Smartphone-compatible high-precision positioning using position correction information: LRTK's labor- and wiring-saving solution

By utilizing position correction information, centimeter-level high-precision positioning has become possible even with smartphones. This is leading to labor savings・reduced wiring at construction sites and in surveying operations, and is dramatically improving on-site work efficiency.

In this article, we carefully explain why position correction information is necessary for high-precision positioning and provide a technical overview (RTK and network-based corrections, etc.). We also concretely describe how to implement RTK surveying using smartphones and the on-site benefits (reduced manpower, reduced cabling, fast startup, cloud integration). While comparing with conventional surveying instruments, we introduce practical use cases of smartphone RTK such as coordinate acquisition, point cloud measurement, as-built documentation, and AR guidance. At the end of the article, we touch on a simplified surveying solution from LRTK that leverages position correction information and present the benefits of its adoption.

Table of Contents

• What is position correction information? Why it is necessary for high-precision positioning

• How RTK positioning works and network-based correction

• Implementation and benefits of smartphone RTK surveying

• Comparison with traditional surveying instruments: What changes with smartphone RTK

• Field use cases for smartphone RTK: from coordinate measurement to point clouds and AR

• Potential for applications across multiple fields

• Simplified surveying solutions utilizing position correction information via LRTK

• FAQ

What is position correction information? Why it's necessary for high-precision positioning

First, let's clarify what "position correction information" is. In positioning using satellites (GNSS satellites such as GPS, GLONASS, and Michibiki), small errors in the satellite signals typically cause position offsets of several meters. For example, errors in the satellite's orbit or clock, and signal delays caused by the atmosphere (ionosphere and troposphere), mean that positioning accuracy when relying only on a smartphone's built-in GPS is generally about 5-10 m (16.4-32.8 ft). To reduce this error to a few centimeters, it is necessary to apply correction information to the raw data received from GNSS and refine the positioning calculations.

Position correction information is, simply put, "data used to compensate satellite positioning errors relative to an accurate reference position." On the ground, a reference station (a GNSS receiving station that serves as the reference point) with accurately known coordinates is installed. The correction information service is a system that calculates in real time the difference between the position measured at the reference station and the true, accurate position, and transmits that difference to the receiver in motion (mobile station). The mobile station (the worker's GNSS terminal) receives this correction data, cancels out the error components contained in its positioning result, and can determine its current position with an accuracy of a few centimeters (a few inches).

Traditional methods known as DGPS (augmentation using wide-area positioning systems) and RTK (real-time kinematic) have used such correction information to improve positioning accuracy. In particular, because RTK offers high accuracy, its use has expanded across a wide range of fields, such as civil engineering surveying and autonomous operation in agriculture. In Japan, the Quasi-Zenith Satellite System "Michibiki" also provides a centimeter-class positioning augmentation service (CLAS) (cm level accuracy (half-inch accuracy)), making it possible to receive correction information directly from satellites to improve positioning accuracy.

How RTK Positioning Works and Network-Based Corrections

Now, let's take a closer look at the technical mechanisms of RTK and network-based corrections.

RTK (Real-Time Kinematic) positioning is a method in which a reference station and a rover simultaneously perform GNSS positioning at two points and use the difference in their observation data to correct errors on the rover side. Because the reference station knows the "true" position without error, it can calculate in real time how the satellite signals deviate as seen from that station. That information is sent to the rover, and the rover applies it to the satellite signals it receives, reducing errors that would be several meters (several ft) when operating alone to about a few centimeters (a few in). RTK uses the precise measurement called the satellite carrier phase, which enables centimeter-level high accuracy (half-inch accuracy).

In conventional RTK operations, users had to provide their own reference station (base station) with known coordinates and transmit its observation data to the rover by radio. Installing a reference station at each site every time is time-consuming and costly.

This gave rise to an approach called network RTK. It uses correction information generated from networks of multiple reference stations that governments and companies have established nationwide, accessed via the Internet. Users do not need to install their own base station and can perform RTK positioning with just a single rover (+ a communication link).

