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 and Platforms

• By Use Case: Which Service Suits Which Task

• Correction Services Suitable for As-Built Management

• Correction Services Suitable for Piling and Layout

• Correction Services Suitable for Disaster Surveying

• Emergence of Smartphone-Integrated CLAS Support “LRTK”

• FAQ

Introduction

In construction and surveying, positional measurement accuracy directly influences project quality and efficiency. Conventional standalone GPS (GNSS) positioning can have errors on the order of meters; while such errors are acceptable for indicating your position on a map app, they can be critical when marking precise locations for civil engineering works or detecting minute displacements during infrastructure inspections. Many tasks require centimeter-level accuracy: construction management to meet design drawings and as-built verification, boundary surveys, ensuring accuracy in pile driving, measuring ground displacement after disasters, and more.

To achieve such high-precision positioning, using GNSS “correction information services” is indispensable. Correction information compensates for effects such as ionospheric and tropospheric delays and satellite orbit/clock errors; providing these data to receivers in real time dramatically improves positioning accuracy. Recently, various correction service methods have become practical: representative ones include RTK, VRS, the QZSS Michibiki L6 centimeter-level augmentation service (CLAS), IMU assistance, SBAS, and PPP. Each differs in correction methodology, coverage area, and required infrastructure, so choosing the right one for your use case is important.

This article compares the mechanisms and coverage-area differences of major correction information services from a technical viewpoint, and explains differences between nationwide and local coverage, communication infrastructure and reference-station requirements, and suitability by use. With centimeter-level accuracy needs in mind—such as as-built management, piling, and disaster surveying—we analyze key points for selecting optimal correction services. We also introduce the recently emerged smartphone-integrated CLAS-capable device “LRTK” and touch on new possibilities for high-precision field positioning.

Types and Mechanisms of Correction Information Services

RTK (Real-Time Kinematic)

RTK is a method that uses two GNSS receivers—a base station (reference point) and a rover—to correct positioning errors in real time. A base station with a known, precise coordinate is deployed near the survey site, and the error information obtained there is sent to the rover via radio communication or over the Internet. By applying the base station’s correction values to its own GNSS solution, the rover can reduce positioning errors to on the order of several centimeters. Because correction exchanges occur every second, RTK supports dynamic measurements and offers excellent responsiveness: initialization (obtaining a fixed solution) typically takes only a few seconds.

However, accuracy degrades if the rover is too far from the base station, since error factors in the satellite signals received by both stations diverge. Generally, centimeter-level accuracy can be maintained within about 10–20 km of the base station; beyond that, the correction effect weakens and covering a wide area requires relocating base stations sequentially. Operating RTK requires providing base station equipment on site and securing communication between base and rover—via low-power radio, UHF-band radio, or mobile networks (Ntrip-based)—so infrastructure is necessary. Although installing and maintaining base stations incurs cost and effort, RTK using self-managed base stations can operate in areas without cellular coverage, offering the advantage of stable centimeter accuracy for local sites.

VRS (Virtual Reference Station)

VRS (Virtual Reference Station), also called network RTK, is a correction method that uses a network of multiple fixed reference stations to create a virtual reference station near the user and provide correction data. The user (rover) sends an approximate position to the service server via the Internet, and the server generates and returns reference-station data (a virtual reference point) as if a base station existed nearby. This allows the user to receive corrections as if there were a base station very close by, enabling real-time RTK-equivalent accuracy (about 3–4 cm horizontally) with a single rover. Initialization completes in seconds, and dynamic positioning is supported.

