Challenges and Solutions for Indoor Positioning in Factories: How to Achieve High Accuracy Even in GPS-Denied Environments

Table of Contents

• Why centimeter-level positioning is required in factories (cm level accuracy, half-inch accuracy)

• Challenges of indoor positioning: achieving high accuracy in environments where GPS cannot be used

• What is RTK-GNSS: technology that enables positioning to within a few centimeters

• Easy high-precision positioning realized by smartphone × RTK

• What is LRTK: a compact positioning device ideal for factory positioning

• Benefits of LRTK that change factory positioning

• LRTK use cases in factories

• Easy to deploy: start high-precision positioning with smartphone integration

• Summary: Factory DX and simplified surveying realized by LRTK

• Frequently Asked Questions (FAQ)

Why centimeter-level positioning (cm level accuracy (half-inch accuracy)) is necessary in factories

In large work areas such as factories and warehouses, even a deviation of a few centimeters (a few inches) in the equipment installation position can lead to major problems. For example, when installing machinery on a production line, if a machine is not placed in the position specified by the design, parts or piping may not align in subsequent processes, or additional adjustment work may become necessary. Even slight positional deviations, if they accumulate, can cause various issues on the shop floor, such as product quality variability and abnormal wear of machines. Accurate position measurement at the centimeter level (cm level accuracy (half-inch accuracy)) is essential to prevent such problems in advance and to maintain production efficiency and quality.

High-precision positioning is also indispensable for the travel routes of automated guided vehicles (AGVs) and autonomous mobile forklifts used in factory logistics. If an automated vehicle's route deviates by even tens of centimeters, the risk of collision with shelves or equipment increases, and unexpected stops or avoidance maneuvers may occur. If the current position can be continuously and accurately known, vehicle autonomous control becomes more precise, enabling safe and efficient transport. For factory safety management and productivity improvement, positioning information with centimeter-level accuracy (cm level accuracy (half-inch accuracy)) provides an important foundation.

Furthermore, in situations such as maintenance inspections and layout changes of factory equipment, accurate knowledge of coordinates is required. By 3D-scanning and recording the arrangement of existing equipment, or by precisely measuring the positions of reference points when marking the installation locations of newly installed equipment, work can proceed as planned. For example, when bringing in a large machine, if the dimensions of the installation space are not measured correctly, problems such as "the machine not fitting into its designated location" or "interference with other equipment" may arise. Having tools on the factory floor that can perform positioning to the centimeter level (cm level accuracy (half-inch accuracy)) can be said to directly contribute to reducing these risks.

Thus, in manufacturing sites there are a wide variety of needs to determine positions with centimeter-level accuracy (inch-level accuracy). However, achieving that level of accuracy indoors has not been easy until now. Next, let's look at the challenges that have hindered high-precision indoor positioning within factories.

Challenges of Indoor Positioning: Achieving High Accuracy in Environments Where GPS Cannot Be Used

In indoor or underground spaces, GPS signals from satellites do not reach sufficiently, so conventional GNSS positioning hardly functions. Unlike open outdoor areas, signals are blocked by factory roofs and steel-framed ceilings. Even if weak radio signals can be received, reflections from walls and floors (multipath) make it difficult to calculate an accurate position. As a result, large errors occur, and indoors you may obtain location information off by not just a few meters but tens of meters. Therefore, even using the GPS function standard on smartphones, it is not possible to accurately measure the positions of people or equipment inside buildings.

Several technologies have been explored to achieve high-precision indoor positioning. One approach is to install transmitters such as UWB (ultra-wideband) and Bluetooth beacons in the space and estimate position from their signals. UWB is theoretically said to enable accuracy of a few centimeters (a few inches), but it requires placing multiple antennas and tags indoors, leading to high initial deployment costs and substantial maintenance overhead. The beacon approach likewise requires installing dedicated devices throughout the site and periodically replacing batteries and calibrating them, and positioning accuracy tends to become unstable due to surrounding radio interference. Image-recognition using cameras and geomagnetic-based positioning methods have also been studied, but they require placing markers in advance or measuring the magnetic distribution of the environment, so they tend to be difficult to operate in environments like factories where layouts and equipment are frequently changed.

After all, up until now the reality was that to reliably obtain centimeter-level accuracy (half-inch accuracy) you had to rely on manual optical surveying. By using an optical surveying instrument called a total station (electro-optical distance meter), and by establishing reference points and then measuring the relative distances from them to each point you want to measure, even millimeter-level accuracy (0.04 in) can be achieved. However, that requires bringing in a specialized surveying team and large equipment into the factory, and during the work the production line often has to be temporarily stopped or access areas restricted, placing a heavy burden on the site. In response to the on-site voices asking, "Can't we measure indoors more easily and with high accuracy?", new solutions have finally begun to emerge in recent years.

