Table of Contents

• Introduction

• Why RTK is needed even outside coverage

• Challenges of high-precision positioning in mountainous areas without communications infrastructure

• Positioning outside coverage using the Quasi-Zenith Satellite (QZSS, "Michibiki") and CLAS

• High-precision positioning use cases in mountainous areas

• Benefits of smartphone-linked RTK systems (LRTK)

• Conclusion

• FAQ

Introduction

Wouldn’t it be convenient if you could measure positions in real time with centimeter-level accuracy (half-inch accuracy) even at sites that are out of mobile phone coverage, such as mountainous areas? In surveying and infrastructure inspection in forests and mountain areas, conventional GPS (GNSS) alone often produces errors of several meters and is insufficient for high-precision positioning. However, in recent years, Japan’s Quasi-Zenith Satellite System (QZSS) “Michibiki” offering a centimeter-level augmentation service (CLAS), together with smartphone-linked RTK devices called “LRTK,” has made it increasingly possible to achieve positioning with errors of a few centimeters (a few inches) in mountainous areas without relying on communications infrastructure.

This article, under the theme “RTK usable even outside coverage,” explains why high-precision positioning is required in mountainous areas outside coverage, the challenges involved, and the CLAS-compatible RTK technology that is attracting attention as a solution. We also introduce actual use cases in mountain regions and, finally, detail the benefits of the smartphone-integrated RTK solution LRTK from a field perspective. Engineers and workers involved in trail maintenance, forestry, slope inspection, power line patrols, disaster response, and officials in municipalities and companies considering RTK adoption will find this useful.

Why RTK is needed even outside coverage

There are many situations in mountainous sites that require precise positional information. For example, in forest road design and surveying, accurate terrain data are required to determine the layout of curves and grades. In slope stability inspections, monitoring points must be located precisely to detect signs of landslides. In trail maintenance, accurately recording hazardous locations and signpost installation points helps with safety measures and maintenance. In these tasks, conventional GPS positioning with errors of several meters is insufficient; many situations demand centimeter-level accuracy (half-inch accuracy).

A representative technology for achieving high-precision positioning is RTK (Real Time Kinematic). RTK uses two GNSS receivers—a base station and a rover—simultaneously to correct the rover’s position relative to the reference point in real time, achieving centimeter-level accuracy (half-inch accuracy). Typically, correction data are received from the base station via mobile networks or radio, so this method strongly depends on communications infrastructure. In urban or flat areas, network RTK services (VRS or Ntrip) provide correction information within their coverage. However, in deep mountain locations, mobile signals may not reach and network RTK may be unusable, and installing your own base station is often difficult because mountainous terrain hinders radio relaying. As a result, teams have often been forced to give up on high-precision positioning when outside coverage.

Challenges of high-precision positioning in mountainous areas without communications infrastructure

The difficulty of achieving centimeter-class positioning in areas outside coverage stems from several factors. First, in mountainous regions it is harder to observe satellites than on open plains, so GNSS positioning accuracy tends to degrade. In valleys and forests, sightlines are blocked, the number of visible satellites can be insufficient, and signals may reflect off mountains or trees (multipath), increasing errors. In addition, methods for obtaining correction information for higher accuracy are limited. As noted above, RTK requires continuous reception of data from a base station, but in areas outside coverage mobile networks cannot be used, and historically cumbersome measures such as relaying with long-range radios were necessary. Installing a base station in the mountains also incurs significant labor and cost to transport equipment and secure power.

Moreover, right after a disaster in mountainous areas, communications networks themselves may be severed. When surveying damage in areas where roads and communications are cut by landslides or earthquakes, even if responders want to record positions precisely, they could not obtain correction information. In such situations, data collection needed for map creation or emergency recovery planning is also hampered. This is why a “method to perform high-precision positioning without relying on communications infrastructure” was in demand.

Positioning outside coverage using the Quasi-Zenith Satellite (QZSS, "Michibiki") and CLAS

A key solution to the above challenges is Japan’s unique satellite positioning system, the Quasi-Zenith Satellite “Michibiki,” and its centimeter-level augmentation service CLAS (Centimeter-Level Augmentation Service). Michibiki is a constellation launched to complement GPS and is designed to remain near Japan for extended periods. At least one Michibiki satellite is always operated at a high elevation angle over Japan, making signals easier to receive even in mountain valleys. This increases the opportunities to observe satellites and improves positioning stability in mountainous areas.

