Centimeter-Level Positioning with CLAS Even Outside Mobile Coverage! LRTK Phone Enables One-Person Surveying

Table of Contents

• What CLAS (Centimeter Level Augmentation Service) Is

• Differences Between RTK Positioning and CLAS

• How Centimeter-Level Positioning Is Possible Even Outside Mobile Coverage

• What LRTK Phone Is

• One-Person Surveying Enabled by LRTK Phone

• Use Cases for LRTK Phone

• The Future of Simple Surveying with LRTK

• FAQ

In fields where high-precision location information is required, a new possibility—“centimeter-level positioning even outside mobile coverage”—is attracting attention. Today, by utilizing CLAS (Centimeter Level Augmentation Service) provided by Japan’s Quasi-Zenith Satellite System “Michibiki,” it is possible to achieve real-time positioning with accuracy of a few centimeters even in mountainous or remote areas where mobile phone signals do not reach. A groundbreaking device that makes CLAS easy to use is the LRTK Phone. This article explains in detail how CLAS works, how it differs from conventional RTK, and the revolution in one-person surveying that LRTK Phone brings.

What CLAS (Centimeter Level Augmentation Service) Is

First, let’s explain what CLAS is. CLAS stands for Centimeter Level Augmentation Service, a centimeter-level positioning augmentation service provided by the Quasi-Zenith Satellite System “Michibiki,” often referred to as Japan’s version of GPS. Simply put, it is a technology that dramatically improves GNSS (Global Navigation Satellite System) positioning accuracy by using correction information transmitted from satellites. While the GPS built into a typical smartphone has errors on the order of several meters, using CLAS can achieve centimeter-level positioning with errors of only a few centimeters.

The CLAS mechanism distributes positioning error information calculated from data collected by electronic reference points (GNSS reference stations) installed nationwide by the Geospatial Information Authority of Japan to the ground via the Michibiki satellites. Specifically, the errors obtained at ground reference points are sent to the quasi-zenith satellites as correction signals (transmitted in the L6 band), and the satellites broadcast these signals widely and simultaneously. If you have a CLAS-compatible receiver (a dedicated GNSS terminal), you can receive these correction signals directly and apply them to your own positioning data, enabling real-time high-precision position information.

The service’s coverage area is mainly within Japan (and surrounding regions), and it can be used as long as there is a clear view of the sky. In other words, whether in mountains or on remote islands, even in places without base stations, as long as you can catch the CLAS signal from the satellites, high-precision positioning is possible. CLAS is a government-provided service and there is no usage fee. However, as noted later, a dedicated high-precision GNSS receiver is required to use CLAS.

Differences Between RTK Positioning and CLAS

When it comes to high-precision positioning, the RTK (Real-Time Kinematic) method has long been well known. RTK uses a base station receiver at a known point and a rover receiver on the moving unit operated simultaneously, correcting relative errors with respect to the base station to achieve centimeter-level positioning. In recent years, network RTK using the Geospatial Information Authority’s network of electronic reference points (for example, the Ntrip method) has also become widespread; by using a smartphone and a communication line to receive correction information, high-precision positioning has become more accessible. However, RTK has a weakness: dependence on a communication environment. In mountainous areas or underground where mobile signals do not reach, correction information cannot be obtained via the network, and RTK positioning cannot be achieved. Also, in conventional RTK that requires provision of a dedicated base station, there are costs and labor associated with preparing equipment and personnel.

On the other hand, as described above, CLAS’s major difference is that correction data can be received directly from satellites, so no base station or communication line is required. Put simply, if you have a receiver, you can achieve centimeter-level positioning via satellite whether you are in a desert or deep in the mountains. Whereas RTK is relative positioning with a local base station, CLAS can be considered a wide-area augmentation system using a network of government-maintained reference points and satellite communications. Therefore, CLAS is a PPP-RTK method that combines RTK-level accuracy with the wide-area capability of PPP (Precise Point Positioning).

