Basic of RTK Positioning and Traditional Correction Methods

RTK (Real-Time Kinematic) positioning is a method for obtaining centimeter-level positioning accuracy in real-time. The principle involves comparing the carrier phase of GNSS satellites received by both a base station, which has a known coordinate, and a rover (mobile unit), and correcting the error factors in distance measurements to calculate a high-precision position. Typically, RTK positioning achieves high accuracy of about 2 cm horizontally and 4 cm vertically, with the initial fixed solution (integer ambiguity resolution) being calculated within seconds, which is a key advantage.

However, the downside is that continuous correction data from the base station (known point) is required, and without a base station, RTK positioning cannot be established.

As a result, traditionally, the installation of a base station or the acquisition of correction data via communication infrastructure has been the key to the operation of RTK systems.

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The traditional RTK correction methods can be broadly categorized into the following approaches:

• Local Base Station Method (Standalone RTK): In this method, the user installs a base station GNSS receiver at a known point and transmits RTK correction data (such as RTCM format) to a rover via wireless communication (e.g., UHF-band specific low-power radio or digital simplex radio). The base station and rover are configured in a one-to-one setup, which is relatively simple. However, it involves the effort of setting up the base station and is constrained by communication distance. As the distance from the base station increases, the correction accuracy diminishes, and typically, errors become significant when the distance exceeds 10 km.

• Network-based RTK Method (Ntrip method, VRS method): In this method, correction information is received via the internet. By using data from multiple base stations, such as the Geospatial Information Authority of Japan's electronic reference point network (about 1300 stations), a virtual reference station (VRS) is generated near the user’s location, and correction information is distributed. The user can obtain correction data via an Ntrip client (via a dedicated app or receiver functionality) through the mobile network, enabling RTK positioning with only a single rover. In this method, there is no need for a base station installation, and the issue of accuracy loss due to baseline length (distance from the base station) is essentially resolved. However, it requires a contract with a VRS data provider, and there are ongoing costs such as annual or monthly fees. Additionally, this method cannot be used in areas outside of mobile network coverage.

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As mentioned above, traditional RTK positioning required either the "installation of a base station" or the "acquisition of correction data via communication infrastructure." Therefore, in remote areas, locations with poor communication environments, or situations where base stations and communications fail during disasters, high-precision positioning had to be abandoned or dealt with using post-processing methods.

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What is the Centimeter-Level Augmentation Service (CLAS) from QZSS (Michibiki)?

So, what exactly is the Centimeter-Level Augmentation Service (CLAS) provided by QZSS (Michibiki)? CLAS is a free, high-precision positioning augmentation service provided by the Japanese Quasi-Zenith Satellite System (QZSS), which offers "satellite communication-based RTK correction information distribution" covering all of Japan. Technically, CLAS adopts a method called PPP-RTK (Precise Point Positioning-RTK), where error information is calculated from the ground-based electronic reference point network (GEONET), and then transmitted to the user via the Quasi-Zenith satellites.

Specifically, GNSS positioning errors collected by electronic reference points are analyzed for satellite positioning, and the error correction data (such as satellite orbit and clock errors, ionospheric and tropospheric errors) is transmitted via the L6D signal of Michibiki. The user receives the L6D signal using a CLAS-compatible receiver, applying the correction to their own positioning, thus achieving centimeter-level high-precision location.

The biggest difference between CLAS and conventional RTK is that "users do not need to set up their own base stations" and "do not need to use communication lines to obtain correction information." CLAS uses the virtualized correction data from government-operated base stations (electronic reference points), which is directly broadcasted by the Michibiki satellite. This means there is no accuracy degradation due to distance from the base station, and uniform correction information is available anywhere in Japan. Moreover, since there is no need for an internet connection, it can be used in areas where mobile communication is unavailable, such as mountain areas or offshore locations.

Additionally, the fact that receiving the augmentation signal from the satellite is free of charge is a significant advantage (though purchasing a CLAS-compatible receiver is necessary).

