No Network Coverage? No Problem! With LRTK, High-Precision Positioning Anywhere Without Correction Information Services

Table of Contents

• High-precision positioning and correction information services

• Challenges of positioning outside communication coverage

• Is a correction information distribution service really necessary?

• Methods to improve accuracy without relying on communication infrastructure

• Offline correction using Michibiki (CLAS)

• With LRTK, correction information services are not necessary

• The world of simplified surveying realized by LRTK

• FAQ

What if centimeter-level positioning (inch-level) were possible even in mountainous regions or disaster sites where communication infrastructure doesn't reach――. In construction surveying and infrastructure inspection, the demand to measure positions to within a few centimeters (a few inches) is growing year by year. Traditional GPS (GNSS) positioning had errors of about 5-10 m (16.4-32.8 ft), but using a technique called RTK (Real Time Kinematic, real-time kinematic) can reduce errors to several cm (several in). Therefore, RTK positioning is being adopted across a wide range of fields such as drone navigation, automated control of construction machinery, and civil engineering surveying. However, conventional high-precision positioning has been burdened by dependence on communication infrastructure and dedicated services. This article explains the latest technologies that achieve high-precision positioning even outside communication coverage, and the representative example of such technologies: the LRTK solution, while answering the question, "Is a correction data distribution service really necessary?" At the end of the article, we also introduce the world of simple surveying with LRTK that anyone can easily use. How will positioning work change from now on thanks to RTK technology that can be used outside communication coverage? Let's take a closer look right away.

High-precision positioning and correction information services

To reduce errors in satellite positioning and achieve high accuracy, correction information from a reference station is indispensable. In RTK positioning, satellite data observed simultaneously by your own receiver (the rover) and a reference station at a known location are compared, and the differences are used to correct positioning errors in real time. Fundamentally, RTK positioning cannot be performed without a reference station. However, installing a base station at the site each time is burdensome, and a single station is limited when measuring over a wide area.

This is where network-based correction information distribution services come in. Various services have emerged that operate numerous reference station networks installed across the country by government and private companies, generate correction information from their observation data, and distribute that information over the Internet. By using these services, users no longer need to install base stations themselves, and centimeter-level positioning (cm level accuracy (half-inch accuracy)) can be achieved with only a rover. For example, in Japan there are public services based on the Geospatial Information Authority of Japan’s network of continuously operating reference stations, as well as high-precision GNSS correction services provided by mobile carriers, surveying equipment manufacturers, and others. If you sign up in advance and simply connect your receiver to the Internet, you can perform RTK positioning throughout Japan—a convenient arrangement. Using these services provides stable accuracy over a wide area and greatly simplifies on-site preparations. In addition, correction information based on data from multiple reference stations can cancel out error factors such as ionospheric delay over a wide area, so it has the advantage of suppressing accuracy degradation even for long-distance positioning that a single base station cannot fully cover. However, some services may charge monthly or annual subscription fees, and you will also need to prepare a compatible receiver.

Challenges of positioning outside communication coverage

Network-based RTK correction services are extremely useful, but they also have a weakness: dependence on communication infrastructure. Because receiving correction information always requires an Internet connection, this system does not function in environments where mobile phone signals cannot reach, such as deep mountains, remote islands, or at sea. Also, during disasters the communication network itself can be severed, and even if you want to use a contracted correction service, you may find yourself out of range. Relying entirely on the communications network risks losing the ability to perform high-precision positioning when it matters.

To carry out high-precision positioning at such sites outside communication coverage, the conventional approach has been to set up one's own reference station. For example, this is the operation of local RTK in which a base station sited at a pre-known coordinate point transmits correction data wirelessly and the rover (the worker side) receives it. However, with this method, accuracy tends to decrease as the distance between the base station and the work area increases, and, to begin with, positioning can only be performed within the range that radio waves reach (a radius of several km to about 10 km (several km to about 32,808 ft)). In addition, installation and dismantling of base station equipment and securing personnel placed a heavy burden on sites. Therefore, in regions where communication networks do not reach, there were situations where the advantages of high-precision positioning could not be fully utilized.

Is a correction information distribution service really necessary?

So, in order to perform centimeter-level positioning (half-inch level) in environments where communication infrastructure cannot be relied on, are expensive correction information services and heavy base station equipment indispensable? In short, it is "not necessarily required" in some cases. Recent new technologies have opened a path to obtaining correction information and achieving high-precision GNSS positioning even when outside of communication coverage.

One approach is the on-site local RTK operation mentioned above, but a more revolutionary method is using correction signals that come directly down from satellites. In Japan, a Quasi-Zenith Satellite System called "Michibiki" has been deployed, and a service is provided that broadcasts error correction data for RTK via these satellites. In other words, even without relying on ground communications infrastructure, you can receive information for higher accuracy directly from satellites overhead. By leveraging this system, centimeter-level positioning becomes possible even in areas outside communication coverage where positioning was previously difficult.

