RTK-Related Glossary (Categorized)
First, we will organize and explain the main technical terms related to RTK positioning, categorized by basic concepts, positioning accuracy, error correction technologies, positioning environments, and equipment/software.

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Terms Related to Basic Concepts

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• GNSS (Global Navigation Satellite System): A general term for satellite-based positioning systems, including GPS, GLONASS from Russia, Galileo from Europe, and QZSS (Quasi-Zenith Satellite System) from Japan. RTK positioning uses signals from multiple GNSS satellites simultaneously to enhance accuracy and reduce the risk of signal loss.

• RTK (Real-Time Kinematic): A type of relative positioning method using GNSS, where data observed simultaneously by a base station (reference station) and a rover is used to cancel out errors and calculate high-precision positions in real time. By subtracting common error factors (such as satellite orbit errors, clock errors, and ionospheric/tropospheric delays) between the observation data, RTK achieves significantly higher accuracy than standalone positioning. RTK positioning typically allows real-time centimeter-level positioning by receiving correction data sent from the base station, which is then used for position calculation on the rover.

• Base Station (Reference Station): In RTK, a fixed station that is installed at a known coordinate point. The base station sends the raw observation data and correction information to the rover via radio or the internet. The rover receives this data and compares it with its own observation data to calculate high-precision positioning. Base stations can be installed on-site, or public base station data, such as the Geospatial Information Authority of Japan's electronic reference points (a network of around 1,300 GNSS base stations nationwide), can be used. These electronic reference points provide real-time observation data via the internet and are widely used for RTK corrections.

• Rover (Mobile Station): The receiver on the mobile side in RTK. It is mounted on the target object (surveying equipment, vehicles, drones, etc.) and continuously performs GNSS observations while moving. Without corrections, the rover typically provides standard GPS positioning (with an accuracy of a few meters), but by receiving real-time correction data from the base station, its positioning accuracy can be improved to centimeter-level precision.

• Relative Positioning and Standalone Positioning: RTK is classified as relative positioning. Relative positioning uses the difference between the data observed by the base station and the rover to calculate the position. On the other hand, standalone positioning (standalone GNSS positioning) calculates the position using only signals from satellites with a single receiver. Standalone positioning typically results in errors of a few meters, but relative positioning (RTK) corrects errors based on the relative relationship with the base station, allowing for higher precision. RTK is also called "dynamic interferometric positioning" because it can immediately calculate high-precision positions for a moving rover.

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Terms Related to Positioning Accuracy

• Standalone Positioning: This refers to the usual GNSS positioning solution that does not use any correction. The position calculated with a single GPS receiver typically has an error of a few meters, and this is called the standalone positioning solution. For example, GPS on smartphones and general car navigation systems use standalone positioning, and the position can shift by around 5 meters due to factors like building interference. This can be considered the baseline accuracy when RTK is not available.

• Float Solution: In RTK positioning, this refers to a provisional solution where the integer ambiguity (as explained later) has not been resolved yet. In a float solution, the integer part of the carrier wave's cycle number from the satellite remains undetermined, so the accuracy is typically limited to less than a meter, usually within a few tens of centimeters. After starting RTK, depending on the satellite configuration and reception conditions, the solution may remain in the float state, in which case the position accuracy is not sufficiently high. Most RTK receivers display the float solution state on the monitor, and the user must wait until the solution becomes a fixed solution.

• Fix Solution: This is the final solution in RTK positioning where integer ambiguity has been resolved. When the integer carrier wave cycle number (which was previously uncertain) is correctly determined, RTK provides extremely high-precision solutions at the centimeter level. This state is called the fix solution, which is the target state for RTK positioning. Once the fix solution is obtained, the position can be determined with an error of approximately 2-3 cm in horizontal positioning and several centimeters to a few tens of centimeters in vertical positioning. The time it takes to reach the fix solution depends on the environment. With a high number of satellites and good signal conditions, it can take from a few seconds to tens of seconds; under poor conditions, it may take more than a few minutes.

Note: Integer Ambiguity

Integer ambiguity refers to the "uncertainty in the number of carrier wave cycles (integer cycles) included in the distance between the satellite and the receiver" during distance measurement using GNSS carrier waves. In RTK, this uncertain integer value must be correctly resolved, and the provisional value obtained during the calculation process is called the float solution, while the resolved value is called the fix solution.

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(Reference) Here is a summary of the main positioning solution modes and their typical accuracy:

Positioning Method | Description | Typical Accuracy

Standalone Positioning: Positioning method using only one receiver without a base station, with an accuracy of about a few meters.

