Basic Concept of Network RTK

RTK (Real-Time Kinematic) is a positioning method that uses two GNSS receivers, a base station (reference station) and a rover, to simultaneously receive satellite signals and correct the difference in their position information in real-time, achieving high-precision positioning. One of the major advantages of RTK is that the errors, which would typically be several meters in standalone positioning, are reduced to generally just a few centimeters. This makes RTK ideal for applications that require precise positioning, such as drone autonomous flight, machine guidance for construction machinery, and civil engineering surveys, where GPS alone was previously not sufficient.

However, traditional RTK positioning required the installation of a base station with known coordinates, and the observation data from that station needed to be transmitted to the rover via radio. Setting up base stations on-site was time-consuming and costly. To address this issue, network RTK was introduced. This method uses correction data generated from a network of base stations established by governments or companies and is delivered via the internet, allowing RTK positioning with just a single rover.

With network RTK, users no longer need to provide their own base station, which simplifies the preparation on-site while maintaining positioning accuracy even for long-distance measurements.

*Network RTK benefits from having multiple base stations, which improves the correction accuracy for long-distance measurements (such as ionospheric delays), and reduces the loss of accuracy even when the baseline length is long.

​

In Japan, the following services and systems are mainly used to obtain network RTK correction information:

| Correction Service Provider | Features and Description |

Electronic Reference Points (GEONET)
A GNSS reference station network established by the Geospatial Information Authority of Japan (GSI) with approximately 1,300 stations across Japan. It provides various correction data based on real-time observation data (e.g., VRS method) and is also available for public surveying.

Ntrip Correction Services
A service provided by private sector companies (including the GSI) that distributes RTK correction information via the internet. Examples include SoftBank's "ichimill" (which deploys over 3,300 proprietary reference points nationwide) and GNSS correction services from companies like Genova. Available for use nationwide via subscription.

CLAS (Centimeter-Level Augmentation Service by Michibiki)
A correction information service delivered by Japan’s Quasi-Zenith Satellite System (QZSS). This service transmits correction data generated from GSI’s electronic reference point data via L6 signals and can be received by compatible receivers.

​

It is available even in areas without mobile network coverage, and as long as you have a receiver, you can obtain centimeter-level accuracy without additional costs.
Below, we will explain the details of each service.

​

Electronic Reference Points (GEONET)
Electronic reference points are GNSS reference stations established by the Geospatial Information Authority of Japan (GSI) at approximately 1,300 locations across the country. Each electronic reference point continuously observes GNSS data, which is then aggregated in GSI’s servers. In network RTK, the data from this electronic reference point network (GEONET) forms the foundation for high-precision positioning. By utilizing real-time data from the electronic reference points, RTK positioning accuracy can be maintained over long distances, making surveying possible even in areas where it is not feasible to set up a base station.
The GSI itself also conducts correction information provision experiments using electronic reference point data, and network-based RTK-GNSS surveying methods for public surveying have been developed.

​

Ntrip Correction Services (Private Sector and Geospatial Information Authority of Japan)
The Ntrip (Networked Transport of RTCM via Internet Protocol) system is a widely adopted method for delivering correction data over the internet. In Ntrip-compatible correction services, users connect their rover's receiver to the internet via mobile networks or Wi-Fi, accessing a correction data distribution server (Ntrip caster) they have pre-subscribed to, in order to receive correction information. The Geospatial Information Authority of Japan (GSI) also conducts real-time correction information experiments using the electronic reference point network, but the services widely used are those provided by private sector companies.

A representative private service is SoftBank's "ichimill". Ichimill installs its own GNSS reference points at SoftBank's communication base stations and other locations, expanding them to over 3,300 stations nationwide. As a result, users can immediately begin high-precision positioning without the need to set up their own base station.

