No reference station or correction information service required! Start easy, low-cost, high-precision positioning with LRTK

On surveying and civil engineering construction sites, even a positional deviation of a few centimeters (a few in) can have a large impact. In infrastructure works such as highways and railways, quality control of land development, and confirming pile-driving positions for buildings, accurate positional information determines quality and safety. Therefore, technologies that correct satellite positioning (GNSS) errors and obtain centimeter-level accuracy (half-inch accuracy) in real time are extremely important. However, with conventional GNSS (standalone positioning), satellite-signal-derived errors cause positions to be off by several meters (several ft), which does not meet the accuracy required by civil engineering and surveying. The error-correction technology developed to solve this problem is called RTK positioning (Real Time Kinematic, real-time dynamic positioning).

Traditionally, achieving such high-precision positioning has generally required RTK surveying that installs reference stations (base stations) or the use of external position correction information distribution services. However, are reference stations or paid correction services really necessary for achieving higher precision?

In this article, we explain how RTK-based high-precision positioning works and the challenges of conventional methods, and introduce the latest smartphone-compatible RTK solution, "LRTK". With LRTK, no cumbersome base stations or expensive correction service contracts are required, allowing anyone to start centimeter-accurate positioning easily and at low cost.

Table of Contents

• Conventional RTK surveying using reference stations

• What is network RTK?

• Are reference stations and correction information services really necessary?

• What is the smartphone RTK system "LRTK"?

• Advantages of LRTK: simple & low-cost

• Expanded possibilities for simple surveying with LRTK

• FAQ

Conventional RTK surveying using base stations

High-accuracy RTK positioning operates two GNSS receivers simultaneously: a base station (fixed station) and a rover (mobile station). The base station is installed at a point whose precise coordinates are already known, and the rover is set up at the point to be measured; both receive signals from the same satellites. The base station computes the error from the difference between its known accurate position and its GNSS observations, and transmits that correction information wirelessly and sequentially to the rover. The rover applies the received corrections to its own observation data to obtain a high-accuracy position with reduced error in real time. Because relative positioning with the base station cancels out errors from satellite orbits and the atmosphere, a precise positioning accuracy of about 1-2 cm (0.4-0.8 in) in the horizontal direction is typically achieved.

However, to maintain this accuracy, it is necessary to keep the distance between the base station and the rover (the baseline length) short. Because the error-correction effect weakens as the two stations move apart, operation is generally recommended within a few km (a few thousand ft). Therefore, in conventional RTK surveying, the mainstream method has been to set up a base station on a tripod near the work area each time and deliver correction data to the rover via low-power UHF radio, etc. Erecting base station equipment at each site is time-consuming, but if properly installed it can achieve centimeter-level positioning (cm level accuracy (half-inch accuracy)) in a short time, demonstrating accuracy that is in a class apart from conventional GNSS positioning with errors of several meters (several ft). Note that if high-power, long-range radio equipment is used for correction communications, you may in some cases need to apply in advance for a radio station license.

What is Network RTK?

The approach that overcomes the conventional method's weakness—the constraint that 'a reference station must be placed on site every time'—is called network RTK. This method uses a network of reference stations installed at numerous locations across the country (for example, the Geospatial Information Authority of Japan's network of continuously operating reference stations) to set up a virtual reference station near the user and provide correction information.

Specifically, observation data from multiple fixed reference stations are integrated on the server side, and virtual reference data are generated—based on the approximate location of the user, i.e., the mobile station—that make it appear “as if a reference station were nearby.” Then, the correction data created by simulating the signals that would have been received at that virtual reference point are delivered in real time via cellular networks, etc. (they are usually transmitted over the Internet using a protocol called Ntrip). Because the mobile station can perform RTK calculations under the same conditions as if a reference station were right next to it, wide-area, uniform centimeter-level positioning (half-inch-level positioning) can be achieved without worrying about accuracy degradation due to distance.

With the advent of network RTK, users can now perform surveying work on site with just a single GNSS receiver without preparing base station equipment. Not only is the effort of setting up a reference station eliminated, but the complicated procedures associated with radio communications (such as license applications for high‑power radios) are also unnecessary, and constraints when renting equipment are reduced. Because centimeter-level correction information (half-inch accuracy) can be obtained stably almost anywhere in Japan where mobile phone signals are available, positioning methods using network RTK have recently been becoming mainstream. However, it should be noted that receiving such correction data requires a subscription to a dedicated distribution service and an Internet connection via a mobile network.

Are reference stations and correction information services really necessary?

Up to now, I have explained as if there were only two options for achieving centimeter-level high-precision positioning: "setting up your own reference station" or "using an external correction information service". In fact, almost all of the high-precision GNSS currently in widespread use adopt one of these two approaches. However, did you know that by using the latest technology, both reference stations and correction information distribution services may no longer be necessary?