RTK-GNSS: A technology that enables positioning to within several centimeters (a few in)

A representative technology that achieves outdoor positioning with errors kept to within several centimeters (a few in) is RTK-GNSS. RTK (Real Time Kinematic, real-time kinematic positioning) is a method in which the data from a GNSS receiver on a moving platform and a reference station (base station) installed nearby are compared in real time to correct position calculation errors. With conventional GNSS positioning, delays as satellite signals pass through the ionosphere and clock errors in the satellites or receiver inevitably cause discrepancies on the order of several meters (several ft). In RTK, with the idea of “placing another high-precision GPS nearby as a reference and canceling errors relatively,” the reference station sequentially sends correction information to the rover (user side) receiver, and by performing rapid computations it becomes possible to achieve centimeter-level accuracy (cm level accuracy (half-inch accuracy)).

In recent years in Japan, development has progressed for an Internet-accessible electronic reference station network and a Quasi-Zenith Satellite “Michibiki”–based centimeter-level positioning augmentation service (CLAS), creating an environment where the correction data required for RTK positioning can be easily obtained. Traditionally, performing RTK required expensive survey-grade GNSS equipment, but compact, low-cost, high-precision receivers have begun to appear. And now, a new approach—leveraging that RTK-GNSS technology on smartphones—is attracting attention.

Convenient high-precision positioning with Smartphone × RTK

Smartphones are now ubiquitous general-purpose devices, and by combining a smartphone with a compact RTK-capable GNSS receiver, they can be turned into survey-grade high-precision positioning devices. Smartphones themselves are equipped with various sensors such as GPS, cameras, accelerometers, and gyroscopes. By linking a positioning app for the smartphone with an external GNSS receiver and ingesting precise positional information obtained from satellites in real time, the data can be displayed and saved on the smartphone.

Specifically, a GNSS receiver device attached to a smartphone calculates a highly accurate current position using RTK and sends the data to an app on the phone via Bluetooth or a dedicated connector. The user need only tap a button on the smartphone screen at the point to be measured, and that location's latitude, longitude, and altitude are recorded instantly. Because RTK corrections keep errors within a few centimeters (within a few in), the accuracy that once required optical surveying instruments or expensive GNSS equipment can now be achieved with just a smartphone.

You can also take advantage of the intuitive operations and information processing unique to smartphones. For example, you can plot your current location and already-acquired measured points on a map or camera image on the screen and visually confirm them on site. After measuring multiple points, it is easy to create a simple drawing on the smartphone on the spot or immediately upload measurement data to the cloud and share it within the company. Data management that used to require taking handwritten notes from the field back to the office for entry and organization can be greatly streamlined by using smartphones and the cloud.

With the advent of high-precision positioning technology using smartphone × RTK, the act of "measuring position accurately" is becoming significantly easier and more familiar. So then, what exactly is the device LRTK (Eru Aru Tii Kē), which can be regarded as a concrete example of that?

What is LRTK: A compact positioning device ideal for indoor factory positioning

LRTK (El-Arr-Tee-Kay) is an ultra-compact RTK-GNSS receiver device designed to be attached to and used with a smartphone. It was developed by Refixia Inc., a startup originating from Tokyo Institute of Technology, and operates in conjunction with iPhone and iPad via Bluetooth. The main unit weighs only about 125 g and features a robust integrated design with an antenna and battery built in. It can be attached to the back of the device with a single touch via a dedicated smartphone case-style adapter, eliminating the hassle of cable connections. Because it can be carried integrated with the smartphone, positioning work while moving through narrow factory sites can be carried out comfortably.

With LRTK, your handheld iPhone becomes a centimeter-level accuracy surveying instrument (cm level accuracy (half-inch accuracy)). Launch the dedicated app "LRTK Phone" and simply press the on-screen button at the point you want to measure to obtain high-precision coordinate values for that location. In actual tests, standalone LRTK positioning yielded horizontal position errors of approximately ±1-2 cm (±0.4-0.8 in) and vertical errors of approximately ±2-3 cm (±0.8-1.2 in). Furthermore, by making multiple observations and averaging, it has been confirmed that errors can be reduced to less than 1 cm (less than 0.4 in). Achieving this level of accuracy with a pocket-sized device is groundbreaking, and its performance approaches that of total stations, which boast millimeter-level accuracy (0.04 in) over short distances.

LRTK does more than just measure point coordinates; it also includes various functions that leverage high-precision positioning information. For example, when combined with the LiDAR scanner and camera built into iPhone Pro models, it can record the surrounding environment as 3D point cloud data. Tasks that previously required a dedicated 3D laser scanner can now be simply scanned with just LRTK and a smartphone. Because the acquired point cloud data is tied to position coordinates measured by LRTK, you can reproduce measured data at actual scale on CAD drawings or easily integrate it with other surveying data.

