Table of Contents

• What is Network RTK (NRTK)?

• What is a cloud-based (PPP) correction service?

• What is Michibiki’s Centimeter-Level Augmentation Service (CLAS)?

• How to choose correction methods by use case and region

• Smartphone-integrated operation and cloud integration

• Smartphone-integrated positioning solutions using LRTK

• Frequently Asked Questions

High-precision positioning enabled by GNSS correction information services has become increasingly important in recent years and is being widely applied in fields requiring centimeter-level accuracy, such as surveying, construction, agriculture, and autonomous driving. Even within the category of correction information services, there are various types—<u>network RTK</u>, <u>cloud-based PPP</u>, and <u>satellite augmentation CLAS</u>—and each service differs in the coverage area available and the required infrastructure (e.g., communications). Therefore, it is important to compare and evaluate each service based on local conditions and intended use to select the optimal method.

Industry trends also show demand not merely for providing correction data but for integrated positioning solutions and cloud connectivity, and whether a service can be used in areas with unstable communications—such as mountainous regions or at sea—is expected to be a key competitive factor. Against this backdrop, next-generation devices that are smartphone-integrated and support satellite augmentation services (CLAS) have emerged and attracted attention. Can “smartphone-integrated CLAS support” be the deciding factor when choosing a correction information service? This article comprehensively compares the characteristics of various services and discusses the advantages by coverage area and use case.

What is Network RTK (NRTK)?

Network RTK (NRTK) is a method that networks multiple fixed reference stations and provides correction data as if a virtual reference station were placed near the user. In Japan, the Geospatial Information Authority’s continuous GNSS network (GEONET) forms the basis of this system, and a high-density observation network of about 1,300 sites (average spacing about 20 km) has been established nationwide. GNSS data observed at each continuous station are aggregated and analyzed in real time, and this data is delivered to users via the Ntrip protocol (RTCM distribution over the Internet) to realize correction information services. A user (rover) connects to the service via mobile networks or similar, transmits their approximate position, and the server generates and provides correction data corresponding to the optimal virtual reference station. This creates a situation where it is “as if a reference station is nearby,” enabling centimeter-level RTK positioning even when the base station is tens of kilometers away.

The coverage area of network RTK is, in principle, limited to the range covered by the reference station network. In Japan, GEONET covers almost the entire country, and as long as there is network connectivity, RTK positioning using public infrastructure is available nationwide. In practice, by connecting to an NRTK service based on known point coordinates, high-precision positioning in a public coordinate system can be obtained immediately without installing a dedicated base station on site, enabling tasks that previously required two-person crews for setting reference points or surveying to be carried out efficiently by a single person. Because of this convenience, NRTK has expanded beyond surveying and construction sites to diverse fields such as self-positioning for autonomous vehicles, autonomous operation of agricultural machinery (smart agriculture), geotag correction for drone photogrammetry, and displacement monitoring for disaster-prevention infrastructure.

Advantages: Because NRTK leverages existing reference station infrastructure, users only need rover equipment and a communication method to relatively easily obtain centimeter-level correction information. Thanks to VRS methods using multiple reference stations, accuracy is maintained over long distances, and positioning results are tied to national coordinates, directly matching public surveying coordinate systems. Since positioning is relative to fixed stations, high-precision positions are obtained immediately after initialization, making it suitable for real-time control of moving objects.

Disadvantages: On the other hand, the requirement for Internet connectivity is a limitation. Correction data must be received continuously through a communication link, so the service is unavailable in areas outside mobile network coverage. Also, use often requires a contract with a private service provider, meaning monthly fees and other costs must be considered. Moreover, in countries or regions without a well-developed reference station network, NRTK services themselves may not exist, so alternative methods are needed for overseas or offshore use. In summary, NRTK is the best choice in areas with robust communication infrastructure and within existing service coverage for its accuracy and responsiveness, but note that it is unusable during communication outages or in unsupported regions.

What is a cloud-based (PPP) correction service?

A correction method that is less affected by communication infrastructure or regional constraints is the cloud-based PPP (Precise Point Positioning) service. PPP improves standalone positioning accuracy using precise orbit and clock corrections and atmospheric error models that can be applied over wide areas without placing reference stations near the observation point. Typical PPP corrects GNSS error sources globally and therefore requires time to converge initially, but it has the advantage that the geographical coverage is effectively worldwide. Such PPP correction data are distributed “cloud-based,” i.e., via the Internet, and in some cases are provided globally via L-band communication satellites on a subscription basis. If users have a compatible receiver, high-precision positioning is possible even in areas without ground station infrastructure, including offshore.

