Comparison of Correction Information Service Coverage Areas: How Much Difference Does Smartphone-Integrated CLAS Support Make!

Table of Contents

• Introduction

• Types and Mechanisms of Correction Information Services

• RTK (Real-Time Kinematic)

• VRS (Virtual Reference Station)

• L6 / CLAS (Centimeter-Level Augmentation Service)

• IMU Assistance (Tilt Correction & Inertial Navigation)

• SBAS (Satellite-Based Augmentation System)

• PPP (Precise Point Positioning)

• Differences between Nationwide and Local Coverage

• Differences in Required Infrastructure / Foundations

• By Use Case: Which Service Suits Which Task

• Correction Services Suitable for As-Built Control

• Correction Services Suitable for Piling and Layout

• Correction Services Suitable for Disaster Surveying

• Emergence of Smartphone-Integrated CLAS-Supporting “LRTK”

• FAQ

Introduction

In construction and surveying, positional measurement accuracy determines project quality and efficiency. Conventional standalone GPS (GNSS) positioning can incur errors of a few meters; while such errors are acceptable for merely showing a current position in a map app, they can be fatal when accurately laying out structures in civil engineering, detecting minute displacements in infrastructure inspections, and other tasks requiring high precision. Many tasks require centimeter-level accuracy, such as construction control and as-built verification according to design drawings, boundary surveys, ensuring accuracy in piling, and measuring ground deformation during disasters.

To achieve such high-precision positioning, using GNSS “correction information services” is essential. Correction information compensates for effects such as ionospheric and tropospheric delays and satellite orbit errors associated with satellite positioning, and providing these data to receivers in real time dramatically improves positioning accuracy. Recently, various correction service methods have been implemented in practice; representative ones include RTK, VRS, the L6-band centimeter-level augmentation service (CLAS) of the QZSS (Quasi-Zenith Satellite System) “Michibiki,” IMU assistance, SBAS, and PPP. Each differs in correction methodology, coverage area, and required infrastructure, so choosing according to the use case is important.

This article compares the mechanisms and coverage differences of major correction information services from a technical perspective, explains differences between nationwide and local support and the communication infrastructure and base station requirements, and discusses suitability by use case. Focusing on on-site tasks that require centimeter-level accuracy—such as as-built control, piling, and disaster surveys—we analyze key points for selecting the optimal correction service. We also introduce the recently emerged smartphone-integrated CLAS-compatible device “LRTK,” highlighting new possibilities for high-precision positioning in the field.

Types and Mechanisms of Correction Information Services

RTK (Real-Time Kinematic)

RTK is a method that uses two GNSS receivers—a base station (reference) and a rover—to correct positioning errors in real time. A base station with a known precise coordinate is installed near the measurement site, and error information obtained there is sequentially transmitted to the rover via radio communication or the Internet. The rover applies the base station’s correction values to its own GNSS positioning results, reducing positioning errors to on the order of a few centimeters. Because correction information is exchanged multiple times per second, RTK supports dynamic measurements and achieves fast initialization (obtaining a fixed solution) in just a few seconds.

However, accuracy decreases if the rover is too far from the base station because the error factors in the satellite signals received by each station diverge. Generally, centimeter-level accuracy can be maintained if the distance to the base station is within about 10–20 km, but beyond that the correction effect weakens and covering a wide area requires relocating base stations sequentially. Operating RTK also requires the infrastructure to provide a base station device on-site and to ensure communication between the base and rover, such as specified low-power radios, UHF-band radio, or a cellular network (Ntrip method). Although installing and maintaining a base station incurs cost and effort, RTK using one’s own base station can operate even outside communication coverage and offers the advantage of stable centimeter accuracy locally.

VRS (Virtual Reference Station)

VRS (Virtual Reference Station), also called network RTK, is a correction method that uses an observation network composed of multiple fixed reference stations to create a virtual reference station near the user and provide correction information. The user (rover) sends an approximate position to the positioning service’s server over the Internet, and the server generates and returns reference station data (a virtual reference point) as if a station existed nearby. Because this enables corrections as if “a base station were right next to you,” a single rover can obtain RTK-equivalent accuracy in real time (about 3–4 cm horizontally). Initialization is also completed within a few seconds, and dynamic positioning is supported. Major advantages of VRS are that users do not need to install their own base stations and can obtain stable centimeter-level positioning over wide areas. As long as the reference network covers the area, you can move within a prefecture or nationwide and continuously receive corrections.

