Centimeter-level accuracy (half-inch accuracy) even in mountainous areas without cellular coverage!? High-precision positioning made possible by the Quasi-Zenith Satellite System

Would you believe that you could measure your position with an error of only a few centimeters (a few in) even in a deep mountain survey site where mobile phone signals do not reach? In recent years, the Japanese Quasi-Zenith Satellite System (QZSS) "Michibiki" has attracted attention as the key to achieving this astonishing high-precision positioning. While GPS positioning—commonly used in urban and flat areas—can become difficult in mountainous regions because signals are blocked by mountains or the number of visible satellites is insufficient, achieving real-time centimeter-level accuracy has traditionally required communication with a base station via RTK positioning or similar methods, forcing users to give up on high-precision positioning outside of coverage areas. However, new positioning technologies that utilize the Quasi-Zenith Satellite are making it increasingly possible to obtain position information with centimeter-level accuracy without relying on terrestrial communications infrastructure, even in mountainous areas.

This article explains how the Quasi-Zenith Satellite "Michibiki" works and why it is particularly effective in mountainous areas, covers the basics of satellite positioning, and introduces the augmentation services (SLAS and CLAS) that support centimeter-level accuracy and how they differ from RTK. We will also consider the benefits of autonomous positioning that works outside communication coverage and examine practical survey and investigation use cases in mountainous regions. Finally, we touch on the latest method called "LRTK", which leverages Michibiki’s augmentation signals in conjunction with smartphones to easily achieve centimeter accuracy, and highlight points that lead to labor savings and improved positioning accuracy on site.

Features of the Quasi-Zenith Satellite "Michibiki" that make it strong in mountainous areas

In addition to satellite positioning systems represented by GPS, Japan operates its own satellite positioning network, the Quasi-Zenith Satellite System (QZSS). Nicknamed "Michibiki," this satellite constellation has a distinctive orbital design: it uses special orbits so that the satellites remain over Japan for extended periods. By employing elliptical orbits called quasi-zenith orbits and operating multiple satellites alternately, at least one Michibiki satellite is always positioned at a high elevation angle over Japan. For example, over Tokyo the system is designed so that one satellite is always visible at a high elevation angle of over 70 degrees. This “overhead satellite” is easier to receive even in valleys surrounded by mountains or in urban canyons, increasing the chances of positioning in locations where conventional GPS satellites (which tend to be at low elevation angles as they orbit near the equator) have difficulty delivering signals.

Currently, Michibiki operates with four satellites (three in quasi-zenith orbits plus one in geostationary orbit), providing service across Japan. Furthermore, three more satellites are planned to be launched in 2024–2025, and if a seven-satellite constellation is realized, it is expected to enable “sustained positioning” by independently combining multiple Michibiki satellites. As the number of satellites increases, the ability to receive multiple satellite signals stably at any time of day—even in mountainous regions—will improve dramatically, boosting positioning availability and reliability.

Michibiki is sometimes called the “Japanese GPS” and is used in combination with other countries’ positioning satellites such as the U.S. GPS, Russia’s GLONASS, Europe’s Galileo, and China’s BeiDou. With a compatible receiver, Michibiki’s signals can be treated as if they were additional GPS satellites and incorporated into positioning calculations. As a result, the total number of satellites visible overhead increases even in mountainous areas, significantly reducing situations where positioning cannot be obtained because no positioning satellites are visible. In addition, as described later, Michibiki not only broadcasts position signals like other GNSS, but also provides augmentation services that complement and enhance other GNSS positioning, playing an important role in both accuracy and stability.

How satellite positioning works and challenges to improving accuracy

The basic principle of satellite positioning (GNSS) is to receive radio signals transmitted from multiple satellites and determine the distances to those satellites from the signal propagation times, thereby calculating your position. At least four satellites are required to determine a position on Earth (latitude, longitude, altitude), and your current location is measured three-dimensionally based on the distances to each satellite. However, this standalone positioning (normal GNSS positioning without correction information) is subject to various sources of error. For example, delays and disturbances as radio waves pass through the atmosphere, slight errors in transmission time or satellite orbit information, and radio reflections (multipath) near the receiver typically cause ordinary GPS positioning errors of about 5–10 m (16.4–32.8 ft). In mountainous or forested areas, the satellite signal reception environment is worse than in open sky locations, and not only is satellite visibility limited, but multipath caused by trees and terrain further degrades positioning accuracy.

