Centimeter-level surveying with just a smartphone! The on-site revolution enabled by CLAS

In recent years, an era is approaching in which centimeter-level positioning accuracy can be achieved using only a smartphone. The key to this is Japan’s satellite positioning service, “CLAS”. Traditionally, achieving centimeter-level surveying required specialized GNSS equipment or RTK base stations. However, by combining CLAS-compatible technology with smartphones, a revolution is beginning in site work efficiency and labor savings across surveying, construction, agriculture, and even municipal disaster response and infrastructure inspections. This article explains, in beginner-friendly terms, the potential of smartphone+CLAS positioning from the perspectives of accuracy, ease of use, cost, service coverage, and communication environment. It also introduces the differences from RTK and SLAS, concrete benefits and use cases on site, and finally touches on the smartphone-compatible surveying solution “LRTK” that leverages CLAS.

What is CLAS? Centimeter positioning achieved on a smartphone

CLAS (Centimeter Level Augmentation Service) is a centimeter-level positioning augmentation service provided by Japan’s Quasi-Zenith Satellite System “Michibiki” (QZSS). Simply put, it is a technology in which high-precision correction information broadcast from satellites is received by a receiver such as a smartphone and applied to GPS positioning errors, enabling centimeter-level positioning that was previously difficult. CLAS uses a technique technically called PPP-RTK: error information calculated from the national network of continuously operating reference stations is broadcast nationwide by the Michibiki satellites. Users receive this information with a CLAS-compatible receiver (a dedicated high-precision GNSS antenna/chip) and apply it to their position calculations. As a result, smartphones alone can achieve RTK-level high-precision positioning as a standalone solution.

The biggest feature of CLAS is that it is a satellite-based correction service covering the entire country. Whereas the accuracy of built-in smartphone GPS used to be at best around 5–10 m (16.4–32.8 ft), leveraging CLAS can dramatically improve standalone positioning to the level of several centimeters. Moreover, this service is provided basically for free (no usage fee to receive the satellite signal itself), and it can be used in areas without terrestrial communication infrastructure such as mountainous regions or offshore. However, as described later, using CLAS requires a compatible receiving device, and current general-purpose GPS chips built into smartphones cannot directly receive CLAS signals (L6 band). Therefore, the mainstream approach is to use a small CLAS-compatible GNSS receiver paired with the smartphone. With that in hand, a new on-site style of centimeter-level positioning using just a smartphone becomes a reality.

Differences between RTK, SLAS, and CLAS

For high-precision positioning, the long-standing methods include RTK positioning (Real-Time Kinematic) and another augmentation service from Michibiki, SLAS (Sub-meter Level Augmentation Service). Let’s summarize the main differences between CLAS and these conventional methods.

• RTK positioning: A method that corrects GNSS errors in real time between a reference station and a rover to obtain horizontal accuracy of several centimeters and vertical accuracy of several centimeters. In local RTK, the user sets up a base station at a known point and transmits correction data by radio; in network RTK (VRS methods, etc.), correction information is obtained from a distribution service via mobile communications. RTK offers fast initial fixed solutions within seconds and high accuracy, but it requires base stations and communication lines, and accuracy degrades with distance from the base station. High initial and operational costs associated with dedicated equipment and service fees have also been issues.

• SLAS (sub-meter augmentation): A free positioning augmentation service provided by Michibiki that improves standard GPS accuracy (several meters) to sub-meter level (on the order of tens of centimeters). SLAS is mainly provided on the L1 frequency and can be used with compatible receivers and some commercial GPS devices. If a smartphone supports it, location accuracy can improve somewhat, but it does not reach the centimeter levels required for surveying. SLAS is intended for applications such as car navigation and simple positioning, and is insufficient where centimeter accuracy is required.