In Japan, the Geospatial Information Authority’s GEONET and private-sector network RTK services are established, enabling positioning across the country in a unified high-precision coordinate system. Coordinates obtained via VRS conform to the official geodetic system based on the reference station network (in Japan, JGD2011/2020), which makes comparing with design coordinates and managing relative accuracy between control points straightforward. However, using VRS requires Internet connectivity such as a cellular connection, so it cannot be used in areas outside communication coverage. Many network correction services are subscription-based and incur monthly fees. Nonetheless, because VRS allows easy acquisition of cm level accuracy (half-inch accuracy) without adding equipment on-site, its use has expanded particularly for long-term and wide-area projects.

L6 / CLAS (Centimeter-Level Augmentation Service)

CLAS (Centimeter Level Augmentation Service) is a centimeter-level augmentation service provided by Japan’s Quasi-Zenith Satellite System “Michibiki.” Whereas RTK and VRS depend on ground reference stations, CLAS provides correction information directly from quasi-zenith satellites. Technically, CLAS adopts a “PPP-RTK” method that fuses the global PPP (Precise Point Positioning) concept with the immediacy of RTK, enabling a single receiver to achieve centimeter-level accuracy across Japan via satellite distribution. In practice, as long as you have a device capable of receiving Michibiki (QZSS) L6-band signals, you can achieve a few-centimeter accuracy with a single receiver even in mountainous areas or remote islands lacking communication infrastructure. Although initial convergence takes a few minutes, thereafter position updates achieve accuracy comparable to RTK.

CLAS’s greatest feature is that it provides the same accuracy nationwide without the need for communication, making it powerful in scenarios that were difficult for conventional RTK. For example, CLAS is effective for surveying deep in the mountains where cellular signals do not reach, or at large disaster sites where base stations and communication networks have been disrupted; a single CLAS-compatible receiver can continue positioning. CLAS correction signals are provided free of charge as a national satellite service, making them advantageous for wide-area positioning without subscription concerns. There are, however, caveats. First, CLAS requires a dedicated high-precision GNSS receiver; conventional single-frequency GNSS or consumer devices cannot receive it. Second, achieving a centimeter-level fixed solution requires several minutes of initialization, so if high precision is needed immediately at the start of work, combining CLAS with conventional RTK is effective. Furthermore, because augmentation signals are still part of GNSS positioning, they are subject to signal blockage and multipath in urban canyons or forests. By using VRS in urban areas and CLAS where communications are unavailable, you can exploit the strengths of each according to environment. Recently, CLAS use has expanded in applications needing wide-area, communication-less centimeter accuracy—such as drone surveying and smart agriculture—gaining attention as a new high-precision positioning infrastructure.

IMU Assistance (Tilt Correction & Inertial Navigation)

IMU (Inertial Measurement Unit) assistance augments positioning by incorporating inertial sensors such as accelerometers and gyroscopes into GNSS receivers. Increasingly, high-precision GNSS devices implement a “tilt correction” function, which can compute the precise coordinate of a pole tip even if the surveying pole is not vertical. The built-in IMU detects the pole’s tilt angles and corrects the tip position from the pre-set pole height. This enables obtaining coordinates of a target point even when the pole cannot be placed directly above the point due to obstacles—by inserting the pole at an angle and measuring, you can still obtain the coordinate of the intended point. Tilt correction also eliminates the need to make the pole exactly vertical each time, allowing efficient one-person surveying.

IMU assistance has another advantage: it can estimate position during brief GNSS outages. In situations where satellite visibility is temporarily lost—such as tunnel entrances or under viaducts—the IMU can autonomously integrate motion for that short period and estimate relative movement from the last high-precision position, allowing positioning to continue for several seconds. While this is not a complete replacement for GNSS, it helps maintain position tracking during abrupt signal loss and shortens reinitialization time after GNSS recovery. GNSS receivers with integrated IMUs are more expensive than conventional units, but because they reduce on-site labor and improve safety, they are increasingly becoming standard in modern high-precision positioning systems.

SBAS (Satellite-Based Augmentation System)

SBAS (Satellite Based Augmentation System) broadcasts wide-area correction data from geostationary satellites and is widely used in aviation navigation. SBAS provides information on satellite orbit and clock errors and ionospheric delays via geostationary satellites, improving standalone positioning accuracy from several meters to under one meter. In Japan, MSAS, and systems like WAAS in North America and EGNOS in Europe are examples of SBAS; they are all free to use. Many general GNSS receivers and some high-performance smartphones receive SBAS signals to improve positioning accuracy. However, SBAS’s augmentation is primarily intended to improve navigation and positioning reliability; its accuracy generally remains on the order of tens of centimeters to several meters. Therefore, for civil surveying and construction management that require centimeter-level accuracy, other methods such as RTK or PPP-RTK (CLAS) are used.

