For Surveyors: LRTK CLAS-Compatible Receiver Achieves Centimeter-Level Positioning Without a Base Station (cm level accuracy (half-inch accuracy))

Table of Contents

• What are Japan’s Quasi-Zenith Satellite System “Michibiki” and CLAS?

• Basic Structure and Benefits of CLAS-Compatible Receivers

• Practical Positioning Accuracy and How to Use It

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

• Conditions and Points to Note for Using CLAS

• Features and Implementation Benefits of the LRTK Smartphone-Type CLAS-Compatible Receiver

• Field Use Cases of LRTK CLAS in Surveying

• FAQ

What are Japan’s 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 there is always at least one satellite at a high elevation angle (near the zenith) over Japan, making satellite reception easier even in mountainous areas or urban canyons. As a result, positioning that had errors of about 10 m (32.8 ft) with GPS alone can be greatly improved to around 1 m (3.3 ft) down to a few centimeters (cm level accuracy (half-inch accuracy)) by using Michibiki.

One particularly notable new technology is the Centimeter Level Augmentation Service (CLAS). CLAS (Centimeter Level Augmentation Service) creates correction data based on positioning error information obtained from the Geospatial Information Authority of Japan’s (GSI) Continuously Operating Reference Stations (GEONET) and broadcasts that data via the Michibiki satellites. Simply put, it is a satellite-communication-type RTK correction information service that can be received anywhere in Japan. When a user’s receiver obtains the CLAS signal (QZSS L6 band), it applies those corrections to the GNSS positioning from GPS, GLONASS, etc., and can compute real-time positions with errors of only a few centimeters.

The biggest feature of CLAS is that it enables centimeter-level positioning without surveyors having to provide their own base stations. With conventional RTK, users needed to install a GNSS base station at a known point and transmit corrections by radio to rovers, or obtain correction data from a reference station network via mobile internet (e.g., VRS). CLAS, in contrast, virtually uses the nation-maintained Continuously Operating Reference Stations as the reference stations, and those corrections are broadcast directly from the satellites, so users don’t need to prepare base station equipment or communication lines. Because it does not depend on terrestrial communications infrastructure, it can be operated in mountain areas or at sea where mobile signals do not reach, and receiving the correction signals themselves is free (the satellite broadcast service is provided publicly). CLAS truly realizes “RTK usable nationwide without a base station.”

Basic Structure and Benefits of CLAS-Compatible Receivers

To benefit from CLAS, you need a compatible GNSS receiver. The point is that ordinary standalone GNSS units or smartphone GPS cannot receive Michibiki’s CLAS signal (L6 band). CLAS-compatible receivers are equipped with multi-band GNSS antennas and receiving modules that include the L6 band. Specifically, they track GPS, GLONASS, Galileo, BeiDou, and the Quasi-Zenith Satellite, and their dedicated hardware and software can process the L6 augmentation signal.

Because such CLAS-compatible GNSS receivers can perform high-precision positioning standalone without relying on base stations or external communications, they offer the following advantages compared to conventional RTK equipment:

• No internet required—usable anywhere: CLAS delivers correction information directly from satellites, so you don’t need to connect a mobile router or tether a smartphone on site. As long as the satellites are visible, centimeter-level positioning is possible even in deep mountains or areas with no mobile coverage. For example, at disaster sites where cellular networks are down, a CLAS-compatible receiver allows survey work to start immediately.

• Low power consumption for long operation: Not using external communication modules reduces the receiver’s power consumption. Compared to continuous use of UHF radios or 4G communication, battery load is lighter and long continuous positioning on site is possible. You don’t have to worry about draining your smartphone battery searching for a mobile signal in remote mountains.

• Compact and highly mobile: CLAS-compatible receivers have become smaller recently, and compact models with integrated antennas are available. Because the receiver is self-contained, you don’t need to transport large base station equipment or long antenna poles, making them easy to carry and set up. This suits agile tasks like a single surveyor quickly visiting and capturing multiple points.