However, there are caveats with CLAS. CLAS alone does not magically improve positioning accuracy; the receiver needs multi-frequency GNSS positioning and high-performance processing. Also, initial convergence to a precise solution can take time, and there remain challenges in real-time application to fast-moving vehicles (for example, autonomous driving). For handheld or stationary surveying, though, CLAS’s advantages are well realized. In particular, the fact that high-precision positioning becomes possible in places where the internet is unavailable is significant.

How Centimeter-Level Positioning Is Possible Even Outside Mobile Coverage

So why is “centimeter-level positioning even outside mobile coverage” possible? It is due to the combination of CLAS as described above and the latest receiver devices. Being outside mobile coverage means you cannot rely on mobile networks, but with CLAS the correction information arrives via satellite communications. As long as the sky is visible, a smartphone in an area with no mobile signal can still receive position augmentation signals from satellites overhead.

To summarize the mechanism concretely:

• Previously: In mobile coverage blackspots you could not obtain RTK correction data → high-precision positioning was impossible

• Now: Even outside coverage, CLAS signals can be received directly from satellites → high-precision positioning is possible

The key to realizing this difference is a CLAS-compatible high-precision GNSS receiver. Ordinary smartphone GPS cannot receive CLAS signals, so a dedicated device like the LRTK Phone described later must be combined with the smartphone. CLAS signals are transmitted in a special frequency band called L6, and general consumer GPS receiver chips do not support it. However, by using dedicated GNSS chips or receivers designed for high-precision positioning, the L6-band signal can be incorporated and used as correction information.

For example, consider a construction site in the mountains with no mobile coverage. Usually, to perform high-precision surveying you would need to set up your own base station receiver and transmit corrections wirelessly to the rover. But with a CLAS-compatible receiver, you can complete the positioning on-site using correction information coming from the sky. In fact, a municipality in Japan (Fukui City) introduced a CLAS-compatible device that attaches to an iPhone and used it for surveying disaster-affected areas after heavy rains. This allowed them to quickly perform 3D measurements and surveys of damage in areas without communication infrastructure, speeding up recovery planning and reducing costs.

In short, because a “surveying infrastructure raining down from the sky” has been established, the era has arrived in which precise surveying can be performed by a single person anywhere.

What LRTK Phone Is

So what is the crucial device that uses CLAS, the LRTK Phone? The LRTK Phone is a compact RTK-GNSS receiver device developed by Reflexia Co., Ltd. (a startup originating from Tokyo Institute of Technology). It is an external positioning unit that attaches to a smartphone (currently iPhone and iPad) and turns the phone into a high-precision GNSS surveying instrument. Despite its pocketable compact size, it contains a high-performance antenna, a GNSS receiver engine, and a battery, and can achieve centimeter-level positioning in real time by itself.

Main features of the LRTK Phone:

• Lightweight and slim: Weighs approximately 165 g, thickness about 1 cm (0.4 in), similar in size to a smartphone case. Its portability makes it easy to carry on site without burden.

• Supports high-precision GNSS positioning: Compatible with multiple satellite positioning systems beyond GPS and receives multiple frequencies such as L1/L2. It also supports the CLAS L6 signal, providing positioning accuracy of approximately horizontal ±1〜2 cm (±0.4〜0.8 in) and vertical ±3 cm (±1.2 in) (typical smartphone GPS accuracy is about 5〜10 m (16.4〜32.8 ft)).

• Built-in battery: Rechargeable battery supports about 6 hours of continuous operation. It can be charged via USB Type-C and operated while powered by a mobile battery.

• Easy smartphone integration: Attaches with a dedicated smartphone case or mount for one-touch installation and communicates with the phone via Bluetooth or Lightning. No complicated setup is required; positioning begins automatically by launching the dedicated app.

• CLAS and network RTK support: LRTK Phone can receive Michibiki’s CLAS signals, enabling correction data acquisition in coverage-free environments. Where mobile communications are available, it can also connect to network RTK via the Ntrip protocol. It can seamlessly switch between CLAS and network RTK depending on the situation.

