High-Precision Positioning Even at Disaster Sites and in Mountainous Areas! Achieving Centimeter-Level Positioning with an Out-of-Coverage LRTK Antenna (208.2 mm Offset)

Table of Contents

• Why high-precision positioning is difficult at disaster sites and in mountainous areas

• RTK positioning and the barrier of being out of coverage

• What is an out-of-coverage LRTK antenna?

• Meaning of the 208.2 mm offset and how to set it

• QZSS (CLAS) enabling high-precision positioning out of coverage

• Examples of LRTK use at disaster sites and in mountainous areas

• Streamlining fieldwork with simple surveying using LRTK

• FAQ

Why high-precision positioning is difficult at disaster sites and in mountainous areas

Accurate location information becomes critically important at disaster sites and in remote mountainous areas. At disaster sites, it is necessary to quickly record changes to buildings and terrain, and in mountainous areas precise surveying is required for infrastructure development and investigation. However, in such locations it is often very difficult to perform high-precision positioning. One major reason is the lack of communication infrastructure.

Typically, achieving centimeter-level positioning requires advanced GNSS techniques such as RTK (Real-Time Kinematic), which depend on a stable communications environment. However, immediately after a disaster, cellular base stations may be damaged and areas may be out of coverage, and in deep mountain regions cellular signals may never have reached in the first place. It is also often difficult to bring in large surveying equipment, and there may be no manpower to set up and transport instruments. When these adverse conditions overlap, conventional methods cannot achieve high positioning accuracy, and locations may only be determined with errors of several meters at best.

RTK positioning and the barrier of being out of coverage

RTK positioning is a technique that reduces GNSS positioning errors to a few centimeters by using correction information from a reference station. Both the reference station (a receiver with known coordinates) and the rover (the field receiver) receive satellite signals, and the differences are used to correct errors and compute highly accurate positions. Normally, RTK achieves centimeter-level accuracy in horizontal and vertical dimensions. While RTK is very high-precision, it requires as a prerequisite that correction information from the reference station be continuously received.

There are two main ways to obtain correction information: setting up your own nearby reference station, or using a network-based correction service. The former involves placing equipment for a base station near the field and transmitting data wirelessly, which requires effort to erect the station and is limited by radio range. The latter uses network RTK services such as Ntrip or VRS, where the rover (the surveyor’s receiver) downloads correction data via the Internet. Network RTK is efficient because you don’t need to prepare your own base station, but its weakness is that it cannot be used where cellular networks are out of coverage because it relies on mobile networks.

In other words, in places without established communications—such as mountainous regions or disaster sites—you encounter the wall that “RTK cannot be used = high-precision positioning cannot be achieved.” Until now, obtaining centimeter-level real-time accuracy in out-of-coverage areas required either post-processing surveys (the PPK method) or abandoning the survey altogether.

What is an out-of-coverage LRTK antenna?

The out-of-coverage LRTK antenna was developed to solve the challenge of high-precision positioning in out-of-coverage environments. LRTK is a pocket-sized high-precision GNSS receiver series designed to be used in combination with smartphones (mainly iPhones), aiming to dramatically simplify field surveying tasks. Among them, the “out-of-coverage LRTK antenna” is a special antenna module that performs effectively even in environments without coverage.

Standard LRTK terminals (LRTK Phone series) support network-based RTK via cellular networks and can receive correction information through a smartphone where signals are available, enabling centimeter-level positioning. The model with the out-of-coverage LRTK antenna, however, can additionally receive centimeter-level augmentation signals directly from Japan’s Quasi-Zenith Satellite System, QZSS (known as “Michibiki”). The major feature is that simply changing the antenna makes it possible to operate out of coverage: by attaching this antenna option to a standard model, you can achieve high-precision positioning independently even in deep mountains or remote islands where cellular signals do not reach.

This out-of-coverage antenna integrates a high-sensitivity GNSS antenna and receiver that support multiple frequency bands such as L1, L2, and L6, and also includes an integrated battery. Weighing only about 125 g and with a thickness of approximately 13 mm (0.51 in), it is extremely compact and lightweight, and is attached to the back of a smartphone via a dedicated cover. Despite its pocket-sized portability, it provides all functions necessary for positioning, eliminating the need for complicated cabling or equipment setup in the field. It is also highly durable with dust- and water-resistant features, so it can be used reliably in harsh outdoor environments. Of course, in coverage areas it can still use network-based RTK as before, and it can automatically switch positioning modes according to signal conditions. Therefore, it builds reliability by allowing positioning to continue using the best available method regardless of the presence or absence of communications.

