To utilize satellite positioning such as GPS with centimeter-level accuracy (half-inch accuracy), data called "position correction information" is indispensable. With normal standalone GNSS (Global Navigation Satellite System) positioning, positioning results can have errors of several meters (several ft), but applying correction information can greatly reduce those errors and make high-precision positioning possible. In this article, we explain what position correction information is, why it is necessary, and how it works in a way that is easy for beginners to understand. We also introduce the representative method for achieving high-precision positioning, RTK, and the latest augmentation methods provided by the Quasi-Zenith Satellite System "Michibiki", CLAS and MADOCA, and explain in detail the advantages 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 position correction information?

"Position correction information" is a general term for data used to correct various errors associated with GNSS positioning. Signals transmitted from GNSS satellites are affected by several main error sources before they reach a ground receiver from space. Representative error sources include (A) errors caused by satellite orbit and clock offsets, (B) errors due to delays when radio waves pass through the atmosphere, such as the ionosphere and troposphere, and (C) errors caused by multipath (reflections) near the receiver and receiver noise. Position correction information is the data used to correct these errors, and specifically includes "precise correction values for satellite orbit and clock," "correction values for ionospheric and tropospheric delays," and "differential information based on distance to a reference station," among others.

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 source (A); PPP-RTK, which uses information that corrects error sources (A) + (B); and RTK, which uses real-time differentials depending on the distance to a reference station.

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 set up at a point with accurately known coordinates (a known point), and the rover is operated at the location to be measured. Both receivers simultaneously receive signals from multiple GNSS satellites; the reference station calculates the positioning error (the discrepancy in the satellite distance measurements) and sends this correction information to the rover via radio or the Internet. The rover applies the correction to its own positioning solution, yielding high-precision coordinates with reduced error.

In traditional surveying, optical instruments called total stations (TS) required two or more workers, but with RTK surveying, as long as GNSS signals can be received, ensuring line-of-sight (visibility) is unnecessary, and basically one person can survey wide areas. Also, RTK can acquire coordinates in a few seconds to a dozen or so seconds per point, making it highly efficient for measuring many points in a short time. In terms of accuracy, RTK positioning calculates each point’s position by relative positioning to the same reference station, so even when surveying a large area systematic errors between points are less likely to accumulate and stable accuracy can be maintained. In general, RTK-GNSS equipment can achieve planar-position accuracy on the order of a few centimeters (a few in), and can meet the accuracy requirements commonly required for civil engineering and construction surveying (a few cm (a few in)).

In recent years, services for network RTK (e.g., the VRS method) that deliver correction information over the Internet have become widespread. With network RTK, you can achieve RTK positioning with just a single rover by receiving correction data generated from a network of multiple regional reference stations from the service provider via mobile communications, without installing your own reference station. Using a network RTK eliminates the need to prepare equipment or install a reference station, and maintenance of a base station is also unnecessary. However, note that this method cannot receive data outside the communication area, and subscription fees for the correction information service (running costs) will apply.

What is PPP-RTK (CLAS/MADOCA)?

PPP-RTK (P-P-P R-T-K) is the latest positioning method that combines the advantages of the above PPP (Precise Point Positioning: precise standalone positioning) and RTK. With PPP, although it can be used anywhere in the world, because it does not correct ionospheric and tropospheric errors (B), accuracy remains at the level of several tens of centimeters, and there was the issue that initial convergence (the time until accuracy stabilizes) takes about 15–30 minutes. PPP-RTK was developed to address this: in addition to precise satellite orbit and clock correction information (A), it also provides regional ionospheric and tropospheric correction information (B) from the satellite, achieving RTK-level accuracy without a reference station.

In Japan, the Quasi-Zenith Satellite System Michibiki's CLAS (Centimeter Level Augmentation Service: centimeter-level positioning augmentation service) is a representative example of PPP-RTK. CLAS utilizes data from the Geospatial Information Authority of Japan's nationwide network of electronic reference points (approximately 1,300 GNSS reference stations), divides the Japanese archipelago into several areas, and broadcasts ionospheric and tropospheric correction information tailored to each region via satellite. As a result, anywhere in Japan—even outside communication coverage—centimeter-level positioning is available free of charge, provided the augmentation signal (L6-band radio signal) from the satellite can be received.

