To utilize satellite positioning such as GPS with centimeter-level accuracy, data called "position correction information" is indispensable. Standalone GNSS (Global Navigation Satellite System) positioning typically results in errors of several meters, but applying correction information can greatly reduce those errors and enable high-precision positioning. This article explains what position correction information is, why it is necessary, and how it works in a way that is easy for beginners to understand. It also introduces the representative method for achieving high-precision positioning, the RTK method, and the latest augmentation methods provided by the Quasi-Zenith Satellite System "Michibiki", CLAS and MADOCA, and provides a detailed explanation of the benefits of choosing the new positioning solution "LRTK" that supports them.

Table of Contents

• What is position correction information?

• What is the RTK method?

• What is PPP-RTK (CLAS/MADOCA)?

• What is LRTK?

• Advantages of LRTK

• Simple surveying with LRTK

• FAQ

What is positioning correction information?

"Positioning correction information" is a collective term for data used to correct various errors associated with GNSS positioning. Signals transmitted from GNSS satellites are affected by several major error factors before reaching receivers on the ground. Typical error factors include (A) errors due to satellite orbit and clock offsets, (B) errors caused by delays as radio waves pass through the atmosphere, such as the ionosphere and troposphere, and (C) errors due to multipath (reflections) and equipment noise occurring around the receiver. The information that corrects these errors is positioning correction information, and specifically includes items such as "precise correction values for satellite orbit and clock," "correction values for ionospheric and tropospheric delays," and "differential information based on the distance to a reference station."

By applying position correction information to a receiver, GNSS positioning errors that normally amount to several meters can be reduced to the centimeter level. For example, various methods use this position correction information to achieve high-precision positioning, such as PPP (Precise Point Positioning), which uses precise satellite orbit and clock information to correct error factor (A); PPP-RTK, which uses information to correct error factors (A)+(B); and RTK, which uses real-time differential corrections based on the distance to reference stations.

What is the RTK method?

RTK method (Real-Time Kinematic positioning) is a technique that uses two GNSS receivers—a reference station (base station) and a rover (mobile station)—to correct positioning errors in real time. First, the reference station is installed at a point whose coordinates are already known (a known point), and the rover is operated at the location to be measured. Both receivers simultaneously receive signals from multiple GNSS satellites, and the reference station sends the positioning error it has calculated (the discrepancy in distance measurements from the satellites) as correction information to the rover via radio or the Internet. By applying that correction to its own positioning solution, the rover can obtain high-precision coordinates with reduced error.

In conventional surveying, optical instruments called total stations (TS) required two or more operators, but with RTK surveying, as long as GNSS signals can be received, ensuring line-of-sight (visibility) is unnecessary, and in principle a single person can survey large areas. In addition, RTK can obtain coordinates in a few seconds to around a dozen seconds per point, making it highly efficient for measuring many points in a short time. In terms of accuracy, RTK positioning always computes each point’s position by relative positioning to the same reference station, so even when surveying a wide area, systematic errors between points are unlikely to accumulate and stable accuracy can be maintained. Generally, RTK-GNSS equipment can achieve planimetric accuracies on the order of a few centimeters, meeting the accuracy requirements (a few cm) typically required for routine civil engineering and construction surveying.

Recently, services that deliver correction information over the Internet—known as network-type RTK (e.g., the VRS method)—have become widespread. With network-type RTK, you do not install your own reference station; by receiving correction data generated from a network of multiple regional reference stations from a service provider via cellular communications, RTK positioning can be achieved with just a single rover. Using the network-type approach saves the effort of preparing equipment and installing reference stations, and eliminates the need to maintain base stations. However, it should be noted that this method cannot receive data outside the communication area and that subscription fees for the correction information service (running costs) will be incurred.

What is PPP-RTK（CLAS/MADOCA）?

