Correction Information Service Comparison: How Widely Usable Is Smartphone-Integrated CLAS Support Nationwide?

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 usable is smartphone-integrated CLAS support nationwide?

• Summary

• FAQ

What is a correction information service?

To obtain high-precision positions from satellite positioning (such as GPS), it is necessary to compensate for positioning errors using data called "correction information." Ordinary GPS positioning can have errors of about 5-10 m (16.4-32.8 ft) due to atmospheric effects and satellite orbit/clock errors. Correction information services distribute error information observed at reference points and correct these deviations in real time. For example, traditional RTK (Real Time Kinematic) positioning installs a reference station (fixed station) and sends correction data to the rover, canceling errors and achieving accuracy on the order of 1-3 cm (0.4-1.2 in). Correction information services generally refer to services that provide the correction data necessary for such high-precision positioning and are essential for achieving high accuracy.

In recent years, the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations and reference station networks by telecom operators have been developed, making it possible to obtain correction information without users providing their own base stations. The provision methods broadly fall into two types: distribution via satellites and distribution via internet connections. Below, as representative examples, we look at the characteristics of the satellite-based service CLAS and internet-based network RTK services.

Satellite-based correction information service (CLAS)

Japan operates the Quasi-Zenith Satellite System (QZSS) "Michibiki," and one of its services is the Centimeter-Level Augmentation Service (CLAS). CLAS generates error information based on reference point data distributed across Japan and delivers it directly from the Michibiki satellites to user terminals. As long as a compatible GNSS receiver is available, this groundbreaking service can correct positioning errors to the order of a few centimeters simply by receiving satellite signals from above. Using CLAS-compatible equipment can reduce the several-meter errors that were unavoidable with standalone GPS down to a few centimeters. This is an advanced initiative even by global standards, and a major advantage is that centimeter-level accuracy can be obtained anywhere in Japan solely via satellite communication.

Technically, CLAS is a method known as PPP-RTK, which provides a wide-area error model via satellite so users do not need their own base stations. The correction signals are transmitted from the QZSS Michibiki satellites on the L6 band, and a receiver capable of decoding that signal is required. The service itself is an open service provided by the government, so receiving it is free of communication or usage fees (only compatible equipment is required). Because correction information is obtained directly from the satellite, CLAS can provide high-precision positioning even in mountainous areas where cellular signals do not reach or in situations where terrestrial communication networks are disrupted by large-scale disasters.

There are, however, some caveats when using CLAS. For example, it may take several tens of seconds to about one minute to converge to full positioning accuracy on first fix, so it can take slightly longer than conventional RTK, which can achieve high precision almost instantly. The achieved accuracy is reported to be slightly inferior to the ideal accuracy of network RTK (horizontal 2-3 cm (0.8-1.2 in), vertical a few cm (several in))—for CLAS, horizontal accuracy is on the order of a few centimeters (measured RMS about 5-6 cm (2.0-2.4 in)), and vertical accuracy is around 10 cm (3.9 in). However, this is acceptable for most practical uses, and given the advantage of not requiring communications infrastructure and being usable uniformly nationwide, CLAS is an extremely useful correction service. Note that at present CLAS is available only within Japan (due to the satellite service area).

Network RTK services

Correction information distributed via internet connections rather than satellites is called a network RTK service. This service integrates observations from multiple GNSS reference stations (continuously operating reference stations or proprietary fixed stations) installed in various locations, performs error analysis in the cloud, and delivers corrections to users (rovers) in real time. Users connect to the internet via cellular networks and receive correction information from the service provider's distribution server (Ntrip caster). Previously, RTK positioning was limited to a few kilometers around a base station that one set up in the field, but with network RTK, centimeter-level positioning is possible anywhere within the service area without a private base station. Eliminating the need to set up or remove base stations dramatically improves efficiency for surveying and construction work. Moreover, corrections based on multiple reference station networks suppress accuracy degradation at distant locations compared to a single base station.

Commercial network RTK services that cover all of Japan are now available, and if within a communication area they can be used widely from Hokkaido to Okinawa. For example, major telecom operators install many GNSS receivers at their cellular base stations to build their own correction networks and provide centimeter-accuracy correction information. Survey equipment manufacturers and private companies also offer VRS (Virtual Reference Station) services nationwide. With these services, not only construction firms and survey offices but also small and medium-sized local companies and municipalities without specialized survey teams can make use of high-precision positioning.

Using network RTK requires a contract with the provider. In many cases there is a monthly fee, and depending on the plan and region, a monthly cost on the order of tens of thousands of yen is common. Users set the ID and password issued after contracting in their GNSS receiver or positioning app to connect to the correction distribution service (Ntrip). Because network RTK uses communication lines, real-time correction reception is not possible outside cellular coverage. This inability to use the service in remote mountainous or offshore areas is a disadvantage, but conversely, if a communication environment is available, the service can be used over wide outdoor areas. The achievable accuracy is comparable to local RTK, delivering centimeter-level errors almost immediately, so network RTK is already being practically applied in many fields such as construction ICT sites and autonomous vehicle fleet management.