Meaning of the 208.2 mm offset and how to set it

One frequently discussed topic when using the out-of-coverage LRTK antenna is the “208.2 mm offset.” What does this number mean? In fact, when the out-of-coverage antenna is attached to a smartphone, the vertical distance from the antenna’s reference point to the bottom surface of the smartphone (the lower edge when holding the device) is approximately 208.2 mm (8.20 in). In other words, if you place the LRTK terminal directly on the ground to measure, the antenna is measuring from a position 208.2 mm (8.20 in) above the ground.

To obtain an accurate elevation, you need to subtract this antenna height in advance. The LRTK app therefore allows you to set an offset value of “208.2” mm so it automatically subtracts that height. For example, if you place the bottom of the LRTK terminal directly on a point on the ground and press the measurement button, the 208.2 mm offset is subtracted and the app returns the elevation of the ground surface at that point.

On site you may also measure using a tripod or monopod (pole); in that case you can similarly correct by entering the height from the instrument tip to the antenna. The 208.2 mm value when the out-of-coverage LRTK antenna is attached is the standard height correction amount to remember for direct attachment measurements. Setting it is simple: from the positioning settings screen in the LRTK dedicated app, just enter 208.2 in millimeters in the offset field. Once set, the app will automatically apply the height correction thereafter, allowing users to obtain accurate elevation information without conscious effort.

QZSS (CLAS) enabling high-precision positioning out of coverage

So what exactly is “Michibiki (CLAS),” the key to achieving high precision even out of coverage? Michibiki is the popular name for Japan’s Quasi-Zenith Satellite System (QZSS), a Japanese GNSS constellation launched to complement GPS. Michibiki offers multiple services, and among them the Centimeter-Level Augmentation Service (CLAS) is especially important. CLAS broadcasts error correction information, computed based on data from electronic reference points (the Geospatial Information Authority of Japan’s GNSS reference station network) across Japan, directly from Michibiki satellites to the ground.

Users with CLAS-compatible receivers (for example, the out-of-coverage LRTK antenna) can receive these correction signals from the satellite. The receiver applies those corrections to its own GNSS positioning data, enabling real-time centimeter-level positioning. The revolutionary point is that this process requires no Internet communication at all. In other words, the satellites act as “reference stations in the sky,” broadcasting error information down to receivers on the ground.

By using CLAS, users do not need to prepare their own base station nor rely on cellular networks. Anywhere within Japan, the unified correction information can be received free of charge, enabling high-precision positioning in places where communication is hard to reach, such as deep forests, remote islands, or offshore. In fact, during the 2023 Noto Peninsula earthquake, compact RTK receivers (LRTK) compatible with CLAS were used effectively in field surveys while cellular networks were cut off, demonstrating great capability. Thus, Michibiki’s CLAS is extremely useful as an emergency backup and can truly be said to break through the “out-of-coverage” barrier.

Furthermore, CLAS has advantages in terms of running costs. Traditionally, using network RTK required subscribing to private correction data distribution services with monthly fees. Receiving CLAS signals from satellites eliminates such service charges. Because no communications are used, there are no data communication fees either. Once the initial hardware is introduced, CLAS can be used thereafter without worrying about additional costs, which is a welcome point for field operations.

Examples of LRTK use at disaster sites and in mountainous areas

The combination of the out-of-coverage LRTK antenna and CLAS has made practical accuracy attainable even in sites where positioning was previously difficult. A representative example is use at disaster sites. For instance, at the Noto Peninsula earthquake site, LRTK terminals were deployed even when cellular networks were unavailable, playing a major role in surveying and recording damage. Even in emergencies where large equipment cannot be operated, small devices that fit in one hand can acquire local position information, and measured data can be shared via the cloud afterwards, speeding decision-making for recovery and damage assessment.

LRTK also performs strongly in surveys and construction sites in mountainous areas where communication infrastructure does not reach. In forest surveying, it used to be necessary to set up base stations or arrange survey teams in advance, which was time-consuming. With LRTK, a person can enter deep mountains alone and still perform accurate surveys. Tasks that were often postponed because they were “out of coverage,” such as boundary confirmation in forests, construction management at mountain tunnel portals, and topographic surveys at dam construction sites, can now be handled in real time. In fact, LRTK has been used at construction sites near tunnel entrances where cellular coverage is absent for layout marking and as-built measurements according to design.

Additionally, for operations far from the office—such as remote islands or offshore work—LRTK provides significant reassurance. For example, in disaster surveys on remote islands or buoy installation work offshore, positions that would normally be unstable can be determined to the centimeter level as long as augmentation signals from the sky are received. This is groundbreaking for marine surveys and coastal construction, dramatically improving positioning reliability.