A major advantage of VRS is that users need not install their own base stations, and it can provide stable centimeter-level positioning over wide areas. As long as the reference-station network covers the area, users can move within a prefecture or nationwide and continuously receive corrections. In Japan, the Geospatial Information Authority’s Continuous GPS Network (GEONET) and private-sector network RTK services are established; leveraging these enables positioning within a unified nationwide high-precision coordinate system. Coordinates obtained via VRS align with the official geodetic datum of the reference network (in Japan, JGD2011/2020), facilitating comparisons with design coordinates and relative accuracy management between control points. However, VRS requires Internet connectivity (e.g., cellular service), so it cannot be used in areas without coverage. Many network correction services are subscription-based, incurring monthly fees. Nevertheless, because VRS offers centimeter accuracy without installing equipment on site, its use is expanding—especially for long-term, 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 (QZSS), “Michibiki.” Unlike RTK or VRS, which depend on ground reference stations, CLAS delivers correction information directly from quasi-zenith satellites. Technically, it adopts a “PPP-RTK” approach that fuses PPP (Precise Point Positioning) wide-area standalone corrections with RTK-like immediacy, enabling a single receiver to achieve centimeter-level accuracy via nationwide satellite broadcast. In practice, as long as you have equipment capable of receiving Michibiki’s L6-band signals, you can achieve several-centimeter accuracy even in mountainous or island areas lacking communication infrastructure. Although initial convergence takes a few minutes, subsequent positioning updates can match RTK accuracy.

CLAS’s main feature is that “the same accuracy can be obtained nationwide without communication,” making it powerful in scenarios where conventional RTK is difficult. For example, it enables high-precision surveying in remote mountain locations with no cellular signal, or continued positioning at disaster sites where base stations and communication networks are disrupted—provided you have a CLAS-capable receiver. CLAS corrections are offered free as a national satellite service, allowing wide-area use without subscription costs. Some caveats: first, CLAS requires a dedicated high-precision GNSS receiver; conventional single-frequency GNSS or consumer devices cannot receive it. Second, achieving centimeter-level fixed solutions requires several minutes of initial convergence, so combining CLAS with RTK can be beneficial when immediate high precision is needed at the start of work. Also, since the augmentation signal is part of GNSS, satellite signal blockage and multipath in dense urban canyons or forests still affect performance. Using VRS in urban areas and CLAS in communication-less areas allows you to exploit their respective strengths. CLAS adoption has been increasing in fields requiring wide-area, communication-free centimeter accuracy—such as drone surveying and smart agriculture—drawing attention as a new high-precision positioning infrastructure.

IMU Assistance (Tilt Correction / Inertial Navigation)

IMU (Inertial Measurement Unit) assistance augments GNSS receivers with inertial sensors such as accelerometers and gyros to assist positioning. In modern high-precision GNSS units, “tilt correction” functionality is increasingly implemented, allowing accurate computation of the pole tip coordinate even when the survey pole is not perfectly vertical. The built-in IMU detects the pole’s tilt angles and corrects the tip position based on a preset pole height. This enables obtaining coordinates of the intended point even when you cannot place the pole directly over the point—by measuring from an angle. Tilt correction eliminates the need to make the pole strictly vertical each time, enabling efficient single-person surveying.

Another advantage of IMU assistance is estimating position during brief GNSS signal loss. In cases where satellite visibility is temporarily lost—such as tunnel entrances or under overpasses—the IMU can perform short-term dead-reckoning by integrating motion, estimating relative changes from the last high-precision position, and allowing positioning to continue for several seconds. While not a full replacement for GNSS, this helps maintain position tracking during sudden signal outages and shortens reinitialization time after GNSS reacquisition. GNSS receivers with integrated IMUs tend to be more expensive than traditional units, but they reduce field labor and improve safety, so IMUs are increasingly standard in modern high-precision positioning systems.

SBAS (Satellite-Based Augmentation System)

SBAS (Satellite-Based Augmentation System) broadcasts wide-area correction data via geostationary satellites and is widely used in aviation. By providing satellite orbit/clock and ionospheric delay information via geostationary satellites, SBAS can improve standalone GNSS accuracy from several meters to under one meter. In Japan, MSAS is the SBAS; in North America, WAAS; in Europe, EGNOS—these are all free to use. Many general GNSS receivers and some high-end smartphones receive SBAS signals to improve positioning. However, SBAS is primarily intended to improve navigation reliability (e.g., for vehicles and aircraft), and its accuracy is limited to tens of centimeters to a few meters. It does not reach the centimeter-level accuracy required for civil engineering surveying or construction management, so RTK or PPP-RTK (CLAS) are used for serious high-precision needs.