However, there are some considerations when using CLAS. First, a dedicated GNSS receiver capable of receiving the CLAS signal (L6D) is required. Regular GPS-only receivers or smartphone-integrated GPS cannot handle L6 signals, so equipment that supports CLAS must be used. Secondly, the accuracy of CLAS may be slightly lower compared to conventional RTK. For example, horizontal accuracy is around 6 cm in CLAS, while RTK fixed solution is about 2 cm, and there may be a delay of about 30 seconds to 1 minute for accuracy to converge to high precision. During this time, the accuracy may remain at a float solution level (decimeter-level), so care should be taken for tasks requiring immediate, extreme precision. However, after about a minute, the accuracy converges to within a few centimeters, and it maintains stable, high precision afterward.

Furthermore, CLAS is only available within Japan (within the visible range of Michibiki), so it cannot be used outside of Japan.

Given these points, RTK and CLAS should be considered complementary rather than competitive. In fact, there are many cases where "CLAS is sufficient for tasks that do not require millimeter-level accuracy or immediate results," and it is expected that some tasks that traditionally required RTK will be replaced by CLAS.

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Comparison of RTK and CLAS The main differences between RTK positioning and CLAS positioning are summarized as follows:

ItemConventional RTK Positioning (Base Station/Ntrip/VRS Method)MICHIBIKI CLAS Positioning (PPP-RTK Method)

Need for Base StationRequired (set up your own base station or use a base station network)Not required (uses electronic reference point data via satellite)

Communication InfrastructureRequired (receive correction data via radio or mobile internet)Not required (directly receives correction signal from MICHIBIKI satellite)

Positioning AccuracyHorizontal: about 2 cm, Vertical: about 4 cm (Fixed Solution)Horizontal: about 6 cm, Vertical: about 12 cm (After convergence)

Initial Convergence TimeFixed solution within a few secondsHigh-accuracy position obtained within about 1 minute

Accuracy Degradation with DistanceYes (Accuracy decreases as the distance from the base station increases)No (Uniform accuracy across Japan)

Service FeesPaid (Requires equipment cost, VRS, and service contracts)Free (No cost to receive correction signals)

Available AreaWithin a few dozen km from the base station or VRS service areaNationwide (within MICHIBIKI service area in Japan)

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Notes
While high precision is achieved immediately, it is vulnerable during communication interruptions or in disaster situations. Initial convergence takes time and requires specialized equipment. However, it is strong in disaster situations as it does not depend on communication.

Thus, CLAS is an "RTK augmentation service that can be used over a wide area without relying on communication infrastructure," and it significantly improves the issues of installation costs, operational burdens, and the inability to use it in areas without communication, which were challenges with traditional methods.

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Benefits of RTK Positioning with Michibiki (QZSS) CLAS and Use Cases

How does using Michibiki (QZSS) CLAS bring specific benefits to RTK positioning? Here, we introduce the main advantages and expected use cases.

• High-precision positioning independent of communication environments: The greatest strength of CLAS is its ability to receive correction information directly from satellites, without relying on ground-based communication systems like mobile networks or Wi-Fi. As long as the satellite signals can be received, positioning continues accurately even in mountainous areas, forests, remote islands, and offshore locations, or in cases where communication infrastructure fails during a disaster. For example, during the 2023 Noto Peninsula earthquake, where local mobile base stations were down and the internet was unavailable, a small RTK receiver compatible with CLAS (LRTK, described below) played a significant role in surveying the disaster site. Thus, CLAS is extremely useful as a backup in emergencies.

• No need for base station setup or service contracts: Traditionally, performing RTK positioning required expensive base station equipment and contracts for paid correction data services. Michibiki's CLAS eliminates these requirements, allowing anyone to use high-precision positioning without additional costs. This is particularly beneficial for small- and medium-sized civil engineering firms and surveyors, lowering the barriers of initial investment and running costs. Moreover, by eliminating the need to set up base stations, work efficiency improves, as surveying or machine operations can begin immediately upon arrival at the site.

• Wide-area mobility and continuous operation across multiple sites: CLAS is a satellite-based correction service that covers all of Japan, so you can continue to use it consistently even when moving over large distances during operations. For instance, in infrastructure inspections or patrols on highways or railroads, or when surveying large properties, there’s no need to move or change base stations or switch between VRS service areas. As long as there’s a clear view of the sky, correction data can be continuously received in real-time, regardless of location. This feature also supports use cases like drone surveying and mobile mapping systems (MMS), enabling continuous positioning and sensing over wide areas.