For car navigation and smartphone map apps, meter-level accuracy is usually acceptable, but applications that require centimeter-level accuracy (half-inch accuracy)—such as precise placement of structures, construction quality control, and boundary marker measurement—cannot be satisfied by conventional standalone positioning. Achieving such high-precision positioning requires using some kind of correction information in addition to the satellite signals. Representative methods for doing this are RTK positioning that uses ground-installed reference points, and satellite-based augmentation systems (SBAS). Japan’s Quasi-Zenith Satellite System also provides its own correction information services, which we will look at in the next section.

High-precision positioning services of the Quasi-Zenith Satellite: SLAS and CLAS

As mentioned above, the Quasi-Zenith Satellite System "Michibiki" not only functions as positioning satellites like GPS, but also provides its own positioning augmentation services that improve satellite positioning accuracy. The two main augmentation signals are SLAS and CLAS. SLAS (Sub-meter Level Augmentation Service) is a correction information service that can reduce positioning errors to below one meter, while CLAS (Centimeter-Level Augmentation Service) is an advanced augmentation service that can reduce errors to within a few centimeters.

SLAS (Sub-meter Level Augmentation Service): SLAS is positioned as Japan’s satellite-based augmentation system (SBAS). Error correction information is provided from Michibiki satellites, including the geostationary satellite, via the L1S signal. Broadcast correction values include GPS satellite orbit and clock errors and ionospheric delay corrections, and when received by a compatible receiver, positioning errors that were typically several meters are improved to within 1 m (3.3 ft) (sub-meter level). For example, a handheld GPS device with an initial error of about 5 m (16.4 ft) can reduce position offsets in simple navigation used in agriculture or logistics if it supports SLAS. SLAS does not require special receivers and can be used by existing SBAS-compatible GPS devices, so its benefits are expected to extend to a wide range of current equipment.

CLAS (Centimeter-Level Augmentation Service): CLAS is a groundbreaking service for real-time centimeter-level positioning. Correction data are generated based on error information from the Geospatial Information Authority of Japan’s continuously operating reference station network and are distributed via Michibiki on the L6 band (1.2 GHz). With a compatible receiver, a mobile unit can receive this correction signal on its own and cancel out satellite positioning errors. Traditionally, centimeter-level positioning required RTK methods that receive radio signals from base stations, but CLAS enables RTK-equivalent high-precision positioning using only satellites—essentially a “satellite-based RTK”—without the need for base stations or communication lines. CLAS-compatible devices can measure positions using a common accuracy standard anywhere in Japan, ensuring interoperability of positioning results. The accuracy when using CLAS depends on the environment, but it has been reported to be roughly horizontal errors of a few cm (a few in) and vertical errors on the order of 10 cm (3.9 in), making it promising for applications in surveying, agriculture, and autonomous driving.

Note that because CLAS achieves high accuracy, it requires dedicated receivers (multi-band GNSS support and antennas compatible with the L6 signal), and it cannot be used directly by typical smartphone-integrated GPS. Also, the correction information is generated from reference station data within Japan, so the service area is limited to Japan (although Michibiki’s radio signals themselves reach Asia and Oceania, CLAS functionality is optimized for Japan). On the other hand, receiving CLAS augmentation signals incurs no usage fee, so as long as compatible devices are available, centimeter-level positioning is possible outside communication coverage without additional cost, which is a significant advantage.

Differences from RTK positioning and network correction methods

When hearing about correction of satellite positioning errors, RTK positioning likely comes to mind first. RTK (Real-Time Kinematic) is a method in which a fixed reference station and a rover (mobile station) simultaneously observe satellite signals and remove errors by differencing the two observations. Since the reference station is installed at a known accurate coordinate, it can calculate the error amount of the satellite signals it receives. That correction information is sent to the rover via radio or the Internet, and the rover applies the correction to its raw positioning data to obtain a position result with largely canceled errors. RTK typically achieves very high accuracy—about 2–3 cm horizontally and a few centimeters vertically—in real time, and has been widely used in surveying and civil engineering. However, RTK operation requires building a communication environment (radio devices or mobile networks) to continuously distribute correction data, and accuracy degrades if the distance between the base station and the rover becomes too large (typically a practical range of a few kilometers to at most a dozen or so kilometers). Establishing one’s own base station involves initial costs and equipment transport burdens, and using commercial correction services (such as VRS) generally entails monthly fees and other running costs.

