Table of Contents

• What is a correction information service?

• Satellite-based correction information service (CLAS)

• Network RTK services

• Emergence and advantages of smartphone-integrated GNSS receivers

• How widely can smartphone-integrated CLAS be used nationwide?

• Summary

• FAQ

What is a correction information service?

To obtain high-precision positions with satellite positioning (such as GPS), it is necessary to compensate for positioning errors using data called "correction information." With standard GPS positioning, atmospheric effects and satellite orbit/clock errors typically cause deviations of about 5–10 meters. In contrast, a correction information service distributes error information observed at reference points and corrects these deviations in real time. For example, traditional RTK (Real Time Kinematic) positioning installs a base station (fixed station) and sends correction data to a rover to cancel errors, achieving accuracies on the order of 1–3 centimeters. Correction information services broadly refer to services that provide the correction data required for such high-precision positioning and are a key element of precise positioning.

In recent years, networks of electronic reference points maintained by the Geospatial Information Authority of Japan and reference station networks by communication providers have been developed, enabling users to obtain correction information without setting up their own base stations. There are two main delivery methods: distribution via satellites and distribution via Internet connections. Below, as representative examples, we will look at the features of the satellite-based service CLAS and the Internet-based network RTK service.

Satellite-based correction information service (CLAS)

Japan operates the Quasi-Zenith Satellite System (QZSS) "Michibiki," and one of its services is the Centimeter-class Positioning Augmentation Service (CLAS). CLAS generates error information based on reference point data placed throughout Japan and transmits it directly from the Michibiki satellites to user terminals. As long as the GNSS receiver supports it, this revolutionary service allows users to correct positioning errors to a few centimeters just by receiving the satellite signal overhead. Errors of several meters that were unavoidable with standalone GPS positioning can be reduced to a few centimeters by using CLAS-compatible equipment. This is an advanced effort even on a global scale, and a major strength is that centimeter-level accuracy can be obtained from satellites alone anywhere within Japan.

Technically, CLAS employs a method called "PPP-RTK," which provides wide-area error models via satellite, so users do not need their own base stations. The correction signal is transmitted from the QZSS Michibiki satellites on the L6 band, and use requires a receiver capable of decoding that signal. The service itself is an open service provided by the government, and the correction signal can be received without communication or usage fees (free) (only compatible equipment is required). Because correction information is obtained directly from satellites, CLAS can perform high-precision positioning even in mountainous areas where mobile phone signals do not reach or in situations where ground communications are severed by large-scale disasters.

On the other hand, there are some caveats when using CLAS. For example, it can take several tens of seconds to about a minute for positioning accuracy to converge the first time, which may be somewhat slower than conventional RTK that can achieve high precision almost instantly. Also, the achievable accuracy is slightly inferior to the ideal accuracy of network RTK—CLAS typically yields horizontal positions on the order of a few centimeters (measured RMS around 5–6 cm) and vertical accuracy around 10 cm, compared to network RTK's ideal horizontal 2–3 cm and vertical a few centimeters. However, this is generally acceptable for practical use, and considering the advantage of nationwide availability without communication infrastructure, CLAS is a very useful correction service. Note that CLAS is currently available only within Japan (due to the satellite service area).

Network RTK services

Services that distribute correction information via Internet connections rather than satellites are called network RTK services. These services integrate observations from multiple GNSS reference stations (electronic reference points or proprietary fixed stations) installed in various locations, perform error analysis in the cloud, and deliver corrections to users (rovers) in real time. Users connect to the Internet via mobile phone lines and receive correction information from the service provider's distribution server (Ntrip caster). Whereas RTK could previously only be used within a few kilometers of a local base station, network RTK enables centimeter-level positioning anywhere within the service area without the need for a personal base station. Eliminating the setup and takedown of base stations dramatically improves efficiency for surveying and construction work. In addition, corrections derived from multiple reference stations reduce accuracy degradation over distance compared to a single base station.

