CLAS delivers high-precision positioning even outside communication coverage — supporting disaster response and mountainous areas

Have you ever wished you could measure precise positions at sites where mobile phone signals don’t reach, such as deep mountains or remote islands? Or experienced situations after a large-scale disaster where you wished you could record the locations of damage with high precision even though the communications infrastructure was down? In situations where conventional GPS struggled, a service that has recently attracted attention is Japan’s quasi-zenith satellite system “Michibiki” and its CLAS (Centimeter Level Augmentation Service). By using CLAS, you can achieve real-time, centimeter-level high-precision positioning without relying on ground communication infrastructure.

This article explains what CLAS is, how it works and its features, and how it differs from conventional RTK positioning and network-based corrections. It also introduces concrete benefits for initial disaster response surveying and use in mountainous and island areas, and at the end of the article touches on a modern method that completes on-site simple surveying by pairing the CLAS-compatible compact GNSS receiver “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 dedicated augmentation signals transmitted from Michibiki, the positioning error of ordinary GNSS positioning such as GPS—typically on the order of about 5-10 m (16.4-32.8 ft)—can be reduced to a few centimeters. In other words, provided you have a specialized receiver, a standalone GNSS receiver alone can achieve survey-grade position accuracy—an innovative arrangement.

This centimeter-level positioning service was initially developed with uses in mind such as surveying and construction ICT work (so-called *i-Construction*) and autonomous agricultural vehicles. However, because of its accuracy and its characteristic of not requiring communications, CLAS is expected to be applied widely in disaster prevention, infrastructure inspection, traffic management, and other fields. Michibiki currently operates with four satellites, but it is planned to be expanded to seven satellites by around 2025, which is expected to further improve the service’s stability and convenience.

Traditionally, centimeter-level positioning was achieved using RTK with a base station installed in the field or network RTK that acquires correction information via the Internet. These methods provide high precision but have constraints related to communications environments and equipment, sometimes making them unusable in deep mountains or disaster zones. By contrast, CLAS receives correction information directly from satellites and does not depend on ground communication infrastructure. As long as Michibiki’s signals covering all of Japan reach a location, high-precision real-time positioning can be performed anywhere—a major advantage. 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 without additional cost.

How CLAS works and its features

How CLAS works: CLAS computes error factors affecting positioning (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 these corrections to its observations to compute a high-precision position. In effect, a “virtual base station” exists on orbit, allowing the same correction data to be shared nationwide. This approach is called PPP-RTK (a fusion of Precise Point Positioning and RTK) and provides uniformly high-precision positioning over wide areas.

CLAS’s augmentation signals are designed for the region around Japan, and some error information is provided on a per-region grid basis. Therefore, full accuracy is obtained only after receiving the grid information corresponding to your current location, but in general centimeter-level positioning can be achieved within about one minute. This initial convergence time is dramatically shorter than that of the conventional satellite-only PPP method, which used to take tens of minutes.

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

• Communication infrastructure not required: Because correction information is received directly from satellites, high-precision positioning is possible in places without mobile networks or Internet connections. CLAS is effective in mountainous areas, remote islands, and regions where communications are disrupted by disasters.

• Centimeter-level accuracy: Positioning at the level of a few centimeters is possible. While conventional standalone GPS positioning produced errors on the order of 5-10 m (16.4-32.8 ft), CLAS allows you to determine your position at survey-grade accuracy.

• Wide-area, uniform service: Because the correction information covers the entire country, users can obtain nearly uniform accuracy nationwide regardless of distance from a reference station. The same mechanism provides high precision in remote areas as well as urban areas.

• Real-time positioning: Correction data are continuously transmitted from the satellites, enabling users to obtain high-precision positions in real time in the field. There is no need to wait for post-processing; positioning results can be used on the spot.

• Low-cost availability: Receiving CLAS signals is free and there are no monthly usage fees. Although initial investment in a dedicated receiver and compatible equipment is necessary, once equipped you can keep ongoing operational costs low. The simple configuration is easy for field personnel to handle, which is another advantage.

Note that CLAS requires a compatible GNSS receiver. Typical smartphone GPS chips do not support CLAS signals or carrier-phase measurements, so separate equipment with an antenna and receiver is needed. However, recently small receivers that make it easy to use CLAS with a smartphone—such as the LRTK described later—have appeared, and equipment is becoming smaller and simpler.

