Is a Position Correction Data Distribution Service Necessary? LRTK Is Changing the New Standard for High-Precision Positioning

Table of Contents

• What is a position correction information distribution service?

• Why correction information is necessary for high-precision positioning

• Types and characteristics of correction information services - Satellite-based correction information service (CLAS) - Network-type RTK services

• The emergence of smartphone-integrated high-precision GNSS receivers

• The new standard for high-precision positioning brought by LRTK

• Summary

• FAQ

What is a Position Correction Information Distribution Service?

To obtain centimeter-level high-precision positions with satellite positioning, it is necessary to correct positioning errors using data called "correction information". In standalone GPS positioning, multiple factors such as signal delays in the atmosphere, satellite orbit and clock errors, and radio wave reflections (multipath) cause discrepancies of about 5-10 m (16.4-32.8 ft). Therefore, for applications that require high positioning accuracy, such as civil engineering surveying and autonomous driving, GPS alone is insufficient and methods to reduce positioning errors to a few centimeters (a few in) are indispensable.

That's where the position correction information distribution service comes into play. This is a service that distributes error information observed at reference points (receivers with known, precise positions) to users and provides a mechanism to correct those errors in real time. For example, in the long-established RTK (Real Time Kinematic) positioning, a reference station (fixed station) is installed on site and correction data is sent to the rover, and by canceling errors through the measurement differences between the two, it achieves approximately 1-3 cm (0.4-1.2 in) accuracy. The position correction information distribution service thus refers broadly to services that provide users with the error-correction data necessary for such high-accuracy positioning, and it is a key to high-precision positioning.

Why correction information is necessary for high-precision positioning

As mentioned above, conventional GNSS (satellite positioning) experiences meter-level position errors due to various error sources. Let's organize the main reasons why correction information is required in the field of high-precision positioning.

• Satellite and clock errors: There are slight prediction errors in the satellites’ orbital information, and tiny drifts also accumulate in the atomic clocks onboard the satellites. These affect position calculations and reduce the accuracy of standalone positioning.

• Ionospheric and tropospheric effects: When radio signals from satellites pass through the Earth's atmosphere, delays in the ionosphere and refraction in the troposphere change the signal propagation time, causing errors in the measured ranges.

• Multipath errors: When radio waves reflected off building façades, the ground, or other surfaces (indirect waves) reach the receiver, they mix with the direct waves and distort range measurements. In urban areas in particular, blockage and reflections from buildings have a large impact and contribute to degraded accuracy.

• Receiver-side errors: Positioning accuracy is also affected by slight offsets in the receiver’s internal clock and by poor satellite geometry (geometric distribution of the satellites).

Because the above factors combine, GPS built into a smartphone and other standalone positioning methods inevitably have errors of 5-10 m (16.4-32.8 ft) or more. This is not a problem in everyday situations such as car navigation or map apps where a deviation of several meters (several ft) is acceptable, but in fields such as infrastructure construction management and surveying where cm level accuracy (half-inch accuracy) is required, a mechanism to correct this error is indispensable. Therefore, by using correction information based on observation data from reference stations, we improve accuracy by subtracting the error components from the positioning results. In short, to achieve high-precision positioning, it is necessary to obtain and utilize correction information in some form.

Types and Characteristics of Correction Information Services

Recently, the Geospatial Information Authority of Japan's electronic reference point network and proprietary reference station networks established by telecom operators have been developed, creating an environment in which users can obtain correction information without having to set up base stations on-site themselves. The service formats for distributing that correction information can broadly be divided into two types: methods that deliver correction information via satellite and methods that deliver correction information via Internet connections. As representative examples, let's look at the characteristics of Japan's satellite communications service CLAS and the communication-network-type RTK correction services.