PPP (Precise Point Positioning)

PPP (Precise Point Positioning) is a method in which a single GNSS receiver uses globally available high-precision orbit and clock corrections to achieve high-accuracy positioning. In principle, PPP corrects all error terms via calculation models and precise observations, enabling absolute positioning anywhere in the world. Specifically, it uses precise satellite orbit and clock data provided by services like the International GNSS Service (IGS), and global ionosphere models, to greatly enhance standalone receiver accuracy without local reference stations. PPP is effective for surveying and positioning in oceans and remote regions because it is not limited by distances between reference stations. However, achieving real-time centimeter-level accuracy with PPP requires long initial convergence times (sometimes 10–30 minutes), and accuracy improves gradually during observation. Real-time PPP typically begins with errors of several tens of centimeters and improves to several centimeters over many minutes. Therefore, PPP is unsuitable for time-sensitive construction surveying or machine control; it is mainly used for static control point surveys, crustal deformation monitoring, and vessel/aircraft positioning. Recently, countries and private companies have offered PPP services, and “PPP-RTK” approaches that combine regional corrections to shorten convergence time have emerged (CLAS is an example).

Differences Between Nationwide and Local Coverage

When selecting a correction information service, the coverage area is a crucial factor. Methods like RTK that install a base station for each site only deliver corrections locally—typically within several kilometers to a dozen kilometers of that base station. In contrast, VRS, CLAS, and PPP can provide wide-area or nationwide coverage without the physical constraints of a base station while maintaining accuracy. For example, VRS uses a nationwide network of reference points, offering the same service across most of Japan, and CLAS, broadcast from quasi-zenith satellites over Japan, provides uniform accuracy nationwide. PPP can be applied globally, enabling positioning at sea or abroad using the same method. SBAS can be used wherever its geostationary satellites’ signals reach (for example, MSAS covers East Asia).

Nationwide services excel for surveying that involves wide movements or projects spanning multiple sites. They remove the need to rebuild reference stations in each region or to reconcile local coordinate systems, allowing consistent positioning under unified standards and improving data consistency. Conversely, for tasks confined to a single site, operating a local RTK base station may suffice. Self-managed base stations can provide a stable communication environment and reduce dependence on external services. The key is to assess whether you need nationwide coverage or can complete work at the site level, considering the area and frequency of movement. Also, even wide-area services can fail in local radio dead zones (valleys in mountainous areas), so combining correction methods flexibly according to field conditions is recommended.

Differences in Required Infrastructure and Platforms

Each correction method requires different infrastructure and equipment. RTK (single reference-station method) requires your own base station device and power supply, plus a communication means to deliver corrections to the rover. Communication options include short-range low-power radio or UHF radio for nearby sites, and Internet-based Ntrip distribution for wider coverage. Radio methods offer real-time performance with minimal communication costs but are limited by range and line-of-sight; obstacles block the signal. Internet methods (mobile networks) are unaffected by terrain and support wide-area communication but are unusable outside coverage and incur mobile data and service charges.

VRS (network RTK) does not require a base station on the user side but presumes continuous Internet connectivity. The rover communicates bidirectionally with the correction service server via the cellular network to receive location-dependent corrections. Therefore, the work area needs to be within cellular coverage and you must prepare a compatible SIM card or router. Methods that receive corrections directly from satellites—like CLAS or SBAS—do not require ground communication infrastructure. CLAS only requires a high-precision GNSS receiver that can receive the QZSS L6 signal, eliminating the need for additional communication devices even in remote mountain areas (though compatible receivers are limited). SBAS signals are automatically received by compatible receivers, but Japan’s MSAS broadcasts from a geostationary satellite over the equator, so an antenna installation may require a clear view toward the southern sky. PPP usage varies by service: correction data might be obtained from the Internet in real time (e.g., IGS streams or commercial feeds) or received via L-band satellite signals. High-precision PPP requires a multi-frequency high-performance receiver and may also require paid service subscriptions.

Infrastructure reliability is another factor. Self-operated RTK base stations let you manage equipment and communications, but failures or misconfigurations can interrupt corrections. VRS providers maintain sophisticated monitoring, but cellular instability or server outages can still cause temporary interruptions. CLAS and SBAS are resilient to ground communication failures—making them strong in disasters—but cannot perform where satellite signals are blocked, such as deep valleys or urban high-rise canyons. Having multiple correction options increases redundancy. For example, normally use VRS but switch to CLAS when out of coverage, or operate a fixed RTK base while also receiving SBAS signals as a backup. Consider field infrastructure constraints and required availability to design the optimal combination and operational plan.