• CLAS: As noted above, CLAS realizes several-centimeter accuracy without a base station and without the need for communications. Unlike RTK, users do not need to set up a base station or receive corrections via the Internet—they only need to receive correction signals directly from the Michibiki satellites. Anywhere in Japan, the same quality of corrections is available, enabling positioning while moving over wide areas and use on remote islands or in mountainous regions. The decisive differences from RTK are therefore “no need for a personal base station” and “not dependent on communications infrastructure.” A caveat is that CLAS requires several tens of seconds to about a minute for initial convergence, so accuracy is somewhat lower (decimeter level) immediately after starting positioning, and it takes a short time to stabilize to centimeter-level accuracy. Also, in terms of absolute precision, CLAS can be slightly inferior to an RTK fixed solution; it may result in horizontal accuracies of about 5–6 cm (2.0–2.4 in) in some cases. Still, the accuracy is sufficient for many on-site tasks, and the benefits of not needing base station setup or communications contracts are significant. Compared to SLAS, CLAS requires dedicated equipment but offers dramatically higher accuracy (more than about tenfold improvement), so they occupy different niches. Overall, RTK is the most precise and immediate option but comes with higher cost and environmental constraints; SLAS is convenient but lacks precision; CLAS fills the middle ground as a new option. Tasks that once required RTK can now in part be replaced by CLAS, and an era has begun in which the two are used complementarily depending on the situation.

Five benefits that expand with Smartphone × CLAS

Smartphone+CLAS positioning solutions offer the following major advantages on site.

• High positioning accuracy: Even with just a smartphone, you can achieve errors within several centimeters, making precision surveying and stakeout work—previously done with total stations or conventional RTK equipment—easy to perform. Vertical measurements also become possible, meeting accuracy requirements for as-built management, batter-board placement, boundary point measurements, and a wide range of other uses.

• Ease of use and rapid measurements: Positioning can be performed via intuitive operations in a smartphone app, with no difficult settings or specialist knowledge required. There is no need to carry heavy tripods or large equipment, and surveying can begin immediately upon arrival at the site. Eliminating setup time and base-station installation makes tasks that involve visiting multiple points in a short time much more efficient. With a lightweight smartphone system, one person can complete surveying tasks, helping to address labor shortages and improve safety (no need for an assistant to enter dangerous areas).

• Low-cost introduction: Compared to conventional high-precision GNSS equipment and surveying instruments, a smartphone plus a small receiver combination can significantly reduce the initial investment cost. Also, because CLAS correction signals are free, there is no monthly fee like those for private VRS correction services. Small and mid-sized companies and municipalities that previously gave up on high-precision positioning for cost reasons can now adopt it at an accessible price point.

• Consistent accuracy nationwide: CLAS covers all of Japan, so uniform accuracy can be maintained over wide work areas. For example, during long-distance road or railway construction where surveying is conducted while moving, there is no need to worry about differences in correction accuracy by area. You do not need to be conscious of distance from a base station or the boundaries of a VRS service area, making broad-area surveying and patrols smooth. If the sky is open, continuous positioning for mobile platforms (vehicle-mounted MMS or drones, etc.) is also consistently supported.

• Positioning even outside of communication coverage: While smartphones typically evoke the need for cellular signals, CLAS usage does not depend on communication environments at all. Because no Internet connection is required to receive correction data, centimeter-level positioning can continue in mountainous areas, forests, remote islands, and offshore locations where mobile communications are out of range. It can also function during disasters when communication infrastructure is down, making it a highly reliable backup method in emergencies. This peace of mind of being able to measure anytime, anywhere is a major advantage for field personnel.

CLAS use cases advancing on site

Smartphone+CLAS positioning technology is already being applied across various fields. Here are some concrete examples and effects.

• Surveying and construction sites: In civil surveying and construction, many tasks require centimeter accuracy, such as control point surveying, as-built management, and ICT construction (machine guidance). With CLAS-compatible equipment, even in places with unstable communications—like tunnel portals in mountain areas or dam construction sites—high-precision stakeout and surveying planning has become possible. Guidance for heavy equipment can also be performed accurately, contributing to improved construction quality and work efficiency. On-site surveying labor savings have progressed; there are reports of tasks that used to require two people—such as batter-board installation and cross-section surveys—being safely completed by one person.

