cm-level high-precision positioning with GNSS receivers — What are the on-site benefits?

In modern surveying and construction sites, centimeter-level high-precision positioning is attracting a great deal of attention. Until now, surveying often relied heavily on experience and manual labor, but with the advances of GNSS receivers (satellite positioning receivers), an era in which a single person can easily perform surveying using a smartphone is coming. In this article, we explain what GNSS receivers are, and the innovations and specific use cases that cm-level positioning brings, furthermore the differences from conventional devices and future prospects, and finally introduce an easy method to implement surveying using LRTK Phone.

What is a GNSS Receiver? (Basic Principles)

GNSS receiver is a device that receives radio signals from satellite positioning systems (GNSS), represented by GPS, and computes its current position (latitude, longitude, altitude). There are multiple GNSS worldwide besides the U.S. GPS, such as Russia's GLONASS, Europe's Galileo, and Japan's quasi-zenith satellite "Michibiki", and GNSS receivers determine their position by simultaneously using signals from these multiple satellites. The basic principle is measuring the distance to satellites, where the receiver calculates the distance from the time differences of signals received from each satellite and determines its position by intersecting them in three dimensions.

However, with conventional standalone GNSS positioning, errors of several meters (several ft) occur. These arise from various factors such as clock errors in the satellites and receivers, signal delays when passing through the atmosphere, and reflections from buildings or the ground (multipath), and it is not uncommon for the GPS built into smartphones to have errors of 5-10 m (16.4-32.8 ft). This is insufficient for the centimeter-level accuracy (half-inch accuracy) required on surveying and construction sites. That is where error correction technologies such as RTK (Real Time Kinematic) methods come in. RTK prepares two GNSS receivers—a base station and a rover—and by sending the error information measured at the base station to the rover and applying corrections, it can cancel out errors in real time down to a few centimeters (a few in). This mechanism has made it possible to achieve high accuracy of a few centimeters (a few in) even with satellite positioning. Recently, cloud-based services that make RTK even more convenient and Japan’s own satellite augmentation signals have appeared, lowering the barriers to on-site use.

Innovations Brought by Centimeter-Level High-Precision Positioning (Including Comparisons with Total Stations)

The benefits that centimeter-level high-precision positioning brings to worksites are immeasurable. Let's examine the innovation of GNSS-based high-precision positioning while comparing it with the Total Station (TS), which has traditionally been the mainstay of field surveying.

First, it's about surveying efficiency and personnel. With total station surveying, operating the instrument and setting up the prism (target) typically required a team of at least two people. On the other hand, with RTK-GNSS surveying a receiver-carrying single person can continuously measure a wide range of points. It eliminates the need to maintain line of sight or to set up targets, and because the equipment needs to be repositioned less often, work time is greatly reduced. For example, even distant points that traditionally could not be measured due to obstructed sightlines can be directly positioned by GNSS as long as the sky above is open. This enables quick surveying of wide-area topography and measurements between distant control points.

Next is a comparison of accuracy and characteristics. Total stations boast extremely high precision on the order of millimeters for short-distance relative measurements, and when combined with leveling for height measurements, errors can be reduced to a few millimeters (a few 0.08-0.35 in). On the other hand, GNSS RTK surveys are considered to have an accuracy of approximately ±1-2 cm (±0.4-0.8 in) in plan position and ±2-3 cm (±0.8-1.2 in) in the vertical direction【※】, and the ability to obtain the precision required for civil engineering surveys fully automatically is revolutionary. For general design surveys that do not require the strictness of TS, it can now be replaced by cm-level positioning (half-inch accuracy) using RTK. Also, because GNSS surveys can directly obtain absolute coordinates, there is the advantage of being able to output results directly in public coordinate systems such as the World Geodetic System. In contrast, TS surveys are relative measurements, so obtaining a consistent coordinate system over a large area required securing known points and tying together multiple stations. GNSS centimeter-level positioning (half-inch accuracy) also simplifies surveying workflows in this respect.