As described above, CLAS-compatible receivers enable a new positioning style that is “base-station-less and communication-less.” They make RTK-style positioning in mountainous areas and high-precision surveying where equipment or communication constraints previously made such work difficult much more straightforward.

Practical Positioning Accuracy and How to Use It

What level of accuracy can be obtained using a CLAS-compatible receiver? In short, you can stably achieve horizontal errors of a few centimeters and vertical errors of several tens of centimeters. For example, experiments and operational results often show horizontal errors around 3–6 cm (1.2–2.4 in) and height errors often around 10 cm (3.9 in). This is a dramatic improvement over ordinary standalone positioning (errors of several meters to tens of meters) and SBAS (errors around 1 m (3.3 ft)), and is sufficiently accurate for surveying and construction management work.

One thing to note in actual use is that it can take a little time from startup to obtain a high-precision Fix solution. With CLAS, even after turning on the receiver and starting satellite acquisition, it may take several tens of seconds to about 1 minute for the corrections to be fully applied and for errors to converge to a few centimeters. In the initial convergence stage, errors may remain on the order of several tens of centimeters (equivalent to a “Float” solution in RTK). Therefore, when starting surveying on site, wait calmly for about a minute for stabilization before taking critical measurements. Once centimeter-level accuracy is reached, positioning will remain stable within a few centimeters, allowing you to proceed with surveying tasks with confidence.

Handling a CLAS-compatible receiver is basically the same as conventional RTK receivers. No complex sighting like with total stations is required—simply place the GNSS antenna at the point to be measured and receive satellites. Depending on the model, many units allow you to check the current positioning mode (standalone → SBAS → CLAS transition status) and accuracy via a dedicated controller or smartphone app. To maintain a Fix solution (centimeter accuracy), an open sky is ideal. Even in mountainous areas, as long as the sky above is visible you can get enough satellites, but under tree canopies or beneath elevated structures where satellite visibility is severely blocked, CLAS accuracy may degrade or solutions may become unstable. This is a common caveat for RTK and GNSS positioning in general.

Differences and Comparison with Conventional RTK (Network Type, UHF Type)

How does CLAS compare with RTK methods long used in surveying (network RTK and standalone RTK using local base stations)? Below are the main points of comparison.

• Presence of a base station: The biggest difference is whether users need to provide a base station themselves. With standalone RTK, users install a base station at a known point and transmit corrections by radio to the rover. Network RTK (VRS, etc.) does not require the user to install a base station, but it does require obtaining correction information over the internet from a reference station network. With CLAS, users do not need any base station equipment at all, nor do they need communication to obtain corrections. The nation-maintained reference stations are effectively used as virtual base stations, and their data are broadcast directly from satellites, so users only need a single receiver to complete the workflow.

• Communication environment and cost: Network RTK depends on mobile networks, so you must consider subscription plans and site signal availability. Many high-precision correction services like VRS are paid (annual or monthly fees). Standalone RTK requires radio links between base and rover, incurring initial costs and radio licensing procedures. CLAS is completed via satellite reception, so there are no communication fees and no service charges (once you purchase the equipment, you can use it without additional recurring costs). It also works in areas outside mobile coverage, and you don’t need to worry about subscription renewals or communication outages.

• Positioning coverage: With local base station RTK, baseline length (distance from the base) greatly affects accuracy, and accuracy degrades when more than 10 km (6.2 mi) away. Network RTK alleviates baseline length issues by providing a reference station network, but it still depends on the service coverage area (e.g., within a particular prefecture). CLAS covers all of Japan, so you can move widely during positioning while using the same method consistently. For islands or offshore work, you don’t need to relocate base stations or worry about VRS area switching. CLAS provides uniform augmentation information anywhere in Japan without baseline-length-related accuracy degradation.

