Overview of Indoor and Underground Positioning Technologies

Representative technologies developed and utilized for positioning in environments where GNSS signals cannot reach include the following. Each of these technologies has different principles and characteristics, and by combining them, their respective weaknesses can be complemented.

• High-Precision Positioning with UWB (Ultra-Wideband Radio): UWB is a wireless technology that uses a wide frequency band in the GHz range, measuring the time of flight (ToF) of radio pulses in the nanosecond order to accurately calculate distance. As a result, UWB allows real-time positioning with an accuracy of less than a few tens of centimeters, even in indoor environments. By installing multiple fixed stations (anchors) and performing bidirectional communication with a moving tag, position is determined through triangulation. Due to its low communication power and minimal interference with other devices, UWB is gaining attention for a wide range of applications, such as asset management in factories and warehouses and tracking the positions of athletes. Companies like Apple and Samsung have started embedding UWB chips in smartphones, accelerating its adoption.

• Self-Positioning Estimation with IMU (Inertial Measurement Unit): An IMU is a device consisting of accelerometers and gyroscopes that integrates acceleration and angular velocity to calculate displacement, and then estimates the current position relative to the previous location. The advantage of IMU is its ability to autonomously compute position without relying on external infrastructure, allowing it to continuously maintain the self-position even when entering GNSS-denied environments. While it can maintain high accuracy over short periods, slight measurement errors accumulate, leading to drift over time, making prolonged use difficult. Therefore, periodic corrections using GNSS or other sensors (zero resets) are necessary. For example, a method may involve combining GNSS positioning before entering a tunnel with corrections from a Kalman filter.

• Integration with SLAM (Simultaneous Localization and Mapping) Technology: SLAM, which stands for Simultaneous Localization and Mapping, is a technique that creates a map of the surroundings while simultaneously estimating the position of the device. Using LiDAR (laser) or camera images, SLAM captures feature points in the environment and continuously computes its own position. This technology is being increasingly used in various fields, including robotic vacuums, autonomous vehicles, and AR (augmented reality) devices, making it a strong method for determining position in environments where GNSS cannot be used, both indoors and outdoors. SLAM does not rely on external satellite signals, but because position estimation is based on matching with a map, ensuring accuracy in an absolute coordinate system requires careful consideration if the initial position is unknown. Additionally, errors accumulate over time, but through a process known as loop closing, where the device returns to a previously visited location, map errors can be corrected and accuracy improved. In indoor positioning, SLAM is typically used in combination with other sensors like IMU or known reference points to enhance accuracy, rather than being used on its own.

• RTK Extension through GNSS Re-broadcasting (Repeaters): When GNSS receivers are needed in underground spaces, one method is to use a device called a GNSS repeater to relay satellite signals. An antenna placed at locations like tunnel entrances or building rooftops receives the satellite signals, amplifies them, and re-broadcasts them indoors via coaxial cables or leaky coaxial cables, creating an environment where satellite signals can be "received" even underground. Simple repeaters, however, result in the same positioning information throughout the entire indoor area. Therefore, advanced systems that generate pseudo-satellite signals tailored to specific locations are now being put into practical use. This enables smartphones and GNSS positioning devices to recognize their location within a tunnel via satellite positioning. In fact, this "tunnel GPS" technology is gaining attention as a means to track the location of workers and rescue teams in case of emergency situations in long tunnels. In a GNSS re-broadcast environment, if correction information from an RTK base station can also be received, centimeter-level RTK positioning can be achieved even in underground spaces.

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Use Cases in Underground Spaces and Tunnel Construction

The technologies mentioned above are not just limited to research and experimentation but are actively being utilized in real-world applications. Here, we introduce use cases in fields where there is a particularly high demand for positioning in underground and indoor environments.