Another important difference is the strength under field conditions. Although TS offers high accuracy, its measurements require an unobstructed line of sight between the instrument and the prism. They can be difficult to use in urban areas with many buildings, dense forests, or inside tunnels. GNSS positioning uses satellite signals from above, so it is not constrained by line-of-sight, and can directly measure distant points as long as there are no obstructions (however, GNSS is also difficult in environments with no view of the sky at all, such as dense forests or under elevated structures; in those cases TS or terrestrial LiDAR still have their role). Regarding weather, TS makes prism sighting difficult in rain, whereas GNSS is less affected by moderate rain and can likewise operate at night【※】. In this way, the ability to survey “alone, anywhere, in a short time” is arguably the greatest innovation brought by cm-class GNSS positioning (cm-level accuracy; half-inch accuracy).

[Reference] In the network RTK using the Geospatial Information Authority of Japan’s continuously operating reference stations (CORS), there have been reports of horizontal positioning errors of about 3–4 cm (1.2–1.6 in), and it has now been demonstrated that satellite positioning can achieve errors within a few centimeters (a few inches). In addition, GNSS surveying does not require line of sight, is less affected by weather, and is attracting attention as a method for efficiently obtaining three-dimensional coordinates (Reference: <a href="https://digital-construction.jp/column/1052" target="_blank">Digital National Land Research Institute column</a>).

Specific use cases in surveying and construction sites

High-precision positioning using GNSS receivers is beginning to play an active role in various surveying and construction scenarios. Here we introduce specific examples of smart surveying applications expected on-site.

• Acquisition of 3D point clouds: By combining a smartphone with a GNSS receiver, it becomes easy to acquire three-dimensional point cloud data. For example, using an iPhone’s built-in LiDAR scanner or photogrammetry apps to scan buildings or terrain and convert them into point clouds, you can obtain on-site 3D models with high-accuracy positional information. Point cloud surveying, which traditionally required specialized equipment, can be completed with just a smartphone by providing accurate positional references via GNSS, and can be used for as-built management and earthwork volume calculations.

• Pile-driving support (design position setting out): Traditionally, setting out the positions of structures (pile-driving work) required surveyors to mark positions with tape measures and chalk lines or to track coordinates with a TS (total station). By using a GNSS receiver, you can display the design coordinates directly on site and guide workers to them, allowing a single person to accurately set out pile survey points. Since the distance and direction to the target point are displayed in real time on a smartphone screen, the operator holding the receiver is guided to the designated location and can immediately install the pile. Even points beyond a curve with no line of sight or points within a vast site can be reached without getting lost, and reverse pile-driving (measurement and verification of existing pile positions) is also smooth.

• Use of AR technology: The combination of GNSS and smartphones also enables intuitive on-site work support through AR (augmented reality). Through the screen of a smartphone or tablet, you can overlay lines from design drawings and equipment models onto the real-world scene, allowing you to check the final image on the spot and visualize the locations of buried objects beneath excavation points. For example, displaying the route of pipes buried underground in AR lets you determine exact positions from the surface and prevent accidental excavation. By superimposing design models on site to check construction accuracy, the flow of GNSS-based positioning → AR-based visualization is spreading as a new construction management method.

• Photograph-based positioning (positioning photo function): By combining a smartphone camera with high-precision GNSS coordinates, you can attach accurate location information to photos and record them. When you take photos of anomalies or construction sites during site patrols, the image files are automatically tagged with the latitude, longitude, and timestamp of that location, so it is immediately clear where each photo was taken when you review them later. For example, when a person in charge discovers a malfunctioning streetlight and records it with a photo, the precise coordinate data determined by the GNSS receiver at hand is linked to the photo, enabling reports that are far more accurate than vague notes such as "about 50 m (164.0 ft) east of the XX intersection." If linked with the cloud, the photos and their locations can be shared with the office at the time of capture, making it faster to identify the sites that require action and to navigate to them. This kind of photo-plus-positioning usage is useful in a wide range of fields, such as disaster documentation and infrastructure inspections.