CLAS provided by Michibiki computes correction information from the GNSS reference station network maintained by the Ministry of Land, Infrastructure, Transport and Tourism (around 1,300 GNSS reference stations nationwide) and broadcasts it from satellites on the L6 band. In effect, it is like “RTK broadcasting error information from satellites,” and any receiver that supports CLAS can perform centimeter-class positioning based on a common accuracy standard anywhere in Japan. The greatest advantage is that it does not require individual communications lines or base stations. Even in mountainous areas outside coverage, receivers can obtain correction data directly from satellites and perform positioning, freeing users from dependence on ground infrastructure.

Positioning accuracy using CLAS varies with the environment, but it is reported to be roughly 2-3 cm (0.8-1.2 in) horizontally and about 10 cm (3.9 in) vertically in many cases. This accuracy is comparable to high-performance network RTK. Compared with RTK, satellite-broadcast wide-area corrections may be slightly inferior in reliably obtaining fixed (integer) solutions, but CLAS is still expected to be practical for many fields such as civil engineering surveying and agricultural automation. Above all, the fact that receiving the augmentation signal itself is free of charge means that once compatible equipment is introduced, it can be used without running costs. Without monthly subscription correction services or expensive specialized equipment, a CLAS-compatible receiver alone enables “centimeter-class positioning (half-inch accuracy) outside coverage with no additional cost.”

Of course, to take advantage of high-precision CLAS you need a compatible GNSS receiver. Ordinary smartphone GPS chips cannot receive CLAS signals on the L6 band, and without multi-band support (L1/L2/L5, etc.) you cannot fully realize the accuracy. Therefore, using CLAS in mountainous areas requires carrying a dedicated high-precision GNSS terminal. Traditional surveying instruments were large and expensive and not suitable for walking along trails. That is where the portable, smartphone-integrated RTK receivers emerged. They combine “satellite augmentation signals” and the “convenience of smartphones,” opening the way for anyone to enjoy centimeter-level accuracy (half-inch accuracy) even deep in the mountains.

High-precision positioning use cases in mountainous areas

CLAS-compatible GNSS that does not rely on communications infrastructure is powerful in many mountain-area applications. Here are some concrete use cases.

• Mountain civil engineering surveying and infrastructure inspections: For surveying roads and tunnels in mountainous areas, previously heavy GNSS equipment or radio relays and significant manpower were required. With a compact CLAS-compatible receiver, one person can perform on-site terrain surveys and as-built checks without worrying about mobile coverage. For example, precise alignment of distant tunnel portals or reference point surveys at dam construction sites can be carried out quickly with a small team.

• Initial disaster surveying and response: After landslides or earthquakes, rapid understanding of the damage is needed. A Michibiki+CLAS-compatible GNSS that works outside coverage allows on-site personnel to record coordinates of damaged areas alone and immediately map collapse extents and the locations of severed lifelines. In the 2024 Noto Peninsula earthquake, a civil engineering contractor that happened to have introduced a coverage-capable high-precision GNSS terminal was able to greatly assist by recording collapse sites in coverage-less disaster zones with photos. Real-time centimeter-level location data enable more accurate recovery planning and hazard area delineation.

• Forest management and forestry surveying: High-precision positioning is useful for boundary confirmation and resource surveys in vast woodlands. Forest interiors are challenging for surveying due to poor visibility, but recording survey points along forest roads or the boundaries of logging areas with a CLAS-compatible GNSS yields higher-precision forest boundary maps and logging plans. Since mobile coverage is often absent in remote forests, having a device that receives satellite augmentation signals allows new access road design and erosion control surveys to proceed without concern for communications. In the future, mounting CLAS receivers on drones could enable aerial monitoring of forest growth and health.

• Trail maintenance and environmental conservation: In high-alpine trails and park walkways, workers often need to record route conditions outside coverage. By measuring hazardous trail points or signpost locations accurately with handheld CLAS-compatible GNSS or smartphone-linked devices, they can easily plot them on maps back in the office and plan improvements. Areas that previously could only be recorded with meter-level errors can now be recorded with centimeter-level accuracy (half-inch accuracy), improving the reliability of long-term observations and environmental impact assessments.