Satellite-based correction information service

Japan operates the Quasi-Zenith Satellite System, and one of its services is the Centimeter-Level Augmentation Service (CLAS). CLAS generates wide-area error information based on observation data from electronic reference stations installed throughout Japan, and broadcasts that information directly from the Michibiki satellites to users' receivers. With only a compatible high-precision GNSS receiver, users can correct positioning errors to within a few centimeters (a few in) simply by receiving the satellite signals — an innovative service. Positioning errors of several meters (several ft) that could not be avoided with standalone GPS can be reduced at once to a few centimeters (a few in) by using CLAS-compatible equipment. This is an advanced initiative even on a global scale, and its major strength is that, anywhere within Japan, centimeter-level accuracy (half-inch accuracy) from satellite signals alone can be obtained.

Because an error model common to a wide area is provided via satellite, users do not need to install their own base stations. The correction signals are transmitted on the L6 band from the Michibiki quasi-zenith satellite, and because the service itself is an open service provided by the government, it can be received free of charge (a compatible receiver must be prepared). Moreover, because correction data are obtained directly from the satellite, high-precision positioning can be maintained even in mountainous areas where mobile phone signals do not reach, or in situations where ground communication networks are cut off by large-scale disasters.

On the other hand, there are some caveats when using CLAS. For example, when initially improving positioning accuracy, a convergence time of tens of seconds to about 1 minute (warm-up) may be required, and in terms of immediacy it can be inferior to conventional RTK, which can quickly achieve high accuracy through relative comparison with a base station. Also, the final achieved accuracy has been reported as on the order of a few centimeters horizontally (measured RMS of about 5-6 cm (2.0-2.4 in)) and about 10 cm (3.9 in) in height, which is slightly inferior to the ideal accuracy of network RTK (horizontal 2-3 cm (0.8-1.2 in), vertical a few centimeters (a few inches)). Nevertheless, this level is acceptable for many practical applications, and considering the advantage of not requiring communication infrastructure and being usable uniformly nationwide, it can be said to be a very useful correction service. Note also that CLAS is currently available only within Japan (due to satellite service area considerations).

Network-based RTK Service

A method of distributing correction information via an Internet connection rather than by satellite is generally called a network RTK service. It integrates observation data from multiple GNSS reference stations installed in various locations (electronic reference stations (CORS) or privately operated fixed stations), performs error analysis in the cloud, and delivers the corrections to users (rovers) in real time. On the user side, a connection to the Internet is made via a cellular network or similar, and correction data are received from the service provider’s distribution server (Ntrip caster).

Traditionally, to perform RTK positioning, users had to set up a base station on site themselves, and high-precision positioning was only possible within a few kilometers (a few thousand ft) of it. However, by using a network RTK service, centimeter-level positioning (cm level accuracy (half-inch accuracy)) is possible anywhere within the service area without owning a base station. Because there is no need for the effort of installing and removing a base station, work efficiency in surveying and construction sites is dramatically improved. In addition, wide-area corrections from a network of multiple reference stations suppress accuracy degradation at distant locations compared with a single-base-station method.

Commercial network RTK services that cover the entire country have emerged, and they are widely available from Hokkaido to Okinawa wherever mobile communications are available. For example, major telecommunications operators install numerous GNSS receivers at their mobile base stations to build proprietary correction networks, and based on that data they provide correction information distribution services with cm level accuracy (half-inch accuracy). In addition, virtual reference station services provided by surveying equipment manufacturers and private companies are being rolled out across many areas. By leveraging these services, the environment is being put in place that allows high-precision positioning, which had previously been limited to large companies and government agencies with dedicated surveying teams, to be adopted by regional small and medium-sized enterprises and local governments.

To use a network RTK service, you need a contract with the service provider. In many cases a monthly subscription fee is charged, and although it depends on the plan and region, a monthly fee of around tens of thousands of yen is common. After contracting, users set the issued ID and password in their GNSS receiver or positioning app and connect to the correction information distribution service (Ntrip) to operate. Also, because it uses internet communication, real-time reception of correction data is not possible in areas without mobile phone signal. It is a disadvantage that the service cannot be used in areas outside communication coverage such as mountainous regions and at sea, but conversely, if a communication environment can be secured, it offers the convenience of wide-area use. The theoretical positioning accuracy itself is not inferior to localized RTK methods, and because it can obtain high accuracy with errors of a few centimeters (a few in) almost immediately, it is now being practically used in various fields such as construction ICT construction management and autonomous vehicle operations.