Commercial network RTK services covering all of Japan are now available, and they can be used widely from Hokkaido to Okinawa wherever there is network coverage. For example, major telecom operators have installed many GNSS receivers at their mobile base stations to build proprietary correction networks and provide centimeter-accuracy correction information based on that data. VRS (Virtual Reference Station) services offered by surveying equipment manufacturers and private companies are also available in many areas. With these services, not only construction companies and surveying offices but also small local companies and municipalities can increasingly use high-precision positioning without maintaining specialized surveying teams.

Using a network RTK service requires a contract with the provider. In many cases, a monthly usage fee applies—depending on the plan and region, a monthly fee of tens of thousands of yen is common. After contracting, the user sets the issued ID and password in the GNSS receiver or positioning app to connect to the correction distribution service (Ntrip). Because it uses a communication line, real-time correction reception is not possible outside mobile network coverage. This is a disadvantage in areas such as deep mountains or at sea, but conversely, if a communication environment is available, the service can be used over wide outdoor areas. Accuracy itself is comparable to local RTK, and centimeter-level positioning is obtained almost immediately, so network RTK is already in practical use in various fields such as construction ICT sites and management of autonomous vehicles.

Emergence and advantages of smartphone-integrated GNSS receivers

Traditional high-precision GNSS surveying systems required large antennas, dedicated terminals, radio equipment for base stations, and other gear, with initial investments of several million yen and the need for skilled operators. This meant only large companies typically used such systems, and high-precision positioning was not widely utilized on site. Recently, however, small GNSS receivers that can be paired with smartphones have emerged. These pocket-sized devices integrate an antenna, high-precision GNSS chip, and battery and attach to a smartphone; connecting via Bluetooth or similar makes the smartphone a high-precision positioning system. For example, attaching a receiver weighing about 100–150 g to the back of a smartphone and tapping "start positioning" in a dedicated app can improve the smartphone’s usual 5–10 m GPS error to a few centimeters. No complex equipment operation or specialized knowledge is required—your smartphone effectively becomes an easy surveying instrument.

The biggest advantage of smartphone-integrated RTK-GNSS receivers is that they dramatically lower the initial adoption barrier. By leveraging an existing smartphone and adding a small device, costs are far lower than assembling a full set of dedicated equipment. Many smartphone-linked solutions also provide positioning apps and cloud services; after purchasing hardware, basic app use is often free and cloud functions can be paid monthly as needed, making operating costs adjustable. This is overturning the conventional notion that "high-precision positioning requires large investment," creating circumstances where small and medium enterprises and municipalities can more readily adopt it.

Additionally, the smartphone-plus-GNSS-device form factor offers benefits in on-site work efficiency. Main use cases and effects include:

• Labor and personnel reduction: With smartphone RTK, surveying tasks and setting out that used to require two surveyors can be done accurately by one person. Machine guidance and as-built management can also be handled solo, making it valuable on worksites facing labor shortages.

• Enhanced data recording: High-precision position-tagged photos and point cloud data can be easily collected on site and used for as-built confirmation and infrastructure inspection records. Because high-precision digital records are saved immediately, later data use and report creation are smoother.

• Real-time information sharing: Many smartphone RTK apps link to the cloud, allowing positioning data and photos obtained on site to be uploaded instantly. office personnel can grasp the situation in real time, smoothing communication between field and office and helping prevent rework and speed decision-making.

• Improved disaster response capability: Even on disaster-stricken sites where mobile networks are down, CLAS-compatible smartphone RTK can perform high-precision positioning as long as satellites are visible. Recording damage with accurate location information can aid recovery planning. Positioning methods that do not rely on communication infrastructure are highly reliable in emergencies.

In this way, smartphone-integrated high-precision positioning systems strongly support on-site digitalization (DX). For the civil engineering and construction industries facing aging workforces and labor shortages, and for municipalities managing wide-area infrastructure, easy and accurate positioning tools are powerful assets for both productivity improvement and ensuring safety and quality.

