How to Obtain Coordinates in Indoor Environments Where GPS Signals Don't Reach: Everything Practitioners Should Know

Table of Contents

• Use cases and challenges for high-precision indoor positioning

• Positioning methods used indoors where GPS is unavailable

• How RTK technology achieves high-precision positioning

• New indoor positioning enabled by smartphones × LRTK

• Worksites transformed by simplified surveying with LRTK

• FAQ

Cases and Challenges Where High-Precision Indoor Location Information Is Required

It is not uncommon on-site to need to measure accurate position information, such as for indoor work at construction sites and equipment management in factories and warehouses. For example, cases such as layout marking work to determine installation positions of machinery and equipment inside buildings, or flow-line analysis to optimize inventory placement in indoor stockyards, are increasing and require centimeter-level positioning (cm (0.4 in)). Recently, on-site digitalization has progressed, and efforts to improve productivity through "visualization", such as construction management and layout improvements using surveying data, are becoming more common.

However, a major challenge is that satellite-based positioning, represented by GPS, cannot be used as-is in indoor or underground spaces. Signals from satellites are blocked by building walls and roofs, making reception indoors extremely difficult. Even if a very weak signal can be received, errors can range from a few meters to several tens of meters, making it unsuitable for precise positioning. Therefore, to obtain accurate coordinates in indoor environments, it is necessary to employ alternatives to GPS.

Positioning methods used indoors where GPS is unavailable

So, what methods have been used so far to measure positions indoors where GPS signals cannot reach? First, a long-established approach is to set reference positions inside the building and then measure distances from them with tape measures or laser distance meters, or determine relative positions using optical surveying instruments such as transits and total stations (TS). The task of marking each point by manually calculating coordinates based on baselines and known points shown on the drawings delivers high accuracy but requires skilled technique and considerable effort. In long indoor corridors or large-scale factory facilities, it is necessary to reposition instruments and take repeated measurements, making it difficult to perform precise surveying within limited time.

In addition to methods that rely on manual effort or optical equipment, various technologies have been researched and put into practical use to realize indoor positioning. Representative indoor positioning technologies include the following.

• Radio positioning using BLE beacons or Wi‑Fi: This approach installs multiple transmitters (BLE beacons or Wi‑Fi access points) indoors and estimates distance from the signal strength or time of arrival received by the device to determine position. It can be deployed at relatively low cost, but accuracy is at best on the order of several meters (several ft), and errors tend to be large due to radio reflections and interference from walls, machinery, etc.

• UWB (ultra-wideband) positioning: This method uses extremely short radio pulses on the order of nanoseconds to measure distance and trilaterates position from time-of-arrival differences between multiple antennas. In some environments, high-precision positioning can reduce errors to several tens of centimeters to about 10 cm (several to about 3.9 in). However, installing dedicated antennas and initial tuning require time and cost, and building a system that covers a wide area is relatively challenging.

• Ultrasonic/acoustic positioning: This method places ultrasonic transmitters on ceilings, etc., and determines position from the time of arrival of sound waves. Compared with radio waves, it is less likely to be blocked by obstacles, and some systems can measure three-dimensional positions (including height). Some claim centimeter-level accuracy (half-inch accuracy), but equipment installation and periodic calibration are required, so the barrier to on-site deployment is not low.

• Self-localization using cameras or LiDAR: This technology scans the surroundings with camera images or LiDAR sensors mounted on mobile devices or robots and estimates the device's position from the obtained features. If a full 3D map of the interior is created in advance, positions can be determined without placing tags on walls or floors. However, equipment and software can be expensive, and creating a detailed scan of the entire building before measurement can be time-consuming, so it is not necessarily an easy-to-use method.

• PDR (pedestrian dead reckoning): This method estimates steps taken and heading from a smartphone's accelerometer and gyroscope and cumulatively computes relative position from a known starting point. It is effective for short periods, but sensor errors accumulate over time and the position gradually drifts (the "drift" problem). During long or extended movement, errors of several meters or more (several ft) can occur, so without periodic corrections to a known position high accuracy cannot be maintained.

As described above, the conventional methods for obtaining position coordinates indoors each have their own advantages and disadvantages. In particular, to achieve centimeter-level accuracy, it has been necessary to deploy dedicated infrastructure or introduce expensive equipment. Also, many technologies are limited to position estimation on the plane (X, Y), and precise positioning in the vertical direction (Z axis) is difficult. However, inside buildings and factories, height information such as elevation differences and floor levels can also be important. So, how can centimeter-class positioning indoors be achieved more easily? One of the keys is the use of a high-precision positioning technology called RTK (Real-Time Kinematic).