PPP-RTK（ピー・ピー・ピー・アールティーケー）は、上記のPPP（Precise Point Positioning：精密単独測位）とRTKの長所を組み合わせた最新の測位手法です。PPPでは全世界どこでも利用可能な代わりに、電離層や対流圏の誤差(B)を補正しないため精度が数十センチ程度に留まり、初期収束（精度が安定するまでの時間）にも15～30分ほど要するという課題がありました。そこで登場したPPP-RTKでは、衛星軌道・時計の精密補正情報（A）に加えて、地域ごとの電離層・対流圏の補正情報（B）も衛星から提供することで、基準局がなくてもRTK並みの精度を達成しています。

In Japan, the Quasi-Zenith Satellite System “みちびき” provides CLAS (Centimeter Level Augmentation Service: centimeter-level augmentation service) as a representative example of PPP-RTK. CLAS utilizes data from approximately 1,300 electronic reference points (a GNSS reference station network) of the Geospatial Information Authority of Japan, divides the Japanese archipelago into several areas, and transmits ionospheric and tropospheric correction information tailored to each region via satellite. As a result, anywhere in Japan—even outside cellular coverage—as long as the augmentation signal from the satellite (L6-band radio waves) can be received, centimeter-level positioning is available free of charge.

Provided alongside CLAS, MADOCA (Multi-GNSS Advanced Orbit and Clock Augmentation) is Michibiki’s wide-area high-precision positioning service. MADOCA supplies global PPP augmentation information and can be used in regions beyond the reach of domestic reference station networks, such as outside Japan and at sea. However, because MADOCA does not include local ionospheric corrections, its accuracy is somewhat inferior to CLAS and it tends to require more time for initial convergence. For reference, under clear-sky, open-sky conditions the indicative observation accuracies are CLAS: approximately horizontal ±3–5 cm; vertical ±5–10 cm (initial convergence 2–3 minutes) and MADOCA: approximately horizontal ±20 cm; vertical ±50 cm (initial convergence about 20–30 minutes). Even so, compared with the several-meter errors of conventional standalone positioning, MADOCA still achieves a dramatic improvement in accuracy and is useful for rough surveying and GIS applications in remote areas where reference stations cannot be installed.

What is LRTK?

*An example of using the LRTK Phone device attached to an iPhone. By attaching a compact receiver with an integrated antenna and battery to a smartphone, you can achieve high-precision positioning with one hand. Bluetooth connectivity eliminates the need for cable connections, making on-site handling simple. Using a dedicated app allows a single person to easily carry out surveying tasks.*

LRTK is a next-generation high-precision GNSS positioning solution that supports both the RTK method described above and the PPP-RTK method (CLAS/MADOCA). LRTK is a system developed by Reflexia Co., Ltd., consisting of a compact GNSS receiver device that integrates with a smartphone and a dedicated app. It is characterized by miniaturizing and reducing the cost of RTK-capable GNSS equipment that was previously stationary and expensive, and by enabling direct reception of Michibiki’s CLAS and MADOCA signals, thereby achieving centimeter-level positioning that does not rely on communications infrastructure.

LRTK devices support multi-GNSS and multi-frequency, and are compatible not only with GPS and GLONASS but also with Japan’s quasi-zenith satellite Michibiki (QZSS). With a dedicated option, they can receive CLAS correction information from QZSS satellites and continue high-precision positioning even in mountainous areas where mobile phone signals do not reach (the so-called "out-of-coverage" model). On the other hand, in areas where mobile networks are available, positioning is also possible using the conventional network RTK method that acquires reference station data via the Internet. A hybrid operation that automatically switches between RTK and PPP-RTK depending on the situation is also envisaged, so with LRTK you can obtain stable, high-precision positioning across a wide range of environments from urban areas to remote mountains.

Benefits of LRTK

By adopting LRTK, you can gain the following benefits.

• Centimeter-level high accuracy: Because LRTK supports RTK methods and CLAS augmentation information, it can provide positioning with an accuracy of a few centimeters horizontally and a few centimeters to around ten centimeters vertically. Centimeter-level accuracy, which was difficult to achieve with conventional standalone positioning, can be attained without installing special reference stations.

