Choosing Augmentation Services by Coverage Area: Is Smartphone-Integrated CLAS Support the Deciding Factor?

Table of Contents

• What is Network RTK (NRTK)

• What is a Cloud-Based (PPP) Augmentation Service

• What is the QZSS Centimeter-Level Augmentation Service (CLAS)

• How to Choose an Augmentation Method by Use Case and Region

• Smartphone-Integrated Operation and Cloud Integration

• Smartphone-Integrated Positioning Solution Using LRTK

• Frequently Asked Questions

High-precision positioning augmentation services for GNSS have become increasingly important in recent years, and they are being widely implemented in fields that require centimeter-level accuracy (cm level accuracy (half-inch accuracy)), such as surveying, construction, agriculture, and autonomous driving. Even within augmentation services there are various types—<u>network RTK</u>, <u>cloud-based PPP</u>, and <u>satellite augmentation CLAS</u>—and each service differs in its available coverage area and required infrastructure (such as communications). Therefore, it is important to compare each service and choose the optimal method according to local site conditions and intended use.

Industry trends also show demand not only for providing correction data but for integrated positioning solutions and cloud connectivity; whether a service can be used in areas with unstable communications—such as mountainous regions and the open sea—is expected to be a key competitive factor. Against this backdrop, next-generation devices that are smartphone-integrated and support satellite augmentation services (CLAS) have emerged and drawn attention. Can “smartphone-integrated CLAS support” be the deciding factor when choosing an augmentation service? This article comprehensively compares the characteristics of each service and considers their advantages by coverage area and use case.

What is Network RTK (NRTK)

Network RTK (NRTK) is a method that networks multiple fixed reference stations and provides correction data as if a virtual reference station were established near the user. In Japan, the Geospatial Information Authority’s continuously operating reference station network (GEONET) forms the foundation, with a high-density observation network of about 1,300 sites (average spacing about 20 km). GNSS data observed at each continuously operating reference station are aggregated and analyzed in real time, and this data is delivered to users via the Ntrip protocol (RTCM distribution over the Internet) to realize the correction service. The user (rover) connects to the service via a mobile data connection or similar and sends its approximate position; the server generates and provides correction data corresponding to the optimal virtual reference station. This creates the effect of “as if a nearby reference station exists,” enabling centimeter-level RTK positioning even from base stations located tens of kilometers away.

The coverage area of network RTK is, in principle, the area covered by the reference station network. In Japan, GEONET almost entirely covers the country, so anywhere nationwide with mobile coverage can perform RTK positioning using public infrastructure. In practice, connecting to an NRTK service based on known point coordinates enables high-precision positioning in the public coordinate system immediately without installing a dedicated base station on site; tasks that used to require two people to set up reference points or perform surveys can now be done more efficiently by one person. Because of this convenience, NRTK is expanding beyond surveying and construction to applications such as self-localization for autonomous vehicles, autonomous operation of agricultural machinery (smart agriculture), geotag correction for drone photogrammetry, and displacement monitoring for disaster infrastructure.

メリット: NRTK leverages existing reference station infrastructure, so users only need to prepare rover equipment and a communications method to relatively easily obtain centimeter-level correction data. Thanks to the VRS approach with multiple reference stations, accuracy is maintained over long distances, and positioning results are tied to national coordinates, directly matching public survey coordinate systems. Because it is relative positioning to fixed stations, high-precision positions are obtained immediately after initialization, making it suitable for real-time control of moving objects.

デメリット: On the other hand, the requirement for an Internet connection is a limitation. Correction data must be received continuously via a communications link, so the service is unavailable in areas outside cellular coverage. Also, use often requires a contract with a private provider, so monthly fees and other costs need consideration. Furthermore, in countries or regions without a reference station network, NRTK services may not exist, so alternative methods are necessary for overseas or offshore use. Overall, where communications infrastructure is well-developed and existing service coverage is available, NRTK is the best choice in terms of accuracy and responsiveness, but note that it cannot be used in the event of communication outages or in areas not supported by the service.

What is a Cloud-Based (PPP) Augmentation Service

A correction method that is less affected by communications infrastructure or regional constraints is the cloud-based PPP (Precise Point Positioning) service. PPP improves standalone positioning accuracy without placing reference stations near the observation point by using wide-area precise orbit and clock corrections and atmospheric error models. General PPP corrects GNSS error factors globally and typically requires time for initial convergence, but it has the advantage of worldwide geographic coverage. PPP correction data are distributed “cloud-based,” i.e., over the Internet, and in some cases are also delivered globally by subscription via L-band communications satellites. Users with compatible receivers can achieve high-precision positioning even in regions without ground station infrastructure, including offshore.