In recent years, Europe’s Galileo has started a global high-accuracy service (HAS), and Japan’s Quasi-Zenith Satellite System (QZSS) has deployed a service called MADOCA-PPP for Asia and Oceania. MADOCA-PPP computes satellite-origin errors from observatory networks in and outside Japan and transmits correction information via QZSS’s L6 signal, enabling PPP-based high-precision positioning across a wide area of the Asia-Pacific region for compatible receivers. This service began trial operations in 2022 and entered full operation in April 2024. Because it can be used for maritime and overseas positioning, it is expected to enable centimeter-level positioning at a global scale that was previously difficult.

Advantages: The strength of cloud-based PPP services is their very wide coverage. Because they do not depend on local reference station networks, they can be used seamlessly for applications that cross national or state boundaries (international surveying projects, navigation for ships and aircraft, etc.). Satellite-based services can also be received in mountainous or remote island areas without communication infrastructure, literally providing correction information “anywhere.” Some services claim global availability with better than 99% uptime and centimeter-level accuracy (actual accuracy and availability depend on the provider and receiver environment).

Disadvantages: Conversely, PPP’s drawback is the initial convergence time required to achieve high accuracy. Standalone PPP generally requires tens of minutes to resolve all error terms, making it unsuitable for dynamic real-time applications. Recently, SSR (State Space Representation) technologies and PPP-RTK methods that integrate some RTK principles have shortened convergence times, but PPP still tends to be slower than RTK by a few minutes before convergence. Also, many commercial global PPP services carry high fees, posing a cost barrier. Overall, PPP is a strong option when wide-area coverage or environments lacking infrastructure are required, but its startup time and cost-effectiveness should be considered when choosing its use.

Reference: For wide-area augmentation, SBAS (Satellite-Based Augmentation Systems: Japan’s MSAS, North America’s WAAS, Europe’s EGNOS, etc.) provides sub-meter to meter-level corrections and is free, but its accuracy (meters to decimeters) differs from the surveying-grade (centimeter-level) needs discussed here, so details are omitted.

What is Michibiki’s Centimeter-Level Augmentation Service (CLAS)?

A highly effective solution limited to Japan is the CLAS (Centimeter Level Augmentation Service) provided by the Quasi-Zenith Satellite System Michibiki (QZSS). CLAS is a satellite-based correction information service that adopts the PPP-RTK method mentioned earlier, and its most notable feature is that no base station is required. A dedicated L6D signal carrying correction data is continuously broadcast from the QZSS Michibiki satellites—often referred to as the Japanese GPS—and compatible receivers can receive this directly for position correction. Since the four-satellite QZSS configuration began operation in 2018, at least one Michibiki satellite is always near the zenith over Japan, enabling centimeter-level correction via satellite communications anywhere in the country. In experiments with CLAS-compatible receivers, exceptionally good results have been obtained—for example, open-sky horizontal RMS error of 36 mm with a 100% fix rate—making CLAS a groundbreaking service that reduced standalone positioning errors from roughly 5–10 m to high precision.

The coverage area of CLAS is almost exclusively limited to Japan (technically within QZSS visibility). As long as the satellite signal can be received, uniform correction information is available nationwide—from mountainous regions to remote islands—so centimeter-level positioning is possible without additional infrastructure. For example, in agriculture the ability to “use CLAS in any field nationwide as long as the satellite is visible” is highly valued, enabling high-precision autonomous operation even in previously difficult mid-mountain or mobile coverage–lacking areas. Also, CLAS can continue positioning even when communication networks are severed during disasters, making it useful for disaster area surveys (in the 2023 Noto Peninsula earthquake, CLAS-compatible devices recorded conditions even when outside mobile coverage).

Technical background: To address PPP’s convergence time issues for wide-area augmentation, CLAS uses a PPP-RTK core that incorporates RTK elements into PPP. Specifically, it estimates satellite orbit and clock errors and ionospheric and tropospheric delays with high accuracy based on nationwide continuous station data and transmits these corrections via satellite. This fusion of global PPP corrections with local RTK corrections significantly shortens convergence time while maintaining high accuracy. In practice, there are reports of positions converging within “under a minute,” making it viable for dynamic applications.