• Usable even outside communication coverage: Since it can directly receive Michibiki’s L6 signal, positioning remains uninterrupted even at sites where cellular networks are out of range or in mountainous areas. Real-time correction information can be obtained from satellites, so high-precision positioning can continue even when mobile networks are down during disasters.

• Low running costs: Satellite augmentation services such as CLAS and MADOCA are provided by the government free of charge. Therefore, using LRTK allows high-precision positioning without subscribing to external paid positioning services. In situations where an internet connection is unnecessary, there are no communication fees, reducing operational costs.

• Portability and ease of use: The LRTK receiver is small and light enough to attach to a smartphone and features a battery-powered wireless design. It does not require cumbersome cables or large tripods, significantly reducing the burden of bringing equipment to the field. With the dedicated app’s intuitive interface, starting positioning and recording data can be done with a single touch, and it is designed to be easy to use even without specialized GNSS knowledge.

• Multi-functional positioning app integration: The dedicated app "LRTK" offers a wealth of features useful for surveying and inspection work, including single-point positioning and continuous positioning (track log recording), high-precision geotagging for photos, and AR-based navigation functions. Measurement data and photos can be uploaded to the cloud service "LRTK Cloud" for storage and sharing, making team information sharing and report preparation easier. With these features, LRTK is not just a GNSS receiver but a comprehensive solution that directly improves fieldwork efficiency.

Simplified surveying with LRTK

*An example of displaying high-precision 3D point cloud data acquired with an LRTK system on the cloud. By combining it with a smartphone’s LiDAR scanner function, surrounding terrain and structures can be scanned in a short time, and high-precision position coordinates can be assigned to each point. These data can be visualized on a map and used to check distances and bearings of measurement points, compare with photographs, and so on.*

By utilizing LRTK, tasks that previously required specialized surveying equipment and skilled technicians can be performed easily by anyone as simplified surveying. For example, using an LRTK receiver attached to a smartphone and an app, you can automatically append precise positioning information (latitude, longitude, altitude, and camera orientation) to photos taken on site, and place and record those photos on a map with a single tap. This is highly effective for recording conditions at construction sites and preparing reports for infrastructure inspections, greatly streamlining work that previously required time-consuming position post-processing and verification.

Also, by combining a smartphone equipped with a LiDAR sensor and LRTK, you can easily acquire terrain and structures' 3D point cloud data. Because high-precision coordinates are assigned to each point in the acquired point cloud, it can also be used for simple as-built surveys and volume calculations. Furthermore, using the AR function of the LRTK app, the direction and distance to a specified on-site coordinate can be displayed on the screen with an arrow, guiding the operator to the target location. As a result, staking-out and point-identification tasks that traditionally required two surveyors working as a pair can be performed intuitively by a single person.

In this way, simplified surveying with LRTK proposes a new surveying style for a wide range of users, from GNSS beginners to surveying professionals. By leveraging the power of high-precision position correction information and combining it with the convenience of smartphone apps, it lowers the barriers to fieldwork and enables obtaining sufficient surveying results with a small team and in a short time. If you are thinking, "I want to perform high-precision positioning easily without relying on communication environments" or "I want to introduce GNSS surveying to improve operational efficiency," LRTK can be a strong option.

FAQ

Q: What is positioning correction information? A: It refers to various data used to correct errors in GNSS positioning. It includes information that compensates for satellite orbit and clock errors and for signal delays in the atmosphere; by using this, GPS positioning errors can be reduced from several meters to a few centimeters. Specifically, positioning correction information includes precise satellite orbit and clock data, differential data relative to reference stations, and correction values for ionospheric and tropospheric delays.