A representative example of network RTK is the Geospatial Information Authority of Japan's electronic reference points (GEONET), which has approximately 1,300 GNSS reference stations nationwide. Based on data from these observation networks, correction information is distributed using virtual reference stations (VRS), enabling accurate positioning even at distant locations. In the private sector, telecommunications companies and surveying equipment manufacturers also provide correction data distribution services over the Internet. By simply accessing a subscribed distribution service from a smartphone or receiver to receive correction information, you can begin high-precision positioning almost anywhere in Japan.

On the other hand, as a correction method that does not rely on networks, there is the aforementioned Michibiki's CLAS. CLAS (Centimeter-Level Augmentation Service) is a service that directly provides error correction information via L6-band radio signals from Japan's Quasi-Zenith Satellite "Michibiki". Satellite orbit errors and atmospheric errors are corrected using data from the government-maintained network of reference stations, and a major advantage is that, with a compatible receiver, positioning accuracy of a few centimeters can be obtained even in mountainous areas or outside communication coverage.

However, to use CLAS you need a dedicated CLAS-compatible GNSS receiver. Because the GPS chips in ordinary smartphones and car navigation systems cannot decode the L6 signal, investment in compatible equipment is required. Also, because the correction information comes from satellites, a slight time lag has been reported when using it while moving. Even so, CLAS — which can secure stable accuracy over wide areas without relying on communications infrastructure — is a technology expected to see increasing use in various fields such as surveying, construction, and agriculture.

How to Realize Smartphone-Compatible RTK Surveying and Its Benefits

High-precision positioning that utilizes the above-mentioned position correction information has recently become feasible on smartphones. Specifically, it involves using a compact RTK-capable GNSS receiver that can be paired with a smartphone, and obtaining correction information via the smartphone’s communication functions to perform positioning. With a smartphone there is no need to carry dedicated large controllers or surveying instruments, and one person can easily perform surveying with cm level accuracy (half-inch accuracy).

There are two major requirements to realize smartphone RTK. One is a high-precision GNSS antenna receiver that supports multi-GNSS and multiple frequencies, and the other is a means of accessing a service that provides correction information. As for the former, fixed survey instruments were traditionally dominant, but recently pocket-sized GNSS devices that can be attached to a smartphone have emerged. For example, using a compact receiver such as LRTK that attaches to an iPhone/iPad, you can mount a device with an integrated antenna and battery on your smartphone and easily begin high-precision positioning. Regarding the latter correction information service, you either connect from the smartphone over the internet to an Ntrip distribution server to receive data, or, if the device supports CLAS, obtain correction signals directly from satellites.

The advantages of smartphone RTK surveying are significantly greater compared with conventional methods. First, reduced manpower. Traditionally, surveying that involves heavy equipment required multiple people and specialist operators, but with smartphone RTK, on-site workers can complete surveys single-handedly, greatly reducing the need for dedicated personnel.

Next is reduced wiring. Because the connection between a smartphone and a GNSS device can be made wirelessly via Bluetooth, cumbersome cabling is not required. Furthermore, wiring to external power supplies and radios that was necessary when installing a base station is also eliminated, shortening the time required to set up equipment and allowing high-precision positioning to begin immediately upon arrival at the site. Fast startup is also a major advantage. For example, with network RTK, after powering on the device it takes approximately several tens of seconds to about 1–2 minutes for the receiver’s solution to switch from a “Float” solution to a “Fix” solution, at which point centimeter-level positioning becomes possible. If reference station information and coordinate system settings are preset in the dedicated app, it also enables the convenience of starting measurements with a single button press.

Furthermore, one advantage of using a smartphone is cloud integration. Data obtained through positioning can be immediately synced to the cloud on site and shared in real time with colleagues in the office or stakeholders located remotely. Tasks that traditionally involved saving data to USB flash drives or SD cards and taking it back can, with smartphone RTK, be sent directly from the field, dramatically improving efficiency and speed. Dedicated apps on the smartphone also integrate functions such as map display, loading drawing data, taking photos, and entering notes, allowing work to be completed digitally without carrying paper field notebooks or drawings. By synchronizing data from the field to the cloud, same-day as-built checks and daily report creation can be carried out, among other things, enabling a major transformation of the workflow itself.