How RTK Technology Achieves High-Precision Positioning

RTK (Real Time Kinematic) is a technique that dramatically improves accuracy in satellite positioning such as GPS by using a method called differential correction. The principle is that two receivers—a base station (a receiver with a known position) installed nearby and a rover (the receiver at the point to be measured)—simultaneously receive satellite signals, and by sending the error information computed at the base station to the rover in real time and applying corrections, standard GPS positioning with errors of several meters can be reduced at once to an accuracy of a few centimeters. It is, in effect, a system that realizes "ultra-high-precision real-time GPS," and it has long been used in various outdoor fields such as civil surveying, machine control guidance, and autonomous driving.

To perform conventional RTK positioning, it has been necessary for the base station to transmit data to the rover via radio or the Internet. Also, because both receivers need to track the same satellites simultaneously, RTK has difficulty delivering its intended performance in environments where satellite signals are weak, such as inside tunnels or in the canyons between high-rise buildings. However, in recent years solutions have emerged that make it easy to utilize this RTK technology with compact all-in-one devices and smartphones. This is the high-precision positioning using smartphone × LRTK that will be introduced next.

New indoor positioning enabled by Smartphone × LRTK

A new positioning solution has appeared that combines a smartphone with an ultra-compact RTK receiver called LRTK (Eru-Aru-Tii-Kee). The LRTK is a palm-sized, integrated GNSS receiver weighing about 150 g, which is attached to the top of a smartphone or tablet for use (wirelessly connected via Bluetooth, etc.). With an all-in-one design that combines the antenna, receiver, and battery, you can turn it on and go to a place with a view of the sky; initial positioning completes in tens of seconds, and you can immediately obtain your position with centimeter-level accuracy.

LRTK devices support the centimeter-level (half-inch-level) augmentation service (CLAS) provided by Japan's Quasi-Zenith Satellite System "Michibiki" and can receive correction signals directly from the satellites even outside cellular coverage. Therefore, even at sites in mountainous areas or underground facilities where mobile phone signals do not reach, as long as the sky is visible overhead, high-precision positioning is possible without the Internet.

Also, if a known reference point exists on site, you can place one LRTK unit there and operate it in a simple base-station mode, and use another unit as a rover, enabling flexible uses such as measuring relative coordinates from the reference point even within enclosed spaces.

On the other hand, what happens after you go indoors and satellite signals are lost? In this regard, built-in sensors in smartphones and AR (augmented reality) technology come into play. Modern smartphones are equipped with high-quality cameras and gyroscopes that can capture the device's motion in real time. In the LRTK system, even after entering a building and losing GNSS signal reception, the smartphone's AR capabilities continue to track the device's position and compute the relative movement from the high-precision reference position it had acquired up until that point. Simply put, while moving indoors the smartphone acts as a high-performance pedestrian navigation device, and for short periods it can measure position without satellites while maintaining centimeter-level accuracy (half-inch accuracy).

With this mechanism, for example when recording multiple survey points inside a building, you can obtain an accurate reference point using LRTK outside the building or on the roof before starting, then simply carry the device indoors and press the positioning button at each point to record coordinates. There is no need to bring surveying equipment inside and struggle with complex setup or ensuring line of sight, or to repeatedly perform coordinate calculations on drawings as was traditionally required. The smartphone screen instantly displays the current latitude, longitude, and height, and measured points are automatically recorded along with names and notes. Measurement data can be shared via the cloud as is, enabling real-time collaboration such as immediately checking coordinates obtained on site from the office PC.

Simplified Surveying with LRTK Transforms the Jobsite

The emergence of positioning systems that combine smartphones and LRTK means that "surveying" is no longer a special task reserved only for specialist technicians; it is increasingly becoming something that field personnel can handle as an extension of their daily work. Traditionally, to perform precision surveying at the centimeter level (half-inch precision) you would need to hire a team of surveyors or use expensive equipment, but with simple surveying using LRTK each worker can quickly complete measurements at the necessary time. For example, in situations where a heavy equipment operator might otherwise wait for the layout results of construction reference points, they can use an LRTK-equipped smartphone on the spot to check for themselves, greatly reducing waiting times and rework. Because positioning results are shared to the cloud in real time, data measured on site can be immediately shared with stakeholders and decisions about next steps can be made on the spot.

High-precision indoor positioning with LRTK can be utilized in a variety of scenarios. For example, the following applications can be mentioned.

• Indoor layout marking and installation surveying: When installing or relocating mechanical equipment inside a building, LRTK can be used to accurately measure the installation position coordinates on the floor surface. By installing equipment while comparing the planned values on the drawings with the on-site survey results, accurate construction without positional displacement can be achieved.

• Flow analysis inside factories and warehouses: In logistics warehouses, movement paths of forklifts and staff can be recorded with LRTK and visualized for analysis, helping to consider layout improvements and safety measures. Verification of subtle improvement effects that was difficult with errors of several meters (several ft) can be quantitatively evaluated using centimeter-level accuracy data (cm level accuracy (half-inch accuracy)).

• Surveying in tunnels and underground spaces: Even inside tunnels where satellite signals do not reach, surveying inside the tunnel can be advanced using LRTK from control points acquired near the entrance. Tasks that were previously repeatedly measured with TS are simplified, enabling as-built control and installation measurements to be performed with fewer personnel and in shorter time.