Float Solution: A provisional solution in RTK where integer ambiguity is unresolved, with an accuracy of a few tens of centimeters to less than 1 meter.

Fix Solution: The final solution in RTK where integer ambiguity is resolved, with an accuracy of about a few centimeters.

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Terms Related to Error Correction Technology

• DGPS (Differential GPS): DGPS is a method for correcting GNSS positioning errors using a base station and a mobile station, similar to RTK, but the correction uses code (pseudo-range) measurements. The basic structure is similar to RTK, where a fixed base station calculates the error between its observed GPS positioning and its known accurate location, then sends this correction data to the mobile station. The mobile station applies this data to correct its positioning, resulting in higher accuracy than standalone positioning. In DGPS, certain errors such as satellite signal delay, orbital errors, and ionospheric/tropospheric delays are canceled out, improving accuracy to within several meters to one meter. However, since DGPS does not use carrier phase measurements like RTK, it cannot achieve the same centimeter-level precision.

• Network RTK: This RTK method uses data from multiple base stations, integrated via a network, to generate correction data that can be applied over a wide area. In traditional RTK, correction data is obtained from a single base station, and the positioning accuracy decreases as the distance between the base station and the mobile station (baseline) increases. Network RTK overcomes this by generating virtual reference stations (VRS) or providing wide-area correction data based on real-time observation data from surrounding base stations (e.g., electronic reference point networks). This allows for stable centimeter-level positioning over long distances. Common methods include VRS (Virtual Reference Station), FKP, and MAC, with network RTK services utilizing electronic reference points provided in Japan. Users receive correction data from virtually located base stations near their mobile station, ensuring accuracy similar to short-distance RTK, even if the base station is tens of kilometers away.

• NTRIP (Networked Transport of RTCM via Internet Protocol): NTRIP is a communication protocol for transmitting and receiving RTK correction data (DGPS/RTK data) via the internet. The mobile station's equipment connects to an NTRIP caster, which transmits correction data via mobile or wireless networks in real-time. Previously, dedicated radio frequencies (such as UHF) were commonly used for communication between base stations and mobile stations. However, with the widespread adoption of NTRIP, high-precision positioning using local base station networks is now easily accessible as long as there is an internet connection. In Japan, there is a growing network of NTRIP services using electronic reference points, including paid services by private companies and free services by local governments. It is now common for RTK receivers to use SIM cards to access this service.

• CLAS (Centimeter-Level Augmentation Service): CLAS is a satellite-based error correction service provided by Japan's Quasi-Zenith Satellite System (QZSS). CLAS-enabled receivers can receive correction signals directly from satellites, allowing for RTK-like centimeter-level positioning even without an internet connection. CLAS broadcasts wide-area correction information (known as SSR—State Space Representation) via satellites, providing uniform high-precision correction across Japan for any GNSS receiver. Unlike RTK, it does not require direct communication with a base station, but its error correction method is similar to network-based RTK, as the correction values calculated by base station networks (such as electronic reference points) are broadcast via satellites. CLAS has gained attention for its ability to function in areas where mobile networks are unavailable, such as mountainous regions. Recently, RTK devices supporting CLAS reception have also become available.

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Terms Related to Positioning Environment

• Baseline Length: This refers to the distance between the base station and the rover. Generally, the accuracy and initial solution time of RTK positioning depend on the length of the baseline. A shorter baseline means that the error factors at both stations are nearly identical, allowing for a more accurate and quicker fix solution. However, as the distance increases, local differences in atmospheric errors and other factors can prolong the float solution period or reduce accuracy. In practical applications, a baseline of around 20 km is considered a guideline for single base station RTK. However, with network RTK (as mentioned earlier), this limitation can be effectively mitigated.

• Multipath: This phenomenon occurs when satellite signals are reflected by surrounding structures, such as buildings or the ground, and reach the receiver. The interference between the direct wave and reflected waves causes errors in distance measurement and can degrade RTK positioning accuracy or make the solution unstable. In urban areas with many buildings, under bridges, or in forests, multipath effects can be significant, making high-precision positioning more difficult. Solutions to mitigate multipath include using antennas that reduce multipath (such as setting up ground planes or using choke ring antennas), removing bad satellites in the positioning algorithm, choosing times with favorable satellite configurations, and increasing the number of satellites by using Multi-GNSS.

• Satellite Geometry (Geometric Distribution): This refers to the arrangement (geometry) of satellites overhead. Good satellite geometry means that the satellites are evenly distributed in space, resulting in high geometric accuracy for positioning. When satellites are clustered in only one direction (e.g., aligned in a straight line or concentrated directly above), the PDOP (Position Dilution of Precision) value deteriorates, which reduces positioning accuracy. RTK ensures a good satellite configuration by increasing the number of available satellites through Multi-GNSS, leading to high-precision and stable positioning. In environments like Japan, where there are many obstructions, using GPS along with GLONASS, Galileo, and QZSS improves the maintenance of fix solutions.