Differences from RTK positioning and network corrections

Before CLAS emerged, RTK-GNSS positioning was the common method to obtain centimeter-level accuracy in the field. RTK (Real-Time Kinematic) uses two GNSS receivers—a base station at a known coordinate (reference point) and a rover—and continuously sends correction information from the base station to the rover in real time. If the distance to the base station is short, initialization can be achieved in a few seconds, yielding horizontal and vertical accuracies on the order of several centimeters. However, RTK requires that signals or communications from the base station be available at all times.

In recent years, network RTK (VRS and other methods), which allows users to obtain high-precision correction information via the Internet without providing their own base station, has become widespread. This service distributes data from a network of reference stations to users via a service provider, and users perform positioning while receiving corrections (RTCM data, etc.) over a mobile data connection. Network RTK enables centimeter-class positioning over a wide area without a local base station, but it still requires a mobile communications connection.

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

• Difference in communication dependence: CLAS receives corrections via satellite and does not require a communications line. Conventional RTK requires reception of base station data via radio or Internet communications and cannot perform high-precision positioning outside communication coverage.

• Need for a base station: CLAS users do not need to set up their own base station (correction information based on the nationwide electronic reference station network is distributed via satellite). With conventional standalone RTK, users must place a nearby base station themselves, or with network RTK they must receive virtual reference station information via a service contract.

• Coverage area: CLAS functions uniformly anywhere its satellite signals reach. Standalone RTK has a practical range of several km - 20 km (several mi - 12.4 mi) from the base station (errors increase with distance), and network RTK cannot be used outside the provider’s service area.

• Time to initial solution: CLAS can achieve high-precision positioning in on the order of tens of seconds to about one minute. RTK can obtain a fixed solution in a few seconds under favorable conditions, so initialization can be faster, but CLAS’s convergence time is practically short enough for most uses.

• Positioning accuracy and stability: Both methods can ultimately achieve centimeter-level accuracy. Theoretically, RTK near a reference station may have a slight advantage in accuracy, but CLAS also limits planar position errors to a few centimeters, providing comparable accuracy for many applications.

• Equipment and cost: RTK surveying may require base station equipment, communication devices, and service fees. With CLAS, once you have a receiver, no additional communication equipment or communication costs are required, lowering the barrier to high-precision positioning. The simple setup is also easy for less experienced field staff to operate.

As described above, CLAS, which is not affected by the communications environment, is drawing attention as a complement and enhancement to conventional methods. Of course, network RTK remains very effective where communications are available, but CLAS’s ability to secure accuracy standalone “when it matters” is highly valuable.

CLAS is powerful at disaster sites

Immediately after large-scale disasters such as earthquakes or heavy rain, initial response surveying at the affected site 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, immediately after a disaster, power outages and communications failures often render surveying infrastructure unusable, and it may be too dangerous to bring in conventional large equipment.

CLAS-capable GNSS positioning can autonomously measure positions at disaster sites. For example, at a large landslide site, workers carrying GNSS receivers can sequentially measure the toe of the slide and the locations of damaged roads and bridges, plotting them on a digital map. These precise coordinates, available in real time even outside communication coverage, help to quickly grasp the overall extent of damage. If surveying results are shared immediately with relevant agencies, decisions such as which areas should be prioritized for assistance or where to start restoration work can be made more smoothly.

CLAS is also powerful for assessing isolated communities whose road and communications links have been severed. Staff who reach such communities by helicopter or on foot can record coordinates of damaged sites or clearings suitable for dropping relief supplies with CLAS, enabling accurate location information to be shared with headquarters even when communications are cut. This allows an overview of isolated areas to be formed on maps and facilitates efficient planning of relief activities and routes.

In addition, CLAS contributes to disaster response through mobility and safety. With small GNSS receivers and antennas, a single worker can survey multiple points in a short time. Where conventional total stations required setup and line-of-sight, GNSS allows broad coverage simply by carrying the equipment on foot. The need to carry heavy tripods or instruments into hazardous areas is reduced, lowering the risk of secondary disasters while surveying. Especially because CLAS has low dependence on communications and power, it gives field personnel the reassurance that they can measure on-site whenever needed, even amid the chaos immediately after a disaster.