• Farmland management and satoyama surveys: In terraced fields and valley-side farmland of mountain communities, there is a need to precisely understand parcel boundaries and irrigation channels. Even in coverage-less mountain settlements, GNSS surveying can accurately measure parcel boundaries and areas, streamlining boundary confirmation and agricultural facility planning. Municipal staff carrying simple high-precision GNSS terminals can collect wide-area field data with few people and integrate them into municipal GIS maps. This enables centimeter-level location information to be used for regional infrastructure inspections (e.g., slope faces and check dams) and identifying abandoned farmland.

• Power line patrols and equipment inspections: High-precision GNSS is useful for patrolling power line routes across mountains. Accurately recording tower locations and obstacles along routes improves maintenance planning. Even in radio-dark mountain zones, CLAS-compatible GNSS allows reconfirmation of tower coordinates or precise pinpointing of fallen trees and line breaks for reporting with centimeter-level accuracy (half-inch accuracy). This improves the efficiency and accuracy of restoration work and asset management. For utilities and railway/road authorities managing wide-area infrastructure, field-capable high-precision positioning tools directly translate into labor savings during inspections.

Benefits of smartphone-linked RTK systems (LRTK)

Figure: LRTK Phone device attached to a smartphone – LRTK is a smartphone-linked RTK solution in which a compact GNSS receiver with an integrated antenna is attached to a smartphone and operated via a dedicated app. The slim device that mounts to the smartphone’s back (weighing only a few hundred grams) contains a battery and a high-performance antenna, and by wirelessly connecting to the phone via Bluetooth, centimeter-level positioning starts. There is no need for tripods or cable routing as before; it truly functions as a “surveying instrument that fits in your pocket,” enabling handheld positioning and recording even in rugged sites.

The strength of the LRTK system is that it is both high-precision and easy to handle. The smartphone displays the current position and provides an intuitive interface for recording survey points with one tap. Technicians with limited surveying expertise can complete precise positioning tasks by following on-screen guidance. For example, if a target point’s coordinates are specified in the app, the map’s arrow and distance indications guide the user to the point. As the user approaches the target, a crosshair and “X cm remaining” guidance appear on the screen to support final fine-tuning. This system enables anyone to achieve the same accuracy without complex calculations or surveying know-how, transforming the traditionally skill-dependent surveying process.

LRTK also offers AR (augmented reality) guidance and recording by combining smartphone camera imagery—an advantage unique to LRTK. Even when physical stakes or markings cannot be placed on site, virtual markers (AR markers) can be displayed on the smartphone screen to indicate “this is the point.” For steep slopes or otherwise hazardous areas inaccessible to people, virtual stakes can be confirmed from a safe distance through the phone, and the location can be referenced later. AR can also overlay planned lines or installations on the real scene, helping users visualize the completed work and check discrepancies between design and actual conditions. These AR guidance and visualization functions are more intuitive and reduce errors compared with paper drawings or verbal instructions.

LRTK significantly contributes to labor-saving and efficiency improvements. With one smartphone RTK per person, survey tasks that previously required two-person teams can be completed solo. Waiting times for surveyors are reduced and site workflows become smoother. In practice, pile-driving tasks using RTK and AR navigation have been completed in about one-sixth the time of conventional methods in some reported cases. For construction and surveying industries facing labor shortages and municipalities managing large areas with limited staff, technologies like LRTK are a trump card that achieves both solo surveying labor savings and high precision. As veteran surveyors retire and shortages grow, there are reports that young staff can become productive after short training.

Furthermore, LRTK excels in post-installation data utilization and extensibility. Collected coordinate data and photos can be uploaded to the cloud on site and immediately shared and reviewed from office PCs. Position-tagged photos can be automatically attached with high-precision location metadata, and integration with a smartphone’s LiDAR scanner can produce image records with point cloud data. Using continuous logging, trajectories can be recorded at up to 10 Hz, which is useful for patrol path surveys and detailed logs of work routes. In addition to Michibiki’s CLAS signals, LRTK receivers also support network RTK corrections (such as Ntrip), allowing the use of internet-based corrections when in coverage. By switching to satellite augmentation outside coverage and internet corrections inside coverage, you can flexibly maintain centimeter-level accuracy (half-inch accuracy) with the optimal method. Future expansions might include linking collected data with GIS or CAD drawings, or simultaneous positioning by multiple devices with real-time cloud-based progress management.