By Use Case: Which Service Suits Which Task

Correction Services Suitable for As-Built Management

For as-built management after construction completion (measuring final shapes and dimensions), high accuracy and many measurement points are required, so RTK or VRS—providing immediate centimeter-level coordinates—are mainly used. To check deviations from reference points or to compare against design values on site, real-time precise coordinates are essential. Where communication is available, using VRS yields measurements directly linked to public coordinate systems (plane rectangular coordinates and elevations), facilitating as-built drawings and inspection submissions. In locations where communication is difficult, deploying your own RTK base station is common; once the base is set up, stable accuracy can be ensured. Recently, CLAS-capable receivers have also been used for as-built surveys: although initial convergence takes time, they enable as-built surveys in mountain-site completion without network access. PPP is impractical for as-built management due to convergence time, and SBAS lacks sufficient accuracy.

Correction Services Suitable for Piling and Layout

Processes such as pile driving and stakeout/layout require continuous, real-time centimeter accuracy. Machine guidance on pile-driving equipment sometimes uses GNSS, but controlling the machine tool tip within a few centimeters requires continuous high-precision corrections. Therefore, RTK or VRS continuous positioning is usually chosen for these operations. On large development sites, companies often install their own base stations so multiple machines and surveyors share the same corrections. In such conditions, radio-based RTK is advantageous because it is independent of external infrastructure and can broadcast simultaneously to multiple machines and rovers. Where communications are good, VRS can allow each machine to receive corrections individually, but it is not realistic in tunneling or mountainous construction outside cellular coverage. In the future, if CLAS-capable equipment spreads to construction machinery and survey instruments, it could enable pile driving and layout without communication infrastructure (examples of autonomous agricultural machinery already using CLAS-based high-precision guidance exist). PPP’s long initialization and gradual accuracy improvement make it unsuitable for operations demanding immediate responsiveness. SBAS’s tens-of-centimeters error is insufficient for pile position control and thus not used.

Correction Services Suitable for Disaster Surveying

In disaster surveys after earthquakes or landslides, control points may have been lost and communication infrastructure may be down. For rapid, high-precision measurement of terrain displacement and damage extent under such conditions, correction services that permit standalone operation are valuable. CLAS is a prime example: its ability to operate without relying on communication networks is highly valued in major disasters. In the Noto Peninsula earthquake (2023), portable CLAS receivers were effective for assessing disaster conditions when local cellular base stations were offline. With CLAS, a single receiver can record ground displacements in the affected area and immediately provide coordinates in the global geodetic system, facilitating wide-area displacement comparisons and mapping. If communications are available, VRS positioning is possible, but immediately after a disaster, communication restrictions or server outages are a risk. When there is no time or capacity to set up an RTK base station, CLAS—or, in some cases, PPP positioning—can be a valuable tool despite longer initialization. PPP is less suitable for rapid response because it requires long, continuous reception, but it may be used for continuous monitoring during aftershock periods or for precision surveying to restore control networks. SBAS lacks the necessary precision and reliability for disaster-scale analysis. For disaster surveying, selecting CLAS or RTK depending on site conditions and preparing flexible operation plans is key.

Emergence of Smartphone-Integrated CLAS Support “LRTK”

Finally, as a notable new application of correction services, we introduce the smartphone-integrated device “LRTK.” LRTK is a compact RTK-GNSS receiver that attaches to smartphones (mainly iPhone/iPad currently), weighing only about 125 g and about 1.3 cm thick, yet achieving centimeter-level positioning. It mounts to a dedicated smartphone case with one-touch attachment; since it integrates the antenna, receiver circuitry, and battery, having just this device and a smartphone enables positioning comparable to conventional stationary GNSS survey equipment.

