Table of Contents

• Challenges of Conventional RTK Positioning and Communication Environments

• Field Examples Where High-Precision Positioning Is Required Outside Communication Coverage

• Mechanism of Satellite-Delivered RTK "Michibiki CLAS"

• Features of CLAS-Compatible RTK Receivers

• A New Positioning Style Realized by Smartphones × LRTK

• Comparison with Other Positioning Methods and When to Use Them

• Summary

• FAQ

Challenges of traditional RTK positioning and communication environments

RTK (Real-Time Kinematic) positioning is a technique that, by delivering error correction information from a base station to a rover in real time, can reduce errors of about 5-10 m (16.4-32.8 ft) in standalone positioning to a few centimeters (a few in). It is being widely used in surveying and construction management sites that require high-precision positioning, and has become a foundational technology in the DX era for applications such as drone surveying and machine guidance. However, RTK operation is premised on the assumption that communication infrastructure is always required. This is because the base station (known point) and the rover (mobile station) must perform GNSS observations simultaneously and transmit correction data (such as RTCM) via radio or cellular internet. Therefore, in environments where cellular signals do not reach, real-time RTK positioning cannot be established, and even when using a GNSS surveying instrument you will only obtain accuracy comparable to ordinary GPS (on the order of a few meters (a few ft)).

With network-type RTK that uses cellular networks (such as the VRS method), you do not need to set up your own base station, but the same point remains that it cannot be used outside of coverage. In out-of-coverage environments, as an alternative you can set up your own receiver for a base station and send correction information via radio communications using a local RTK method. However, there is the labor of transporting and installing base station equipment near the site, the limitation of the radio range (several km to about 10 km (several thousand ft to about 32,800 ft)), and the unavoidable issue that accuracy degrades as you move away from the base station. Installing such base stations itself is often difficult in remote areas, especially mountainous regions or remote islands. Also, during large-scale disasters there is a risk that cellular networks and existing reference station systems will go down, making RTK positioning itself impossible. As a result, at sites where communication cannot be secured, it is often unavoidable to give up on real-time, high-accuracy positioning and instead rely on post-processed positioning (PPK) by bringing observation data back.

Examples of field sites that require high-precision positioning without network coverage

There are many situations in environments where communication infrastructure can't be relied on—such as mountainous areas or remote islands beyond mobile phone coverage, surveying along forest work roads, upstream river investigations, and inspections of underground infrastructure—where centimeter-level positioning (half-inch-level positioning) is required. Also, in disaster zones affected by major earthquakes or heavy rain, it is necessary to accurately record damage during the initial response, but immediately after a disaster power outages and communication failures can prevent conventional RTK surveying equipment from functioning. For example, when precise as-built control is required at a construction site in mountainous terrain, or when the locations of collapsed terrain in a disaster area need to be surveyed quickly, traditionally the need to establish communication infrastructure or deploy large equipment has been a bottleneck, making it difficult to achieve high-precision positioning as desired.

This constraint that "RTK cannot be used outside communication coverage" is a major headache not only for surveyors and construction management engineers but also for disaster-response personnel and infrastructure maintenance managers. In fact, during the 2023 Noto Peninsula earthquake, communications infrastructure at disaster sites was cut off, and sufficient surveying with conventional RTK-GNSS equipment became difficult. For positioning in forests or inside tunnels, and for on-site measurements immediately after disasters, the need to perform positioning even when outside communication coverage exists throughout the field.

How the satellite-broadcast RTK Michibiki CLAS works

One new high-precision positioning method attracting attention to solve the above issues is CLAS (Centimeter-Level Augmentation Service) provided by Japan's Quasi-Zenith Satellite System "Michibiki". CLAS is a system that corrects GNSS positioning errors in real time via satellite communications, and, provided a specially compatible receiver is available, it can achieve positioning accuracy of several centimeters (a few inches) without relying on ground communications infrastructure. Errors of about 5-10 m (16.4-32.8 ft) seen in standalone GPS positioning can be reduced to on the order of a few centimeters (a few inches) when using CLAS. The service is also free of charge, and within Japan, as long as the "Michibiki" satellites are visible overhead, service can be received even in mountainous areas or at sea.

Technically, CLAS employs a method called PPP-RTK (a fusion of Precise Point Positioning and RTK). It takes various error information—such as satellite orbit errors, satellite clock errors, and ionospheric and tropospheric delays—collected by the Geospatial Information Authority of Japan’s Continuously Operating Reference Station network (about 1,300 GEONET observation sites) and broadcasts that information nationwide on the quasi-zenith satellites’ L6-band signal so users can perform correction computations on their side. In other words, an "invisible base station" exists in space, allowing common correction data to be shared anywhere in the country. This augmentation signal is designed specifically for the area around Japan, and some error information is provided on a regional grid (mesh) basis. A receiver only attains full accuracy once it has received the grid information corresponding to its position, but it generally converges to centimeter-level accuracy within 1 minute. This is a dramatic reduction compared with conventional static PPP positioning, which required tens of minutes for initial convergence.