• Obstruction: This refers to situations where satellite signals are blocked, such as in the shadow of buildings, inside tunnels, or under trees. RTK positioning requires receiving signals from a sufficient number of satellites, so in environments with many obstructions, the positioning may be interrupted or revert to a float solution. Modern receivers have improved signal reacquisition performance, allowing faster recovery from temporary obstructions, but it is ideal to perform positioning in open areas with a clear line of sight to the sky. If obstructions caused by terrain or structures cannot be avoided, solutions like Multi-GNSS or network RTK can be used to mitigate these issues.

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Terms Related to Equipment and Software

• RTK-GNSS Receiver: A specialized receiver used for RTK positioning. It is equipped with an antenna and receiving circuitry that supports high-precision GNSS positioning and performs real-time transmission and reception of correction data and computation of the solution. Traditionally, surveying-specific equipment (expensive GNSS receivers and controllers from specialized surveying manufacturers) was the norm, but recently, low-cost receiver modules and compact receivers that can be used with smartphones and tablets have been introduced. The basic structure of receivers for both base stations and mobile stations is similar, but base station receivers are designed for stationary use, featuring enclosures that can withstand harsh weather conditions and interfaces for connecting external communication devices. Mobile station receivers are designed for portability and installation, with an emphasis on lightweight, compact design and improved mobility through integrated batteries.

• GNSS Antenna: An antenna that receives GNSS signals. The antenna is a crucial hardware component that directly impacts positioning accuracy, as its reception sensitivity, noise characteristics, and multipath rejection performance affect accuracy. RTK antennas are typically designed to efficiently receive circularly polarized satellite signals and are stable at the phase center. For example, surveying antennas use a metal plate called a ground plane to block unwanted reflected signals, and choke ring designs reduce multipath from low elevations. Recently, antennas have been miniaturized and integrated for small devices, and high-performance Multi-GNSS antennas are now also integrated into smartphone-mounted devices.

• Positioning Software and Apps: Software used to process the observation data obtained from GNSS receivers and calculate, record, and display coordinates. In the surveying field, these are provided as apps on dedicated controllers or PC software, enabling RTK solution monitoring, recording measurement points, and outputting to drawing applications. A well-known open-source RTK library is RTKLIB (developed by Kohsho Takeuchi of Tokyo University of the Marine Science and Technology), which is widely used as an RTK computation engine. Additionally, recent developments have made advanced positioning possible with smartphone apps, which now feature NTRIP connections to reference station services, point management, and cloud integration. The availability of easy-to-use apps on-site has made RTK positioning, which was previously limited to specialized technicians, more accessible.

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Latest Trends in RTK Technology

RTK technology continues to evolve rapidly. Below, we introduce some of the latest trends.

• Expansion of Multi-GNSS and Multi-Frequency Utilization: As mentioned earlier, using multiple satellite systems simultaneously, rather than relying on GPS alone, has become standard with Multi-GNSS. Additionally, receivers that can use multiple frequency signals, such as L5 in addition to L1, are becoming more common. This allows for an increase in the number of visible satellites and more precise ionospheric error correction, contributing to a reduction in initialization time and improved stability in maintaining high-precision solutions. In fact, even the latest low-cost receivers, with dual-frequency support, have achieved stable centimeter-level positioning, even with longer baselines. Going forward, the full use of signals from all GNSS satellites will enable stable RTK positioning in urban and mountainous areas where signal interruptions were previously common.

• Smart RTK and Mobile Utilization: Miniaturization and mobility of high-precision positioning are also major trends. Traditionally, RTK receivers were stationary or mounted on tripods, but now devices that fit in your pocket or are integrated into smartphones are emerging. For example, with the advent of RTK devices that attach to smartphones, the era of "one device per person" has become a reality. Now, all field workers can easily carry centimeter-accuracy positioning devices and immediately perform surveys and location checks whenever necessary. Additionally, the use of smart RTK systems is becoming widespread, where high-precision data is shared in real time on the cloud, syncing information between the office and the field. The focus on high-precision positioning as a core part of DX (Digital Transformation) is driving efforts to improve construction management and infrastructure maintenance efficiency.

