How can cm level accuracy (half-inch accuracy) be achieved in indoor positioning? A comprehensive guide to the latest technologies and implementation case studies

High-precision indoor positioning is seeing rapidly growing demand across many fields such as manufacturing, logistics, construction, and inspection. Outdoors, GNSS can provide a certain level of accuracy, but indoors satellite signals do not reach, so different methods are required. In particular, at sites where "cm-level (centimeter-level; half-inch accuracy)" precision is required, the choice of technology can determine success or failure.

This article provides a detailed explanation of the mechanisms and latest trends of the technologies for achieving cm level accuracy (half-inch accuracy) in indoor positioning, as well as real-world deployment case studies.

Table of Contents

• What is indoor positioning, and why is cm level accuracy (half-inch accuracy) necessary?

• Differences between outdoor GNSS and indoor positioning

• Overview of major indoor positioning technologies

• Technical points for achieving indoor positioning with cm level accuracy (half-inch accuracy)

• Representative implementation cases

• Challenges in seamlessly connecting indoor and outdoor positioning

• LRTK Phone: an iPhone-mounted device that achieves seamless indoor-outdoor positioning with cm level accuracy (half-inch accuracy)

• Future outlook: toward the widespread adoption of indoor positioning with cm level accuracy (half-inch accuracy)

• Steps for implementing indoor positioning with cm level accuracy (half-inch accuracy)

• Cost-effectiveness of indoor positioning with cm level accuracy (half-inch accuracy)

• Summary

What is indoor positioning, and why is cm level accuracy (half-inch accuracy) necessary?

Indoor positioning refers to the technologies used to determine the location of objects or people inside buildings. Its use cases are diverse, including navigation in shopping malls, patient management within hospitals, and inventory management in warehouses.

For typical indoor positioning, accuracy on the order of several meters to ten or so meters is sufficient in many cases. However, in situations that require more precise work, such as quality control on manufacturing lines, as-built verification at construction sites, and position recording during equipment inspections, cm-level accuracy (half-inch accuracy) is essential.

For example, if an abnormality occurs in equipment inside a factory, being able to record the exact position of the equipment with cm level accuracy (half-inch accuracy) would greatly streamline maintenance and repair planning. Also, on construction sites, measuring the installation status of interior wall and floor surfaces with cm level accuracy (half-inch accuracy) allows for precise checks of consistency with the design drawings.

To meet these needs, a variety of indoor positioning technologies have been developed and improved in recent years, and their deployment in the field has been progressing. In particular, in industrial applications, because positioning accuracy is directly linked to productivity, quality, and safety, expectations and demand for cm level accuracy (half-inch accuracy) have been rapidly increasing.

Differences between Outdoor GNSS and Indoor Positioning

With high-precision positioning using GNSS now widespread outdoors, many people wonder whether it can be used the same way indoors. However, GNSS relies on extremely weak radio signals transmitted by satellites above the Earth, and those signals are greatly attenuated or blocked by building walls, roofs, and ceilings.

Inside buildings with concrete or metal structures, satellite radio signals hardly reach at all. Even if some signals do get through, multiple reflections off walls and floors distort them, making accurate distance calculations impossible. Therefore, indoors, independent positioning technologies that do not rely on GPS/GNSS are required.

On the other hand, at worksites where tasks involve moving between outdoor and indoor areas, a key challenge is seamlessly integrating the absolute coordinates obtained from outdoor GNSS with the coordinate system used for indoor positioning. The design of this integration is the key to achieving high-precision position information management across the entire site.

Overview of Major Indoor Positioning Technologies

There are several technology categories for indoor positioning. Each has its own strengths and weaknesses, and you need to choose according to the desired accuracy and the environment.

Positioning Using Ultra-Wideband (UWB)

UWB transmits radio waves over a very wide bandwidth in short bursts and calculates position based on the time difference of arrival (TDoA) or the round-trip time of flight (TWR) between multiple anchors (fixed stations). In commercial products, accuracies of about 10-30 cm (3.9-11.8 in) are often achievable, and accuracy can be further improved with measures to increase precision.

Although a drawback is that accuracy tends to decrease in non-line-of-sight (NLoS) environments, on factory floors and in warehouses with few obstacles stable positioning at the cm (in) to tens of cm (in) level is possible. Equipment costs are relatively high, but the advantage is that real-time tracking is possible. In recent years chip prices have fallen, and standard integration into smartphones and wearable devices has been increasing.