While general GNSS positioning typically has an error of around 5 to 10 meters, Ichimill reduces this error to just a few centimeters by distributing correction information via the network and performing RTK calculations in real-time. By utilizing such a service, users can achieve high-precision positioning almost anywhere in Japan as long as there is an internet connection. Other services such as Genova's VRS system, Topcon's Network RTK service, and Nikon's Trimble VRS Now are also available from various surveying equipment manufacturers, allowing users to choose a service based on their needs and region. Since the pricing (monthly or yearly) and compatible equipment vary by service, it is recommended to evaluate plans that best suit your company's requirements.

​

CLAS (Centimeter Level Augmentation Service by Michibiki)
CLAS (Centimeter Level Augmentation Service, also referred to as "SIRUS") is a centimeter-level positioning augmentation service provided by Japan’s Quasi-Zenith Satellite System (QZSS), known as "Michibiki."

Using the Geospatial Information Authority of Japan (GSI) electronic reference point data, CLAS calculates errors and broadcasts the correction information via L6-band radio waves from the quasi-zenith satellite. By receiving this signal with compatible GNSS receivers, RTK-like corrections to single positioning errors can be made in real-time, enabling high-precision positioning.

One of the main advantages of CLAS is that it does not require an internet connection. Even in areas where mobile communications are unavailable, such as mountainous regions or offshore locations, as long as the sky is clear and the Michibiki signal is receivable, positioning accuracy within a few centimeters can be achieved.

Moreover, since correction information is directly received from satellites, it is also efficient when multiple moving stations are used simultaneously over a wide area. Currently, CLAS signals are available for use in most of Japan (except for some regions, such as the northern areas, which are still under verification).

A point to note is that CLAS requires a CLAS-compatible GNSS receiver. Standard single-positioning receivers cannot decode L6 signals, so investment in compatible equipment is necessary. Additionally, CLAS is an augmentation service, meaning that the positioning itself still relies on standard GNSS positioning. Therefore, a clear sky with sufficient satellite visibility is essential for proper usage.

Some time lag has been reported in its application for moving objects, and it may face challenges in high-speed, real-time applications such as autonomous driving. Nonetheless, CLAS, which provides stable, high-precision results without relying on communication infrastructure, is expected to be increasingly utilized in various fields such as surveying, construction, and agriculture in the future.

​

How to Use Network RTK for High-Precision Positioning
So, how should you proceed to perform high-precision positioning using Network RTK (correction services)? The basic steps are as follows:

1. Prepare an RTK-Compatible GNSS Receiver
   First, prepare an RTK-compatible GNSS receiver. Recently, GNSS equipment manufacturers have introduced RTK-compatible devices, including small modules that can be paired with smartphones and tablets (such as the LRTK system mentioned below). The key is to select devices that support multi-GNSS and multi-frequency, as well as those equipped with Ntrip client functions and CLAS receiving capabilities. If you only have a GPS receiver, it may not support RTK, so it is important to check the specifications. Also, prepare a controller (e.g., a field computer or tablet) for connecting to the receiver, and install the required positioning software if necessary.

2. Subscribe to a Correction Service and Configure Settings
   Subscribe to the correction information service you wish to use and obtain the necessary configuration information. In the case of Ntrip, the service provider will provide the URL (IP address), port number, mount point name, and user ID/password for connection. These settings need to be entered into the receiver or positioning software’s Ntrip client configuration screen. If you are using the Geospatial Information Authority of Japan’s electronic reference points directly, use the connection information provided by them. For CLAS, no special contract is required, but you may need to enable CLAS reception mode on the receiver. Additionally, ensure the means of internet connection for the moving station (e.g., a SIM-equipped communication module or a tethering-enabled smartphone) to maintain an online status at the site.