Comparison with Traditional Surveying Instruments: What Changes with Smartphone RTK

What will change—and how—when using smartphone-compatible RTK positioning solutions compared with traditional surveying instruments? We will compare from several perspectives.

• Portability and ease of handling of equipment: Conventional GNSS surveying equipment required many peripherals such as large receivers mounted on tripods, long poles, external batteries, and radios. In contrast, smartphone RTK can be completed with only a small device that attaches to a smartphone. The weight is light, around a few hundred grams, and it can be carried in a pocket, so it can be taken out immediately when needed.

• Initial deployment cost: High-precision GNSS surveying instruments are extremely expensive, and a base station plus rover set can cost several million yen. In smartphone RTK solutions, the smartphone itself can be one you already own, and the additional GNSS devices are offered at relatively low cost. Therefore, provisioning one per person becomes realistic, and even organization-wide deployment can significantly reduce costs.

• Ease of operation: With conventional equipment, on-site equipment setup and calibration and configuration of positioning software required specialized knowledge. There was also a time lag from importing measurement results into the office PC to reflecting them in drawings. With smartphone RTK, intuitive operation via a dedicated app enables a one-stop workflow from starting positioning to data sharing. It is easy for personnel without special training to use, and its responsiveness—the ability to take measurements immediately when needed—is also improved.

• Function integration and extensibility: Smartphones have a variety of functions beyond positioning, such as cameras, accelerometers, and AR display. With smartphone RTK, you can not only obtain position coordinates but also add high-precision location tags to photos, measure point cloud data on site, and verify construction accuracy with AR overlays of design drawings—tasks that used to be performed with separate devices can be handled by a single unit. Data can also be centrally managed in the cloud, and adding new features via app updates is easy.

As described above, smartphone RTK holds the potential to significantly change conventional surveying methods. However, traditional equipment also offers advantages such as stable operation in all weather conditions and robust durability. By using both appropriately according to on-site needs and actively incorporating smartphone RTK into routine surveying and measurement tasks, improvements in efficiency and productivity can be expected.

Smartphone RTK On-site Use Cases: From Coordinate Measurement to Point Clouds and AR

What becomes possible when smartphone RTK is used in the field? By combining a smartphone with high-precision positioning, a single device can handle a variety of tasks that previously required specialized equipment. Here are the main use cases.

• High-precision coordinate acquisition: On surveying sites, high-precision GNSS has been used for establishing control points and measuring coordinates of features. With smartphone RTK, you can simply hold the device over any point and press a button to obtain latitude, longitude, and elevation within an error range of a few centimeters (a few in). The recorded coordinates are automatically converted not only to the World Geodetic System (WGS84) but also to Japan’s plane rectangular coordinate system and elevations (geoid height). Point names and notes can be entered on the spot, eliminating the need to copy them into paper field notebooks and streamlining site records.

• Point cloud data measurement: By using a smartphone camera or LiDAR sensor, it is also possible to acquire local 3D point cloud data. For example, combining the LiDAR on the latest iPhones with high-precision position information from LRTK allows you to walk and scan terrain and structures to generate high-precision point clouds. Because each point is assigned absolute coordinates, they can be used for as-built management and displacement measurement. A major advantage is that tasks that used to require expensive 3D laser scanners can now be achieved with just a smartphone. The acquired point cloud can be displayed in a cloud-based 3D viewer or overlaid with design data to analyze differences.

• As-built records and quality control: Smartphone RTK is also useful for recording and surveying the as-built condition in civil engineering works. If post-construction terrain and structures are uploaded to the cloud as point clouds or coordinate sets, the office can immediately create as-built drawings and display deviations from the design as a heat map to check for excesses or deficiencies. Photos taken on site are tagged with high-precision location information, making it easy to identify on a map where each photo was taken. Distances between survey points and calculations of enclosed area and volume can be performed in the app, improving the speed and accuracy of quality control tasks.