• Positioning accuracy and initialization time: In terms of accuracy, conventional RTK can be slightly better in some cases. A well-operated local base station close to the rover can readily provide horizontal accuracy of about 2 cm (0.8 in) and vertical about 5 cm (2.0 in) almost instantly, with initial Fix times within a few seconds. CLAS, as noted above, typically converges to horizontal few cm and vertical around 10 cm (3.9 in) in about 30 seconds to 1 minute. Height accuracy tends to be slightly inferior to RTK. However, once CLAS converges it maintains practically sufficient accuracy, and considering the advantage of needing no communications it is useful on many sites. RTK and CLAS are best thought of as complementary technologies to be used according to requirements.

• Reliability and risk: Network RTK depends on communications, so if communication infrastructure fails in a disaster it becomes unusable. Standalone RTK also has risks such as base station power loss, equipment failure, or radio interference. CLAS depends on the satellite system, which is operated with high stability as national infrastructure. However, CLAS signals may be temporarily suspended (taken offline) due to Michibiki operational schedules, so for critical surveys it is recommended to check official schedules in advance. Overall, it is reassuring to use CLAS for routine ease and have conventional RTK as a fallback in case CLAS becomes unavailable.

Conditions and Points to Note for Using CLAS

To effectively use CLAS for centimeter-level positioning, be aware of the following conditions and cautions.

• Prepare a compatible receiver: As repeated above, CLAS requires a CLAS-compatible GNSS receiver capable of receiving the L6 band. Older survey GNSS units or models without L6 support cannot use CLAS, so confirm compatibility when introducing equipment.

• Service area: CLAS correction information is provided within Japan (roughly where Michibiki is visible). CLAS cannot be received overseas, so when you need high-precision positioning outside Japan you must use other countries’ SBAS or local RTK services. Even within Japan, deep valleys or severe terrain shielding can temporarily block Michibiki visibility.

• Initial convergence time: Be aware that centimeter accuracy may not be available immediately after starting positioning. As noted, it can take about 30 seconds to 1 minute for corrections to stabilize and for high-precision positions to be obtained. When starting surveys or re-acquiring satellites, wait a bit rather than expecting immediate centimeter results.

• Signal suspension risk: Michibiki may occasionally suspend CLAS broadcasts for satellite maintenance or orbit adjustments. Such planned suspensions are announced in advance by the Cabinet Office or the QZSS official site, so check schedules to avoid critical workdays. If CLAS cannot be received on site, remain calm and switch to other correction methods as needed.

• Satellite reception environment: As with any high-precision GNSS, ensure an environment with minimal obstructions. CLAS is a one-way satellite broadcast, so it cannot be received in tunnels, indoors, or very dense forests. During positioning, periodically check visible satellite count and DOP values, and if necessary relocate to a site with better sky visibility.

Features and Implementation Benefits of the LRTK Smartphone-Type CLAS-Compatible Receiver

CLAS has made “RTK without a base station” a reality. However, to use CLAS you need a device that can receive and process the signal. Enter the LRTK series of small, high-precision GNSS receivers that can work with smartphones. LRTK is a product line developed by Reflexia, a startup originating from Tokyo Institute of Technology, and it presents a concept distinct from traditional fixed GNSS receivers. Variants include the smartphone-mounted LRTK Phone and the pole-mounted or vehicle-mounted LRTK Pro2, all of which are CLAS-compatible high-precision GNSS receivers.

A major characteristic of the LRTK series is that they allow 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 (0.51 in) thick into a dedicated case that attaches to a smartphone (iPhone/Android), and connects via Bluetooth, etc. The antenna, receiver, battery, and communication module are all integrated, so with just a smartphone you can perform centimeter-level positioning on site. Launch the dedicated app and you can select correction modes and start positioning with a single tap; the high-precision location data obtained can be shared and stored in the cloud for team use. Enabling surveying tasks that formerly required expensive specialist equipment and skilled operators to be done with one smartphone per person can greatly improve field productivity.