Advantages: CLAS’s biggest repeated benefit is that it is independent of communications infrastructure. The ability to obtain centimeter positioning alone from satellites—even in remote mountains or at sea—offers great practical value on-site. Because the correction signals are provided by the Cabinet Office, there is no usage fee, and operation without running costs is possible if compatible equipment is obtained. Uniform correction quality over a wide area also eliminates concerns about switching reception between reference points or regional coordinate system discrepancies. CLAS is being used for agricultural auto-steering, machine guidance for construction equipment, and other applications, with measured performance comparable to traditional RTK—centimeter-level accuracy and reproducibility.

Disadvantages: There are several challenges. First, CLAS requires compatible receivers and antennas. Traditional GNSS receivers could not receive the L6 band signals of CLAS, and compatible devices were limited. However, domestic manufacturers have recently released CLAS-capable chips and devices, and compact receivers that can be attached to smartphones are now commercially available (discussed later). Second, CLAS depends on the reception environment. While open-sky areas with many visible satellites can achieve RTK-equivalent fix rates, maintaining a fix is difficult in environments surrounded by obstructions. For example, in forests or urban canyons where satellite visibility is partially blocked, CLAS may drop from fix to float solutions (in covered areas like under eaves where visibility is lost entirely, it reverts to standalone positioning). In general, RTK tends to maintain fixes more reliably under the same obstructed conditions, so in urban areas or sites with many obstructions, it is advisable to use CLAS in combination with NRTK or inertial navigation as support. Finally, note that CLAS’s service area is limited to Japan. While other countries are developing similar satellite augmentations (European Galileo HAS, etc.), CLAS remains a powerful Japan-only tool for now.

How to choose correction methods by use case and region

We have reviewed the main correction information services (NRTK, cloud PPP, CLAS). How should these be selected and operated in the field? Below is an organization focused on use cases and regional conditions.

• Urban areas and regions with good communications: In sites with urban infrastructure and easy mobile communications, network RTK (NRTK) is the first choice. Connecting to an existing service yields centimeter accuracy from a rover alone, and accuracy is stable. For public works requiring national coordinate systems, NRTK is advantageous because correction values are tied to public coordinates, reducing post-processing. However, in very narrow urban canyons where satellite visibility is extremely poor, RTK solutions can also become unstable; GNSS positioning is impossible in tunnels or indoors, so IMS (inertial navigation) or known-point methods must be used.

• Suburban, mountainous, or infrastructure-poor areas: In fields such as rural mountains, remote islands, or immediately after disasters when communication infrastructure is unavailable, CLAS’s advantages stand out. Because positioning can be performed solely using augmentation signals from satellites, precise positioning is possible even outside mobile coverage. In fact, case studies in mid-mountain agricultural fields that introduced CLAS have achieved centimeter-level precision for tractor autonomy where communications previously prevented automatic driving. For disaster response, the ability to begin positioning and photographing with CLAS-compatible devices before communications are restored is highly useful. Note, however, that CLAS requires bringing a compatible receiver to the site—commercial smartphones alone cannot receive CLAS (there are aftermarket solutions to make smartphones CLAS-compatible, discussed later).

• Wide-area movement and international projects: For positioning involving ships, aircraft, or surveying projects spanning multiple countries, cloud-based PPP services are suitable for seamless wide-area coverage. NRTK and CLAS become unusable across borders, but global PPP can maintain accuracy across long-distance travel with a single service. For open-ocean positioning, PPP augmentation via quasi-zenith or L-band communication satellites is often the only option. As noted, PPP requires convergence time, so operational measures—such as pre-converging before movement or using auxiliary sensors for bridging in dynamic applications—are necessary.

• Initial cost-sensitive or simple surveys: When budgets or personnel expertise for expensive survey equipment are limited, combining smartphones with low-cost correction services is attractive. For example, establishing a dedicated local RTK with your own base station for a few kilometers of surveying can yield high accuracy with no communication fees. However, setting up and maintaining a base station is labor-intensive, so recently solutions have appeared that connect a smartphone to an external receiver and an NRTK service, achieving “zero initial investment” high-precision positioning. The LRTK system described later is one example: it leverages a user’s smartphone and obtains necessary corrections from the cloud, achieving significant cost savings and simplified operation.

In summary, the basic strategy is NRTK in areas with good communications, CLAS in areas without communications or with poor infrastructure, and PPP for wide-area movement. Recently, hybrid products that support multiple correction sources in a single receiver and can switch based on conditions have also appeared. The next section introduces smartphone-integrated positioning solutions as a representative example.