High-Precision Wi-Fi Positioning

Typical Wi‑Fi-based positioning can only achieve accuracy on the order of a few meters to several tens of meters (a few m to several tens of m, a few ft to several tens of ft), but the new IEEE 802.11az standard enables high-precision distance measurements using TWR (two-way time-of-flight). If devices compliant with this standard become widespread, sub-meter-level accuracy (under 1 m (<3.3 ft)) can be expected while leveraging existing Wi‑Fi infrastructure.

However, at present supported devices are limited, and additional investment is required to reliably achieve full-fledged cm level accuracy (half-inch accuracy). Future developments will be of interest from the perspective of leveraging existing infrastructure.

Localization using LiDAR and point clouds

Devices equipped with 3D laser scanners (LiDAR) can rapidly acquire three-dimensional point clouds of their surroundings and estimate their own position by matching them to known map data. This method is called SLAM (Simultaneous Localization and Mapping), and it can perform localization while simultaneously creating a map even in environments without a preexisting map.

Accuracy varies with the environment, but combining a high-precision LiDAR and sufficient processing power can achieve positioning accuracy of several cm to several tens of cm (several in to several tens of in). Adoption on robots and automated guided vehicles (AGVs) is progressing. It is rapidly spreading, especially as an indispensable technology for autonomous mobile robots in the manufacturing and logistics sectors.

Fusion with an Inertial Measurement Unit (IMU)

An IMU, which accumulates error over time when used alone, can function complementarily when combined with other sensors. Sensor fusion combining UWB, LiDAR, and IMU makes it easier to achieve stable cm level accuracy (half-inch accuracy) positioning even in dynamic environments. Data integration using Kalman filters and particle filters has been implemented in practice, allowing accuracy to be maintained even when tracking high-speed moving objects.

Positioning Using Optical Markers

There are also positioning methods that combine cameras and markers (AR markers or dedicated targets). They can achieve sub-mm level (sub-0.04 in) precision in certain environments, but challenges include the requirement for a constant line of sight between the camera and the markers and susceptibility to lighting conditions. Adoption for positioning verification on manufacturing lines and for quality inspection processes is progressing.

Technical points for achieving cm level accuracy (half-inch accuracy) in indoor positioning

To reliably achieve cm level accuracy (half-inch accuracy), not only technology selection but also deployment design is important.

First, anchor placement has a major impact on accuracy. Using UWB as an example, if the number of anchors and the geometric balance of their placement to cover the positioning area are poor, accuracy will drop dramatically. Based on site drawings, it is fundamental to place a sufficient number of anchors evenly. It is recommended to simulate, during the design phase, placements that minimize GDOP (geometric dilution of precision).

Next, you need to check the radio environment. In factory environments where metal shelves, pipes, and large machinery are densely clustered, reflections, diffractions, and shielding of radio waves occur, making it easy to fall into non-line-of-sight (NLoS) positioning. A prior radio survey and measures to mitigate the effects of obstructions are indispensable. Introducing NLoS detection algorithms is also effective for maintaining accuracy.

Time synchronization accuracy is also important. In time-of-flight difference methods such as UWB, the time synchronization accuracy between anchors directly affects positioning accuracy. Because synchronization accuracy on the order of nanoseconds is required, wired networks or dedicated synchronization protocols (IEEE 1588 PTP) are often used.

Measures against multipath propagation (multipath) should not be overlooked. When radio waves reach the receiver via multiple paths, the apparent time-of-flight becomes longer and leads to positioning errors. Algorithm-level countermeasures (selecting the shortest path or integrating data from multiple anchors) are important.

It is also extremely important to measure and register anchor coordinates with an accuracy within 1 cm (0.4 in) — cm level accuracy (half-inch accuracy). The positioning engine calculates using the anchor coordinates as "known", so errors in the anchor coordinates are directly reflected in positioning accuracy. Precise coordinate measurement using laser rangefinders and 3D measurement equipment is required.

Representative Case Studies

Parts position management in manufacturing

An automotive parts manufacturer has implemented a system that determines with cm level accuracy (half-inch accuracy) exactly which processing station a parts tray is located at within the factory. By attaching UWB tags to each tray and combining them with anchors installed on the factory ceiling, they track the movement of parts in real time. This has successfully reduced part mix-ups and process delays significantly. Because the current positions of parts are visualized digitally, floor supervisors can monitor the process in real time and respond early to bottlenecks.