Recently, Europe’s Galileo has launched a global high-accuracy service (HAS), and Japan’s Quasi-Zenith Satellite System (QZSS) is providing a service called MADOCA-PPP for Asia and Oceania. MADOCA-PPP computes satellite-induced errors from observation network data domestically and abroad and transmits correction information via QZSS L6 signals, allowing compatible receivers to use PPP-based high-precision positioning across wide areas of the Asia–Pacific region. This service began trial operation in 2022 and entered full operation in April 2024. Because it can be used for marine and overseas positioning, PPP is expected to enable centimeter-level positioning at a global scale that was previously difficult.

メリット: The strength of cloud-based PPP services is their very wide coverage. Because they do not depend on local ground reference station networks, they can be used seamlessly across national and state borders for applications that move across regions (international surveying projects, navigation for ships and aircraft, etc.). Satellite-based services can be received in mountainous or island areas lacking communications infrastructure, providing correction data literally “anywhere.” Some services claim global availability exceeding 99% uptime with centimeter-level accuracy (specific accuracy and availability depend on the service provider and the reception environment).

デメリット: Conversely, PPP’s weakness is the initial convergence time to reach high-precision positioning. Standalone PPP typically requires tens of minutes to resolve error terms and is considered unsuitable for dynamic real-time applications. Recent advances such as SSR (State Space Representation) and PPP-RTK methods that incorporate RTK principles have reduced convergence time; still, convergence tends to be several minutes slower than RTK, so applications requiring instantaneous high accuracy should be cautious. Also, many commercial global PPP services have high subscription costs, which can be a barrier to adoption. In summary, PPP is a powerful option where wide-area coverage is needed or where no communications infrastructure exists, but trade-offs regarding time-to-fix and cost must be considered.

Reference: For wide-area augmentation, SBAS (Satellite-Based Augmentation Systems: Japan’s MSAS, North America’s WAAS, Europe’s EGNOS, etc.) provide sub-meter to meter-level corrections free of charge, but their accuracy is on the order of meters to tens of centimeters and differs from the surveying-grade (centimeter-level) needs discussed here, so details are omitted.

What is the QZSS Centimeter-Level Augmentation Service (CLAS)

A highly effective solution limited to Japan is CLAS (Centimeter Level Augmentation Service) provided by the QZSS (Quasi-Zenith Satellite System). CLAS is a satellite-based correction service that adopts the PPP-RTK approach, and its greatest feature is that no base station is required. Dedicated L6D transmissions from QZSS “Michibiki,” Japan’s quasi-zenith satellites (sometimes called the Japanese GPS), continuously broadcast correction signals, and compatible receivers can receive these signals directly to correct their positions. Since the four-satellite QZSS configuration began operation in 2018, at least one Michibiki satellite is always near the zenith over Japan, enabling centimeter-level correction via satellite communications alone anywhere in the country. In experiments with CLAS-compatible receivers, extremely good results have been obtained—for example, horizontal RMS error of 36 mm (1.42 in) and a 100% fix rate in open-sky conditions—making it a revolutionary service that can dramatically improve errors from about 5–10 m (16.4–32.8 ft) in standalone positioning to high precision.

CLAS coverage is largely limited to within Japan (strictly speaking, within QZSS visibility). If the satellite signal can be received, uniform correction information is available nationwide—from mountainous regions to remote islands—without additional infrastructure, which is a major advantage. For example, in agriculture the fact that “if the satellite is visible outdoors, CLAS can be used at fields nationwide” has been well received; areas that previously had difficulty obtaining correction information, such as mid-mountain regions or places outside cellular coverage, have achieved high-precision autonomous operation. CLAS is also expected to be useful in disaster response: because CLAS can continue positioning independently even if communications networks are severed, it is useful for surveying disaster-affected areas (in fact, during the 2023 Noto Peninsula earthquake, CLAS-compatible devices were used to record conditions even in areas without cellular coverage).

Technical background: To address PPP’s issue of convergence time, CLAS uses a PPP-RTK core that incorporates RTK elements into PPP. Specifically, satellite orbit and clock errors, ionospheric and tropospheric delays are estimated at high accuracy based on data from nationwide continuously operating reference stations, and these correction data are transmitted via satellite. This fuses PPP’s global error corrections with RTK’s local corrections, resulting in much shorter convergence times while maintaining high accuracy. In practice, cases reporting “position accuracy converging within one minute” have been observed, bringing the technique to a level usable in dynamic applications.

