartphones’ built-in GPS cannot receive Michibiki’s CLAS signal (the L6 band). CLAS-compatible receivers are equipped with multi-band GNSS antennas and reception modules that include the L6 band. Specifically, they are designed to track GPS, GLONASS, Galileo, BeiDou, and the quasi-zenith satellites, and to process the L6 augmentation signal in dedicated hardware and software.

Because these CLAS-compatible GNSS receivers can provide high-precision positioning standalone without relying on a base station or external communications, they offer the following advantages over conventional RTK equipment.

• No internet needed; usable anywhere: CLAS delivers corrections directly from satellites, so there is no need to connect to the internet via a mobile router or smartphone tethering on site. As long as the satellites are visible, centimeter-level positioning is possible even in remote mountains or areas without cellular coverage. For example, in disaster sites where mobile communications are cut off, CLAS-compatible receivers allow surveying to begin immediately.

• Low power consumption for long operation time: Because external communication modules aren’t used, the receiver’s power consumption can be lower. Compared with continuously using UHF radios or 4G communication, battery load is lighter and continuous on-site positioning for longer periods is possible. You won’t need to worry about battery drain from searching for mobile signals in areas without coverage.

• Compact and highly mobile: CLAS-compatible receivers have become smaller in recent years, and compact integrated-antenna models are now available. Because the receiver is self-contained, you don’t have to carry large base station equipment or long antenna poles, making transport and on-site setup easier. This suits agile tasks like a single surveyor quickly moving between multiple points to take measurements.

As described above, CLAS-compatible receivers enable a new positioning style that is “base-station-less and communication-less.” They make RTK positioning feasible in mountainous regions where it was previously difficult and allow high-precision surveying under constraints on equipment and communications with greater ease.

Practical Positioning Accuracy and How to Use Them

What level of accuracy can be achieved using CLAS-compatible receivers in practice? In short, it is possible to stably obtain horizontal accuracy on the order of a few centimeters and vertical accuracy on the order of a few tens of centimeters. For example, experimental and operational results often show horizontal position errors of about 3–6 cm, and vertical (height) errors typically around 10 cm. This is a dramatic improvement compared with ordinary standalone positioning (errors of several meters to tens of meters) and SBAS (around 1 m), and is sufficiently accurate for surveying and construction management tasks.

One point to note in actual use is that it can take a little time from the start of positioning until a high-precision Fix solution is obtained. With the CLAS method, even after starting the receiver and acquiring satellites, it may take tens of seconds to about a minute for the corrections to be reflected and the error to converge to a few centimeters. In the initial convergence phase, errors may remain on the order of several tens of centimeters (equivalent to a “Float” solution in RTK). Therefore, when starting measurements on site, it is advisable not to rush and to wait about a minute for stabilization before taking official measurements. Once centimeter-level accuracy is reached, positioning will continue stably within a few centimeters, allowing surveying work to proceed with confidence.

Handling a CLAS-compatible receiver is fundamentally the same as a conventional RTK receiver. No complicated sighting like with a total station is required; simply place the GNSS antenna at the point to be measured and receive satellites. Depending on the model, many devices let you check the current positioning mode (transition from standalone → SBAS → CLAS) and accuracy via a dedicated controller or smartphone app. Maintaining a Fix solution (centimeter-level) is best achieved with a clear view of the sky. Even in mountainous areas, satellite availability is sufficient if the sky above is open, but in locations where satellite visibility is severely blocked—such as directly under dense tree cover or under overpasses—accuracy may degrade or the solution may become unstable even with CLAS. This point is a common caveat across RTK and GNSS positioning in general.

Differences and Comparison with Conventional RTK (Network-type, UHF-type)

How does the CLAS method compare with RTK methods traditionally used in surveying (network RTK or standalone RTK using a local base station)? Below are the main comparison points.

• Presence of a base station: The biggest difference is whether the user must provide a base station. With standalone RTK, the user installs a base station unit on a known point and transmits corrections to the rover via radio. Network RTK (VRS, etc.) doesn’t require the user to install a base station, but it does require obtaining correction data over the internet from a reference station network. CLAS, on the other hand, requires no base station equipment on the user side and does not require communications to obtain corrections. The national reference station network is used as a “virtual base station,” and its data are distributed directly from satellites, so the user only needs to take a single receiver into the field.