PPP (Precise Point Positioning)

PPP (Precise Point Positioning) is a method that uses a single GNSS receiver and high-precision orbit and clock correction information obtained from a global positioning network to achieve high-accuracy positioning. In principle, PPP corrects all error terms through calculation models and precise observations, allowing absolute position measurement anywhere in the world. Specifically, it uses precise satellite orbit and clock data provided by services like the International GNSS Service (IGS) and global ionospheric models, enabling high accuracy without reference stations. PPP is useful for surveying and positioning at sea or in remote locations because it is not limited by distances between reference stations. However, achieving centimeter-level accuracy in real time with PPP requires a long initial convergence time (sometimes 10–30 minutes), and accuracy gradually improves during measurement. Real-time PPP typically starts with errors on the order of tens of centimeters and improves to a few centimeters over tens of minutes. For this reason, PPP is unsuitable for time-critical construction surveying or machine control and is mainly used for static control point surveys, crustal deformation observations, and positioning of ships and aircraft. Recently, various nations and private companies have started providing PPP services, and “PPP-RTK” approaches that combine regional correction information to reduce convergence time have appeared (CLAS is an example).

Differences between Nationwide and Local Coverage

When selecting a correction information service, coverage area size is an important factor. Methods like RTK that install a base station for each site can generally only deliver corrections locally within several kilometers to tens of kilometers from the base. Conversely, VRS, CLAS, and PPP can provide wide-area or nationwide coverage without being constrained by physical base station locations while maintaining accuracy. For example, VRS leverages nationwide reference station networks to offer the same service across almost all of Japan, and CLAS delivers uniform accuracy nationwide via quasi-zenith satellites over Japan. PPP’s service area is global, enabling the same positioning method at sea or overseas. SBAS is usable wherever the geostationary satellite signal reaches (for example, MSAS covers much of East Asia).

Nationwide services are powerful for surveys that move over wide areas or projects spanning multiple sites. They eliminate the need to rebuild base stations or reconcile local coordinate systems for each region, enabling consistent positioning and improving data consistency. Conversely, for tasks limited to a single site, operating a local RTK base station may be sufficient. A self-managed base station allows building a stable communication environment and reduces dependence on external services. The key is to consider the scale of the area needing positioning and frequency of movement to decide whether nationwide coverage is necessary or on-site completion suffices. Also, even wide-area services can fail in local dead zones where signals do not reach (such as valleys in mountainous regions), so flexibility to combine correction methods according to site conditions is needed.

Differences in Required Infrastructure / Foundations

Required infrastructure and equipment vary by correction method. For RTK (single base station method), your own base station device and power supply, plus a communication means to deliver corrections to the rover, are essential. Communication options include specified low-power radios or UHF-band radios for short-range on-site use, and Ntrip distribution via the Internet for wider areas. Radio methods are highly real-time and have little communication cost, but range is limited and obstacles in line-of-sight can block signals. Internet-based (cellular) methods are unaffected by terrain and can cover wide areas, but cannot be used outside cellular coverage and incur cellular charges and correction service fees.

VRS (network RTK) does not require the user to deploy base stations, but it assumes continuous Internet connectivity. The rover communicates bidirectionally with the correction service server via the cellular network to receive location-specific correction data; therefore, the work area must be within cellular coverage and the appropriate SIM card or router must be available. Conversely, methods like CLAS or SBAS that receive correction information directly from satellites do not need ground communication infrastructure. CLAS only requires a high-precision GNSS receiver that can receive the Michibiki L6 signal, so there is no need to prepare additional communication equipment even in remote mountain areas (though, as noted, compatible receivers are limited). SBAS will be received automatically by compatible receivers, but since the MSAS signal is transmitted from a geostationary satellite above the equator, an antenna placement with a clear southern sky may be necessary. PPP usage varies by service: some receive real-time correction data from the Internet (e.g., IGS streams or commercial feeds), while others receive corrections via L-band satellite signals. In any case, high-precision PPP requires a multi-frequency high-performance receiver and, in some cases, subscription to a paid service.