Automated Conveyance System for Warehouses

In logistics warehouses, cases are increasing in which AGVs autonomously travel between shelves while transporting goods. By having AGVs determine their position with cm level accuracy (half-inch accuracy), they can stop precisely in front of shelves, enabling picking by robotic arms. Systems that combine LiDAR-based SLAM and high-precision encoders are widely used. To operate safely in environments where humans and robots coexist, integration with collision-avoidance sensors is being implemented.

Construction Management at Construction Sites

Indoors on construction sites, it is required to measure the installation status of walls, floors, and ceilings with cm level accuracy (half-inch accuracy). By tracking differences from the design data in real time, rework costs can be greatly reduced. For this purpose, LiDAR scanners and high-precision indoor positioning devices are utilized. By integrating BIM (Building Information Modeling) data with positioning data, construction progress and quality can be managed simultaneously.

Location Management for Equipment Inspection

In plants and large facilities, routine inspections involve patrolling numerous pieces of equipment to take measurements and record data. If inspectors can record which equipment they inspected with cm level accuracy (half-inch accuracy), the risk of missed inspections can be minimized. Position management systems that combine UWB and high-precision GNSS augmentation technologies have a proven track record; because location information is automatically appended to inspection records, inspection status can later be reviewed on a map, greatly improving management efficiency.

Positioning in underground facilities and tunnels

In underground facilities such as subway tunnels and sewer conduits, GNSS signals do not reach at all. In such environments, autonomous positioning that combines LiDAR and IMU, and fixed-infrastructure positioning using installed radio beacons, are used. By managing positions with cm level accuracy (half-inch accuracy), tunnel displacement measurements and monitoring of long-term equipment deterioration can be conducted precisely.

Challenges of seamless indoor-outdoor positioning

On-site work often does not take place entirely indoors. At construction sites, checks are performed while moving between indoor and outdoor areas, and infrastructure inspections require continuous management of outdoor and indoor equipment.

GNSS is effective outdoors, but its signals do not reach indoors. The continuity of positioning at this boundary is a challenge. Even if GNSS can be used outdoors, the moment you enter a building the GNSS signal is lost, and because the coordinate system changes when the positioning system switches, seamless management becomes difficult.

For this challenge, it is important to integrate an indoor-only positioning system with outdoor GNSS and manage them within a common coordinate system. Also, by properly designing the handover process near building entrances, the continuity of positioning accuracy can be ensured. Designing building openings so that GNSS and indoor positioning can both collect overlapping data makes it easier to align the coordinate systems.

LRTK Phone: an iPhone-mounted device that enables seamless indoor and outdoor positioning with cm level accuracy (half-inch accuracy)

In settings where cm-level accuracy (half-inch accuracy) is required for indoor positioning, an iPhone-mounted GNSS high-precision positioning device using LRTK has attracted attention in recent years. With LRTK, simply attaching it to an iPhone allows you to obtain cm-level position information (half-inch accuracy) outdoors via GNSS/RTK positioning.

Moreover, with a system that leverages the LRTK Phone, it is possible to bring high-precision coordinate references established outdoors into indoor work operations. Even indoors where GNSS signals do not reach, construction management and inspection position records can be made based on the precise reference coordinates obtained outdoors.

LRTK's key feature is its simplicity: without the need to purchase or install a dedicated receiver, you can start high-precision positioning simply by attaching the device to your iPhone. It is designed with on-site usability in mind, allowing you to begin using it without complex calibration procedures.

In addition, the cloud platform provided by LRTK allows acquired point cloud data and positioning data to be shared and visualized on the cloud as is. Because data collected on-site can be shared with stakeholders in real time, the speed of report preparation and decision-making is improved.

In construction sites, infrastructure inspections, surveying, factory management, and any other settings that require position information with cm level accuracy (half-inch accuracy), LRTK functions as an immediate asset. For those considering achieving cm level accuracy for indoor positioning, it is realistic to start by deploying outdoor high-precision positioning with the LRTK Phone and then consider expanding into indoor environments.

Future Outlook: Toward the Widespread Adoption of Indoor cm level accuracy (half-inch accuracy) Positioning

Indoor positioning technology will continue to evolve. The reduction in semiconductor costs has made UWB chips cheaper, and more devices are being equipped with them as standard. Research into AI-powered NLoS detection and correction techniques is also progressing, making it easier to achieve stable cm level positioning accuracy (half-inch accuracy) even in complex environments.