Smartphone-integrated operation and cloud integration

To fully exploit the convenience of the correction information services described so far, device usability is also important. Traditional RTK positioning required bulky equipment like tripod-mounted receivers and radios, but recent technological advances have made the concept of “turning a smartphone into a surveying instrument” a reality. The key is combining a smartphone with a compact high-precision GNSS receiver to perform RTK or CLAS positioning. For example, the ultra-compact GNSS device “LRTK Phone” attachable to an iPhone integrates an RTK receiver into a dedicated smartphone case weighing about 125 g with a thickness of 13 mm, creating a unified smartphone-based system. Connections are made via Bluetooth, eliminating complex wiring. Using such smartphone-integrated devices, if an app on the smartphone connects to Ntrip to receive correction information (e.g., NRTK), the usual 5–10 m accuracy of a smartphone’s built-in GPS can be improved to centimeter-level.

The advantage of smartphone-integrated systems is a significant reduction in initial investment and operational barriers. High-precision GNSS receivers traditionally cost hundreds of thousands of yen or more, but smartphone-linked systems require adding only a palm-sized device and are orders of magnitude cheaper than dedicated units. Usability is dramatically improved as well: positioning, recording, and map display on the smartphone app are consolidated, and geotagging photos is automated. Collected data can be synced to the cloud for immediate viewing and sharing on office PCs. For instance, the LRTK system’s dedicated app integrates continuous positioning modes at 1–10 Hz, single-point modes, high-precision orientation measurement for photography and cloud map placement, and AR guidance to past recorded points—allowing high-precision field data to be synchronized with the cloud in real time. This enables non-specialist technicians and field workers to intuitively perform positioning and recording, contributing to labor savings and efficiency.

Compatibility with communications infrastructure is another benefit. Smartphone integration allows receiving correction data via 4G/5G as well as uploading data to cloud services with a single device. Point cloud data and photos stored in the cloud can be shared instantly among stakeholders, enabling remote progress checks and analysis. This is truly a fusion of correction information services and cloud technology, bringing next-generation positioning operations beyond traditional time-space constraints. This trend is expected to accelerate, and positioning services built around smartphone-cloud integration are competing in development.

Smartphone-integrated positioning solutions using LRTK

As a concrete example embodying the above trend, we introduce LRTK, a notable smartphone-integrated solution at the time of writing. LRTK is a positioning system developed by a startup originating from Tokyo Institute of Technology, consisting of a small GNSS receiver attachable to a smartphone (currently iPhone), a dedicated app, and a cloud service. Its defining feature is that, while enabling streamlined operation by integrating with a smartphone, it supports both network RTK and Michibiki CLAS as correction sources. The company originally released a smartphone-mounted RTK receiver, “LRTK Phone 4C,” in 2022, enabling real-time positioning on a smartphone by receiving NRTK corrections via mobile networks. Responding to requests for “coverage in non-service areas,” they later developed and released a CLAS-capable model as an optional kit. With the CLAS-capable version, positioning is possible even when mobile signals are unavailable as long as the satellite augmentation signal can be received, giving users great reassurance. Users have reported experiences such as “I could successfully position and record in remote mountains outside network coverage” and “I could record high-precision site data during disasters even when communications were down,” demonstrating the significance of zero-dependency on communication infrastructure for smartphone positioning.

Another strength of the LRTK system is its cloud integration and 3D measurement capabilities. Point cloud data and high-precision photos captured in the dedicated app are synced to the cloud with one tap and can be instantly viewed and measured in a 3D viewer in the office. This facilitates smooth data sharing and collaboration between field and office. Combining the smartphone’s camera and sensors enables advanced features such as point cloud scanning and AR-based as-built checks. Completing 3D measurement and analysis—previously requiring specialized equipment and software—using only a smartphone and the cloud is revolutionary. True to its claim of being a “next-generation surveying system anyone can easily start using,” LRTK is expected to democratize high-precision positioning.

From the perspective of selecting correction information services, LRTK’s hybrid approach makes sense. By supporting both NRTK and CLAS, it enables seamless centimeter positioning from urban areas to off-network regions, adopting the strengths and compensating for weaknesses of each service. For example, it can use NRTK for fast, stable positioning under normal conditions and automatically switch to CLAS reception when entering mountainous areas to maintain positioning continuity (actual switching behavior is managed in the app). In this way, LRTK removes the user burden of selecting correction services by handling it within the system, allowing users to obtain optimal high-precision data without conscious effort. As satellite counts increase and devices further miniaturize, integration into drones, small work machines, and mobile devices is likely to expand. In the world of correction information services, “smartphone-integrated CLAS support” is clearly becoming a key technology.