Methods for Improving Accuracy Without Relying on Communication Infrastructure

As concrete measures for obtaining correction information without relying on communication infrastructure, the following main methods can be listed.

• Using a private reference station and radio: This is an operation in which a base station is set up at a known point on site and correction data is transmitted over a specific radio frequency (such as the UHF band or certain low-power radio services). It is a long-established method and, because it does not use the Internet, can be used even outside network coverage. However, it has the drawback that accuracy tends to degrade with distance from the base station due to effects such as ionospheric errors, and positioning is only possible within the range that the correction signal reaches (generally a radius of several km to about 10 km). It also incurs high equipment costs and operational burdens, and may require a radio station license and the expertise of surveying technicians.

• Method using satellite positioning augmentation signals: This method uses the centimeter-level augmentation service (CLAS) provided by the Quasi-Zenith Satellite "Michibiki". If you have a compatible GNSS receiver, you can receive correction information from satellites maintained by the government at no additional cost. Because the correction information is transmitted from overhead, it is revolutionary that high accuracy can be achieved almost uniformly across Japan, from mountainous areas to remote islands and offshore. Since there is no need to install base stations, equipment hassle and costs are kept to a minimum.

Of these, the latter — the method of using satellite augmentation — is precisely what enables "high-precision positioning even without correction information services." In particular, Michibiki's CLAS (Centimeter Level Augmentation Service) broadcasts error correction information that covers all of Japan and can be received even in deep mountains or remote islands, provided there is an open view of the sky. Note that using CLAS requires a dedicated, compatible GNSS receiver and cannot be received by the GPS built into typical smartphones. In the next chapter, we'll take a closer look at this CLAS.

Offline correction by Michibiki (CLAS)

Michibiki (Quasi-Zenith Satellite System) is a satellite system that tends to remain over Japan for long periods and serves to complement GPS to improve positioning accuracy. The service CLAS (Centimeter-Level Augmentation Service) transmits error information calculated from the Geospatial Information Authority of Japan's network of continuously operating reference stations via radio signals in the L6 band from the satellites. When a compatible receiver picks up this signal, it can correct positioning errors in real time, resulting in positions with an accuracy of a few centimeters. Users do not need to apply or pay usage fees; as long as they have a compatible GNSS receiver, high-precision positioning can be achieved without additional cost even outside communication coverage.

For example, even in mountainous forest surveys where high-precision positioning has been difficult until now, or in assessing situations at disaster sites cut off from communications, CLAS-compatible equipment can secure accuracy using correction information beamed directly from satellites. In fact, even in areas where mobile networks were down due to heavy rain disasters, using CLAS-compatible RTK receivers enabled accurate location recording and information sharing of affected sites, proving highly effective. Thus, the advent of CLAS can be said to have opened the door to an era in which positioning is possible anywhere, even without communication infrastructure. Of course, using CLAS augmentation signals requires a dedicated compatible GNSS receiver, but LRTK, introduced next, dramatically lowers that hurdle.

With LRTK, correction information services are not required

A solution that can make the most of the satellite augmentation benefits described above is LRTK. LRTK is the name of a compact integrated RTK-GNSS receiver device and suite of services developed by Reflexia, a startup originating from Tokyo Institute of Technology, and is used by attaching it to a smartphone (iPhone or Android device). Despite its palm-sized compact housing, it contains a high-performance antenna and positioning chip that support multi-GNSS and multi-frequency, and by supporting three frequencies L1/L2/L5 for multiple satellite systems including GPS, it achieves shorter time to fixed solution and improved stability. Although small, it achieves positioning accuracy comparable to professional large equipment (errors within a few centimeters (within a few inches)), and it also includes a dedicated LRTK app that can input correction data via the Ntrip method. Most notably, it supports the QZSS CLAS signal.

If you attach an LRTK device to your smartphone, you can continue high-precision positioning by receiving augmentation information directly from the Michibiki satellites overhead, even outside cellular coverage. Even if the site is in mountainous areas and outside mobile communication coverage, as long as you have LRTK you can determine your position in real time with an accuracy of a few centimeters (a few in). This overcomes the network RTK weakness of "dependence on the communication environment" and can be extremely powerful even if communication infrastructure is cut off by a disaster. Also, when using CLAS, connection to external services is unnecessary, so there is the advantage that no running costs such as monthly fees are incurred.

Furthermore, LRTK offers multiple lineups—from professional dust- and waterproof models to helmet-mounted and smartphone-attached types—and all support CLAS reception. You can choose according to on-site needs, and any type will provide the reassurance of being able to "measure anywhere without correction information services." Of course, when within communication range, you can also perform RTK positioning using your network correction service (Ntrip streaming), so you can always switch between online and offline environments to ensure optimal accuracy. In addition, LRTK meets the requirements of i-Construction promoted by the Ministry of Land, Infrastructure, Transport and Tourism, and is attracting attention as a solution that supports DX (digital transformation) in the construction and surveying fields from the field.