3. Connect to the Ntrip Caster
   Power on the GNSS receiver and begin positioning, establishing a connection to the correction service. Specifically, activate the "Network RTK" mode from the receiver or connected device’s menu, and use the correction service information entered earlier to initiate Ntrip connection. Once successfully connected, correction data (such as RTCM messages) from the base station will be transmitted to the receiver in real-time. In the case of CLAS, the receiver will automatically receive the Michibiki L6 signal and incorporate the correction information. After connection, the receiver's solution will change from "Float" to "Fix" within a few seconds to a few minutes. This indicates that the integer ambiguity resolution is complete, and high-precision positioning is possible.

4. Configure Positioning Software and Start Measurements
   In the positioning software, set up the coordinate system and positioning mode. For surveying in Japan, you will likely select a plane rectangular coordinate system based on the world geodetic system (such as JGD2011). Confirm that the positioning results based on the correction data match the known public coordinate system and make any necessary adjustments for origin offsets or geoid model application. Once setup is complete, begin the actual measurements. The positioning software screen will display the current status (whether it is a FIX or FLOAT solution) and accuracy indicators, so confirm that you have a FIX solution before recording the points or conducting machine guidance tasks. Once a FIX solution is obtained, high-precision coordinates within a few centimeters will be immediately available.

5. Check Data and Post-Processing
   After the survey is completed, check the obtained coordinate data. If necessary, verify the error with the known points, and if there are any discrepancies, review the correction data and positioning mode settings and re-measure. Positioning data can be immediately uploaded to the cloud in real-time or checked against the design drawings on-site. Recent software also includes post-processing (PPK) features and accuracy improvement features through averaging calculations. Sharing RTK positioning data acquired on-site via the cloud allows immediate verification from remote offices and contributes to the digital transformation of operations.

​

Precautions for Using Network RTK

To operate Network RTK reliably, there are several important points to keep in mind. Below are some key considerations:

​

Be Aware of Mobile Network Influence
Network RTK relies on receiving correction data via the internet, so the quality of the communication network directly affects the positioning accuracy and stability. If the communication on the moving station side is unstable, the correction data may be interrupted, causing the solution to revert to a float solution or, in the worst case, interrupting the RTK positioning. Therefore, it is important to use a communication network with good signal quality at the surveying site. Typically, in urban areas, 4G/LTE or 5G networks should suffice, but in mountainous areas or underground spaces, pocket Wi-Fi or smartphone tethering may lose signal. In such cases, consider using CLAS as mentioned later or temporarily switching to the base station method (local RTK using your own mobile communication devices) as a workaround. Additionally, large communication delays can affect the timing of the correction data application, so avoid unnecessary delays, such as through VPNs, which may impact accuracy.

Moreover, recently, solutions have emerged that allow positioning even in areas without mobile network coverage. For example, with a receiver compatible with CLAS from Michibiki like LRTK, high-precision positioning is possible in mountainous areas or disaster sites without internet connectivity, as the receiver can still receive satellite augmentation signals. Having a system that does not rely on communication infrastructure as a backup provides added peace of mind in critical situations.

​

Key Points for Obtaining a FIX Solution
In RTK positioning, a FIX solution is required to ensure centimeter-level accuracy. A FIX solution refers to the state when the integer ambiguity resolution in satellite signal phase observation is completed. On the other hand, an unresolved state is called a FLOAT solution, where the accuracy typically remains within tens of centimeters. When working with RTK, it is important to always check whether the current solution is FIX or FLOAT and ensure that measurements are recorded only once the FIX solution has been achieved.

To quickly and reliably obtain a FIX solution, ensuring a proper satellite reception environment is critical. If there are tall buildings or trees nearby, the satellite visibility may be obstructed, or multipath (reflected waves) may occur, leading to unstable positioning. It is advisable to place the antenna in as open an area as possible and ensure that it is sufficiently separated from surrounding obstacles. Additionally, setting an appropriate elevation mask on the receiver to eliminate radio signals from low elevation angles (which tend to have greater errors) is also effective. Using a multi-GNSS compatible receiver will enhance stability by increasing the number of satellites, as it can use not only GPS but also GLONASS, QZSS (Michibiki), Galileo, and other systems.