Finally, these technological advances provide significant benefits on site. With affordable, easy-to-use high-precision GNSS available not only to surveying and construction specialists but also to fields that previously had no need for such systems, substantial improvements in work efficiency and new application scenarios are expected. From the viewpoint of choosing correction services by coverage area, each method still has its proper place, but the emergence of smartphone-integrated solutions is steadily blurring those boundaries. In the future, seamlessly switching among NRTK, PPP, and CLAS according to field conditions will become commonplace, and an era in which anyone can enjoy centimeter positioning with a smartphone in hand will arrive. Watch developments of next-generation solutions like LRTK as a forerunner.

Frequently Asked Questions

Q: Do I need to pay or apply to use CLAS? A: No, the CLAS signal itself is provided free of charge by the Cabinet Office’s Quasi-Zenith Satellite System, and there is no usage fee. However, you must acquire compatible equipment (a high-precision GNSS receiver that supports the L6 band), which incurs purchase costs. Receiving CLAS does not require any special license or application; anyone who obtains compatible equipment can use the satellite augmentation service nationwide in Japan. Meanwhile, many private NRTK services charge monthly fees, so CLAS’s free provision is a major cost advantage.

Q: Can a smartphone alone achieve centimeter-level positioning? A: Currently, it is difficult for a commercial smartphone alone to achieve centimeter-level accuracy. High-end phones include dual-frequency GNSS receivers and can improve accuracy to the meter level, but they do not typically have built-in functionality to receive and apply corrections like RTK or CLAS. Therefore, achieving centimeter positioning with a smartphone requires an external high-precision GNSS module or compatible services. Using smartphone-mounted devices like LRTK can transform a smartphone into an RTK receiver. Some Android devices can access raw GNSS data and, combined with external correction sources and RTK processing apps, may enable advanced setups, but these approaches require specialized knowledge. Practically, using a commercial solution (smartphone-integrated receiver + app) is the realistic option.

Q: Is high-precision positioning possible in areas outside network coverage? A: Yes. A representative method is to use CLAS. In Japan, even where mobile signals are unavailable, receiving CLAS augmentation signals from overhead Quasi-Zenith Satellites enables real-time centimeter-level positioning. There are case reports of successful measurements with CLAS-compatible equipment in remote mountain construction sites and of CLAS being helpful in disaster areas. Another approach is to record base station data beforehand and perform precise post-processing (PPK or static surveying), but that sacrifices real-time capability. If real-time high-precision positions are required on site, satellite-based CLAS is a strong option.

Q: How does CLAS accuracy and stability compare to network RTK? A: CLAS and NRTK both can achieve horizontal accuracy on the order of several centimeters, but there are differences. In terms of accuracy, in open environments CLAS and RTK are similar and both achieve centimeter-level errors. However, RTK—being relative to fixed stations—typically provides centimeter accuracy immediately, whereas CLAS (PPP-RTK) may require tens of seconds to minutes to converge to a full fix. Regarding stability, behavior differs when satellite visibility degrades. Generally, RTK is more robust to obstructions; in underpasses or forests where satellite signals are partially occluded, CLAS may more readily drop from fix to float or revert to standalone positioning. RTK also loses positioning capability without satellite visibility, but because it uses local reference stations for error correction, it can sometimes maintain positions even with fewer visible satellites. Overall, experiments suggest “both methods are highly accurate in open skies; RTK has an edge in obstructed environments.” Therefore, using NRTK in urban areas and CLAS in remote or out-of-network areas, or combining both for redundancy, is recommended.

Q: What is LRTK? A: LRTK is the smartphone-integrated high-precision positioning solution introduced in this article. It consists of a small GNSS receiver that attaches to a smartphone, a positioning app, and a cloud service, enabling centimeter-level positioning with RTK or CLAS on a smartphone. Designed as a next-generation surveying system that anyone can easily start using without specialized equipment, it is already being adopted in construction sites, infrastructure inspections, disaster surveys, and other fields. Attaching an LRTK device to a smartphone makes GNSS surveying—previously requiring tripod-mounted equipment—handheld, and collected point clouds and photo data can be shared to the cloud in real time. LRTK supports both mobile-network NRTK and direct satellite CLAS reception, allowing continuous high-precision positioning regardless of communications environment. In short, LRTK is an innovative solution that leverages the latest correction information services to “turn a smartphone into a high-precision surveying instrument.”