• Extensibility: By using the smartphone as a host, acquired position data can be uploaded to the cloud, combined with photos and AR functions, and extended in various ways. For example, you can take photos with embedded positioning data, calculate distances and areas from acquired coordinates, or even perform 3D point cloud scanning.

The latest LRTK Phone released at the end of 2023 offers an optional “Offline Starter Kit,” which includes a CLAS-compatible antenna and a dedicated pole (monopod). This makes it even easier to perform positioning in areas without mobile coverage and to set up an antenna for stable accuracy.

One-Person Surveying Enabled by LRTK Phone

The biggest impact of LRTK Phone is that it has made one-person surveying a reality. Traditionally, surveying has been performed by teams of at least two. For example, total station surveying typically required an operator and a prism holder, and RTK surveying often involved multiple people for base station setup and transporting equipment. However, with LRTK Phone, surveying tasks can be completed by a single person.

Tasks that previously required an experienced surveyor and an assistant can now be handled by a single technician holding a smartphone with LRTK Phone, contributing significantly to alleviating manpower shortages and improving operational efficiency. On site, you simply take out the smartphone with the LRTK device, point to the location you want to measure, and press a button. Latitude, longitude, and elevation are recorded with centimeter accuracy instantly, and the data are automatically saved and organized. Little specialized skill is required; the dedicated app’s simple interface is designed so anyone can operate it.

Supporting one-person surveying is the dedicated monopod (pole). While LRTK Phone can be used handheld, attaching it to the included pole enables more accurate single-point measurements. Place the tip of the pole on the ground or structure point you want to measure and press the measurement button on the phone to capture the 3D coordinates of the pole tip. The app automatically corrects for pole offset (pole length), so no complicated calculations are necessary. Combined with the monopod, LRTK Phone allows one person to walk around holding the pole and perform precise surveying points in place of the traditional staff and surveying instrument.

Thus, LRTK Phone is a powerful solution in worksites where reducing personnel is required. For example, in large-scale site quality control or disaster damage recording, surveying that previously required many people and long durations can now be completed by one person in a short time, and the data can be shared digitally immediately. The realization of one-person surveying has the potential to greatly change how fieldwork is conducted.

Use Cases for LRTK Phone

Next, here are several concrete examples of using LRTK Phone and CLAS.

• Disaster recovery site surveying: In the Fukui City case mentioned above, staff used an iPhone equipped with an LRTK Phone to survey sites affected by landslides after concentrated heavy rain. Where surveying previously required multiple people and time, introducing LRTK enabled immediate topographic measurement and calculation of collapsed soil volumes even in mountainous areas without mobile coverage. This sped up damage assessment and recovery planning while reducing personnel costs. It also reduced the time spent working on dangerous slopes, improving safety.

• As-built management at construction sites: In construction and civil engineering, surveying tasks such as setting out foundations and checking embankment as-built conditions occur frequently. At one construction site, workers carry a holstered smartphone with LRTK Phone on their waist and perform measurements as needed. Its small, lightweight form factor makes it unobtrusive to carry, serving as a “one-person, one-device surveying instrument” that allows quick measurements whenever necessary. For example, dozens of height measurements that once took half a day can be completed in a short time by one person walking around, with data shared to the cloud on the spot for quality control.

• Civil engineering for agriculture and forestry: In cases such as farmland consolidation or forest thinning planning—where precise location information is needed but calling a surveying specialist is unnecessary—LRTK is effective. In agriculture, farmers use LRTK Phone to check field elevation differences and irrigation channel gradients, collecting their own field data. In forestry, it is being used for confirming routes for service roads and measuring logging areas. Because CLAS allows stable positioning even in remote forested areas without mobile coverage, it is powerful for deep-forest work.

• Infrastructure inspection and maintenance: Accurate location recording is required for managing roads, bridges, and sewer pipes. Using LRTK Phone, inspection photos can automatically be tagged with high-precision coordinates. One local government had road patrol staff take photos of pavement damage with an LRTK-enabled smartphone; the photos recorded coordinates and timestamps, enabling precise location identification on GIS later. Tasks that used to suffer from GPS error in location identification have been dramatically streamlined.