• Communication environment and cost: Network RTK assumes the use of cellular networks, so communication contracts and on-site signal conditions must be considered. High-precision correction services like VRS are often paid services (annual contracts or monthly fees). Standalone RTK also requires radio communications between base and rover, incurring initial equipment costs and potentially radio licensing procedures. CLAS completes corrections via satellite reception, so there are no communication fees and the service itself is free (once you have the equipment). It works without cellular coverage, and you don’t need to worry about service contract renewals or communication outages.

• Positioning coverage: With local base-station RTK, baseline length (distance from the base) significantly affects accuracy; beyond about 10 km the correction effect diminishes and errors increase. Network RTK with a widespread reference station network alleviates the baseline problem but still requires being within the provider’s service area (for example, within a given prefecture). CLAS covers all of Japan, so you can use the same method consistently even while moving over wide areas. Even when surveying on remote islands or offshore, you don’t need to relocate base stations or switch VRS service areas. CLAS provides uniform correction information regardless of baseline length.

• Positioning accuracy and initialization time: In terms of accuracy, conventional RTK can be slightly superior in some cases. A high-quality local base station operated very close to the rover can readily achieve horizontal accuracy around 2 cm and vertical around 5 cm almost instantaneously, with initial Fix often in seconds. CLAS, as mentioned above, may require about 30 seconds to a minute to converge to horizontal accuracy of a few centimeters and vertical around 10 cm. Vertical accuracy tends to be somewhat inferior compared with RTK. However, once CLAS converges it maintains practically useful accuracy stably, and considering the advantages of no communication needs, CLAS can be effectively used at many sites. Rather than being strictly better or worse, RTK and CLAS are complementary technologies that are best used according to their respective strengths.

• Reliability and risks: Network RTK depends on communications, so it becomes unusable if communications infrastructure is down during disasters. Standalone RTK also faces risks such as base station power failures, equipment malfunction, or radio interference. CLAS depends on the satellite system, but it is operated with high stability as national infrastructure. However, there are times when the Michibiki CLAS signal may be temporarily suspended (turned off) according to the satellite operation schedule, so it is recommended to check official information and schedules before critical surveys. Overall, it is sensible to use CLAS as a convenient everyday solution while preparing to switch to conventional RTK if CLAS becomes unavailable, treating the two as mutual backups.

Conditions and Points to Note for Using CLAS

When leveraging centimeter-level positioning with CLAS, there are several conditions and points to be aware of in advance. Key items are summarized below.

• Prepare a compatible receiver: As repeated above, a CLAS-compatible GNSS receiver capable of receiving the L6 band is essential to use CLAS. Many survey GNSS units, especially older models or those not L6-capable, cannot use CLAS, so confirm compatibility when purchasing equipment.

• Service area: CLAS correction information is provided only within Japan (roughly within the area where Michibiki is visible). CLAS cannot be received overseas; for high-precision positioning outside Japan you must use other countries’ SBAS or local RTK services. Even within Japan, deep valleys or areas with strong terrain masking might temporarily block Michibiki from view.

• Initial convergence time: Note that CLAS positioning may not provide centimeter accuracy immediately after startup. As mentioned, it can take about 30 seconds to 1 minute for high-precision positioning to converge. This is the time required for correction information received from satellites to be fully reflected in the receiver. When beginning measurements or re-acquiring satellites, be prepared to wait briefly before relying on results.

• Suspension risk: The Michibiki satellites occasionally suspend CLAS signal transmission for maintenance or orbit adjustments a few times a year. Suspension schedules are published in advance by relevant authorities such as the Cabinet Office and QZSS official sites, so check schedules to ensure they do not conflict with important workdays. If CLAS is not receivable on site, calmly switch to other correction means as described later.

• Satellite reception environment: As with any high-precision GNSS positioning, ensuring as little obstruction as possible is fundamental for quality improvement. In particular, since CLAS is one-way communication from satellites, it cannot be received inside tunnels, indoors, or in extremely dense woodlands. During positioning, regularly check the number of satellites being tracked and DOP values (dilution of precision), and if necessary move location, secure a clearer view of the sky, or take other countermeasures.

Features and Adoption Benefits of LRTK Smartphone-Type CLAS-Compatible Receivers

As described above, CLAS has made “RTK positioning without a base station” a reality. However, to utilize CLAS you need a device capable of receiving and processing the signal. Enter the small, high-precision GNSS receivers that pair with smartphones—the LRTK series. LRTK is a product lineup developed by Refixia, a startup originating from Tokyo Institute of Technology, and it follows a concept distinct from traditional fixed GNSS receivers. Variants include the smartphone-mounted LRTK Phone and the LRTK Pro2 suitable for pole mounting or vehicle installation; all are CLAS-compatible high-precision GNSS receivers.