How widely can smartphone-integrated CLAS be used nationwide?

So, can CLAS-compatible GNSS receivers integrated with smartphones be useful anywhere in Japan? The bottom line is that they can be used almost everywhere within Japan. Michibiki operates with a four-satellite constellation designed so that at least one satellite is always at a high elevation above Japan. Therefore, from Hokkaido to Okinawa and even remote islands, CLAS correction signals can be received wherever satellite visibility is ensured. Because CLAS does not rely on ground communication infrastructure, it is a major advantage that high-precision position information can be obtained for forest surveys in mountainous areas or remote surveys outside mobile coverage.

For example, in disaster response, there are cases where CLAS-compatible devices attached to smartphones have been used to position and record damage in areas where communication networks were down. Previously, real-time high-precision positioning had to be abandoned outside mobile coverage, but satellite-based CLAS makes positioning possible even offline. This is a benefit unique to CLAS, which can provide stable accuracy nationwide.

That said, "being in a location where the sky is visible" is a common precondition everywhere. As with all GNSS, high-precision positioning cannot be performed in environments where satellite signals cannot reach, such as inside tunnels, indoors, or deep inside forests. In urban canyons, satellite signal blockage and multipath (reflections) can make accuracy unstable with either CLAS or network RTK. While the constant presence of Michibiki satellites overhead in urban areas increases the chance of receiving CLAS, both methods have limits where satellite positioning itself is difficult.

In summary, it is reasonable to consider that smartphone-integrated CLAS-compatible positioning can be used "in all regions of Japan where the sky is visible." This is a strength not found in other correction information services and brings nationwide possibilities for high-precision positioning.

Summary

This article introduced correction information services that achieve high-precision positioning by comparing satellite-based CLAS and network-based RTK. Each has strengths and weaknesses, but recent smartphone-integrated small receivers make it easy to enjoy the advantages of both. Here are the main points summarized:

• Infrastructure required: CLAS receives correction signals directly from satellites and does not require communication infrastructure. Network RTK requires Internet connections such as mobile networks.

• Coverage: CLAS’s service area is nationwide in Japan (within satellite visibility). Network RTK is within the service provider’s coverage area (generally nationwide but limited to areas with network connectivity).

• Positioning accuracy: Both can achieve errors on the order of a few centimeters. RTK is theoretically somewhat more accurate and stable, but CLAS provides practically comparable accuracy.

• Initial convergence time: Network RTK can achieve high-precision positioning almost immediately due to differences observed with base stations. CLAS may require about several tens of seconds for initial convergence because it uses wide-area corrections.

• Equipment requirements: Using CLAS requires a compatible high-precision GNSS receiver (current built-in smartphone GPS alone is not sufficient). Network RTK also requires a high-precision GNSS receiver and a communication terminal, but recently both methods can be supported by a smartphone plus a small device.

• Usage cost: CLAS correction signal reception is free (Michibiki is a public service). Network RTK generally incurs monthly fees (contracts for high-precision services are required).

• Environmental adaptability: CLAS can position independently even outside network coverage or during disasters. Network RTK cannot be used when communications are cut. However, both methods share limitations in places where satellites are hard to see, such as dense urban areas or forests.

As described above, CLAS and network RTK can be said to complement each other. In normal conditions, users might obtain quick accuracy with network RTK and switch to CLAS when communications are difficult. Recently, positioning solutions that let users choose either correction source in one system or app have emerged. For example, in the next-generation smartphone RTK system LRTK, attaching a small device to a smartphone and pressing a button starts positioning, and the app allows one-touch switching between `Ntrip` network RTK and Michibiki CLAS mode. Designed so anyone can use it without expert knowledge, such systems aim to enable "anyone, anywhere, anytime" centimeter positioning.

Correction information services for high-precision positioning will continue to evolve and improve usability. By using easy smartphone-integrated positioning devices, centimeter-level positioning that was previously difficult becomes an accessible tool. Why not take this opportunity to experience the latest smartphone RTK technologies and feel their power? They may significantly change how on-site surveying and location information are used.

