Powerful Even When Out of Coverage! CLAS-Supported High-Precision Positioning for Disasters and Mountainous Areas

Have you ever wished you could measure precise positions at sites deep in the mountains or on remote islands where mobile signals don’t reach? Or experienced the need to record the exact locations of damage in areas where communication infrastructure has been severed by a major disaster? In situations where conventional GPS has struggled, a service that has recently attracted attention is Japan’s Quasi-Zenith Satellite System “Michibiki” offering CLAS (Centimeter Level Augmentation Service). By using CLAS, you can achieve real-time, centimeter-level high-precision positioning without relying on terrestrial communication infrastructure.

This article explains what CLAS is, how it works, and its features, and compares it with conventional RTK positioning and network-based corrections. It also concretely introduces the benefits of CLAS for initial disaster-response surveys and for use in mountainous and island areas. At the end, it touches on a modern approach that enables simple on-site surveying by linking a CLAS-compatible compact GNSS receiver, the LRTK, with a smartphone.

What is CLAS (Centimeter Level Augmentation Service)

CLAS (Centimeter Level Augmentation Service) is a high-precision positioning service provided via Japan’s Quasi-Zenith Satellite System, Michibiki. By receiving a dedicated augmentation signal transmitted from Michibiki, positioning errors that in typical GNSS positioning such as GPS would be on the order of 5–10 meters can be reduced to a few centimeters. In other words, with a specialized receiver, a single GNSS receiver alone can achieve survey-grade positioning accuracy — a revolutionary capability.

This centimeter-level positioning service was originally developed with applications such as surveying and ICT construction in the construction industry (so-called *i-Construction*) and autonomous agricultural machinery in mind. However, because of its accuracy and the fact that it does not require terrestrial communications, it is expected to be useful across a wide range of fields including disaster prevention, infrastructure inspection, and traffic management. Michibiki currently operates with four satellites, but plans call for expansion to a seven-satellite constellation by around 2025, which is expected to further improve the service’s stability and convenience.

Traditionally, centimeter-level positioning required establishing a base station for RTK or obtaining correction data over the internet via network RTK. While these methods provide high precision, they are constrained by communication environments and equipment, and may be unusable in deep mountains or disaster zones. In contrast, CLAS receives correction information directly from satellites, so it does not depend on terrestrial communication infrastructure. As long as Michibiki’s signals cover the area in Japan, high-precision real-time positioning is possible anywhere. Another attractive point is that CLAS usage itself is free of charge; as long as you have a compatible receiver, you can use the service with no additional cost.

How CLAS Works and Its Features

How CLAS works: CLAS computes the factors of positioning error (satellite orbit and clock errors, ionospheric and tropospheric delays, etc.) based on observation data from the electronic reference points (GNSS reference stations) that the Geospatial Information Authority of Japan has installed nationwide. These error data are embedded in a dedicated satellite signal (Michibiki’s L6 band signal) and broadcast across Japan. The user’s receiver applies corrections to its own observations to compute high-precision positions. It is as if an “invisible base station” exists in satellite orbit, allowing the same correction data to be shared nationwide. This approach is called PPP-RTK (the fusion of Precise Point Positioning and RTK) and provides uniform high-precision positioning over wide areas.

CLAS’s augmentation signals are designed for the region around Japan, and part of the error information is provided in regional grids. Therefore, full accuracy is achieved only after receiving the grid information corresponding to the current location, but in general centimeter-level position fixes can be obtained within about one minute. This initial convergence time is a dramatic reduction compared to conventional satellite-only PPP, which used to take tens of minutes.

CLAS features: The main benefits provided by CLAS are summarized below.

• No communication infrastructure required: Because corrections are received directly from satellites, high-precision positioning is achievable in locations without mobile networks or internet connectivity. This is especially effective in mountainous regions, remote islands, and areas affected by communication outages due to disasters.

• Centimeter-level accuracy: Positioning at the level of a few centimeters is possible. While conventional standalone GPS positioning had errors around 5–10 meters, CLAS can provide survey-grade accuracy for pinpointing current positions.

• Wide-area, uniform service: Augmentation information covering the entire country provides nearly uniform accuracy nationwide, without constraints related to distance from a base station. The same high precision is available in remote areas and urban settings alike.

• Real-time positioning: Since correction data are continuously transmitted from satellites, high-precision positions can be obtained in real time on site. There is no need to wait for post-processing; the positioning results can be used immediately.

• Low-cost availability: Receiving CLAS signals is free, with no monthly usage fees. Although you need a dedicated receiver and compatible equipment for initial setup, once procured, ongoing operational costs can be kept low. The simple configuration also makes it easy for field personnel to operate.