A major feature of the LRTK series is that it enables easy RTK positioning integrated with a smartphone. For example, the LRTK Phone incorporates an ultra-compact receiver module weighing about 125 g and about 13 mm thick in a dedicated case that attaches to a smartphone (iPhone/Android), and is used via Bluetooth connection. Antenna, receiver, battery, and communication modules are all integrated, so with just a smartphone you can achieve centimeter-level positioning on site. Launching the dedicated app allows one-tap selection of correction modes and start of positioning, and the high-precision position data obtained can be shared and stored in the cloud for team use. Enabling surveying that previously required expensive specialist equipment handled by skilled technicians to be done with “one smartphone per person” can significantly improve on-site productivity.

The LRTK series is also designed to be small and lightweight yet rugged, suited for harsh construction environments. It offers dust and water protection equivalent to IP67, so sudden rain or dusty sites are not a concern. The LRTK Pro2 includes an antenna tilt compensation function, allowing accurate coordinate computation even when the pole is tilted. This is useful under branches, on cliff edges, or other situations where the antenna cannot be held perfectly vertical, enabling surveying in places that were previously difficult.

Technically, LRTK supports multi-GNSS and multi-frequency reception to provide stable positioning in urban and mountainous areas. It can track major satellites such as GPS, GLONASS, Galileo, BeiDou, and Japan’s quasi-zenith satellites. Furthermore, because it supports three or more frequencies including L1/L2/L5/L6 (including the CLAS L6 band), it is effective at removing ionospheric errors and stabilizing integer ambiguity resolution (Fix solution). As a result, it captures as many satellites as possible even in masked environments and demonstrates unique convenience as a device that “can be carried anywhere and used immediately” for high-precision positioning. There are reports of LRTK maintaining standalone surveying at disaster sites where cellular coverage was lost, so it is gaining attention as a backup measurement method that does not depend on communications infrastructure.

The adoption benefits from these features are clear. In cost terms, equipment that previously cost several million yen for RTK surveying can be dramatically reduced in price, and using free CLAS reduces running costs. Operationally, high portability means each worker can carry a device, enabling immediate positioning and recording when needed. Without the need to set up base stations or configure communications, work can start right after arriving on site, saving time. LRTK’s ability to position without cellular service or during disasters is also useful for risk hedging, contributing to resilience in infrastructure maintenance. Moreover, intuitive smartphone-linked operation makes the devices easier to use for personnel without specialized training, promoting on-site adoption.

By introducing CLAS-compatible LRTK receivers, surveying work can be dramatically streamlined, enabling a more flexible and resilient operation. Lowering the barrier to deploying high-precision positioning will strongly support DX (digital transformation) in construction management and inspection workflows.

Use Cases of LRTK CLAS in Surveying Fields

Finally, here are representative use cases where CLAS-compatible receivers (LRTK) are proving useful in actual surveying and construction sites. The power of base-station-free high-precision GNSS is evident across many scenarios; below are typical examples.

• As-built surveys (post-construction shape verification): In managing as-built conditions for roads and landfills, it is important to detect differences from design values at the centimeter level. Using CLAS-compatible receivers, completed structures and terrain can be measured with high accuracy in a short time. On remote dam construction sites where communications are unstable, LRTK enables standalone as-built surveys and immediate data sharing, speeding up and simplifying quality control.

• Disaster response and surveying at disaster sites: Immediately after earthquakes or landslides, rapid assessment of current conditions and planning for recovery are required. Even if communications infrastructure is damaged, CLAS-compatible GNSS allows surveying on site. For example, during the 2024 Noto Peninsula offshore earthquake, LRTK receivers were reportedly effective in measuring cracks and subsidence in areas with communication outages. In peacetime, LRTK is also being used for autonomous monitoring at observation points without network coverage, such as hillside landslide monitoring.

• Improving record accuracy in infrastructure inspections: High-precision GNSS is becoming a new tool for road, railway, and bridge inspections. For example, when crew members carry LRTK receivers to巡回測定(sequentially measure) track or pavement deformations, accurate coordinates of anomalies can be obtained, significantly improving positional reliability compared with visual inspection and paper records. In bridge inspections, attaching high-precision position tags from LRTK to photos of damage makes it easier to quantitatively compare future changes over time. Enhancing position information with CLAS-compatible receivers is expected to contribute to DX in infrastructure maintenance.