Emergence and advantages of smartphone-integrated GNSS receivers

Traditional high-precision GNSS surveying equipment required large antennas, dedicated terminals, radio equipment for base stations, and many other components, with initial investments of several million yen and the need for specialized operators being the norm. That meant only large companies could afford such systems, and high-precision positioning technology was not being fully utilized on site. Recently, however, small GNSS receivers that can interface with smartphones have appeared. These pocket-sized devices integrate an antenna, high-precision GNSS chip, and battery, and when attached to a smartphone and connected via Bluetooth, the smartphone itself becomes a high-precision positioning system. For example, a receiver weighing about 100-150 g attached to the back of a smartphone and started with a dedicated app can improve the typical smartphone GPS error of about 5-10 m (16.4-32.8 ft) to a few centimeters (a few in). No complicated equipment operation or specialized knowledge is required—essentially turning a smartphone into 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 only a small device, the cost is far lower than procuring a full set of dedicated equipment. Many smartphone-linked solutions also provide positioning apps and cloud services, and after purchasing the hardware the basic app use can be free while optional cloud functions are subscription-based, making operational costs adjustable. This is overturning the notion that high-precision positioning requires a large investment, and making adoption easier for small and medium-sized enterprises and municipalities.

Furthermore, the smartphone + GNSS device form factor offers benefits in fieldwork efficiency. Main use cases and effects include:

• Labor and personnel reduction: With smartphone RTK, tasks that traditionally required a two-person survey team, such as surveying and setting batter boards, can be performed accurately by one person. Positioning for heavy machinery and as-built management can also be handled solo, helping sites with labor shortages.

• Enhanced data recording: High-precision location-tagged photos and point cloud data can be easily acquired on site and used for post-construction as-built checks and infrastructure inspection records. Since high-accuracy digital records are created on the spot, later data utilization and report preparation are streamlined.

• Real-time information sharing: Many smartphone RTK apps integrate with the cloud, enabling immediate upload of positioning data and photos from the field. Office staff can monitor conditions in real time, improving communication between field and office and enabling quicker decision-making and reduced rework.

• Improved disaster response: Even immediately after a disaster when cellular networks are down, CLAS-compatible smartphone RTK can perform high-precision positioning as long as satellites are visible. It can record damage with accurate location information and support recovery planning. A positioning method that does not rely on communication infrastructure provides significant reliability in emergencies.

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

How widely usable is smartphone-integrated CLAS support nationwide?

So, can a smartphone-integrated CLAS-compatible GNSS receiver be useful anywhere across Japan? In short, yes: it is usable almost everywhere within Japan. Michibiki operates with a four-satellite constellation designed so that at least one satellite is always positioned at a high elevation angle over Japan. Therefore, from Hokkaido to remote Okinawan islands, as long as satellite visibility is maintained you can receive CLAS correction signals. Because it does not rely on terrestrial communication infrastructure, CLAS’s ability to obtain high-precision position data even for forest surveys in mountainous areas or surveys in zones outside communication coverage is a major strength.

For example, in disaster response, CLAS-enabled devices attached to smartphones have been used to capture damage on sites where communication networks are down. Previously, real-time high-precision positioning had to be abandoned in non-covered areas, but CLAS via satellite enables positioning even offline. This is a benefit unique to CLAS, which can provide stable accuracy nationwide.

That said, the common prerequisite everywhere is that the sky must be visible. As with any GNSS system, high-precision positioning is not possible in tunnels, indoors, or deep forests where satellite signals cannot reach. In urban canyons, satellite signal blockage and multipath reflections can render accuracy unstable even with CLAS or network RTK. Although the constant presence of Michibiki satellites over city skies increases the chances of receiving CLAS, both methods have limitations in places where satellite-based positioning itself is difficult.

In summary, smartphone-integrated CLAS-enabled positioning can be considered usable 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 enable high-precision positioning, comparing the satellite-based CLAS and internet-based network RTK. Each has strengths and weaknesses, but recent smartphone-integrated small receivers make it easy to enjoy the benefits of both. Key points:

• Infrastructure required: CLAS receives correction signals directly from satellites, so no communications infrastructure is needed. Network RTK requires internet connectivity such as cellular networks.

• Coverage: CLAS’s service area is the entirety of Japan (within satellite visibility). Network RTK is limited to the service provider’s area (basically nationwide but only within communication coverage).

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

• Initial convergence time: Network RTK can achieve high-precision positioning almost immediately due to observation differences with base stations. CLAS, using wide-area corrections, may require several tens of seconds to converge on the first fix.

• Equipment requirements: CLAS requires a compatible high-precision GNSS receiver (current smartphone internal GPS alone is insufficient). Network RTK also requires a high-precision GNSS receiver and communication terminal, but recently smartphone + small devices can support both methods.