The world of simplified surveying enabled by LRTK

With the introduction of LRTK, on-site surveying styles have begun to change dramatically. Traditionally, high-precision surveying required large-scale equipment and specialized knowledge, such as tripod-mounted transits and fixed GNSS units. But with LRTK, preparation is complete simply by attaching a small device to a smartphone. A single worker can hold the device in one hand and, with the press of a button, instantly obtain high-precision coordinates for that location. There is no need to arrange auxiliary staff or transport heavy machinery, making on-the-spot, ready-to-measure simplified surveying a reality.

For example, a smartphone equipped with LRTK proves powerful even for everyday tasks such as checking the finished elevation of foundation work at a construction site or measuring the area of a field on farmland. Because it is easy to take photos simultaneously with positioning and save them to the cloud with location information, its use is expanding to recording locations of concrete cracks and comparing workmanship before and after construction. Furthermore, verifying land boundaries in mountainous areas can be handled by a single person if they have a smartphone and LRTK. Even on sites where a professional surveyor cannot be called immediately, being able to grasp the approximate positions of boundary markers on the spot dramatically improves work efficiency. The latest LRTK apps can visualize boundary lines and measurement points on the camera screen using AR (augmented reality) features, allowing intuitive understanding of on-site conditions. Also, some LRTK devices have tilt-compensation functions that allow accurate coordinates to be obtained even if the pole tip is tilted, which is useful when measuring points while avoiding obstacles. They are easy enough to handle even for non-specialist surveying technicians, and it is revolutionary that site supervisors and technical staff themselves can create an environment where they can "measure whenever they want". Because the devices themselves are priced lower than conventional dedicated surveying instruments, they can be said to be playing a role in the “democratization of surveying,” enabling anyone to use centimeter-level accuracy (cm level accuracy (half-inch accuracy)) when needed.

LRTK enables the previously difficult precise positioning outside communication coverage and further lowers the barriers to surveying work itself. It will become an increasingly reliable presence at any site that requires high-precision positioning. If you are interested, you can view product information and case studies on the LRTK official website. You can also request materials or ask for a demo, so please feel free to contact us if you have any questions. Experience high-precision positioning that works even outside communication coverage at your company’s sites.

FAQ

Q: Is subscription to a correction information distribution service required for RTK positioning? A: If you use the conventional network RTK that receives correction information via a communication link, you need to subscribe to a dedicated distribution service. However, if you use equipment that can receive augmentation signals (CLAS) from satellites, such as LRTK, centimeter-level positioning is possible without a service contract.

Q: What is CLAS of Michibiki? Is there an additional fee to use it? A: CLAS is a centimeter-level error correction information service provided by Japan’s quasi-zenith satellite "Michibiki". Correction information based on the Geospatial Information Authority of Japan’s control point data is broadcast directly from the satellite. A compatible receiver is required, but no application or monthly fee is necessary, and anyone can use it as long as they have a reception environment.

Q: What kind of product is LRTK? A: LRTK is a compact, high-precision GNSS receiver device that attaches to smartphones and tablets. It is a product line developed by Refixia, a venture originating from Tokyo Institute of Technology, and incorporates an antenna and positioning engine that support multi-GNSS and multi-frequency, enabling RTK positioning and CLAS reception for centimeter-level positioning (cm level accuracy, half-inch accuracy).

Q: Are there any devices or conditions required to use LRTK? A: Basically, you can get started with the LRTK unit and a compatible smartphone (iPhone or Android). Install the dedicated app on your phone and simply attach the LRTK device to begin positioning. If you use network RTK within a coverage area, a separate subscription to a correction information service and a mobile communication environment are required, but those are not necessary when using CLAS.

Q: Can I use LRTK without specialized knowledge? A: Yes. LRTK is designed with simple operation in mind so that on-site non-surveying engineers can handle it. On the app, simply pressing the "Start Positioning" button automatically activates RTK, and measurement results are displayed and saved in real time. Because of its intuitive interface and comprehensive support features, you can use it with confidence even without specialized knowledge.

Q: Does LRTK support network-based correction services? A: Yes. The LRTK app includes an Ntrip client function, and if you already have a subscription to a correction information distribution service, you can enter its connection details and use it. In areas with network coverage you can use conventional network RTK, and when you move out of coverage you can switch to CLAS to continue positioning, enabling flexible operation.

Q: How accurate is the positioning? A: If an RTK Fix solution (fixed solution) can be obtained in a good satellite reception environment, errors in both the horizontal and vertical directions are generally within a few centimeters (a few in). This is orders of magnitude more accurate than typical standalone positioning (errors of about 5–10 m (16.4–32.8 ft)).