Q: What is the difference between RTK and PPP-RTK (CLAS)? A: The RTK method places a reference station with known coordinates nearby and removes errors by performing relative positioning with that station. Because correction data are received in real time over a communication link, initialization is fast and centimeter-level accuracy can be achieved within seconds. However, it depends on the communication environment and the presence of base stations. On the other hand, PPP-RTK (CLAS) is a method that receives correction information directly from satellites without a reference station; in Japan this is realized by receiving the CLAS signal from Michibiki. It can be used outside communication coverage and the correction service is free of charge, but it differs in that initial convergence takes a few minutes and an open sky view is required to receive the signals. In summary, RTK offers immediacy and proven performance, while PPP-RTK offers convenience and area-free availability.

Q: What is required to use CLAS or MADOCA? A: To receive correction signals such as CLAS or MADOCA, you need a compatible multi-frequency GNSS receiver. Standard smartphone built-in GPS cannot receive these signals (L6 band), so you connect and use a compatible device like an LRTK with your smartphone. Within Japan, CLAS is available without a special contract in areas where the CLAS signal can be received. Overseas (outside the coverage area of Japan’s reference stations), you can augment positioning by receiving MADOCA signals, but outside the vicinity of Japan you should be mindful of satellite visibility and service availability. Note that simply powering on a CLAS/MADOCA-compatible receiver will automatically start positioning in satellite augmentation mode.

Q: What is the positioning accuracy of LRTK? A: It depends on the environment, but you can generally expect horizontal-position accuracy of approximately ± a few centimeters. For example, in open areas using CLAS, planar position typically falls within about 3–5 cm, and vertical errors are often within about 5–10 cm. Standard RTK (using a base station) also achieves similar centimeter-level accuracy. The MADOCA method has lower accuracy than CLAS—around 20 cm horizontally—but is still far more precise than standalone positioning. LRTK supports these methods, and when operated properly yields results that sufficiently meet the accuracy required for surveying tasks and facility/asset management.

Q: Does it take time to start positioning? A: When using LRTK for RTK positioning, if you receive corrections from a base station you can achieve high-precision positioning almost immediately (within a few seconds). For the CLAS method (PPP-RTK), an initial convergence time of about 2–3 minutes is required for the solution to stabilize as correction information from satellites is applied. Once centimeter-level positioning is established, high precision can be maintained continuously thereafter. With the MADOCA method, it starts with a float solution (a somewhat lower-precision state), and it can take more than 20 minutes to fully converge to centimeter- to decimeter-level. However, since accuracy gradually improves after starting work, it is efficient to carry out other preparations in parallel while waiting.

Q: I'm worried about the communication environment and costs. Is LRTK okay? A: LRTK is designed to utilize satellite communications so it can perform positioning even in locations without communications infrastructure. As long as you use CLAS, mobile communications are not required, so it can be used with confidence even at sites in mountainous areas where mobile phone service does not reach. In addition, receiving CLAS or MADOCA does not incur additional charges, so the running cost for using correction information is zero. However, when using network-based RTK, as before a communication contract and service fees are required. Even in that case, the volume of correction data is not large, so communication costs are minimal. Overall, LRTK can be considered an economical option that achieves high-precision positioning while keeping operating costs low.

Q: Can it be used in environments where satellites are hard to see, such as forests or urban areas? A: GNSS positioning generally performs best in environments with a wide, unobstructed view of the sky. In forests covered by trees or urban areas lined with high-rise buildings, satellite signals can be blocked or reflected, which may degrade accuracy. LRTK, like other GNSS devices, achieves its best performance where a clear view of the sky can be ensured. However, because LRTK receivers support multi-GNSS and can capture signals from many satellites beyond GPS, they are designed to continue positioning as much as possible even under partial obstruction. It can be difficult to maintain centimeter-level accuracy deep inside dense forests or in building shadows, so in such environments it is advisable to take measures such as temporarily positioning in an open area to supplement the results or combining LRTK with other surveying methods. Even so, if the sky is at least partially visible, LRTK may automatically acquire satellites and obtain a high-precision solution.