The key to that is the smartphone-based RTK system "LRTK" developed by the startup Refixia. With LRTK, there is no need to install a base station on-site, nor to sign a subscription contract for correction services via communications. In the next chapter, let’s take a closer look at how this LRTK works and the methods for improving its accuracy.

What is the smartphone RTK system "LRTK"?

LRTK is a new RTK solution that, unlike conventional stationary surveying instruments, leverages smartphones. Its concept is "an RTK surveying instrument that fits in your pocket," and it consists of an ultra-compact GNSS receiver device that operates in conjunction with a smartphone. The receiver itself weighs only a few hundred grams, is palm-sized with a thickness of about 1 cm (0.4 in), and is used by attaching it to the back of a smartphone or mounting it on a commercially available monopod or pole. This small enclosure houses a GNSS antenna and receiver chip capable of high precision, a battery, and a wireless communication module, and it connects to the smartphone via Bluetooth or Wi‑Fi and is controlled from a dedicated app.

肝心の高精度化の仕組みですが、LRTKも基本的には従来と同じRTK positioning methodに則っています。違いは、基準局からの補正情報を得る方法です。LRTKでは補正データをインターネット経由で既存のネットワーク型RTKサービスから受信することもできますが、さらにJapan’s Quasi-Zenith Satellite System, centimeter-class positioning augmentation service (CLAS) (cm level accuracy (half-inch accuracy))にも対応しています。上位モデルであれば専用受信機によりCLAS信号を衛星から直接受信できるため、たとえ携帯圏外の山間部であってもリアルタイムに補正データを取得してセンチメートル精度を維持できます (cm level accuracy (half-inch accuracy))。つまりLRTKは、通信環境に応じてInternet-based and satellite-basedを使い分けられるハイブリッドなRTKシステムと言えます。

Using LRTK in this way, high-precision positioning that traditionally required stationary surveying instruments or large base-station sets can be completed with a palm-sized receiver and just one smartphone. There is no need to perform complicated on-site equipment setup; simply launch the positioning app and wait tens of seconds, and centimeter-level (half-inch-level) "Fix solution" (the RTK fixed solution) will begin to be obtained. Because it can be operated simply by following the instructions on the smartphone app without specialized knowledge, it is also easy for technicians with limited surveying experience to use. Furthermore, the accuracy of the positioning results is not inferior to conventional methods. Under good reception conditions, it has been confirmed that LRTK can achieve the same level (several centimeters (a few inches) in horizontal position, several-centimeter range (a few-inch range) in elevation) as conventional high-precision GNSS equipment (even in comparative experiments at the same point, the difference in positioning results was within a few millimeters (a few tenths of an inch)).

Benefits of LRTK: Easy & Low-cost

We will organize the points in which LRTK is superior to conventional methods, focusing on cost and ease of use.

• Low cost: The hardware configuration is simple—just a smartphone + a compact receiver—so initial deployment costs are drastically lower compared to dedicated, expensive GNSS surveying equipment. The falling prices of high‑precision GNSS-capable chips and the use of general-purpose smartphone components have achieved a price point more accessible than typical surveying instruments. Because the smartphone handles communication and processing, there is no need to provide a separate dedicated terminal for data collection. Furthermore, in terms of operational costs, by using the free CLAS correction signal instead of paid network RTK services, you can reduce the communication fees and service charges for obtaining correction information to zero. In other words, with LRTK you can significantly reduce the total costs of hardware, software, and services required for high-precision positioning.

• Portability and Setup: All you need to carry are a pocket-sized receiver and a smartphone, so there is no need to transport and install tripods, large antennas, and a full set of radio equipment at each site as with conventional systems. Because the equipment is compact and lightweight, it provides high mobility to the site and can be handled easily even by women or people with limited strength. Setup after arrival is completed with one touch, freeing you from searching for a place to install a base station and from long preparations before measurements. Also, because a dedicated radio is not used between the mobile station and the base station, the bothersome application for a radio station license is unnecessary.

• Simple operation: By simply following the guidance on the dedicated smartphone app, it can be used intuitively even without specialized knowledge. When you arrive on site, just attach the receiver to your smartphone or a pole, launch the app, and start positioning. A Fix solution can be obtained in a matter of tens of seconds, allowing you to begin acquiring high-precision positioning data immediately. Saving and sharing positioning data is also easy within the app, so even those who are not experienced surveyors can readily utilize high-precision positioning results in-house.

• Cloud integration: Positioning data acquired by LRTK can be automatically synchronized with cloud services. Large-volume 3D point cloud data can also be stored, shared, and analyzed on the cloud, allowing use without separately purchasing point cloud processing software that was previously costly. By using the cloud in combination, you can eliminate the hassle of bringing data collected in the field back to the office for processing, and it also promotes DX (digital transformation)-style use such as real-time information sharing.