In this way, LRTK Phone is making it easy to “measure accurately on the spot” across a wide range of fields—from construction and civil engineering to agriculture and forestry, public infrastructure management, and disaster response. The ability to obtain necessary surveying data without commissioning a specialist accelerates DX (digital transformation) on site.

The Future of Simple Surveying with LRTK

The wave of one-person surveying sparked by LRTK Phone is transforming the concept of simple surveying. Until now, even when one just wanted to “measure roughly,” you often had to compromise to meter-level accuracy without specialist equipment or personnel. But with LRTK, anyone can get centimeter-level results when needed, vastly improving the accuracy of everyday simple surveys.

For example, in preliminary site reconnaissance before construction or when preparing quick damage reports after a disaster—situations where speed is prioritized—LRTK is powerful. You can quickly capture position-tagged photos or point clouds with a smartphone and share them, making information flow between the field and the office seamless and shortening decision-making times. By creating an environment where you can “measure immediately when you want to,” LRTK will change processes where surveying was a bottleneck.

In the future, as mobile high-precision positioning devices like LRTK Phone become more widespread, each construction worker may own a personal surveying tool. That would reduce work stoppages while waiting for surveying and enable more autonomous and efficient site management—truly an era of “one person, one surveying instrument.”

Simple surveying with LRTK has great value because anyone on site—not just experts—can collect data during available spare time. As this balance of accuracy and ease continues to be refined, the way we perform measurements in many everyday scenes will surely be transformed.

FAQ

Q1. What is CLAS? How is it different from regular GPS? A. CLAS stands for Centimeter Level Augmentation Service. It is a correction information service distributed from Japan’s Michibiki satellites for high-precision positioning. Regular GPS positioning has an error of around 5〜10 m (16.4〜32.8 ft), but by using a CLAS-compatible receiver and applying correction signals from satellites, errors can be reduced to a few centimeters. In other words, CLAS enables far more precise position measurements than conventional GPS when used with dedicated equipment.

Q2. Do I need the internet or a mobile phone connection to use LRTK Phone? A. Basically, no. LRTK Phone can receive CLAS satellite signals directly, so it can achieve centimeter-level positioning even in areas outside mobile coverage. Where communication is available, network RTK correction services can also be used, but they are not required. In mountainous areas or other places without mobile signals, LRTK Phone and the satellites are sufficient for high-precision positioning.

Q3. Can I really survey alone? Can a beginner use it? A. Yes. LRTK Phone is designed for one-person surveying. The dedicated app offers intuitive operation; simply press a button at the location you want to measure and high-precision coordinates are automatically acquired. The app handles tasks that would normally require specialist knowledge, such as height correction, so even those with limited surveying experience can operate it. However, basic safety precautions—such as checking the surroundings during surveying—are still necessary.

Q4. What level of positioning accuracy can I expect? A. In practice, horizontal positioning of about 1〜2 cm (0.4〜0.8 in) and vertical positioning of about 3 cm (1.2 in) can be obtained. Accuracy depends on satellite reception conditions and the surrounding environment. With a clear view of the sky and stationary measurement, centimeter-level results are stable; under trees or in urban canyons, satellite signal multipath and attenuation can reduce accuracy.

Q5. What do I need to use LRTK Phone? A. You need the LRTK Phone device itself, a compatible smartphone (currently mainly iPhone/iPad), and the dedicated app. Attach the LRTK Phone to the smartphone and connect via Bluetooth or wired connection, then start the app to begin positioning. Measurement data can be displayed on a map in the app and saved to the cloud for sharing. No special settings or contracts are required to receive CLAS signals; corrections are applied automatically within the coverage area.

Going forward, easy-to-use high-precision positioning technologies like LRTK Phone will continue to evolve and be adopted across many fields. Experience the era when even a single person, anywhere, can obtain centimeter-level position information.