Infrastructure reliability is also a factor in system choice. Operating your own RTK base station allows you to manage devices and communication environments but carries the risk that equipment failure or misconfiguration will interrupt corrections. VRS providers implement advanced system monitoring, but instability in cellular communication or server outages can still cause temporary service interruptions. CLAS and SBAS, being independent of ground communications, are robust during disasters but perform poorly in deep valleys or dense urban high-rise areas where satellite signals are blocked. Providing multiple correction methods increases redundancy—for example, normally use VRS but switch to CLAS when leaving cellular coverage, or operate a fixed RTK base while concurrently receiving SBAS as backup. Consider the site’s infrastructure constraints and required availability when planning the optimal combination and operation.

By Use Case: Which Service Suits Which Task

Correction Services Suitable for As-Built Control

For as-built control after construction (measuring the shape and dimensions of completed work), high accuracy and many measurement points are required, so RTK and VRS, which provide centimeter-level coordinates immediately, are primarily used. To check deviations from control points or compare with design values on-site, obtaining precise real-time coordinates is essential. In sites with good communications, using VRS enables measurements directly tied to public coordinate systems (plane rectilinear coordinates and elevations), facilitating creation of as-built drawings and submission for inspection. In areas with difficult communication, many operators install their own RTK reference stations to ensure stable accuracy—if a base station can be set up, stable precision is achievable. Recently, as-built surveys using CLAS-compatible receivers have also appeared. Although initial convergence takes time, the ability to complete surveys without network connectivity is advantageous at mountain construction sites. PPP is impractical for as-built control due to long convergence times, and SBAS is not adopted because it lacks sufficient accuracy.

Correction Services Suitable for Piling and Layout

Piling and layout tasks require continuous, real-time, stable centimeter accuracy. Machine guidance on piling equipment sometimes uses GNSS-equipped machines, but controlling a blade tip within a few centimeters requires continuous high-precision corrections. Therefore, RTK or VRS continuous positioning is typically chosen for layout and piling. On large development sites, it is common to install an in-house base station so that machines and surveyors share the same correction data. In such projects, RTK via radio is stable and does not depend on external infrastructure, and it is easy to distribute corrections to multiple machines and rovers simultaneously. Where communications are good, each device can individually receive corrections via VRS, but VRS is impractical for tunnel boring or mountain construction outside cellular coverage. If CLAS-compatible equipment becomes widespread in construction machinery and survey instruments, CLAS could enable piling and marking without communications infrastructure (there are already examples of CLAS-based high-precision guidance in autonomous agricultural machinery). PPP’s long initialization and delayed updates make it unsuitable for such responsive work, and SBAS’s tens-of-centimeters error is insufficient for pile positioning management.

Correction Services Suitable for Disaster Surveying

In surveys after earthquakes, landslides, or other disasters, reference points may be lost and communication infrastructure may be down. To rapidly and accurately measure terrain displacement and damage extent, correction services that can operate standalone are effective. CLAS is a representative option, valued for the ability to position without relying on communications during large-scale disasters. In the Noto Peninsula earthquake (2023), handheld CLAS receivers were effective in capturing disaster conditions while local cellular base stations were down. With CLAS, a receiver alone can record ground displacement in the affected area, and measurement results are immediately available in global geodetic coordinates, facilitating broad comparison of displacement magnitudes and plotting on maps. If communications are available, VRS measurement is also possible, but immediately after a disaster, communication restrictions and server downtime risks make it unreliable. When setting up a personal RTK base station is infeasible, CLAS—or in some cases PPP positioning—becomes a valuable option despite longer initialization. PPP requires long continuous reception and is therefore not ideal for rapid-response surveys, but it can be used for prolonged observations during aftershock periods or for precise positioning needed to restore control points. SBAS is limited in both accuracy and reliability and is insufficient for disaster-scale verification. Thus, disaster surveys require flexible operation that uses CLAS and RTK as appropriate to site conditions and available infrastructure.

Emergence of Smartphone-Integrated CLAS-Supporting “LRTK”

Finally, as a newly notable example of leveraging correction information services, we introduce the smartphone-integrated device “LRTK.” LRTK is a small RTK-GNSS receiver that can be attached to a smartphone (mainly iPhone/iPad at present) and achieves centimeter-level positioning despite being pocket-sized—weighing only about 125 g and about 1.3 cm (0.5 in) thick. Its integrated design attaches to a dedicated smartphone case with one touch and contains the antenna, receiver circuitry, and battery, so with just this device and a smartphone you can achieve positioning accuracy comparable to conventional stationary GNSS survey instruments.