The biggest feature of CLAS is that real-time positioning is completed without relying on terrestrial communications. Traditionally, even when performing RTK surveys deep in the mountains, preparations such as installing base station equipment in advance or finding a location with mobile reception were necessary. With a CLAS-compatible receiver, simply turning it on at the site causes correction data to begin arriving from satellites, allowing you to start positioning immediately on the spot. There is no need to install a base station or worry about coverage areas, making it possible to measure positions with almost uniform accuracy across virtually all of Japan. Furthermore, CLAS usage itself is free of charge, and once you have the compatible equipment, you can reduce running costs.

Characteristics of CLAS-compatible RTK receivers

To use CLAS, a compatible GNSS receiver is required. Typical built-in smartphone GPS chips do not support reception of CLAS signals on the L6 band or carrier-phase positioning, so you need to prepare a device equipped with a dedicated antenna and receiver circuitry. In recent years, compact receivers that can be easily used with smartphones to realize CLAS-augmented positioning—such as LRTK terminals—have appeared, and equipment miniaturization and simplification have been progressing rapidly. Here we introduce the main features common to CLAS-compatible RTK receivers.

Standalone positioning without communication infrastructure: CLAS-compatible receivers receive correction information directly from the 'Michibiki' satellite, enabling centimeter-level positioning (cm level accuracy, half-inch accuracy) as a standalone unit even at sites where cellular or radio communications cannot reach. Because it does not rely on communications infrastructure, surveying can continue stably even when networks are cut off during disasters or in remote locations where installing base stations is difficult. The ability to eliminate the previously essential "communications" and "base stations" is itself a major factor that fundamentally changes RTK surveying.

Energy-efficient and long-lasting operation: The dedicated receiver’s energy-saving design provides long battery runtimes, making it suitable for continuous surveying in the field. For example, some models can operate for over 10 hours on their internal battery, so you can survey from morning until evening without worrying about running out of charge. In the past there were GNSS surveying instruments that required external power or large-capacity batteries, but CLAS-compatible units are compact and lightweight, so you can carry them cable-free all day. They offer peace of mind by allowing operation without battery changes even in remote mountainous areas where securing power is difficult or during long work periods.

Small, lightweight, and highly portable: A major attraction of CLAS-enabled receivers is their compactness and ease of use. Small enough to fit in the palm of the hand and weighing under a few hundred grams, they allow workers to carry them at all times and "take a quick measurement when needed". The convenience of being able to obtain positioning simply by holding a smartphone-integrated receiver in one hand—without a dedicated pole or tripod—is revolutionary. They also feature a rugged, dustproof and waterproof design, so they can be used reliably even in the harsh conditions of civil engineering and construction sites. Unlike traditional fixed GNSS equipment, they impose less burden on the operator for surveying in confined or elevated locations or for tasks involving on-foot movement, and they can also be mounted on a pole and operated in the conventional manner when required.

Real-time processing and low latency: Because CLAS correction information is delivered immediately via satellite, position computations are performed on the receiver side almost in real time. As noted above, initial convergence takes a short time (tens of seconds to about one minute), but once a stable fixed solution (Fix) is obtained, delays during continuous positioning are minimal. Even when positioning while moving, high-precision positions can be recorded sequentially and results can be used on the spot without waiting for post-processing. With error corrections applied in real time, you can instantly verify as-built conditions on site or use the data for progress management, dramatically improving operational responsiveness.

A new positioning style enabled by smartphones × LRTK

An example of a smartphone equipped with an LRTK receiver. Because it can autonomously determine position on its own, solo surveying in disaster sites and mountainous areas that was previously difficult becomes possible. Another major advantage is that, being compact and lightweight, it can quickly perform positioning when needed.

In recent years, smartphone-mounted CLAS-compatible receivers such as the "LRTK Phone" have emerged and are significantly changing on-site surveying workflows. Using tools that integrate a smartphone and a GNSS receiver enables even non-specialists in positioning to easily obtain centimeter-level positional information. Below, we introduce several use cases and benefits of adopting smartphone×LRTK.