The LRTK series is also compact and lightweight yet robustly built to withstand harsh environments such as construction sites. It has dust and water resistance equivalent to IP67, so it is safe in sudden rain or dusty conditions. The LRTK Pro2 includes an antenna tilt compensation feature, enabling correct coordinate calculation even when the pole is leaned. This is useful under tree branches or on cliff edges where you cannot set the antenna perfectly vertical, allowing surveying in locations that were previously difficult.

Technically, multi-GNSS and multi-frequency support provide stable positioning in both urban and mountainous areas. In addition to GPS, GLONASS, Galileo, and BeiDou, they can track Japan’s Quasi-Zenith Satellites. Because they support three or more frequencies including L1/L2/L5/L6, they excel at removing ionospheric errors and stabilizing integer ambiguity resolution (Fix solution determination). As a result, they capture as many satellites as possible even in obstructed environments and deliver a unique convenience as a “carry-anywhere, ready-to-measure” high-precision device. There are reports of LRTK continuing standalone surveys at disaster sites that lost cellular coverage, making it noteworthy as a backup measurement method that does not rely on communications infrastructure.

The implementation benefits are clear. Cost-wise, equipment that used to cost several million yen can be greatly reduced, and by using free CLAS, running costs are also lower. Operationally, high portability allows each worker to carry a device, enabling immediate positioning and recording when needed. Eliminating base station setup and communication configuration reduces setup time on site. LRTK’s ability to operate without cellular coverage or during disasters provides risk-hedging benefits and enhances resilience in infrastructure maintenance. The intuitive smartphone-linked operation also means staff without specialized training can use it, promoting on-site adoption.

By introducing LRTK’s CLAS-compatible receivers, surveying work can be dramatically streamlined, building a more flexible and resilient system. Lowering the barrier to field deployment of high-precision positioning will strongly support the DX (digital transformation) of construction management and inspection tasks.

Field Use Cases of LRTK CLAS in Surveying

Finally, here are some use cases where CLAS-compatible receivers (LRTK) are proving valuable on surveying and construction sites. The power of base-station-free high-precision GNSS is applicable in many situations; below are representative examples.

• As-built surveys (post-construction shape verification): In managing as-built conditions for roads or development sites, detecting deviations from design in the order of centimeters is important. Using a CLAS-compatible receiver, you can quickly and accurately measure completed structures and terrain. At a dam construction site in mountainous terrain where communications are unstable, LRTK can perform standalone as-built surveys and immediately share data, accelerating and simplifying quality control.

• Disaster response and surveying at disaster sites: After earthquakes or landslides, rapid assessment and restoration planning are required. Even if communication infrastructure is damaged, CLAS-compatible GNSS allows on-site surveying. For example, during the 2024 Noto Peninsula offshore earthquake, LRTK receivers were useful at communications-degraded disaster sites for measuring cracks and subsidence. In peacetime disaster prevention, LRTK is beginning to be used for autonomous monitoring of slopes and landslide-prone areas where network connectivity is unavailable.

• Improved recording accuracy for infrastructure inspections: High-precision GNSS is becoming a new tool for inspecting roads, railways, and bridges. For example, when巡回測定 (patrol measurement) of track or pavement deformation, staff carrying LRTK receivers can accurately record coordinates of anomalies, greatly improving positional reliability compared to visual inspection and paper records. For bridge inspections, attaching high-precision position tags from LRTK to photographed damage makes it easier to quantitatively compare future aging changes. Upgrading positional information with CLAS-compatible receivers is expected to contribute to the DX of infrastructure maintenance.

• Other applications: Beyond the above, CLAS-compatible high-precision GNSS is being applied in many fields. For example, machine guidance and ICT construction can achieve precise machine control without depending on communications, improving safety and construction accuracy. In agriculture, CLAS corrections enable centimeter-level precision farming with autonomous tractors, and in marine surveys CLAS is used for offshore positioning. Wherever “base-station-free” high precision is valuable, CLAS applications will continue to grow.