• Positioning Management in Large-Scale Infrastructure Projects: In large-scale infrastructure projects such as dam construction or underground power plants, it is crucial to accurately track and record the positions of heavy machinery and workers for safety and project management. For outdoor areas, RTK-GNSS or network-based RTK is used to achieve centimeter-level positioning, while in tunnel interiors or excavation sites, dead reckoning using IMU (Inertial Measurement Unit) connects the positions. Spot corrections are made as needed using UWB tags or Wi-Fi beacons. This system allows for centralized tracking of personnel and machinery from above ground to underground, enabling real-time monitoring of excavation progress and material transportation.

• Surveying and Management in Subway and Road Tunnel Construction: In tunnel construction, advanced positioning technology is essential for accurate control of excavation positions and for measuring the shape of the excavation (as-built geometry). In underground environments where GPS signals cannot be used, total station surveying based on reference points obtained from the surface is commonly used. More recently, SLAM positioning using QR code markers attached to tunnel walls or LiDAR has been increasingly employed. For example, by equipping an excavator with an IMU and laser scanner, its position and the surrounding cross-sections can be measured in real-time using SLAM, enabling immediate detection of over-excavation or insufficient excavation. Furthermore, after tunnel completion, GNSS repeaters or Bluetooth beacons are installed within the tunnel to track the positions of workers and inspection robots during maintenance. These technologies are making it possible to achieve positioning and navigation in underground spaces with the same level of precision as on the surface.

• Utilizing Indoor Positioning for Logistics and Warehouse Management: In large factories and logistics warehouses, forklifts and carts move through narrow aisles, and a large number of items are stored. Recently, real-time location tracking systems (RTLS) using UWB and BLE beacons have been introduced, increasingly automating the visualization of asset and vehicle positions. By attaching UWB tags to forklifts or pallets, the precise location of items—within about 30 cm accuracy—can be instantly identified, allowing for more efficient inventory management and picking processes. Additionally, navigation systems integrated with indoor maps help reduce the time spent by workers searching for materials, contributing to the realization of smart logistics. In this way, indoor positioning technology has become an indispensable tool for digital transformation (DX) in production environments.

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Integration of LRTK with Next-Generation RTK Technology

Traditionally, it was necessary to use different positioning systems for indoor and outdoor environments. However, we are now entering an era where these systems are integrated to provide seamless high-precision positioning. Our LRTK solution also supports this integrated indoor and outdoor positioning approach. Here, we will explain how LRTK products collaborate with other technologies, how their data is managed, and how they are integrated with smart devices.

• Integration with Indoor Positioning Technologies: LRTK is a device that easily achieves high-precision RTK-GNSS positioning, but indoors, where GNSS signals are unavailable, it works effectively when combined with other positioning technologies. For example, by using the reference position obtained outdoors with the LRTK receiver as the starting point, it can switch to relative position estimation using an IMU (Inertial Measurement Unit) inside buildings to maintain continuity. Additionally, by pre-aligning the coordinate system of UWB anchors placed within a factory with the LRTK positioning coordinate system, it is possible to continue tracking the forklift’s position using the same coordinate reference after entering the building from outdoors. LRTK serves as the high-precision calibration for initial positioning through RTK, and then seamlessly hands over to complementary technologies to ensure uninterrupted high-precision positioning. Integration with Indoor Positioning Technologies LRTK is a device that easily achieves high-precision RTK-GNSS positioning, but indoors, where GNSS signals are unavailable, it works effectively when combined with other positioning technologies. For example, by using the reference position obtained outdoors with the LRTK receiver as the starting point, it can switch to relative position estimation using an IMU (Inertial Measurement Unit) inside buildings to maintain continuity. Additionally, by pre-aligning the coordinate system of UWB anchors placed within a factory with the LRTK positioning coordinate system, it is possible to continue tracking the forklift’s position using the same coordinate reference after entering the building from outdoors. LRTK serves as the high-precision calibration for initial positioning through RTK, and then seamlessly hands over to complementary technologies to ensure uninterrupted high-precision positioning.