LRTK supports very flexible correction methods: in areas with cellular service, it can receive VRS corrections from the smartphone via Ntrip while positioning, and when out of coverage it can directly receive Michibiki’s CLAS signals to continue centimeter-level positioning. With triple-frequency GNSS and CLAS capability, LRTK can secure accuracy in mountainous areas and offshore locations without Internet, and it has proven useful in disaster-site surveying. Higher-end models (LRTK Pro series) equipped with IMU-based tilt correction are also developed to meet advanced needs such as measurement in narrow areas or continuous measurements while moving. The dramatic miniaturization and cost reduction of RTK surveying equipment—previously costing millions of yen—make a “one person, one unit” era more realistic. If site managers and technicians can carry a high-precision positioning tool in their pocket at all times, new workflows become possible: measuring immediately when needed and sharing data via the cloud. LRTK exemplifies innovation born from the convergence of correction information services and smart devices.

FAQ

Q: How should I choose a correction information service? A: First, consider required accuracy, positioning area, and the field’s communication environment. If you need immediate centimeter accuracy on a small site, a self-managed RTK base station or VRS service is appropriate. For wide-area work or multiple sites, nationwide services like VRS or CLAS are convenient. If out of cellular coverage, use CLAS or operate a base station; if some connectivity is expected, VRS is easy to use. Also factor in initial setup costs versus ongoing fees (base station purchase vs. service subscriptions). Overall, selecting a combination of methods based on communication infrastructure, work area, and budget provides the greatest assurance.

Q: Which correction method is effective in mountainous areas without coverage? A: In areas without communication infrastructure, CLAS is the most promising option. With a CLAS-capable receiver, you can achieve centimeter accuracy in mountainous regions. A traditional approach is to set up a portable RTK base station on site and broadcast corrections by radio; within radio range, high accuracy is achieved without cellular service. PPP is usable without communications, but its long convergence time reduces responsiveness and makes it inefficient for dynamic survey work. SBAS requires no communications but its accuracy is limited to tens of centimeters and is insufficient for high-precision tasks.

Q: What costs are associated with using correction information services? A: There are hardware costs and service fees. For RTK, purchasing a base station plus rover receiver set costs roughly ¥1,000,000–¥3,000,000, though communication costs are minimal if you use your own radio. For VRS, you must buy a rover receiver (several hundred thousand yen and up) and pay monthly service fees and communication charges. CLAS itself is free, but you must purchase a compatible high-performance receiver. PPP commercial services may require contracts and fees (though free PPP solutions using open precise orbit data also exist). SBAS requires only a compatible receiver with no extra cost. In general, to minimize upfront costs consider VRS subscriptions or equipment rental; to minimize running costs, consider self-operated RTK or free CLAS.

Q: What is needed to use CLAS? A: CLAS requires a GNSS receiver capable of centimeter-level positioning—specifically, one that can receive and decode Michibiki’s L6-band signals, typically a multi-frequency high-precision GNSS device. Many commercial surveying receivers are becoming CLAS-capable via firmware updates. Recently, smartphone-mountable CLAS-capable receivers (e.g., LRTK) have appeared, lowering 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 the corrections will be applied and high-precision positioning will begin (no registration or fees are required).

Q: What is tilt correction and is it useful in the field? A: Tilt correction detects the pole or device tilt with internal sensors and automatically corrects the measured position so the coordinate corresponds to the point directly beneath the pole tip. This allows measurements from an angle when you cannot place the instrument directly over the measurement point—useful for corners of buildings or points behind obstacles. It also eliminates the need to make the pole exactly vertical each time, enabling faster single-person surveying and efficient continuous observations at varying heights. Tilt correction is very useful in the field, where some pole tilt is common. Accurate correction requires prior sensor calibration, and excessive tilt increases error, so care is needed.

Q: Can a smartphone alone achieve centimeter-level positioning? A: Currently, standalone consumer smartphones (internal GPS) cannot achieve centimeter accuracy without augmentation. Recent phones include high-sensitivity, multi-frequency GNSS chips and can improve accuracy to tens of centimeters with SBAS or simple corrections, but achieving a few centimeters requires dedicated hardware and correction data. However, combining a smartphone with an external device can make it possible. For example, using a smartphone-attached high-precision receiver like the LRTK allows centimeter positioning via RTK or CLAS through a phone app. On Android, there are also approaches to access raw GNSS data and combine with external corrections, and future improvements may enable high-precision positioning using only smartphone internals. As of the mid-2020s, though, professional-level centimeter accuracy realistically requires dedicated augmentation services and receivers.