• AR-based guidance and layout marking: Accurate position information obtained with smartphone RTK becomes an intuitive on-site work support tool when combined with AR technology. For example, projecting structural models or baseline lines from the design onto the smartphone’s AR display lets you overlay them on the real scene. Color-coded displays—green when the position is correct, red when it is misaligned—make it possible to instantly judge construction accuracy. Point the smartphone toward a pre-registered target point and arrows or guide lines will appear to navigate the operator to stake positions or hidden control points. With a stable AR display where virtual objects on the smartphone screen do not shift even while walking, even non-experts can accurately identify locations with an accuracy of a few centimeters (a few in).

As described above, a single smartphone RTK can cover a wide range of tasks from positioning to recording, analysis, and guidance. By combining these functions as needed, on-site work can be made significantly more efficient than before, and it also contributes to improved quality control and safety.

Wide-ranging Application Potential

Smartphone-compatible high-precision positioning technology is expected to find applications not only in construction and surveying but across a variety of industries.

• Construction and civil engineering: On construction sites and civil works, high-precision positioning is indispensable for tasks such as setting out reference points, as-built measurements, and machine guidance for heavy equipment. By using smartphone RTK, site supervisors and workers can perform the necessary measurements even without a surveyor on site, leading to labor savings in construction management. Even in confined sites or night work, pocket-sized smartphone RTK enables agile response.

• Infrastructure inspection and maintenance: For inspections of infrastructure such as bridges, tunnels, and roads, it is required to embed location data in photos and to record the precise locations of damage. With smartphone RTK, you can record high-precision latitude, longitude, altitude, and camera orientation simultaneously with photography, improving the accuracy and reliability of inspection reports. Advanced methods such as overlaying past inspection records with current conditions in AR for comparison are also possible.

• Surveying and GIS: In mapmaking and GIS data collection, smartphone RTK becomes a powerful tool. Traditionally, high-precision surveying required licensed surveyors and specialized equipment, but smartphone RTK enables anyone to perform simplified field surveys and capture current conditions. Tasks that used to take time—such as cadastral surveys, boundary verification, and mapping damage in disasters—can be carried out quickly, and by importing the obtained data directly into GIS systems, the time to decision-making can be greatly reduced.

• Agriculture and forestry: Centimeter-level positioning (cm level accuracy, half-inch accuracy) plays an important role in smart agriculture and precision forestry. Autosteering of farm machinery and drone spraying require high-precision position information, but traditionally RTK that requires installing a private base station or using paid correction services posed a barrier. By applying smartphone RTK technology, inexpensive and precise position data can be obtained for tasks such as field parcel measurement and recording crop growth. Also, in situations where positioning in vast forests is difficult, CLAS-compatible devices can obtain positioning even outside communication coverage, making them useful for vegetation surveys and resource management.

• Disaster prevention and response: High-precision GNSS is also useful when recording damage at disaster sites or determining drop points for relief supplies. With a smartphone RTK device, even if cellular networks are down, you can obtain accurate coordinates via CLAS and record the disaster situation with photos. Even in disaster areas where the terrain has changed, reliable positioning data can help plan recovery measures.

In this way, high-precision smartphone positioning that utilizes position correction information can be regarded as a foundational technology with promising new applications across a variety of fields. By flexibly introducing it to meet each industry's needs, it will likely lead to the development of new workflows and the creation of services.

LRTK-based simple surveying solution utilizing positioning correction information

As a concrete example of smartphone-compatible high-precision positioning, we introduce the solution LRTK, developed by a startup spun out of Tokyo Institute of Technology. LRTK (El-Ar-Tee-Kay) is a simple surveying system composed of a small RTK-GNSS receiving device that attaches to a smartphone and a dedicated app and cloud service. By using this solution, field operations that fully leverage the aforementioned smartphone RTK advantages become possible.