Differences from previous models and benefits (weight reduction, smartphone integration, single-person operation, cloud synchronization, localization, etc.)

The latest GNSS receivers (smartphone-connected devices) offer many advantages and advancements compared with traditional surveying instruments and legacy GNSS terminals. Below is a summary of the main differences.

• Lightweight and compact: Traditional high-precision GNSS receivers, including the antenna and battery, were large devices weighing several kilograms, but the latest GNSS receivers are smartphone-sized and pocketable. Weighing just a few hundred grams or less (for example, the LRTK Phone mentioned later is about 165 g) and with a thickness of around 1 cm (0.4 in), they have been slimmed down and lightened, drastically reducing the burden of bringing equipment to the field. Because they can be kept in a dedicated case and carried at all times, they can be quickly taken out and used to start positioning whenever needed.

• Smartphone integration: The new GNSS receiver pairs with smartphones and tablets via Bluetooth or a wired connection, and is operated and records data through a dedicated app. This enables coordinate measurement and data management with an intuitive touch UI, removing the need for the complex control panels and specialist knowledge common in conventional units. By leveraging the smartphone’s processing power and communication capabilities, it is also easy to verify survey points on a map and perform calculations and plotting on the spot. Because the smartphone itself becomes the survey controller and data logger, there is no need to prepare a separate dedicated device.

• Ease of single-person operation: As mentioned above, manpower requirements are greatly reduced, and survey work can be completed by one person. Since base stations can also be accessed over the network, you can go to the site alone carrying only the rover. For example, if you attach a GNSS receiver to a dedicated monopod (pole) and carry it, you can survey any desired point while carrying the pole yourself. The height offset (the height correction from the tip of the pole to the antenna) is also automatically calculated in the app, so you don’t need to worry about cumbersome correction tasks. With conventional units, many tasks required experience—such as setting up a tripod and leveling the equipment—so the presence of a skilled operator was assumed, but the latest devices are designed to be easy for beginners to use, which is another major advantage.

• Cloud sync and data utilization: Recently, more smartphone-connected receivers can integrate with cloud services via internet connectivity. Because coordinates obtained during positioning are automatically saved to the cloud, there is no need to return to the office and extract data via cable. Data uploaded to the cloud can be immediately shared within the company and with partner firms, enabling real-time visualization of progress. In some cases, the cloud service can perform advanced processing such as coordinate transformations, plotting onto drawings, and point cloud merging, allowing the field and the office to connect seamlessly. In disaster response, geotagged photos and point clouds collected in the field can be transmitted to headquarters via the cloud and used to support recovery planning on the same day.

• Localization support: GNSS is well-suited to earth-referenced absolute coordinates, but in practice there are situations where work needs to be done in a local coordinate system (such as a site-specific plane rectangular coordinate system). The software in the latest GNSS receivers includes a localization feature that can transform positioning results into any coordinate system by aligning them with known points on site. For example, if you measure and register a few site reference stakes or regional coordinates, measured points can thereafter be displayed and saved in those local coordinate values. This enables flexible handling of both public coordinate systems and the site’s local coordinate system, automating coordinate transformation tasks that previously required post-processing.

• All-in-one multifunctionality: Traditional surveying instruments were single-purpose, such as "only measuring angles and distances" or "only recording positions", but the latest GNSS receiver + smartphone is an all-purpose surveying tool that can perform many functions with a single device. As mentioned above, it can perform point cloud measurement, AR, and photogrammetric positioning on site without additional equipment, enabling various measurements and recordings. Furthermore, with the evolution of apps, features such as distance measurement between two points and area and volume calculations are included, allowing necessary information to be calculated on the spot even without surveying expertise. It is truly becoming an era in which "simply walking with a GNSS receiver" can meet all on-site measurement needs.