Advanced photogrammetry and record-keeping：By using an LRTK receiver attached to a smartphone and a dedicated app, you can append high-precision position coordinates (latitude, longitude, elevation) to every photo you take. This allows you to plot site photos accurately on maps or drawings, and enables alignment without control points even when creating 3D models from multiple photos (photogrammetry). For example, by combining positioning data obtained with LRTK with aerial photos of a disaster site, you can instantly build a detailed 3D point-cloud model of the damaged areas, which helps in assessing the extent of damage and calculating quantities. Even in routine inspection work, accurate positioning data can be obtained at the time of capture, dramatically improving the reliability of photo-based records.

Disaster Surveying and Emergency Response: Even in disaster areas where communication infrastructure has been severed, a CLAS-compatible smartphone positioning tool enables an individual to autonomously measure positions. At large landslide sites, operations could involve workers carrying lightweight LRTK receivers to successively measure the locations of the toe of the slide and collapse points, plotting them on a digital map. In fact, in one earthquake-affected area, local technicians used an "out-of-coverage model" of an LRTK terminal and succeeded in recording the positions of damaged structures with photographs even where radio signals did not reach. Surveying with small GNSS receivers and smartphones excels in mobility and safety, allowing rapid multi-point surveying even in hazardous sites where heavy tripods or generators cannot be brought in. Because of their low dependence on communications and power, they provide the reassurance of "being able to measure whenever you want to," and strongly support information gathering during initial response.

Fixed-point observation and monitoring: By using LRTK, fixed-point observation of remote structures and ground conditions can also be streamlined. For example, when continuously monitoring slope displacement in mountainous areas, periodically visiting the site and surveying with a smartphone equipped with LRTK allows you to obtain high-precision coordinates of the same points each time and record temporal changes. Because positioning can be performed without concern for communication conditions, shortening measurement intervals and increasing observation points is also easy. Monitoring that traditionally required large fixed sensors can be replaced by simple GNSS measurements, leading to cost reductions and labor savings. Acquired data can be shared instantly via the cloud, so changes can be understood from the office without visiting the site, aiding rapid decision-making.

In addition to these use cases, situations such as simple as-built surveys by less-experienced surveyors, position recording by municipal staff during infrastructure inspections, and high-precision positioning support during drone flights mean the range of applications for convenient surveying combining smartphones and LRTK will continue to expand. The fact that its use can be flexibly adapted on site to suit the circumstances is also one of the attractions of this approach.

Comparison with other positioning methods and how to use them

In addition to CLAS, methods that achieve centimeter-level positioning include local RTK (single-base station method), network RTK (VRS/Ntrip method), and PPK (Post-Processed Kinematic) by post-processing. Since each of these has different characteristics, it is important to select and use them according to the conditions on site. Here we outline the main differences among the major methods.

Dependence on communications: CLAS receives correction data directly from satellites, so ground communication lines are unnecessary. By contrast, local RTK requires radio communication with a base station, and network RTK requires a cellular internet connection, and they cannot function outside coverage. Therefore, at sites where there is no cellular signal, the CLAS method is overwhelmingly advantageous.

Requirement for a base station: CLAS users do not need to install their own base station (it treats the government-maintained network of electronic reference points as virtual base stations). With local RTK, the user must deploy a nearby base station, while network RTK requires a contract to receive virtual reference station information from a service provider. In remote locations where preparing base station equipment is difficult, or for work that moves between multiple sites, the base-station-free CLAS method significantly reduces effort.

Coverage and Positioning Stability: CLAS provides almost uniform accuracy anywhere in the country as long as satellite signals can be received. Standalone RTK is practical within several km (several mi) to more than ten km (more than ten mi) from the base station (errors increase with distance), and network RTK cannot be used outside its service area either. In that respect, CLAS’s advantage is that it is a wide-area, homogeneous service. In terms of accuracy, theoretically RTK positioning very close to a reference station can be slightly superior, but CLAS also keeps planar position errors to the order of a few centimeters (a few in), so it is comparable for many applications. Rather, because accuracy is uniform even between remote locations and corrections remain effective while moving, CLAS offers high operational stability in the field.

Time required for initial measurement: The RTK method can obtain a fixed solution in a few seconds when conditions are met, so initialization is rapid. CLAS takes tens of seconds to about one minute after receiving correction information to converge to full accuracy, but in practice this is a short period that does not cause significant problems. On the other hand, PPK does not produce immediate results in the field and lacks real-time capability because data processing takes time. For tasks that require real-time positioning, use CLAS or RTK; conversely, when the communication environment is poor and only data logging is possible, or when higher-precision post-processing is required, consider PPK.