• Utilization of the Quasi-Zenith Satellite System (QZSS): Japan's unique Quasi-Zenith Satellite System (QZSS), also known as "Michibiki," is gaining importance in the RTK field. QZSS offers services such as CLAS (Centimeter-Level Augmentation Service) and SLAS (Sub-meter Level Augmentation Service), with CLAS being particularly groundbreaking for providing RTK-level precision across Japan. Receivers and services that support this are emerging in the 2020s, and due to the advantage of receiving satellite corrections in areas without mobile communication, such as forests and mountainous regions, it has garnered attention in the fields of surveying, agriculture, and positioning IoT. Moving forward, hybrid operations combining RTK with CLAS (PPP-RTK) and further improvements in accuracy through combinations of multiple base stations and satellite augmentation are expected.

• Cost Reduction and Widespread Adoption: Reducing the cost of RTK equipment is another key trend. High-precision GNSS receivers that once cost several million yen are now available for tens of thousands of yen, and in some cases, modules costing as little as several thousand yen are capable of centimeter-level accuracy. With the advent of affordable devices and subscription-based models, even small-scale civil engineering contractors and farmers can now more easily adopt RTK. Additionally, free base station data provision (e.g., RTK base station services for agriculture provided by local governments) is being introduced in various regions, further reducing usage costs. These developments are expanding high-precision positioning to non-experts and are expected to accelerate the DX of construction sites.

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LRTK Features and Use Cases

Building on the trends mentioned above, one of the most notable products is LRTK. LRTK is a smartphone-compatible RTK-GNSS receiver device developed by Lefixea Inc. (a startup from the Tokyo Institute of Technology) that brings innovation to on-site positioning tasks.

The key feature of LRTK is its pocket-sized RTK receiver that can be easily attached to smartphones and tablets. Weighing only around 125g and with a thickness of 13mm, it integrates the battery and antenna into a compact device, enabling centimeter-accurate surveying simply by attaching it to a smartphone.

Once the dedicated app "LRTK" is launched, positioning starts immediately, making RTK positioning easily accessible without complex setup. This transforms high-precision positioning, previously reserved for professional surveyors, into a tool that can be used by construction managers, inspection personnel, and anyone on-site. In fact, the LRTK Phone, as this device is called, is very reasonably priced compared to traditional equipment, making it feasible for each worker to have one, significantly improving on-site productivity.

LRTK is also designed as an all-in-one positioning solution. With it, users can confirm their position in real-time on their smartphone screen, record measurement points, plot them on drawings, and even integrate with photo-taking features. For example, when inspecting structures, a smartphone camera can take photos while simultaneously recording high-precision coordinates (latitude, longitude) and orientation, all automatically tagged. The photos can then be uploaded to the cloud and plotted on a map, enabling accurate sharing of the exact location and orientation of the photo taken.

This technology is especially useful for inspecting concrete cracks and disaster sites, as LRTK enables high-precision location information to be attached to smartphone photos and stored in the cloud, improving the reliability of records and boosting work efficiency.

Additionally, LRTK offers extended functionality, such as point cloud scanning and use in construction progress management (e.g., volume calculations for embankments), making it a versatile surveying tool for various use cases.

Moreover, LRTK also provides models that support Japan’s unique satellite augmentation signal, CLAS (Centimeter-Level Augmentation Service). The latest model, the LRTK Phone 4C (Out-of-Range Model), has CLAS reception capabilities, allowing high-precision positioning even in remote areas like mountainous regions or construction sites without cellular coverage.

This means that even in areas where internet-based corrections from base stations are unavailable, users can receive correction data directly from the Quasi-Zenith Satellite System (QZSS) and achieve centimeter-level positioning. This feature is particularly useful for surveying bridges in mountainous areas during infrastructure inspections or for disaster response where communication infrastructure may be unstable. Additionally, the Bluetooth connection allows wireless integration with smartphones, eliminating the need for cumbersome cables.

For stationary observations, the device can be mounted on a dedicated monopod or tripod, offering a combination of convenience and practicality in design.

In this way, LRTK has garnered attention as an RTK solution that balances "ease of use" and "high accuracy." As for real-world use cases, it has been reported that LRTK has been used for tasks such as setting up reference points (marking) in civil construction sites, recording locations during infrastructure inspections, and even performing simple surveying by site managers without measuring equipment.

For example, at a construction site, the introduction of LRTK allowed on-site staff to quickly perform terrain surveys and confirm construction progress without relying on specialized surveying teams. This led to a reduction in project timelines and labor costs. Additionally, by immediately sharing positioning data to the cloud, supervisors in remote offices can review results and provide instructions in real-time, enhancing coordination between the field and the office.

As the adoption of smart RTK devices like LRTK continues to grow, it is expected that the digital transformation (DX) of the industry will advance, leading to a revolution in productivity.