Future Outlook for GNSS Receiver Technology

Rapidly advancing GNSS receiver technology is expected to continue transforming field operations. Let's look at some future prospects.

• Further increases in satellites and frequencies: The number of satellites comprising GNSS is increasing year by year, improving positioning accuracy and stability. For example, Japan’s quasi-zenith satellite system "Michibiki" will add satellites during 2024–2025 and is expected to operate a seven-satellite configuration by 2026. This will increase the number of visible satellites over Japan, enabling more stable positioning even in mountainous areas and urban canyons with high-rise buildings. Receivers that support new frequency signals broadcast by various countries’ positioning satellites (such as GPS L5 and Galileo E5) are also becoming more widespread, reducing ionospheric errors and improving multipath resilience. Future GNSS receivers, through the full use of multiple frequencies and multiple constellations, will be able to obtain centimeter-level positioning solutions (cm level accuracy (half-inch accuracy)) even faster than at present.

• Development of augmentation signals and correction services: Error correction information, indispensable for high-precision positioning, is also being developed. In Japan, QZSS (Michibiki) already operates the Centimeter-Level Positioning Augmentation Service (CLAS), and compatible receivers can receive correction information directly from satellites even outside communication coverage to maintain cm level accuracy (half-inch accuracy) (Reference: <a href="https://qzss.go.jp/overview/services/sv06_clas.html" target="_blank">Michibiki official site | Centimeter-Level Positioning Augmentation Service</a>). Going forward, not only will such satellite-based corrections evolve, but internet-based cloud correction services will also advance, and movements by local governments and private companies to build their own GNSS reference station networks will likely become more active. In the future, an environment may be established in which correction information is available nationwide without having to provide reference stations yourself, and an era may come when starting a GNSS receiver will immediately provide centimeter-level positioning.

• Further miniaturization and integration of devices: High-precision GNSS, which has already been reduced to smartphone size, could become even more chip-integrated and built-in in the future. In recent years, some smartphones have been equipped with L1 and L5 dual-frequency GNSS chips, enabling positioning with higher accuracy than before. At present, due to constraints such as antenna performance, accuracy is on the order of tens of cm (tens of in), but in the future smartphones themselves may support RTK corrections and achieve cm level accuracy (half-inch accuracy). However, because of issues like antenna diameter and reference signals, dedicated devices will likely retain their value for full centimeter-class precision. Going forward, the boundary between smartphones and GNSS receivers may become even more blurred, and integrated devices may appear that allow users to take advantage of high-precision location information without being aware of it.

• Expansion of new applications for high-precision positioning: With advances in GNSS receivers, their range of applications is expanding. In the construction industry, the Ministry of Land, Infrastructure, Transport and Tourism-led *i-Construction* initiative is promoting ICT utilization, and machine control using GNSS-equipped construction machinery and same-day as-built management combined with drone aerial photography are becoming widespread. Going forward, combining AR glasses and GNSS for construction management and surveying by autonomous mobile robots will become increasingly realistic. In disaster response, leveraging high-precision GNSS and real-time communications will advance efforts to rapidly create and share 3D maps of affected areas. Future field personnel may come to take it for granted to work while viewing centimeter-level position information (half-inch accuracy) and design information in real time via tablets or helmet-mounted devices. High-precision GNSS receivers will continue to evolve and proliferate as the key to on-site DX (digital transformation).

Introduction to LRTK Phone and Simple Setup

Finally, we introduce one device that makes centimeter-level positioning easy to achieve: LRTK Phone (Eru Aru Tii Kē Fon). The LRTK Phone is an ultra-compact high-precision GNSS receiver that can be attached to and used with a smartphone, and is a groundbreaking product that transforms mobile devices, including the iPhone, into surveying instruments capable of cm-level positioning. Weighing approximately 165 g, it houses a high-performance antenna and a battery in a compact body about 1 cm (0.4 in) thick, and is attached to the back of the smartphone with a dedicated holder. It connects to the smartphone via Bluetooth, and positioning is controlled from a dedicated app.