With the emergence of the out-of-coverage LRTK antenna, the conventional wisdom that “RTK cannot be used out of coverage” is being overturned, and high-precision positioning is becoming accessible across a wide range of scenes from disaster response to remote infrastructure management.

Streamlining fieldwork with simple surveying using LRTK

LRTK brings significant benefits to the field not only for out-of-coverage capability but also as a tool for “simple surveying.” Traditionally, achieving centimeter-level surveys required expensive dedicated equipment and teams of skilled surveyors with qualifications. However, with the LRTK series, anyone in the field can easily perform surveys. No complicated settings or specialized knowledge are necessary; position measurements can be completed with the push of a button in the dedicated app.

For example, using a smartphone equipped with an LRTK terminal, you simply place the device on the point to be measured and press the button to record latitude, longitude, and elevation. Results are automatically converted on site into plane coordinates and elevation (geoid height) in the Japan Geodetic Datum and saved with notes. There is no need to transcribe numbers into a paper field book, eliminating input mistakes. Measured data can also be uploaded to the cloud with one tap and shared in real time with office staff, smoothing communication between field and office and greatly reducing post-survey data processing time.

LRTK is not only for point surveying; it also supports point cloud measurement and AR functions using the smartphone’s camera and sensors, serving as a versatile “all-purpose surveying instrument.” For example, simply walking around the site can acquire high-precision 3D point clouds for use in construction as-built management, or photos can be automatically tagged with position coordinates and orientation for records. These functions previously required specialized equipment and advanced skills, but LRTK is designed for intuitive operation within the app.

Positioning accuracy itself is more than sufficient. Under good conditions, standalone positioning can achieve accuracy within a few centimeters, and by averaging measurements over a certain time it can converge to sub-centimeter accuracy. Practically speaking, this level of accuracy meets the needs for on-site calculations of distances, areas, and volumes in civil engineering work.

Thus, simple surveying with LRTK brings a new surveying style in which each person can measure anytime, anywhere with one device. Without relying on heavy equipment, having only a smartphone allows immediate measurement when needed, dramatically improving work efficiency and productivity. The era is shifting from sharing expensive equipment among several people to each worker carrying an affordable device and conducting measurements independently. Simple surveying with LRTK will make fieldwork faster and more flexible than ever. Try this new surveying tool on site and experience its convenience and effectiveness.

FAQ

Q. How is centimeter-level positioning achieved when out of coverage? A. The out-of-coverage LRTK antenna achieves high-precision positioning by directly receiving CLAS correction information broadcast from Japan’s Michibiki satellites. Instead of receiving base station data over a cellular connection, it uses augmentation signals from the satellite. Therefore, even without Internet connectivity, real-time centimeter-level accuracy is possible.

Q. Why is the 208.2 mm offset value necessary? A. It is to correct for the antenna height. When the out-of-coverage antenna is attached to a smartphone, the antenna reference point is raised about 208.2 mm (8.20 in) above the ground. If uncorrected, the elevation will appear higher than actual, so an offset of 208.2 mm is entered to subtract that height. If you register this value in the LRTK app settings, 208.2 mm will automatically be subtracted from reported elevations.

Q. Are there additional costs to use CLAS? A. No, there are no additional costs to use CLAS itself. CLAS is a government-provided satellite positioning augmentation service and reception is free (corresponding equipment is required). There is no need to subscribe to a monthly paid correction service like network RTK, and no communication charges are incurred when out of coverage.

Q. Which smartphones and tablets are compatible with LRTK? A. Currently, the LRTK series supports iPhone and iPad. Using dedicated cases and apps, these devices support high-precision positioning. Using the latest iPhone/iPad allows you to leverage built-in sensors (such as LiDAR scanners). In particular, iPhone Pro models can utilize advanced features such as point cloud scanning and subject positioning.

Q. What level of positioning accuracy can be expected? A. It depends on conditions, but typically horizontal accuracy is about 2–3 cm (0.8–1.2 in) and vertical accuracy is about 3–5 cm (1.2–2.0 in). With good satellite visibility, even better accuracy can be achieved. For example, averaging while stationary can achieve sub-centimeter accuracy. In any case, this is far more accurate than conventional GPS standalone positioning (5–10 m (16.4–32.8 ft) error) and is adequate for common surveying and construction management tasks.

Q. How long does the battery last? A. LRTK terminals are powered by an internal battery and can perform continuous positioning in the field for several hours to about half a day. A single charge provides long operation time, so you generally don’t need to worry about running out of power during a typical workday. Charging is done via USB, and using a mobile battery makes it easy to replenish power while traveling or outdoors.