If communication networks are restored, CLAS can be used in combination with network RTK, but communications may remain restricted depending on the damage. The ability to use CLAS under such conditions provides significant redundancy (backup) for positioning. High-precision position data collected by CLAS in the initial phase can also be used long-term for restoration planning and for recording and verification of damage.

High-precision positioning use in mountainous areas and remote islands

CLAS is a major ally for high-precision positioning even in mountain zones and remote islands far from urban areas. Previously, carrying out precision positioning in areas outside mobile coverage required installing your own base station for RTK surveying or collecting GNSS records without corrections and performing post-processing (static analysis) later. With CLAS, however, centimeter-level positions can be obtained in real time even in remote locations. Even in vast forests or remote areas, as long as Michibiki’s signals can be received, you can grasp precise latitude, longitude, and elevation on the spot.

In mountain civil engineering or forest road maintenance, just transporting surveying equipment can be a major effort. With a CLAS-capable GNSS receiver, setting out positions for heavy equipment or structures (staking out) can be done efficiently by a small crew. For example, when determining the location of valley bridge piers or tracing a mountainside service road, you do not need optical distance measurement in poor visibility terrain; as long as a clear sky view for GNSS reception is ensured, work can proceed. Even inside forests, selecting points with an open sky allows GNSS positioning, so CLAS is useful for boundary checks and topographic surveys in logging or afforestation work.

Michibiki uses a special orbit that keeps it near the zenith over Japan for long periods. Because of the satellites’ high altitude over Japan, signals are easier to receive even in complex valleys or wooded areas. This orbital design is one reason CLAS positioning is useful in mountainous environments.

High-precision positioning logs are also powerful for trail records and mountain rescue measures. Recording accurate coordinates for mountain huts and signs makes it easier to locate them later on maps and can be used in trail maintenance plans. If hikers carry portable CLAS receivers, they can record their routes with centimeter accuracy, which could help narrow search areas in the event of an accident. Similarly, in remote islands where infrastructure inspection or farmland management was difficult in the past, CLAS enables recording of accurate location data.

Japan’s quasi-zenith satellites essentially cover the whole country, from remote northern Hokkaido to southern islands, so satellite positioning augmentation service is available nationwide. Therefore, even in geographically isolated areas, if you bring CLAS-compatible equipment you can determine where a point is on Earth to centimeter accuracy. This provides peace of mind in regional disaster prevention planning and infrastructure maintenance by enabling precise location information regardless of place.

Complete simple on-site surveying with smartphone-linked CLAS-capable LRTK

As described above, CLAS’s high-precision positioning is a powerful tool in sites outside communication coverage, but using it practically requires carrying a CLAS-compatible receiver. Enter the compact GNSS receiver “LRTK.” LRTK is a very small RTK-GNSS device that supports CLAS signals, and when used in conjunction with a smartphone it provides a solution that completes simple on-site surveying.

LRTK functions like a smartphone expansion module: compact enough to fit in a pocket yet a full-fledged multi-GNSS, multi-frequency receiver that enables centimeter-level positioning. With a dedicated antenna that captures CLAS augmentation signals and other GNSS satellite signals, coordinates are displayed and recorded in real time on a smartphone app. Surveying tasks that used to require two people can be done immediately by one person with an LRTK and a smartphone. The obtained position information can be plotted on maps or drawings on the spot, or linked with photos and notes and shared to the cloud for immediate digital use.

LRTK truly shows its value in situations requiring mobility and autonomy, such as mountainous areas or immediately after disasters. Because stable high-precision positioning is possible with CLAS even outside communication coverage, surveying can proceed without worrying about network connections. With built-in battery and excellent portability, it is easy to take out and measure when needed—something unattainable with bulky conventional equipment. It is fair to say that “a smartphone becomes a universal surveying instrument.” Adoption of such CLAS-compatible simple positioning tools is expanding not only among surveying professionals but also among disaster prevention personnel and infrastructure inspection technicians who perform fieldwork using GPS.

Centimeter-level positioning that once required specialized surveying equipment and a stable communications environment is becoming dramatically more accessible thanks to CLAS and compatible compact devices. The world of high-precision positioning is being transformed by the evolution of CLAS and the devices that support it. The reassurance of obtaining high-precision location information without relying on communications networks will strongly support everything from disaster response to everyday surveying and construction management. Why not try achieving unprecedented efficiency and safety at your site using smartphone surveying with CLAS and LRTK?