• Cost: Receiving CLAS correction signals is free (Michibiki is a public service). Network RTK generally incurs a monthly subscription fee (a contract for a high-precision service is required).

• Environmental adaptability: CLAS can operate independently in areas without communications or during disasters. Network RTK cannot be used when communications are cut. However, both methods face difficulties in urban areas or forests where satellites are hard to observe.

As the above shows, CLAS and network RTK can complement each other. In normal situations you can use network RTK for quick, high-accuracy fixes and switch to CLAS when communications are difficult. Newer systems and apps can select between both correction sources. For example, next-generation smartphone RTK systems like LRTK allow users to attach a small device to a smartphone, press a button to start positioning, and switch between "Ntrip network RTK" and "Michibiki CLAS mode" with a single tap in the app. Designed so anyone can use them without specialized knowledge, these systems aim to make centimeter-level positioning accessible to anyone, anywhere, at any time.

Correction information services will continue to evolve and become more convenient. With smartphone-integrated, easy-to-use positioning devices, centimeter-level positioning that was once difficult to access becomes a familiar tool. Take the opportunity to try the latest smartphone RTK technology and experience its power—your field surveying and positional data usage style may change significantly.

FAQ

Q: What is a correction information service? A: It is a service that provides satellite error information to improve GNSS (such as GPS) positioning accuracy. Based on error data measured at reference stations, it sends correction information to users’ receivers to correct positional deviations in real time. This can reduce typical meter-level positioning errors to the order of a few centimeters.

Q: What is the difference between a smartphone’s built-in GPS and positioning using correction information services? A: The typical accuracy of a smartphone’s built-in GPS alone is about 5-10 m (16.4-32.8 ft), and it can be worse in areas surrounded by buildings. Using correction information services with a dedicated receiver and service corrects GNSS errors and provides centimeter-level accuracy. In short, a smartphone GPS gives a "rough position," while correction information services give "survey-grade accurate positions."

Q: Is a communication environment necessary to use CLAS? A: No. CLAS receives correction information directly from Michibiki satellites by radio, so it can be used without cellular or internet connections. In extreme cases, CLAS can provide centimeter-level positioning in remote mountains or at sea as long as the sky is visible. However, a CLAS-compatible receiver must be able to capture the satellite signal, so it cannot be used indoors or in tunnels.

Q: Can a smartphone alone receive CLAS signals? A: At present, general smartphones cannot directly receive CLAS. Smartphone GNSS chips mainly receive basic signals like GPS and GLONASS and do not support Michibiki’s CLAS signal on the L6 band. To use CLAS you need a compatible high-precision GNSS receiver connected to the smartphone. Small smartphone-integrated receivers are commercially available that enable smartphones to use CLAS. In the future, CLAS-capable chips integrated into smartphones may appear.

Q: What level of accuracy can be achieved using correction information? A: Under good conditions, horizontal accuracy of about 2-3 cm (0.8-1.2 in) and vertical accuracy of several centimeters to around 10 cm (3.9 in) can be expected. Network RTK typically achieves this level almost instantly. CLAS can reach a similar final accuracy level but tends to converge from a slightly larger initial error (within several tens of centimeters) over about a minute. In any case, this is a dramatic improvement over standalone positioning (errors over 5 m).

Q: Is it really usable anywhere in Japan? A: Yes—if the sky is visible outdoors, CLAS can be used in almost all parts of Japan. CLAS is designed to cover the entire country, and Michibiki satellites can provide correction information even for remote islands and mountainous areas. Network RTK services are also being deployed nationwide where cellular coverage exists. However, both services cannot be used where satellites are not visible, such as underground or indoors. Also, CLAS is not available overseas (outside the service area), so consider it a Japan-only service.

Q: How much does it cost? A: Costs vary by method. CLAS signal reception from satellites is free. Costs are associated with purchasing a compatible receiver, and small GNSS devices are now available starting in the hundred-thousand-yen range. Network RTK typically requires a contract with a provider and incurs a monthly fee of around tens of thousands of yen. Depending on your needs and budget, you can choose to use free CLAS or contract a paid service for stability and support.

Q: Can non-experts use it? A: Modern smartphone-linked GNSS systems are designed to be user-friendly for non-experts. Dedicated apps provide intuitive interfaces, and starting/stopping positioning or connecting to correction services can often be done with a single tap. Systems like LRTK allow you to attach the device to a smartphone and start positioning with just a button press, so complex GNSS settings and knowledge are not required. With basic familiarity, field personnel can use these systems immediately.

Q: In what situations can this technology be used? A: Applications are varied. Typical examples include general surveying (boundary surveys and as-built management), construction site control (machine guidance and quantity control), infrastructure inspections (managing deterioration locations with precise position records), and agriculture (autonomous tractor positioning). Municipalities can use it for road and public facility maintenance management, and it is also powerful for disaster mapping. In short, smartphone-integrated high-precision positioning has potential in any situation where position accuracy is important, enabling more efficient measurements and new data utilization that previously required much time and manpower.