FAQ

Q: What is a correction information service? A: It is a service that provides error information for satellite signals (such as GPS) to improve positioning accuracy. Based on measurements at reference stations, correction data are sent to user receivers to correct positional deviations in real time. This can reduce typical meter-level positioning errors to the order of centimeters.

Q: What is the difference between a smartphone’s built-in GPS and using a correction information service? A: With only a smartphone’s built-in GPS, typical positioning accuracy is around 5–10 m and can be worse in built-up areas. Using a correction information service, a dedicated receiver and service correct GNSS errors, achieving centimeter-level accuracy. In short, a smartphone GPS gives a "rough location," whereas a correction information service provides "survey-grade accurate location."

Q: Is a communication environment required to use CLAS? A: No. CLAS receives correction information directly from Michibiki satellites by radio, so it can be used without mobile phone or Internet connections. In extreme cases, CLAS can provide centimeter-level positioning in remote mountain or maritime areas as long as the sky is visible. However, because CLAS-compatible receivers must capture signals from the satellites, it cannot be used indoors or inside tunnels.

Q: Can a smartphone alone receive CLAS signals? A: Currently, general smartphones cannot directly receive CLAS. Smartphone GNSS chips mainly handle basic signals like GPS and GLONASS and do not support Michibiki’s CLAS signal on the L6 band. To use CLAS, a compatible high-precision GNSS receiver must be connected to the smartphone. Small smartphone-integrated receivers are now commercially available, and attaching one enables CLAS use with a smartphone. In the future, CLAS-capable chips may be integrated into smartphones.

Q: How accurate can positioning with corrections be? A: Under good conditions, horizontal accuracy of about 2–3 cm and vertical accuracy of a few centimeters to around 10 cm can be expected. Network RTK can achieve this level almost immediately. CLAS can reach equivalent final accuracy, but initial convergence within about the first minute may show larger errors (within several tens of centimeters) before settling. In any case, this is orders of magnitude more precise than standalone positioning (errors of 5 m or more).

Q: Is it really usable anywhere in Japan? A: Yes; as long as you are outdoors where satellite signals can be received, CLAS can be used almost anywhere in Japan. CLAS is designed to cover all of Japan, and even remote islands and mountainous regions can receive correction information from Michibiki overhead. Network RTK is also deployable nationwide where mobile networks reach. However, neither service works underground or indoors where satellites cannot be seen. Also, CLAS is not available overseas (outside the service area), so consider it a Japan-only service.

Q: What are the costs involved? A: Costs for corrections vary by method. CLAS correction signal reception is free. Costs are mainly for purchasing a compatible receiver, which can be obtained in the hundred-thousand-yen range for small GNSS devices. Network RTK typically requires a contract with a provider and incurs a monthly fee on the order of tens of thousands of yen. Depending on purpose and budget, consider using free CLAS or contracting a paid service that provides greater stability and support.

Q: Can people without expertise use these systems? A: Recent smartphone-linked GNSS systems are designed to be user-friendly without specialist knowledge. Dedicated apps have intuitive interfaces for starting/stopping positioning and connecting to correction services with one touch. For example, systems like `LRTK` let you attach a device to a smartphone and start obtaining correction information and high-precision positioning simply by pressing an app button. If users learn the basic operation in advance, field staff can start using them immediately.

Q: In what scenarios can this technology be applied? A: Applications are wide-ranging. Typical examples include general surveying (boundary measurement, as-built management), construction site management (machine guidance, quantity control), infrastructure inspections (managing deterioration locations with precise positional records), and agriculture (autonomous tractor positioning control). Municipalities can use it for accurate data collection in road and public facility maintenance, and it is powerful for mapping disaster damage. In short, wherever positional accuracy is required, smartphone-integrated high-precision positioning technology has potential. Tasks that previously required manpower and time can be streamlined, enabling new data utilization.