• Position Data Management Using LRTK Cloud: When combining multiple positioning methods, a platform to integrate and manage the data obtained from each is crucial. LRTK Cloud not only consolidates GNSS positioning data transmitted from LRTK devices, but it also has the ability to aggregate and centrally manage data from future technologies, such as positioning results from UWB tags and estimated trajectory data from IMUs. By fusing various sensor information in the cloud, visualizing the movement paths of assets across both indoor and outdoor environments, as well as saving and analyzing positioning logs, becomes much easier. For example, in construction sites, the trajectories of heavy machinery outdoors and work vehicles inside tunnels can be displayed on a single map, aiding progress management. LRTK Cloud serves as the hub for this hybrid positioning data, supporting the digital transformation (DX) of job sites.

• Easy High-Precision Positioning Through Integration with Smart Devices: LRTK is designed to work seamlessly with smartphones and tablets, allowing for centimeter-level positioning without the need for specialized equipment. For example, by attaching an LRTK receiver to a commercially available iPhone, users can achieve positioning accuracy comparable to expensive surveying devices that were traditionally out of reach. Since existing iPhones and iPads can be utilized, there is no need to invest in new specialized devices, which lowers the barriers for field implementation. Additionally, the features of the smart device itself can be leveraged. For instance, by combining the device’s camera and AR technology, annotations can be added directly to the positioned location and photos can be sent to the cloud. Or, by integrating with indoor mapping apps, the current location obtained via LRTK can be displayed in real time on building floor plans. This significantly simplifies the utilization of location information. LRTK serves as a bridge between next-generation RTK technology and digital devices, offering a unique positioning solution that balances both precision and ease of use.

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Future Outlook for Indoor RTK Positioning Technology

In the future, the demand for real-time position tracking, both indoors and outdoors, will continue to grow. To meet this demand, high-precision positioning technologies, including RTK, are evolving further by integrating with technologies from other fields, leading to continuous advancements.

• Real-Time Positioning Through Integration with 5G: The next-generation mobile communication system, 5G, not only enables high-capacity, low-latency communication but is also expected to play a key role in positioning by utilizing the radio wave characteristics of base stations. For example, techniques are being researched that can determine position with sub-meter accuracy by measuring the time of arrival (ToA) of 5G signals or estimating the direction of arrival (AoA), even without GPS. Furthermore, the high-speed communication capabilities of 5G enable real-time data exchange between RTK correction information and multiple sensors with the cloud, allowing for the application of precision corrections to location data without delay. In the future, the 5G base station network will become part of the positioning infrastructure for both indoor and outdoor environments, enabling centimeter-level hybrid positioning in combination with RTK-GNSS.

• Applications in Autonomous Mobile Robots and Smart Cities High-precision positioning is crucial for autonomous mobile robots, such as self-driving cars, delivery robots, and security drones. These robots navigate outdoors using GPS or RTK positioning, and switch to SLAM or indoor positioning when moving through buildings or underground spaces. In the future, infrastructure will also collaborate with robots, with systems such as UWB anchors placed at street corners or 5G units along roadsides providing high-precision self-positioning for vehicles. In smart cities, numerous sensors will be deployed to track pedestrian and vehicle movements. By integrating this data with the positioning of individual devices, it will be possible to plot all moving objects on real-time 3D maps of the urban space. High-precision positioning directly contributes to urban safety management, optimal traffic control, and efficient logistics, making location information platforms an essential part of social infrastructure in the future.

• Evolution of Seamless Indoor and Outdoor Positioning Technologies: The ideal future scenario is for devices to automatically switch to the optimal positioning mode, whether indoors or outdoors, providing continuous, high-precision location information. Efforts toward achieving seamless positioning are already underway. For example, in the area around Tokyo Station, a demonstration experiment is being conducted where satellite positioning and beacons are combined to display the current location from above ground to underground. In this system, when entering an underground area, the smartphone switches to positioning estimated by BLE beacons or Wi-Fi, and when returning above ground, it instantly switches back to GNSS positioning. In the future, advanced integration algorithms that leverage the strengths of each positioning method and enable AI to automatically select and correct modes will likely emerge, allowing users to continuously receive optimal location information without being aware of the positioning process. High-precision RTK technologies, such as LRTK, are expected to play a key role in such seamless positioning networks, providing a consistent, high-precision positioning service for users both indoors and outdoors.