MADOCA (Multi-GNSS Advanced Orbit and Clock Augmentation), offered alongside CLAS, is a wide-area high-precision positioning service provided by Michibiki. MADOCA provides global PPP augmentation information and can be used in areas beyond the coverage of domestic reference station networks, such as outside Japan or at sea. However, because MADOCA does not include local ionospheric corrections, its accuracy is somewhat inferior to CLAS, and initial convergence tends to take longer. For reference, under clear skies and open-sky conditions the typical observational accuracies are CLAS: approximately horizontal ±3-5 cm (±1.2-2.0 in) · vertical ±5-10 cm (±2.0-3.9 in) (initial convergence 2-3 minutes), MADOCA: approximately horizontal ±20 cm (±7.9 in) · vertical ±50 cm (±19.7 in) (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 an LRTK Phone device attached to an iPhone. By attaching a compact receiver with an integrated antenna and battery to a smartphone, high-precision positioning can be achieved with one hand. Bluetooth integration eliminates the need for cable connections, making handling on site simple. Using a dedicated app, surveying tasks can be easily performed by one person.*

LRTK is a next-generation high-precision GNSS positioning solution that supports both RTK and PPP-RTK (CLAS/MADOCA) methods. LRTK is a system developed by Reflexia Inc., consisting of a compact GNSS receiver device that integrates with smartphones and a dedicated app. It miniaturizes and reduces 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, it achieves centimeter-level positioning that does not depend 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. With a dedicated option, they can receive CLAS correction information from QZSS satellites even in mountainous areas where cellular signals do not reach, allowing continued high-precision positioning (the so-called "out-of-coverage" model). On the other hand, in areas where cellular networks are available, they can also perform positioning using the conventional network RTK method that acquires reference-station data via the Internet. Hybrid operation that automatically switches between RTK and PPP-RTK depending on the situation is also envisaged, and with LRTK you can obtain stable, high-precision position information across a wide range of environments from urban areas to remote mountains.

Advantages of LRTK

By implementing LRTK, you can gain the following benefits.

• Centimeter-level high accuracy: Because LRTK supports the RTK method and CLAS augmentation information, it can provide positioning with an accuracy of several centimeters (a few inches) horizontally and several centimeters to around 10 cm (several inches to around 3.9 in) in height. It can achieve centimeter-class accuracy without installing a special reference station (cm level accuracy (half-inch accuracy)).

• Available even outside communication coverage: Because it can directly receive Michibiki’s L6 signal, positioning will not be interrupted even at sites where the cellular network is out of range or in mountainous terrain. Real-time correction information can be obtained from the satellites, so high-precision positioning can continue even in disaster situations where the mobile network is down.

• Low running costs: Satellite augmentation services such as CLAS and MADOCA are provided by the government as free services. Therefore, using LRTK enables high-precision positioning without subscribing to external paid positioning services. In situations where an internet connection is not required, there are no communication fees, reducing operational costs.

• Portability and ease of use: The LRTK receiver is small and lightweight enough to attach to a smartphone and features a battery-powered wireless configuration. It does not require cumbersome cables or large tripods, greatly reducing the burden of bringing equipment to the site. The dedicated app’s intuitive interface allows positioning start and data recording with one touch, and it is designed to be easy to use even without specialized GNSS knowledge.

• Multifunctional positioning app integration: The dedicated app “LRTK” offers single-point positioning and continuous positioning (track log recording), high-precision geotagging for photos, AR-based navigation functions, and many other features useful for surveying and inspection work. Measurement data and photos can be uploaded to the cloud service “LRTK Cloud” for storage and sharing, making team information sharing and report creation easy. These features make LRTK not just a GNSS receiver but a comprehensive solution that directly improves on-site work efficiency.

Simplified Surveying with LRTK

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

By leveraging LRTK, tasks that previously required specialized surveying equipment and experienced technicians can now 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 accurate positioning information (latitude, longitude, elevation, and camera orientation) to photos taken on site, place those photos on a map, and record them—all with a single tap. This is highly effective for documenting conditions at construction sites and preparing infrastructure inspection reports, and it greatly streamlines work that traditionally required time-consuming post-processing and position matching.

By combining a smartphone equipped with a LiDAR sensor and LRTK, you can easily acquire 3D point cloud data of terrain and structures. Because each point in the acquired point cloud is assigned high-precision coordinates, it can also be used for simple as-built surveys and volume calculations. Furthermore, using the LRTK app's AR features, the direction and distance to a specified coordinate on site can be displayed on the screen as arrows, guiding the operator to the target location. This makes staking out and identifying points—tasks that traditionally required two surveyors working as a pair—doable intuitively by a single person.

In this way, simplified surveying with LRTK proposes a new surveying style to a wide range of users, from GNSS beginners to professional surveyors. By leveraging the power of high-precision position correction information and combining it with the convenience of smartphone apps, it lowers the barrier to fieldwork and enables obtaining sufficiently accurate survey 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 position 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 within the atmosphere, and by using this information GPS positioning errors can be reduced from several meters (several ft) down to several centimeters (several in). Specifically, position correction information includes precise satellite orbit and clock information, differential data from reference stations, and correction values for ionospheric and tropospheric delays.