Furthermore, you can save measured points with attached photos and notes, and apply features such as actually moving to a specified coordinate and marking it using the AR (augmented reality) function. For example, if you specify on your smartphone the position on the floor where you want to place a mark, an arrow displayed on the screen will guide the user to that location, making it useful for layout marking (positioning) tasks. All of these functions can be completed with just an LRTK device and a single smartphone.

Benefits of LRTK That Transform In-Factory Positioning

The greatest benefit of using LRTK inside factories is the point that anyone on site can immediately perform centimeter-level positioning (cm level accuracy; half-inch accuracy). As described above, precision surveying that previously required specialist surveyors using expensive equipment can now be carried out at any time by on-site staff themselves. For example, a production line construction supervisor can measure reference points personally to confirm machine installation positions, or a maintenance engineer can record the exact coordinates of repair locations—enabling easy positioning according to the task. Being able to take measurements immediately when needed without calling in a skilled surveyor will dramatically improve on-site agility.

Furthermore, because the LRTK is compact and lightweight, it is suitable for measurements in confined spaces and at height within factories. Even in locations with poor footing where there is no room to set up a tripod, as long as there is a gap large enough for a person to enter, you can hold a smartphone in your hand and bring it closer to the target point. Even inside plant facilities where stairs and piping are intricate, an LRTK that fits in one hand is easy to maneuver. There is no need to carry heavy equipment or perform complicated setup, and preparation time for measurements is minimal. The ease of being able to take it out and measure as soon as you think of it will prove valuable for routine inspections and sudden verification tasks.

There are also benefits in terms of data utilization. Measurement data acquired with LRTK and a smartphone can be uploaded to the cloud on site or shared in real time with other departments within the company. For example, point cloud data of equipment layouts collected in a factory can be immediately reviewed by engineers at headquarters, enabling smooth collaboration such as issuing remote instructions to the field. Traditionally, measurements taken on site had to be converted into drawings and it took time to share them, but using LRTK allows data to be shared and visualized instantly, greatly speeding up decision-making.

Furthermore, the cost of adopting LRTK is kept very low compared with conventional surveying equipment. Because it is far less expensive than purchasing a single large total station, it becomes realistic to provide multiple units and have each worker carry one. With the cost barrier lowered, one could say the era of "one positioning device per person" is now within reach.

LRTK use cases in factories

Let's take a look at some scenarios that show how LRTK can actually be utilized inside factories.

• Installation of new equipment and layout changes: When introducing new machines or production lines, if you accurately measure the coordinates of the installation locations in advance using LRTK, you can reconcile the plans with on-site conditions without discrepancies. By correctly measuring distances and heights from reference points, you can prevent rework on the day of equipment installation such as "the position is wrong" or "it's not level." Furthermore, by using AR features you can work while projecting placement markers onto the floor, enabling marking tasks to be carried out quickly.

• Equipment inspection and maintenance records: In factory equipment maintenance inspections, it is important to accurately record the locations of defects and replacement parts. With LRTK, for example, "the piping connection 2.3 m (7.5 ft) east of the motor mounting on Line 3"—subtle positional relationships that were previously described in text can be preserved as numerical coordinates. By attaching photos and notes to each measurement point and saving them to the cloud, you can easily identify exactly the same location during the next inspection, leading to improved accuracy in equipment management.

• Dimensional Measurement and Current-State Assessment in Factories: In manufacturing sites, there are many situations where you need to measure clearances (gaps) between machines or aisle widths. Measurements can be made with tape measures or laser distance meters, but with LRTK you can acquire distances between multiple points and later compute precise dimensions digitally. Also, by analyzing point cloud data captured with a smartphone, it is possible to understand the overall shape of the site as a 3D model. For example, one application would be deriving slight slopes of the floor surface from point cloud data to help check drainage gradients.

• Outdoor-to-indoor surveying: LRTK also proves powerful when surveying continuously from outdoors into building interiors within a factory site. Outdoors, RTK obtains high-precision absolute coordinates, and when you enter a building and can no longer capture satellites, you switch to relative positioning using a smartphone’s inertial sensors and camera (PDR (pedestrian dead reckoning) or ARKit) to continue measurements. For example, you might take a reference point near a building entrance and then patrol the factory while estimating position by walk-based PDR. Some error accumulates with movement, but over short distances continuous positioning is possible with practically acceptable accuracy. This enables surveying tasks that seamlessly connect indoor and outdoor areas to be completed by a single person.