• Wide-area mobility and continuous operation across multiple sites: CLAS is a satellite-based correction service that covers all of Japan, so you can continue to use it consistently even when moving over large distances during operations. For instance, in infrastructure inspections or patrols on highways or railroads, or when surveying large properties, there’s no need to move or change base stations or switch between VRS service areas. As long as there’s a clear view of the sky, correction data can be continuously received in real-time, regardless of location. This feature also supports use cases like drone surveying and mobile mapping systems (MMS), enabling continuous positioning and sensing over wide areas.In summary, Michibiki CLAS is expected to be widely utilized across various fields, including civil surveying, construction, infrastructure maintenance, disaster response, and agriculture.

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Leveraging LRTK: Key Features and Adoption Benefits

Thanks to the advent of CLAS, “RTK positioning that is not tied to a cellular link” has finally become a reality. To make practical use of CLAS, however, you still need a compatible receiver—and that is exactly what LRTK, a palm-sized, high-precision GNSS receiver that pairs with your smartphone, provides.
Developed by Lefixea, a Tokyo Institute of Technology spin-off, LRTK departs radically from conventional, fixed-mount GNSS gear. Its main features and the advantages they deliver are outlined below.

• Seamless RTK positioning with your smartphone:
  LRTK is an RTK-GNSS receiver that attaches directly to an iPhone or Android device. The ultra-compact module snaps into a dedicated phone case and connects to the handset via Bluetooth, delivering real-time centimetre-level accuracy. Weighing about 125 g and only 13 mm thick, it slips easily into a pocket and includes its own battery—turning any smartphone into a professional-grade surveying instrument. Launch the companion app and positioning begins instantly; the high-precision data can then be shared and stored through the cloud. As a result, surveying tasks that once demanded specialised equipment can now be performed by a single person with a single phone, greatly improving on-site productivity.

• Compact, lightweight, and rugged all-in-one design:
  The LRTK line includes several form factors— the phone-integrated “LRTK Phone,” the tripod-ready “LRTK Pro 2,” and even a helmet-mounted model. Every version houses the antenna, GNSS receiver, battery, and communication module in a single enclosure that is both small and tough enough for harsh construction sites.LRTK Pro 2, for example, offers superior dust- and water-proofing plus built-in tilt compensation, allowing you to capture accurate tip coordinates even when the survey pole is angled. This proves invaluable when trees or other obstacles prevent holding the pole perfectly vertical, turning previously challenging points into quick, reliable measurements.

• Stable positioning through multi-GNSS and multi-frequency support:
  LRTK tracks not only GPS but also GLONASS, Galileo, BeiDou, and Japan’s QZSS (Michibiki). The receiver handles three frequency bands—L1, L2, and L5—and even decodes Michibiki’s L6-band CLAS corrections. That breadth means more satellites in view and dependable fixes, whether you’re in urban canyons or mountain valleys. CLAS compatibility is a particularly powerful edge, enabling standalone centimetre-level accuracy in areas with no cellular coverage. In fact, surveys have successfully continued at disaster sites where communications were down, thanks to LRTK’s self-contained precision. By combining multi-GNSS and CLAS, LRTK delivers unrivalled “carry-anywhere, measure-anytime” convenience in a high-accuracy positioning tool.

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The “LRTK Phone” is a compact RTK receiver that attaches to your smartphone. By clipping on a device weighing only about 125 g, your phone instantly turns into a centimetre-level surveying instrument. Thanks to its cloud-connected app, anyone can make use of high-precision positioning with ease.

These features make the benefits of adopting LRTK clear. Cost-wise, equipment that once cost several million yen can now be introduced at a fraction of the price, while running costs stay low thanks to the free CLAS service. Operationally, its portability lets every field worker carry a unit and obtain positioning and record data instantly whenever needed.

This boosts workforce efficiency and shortens task times, accelerating the digital transformation (DX) of construction and inspection workflows. Because LRTK can still provide positioning outside cellular coverage or in disaster situations, it also functions as a risk-mitigation tool that strengthens the resilience of infrastructure maintenance. And with its familiar, smartphone-based interface, even personnel without specialized training can operate the system intuitively—further easing on-site adoption.