The arrival of a smartphone-integrated high-precision GNSS receiver

Thus, CLAS and network RTK each have their own advantages, but a common challenge for both approaches is that a compatible high-precision receiver is required. Conventional high-precision GNSS survey equipment has demanded a lot of gear—large antennas, dedicated controllers, communications equipment for base stations, and so on—and deployment typically required substantial investment and the securing of specialized technicians. As a result, only a limited number of organizations could utilize it, and excellent high-precision positioning technology was not being fully leveraged in the field.

Recently, what has emerged is a compact GNSS receiver that can be paired with a smartphone. By attaching a pocket-size device that integrates an antenna, a high-precision GNSS chip, and a battery to a smartphone and connecting it via Bluetooth, etc., your smartphone becomes a centimeter-level positioning system (half-inch-level positioning system). For example, if you attach a receiver weighing about 100–150 g to the back of your smartphone and tap "Start positioning" in the dedicated app, the smartphone GPS error, which is normally about 5–10 m (16.4–32.8 ft), can instantly improve to just a few centimeters (a few in). No complicated equipment operation or expert knowledge is required, and it can truly be said that the smartphone is effortlessly transformed into a convenient surveying instrument.

The biggest advantage of using a smartphone-integrated RTK-GNSS receiver is that the hurdle to initial adoption is greatly reduced. Because you only need to add a small device while leveraging an existing smartphone, you can get started at a dramatically lower cost compared to acquiring a full set of dedicated equipment. Also, many smartphone-connected solutions offer positioning apps and web cloud services; basic use of the app after purchasing the hardware is free, and advanced cloud features can be subscribed to on a monthly basis as needed, making it easier to adjust operational costs. As a result, the conventional belief that "high-precision positioning = high-cost investment" is being overturned, creating a situation in which small and medium-sized enterprises and local governments can more easily adopt it.

Moreover, the smartphone + GNSS device setup also offers the benefit of streamlining field work. For example, because photos and point cloud data with high-precision positional information can be captured immediately on-site, they can be used for as-built verification after construction and for infrastructure inspection records. Since highly accurate digital location data is recorded right where measurements are taken, preparing later reports and performing data analysis is smoother. In addition, many smartphone RTK apps have cloud integration features that allow positioning data and photos acquired in the field to be uploaded to the cloud and shared instantly. This enables office staff to understand the situation in real time, smoothing communication between field and office, preventing rework, and speeding up decision-making.

LRTK: Redefining the New Standard for High-Precision Positioning

With the advent of smartphone-connected high-precision GNSS receivers like those described above, the range of use cases for high-precision positioning has begun to expand rapidly. LRTK is a representative new-generation system of this trend, a high-precision positioning solution composed of a small device that attaches to a smartphone and a dedicated app. The name LRTK conveys the meaning of "an RTK that can be used smartly," and it was developed with the aim of being usable by anyone without specialized knowledge.

With the LRTK system, simply attaching the device to a smartphone and pressing a single button in the app starts positioning, and correction information is automatically acquired and high-precision positioning is performed without complicated settings. Also, within the app you can switch between the "network RTK (Ntrip) via the Internet" mode and the "Michibiki CLAS reception" mode with a one-touch operation. In other words, in situations where a communication line is available, use network RTK, and in environments where communication does not reach, use satellite CLAS, allowing you to easily enjoy the advantages of both depending on the situation. Because methods that used to require separate devices or services can now be seamlessly switched within a single system, it truly embodies a new standard that enables "anytime, anywhere, anyone" to utilize centimeter-level positioning (cm level accuracy (half-inch accuracy)).