• Other application scenes: Beyond the above, CLAS-compatible high-precision GNSS is being applied across many fields. For example, in machine guidance for construction equipment or ICT construction, machines can be guided with high precision without relying on communications, improving safety and construction accuracy. In agriculture, CLAS corrections can enable precision farming with centimeter-level control of autonomous tractors, and in marine surveys CLAS is used for offshore positioning. Regardless of location or application, the “base-station-free” high precision of CLAS will find increasing opportunities for use.

FAQ

Q: What is CLAS? A: CLAS stands for the Centimeter Level Augmentation Service and is a high-precision positioning service provided by Japan’s Quasi-Zenith Satellite System “Michibiki.” It distributes correction data derived from the Geospatial Information Authority of Japan’s Continuously Operating Reference Station network via satellite, and CLAS-capable receivers apply these corrections to GNSS positioning to improve real-time accuracy to the centimeter level. In short, CLAS is a mechanism that realizes RTK-level positioning using only satellites, eliminating the need for a local base station or communications.

Q: What do I need to use CLAS? Do I need a base station or a communication line? A: To use CLAS you only need a CLAS-capable GNSS receiver. You do not need to set up a base station yourself, nor do you need an internet connection to receive correction information. Simply power on the receiver and receive the L6 signal from the satellites, and high-precision positioning will begin automatically. Receiving the CLAS signal itself is free (the service is provided without charge). Therefore, while you must purchase a compatible receiver as an initial cost, there are basically no ongoing usage fees.

Q: What accuracy can I expect with CLAS? A: In general, horizontal positioning errors are on the order of a few centimeters, and height errors are around 10 cm. In open-sky conditions you can often achieve horizontal accuracy within 5 cm and vertical within 10 cm. However, immediately after starting positioning, corrections may not have fully propagated and accuracy can be lower (on the order of several tens of centimeters). After tens of seconds to about a minute the corrections stabilize and centimeter-level accuracy is maintained. Note that accuracy can temporarily degrade in urban canyons or forests where satellite reception is poor, so environmental conditions will affect results.

Q: How does CLAS differ from conventional RTK positioning? A: The main difference is the presence or absence of a base station and communications. Conventional RTK requires a base-station GNSS set up by the user, which transmits corrections to the rover to achieve centimeter-level accuracy. Network RTK lets you avoid setting up a base station but requires obtaining correction data via the internet from a service provider. CLAS uses data from the national reference network distributed via Michibiki satellites, so users do not need base stations or communications. CLAS is groundbreaking in terms of ease of use, but there are some caveats: RTK can often produce an initial Fix in seconds, whereas CLAS may take tens of seconds to fully converge; RTK horizontal accuracy can be about 2 cm while CLAS is around 5–6 cm in typical cases. Nonetheless, CLAS offers practical accuracy and nationwide availability—choose RTK or CLAS based on the application and consider using both complementarily.

Q: What is LRTK? A: LRTK is a smartphone-integrated high-precision GNSS receiver series developed by Refixia. A small receiver attaches to a smartphone to receive CLAS signals from Michibiki and other GNSS satellites, enabling centimeter-level positioning. Antenna, receiver, and power supply are integrated in an all-in-one design, dramatically improving portability compared with conventional large RTK equipment. The receiver connects to a smartphone via Bluetooth, and positioning start and data recording are easily controlled via a dedicated app. The LRTK series includes the smartphone-case-integrated "LRTK Phone" and the "LRTK Pro2" for fixed or vehicle-mounted use; both are CLAS-compatible. With LRTK, your smartphone becomes a high-precision surveying device without carrying a base station, allowing anyone on site to use high-precision positioning easily.

Q: What should I do if the CLAS correction signal cannot be received? A: Common reasons CLAS might be temporarily unavailable include satellite visibility being blocked or the satellite being out of service (suspended transmission). If CLAS cannot be used on site, the receiver will automatically switch to SBAS or standalone positioning modes, which will reduce accuracy to the meter level. If cellular communication is available, you can switch to network RTK (NTRIP, etc.) to receive corrections. Many high-precision GNSS receivers, including LRTK, support both CLAS and network RTK and can change modes depending on the situation. If there is no communication and no alternative, you can wait for re-acquisition of satellites or improve accuracy later with post-processing. Planned CLAS suspensions are usually brief and scheduled, so checking schedules in advance can prevent major issues. Once conditions allow, CLAS will resume and centimeter-level positioning will be possible again.