メリット: CLAS’s biggest advantage, as noted, is that it does not depend on communications infrastructure. The assurance that satellite visibility alone suffices for standalone centimeter-level positioning—even in deep mountains or at sea—has great value on site. Because the correction signal is provided by the Cabinet Office, it is free to use, and operation without running costs is possible if compatible equipment is available. Since uniform correction quality is available across wide areas, there is no need to worry about switching between reference stations or regional coordinate system discrepancies. CLAS is being used in autonomous steering of agricultural machinery and machine guidance for construction equipment, with measured accuracies of a few centimeters comparable to existing RTK services and good repeatability.

デメリット: There are, however, several challenges. First, CLAS requires compatible receivers and antennas. Conventional GNSS receivers cannot receive CLAS L6 signals, and compatible equipment was limited. However, in recent years domestic manufacturers have released CLAS-capable chips and devices, and small receivers that attach to smartphones have become commercially available (described later). Second, reception depends on the radio environment. Open-sky environments with many visible satellites can achieve RTK-equivalent fix rates, but maintaining a fix is difficult in environments surrounded by obstructions. For example, if satellite visibility is partially blocked by forest or urban canyons, CLAS fix solutions may drop to float, and in situations where visibility is completely lost (e.g., under eaves or deep shelters) it may revert to standalone positioning. Generally, RTK tends to maintain fixes more easily in the same environment, so in urban or heavily obstructed sites, combining CLAS with NRTK or inertial navigation as a supplement is desirable. Finally, note that CLAS’s service area is Japan-only. Other countries are developing similar satellite augmentation systems (e.g., Galileo HAS), but at present CLAS is a powerful tool primarily for domestic use.

How to Choose an Augmentation Method by Use Case and Region

We have reviewed the main augmentation services (NRTK, cloud PPP, CLAS). How should these be selected and operated in the field? Below is an organization by use case and regional conditions.

• Urban areas / regions with good communications: In sites with urban infrastructure and easy cellular connectivity, network RTK (NRTK) is the first choice. By connecting to existing services the rover alone can immediately achieve centimeter accuracy, and accuracy is stable. Especially for public works where surveying in the national coordinate system is required, NRTK’s corrections are directly tied to public coordinates, reducing post-processing. However, in severely obstructed areas such as urban canyons RTK solutions can also be unstable; in tunnels or indoors GNSS positioning itself is impossible and IMS (inertial navigation) or known-point methods must be used.

• Suburban / mountainous / infrastructure-poor regions: In fields such as remote mountains, islands, or immediately after disasters where communications infrastructure is unavailable, CLAS’s advantages stand out. Because positioning can be performed using correction signals directly from satellites, precise positioning is possible even outside cellular coverage. In actual deployments at mid-mountain agricultural fields, CLAS enabled centimeter-level autonomous tractor operation in places where communications previously did not reach. In disaster response, the ability to begin positioning and photography with CLAS-compatible equipment before communications are restored is highly useful. Note, however, that CLAS requires bringing compatible receivers to the site; standard smartphones alone cannot receive CLAS (though aftermarket devices to make smartphones CLAS-capable exist; described later).

• Wide-area movement / international projects: For ship and aircraft positioning or surveying projects spanning multiple countries, cloud-based PPP services that provide seamless wide-area coverage are suitable. NRTK and CLAS become unusable across borders, but global PPP can maintain accuracy over long distances with a single service. For offshore positioning, PPP augmentation via quasi-zenith or Inmarsat communications satellites is almost the only option. As noted, PPP requires time to converge, so operational measures such as pre-converging before departure or bridging with auxiliary sensors for dynamic use are needed.

• Low initial cost / simple surveying: For cases that cannot afford expensive surveying equipment or specialized personnel, combining a smartphone with a low-cost augmentation service is attractive. For example, establishing a local RTK by installing your own base station can serve simple surveys within a few kilometers (a base + rover setup can provide high accuracy with no communications fees). Because maintaining a base station requires effort, smartphone-attached external receivers that connect to NRTK services to realize “zero up-front investment” high-precision positioning have recently appeared. The LRTK system, described later, is an example: it uses your smartphone while obtaining needed corrections from the cloud, achieving large cost reductions and ease of use.

To summarize: in areas with good communications use NRTK, in areas with communications outages or lacking infrastructure use CLAS, and for wide-area movement use PPP. However, hybrid products that support multiple correction sources with a single receiver and switch according to conditions have recently appeared. The next section introduces smartphone-integrated positioning solutions as a representative example.