The emergence of smartphone-integrated systems like LRTK has begun to change the notion that high-precision positioning information is only for specialized experts. For example, onsite position measurements that previously had to be left to surveyors and other specialists can now be carried out quickly on the spot by general personnel using LRTK-enabled simple surveying. Even at disaster sites where communications infrastructure has been disrupted, LRTK can provide high-precision positioning as long as satellites are visible, so it can be expected to be used for mapping damage with accurate location information. As high-precision positioning technology becomes a familiar tool, on-site work styles and methods of leveraging data will increasingly transform.

Summary

In this article, we introduced the necessity of "position correction information distribution services" for obtaining high-precision location information, and the characteristics of the representative methods, satellite-communication CLAS and network-communication RTK. Each method has its advantages and disadvantages, but in recent years the advent of compact receivers integrated with smartphones has ushered in an era in which the advantages of both can be easily enjoyed. Finally, we summarize the main points.

• 補正情報の入手手段: CLASは衛星から直接補正信号を受信するため携帯回線などの通信インフラが不要。一方ネットワーク型RTKはインターネット接続が必須で、携帯通信網経由で補正データを取得します。

• サービス提供エリア: CLASは準天頂衛星の可視範囲内であれば日本全国がサービスエリアです。ネットワーク型RTKも国内ほぼ全国に対応していますが、利用できるのは通信圏内の地域に限られます。

• 測位精度: どちらの方式でも誤差数センチの高精度を実現可能です。理論上はRTKの方が若干高精度で安定していますが、CLASも実用上はほぼ同等レベルの精度を提供します。

• 初期測定の速さ: ネットワークRTKは基準局との同時観測によりほぼ即時に高精度測位が可能です。CLASは広域補正モデルのため、初回測位時に数十秒程度の収束（コンバージェンス）時間を要する場合があります。

• 必要な機材: CLAS利用には対応する高精度GNSS受信機が必要です（現状、一般的なスマホ内蔵GPSのみではCLAS信号を直接受信できません）。ネットワークRTKも高精度対応の受信機と通信端末が必要ですが、現在はスマホ＋小型デバイスの組み合わせで両方式に対応可能になっています。

• コスト面: CLASの補強信号受信自体は無料です（公共サービスとして提供されているため）。ネットワーク型RTKサービスは通常、契約に月額利用料が発生します。

• 環境適応性: 携帯通信が圏外の環境や災害時でも、CLASなら衛星さえ見通せれば単独で高精度測位が可能です。ネットワークRTKは通信回線が途切れると利用できません。ただし都市部のビル街や森林など衛星信号自体が捉えにくい環境では、どちらの方式でも測位に支障が出る点は共通です。

As described above, CLAS and network RTK can be said to complement each other's weaknesses. Under normal conditions you can quickly obtain high accuracy with a network-based approach, and switch to CLAS in situations where communication is difficult, allowing flexible use. In fact, recently smartphone positioning solutions have emerged that let you choose both types of correction information within a single system or app. For example, in LRTK, a next-generation smartphone RTK system, you can start positioning simply by attaching a small device to the smartphone and pressing a button, and the app makes it easy to switch between "network RTK via Ntrip" and "Michibiki CLAS mode." Designed to be used without complicated settings, the system aims to enable "anytime, anywhere, anyone" use of centimeter-level positioning (half-inch accuracy).

High-precision position correction services will continue to evolve, improving convenience. By leveraging handy smartphone-integrated positioning devices, centimeter-level positioning—previously difficult to achieve—will become accessible to everyone. Take this opportunity to try the latest smartphone RTK technology and experience its power firsthand. Incorporating simplified surveying with LRTK could dramatically change on-site positioning workflows and how location information is utilized.

FAQ

Q: What is a correction information service? A: It is a service that distributes error data observed at reference stations to improve the positioning accuracy of GNSS (such as GPS), allowing users to correct positional offsets in real time. Normally, positioning errors of several meters (several ft) can be reduced to the order of a few centimeters (a few in) by using correction information.

Q: What is the difference between a smartphone's built-in GPS and using a correction information service? A: A smartphone's built-in GPS typically achieves positional accuracy of only about 5–10 m (16.4–32.8 ft). In urban areas surrounded by buildings, it can be even more off. By using a correction information service combined with a high-precision GNSS receiver, you can measure your current position with an accuracy of a few centimeters (a few in). Put simply, while a normal smartphone GPS indicates a "rough position," using a correction information service lets you determine a "survey-level accurate position."