Note that CLAS requires a compatible GNSS receiver. Standard smartphone GPS chips generally do not support CLAS signals or carrier-phase measurements, so you need separate equipment with an antenna and receiver. However, compact receivers that enable easy CLAS use with a smartphone, such as the LRTK described below, have recently appeared, and device miniaturization and simplification are progressing.

Differences from RTK Positioning and Network Corrections

Before CLAS, obtaining centimeter-level positioning accuracy in the field was commonly achieved with RTK-GNSS positioning. RTK (Real-Time Kinematic) uses two GNSS receivers: a base station with a known coordinate and a rover, with the base streaming error correction information to the rover in real time. If the base station is nearby, initialization can be achieved in seconds, yielding horizontal and vertical accuracies of a few centimeters. However, RTK requires that the base station’s radio signal or communication link be continuously available.

More recently, network RTK (VRS, etc.) has become widespread, allowing users to obtain high-precision correction data over the internet without setting up their own base stations. This consists of a network of reference stations whose data are provided by a service provider; the user receives correction information (RTCM data, etc.) via a mobile data connection while positioning. Network RTK enables centimeter-level positioning over wide areas without a local base station, but it still requires a connection to a mobile communications network.

The differences between these conventional methods and CLAS can be summarized as follows:

• Communication dependency: CLAS receives corrections via satellite and does not require a communication line. Conventional RTK requires receiving base station data via radio or internet, so high-precision positioning is not possible outside communication coverage.

• Need for base stations: CLAS users do not need to install their own base stations (corrections are broadcast from satellites based on the nationwide network of electronic reference points). With standalone RTK, users must place a base station nearby or, with network RTK, obtain virtual base station information through a service subscription.

• Coverage area: CLAS functions uniformly wherever satellite signals reach. Standalone RTK is practical within several kilometers to about 20 km of the base station (errors increase with distance), and network RTK is limited to the service area of the provider.

• Time to initial fix: CLAS typically converges to high-precision positioning in tens of seconds to about a minute. RTK can sometimes achieve a fixed solution in seconds when conditions are favorable, so RTK’s initial fix can be faster, but CLAS’s convergence time is practically short enough.

• Positioning accuracy and stability: Both can ultimately achieve centimeter-level accuracy. Theoretically, RTK close to a reference station may have a slight advantage in accuracy, but CLAS also keeps planar position errors to the order of a few centimeters, making it suitable for many applications.

• Equipment and cost: RTK surveying may require base station hardware, communication devices, and service fees. CLAS requires only a compatible receiver, eliminating additional communication equipment and fees, thus lowering the barrier to high-precision positioning. Its simple configuration is also easier for less experienced field personnel to handle.

As described above, CLAS — which is not dependent on communication environments — is attracting attention as a complement and enhancement to traditional methods. Of course, where communication is available, conventional network RTK remains very effective, but the value of CLAS in being able to secure accuracy independently of communications when it matters most is significant.

CLAS Demonstrates Its Strength at Disaster Sites

Immediately after major disasters such as earthquakes or heavy rains, initial survey work in the affected areas is crucial. To accurately understand collapsed terrain and damaged facilities, it is necessary to rapidly record and measure the locations and extents of damage. However, right after a disaster, power outages and communication failures often render surveying infrastructure unusable, and sites may be too hazardous to bring in conventional large equipment.

In such cases, CLAS-capable GNSS positioning enables autonomous, standalone position measurement at disaster sites. For example, at a large landslide site, workers carrying GNSS receivers can measure the toe of the collapsed material and the locations of damaged roads and bridges sequentially, plotting them on a digital map. These precise coordinate data, obtained in real time even outside mobile coverage, help quickly grasp the overall extent of damage. If survey results are shared immediately with relevant agencies, decisions about where to prioritize support or where to begin restoration work can proceed smoothly.

CLAS is also powerful for assessing isolated settlements where roads and communications have been cut. Personnel entering by helicopter or on foot can record coordinates of damaged spots within a settlement or potential drop zones for relief supplies using CLAS, enabling accurate positional information to be shared with headquarters even in areas isolated by communication outages. This makes it possible to overview isolated regions on maps and plan efficient relief operations and search routes.

Moreover, CLAS positioning contributes to mobility and safety in disaster response. With a small GNSS receiver and antenna, a single worker can quickly survey multiple points. Where total stations required setup and line-of-sight, GNSS allows coverage simply by walking with the equipment. The need to carry heavy tripods or instruments into dangerous areas is reduced, lowering the risk of secondary accidents while surveying. CLAS’s low reliance on communication and power supplies also provides the reassurance that surveys can be performed on the spot in the chaotic period immediately following a disaster.