By contrast, the aforementioned CLAS does not require individual base stations like RTK. Using data from the Ministry of Land, Infrastructure, Transport and Tourism’s continuously operating reference station network (approximately 1,300 sites nationwide) as the source, CLAS broadcasts wide-area correction information simultaneously via satellite, so consistent accuracy anywhere within Japan is a major characteristic. While RTK “locally corrects errors from a nearby reference station,” CLAS can be thought of as “broadcasting error information from satellites for the entire area.” The horizontal accuracy on the order of a few centimeters is comparable to high-performance RTK and in some uses may be slightly inferior, but the convenience of achieving centimeter accuracy autonomously in mountainous areas without communication lines as long as dedicated equipment is available is invaluable. Moreover, since receiving Michibiki’s augmentation signals is free, CLAS offers long-term operational cost advantages, making it easier to adopt in budget-constrained contexts such as public surveying and municipal work.

In other words, RTK (network RTK) provides the highest level of accuracy but has dependencies on communication infrastructure and cost hurdles, while CLAS offers somewhat less accuracy but the simplicity of communication-free use and a nationwide unified positioning foundation. By choosing between or combining both approaches according to positioning needs, new solutions have emerged for obtaining positions in locations that were previously difficult.

Benefits of autonomous positioning that does not rely on communication infrastructure

In environments such as mountainous areas, remote islands, and offshore locations where mobile phone and radio communication infrastructure does not reach, traditional high-precision positioning methods could not be used. In the immediate aftermath of disasters, communication networks are often disrupted, limiting means of obtaining high-precision positional information needed for damage assessment and emergency recovery surveys. In such outside-coverage situations, the autonomous positioning provided by the Quasi-Zenith Satellite "Michibiki" is particularly powerful.

Positioning using CLAS can be completed entirely by the receiver, from receiving correction information to position computation, without relying on ground infrastructure. As long as the sky is visible, centimeter-level positioning is possible anywhere in Japan. This is a major advantage for surveying in remote mountainous areas and for initial field surveys at disaster sites where communications have not been restored. For example, measuring the extent of a large landslide blocking a road used to require cumbersome preparations such as “first installing a base station and broadcasting correction information via radio” or “relaying long-distance radio from several kilometers away if communications don’t reach.” With CLAS-compatible receivers, field teams can begin positioning immediately upon arrival without such steps, and a single operator can complete the necessary surveys in a short time.

Furthermore, positioning that does not depend on terrestrial communications is important from the perspective of disaster resilience. Augmentation signals distributed directly from satellites continue to arrive even when ground infrastructure is severed by earthquakes or floods. If municipal disaster management personnel and fire and police units have positioning equipment compatible with Michibiki CLAS, they can autonomously determine their current positions and map damage areas during emergencies, dramatically improving the accuracy and speed of initial response. Thus, the autonomous positioning technology that enables “high accuracy anywhere, anytime” is becoming indispensable for supporting safe and secure social infrastructure.

Main use cases that can be applied in mountainous areas

Here are several use cases where high-precision positioning by the Quasi-Zenith Satellite is especially valuable in mountainous and semi-mountainous regions:

• Mountain civil engineering surveying and infrastructure inspection: Surveying for mountain roads, bridges, and tunnel construction has traditionally required securing communications and significant manpower. With CLAS-compatible devices, a single person can perform terrain surveys and structure quality checks on site without carrying heavy GNSS equipment or radio relays. The ability to perform rapid, high-precision positioning with a small team is especially beneficial for tasks such as aligning tunnel portal positions or establishing control points at dam construction sites.