Q: Do you need a communication connection to use CLAS? A: No, it is not required. CLAS receives correction information directly via radio signals from the Quasi-Zenith Satellite Michibiki, so it can be used without a cellular phone or internet connection. In extreme cases, even in remote mountains or out at sea beyond communication coverage, as long as the sky is visible you can receive CLAS signals and achieve centimeter-level positioning (half-inch accuracy). However, because a CLAS-compatible receiver needs to pick up the satellite’s radio signals, it cannot be used in places where the satellite is not visible, such as indoors or inside tunnels.

Q: Can a smartphone by itself receive CLAS signals? A: At present, a typical smartphone alone cannot directly receive CLAS correction signals. The GNSS chip built into smartphones focuses on receiving basic satellite signals and does not support Michibiki’s CLAS signal (L6 band). To use CLAS, you need to connect a compatible high-precision GNSS receiver to the smartphone. Recently, compact receivers that integrate with smartphones have become commercially available, and if attached to a smartphone you can make use of CLAS corrections. In the future, it is possible that smartphones will include positioning chips that support CLAS.

Q: How accurate can you get using correction information? A: Under good conditions, you can expect accuracy of about 2～3 cm (0.8～1.2 in) horizontally and on the order of several centimeters to 10 cm (a few inches to 3.9 in) vertically. If you use a network RTK service, this level of accuracy can be obtained almost immediately. When using CLAS the final level of accuracy is equivalent, but during the first several tens of seconds the error tends to be somewhat larger (within several tens of centimeters; within several to a few dozen inches) before converging. In any case, compared with conventional standalone positioning (errors of 5 m or more (16.4 ft or more)), positioning is orders of magnitude more precise.

Q: Can it really be used anywhere across Japan? A: Yes, basically it can be used almost anywhere across Japan, as long as you are outdoors in a location where you can receive signals from satellites. CLAS is designed to cover the entire country and can obtain correction information from the MICHIBIKI satellites overhead even on remote islands and in mountainous areas. Network-based RTK services are also deployed nationwide in areas with cellular coverage. However, both methods cannot be used in environments where satellites are not visible, such as underground or indoors. Also, CLAS is not offered overseas, so please consider it a Japan-only service.

Q: How much does it cost to use? A: The cost of using the correction information itself varies by method. With CLAS, receiving signals from satellites is available free of charge. If any cost is involved, it would be the purchase price of a compatible receiver; compact high-precision GNSS devices can be obtained from around ¥100,000. On the other hand, if you use a network-based RTK service you need to contract with a service provider, and it's common to incur a usage fee of around tens of thousands of yen per month. Depending on your application and budget, you should consider either utilizing CLAS, which can be used for free, or contracting a paid service that offers stability and support.

Q: Can it be used without specialized knowledge? A: Recent smartphone-connected GNSS systems are designed to be easy to use even without specialized knowledge. Dedicated apps provide an intuitive interface, allowing start/stop positioning and connection to correction services with a single tap. For example, with systems like LRTK, you simply attach the device to your smartphone and press a button in the app to automatically receive correction data and begin high-precision positioning. No complex setup or GNSS expertise is required. As long as you learn the basic operation, field personnel can start using it immediately.

Q: In what situations can this technology be used? A: The range of applications is extremely broad. Representative examples include general surveying work (measuring boundary points, as-built control, etc.), construction project management (precise guidance of heavy machinery and progress/volume control), infrastructure inspection (managing deteriorated areas using highly accurate positional data), and agriculture (position control of autonomous tractors). For local governments, it helps obtain accurate positioning data for the maintenance of public facilities such as roads and bridges, and in disaster response it proves powerful for mapping damage. In short, a smartphone-integrated high-precision positioning technology can potentially be used in any situation where positional accuracy is required. Measurement tasks that previously required manpower and time can be made more efficient, enabling new uses of the data.