Cost and equipment configuration: Local RTK requires expensive base station equipment and radio modems, and network RTK incurs annual or monthly service fees. In contrast, CLAS only requires purchasing a compatible receiver to use the augmentation signal for free, and there are no communication charges. The equipment setup is also simple and easy to carry, lowering the barriers in both initial deployment cost and operating cost. In particular, if smartphone surveying with one device per person becomes possible, it will eliminate waiting times and wasted labor costs and lead to improved productivity for the entire team.

Summary

Introduced as an RTK positioning method usable outside mobile network coverage, Michibiki CLAS has made real-time high-precision positioning possible anywhere in Japan through a revolutionary system that requires neither base stations nor mobile networks. Even in mountainous areas or in disaster zones immediately after an event, a CLAS-compatible receiver alone can perform standalone surveying with an accuracy of several centimeters (a few in). The emergence of LRTK, which allows that technology to be conveniently used on smartphones, is rapidly transforming field surveying practices. Enabling anyone to handle high-precision location information without being constrained by communication environment limitations brings tremendous value not only to surveying operations but also to disaster prevention and infrastructure management.

Especially in situations where introducing high-precision positioning has been difficult—such as infrastructure inspections in mountainous areas and remote islands, initial surveys for disaster response, and public surveying by local governments—LRTK-based smartphone surveying is delivering dramatic improvements in efficiency and safety. The burden of carrying heavy surveying equipment and communications devices is reduced, and accurate position data can be obtained on site even when specialist surveyors are absent, leading to shorter work times and fewer human errors. Because of these implementation benefits, in recent years a wide range of sites—not only surveying companies but also construction firms, municipalities, and disaster-management agencies—have begun adopting CLAS-compatible simple positioning tools. On your site as well, why not experience unprecedented productivity gains and peace of mind by using LRTK to "transform a smartphone into an all-purpose surveying instrument"?

FAQ

Q: Can CLAS positioning be used with only a smartphone? A: Currently, the GPS built into commercial smartphones alone cannot provide centimeter-level positioning using CLAS. Receiving CLAS correction signals and performing high-precision phase measurements require dedicated hardware. Therefore, you need to prepare an external CLAS-compatible GNSS receiver (e.g., an LRTK unit) that attaches to the smartphone. Once you attach the receiver and use a dedicated app, you can perform CLAS positioning on the smartphone.

Q: Do I need a communications line or service contract to use CLAS? A: No, it is not necessary. CLAS is a correction service delivered directly from the Quasi-Zenith Satellite Michibiki and does not require connection to terrestrial communications networks. Also, the CLAS signal itself is provided as part of a public service by the government and is free of charge. As long as you obtain a compatible receiver, you can operate it without additional communication fees or monthly charges.

Q: What level of positioning accuracy can be obtained using CLAS? A: Generally, CLAS positioning achieves planar position errors on the order of several centimeters (a few inches), and vertical accuracy on the order of several centimeters to several tens of centimeters (a few inches to a few feet). Experimental data in static conditions have reported that with 95% probability horizontal errors are within 6 cm (2.4 in) and vertical within 12 cm (4.7 in). This is markedly higher accuracy compared with conventional standalone positioning and is comparable to that of conventional RTK-GNSS surveying. In actual operation it can vary somewhat depending on satellite visibility and the surrounding environment, but it stably provides accuracy adequate for surveying and design applications.

Q: Does it take time for CLAS positioning to stabilize initially? A: Usually, it reaches centimeter-level positioning accuracy (cm level accuracy (half-inch accuracy)) within a few tens of seconds to about one minute after the receiver is powered on. This is the initial convergence time required for the receiver to receive the grid correction information in the CLAS signal and for the error model to be applied, but it is not a significant burden for routine surveying work. Compared with fixed-base-station RTK, it takes slightly more time to obtain the initial fix, but once a high-precision solution is obtained, the positioning results continue to be updated in real time.

Q: In what sites or applications is CLAS positioning particularly effective? A: In sites where communication infrastructure is lacking, the effectiveness of CLAS positioning stands out. CLAS shines in situations that were traditionally out of communication range and therefore difficult to achieve high-precision positioning, such as construction surveying in mountainous areas, topographic surveying of forests and farmland, infrastructure inspections on remote islands, and damage assessment immediately after disasters. Also, when surveying over large areas while moving, CLAS enables consistent positioning because its accuracy does not decrease as you move away from a base station. Conversely, in urban areas or other environments with good cellular coverage where you already subscribe to a network RTK service, conventional methods can provide adequate positioning. In short, it is effective to use them according to the situation: "use CLAS when you cannot rely on communications; use conventional RTK where a communications environment exists". Furthermore, for observations that do not require real-time results, there is the option of PPK, which post-processes recorded data. Depending on the site conditions and requirements for accuracy and immediacy, it is best to choose among the various methods, including CLAS.