If communication networks are restored, CLAS can be used in combination with network RTK, but communication conditions are often still limited in disaster areas. CLAS’s availability under such circumstances is therefore a major advantage for ensuring redundancy (backup) in positioning. High-accuracy positional data collected by CLAS during the initial phase can be used long-term for restoration planning and for recording and verifying damage.

High-Precision Positioning Use in Mountainous Areas and Remote Islands

In mountain regions and on remote islands far from urban centers, CLAS is also a powerful ally for high-precision positioning. Previously, in areas outside mobile coverage, precise positioning required setting up a dedicated base station for RTK surveying or collecting GNSS records without corrections and performing post-processing later (static analysis). With CLAS, however, even in such remote locations you can obtain centimeter-accurate positions in real time. Even in vast forests or remote areas, you can determine accurate latitude, longitude, and elevation on the spot as long as Michibiki’s signals are receivable.

Transporting surveying equipment into mountainous areas or forest roads is often a major challenge. With a CLAS-compatible GNSS receiver, a small team can efficiently stake out positions for heavy machinery or structures. For example, when determining bridge pier locations in valleys or tracing work roads along slopes, there is no need to use optical distance measurement where visibility is poor; as long as the sky above is visible to receive GPS signals, work can proceed. Inside forests, selecting spots where the sky opens allows GNSS positioning, making CLAS useful for boundary checks and terrain surveys during logging or afforestation work.

Michibiki’s satellites adopt a special orbit that keeps them near zenith over Japan for extended periods. This means that even in convoluted valley terrain or places surrounded by trees, the higher satellite elevation makes it easier to receive signals. This design is one reason CLAS positioning is useful in mountainous environments.

High-precision positioning logs are also effective for recording trails and for mountain rescue measures. If mountain huts and trail markers have accurate coordinates recorded, their positions can be easily located on maps later and used in trail maintenance plans. Hikers carrying portable CLAS receivers can log their routes with centimeter-level accuracy, which could help narrow search areas in the event of an accident. Additionally, CLAS enables accurate location recording for infrastructure inspection and farmland management on remote islands and other areas where surveying was previously difficult.

Japan’s Quasi-Zenith Satellite System fundamentally covers the entire country, from remote parts of Hokkaido in the north to southern islands. Therefore, even in geographically isolated areas, if you bring CLAS-capable equipment you can determine where a point lies on Earth to centimeter precision. This capability provides reassurance in disaster planning and infrastructure maintenance by enabling precise location information regardless of place.

Complete On-Site Simple Surveying by Linking a CLAS-Compatible LRTK with a Smartphone

As described above, CLAS’s high-precision positioning is a powerful tool in areas without communication coverage, but using it requires carrying a CLAS-compatible receiver. Enter the compact GNSS receiver called the LRTK. LRTK is an ultra-compact RTK-GNSS device that supports CLAS signal reception and, when paired with a smartphone, provides a solution that enables simple on-site surveying to be completed.

LRTK functions like a smartphone expansion module: compact enough to fit in a pocket, yet it operates as a full-fledged multi-GNSS, multi-frequency receiver capable of centimeter-level positioning. Its dedicated antenna receives CLAS augmentation signals and signals from various GNSS satellites, and coordinates are displayed and recorded in real time via a smartphone app. Survey tasks that once required two people can now be performed by a single person instantly measuring and recording points with LRTK and a smartphone. The obtained position information can be plotted on maps or drawings on site, or linked with photos and notes and shared to the cloud for immediate digital use.

LRTK’s true value shines in situations requiring mobility and autonomy, such as in mountainous areas or immediately after disasters. Because CLAS provides stable high-precision positioning even outside communication coverage, surveying can proceed without worrying about network connectivity. With built-in batteries and excellent portability, it is quick to take out and start measuring whenever needed — a convenience not possible with traditional bulky equipment. It truly transforms a smartphone into a versatile surveying instrument. Adoption of such CLAS-compatible simple surveying tools is expanding across many fields, not only among surveying professionals but also among disaster-response personnel and infrastructure inspection technicians who use GPS in fieldwork.

Centimeter-level positioning that once required specialized surveying equipment and stable communications is becoming dramatically more accessible thanks to CLAS and the emergence of small devices that support it. The world of high-precision positioning is changing significantly through the evolution of CLAS and its supporting devices. The reassurance of obtaining high-precision location information without relying on communication networks will strongly support activities from disaster response to everyday surveying and construction management. Why not achieve unprecedented efficiency and safety on your site with smartphone surveying that leverages CLAS and LRTK?