Conclusion

Technologies that enable high-precision positioning in mountainous areas without communications infrastructure are becoming a reality. The combination of QZSS Michibiki’s CLAS and the smartphone-linked RTK “LRTK” has realized a new field style of “centimeter-level accuracy (half-inch accuracy) anywhere, for anyone.” This breaks the long-standing constraint of “cannot measure outside coverage” in forestry, civil engineering, and disaster response, providing a powerful tool to support efficient and safe operations.

Surveying and positioning tasks that previously required specialists can now be performed intuitively by field workers themselves with LRTK. Being able to measure many points alone in a short time directly leads to labor savings and speed improvements. Collected precise location data can be linked with photos and drawings for recording and sharing, smoothing report preparation and downstream data use. “RTK that works outside coverage” has the potential to greatly transform infrastructure management and disaster response practices in mountainous areas.

If your site requires high-precision positioning, consider introducing simple surveying with LRTK and AR assistance. This cutting-edge solution that leverages advanced satellite technology and smartphones can bring new efficiency and assurance to your operations.

FAQ

Q1. Can centimeter-level accuracy (half-inch accuracy) really be achieved in places without communications coverage? Yes, it is possible. Using a Michibiki Centimeter-Level Augmentation Service (CLAS)-compatible GNSS receiver, real-time centimeter-level positioning can be achieved in the field even where communications infrastructure is absent, because correction information is delivered from satellites. However, positioning is affected by surrounding terrain and trees; more open areas yield more stable high accuracy. In dense forests or deep gorges, satellite visibility is limited and positioning may take longer or produce larger errors.

Q2. What is CLAS? How is it different from conventional RTK? CLAS (Centimeter-Level Augmentation Service) is an error-correction service provided by Japan’s Quasi-Zenith Satellite System “Michibiki.” It distributes error information from satellites based on data from the national GNSS reference station network maintained by the government. Unlike conventional RTK, which requires regional base stations, CLAS delivers wide-area correction signals via satellites. RTK can achieve very high local accuracy but requires communications and base station setup, whereas CLAS offers the ease of nationwide usability with a single receiver, although it has some environmental dependencies. The LRTK system supports both CLAS and network RTK, allowing users to choose based on conditions.

Q3. Can satellites’ signals be properly received in forests and on slopes? Michibiki satellites are operated in orbits that keep them near the zenith over Japan, so their signals are often received at relatively high elevation angles even in valleys surrounded by mountains. Compared with times when only GPS satellites were available, the number of satellites receivable in mountainous areas has increased and positioning interruptions are less frequent. Nevertheless, in places with extremely poor visibility, such as dense forests, satellite signals can still weaken and accuracy will degrade. In such cases, moving to a location with a view of the sky or retrying positioning after some time is effective. LRTK also allows mounting the receiver on a pole or helmet to increase height and improve signal reception.

Q4. Do I need special qualifications or communications fees to use LRTK? No special qualifications are required. LRTK receivers use radio modules that comply with radio law regulations and can be handled by the general public (unlike traditional UHF radio RTK, no license is required). CLAS correction signals are provided free of charge, so using positioning outside coverage incurs no additional positioning fees. Licensing models for dedicated apps or cloud services depend on the provider, but generally you can start using the system once you purchase the smartphone and receiver. When using network RTK in areas with coverage, separate correction service contracts or communications charges may apply.

Q5. How can collected positioning data and photos be used and shared? LRTK systems allow uploading coordinate data and photos collected in the field to the cloud for management. For example, photos with attached point cloud data can be shared immediately with the office and viewed by all stakeholders. Recorded position data can be exported in common coordinate formats (latitude/longitude or geodetic coordinates) and imported into GIS software or CAD drawings. This makes it easy to update design drawings based on field measurements or reflect precise positions in maintenance ledgers. Even when producing paper reports, centimeter-level coordinate data make the documentation more persuasive.

Q6. Can anyone use LRTK if they have a smartphone? LRTK is designed to be intuitive for field technicians. The dedicated app has a clear interface showing the current position and direction/distance to target points, so even those uncomfortable with machinery can reach a target by following the guidance. Recording points is a one-tap operation; during positioning, accuracy and satellite tracking status are automatically checked. However, basic smartphone operation skills and an understanding of the surveying purpose are necessary. Also, confirming compatible smartphone models, OS versions, and Bluetooth settings is required before use. For first-time users, follow the provider’s manual or support for setup and test in an open outdoor location before performing actual surveys. Once accustomed, LRTK will feel far more convenient than traditional surveying instruments.