• Initial disaster-response surveying for disaster prevention and response: High-precision GNSS is effective for understanding terrain changes caused by landslides or earthquakes. Michibiki positioning that works outside coverage allows personnel rushing to disaster sites to record coordinates of damaged areas alone and instantly map landslide extents or the locations of severed lifelines. In disaster response, where every minute counts, receiving centimeter-level position information in real time on site enables more accurate planning of recovery operations and delineation of hazard zones.

• Forestry management and surveying in forestry operations: Satellite positioning is used for confirming boundaries and resource surveys across vast forested areas. Surveying in forests is challenging because visibility is obstructed, but recording survey points along forest roads or boundaries of logging areas with CLAS-compatible GNSS yields more accurate forest maps and logging plans. Mountain forests are often outside mobile coverage, but receiving Michibiki’s augmentation signals allows road improvement planning and erosion control surveys to proceed without worrying about communications. In the future, combining drones with augmented positioning data from above will further enhance forest resource management.

• Trail maintenance and environmental conservation: In alpine trail and natural park maintenance, workers often record positions in areas without mobile coverage. Accurately measuring locations for hazardous trail sections or signpost installations with CLAS-compatible handheld GNSS or smartphone-linked devices allows easy plotting on maps in the office later for planning improvements. Centimeter-level precision in mountainous areas—where previously only meter-level records were possible—increases the persuasiveness of long-term change observations and environmental impact assessments.

• Farmland management and regional surveys: In terraced rice fields and foothill farmland of semi-mountainous areas, there is demand to accurately locate parcel boundaries and structures. Even in mountain villages outside network coverage, GNSS surveying can accurately measure parcel boundaries and areas, streamlining boundary confirmation and irrigation planning. Municipal staff carrying simple high-precision GNSS terminals can collect extensive field data with a small team for integration into municipal GIS maps. This high-precision positional information also assists in infrastructure inspections in mountain areas (such as slope and erosion control facility location verification) and in identifying abandoned cultivated land.

New surveying method using smartphones × Quasi-Zenith Satellite: Using LRTK

With the advent of Quasi-Zenith Satellites, high-precision positioning in mountainous areas has become possible, but “how to practically use it on site” is also important. Recently, a simple surveying system called LRTK that combines smartphones with compact GNSS receivers has emerged, making it increasingly possible for anyone to carry out centimeter-accuracy surveys easily. LRTK (Lightweight RTK) is a solution composed of a “palm-sized RTK-type positioning device that integrates with a smartphone” and “dedicated apps and cloud services,” turning a commercial smartphone into a high-performance surveying instrument.

Receivers for LRTK (for example, small GNSS terminals that attach to a smartphone) are compact enough to fit in the palm of your hand and weigh only a few hundred grams. By attaching them to the back of a smartphone and connecting via Bluetooth or similar, they can receive Michibiki’s CLAS augmentation signals, and launching a dedicated app outdoors immediately starts centimeter-level positioning. There is no need to set up tripods, carry large batteries, or use radio equipment. As a true “surveying instrument that fits in your pocket,” you can walk through rugged sites and take measurements and records with one hand.

LRTK systems are characterized by high accuracy, ease of use, and cost-effectiveness. Users can record survey points with a single tap while confirming their position on the smartphone screen, so technicians with limited experience can operate them intuitively. Positioning data can be shared instantly with the office via the cloud, and can be linked with smartphone photos or LiDAR scanner functions to save image records with point-cloud data. Moreover, LRTK devices support not only Michibiki’s CLAS signals but also network RTK corrections (Ntrip, etc.), so when in coverage they can use terrestrial corrections, and when out of coverage they can rely on satellite corrections—maintaining centimeter-level positioning by switching between satellite and network corrections as appropriate.

Such smartphone-linked positioning solutions are reducing the need for the large equipment and personnel traditionally required for high-precision surveying. For the surveying and construction industries facing labor shortages and for municipalities managing large areas with limited staff, technologies like LRTK represent a trump card that achieves both labor savings and accuracy improvements. In practice, introducing smartphone-based surveying that is intuitive to operate can enable young technicians to become effective in a short time, addressing the shortage of experienced surveyors. By leveraging LRTK to maximize the benefits of the Quasi-Zenith Satellite, and applying “centimeter accuracy anywhere, by anyone” on site, surveying and civil engineering work in the coming years will surely undergo major transformation.