Smartphone-Integrated Operation and Cloud Integration

To fully realize the convenience of the augmentation services described so far, the usability of the user-side equipment is also important. Traditional RTK positioning required sizable equipment such as tripod-mounted dedicated receivers and radios, but recent technological advances have made “turning a smartphone into a surveying instrument” a reality. The point is to combine a smartphone with a small, high-precision GNSS receiver to perform RTK or CLAS positioning. For example, the ultra-compact GNSS device “LRTK Phone” that can be attached to an iPhone integrates an RTK receiver of about 125 g and 13 mm (0.51 in) thickness into a dedicated phone case, allowing the smartphone and receiver to be used as one unit. Connection is via Bluetooth, eliminating cumbersome wiring. Using such smartphone-integrated devices and connecting to Ntrip from a smartphone app to receive correction data (NRTK, etc.) can improve the typical 5–10 m (16.4–32.8 ft) accuracy of a smartphone’s internal GPS to centimeter-level accuracy (cm level accuracy (half-inch accuracy)).

The advantages of smartphone integration are a significant reduction in initial investment and operational hurdles. High-precision GNSS receivers traditionally cost several million yen, but using a smartphone-linked approach requires only a palm-sized device, drastically reducing cost compared to dedicated equipment. Usability has also dramatically improved. Positioning and recording can be completed within a smartphone app, and map display of positioning results and photo tagging are done automatically. Data can be synchronized to the cloud and immediately viewed and shared on office PCs. For example, the LRTK system’s dedicated app integrates continuous positioning modes from 1 Hz to 10 Hz, single-point positioning, high-precision azimuth measurement for photo capture and cloud map placement, and AR guidance to past recorded points, enabling real-time cloud-linked workflows for high-precision data collected on site. This allows non-surveying specialists and field workers to perform intuitive positioning and recording, reducing labor and improving efficiency.

Smartphone integration also aligns well with communications infrastructure. With a smartphone-integrated device, receiving correction data via 4G/5G and uploading data to cloud services can be completed with a single device. Point cloud data and photos stored in the cloud can be shared immediately among stakeholders, enabling remote progress checking and analysis. The fusion of augmentation services and cloud technology is enabling next-generation positioning operations that transcend time and space constraints, and this trend is expected to accelerate, with service development increasingly centered on smartphone-cloud integration.

Smartphone-Integrated Positioning Solution Using LRTK

As a concrete example of this trend, consider LRTK, a noteworthy smartphone-integrated solution at the time of writing. LRTK is a positioning system developed by a startup originating from Tokyo Institute of Technology, consisting of a small GNSS receiver that attaches to a smartphone (currently iPhone), a dedicated app, and cloud services. Its notable feature is that it enables labor-saving smartphone-integrated operation while supporting both network RTK and QZSS CLAS as correction sources. The company released the smartphone-mounted RTK receiver “LRTK Phone 4C” in 2022, enabling real-time smartphone positioning via mobile network NRTK corrections. Responding to requests for “coverage in no-signal areas,” they later developed and released a CLAS-capable model as an optional kit. With the CLAS-capable model, positioning is possible even where cellular signals do not reach, as long as the satellite augmentation signals can be received, providing users with significant peace of mind. Users of this model report “successful positioning and recording in remote mountains without cellular coverage” and “high-precision on-site records during disasters when communications were down,” demonstrating the value of truly zero-dependency-on-communications smartphone positioning.

Another strength of the LRTK system is data cloud integration and 3D measurement functions. Point cloud data and high-precision photos captured with the dedicated app can be synced to the cloud with one tap and immediately reviewed and measured in an office-side 3D viewer. This facilitates smooth data sharing and collaboration across field and office. Combining the smartphone camera and sensors enables advanced features such as point cloud scanning and AR-based as-built verification. Achieving 3D measurement and analysis—formerly requiring specialized equipment and software—with only a smartphone and cloud services is revolutionary. As billed, LRTK is expected to democratize high-precision positioning by making modern surveying starting points accessible to anyone.

From the viewpoint of choosing augmentation services, LRTK’s hybrid approach is logical. By supporting both NRTK and CLAS, it provides seamless centimeter-level positioning from urban areas to no-signal regions, leveraging the strengths and compensating for the weaknesses of each service. For example, normal operation might use NRTK for fast, stable positioning, and automatically switch to CLAS reception when entering mountainous areas to maintain positioning (the actual switching is managed by the app). Thus, LRTK removes the burden on users to select augmentation services by region or use case, enabling them to acquire optimal high-precision data without conscious decision-making. As satellite numbers increase and device miniaturization continues, integration into drones, small work machines, and mobile devices will likely expand. In the world of augmentation services, “smartphone-integrated CLAS support” is becoming a key technology.