LRTK supports correction methods very flexibly: in areas with cellular connectivity, it can receive VRS corrections via Ntrip from the smartphone while positioning; when outside communication coverage it can directly receive Michibiki CLAS signals to continue centimeter-level positioning. Being triple-frequency GNSS and CLAS-compatible, it can maintain accuracy alone in mountainous or offshore locations without Internet, and it has proven effective in disaster-site surveying. Higher-end models (LRTK Pro series) with IMU-based tilt correction are also being developed to meet advanced needs such as measurement in narrow spaces and continuous measurements while moving. By drastically miniaturizing and reducing the cost of RTK surveying equipment that used to cost several million yen, the era of “one person, one device” is becoming realistic. If site managers and technicians can carry a high-precision positioning tool in their pocket at all times, new workflows—such as immediate measurement when needed and cloud data sharing—become possible. LRTK exemplifies innovation brought to the field by the advancement of correction information services and their integration with smart devices.

FAQ

Q: How should I choose a correction information service? A: First consider the required accuracy, the positioning range, and the site’s communication environment. If centimeter accuracy is needed immediately within a confined site, operating your own RTK base station or using a VRS service is appropriate. For wide-ranging movement or projects spanning multiple sites, nationwide VRS or CLAS is convenient. If outside communication coverage, choose CLAS or operate base stations; if some communication is expected, VRS is convenient. Also consider differences in initial investment versus running costs (buying a base station vs. service subscription). Overall, balancing communication infrastructure, work area, and budget—and maintaining flexibility to combine multiple methods—provides reassurance.

Q: Which correction method is effective in mountainous areas without communication coverage? A: In areas without communication infrastructure, CLAS—which obtains corrections via satellite—is the most promising. With a CLAS-compatible device, you can obtain centimeter accuracy even in mountains. Another traditional method is to set up a movable RTK base station on-site and transmit correction data by radio; if within radio range, high accuracy can be achieved without external communications. PPP is also usable without communications, but its long initial convergence time makes it unsuitable for immediate-response tasks. SBAS does not require communications but its accuracy is limited to tens of centimeters, insufficient for high-precision needs.

Q: What kind of costs are associated with using correction information services? A: Costs involve both hardware and service fees. For RTK, purchasing a base station and rover receiver set may cost roughly ¥1,000,000–¥3,000,000, but ongoing communication costs are minimal if using your own radios. VRS requires purchasing a rover receiver (several hundred thousand yen and up) plus monthly service fees and communication costs. CLAS itself is free, but you must buy a compatible high-performance receiver. PPP commercial services may require subscription fees (though free PPP using open precise orbit data exists). SBAS can be used with any compatible receiver without additional cost. Generally, to reduce initial costs consider using VRS or renting equipment; to reduce running costs consider operating your own RTK base or using free CLAS.

Q: What is required to use CLAS? A: To use CLAS you need a GNSS receiver that supports centimeter-level positioning. Specifically, a high-precision GNSS device capable of receiving and decoding the Michibiki L6-band signal and supporting multiple frequencies is required. Many commercial surveying GNSS receivers are gaining CLAS support via firmware updates. Recently, CLAS-compatible receivers that attach to smartphones (e.g., LRTK) have appeared, lowering the entry cost. Once you have compatible equipment, you only need to start receiving Michibiki’s augmentation signals outdoors with good reception; after a few minutes, high-precision positioning with applied corrections begins (no registration or fees required).

Q: What is tilt correction and is it useful on-site? A: Tilt correction is a function in which internal sensors detect the tilt of a pole or device and automatically correct the measured position so that the coordinate of the point directly below the pole tip is obtained even when measured at an angle. This allows measurements where the instrument cannot be placed directly above the point—such as at building corners or points behind obstacles—and eliminates the need to make the pole exactly vertical each time. Thus, tilt correction greatly improves efficiency for one-person rapid surveying and continuous height observations at points of differing elevation. Tilt correction is highly useful in practice because some pole tilt is common on sites; however, accurate correction requires prior sensor calibration, and extreme tilts increase error, so care is needed.

Q: Can a smartphone alone achieve centimeter-level positioning? A: At present, it is difficult for a commercial smartphone alone (with built-in GPS) to achieve centimeter accuracy without augmentation. Modern smartphones have high-sensitivity, multi-frequency GNSS chips and with SBAS or simple corrections can reach tens of centimeters, but achieving a few centimeters requires dedicated equipment and correction data. However, combining a smartphone with an external device can achieve this. For example, attaching a high-precision receiver like the LRTK to a smartphone enables centimeter positioning via RTK or CLAS through a smartphone app. On Android, efforts to obtain raw GNSS data and combine it with external correction services are underway, so in the future native smartphone capabilities may enable high-precision positioning. As of the mid-2020s, though, achieving professional-level centimeter accuracy realistically requires dedicated correction data and receivers.