The LRTK device is a slim unit that can be attached to an iPhone or iPad; weighing approximately 125 g and light enough to fit in a pocket, it houses a multi‑frequency GNSS antenna and a battery. It connects to the smartphone wirelessly via Bluetooth, so no cumbersome cables are required. Satellite data captured by the device is processed in real time through the smartphone’s dedicated "LRTK" app. Correction information can also be obtained through this app, and in addition to network-type RTK, it supports receiving correction signals from Michibiki (CLAS) when out of communication range. This provides peace of mind by allowing positioning to continue even in places where mobile signals do not reach, such as mountainous areas and underground spaces.

The dedicated LRTK app includes a variety of functions such as single-point positioning, continuous positioning (track logging), geo-tagged photo capture, and AR guidance display. For example, with a single button you can measure and record the coordinates of your current position, or use continuous logging mode to survey terrain while walking. In photo mode, when you take a picture with your smartphone camera the app simultaneously records the latitude, longitude, height and shooting direction for that location, making it easy to later confirm the photo’s position on a map. Measurement data and photos can be synced directly to the cloud (LRTK Cloud) and shared with stakeholders without returning to the office.

Advanced analysis functions are also provided, such as visualizing 3D point cloud data collected on-site in the cloud and displaying measured points color-coded by comparing them with design drawings. For positioning guidance during pile driving and equipment installation, you can set a previously recorded point as the target location and perform arrow-based navigation on a smartphone screen. Because a stable AR display that prevents the model from shifting even while walking is possible, non-experts can accurately identify the target position within an error range of a few centimeters (a few inches).

LRTK is, as described above, a solution that embodies the keywords reduced manpower, reduced wiring, and fast startup. On actual sites it is increasingly used as a "one surveying instrument per person," carried in a pocket and taken out immediately when needed to perform measurements and record data. From a price standpoint it is easier to adopt compared with conventional surveying instruments, enabling even small companies or departments to readily introduce high-precision positioning. With a smartphone and LRTK, anyone on site can measure positions like a surveyor, share data instantly, and improve the quality of construction and inspections — such an era is becoming a reality.

FAQ

Q1. What is position correction information? A1. It is differential data provided by reference stations to compensate for errors in satellite positioning. Standalone GPS positioning can have errors of several meters, but using correction information can reduce those errors to a few centimeters.

Q2. What is required to perform centimeter-level positioning with a smartphone (cm level accuracy (half-inch accuracy))? A2. A high-precision, multi-frequency GNSS receiver and a means of obtaining correction information are required. Specifically, it can be realized with an RTK-capable GNSS device that can connect to a smartphone (e.g., LRTK) and either a subscription to a network-based correction service or a receiver environment that supports CLAS.

Q3. How accurate can it be? A3. It depends on the environment and satellite reception conditions, but when an RTK FIX solution (integer solution) is obtained, you can generally expect accuracy of approximately ± a few centimeters (± a few in) both horizontally and vertically. If you take multiple measurements while stationary and average them, the accuracy can sometimes be below 1 cm (below 0.4 in).

Q4. Can positioning be performed in locations where mobile phone signals do not reach? A4. Yes, it is possible. Network RTK requires a communication environment, but receivers that support Michibiki's CLAS can obtain correction information without an internet connection. Devices that support both, such as LRTK, can continue positioning via CLAS even when out of cellular coverage, so you can rely on them.

Q5. Are there any reliability or stability issues compared to conventional surveying instruments? A5. Smartphone RTK offers great convenience, but compared with dedicated equipment, its durability and stability during long continuous use can be inferior. Nevertheless, it sufficiently meets the accuracy and reliability required for typical surveying and inspection tasks, and it is increasingly being adopted in the field. Rather, automatic backups via cloud integration and the continuous improvement of features through software updates can be considered major strengths.

Q6. How much are the initial setup and running costs? A6. Initial setup costs can be kept significantly lower than for conventional surveying equipment. Because you can get started with a high-precision GNSS device and a smartphone, you can set up a centimeter-level positioning environment (cm level accuracy (half-inch accuracy)) with a budget of several hundred thousand yen per person. Fees for correction information services are free for public services, while private services typically cost several thousand to several tens of thousands of yen per month. However, solutions that include correction information, such as LRTK, have also appeared, and plans are offered that let you use them without worrying about operating costs.