With the spread of 5G, the practical deployment of high-precision positioning that leverages communication infrastructure (5G-NR Positioning) is also anticipated. By utilizing time-of-flight differences and angle information between base stations, positioning ranging from sub-meter (<1 m (3.3 ft)) to centimeter-level (1 cm (0.39 in)) is expected to be achievable both indoors and outdoors.

Also, with the proliferation of spatial computing devices, indoor positioning with cm level accuracy (half-inch accuracy) is positioned as an essential technology for the accurate overlay of AR content and for high-precision integration with digital twins. In the use of digital twins at manufacturing and construction sites, when position information with cm level accuracy (half-inch accuracy) flows in real time, discrepancies between simulation and reality can be detected immediately.

In the future, the mainstream will shift from using individual technologies in isolation to hybrid approaches that integrate multiple technologies. By combining sensor fusion and AI processing, an era in which stable positioning with cm level accuracy (half-inch accuracy) becomes commonplace in any environment is approaching. On-site deployment costs are also falling year by year, reaching a level where they can be considered realistic investments even for small- and medium-sized manufacturing sites and small- and medium-sized logistics warehouses.

Steps to implement indoor positioning with cm level accuracy (half-inch accuracy)

We will organize the basic steps for introducing an indoor positioning system.

The first step is an on-site survey and requirements definition. Investigate the extent of the positioning area, ceiling height, obstacle conditions, and sources of electromagnetic noise. Clarify the types and number of tracked targets, their movement speeds, the required accuracy, and the update rate. Also check the status of existing communications infrastructure (LAN/Wi‑Fi) and power supply.

The next step is technology selection and PoC (proof of concept). We select technologies that meet the requirements and conduct a PoC (proof of concept) in the actual field environment. We proceed with system design after understanding the performance differences between catalog specifications and real-world performance. A PoC usually takes about 1 to 2 weeks, but this investment significantly reduces the risk of failure in the full implementation.

Next are the system design and installation works. We design anchor placement, network architecture, software configuration, and integration with higher-level systems, and formulate the construction plan. It is important to coordinate the construction schedule with the factory so that installation can be carried out without stopping factory or warehouse operations.

In the installation, calibration, and accuracy verification steps, anchors are installed according to the design, and coordinate registration and calibration are carried out. Accuracy verification is performed across the entire positioning area to confirm that the target accuracy has been achieved.

Finally, it is the start of operations and continuous improvement. We put the system into full operation and carry out regular accuracy checks and maintenance. We adjust the system as appropriate in response to environmental changes and maintain stable cm level accuracy (half-inch accuracy) over the long term.

Cost-effectiveness of indoor cm level accuracy (half-inch accuracy) positioning

When evaluating the cost-effectiveness of indoor positioning systems with cm level accuracy (half-inch accuracy), it is important to consider not only direct cost reductions but also indirect effects.

Direct effects include reduced labor costs from AGV implementation, reduced losses due to fewer picking errors, lower equipment maintenance costs, and reduced costs from quality defects. These effects are converted into monetary values and compared with implementation costs to calculate the payback period.

As indirect effects, there are improvements in management efficiency through standardization of work and visualization of the shop floor, reduced employee burden, enhanced safety, and the promotion of data-driven continuous improvement (Kaizen). These effects are difficult to quantify, but over the long term they lead to a significant competitive advantage.

Additionally, by taking advantage of government and municipal subsidy and grant programs, you may be able to reduce the burden of initial investment. We recommend checking relevant programs, such as IT subsidies for small and medium-sized enterprises (SMEs) and manufacturing subsidies.

Summary

To achieve cm level accuracy (half-inch accuracy) in indoor positioning, it is important to appropriately select and combine multiple technologies such as UWB, LiDAR, and sensor fusion. In addition to technology selection, the precision of deployment design, such as anchor placement, radio environment, and time synchronization, also directly impacts positioning accuracy.

In fields such as manufacturing, logistics, construction, and inspection, indoor positioning with cm level accuracy (half-inch accuracy) has already built up a track record. Seamless integration with outdoor GNSS remains a major challenge going forward, and iPhone-mounted high-precision GNSS devices, such as the LRTK Phone, are attracting attention as an easy way to achieve cm level positioning outdoors (cm level accuracy (half-inch accuracy)).

If you have a need for cm level accuracy (half-inch accuracy) positioning indoors, we recommend first clarifying the on-site environment and the required accuracy, and considering an appropriate combination of technologies. Using a tool like LRTK that provides seamless high-precision positioning from outdoors as a starting point and gradually integrating it with indoor positioning systems is a realistic and effective approach.