Furthermore, be familiar with the initialization and re-fix procedures. If it takes time for the RTK positioning to reach a FIX solution, resetting (reinitializing) the receiver may speed up the process. If the solution temporarily returns to FLOAT after a long observation, remain calm, wait for a few seconds, or reconnect the system to restore the FIX solution. It is important to avoid using FLOAT data easily to prevent "misfixes."

Additionally, the distance from the fixed station (baseline length) tends to affect the time it takes to reach a FIX solution. However, in the case of network RTK, this issue is partially mitigated by the virtual reference station method. Nevertheless, if the distance exceeds several tens of kilometers, it can still be disadvantageous, so it is important to select the nearest correction data according to the area of use.

​

LRTK Utilization and Implementation Benefits
For those who want to easily introduce network RTK on site, the LRTK solution is a great help. LRTK, developed by Lefixea, a startup company from the Tokyo Institute of Technology, is a compact, integrated RTK-GNSS receiver device and service.

LRTK is a pocket-sized receiver that can be attached to smartphones (primarily iPhone/iPad). It integrates the antenna, receiver, battery, and wireless communication into one device. The device is easily attached via a specialized magnetic cover, weighing only about 125g, making it portable and convenient to carry. By simply attaching this device to your smartphone, the smartphone transforms into an RTK surveying tool.

​

One of the key benefits of utilizing LRTK is its ease of use and versatility. By using the dedicated LRTK app to enter the network RTK correction service information and connect, a single operator can instantly begin centimeter-level positioning. The operation is simple: "Network RTK on → Input correction data → Start connection," and it is designed to be user-friendly, even without specialized knowledge.

The high-precision positioning data obtained can also be tagged on photos or point cloud scan data in the smartphone. For example, the photographs taken at the site can be accurately overlaid using public coordinates, enabling efficient and precise data use.

Tasks that traditionally required expensive surveying equipment or skilled surveyors can now be completed with one smartphone per worker, significantly improving on-site productivity. Furthermore, LRTK is integrated with cloud services, allowing data to be uploaded and shared instantly on-site. For example, in the aftermath of the Noto Peninsula earthquake in January 2024, workers used smartphones with LRTK to measure ground subsidence and cracks in roads caused by liquefaction. The photo data was uploaded to the LRTK cloud immediately and shared with stakeholders, contributing to a quick understanding of the situation and response.

In this way, real-time sharing and digital record-keeping (DX) at the site level are made possible, which is a significant strength.

​

Here is the English translation of the text:

The scene shows a smartphone equipped with LRTK being used for photo-based surveying on-site. The screen displays the RTK status as "Fix," indicating that high-precision positioning data is being obtained. The acquired coordinate values and photo data can be shared instantly via the cloud.

Another notable feature of LRTK is its capability to function outside of network coverage. LRTK is a GNSS receiver that supports three frequencies and is compatible with receiving CLAS signals from the Michibiki satellite. This allows for continued high-precision positioning even in areas without mobile network coverage, such as remote mountain areas or the sea, by receiving correction data directly from the Michibiki satellites.

In real-life situations, even when communication infrastructure was disrupted due to heavy rain disasters, LRTK's off-network capability allowed for the recording of disaster-related data and high-precision positioning, enabling the rapid sharing of on-site information. This ability to overcome the "dependency on communication" – a common weakness of network RTK – makes it a highly useful feature.

From a cost perspective, LRTK is priced much more reasonably compared to traditional surveying equipment. By reducing initial setup costs, it becomes easier for small to medium-sized civil contractors and surveying professionals to introduce it on an individual basis, making it an attractive tool for on-site digital transformation (DX). Since it utilizes existing iPhones and iPads, there is no need to purchase new specialized devices. As a result, LRTK is becoming a "universal surveying tool" for the field, and as of 2024, it is quietly gaining popularity.