Features of the LRTK Phone include, first, centimeter-level positioning accuracy (cm level accuracy (half-inch accuracy)). It supports network RTK (using the Geospatial Information Authority of Japan's Continuously Operating Reference Station network) and can perform real-time positioning with an accuracy of approximately ±2 cm (±0.8 in) in the horizontal plane and about ±4 cm (±1.6 in) in height. The obtained coordinate data includes latitude, longitude, and elevation, and can be used directly in official reference coordinate systems. It runs for about 6 hours on its built-in battery and can be charged via USB Type-C, so it can withstand long continuous surveying sessions. With a portable power bank for charging, there is no worry about running out of power on site.

Notably, it supports the CLAS satellite augmentation signals provided by Japan's quasi-zenith satellite Michibiki. This allows reception of correction information directly from the Michibiki satellites even in mountainous or other areas outside mobile communication coverage, maintaining centimeter-level accuracy (cm level accuracy (half-inch accuracy)). Because it does not depend on communication infrastructure, it provides peace of mind that accurate positioning can still be performed even if mobile networks are down due to a disaster. Furthermore, if necessary, it can be attached to a surveying pole (monopod) and used, accommodating cases where you want to rigorously measure the coordinates of a single point. By using a dedicated pole, simply setting the pole height in the app will automatically apply height correction, allowing you to carry out even professional surveying tasks with an ease not found in conventional devices. It can also connect to tablet devices such as an iPad, enabling use cases like measuring while displaying drawings on a large screen.

Despite these high specifications, installation of the LRTK Phone is very simple. Below are the basic installation steps.

• Device mounting: Attach the LRTK Phone receiver to the back of the smartphone (e.g., iPhone) using the dedicated mount. Its slim, lightweight design means it can be attached to a smartphone and carried without feeling awkward.

• Preparing the app: Download and install the dedicated "LRTK app" from the App Store. Launch the app and set up the connection with the LRTK Phone unit via Bluetooth (once paired, it will connect automatically thereafter).

• Initial setup and start of positioning: On the app, select the base station mode to use (network-based RTK for sites with a communication link, CLAS mode for locations with poor connectivity, etc.). For first-time use only, complete a simple account registration and set the coordinate system for positioning, and you'll be ready. Then start positioning and check the current position coordinates and accuracy information displayed on the smartphone screen. Within a few tens of seconds you will obtain the RTK's FIX solution (integer solution), and high-precision positioning will begin.

• Surveying Work and Data Storage: On-site, you can move to the point you want to measure while viewing the app's map screen or AR camera screen, and record a measurement point with a single tap. Recorded points can be saved with a name and attribute notes and are automatically synced to the cloud. Using the photo function, location-tagged site photos are also shared to the cloud simultaneously. After surveying is complete, you can download the data from the cloud or share it directly with stakeholders for use.

As described above, by introducing the LRTK Phone, you can bring centimeter-level positioning (half-inch accuracy) to the field with just a handheld smartphone, without preparing special surveying vehicles or large-scale equipment. Some municipalities have already introduced on-site surveying using iPhone + LRTK, successfully improving surveying efficiency and reducing costs during disaster recovery (In Fukui City, it was reported that in 2023 they adopted a smartphone surveying system and achieved rapid recording of damage and a reduction in outsourced costs). For surveying companies and those in the construction industry, switching from conventional equipment to such smart surveying devices can be expected to offer significant benefits in both labor savings and improved accuracy.

If you’re interested, please try a simple survey using LRTK Phone at least once. The once cumbersome surveying work will become surprisingly streamlined, and you should be able to experience improved productivity on site. Why not easily adopt centimeter-level high-precision positioning (cm level accuracy (half-inch accuracy)) at your site as well?