Finally, these technological advances greatly benefit field operations. Not only surveying, construction, and municipal specialists but also fields that previously had little access to high-precision GNSS will have inexpensive, easy-to-use options, leading to dramatic improvements in operational efficiency and new use cases. From the perspective of choosing augmentation services by coverage area, each method still has its appropriate place, but smartphone-integrated solutions are steadily blurring those boundaries. In the future, seamless use of NRTK, PPP, and CLAS according to site conditions will become commonplace, and an era when everyone can enjoy centimeter-level positioning with a smartphone in hand will arrive. Watch trends in next-generation solutions such as LRTK as pioneers of this shift.

Frequently Asked Questions

Q: Do I need to pay or apply to use CLAS? A: No, CLAS signals are provided free of charge by the Cabinet Office’s QZSS, and there is no usage fee. However, a compatible receiver (a high-precision GNSS receiver that supports the L6 band) is required, and purchase costs for that equipment will be incurred. Receiving CLAS does not require special licenses or applications; anyone with compatible equipment can use the satellite augmentation service across Japan. By contrast, many commercial NRTK services charge monthly fees, so CLAS’s free provision is a significant cost advantage.

Q: Can I achieve centimeter-level positioning with a smartphone alone? A: Currently, it is difficult for a commercial smartphone alone to achieve centimeter-level accuracy. High-end smartphones with dual-frequency GNSS can improve accuracy to the meter level, but they do not typically include built-in capabilities to receive and apply corrections from RTK or CLAS. Therefore, external high-precision GNSS modules or compatible services must be combined with the smartphone to achieve centimeter-level positioning. For example, using a smartphone-mounted device like LRTK can turn a smartphone into an RTK receiver. Some Android devices can output raw GNSS data, enabling attempts to apply corrections via apps, but these approaches often require technical expertise; thus, using a commercial solution (smartphone-integrated receiver + app) is the practical option.

Q: Can I get high-precision positioning in places without cellular coverage? A: Yes. A representative method is using CLAS. In Japan, even where cellular signals do not reach, receiving CLAS augmentation signals directly from overhead quasi-zenith satellites enables real-time centimeter-level positioning. There are practical reports of successful CLAS positioning in mountain construction sites without cellular coverage and of CLAS being helpful in disaster scenarios. Alternatively, pre-recording base station data and performing post-processing (PPK or static surveying) can provide precise positions but lose real-time capability. If you need immediate high-precision positions on site, satellite-based CLAS is a leading option.

Q: How does CLAS accuracy and stability compare to network RTK? A: Both CLAS and NRTK can achieve horizontal accuracies on the order of several centimeters (cm level accuracy (half-inch accuracy)), but there are differences. In terms of accuracy, in open environments CLAS and RTK are comparable and fall within a few centimeters. However, RTK, being relative to fixed stations, can produce centimeter-level accuracy immediately after initialization, whereas CLAS (PPP-RTK) may require tens of seconds to several minutes to reach a full fix. Regarding stability, behaviors differ when satellite visibility degrades. Generally, RTK is more robust to obstructions; in environments such as under elevated structures or in forests where satellite signals are partially lost, CLAS is more likely to drop from fix to float or revert to standalone. RTK also fails when satellites are not visible, but because it uses local reference stations for error correction, it can sometimes maintain position with fewer visible satellites. Experimental results suggest: “both achieve high accuracy in open skies; in obstructed environments RTK is somewhat more advantageous.” Therefore, it is desirable to use NRTK in urban areas, CLAS in remote/no-signal areas, or combine both for redundancy.

Q: What is LRTK? A: LRTK, introduced in this article, is a smartphone-integrated high-precision positioning solution. It consists of a small GNSS receiver that attaches to a smartphone, a positioning app, and cloud services, enabling centimeter-level positioning via RTK or CLAS with a smartphone. Designed as an easy-to-start next-generation surveying system requiring no specialized equipment, LRTK is already being used in construction sites, infrastructure inspections, and disaster surveys. Attaching an LRTK device to a smartphone enables GNSS surveying that previously required tripod-mounted equipment to be performed one-handed, and collected point clouds and photos can be shared to the cloud in real time. LRTK supports both cellular-based NRTK and satellite direct reception CLAS, enabling continuous high-precision positioning regardless of communications conditions. In short, LRTK is an innovative solution that “turns a smartphone into a high-precision surveying instrument” by fully utilizing the latest augmentation services.