FAQ

Q: What is CLAS? A: CLAS stands for Centimeter Level Augmentation Service. It is a high-precision positioning service provided by Japan’s Quasi-Zenith Satellite System “Michibiki.” Correction information derived from the Geospatial Information Authority of Japan’s reference station network is broadcast via satellite, and compatible receivers apply the corrections to GNSS positioning to improve real-time accuracy to the centimeter level. In short, CLAS is a mechanism that achieves RTK-like positioning using satellites alone.

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 compatible GNSS receiver (CLAS-compatible receiver). You do not need to install a base station yourself, nor do you need internet connectivity to receive correction information. Simply power on the receiver and receive the L6 signal from the satellites to automatically begin high-precision positioning. Receiving the CLAS signal itself is free (the service is provided without charge). Thus, the only initial cost is purchasing a compatible receiver; there are typically no recurring usage fees.

Q: What accuracy can be achieved with CLAS? A: Typical performance is horizontal errors on the order of a few centimeters and height errors around 10 cm (3.9 in). In open-sky environments, horizontal errors often fall within 5 cm (2.0 in) and vertical within 10 cm (3.9 in). However, immediately after starting positioning, corrections may not have propagated fully and accuracy can be lower (on the order of several tens of centimeters). After tens of seconds to about a minute, correction effects stabilize and centimeter-level accuracy is maintained. In urban areas with many building shadows or inside forests, satellite signal reception may degrade and accuracy can temporarily drop, so keep in mind that environmental conditions affect results.

Q: How does CLAS differ from conventional RTK? A: The biggest difference is the presence or absence of a base station and communications. Conventional RTK requires a base GNSS station set up by the user, which transmits corrections to the rover to achieve centimeter-level accuracy. Network RTK eliminates the need to set up your own base station but requires internet to receive corrections from a network service. CLAS, however, uses the nation-operated reference station network data broadcast by Michibiki, so users do not need base stations or communication lines. While CLAS is revolutionary in convenience, there are trade-offs: for example, RTK often obtains an initial Fix within seconds, whereas CLAS may take tens of seconds to fully converge. RTK can also achieve slightly better theoretical horizontal accuracy (around 2 cm (0.8 in) for RTK vs. around 5–6 cm (2.0–2.4 in) for CLAS). Nonetheless, CLAS provides practical accuracy and nationwide coverage, and using RTK and CLAS according to needs will maximize their respective strengths.

Q: What is LRTK? A: LRTK is a series of smartphone-compatible high-precision GNSS receivers developed by Reflexia. A small receiver attaches to a smartphone and receives the Michibiki CLAS signal and other GNSS satellites to perform centimeter-level positioning. With antenna, receiver, and power integrated, it is far more portable than conventional large RTK equipment. It connects to a smartphone via Bluetooth and the dedicated app lets you easily start positioning and record data. The LRTK product line includes the case-integrated “LRTK Phone” and the stationary/vehicle-friendly “LRTK Pro2,” both CLAS-compatible. With LRTK, your smartphone becomes a high-precision surveying instrument without carrying a base station.

Q: What if I cannot receive CLAS correction signals? A: If CLAS cannot be received temporarily, possible causes include blocked satellite visibility or Michibiki being taken offline for maintenance. In such cases the receiver will automatically switch to SBAS or standalone positioning, reducing accuracy to the meter level. If mobile communications are available at the site, 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 switch modes as needed. If no alternatives exist in communications-outage areas, wait for satellite reacquisition or perform post-processing to improve accuracy later. Note that planned CLAS suspensions are usually short and announced in advance, so checking schedules can prevent surprises. Once environmental conditions are restored, CLAS can resume providing centimeter-level positioning.