Easy Setup: Start High-Precision Positioning with Smartphone Integration

No special infrastructure construction is required to deploy LRTK. Basically, you only need the LRTK device itself and a compatible smartphone (currently mainly iPhone/iPad), install the dedicated app, and you can start high-precision positioning. Correction information required for RTK positioning can be obtained in Japan either by connecting via the Internet to the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations network, or, in supported areas, by directly receiving the QZSS Michibiki CLAS signal. Because the app automatically performs the complicated settings, users can access centimeter-level positioning (cm level accuracy (half-inch accuracy)) simply by selecting the positioning mode.

The connection operation between the smartphone and the LRTK is simple. Once you have paired them via Bluetooth, at the site you only need to turn on the power and the smartphone will automatically detect and connect to the LRTK. After that, all you need to do is follow the instructions on the app screen, so even first-time users should be able to start measurements without hesitation. It is designed to be operated without specialized surveying knowledge, so almost no training costs are required for operational instruction. There is no need to hold large-scale briefing sessions for field staff; simply reviewing a short, few-page manual or a tutorial video should be sufficient to become proficient.

In this way, LRTK also excels in terms of ease of deployment. As one of the tools to advance factory DX (digital transformation), the high-precision positioning that can be started using existing smartphones and tablets is a major attraction. The fact that it can minimize initial investment, begin operation in a short period, and immediately contribute to on-site productivity makes it an easy-to-adopt option for management.

Summary: LRTK-enabled Factory DX and Simplified Surveying

To address the biggest challenge in indoor factory positioning—achieving high accuracy in environments where GPS cannot be used—LRTK has presented a practical solution through smartphone integration. By combining the accuracy of RTK-GNSS with the convenience of smartphones, an era in which anyone, anywhere can perform centimeter-level positioning is becoming a reality. As a result, location data DX will accelerate across every operational scenario in factories. Layout planning, equipment management, logistics optimization, safety management, and other situations where location data can be leveraged are expected to expand even further.

Especially, LRTK has the potential to overturn the conventional belief that surveying is only for specialists and to root a new culture of simplified surveying on-site. If the casual act of taking measurements with a smartphone in hand can, as an extension, lead to understanding the current state of a factory and discovering points for improvement, on-site capabilities will improve dramatically. LRTK, which combines high accuracy with ease of use, is truly a next-generation positioning and surveying tool.

An era may come when workers at factories and construction and civil engineering worksites carry "one high-precision positioning device per person." LRTK, as a pioneer, is strongly driving on-site DX forward. Why not bring the world of centimeter-level accuracy (half-inch accuracy) into your factory? You'll surely be amazed by its effects and soon find you can't do without it.

Frequently Asked Questions (FAQ)

Q: Can you really achieve centimeter-level accuracy (half-inch accuracy) indoors? A: In completely enclosed indoor spaces where satellite signals cannot reach, LRTK estimates position using relative positioning that leverages a smartphone’s inertial sensors and camera (technologies such as PDR and ARKit). Small errors accumulate when moving long distances, but over relatively short ranges it can maintain high accuracy on the order of several centimeters (a few inches). Also, in locations where the sky is temporarily visible—such as near building entrances—it is possible to capture satellites and apply corrections to help maintain indoor positioning accuracy. In facilities where indoor and outdoor areas are contiguous, combining this with high-precision RTK positioning can deliver practically sufficient accuracy for real-world use.

Q: Do you need specialized knowledge or skills to operate LRTK? A: No, you do not need any special surveying knowledge or qualifications. LRTK is designed so that anyone on site can use it, and by following the dedicated app’s on‑screen prompts you can complete measurement, saving, and sharing. Complex settings and operations are automated, so even first‑time users can learn the basic positioning methods after trying it for a short time. If you do get stuck, clear manuals and a support desk are available for peace of mind. The ease of use that lets facility personnel and maintenance staff on site operate it comfortably is a key feature of LRTK.

Q: How can positioning data be stored and utilized? Can it also help manage inspection histories? A: Positioning data acquired with the LRTK app can be recorded per point with latitude, longitude, and altitude coordinates, along with timestamps, optional notes, and photos. These data can be exported in CSV or surveying coordinate formats, or imported to a PC via the cloud and loaded into CAD software or GIS systems. Point cloud data can also be output in common 3D formats such as LAS or PLY, making it easy to compare collected as-built data with design drawings or to construct 3D models. Positioning information shared in real time on the cloud can be displayed on maps in a web browser and supports collaborative work such as simultaneous viewing and editing by other personnel. Because high-precision positioning data recorded on site can be used in these multiple ways, it enables accurate retention of each work location in equipment inspection history management and greatly helps improve efficiency and prevent omissions in subsequent inspections.