Q: RTK方式とPPP-RTK（CLAS）の違いは何ですか？ A: RTK方式は近くに既知座標の基準局を置き、その局との相対測位で誤差を取り除く方法です。通信回線を通じてリアルタイムに補正データを受け取るため、初期化が速く数秒でセンチ級精度が得られます。ただし通信環境や基地局の有無に依存します。一方、PPP-RTK（CLAS）は基準局無しでも衛星から補正情報を直接受け取る方法で、日本ではみちびきのCLAS信号を受信することで実現します。通信圏外でも利用でき補正サービス利用料も無料ですが、初期収束に数分程度かかる点や電波を受信できる開けた空視界が必要な点が異なります。まとめると、RTKは即時性と実績、PPP-RTKは利便性とエリアフリーという違いがあります。

Q: What is the difference between RTK and PPP-RTK (CLAS)? A: RTK places a reference station with known coordinates nearby and removes errors through relative positioning to that station. Because correction data is received in real time over a communications link, initialization is fast and centimeter-level accuracy (cm level accuracy (half-inch accuracy)) can be achieved in a few seconds. However, it depends on the communication environment and the presence of base stations. PPP-RTK (CLAS), on the other hand, 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, but it differs in that initial convergence takes several minutes and an open sky view where signals can be received is required. In summary, RTK offers immediacy and a proven track record, while PPP-RTK offers convenience and area-free availability.

Q: What is required to use CLAS or MADOCA? A: To receive augmentation signals such as CLAS or MADOCA, you need a compatible multi-frequency GNSS receiver. Because the built-in GPS in typical smartphones cannot receive these signals (L6 band), you use a compatible device such as an LRTK connected to the smartphone. In Japan, if you are in an area where CLAS signals can be received, you can use them without any special contract. 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 need to pay attention to satellite visibility and service availability. Note that simply powering on a CLAS/MADOCA-capable receiver will automatically start positioning in satellite augmentation mode.

Q: What is the positioning accuracy of LRTK? A: Depending on the environment, horizontal positioning can typically be expected to be accurate to approximately ± a few centimeters (± a few in). For example, in open areas using CLAS, planar position is often within about 3-5 cm (1.2-2.0 in), and height is often within about 5-10 cm (2.0-3.9 in). Standard RTK (using a reference station) also achieves a similar centimeter-level accuracy (cm level accuracy, half-inch accuracy). With the MADOCA method, horizontal accuracy is lower than CLAS at around 20 cm (7.9 in), but it is still far more accurate than standalone positioning. LRTK supports these methods, and when operated properly will produce results that fully meet the accuracy required for surveying work and facility management.

Q: Does it take long to start positioning? A: When using LRTK in RTK mode, if corrections are received from a reference station, high-precision positioning is possible almost immediately (within a few seconds). For CLAS (PPP-RTK), an initial convergence time of about 2–3 minutes is required for the solution to stabilize after incorporating corrections from satellites. Once centimeter-level (half-inch-level) positioning is established, high precision can be maintained continuously thereafter. With MADOCA, the solution starts as a float solution (a somewhat less precise state) and it can take more than 20 minutes to fully converge to centimeter- to decimeter-level (half-inch to 3.9 in). However, because accuracy gradually improves after starting work, it is efficient to carry out other preparations in parallel while you wait.

Q: 通信環境や費用面で心配があります。LRTKは大丈夫でしょうか？ A: LRTKは衛星通信を活用することで、通信インフラが無い場所でも測位できるよう設計されています。CLASを使う限りモバイル通信が不要なので、山間部など携帯電話の届かない現場でも安心して利用できます。また、CLASやMADOCAの受信に追加料金は発生しませんので、補正情報利用のランニングコストはゼロです。ただし、ネットワーク型RTKを利用する場合は従来通り通信契約やサービス利用料が必要です。その場合でも、補正データの通信量は大きくないため通信費はごくわずかです。総じて、LRTKは運用コストを抑えつつ高精度測位を実現できる経済的な選択肢と言えます。

Q: Can it be used in environments where satellites are difficult to see, such as forests or urban areas? A: GNSS positioning basically works best in environments with a wide, open view of the sky. In forests covered with trees or urban areas lined with tall buildings, satellite signals can be blocked or reflected, causing reduced accuracy. LRTK, like other GNSS devices, delivers its best performance where sky visibility can be ensured. However, LRTK receivers that support multi-GNSS are designed to pick up signals from many satellites, including ones other than GPS, so they are engineered to continue positioning as much as possible even under occlusion. It can be difficult to maintain centimeter-level accuracy (half-inch accuracy) deep inside a dense forest or in the shadow of buildings, but in such environments it is advisable to take measures such as performing positioning temporarily in an open spot to supplement the results or combining 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.