• Infrastructure inspection and maintenance: CLAS is proving effective in infrastructure inspections for roads, railways, and bridges. Portable small receivers carried during patrol inspections can monitor displacements of tracks or pavement to the centimeter level, and during bridge or tunnel inspections, photos can be tagged with high-precision position coordinates for management. Infrastructure inspection data, which used to have vague location records, now benefits from accurate latitude/longitude and elevation embedded in photos taken with a smartphone, greatly facilitating subsequent repair planning and long-term change monitoring. CLAS’s advantage of not needing to relocate base stations is particularly useful for long infrastructures; cases have emerged in which continuous precision positioning is performed while patrolling highways.

• Agriculture (smart agriculture): High-precision positioning is essential in agriculture for autonomous tractors and drone spraying in smart agriculture. With CLAS-compatible tractor steering systems, autonomous travel across large fields with lateral deviation of only a few centimeters becomes possible, enabling overlap-free and uniform tillage and seeding. Demonstrations using Michibiki CLAS for autonomous agricultural machinery have reported results that help address labor shortages and improve operational efficiency. The ability to provide precise guidance using only satellite augmentation is a major advantage in mountainous farmland where communications are unstable. If in the future autonomous agricultural machines that anyone can operate using a smartphone or tablet become widespread, this will revolutionize labor-saving in farming.

• Municipal disaster response: Smartphone surveying + CLAS is also useful in damage surveys and recovery planning after large-scale disasters. For example, one municipality introduced a CLAS-compatible device that attaches to an iPhone for surveying landslide sites and other disaster areas. As a result, they were able to survey affected areas faster and more accurately than before, significantly reducing the time and cost of planning restoration work. Because measurements can continue even when cellular networks are disrupted, the system proved powerful for immediate post-disaster response. There have also been cases where dangerous slope measurements were conducted remotely and safely by a single worker, contributing to labor savings and safety at disaster sites. Municipalities and public agencies are increasingly adopting such smartphone surveying systems as disaster prevention tools, and the technology is期待されています (期待されています should be translated as "expected"—I will correct) expected as a new technology that leads to early recovery and accurate support.

Thus, centimeter-level positioning using CLAS is beginning to deliver results across a wide range of fields—from surveying and construction to infrastructure management, agriculture, and disaster response. For field personnel, the ability to “perform precision positioning with just a smartphone” has the potential to fundamentally change work processes, representing a revolution in both labor savings and skill enhancement.

The emergence of smartphone surveying solutions with LRTK

A concrete solution for using CLAS on a smartphone is the system called LRTK. LRTK is a surveying solution consisting of a compact, high-precision GNSS receiver integrated with a smartphone and a dedicated app, and it supports Michibiki’s CLAS signals. Simply attaching a small device that wirelessly connects to your smartphone enables centimeter-level positioning for anyone. On the dedicated app, recording survey points and taking photos can be done with a single button, and photos are automatically tagged with high-precision position and orientation information. By leveraging the smartphone camera, tasks that previously required expensive surveying equipment—such as 3D point cloud scanning and AR-based stake location confirmation—can also be performed. Acquired data can be linked with cloud services so that point clouds and photos measured on site can be shared and used immediately.

With such smartphone-compatible high-precision surveying systems, even non-specialist field personnel can perform surveying and recording themselves. The ease of “a surveying instrument that fits in your pocket,” requiring no base stations or communications, will strongly promote on-site DX (digital transformation). Municipalities that have introduced LRTK report that surveying work during disaster response has become dramatically more efficient, and that needed data collection is completed simply by “walking the site with surveying equipment.” The centimeter-level surveying made possible by the smartphone+CLAS combination is changing on-site norms right now. Make high-precision positioning more accessible—why not leverage this on-site revolution in your operations?