Thus, the easy high-precision surveying made possible by LRTK becomes a powerful ally for improving productivity at construction and facility management sites. It is a technology that aligns with i-Construction (ICT-based construction) promoted by the Ministry of Land, Infrastructure, Transport and Tourism, and it can be said to be a promising solution for operating sites efficiently with limited personnel amid a declining birthrate and aging population. If your sites also face issues such as "wanting to measure indoors but unable to" or "being troubled by large positional errors," please consider the new option of pairing smartphones with LRTK. Positioning in environments that were previously difficult will become markedly easier, and onsite surveying accuracy and work efficiency should improve dramatically.

FAQ

Q. What do you need to use a smartphone with LRTK? A. The only equipment required is the LRTK device itself and a compatible smartphone. First, install the dedicated LRTK app on an iOS device such as an iPhone or iPad, and connect it to the LRTK device via Bluetooth. Because positioning requires receiving satellite signals in a location with a clear view of the sky, even when measuring indoors you should first perform initial RTK positioning (Fix solution) outside the building or by a window. After that, simply move around indoors while holding your smartphone and press the record button at the points you want to measure. No complicated base station setup or prior calibration is required, so you can start measuring as soon as you arrive on site.

Q. Is the positioning accuracy really at the centimeter level? Won't the accuracy deteriorate indoors? A. Yes—if RTK corrections are applied properly, errors will remain within a few centimeters or less (a few in or less). If you achieve high-precision (Fix state) positioning in an outdoor open-sky location, moving indoors immediately afterward will not cause an immediate large drop in accuracy. Smartphone AR technology can maintain self-position accuracy for short periods, so coordinates can be obtained indoors with roughly a few cm (a few in) of accuracy. However, if satellite reception is completely lost for an extended period, errors will gradually accumulate, so when moving long distances inside a large facility it is reassuring to go outdoors or near a window to reacquire satellites and reapply corrections. In actual operations, even when measuring about 10 indoor points, the variation at each point has been confirmed to fall within a standard deviation of about 1-2 cm (0.4-0.8 in). By measuring dozens of times at a single location and averaging, it is also possible to approach accuracy below 1 cm (below 0.4 in).

Q. I'm concerned about the size and power supply of the LRTK device. How portable is it, and how long can it be used continuously? A. The LRTK device is extremely compact and lightweight, weighing approximately 150 g and about the same as a smartphone, and is only around 1 cm (0.4 in) thick. It is pocket-sized and won't get in the way during field work. The built-in battery enables cable-free operation, and a full charge provides approximately 6 hours of continuous positioning. Charging is done via USB Type-C, and it also supports power supply from a mobile battery. You should be able to operate it for long surveying sessions without worrying about the power running out.

Q. Can it be used in places without an internet connection? A. Yes. As mentioned above, the LRTK device supports CLAS signals broadcast by Japan’s Quasi-Zenith Satellite System (QZSS) "Michibiki". Even in remote mountain areas or underground spaces where cellular signals do not reach, as long as the sky is visible overhead and satellite signals can be received, centimeter-level positioning (cm level accuracy (half-inch accuracy)) is possible without a network connection. However, in locations such as the deepest parts of tunnels or building basements where the sky is completely not visible, satellite signals themselves cannot be received, so positioning is also difficult in those cases. In such environments, this is typically supplemented by measures such as relative surveying from reference points obtained near the entrance or determining position by matching against existing drawing data.

Q. How can the measured data be handled? Can it be shared within the company? A. Positioning data acquired with the LRTK app can be synchronized to and utilized on the cloud. Recorded point coordinates, photos taken, and point cloud data scanned with a smartphone can be uploaded to a dedicated cloud service via the internet. After uploading, you can display the data on 2D maps or in a 3D view from a PC browser, check coordinates and distances, and perform measurements. You can also show results to members of other departments or to clients who do not have dedicated software by issuing a web share URL. In addition, because survey coordinates support Japan’s Plane Rectangular Coordinate System (you can select any system), overlaying with CAD drawings and GIS data is simple. In other words, it becomes smooth to directly compare field-measured data with design drawings and existing documents and to share information among stakeholders.

Q. Does operation require specialized knowledge or skill? Can beginners use it proficiently? A. No special qualifications or surveying knowledge are required. The LRTK system is designed with a simple operation app and automatic processing features so that even first-time users can operate it intuitively. Setting up the equipment is as simple as attaching it to a pole (monopod) and standing it vertically, with no complicated adjustments needed. The app displays the current accuracy (Fix/Float status) and the number of satellites being tracked, allowing users unfamiliar with surveying to understand the situation as they work. Tutorials and support systems are also in place, so even facility managers or other staff without surveying experience can master the LRTK and perform surveys after a short briefing. You can confidently deploy it on-site.