• Extensibility: It can interface with the AR features of smartphone cameras and LiDAR sensors, easily enabling advanced measurements such as appending positional coordinates to photos and scanned point clouds. For example, using LRTK makes it possible to indicate an on-site object on the smartphone's AR display while recording its positional coordinates with high accuracy. The combination of GNSS and AR technologies is expected to enable new surveying workflows that were previously unavailable.

Expanding the Possibilities of Simplified Surveying with LRTK

With the advent of LRTK, simple surveying that allows you to easily obtain centimeter-level accuracy (half-inch accuracy) in situations where you just want to take a quick measurement has become a reality. For example, on civil engineering sites, equipment operators and site supervisors sometimes need to verify on the spot whether excavation has reached the design depth and how many cubic meters of fill are still required. With LRTK, you can measure the current terrain with a smartphone, instantly compare it to the design data in the cloud, and calculate the differences in excavation or fill quantities on site. This allows teams to confirm the as-built condition during construction and make instant decisions and corrections, reducing rework and preventing delays in material procurement. Also, if you suddenly need to measure a distance or elevation difference, there's no longer any need to go back for a tape measure or level. All you need is a smartphone with an LRTK device at hand, and you can immediately measure the horizontal distance and elevation difference between any two points on the screen. The accumulation of these small efficiency gains in everyday measurement tasks leads to improved productivity for the entire project.

Thus, LRTK provides an environment where anyone on site can take measurements when needed without waiting for a specialized surveying team. If high-precision positioning expands from a limited set of tasks to daily operations, the accuracy of quality control and progress management will improve dramatically. Also, in the construction industry, which faces a chronic shortage of surveying personnel, labor-saving and efficiency improvements in field surveying through LRTK could become one of the solutions. In fact, in "i-Construction" (a construction DX initiative led by the Ministry of Land, Infrastructure, Transport and Tourism aiming to improve on-site productivity), the introduction of high-precision GNSS equipment that anyone can easily use is regarded as an important key. Truly embodying "simple, low-cost, high-precision positioning", LRTK can be said to be a solution with the potential to greatly change the future of field surveying. With this new positioning style that requires neither base stations nor correction services, why not incorporate the power of high-precision positioning into your daily operations?

FAQ

Q: Does high-precision positioning require a subscription to a positioning correction information distribution service? A: In conventional GNSS positioning, it has been common to use paid correction information services to obtain centimeter-level accuracy (cm level accuracy, half-inch accuracy). However, with LRTK a subscription is not mandatory. LRTK can directly receive the CLAS satellite augmentation signal provided free of charge from Japan's quasi-zenith satellite "Michibiki", so it can obtain correction information in real time even without network connectivity. In other words, using LRTK enables centimeter-class positioning (cm level accuracy, half-inch accuracy) without additional costs (of course, you can also use network-based RTK services as before).

Q: How does LRTK work? A: It is a compact RTK-GNSS positioning system used in combination with a smartphone. A pocket-sized dedicated GNSS receiver is attached to the smartphone and applies high-precision corrections to positioning signals from satellites. For real-time corrections it uses network RTK services delivered via the Internet or CLAS signals via satellite. In short, the idea is that your smartphone quickly transforms into a high-precision GNSS surveying instrument.

Q: What do I need to use LRTK? A: Basically, you can start positioning with a smartphone (such as an iPhone) and an LRTK receiver. First install the dedicated app on your smartphone and connect to the LRTK receiver via Bluetooth or similar. Then go outdoors to a location where satellites can be received and you can perform positioning. If you use network RTK, you need a mobile data connection on your smartphone and an account for the correction service, but when using CLAS a data plan or additional equipment is not required.

Q: How accurate is positioning with LRTK? A: Depending on the environment, high-precision positioning with errors typically within about ±1-2 cm (±0.4-0.8 in) horizontally and about ±3 cm (±1.2 in) vertically is possible. This is equivalent to conventional professional GNSS surveying equipment, and experiments comparing the same points have confirmed differences of only a few millimeters. In sufficiently open-sky conditions, you can obtain positions with accuracy that is comparable to conventional methods.

Q: Can LRTK be used in places outside mobile coverage? A: Yes, it can be used. The CLAS satellite augmentation signals supported by LRTK receivers are transmitted directly from the quasi-zenith satellites positioned over Japan nationwide. Therefore, even in mountainous areas where mobile phone signals do not reach, as long as the satellites can be received, centimeter-level positioning can be maintained. Because it does not rely on communications infrastructure, it is also highly effective for surveying during disasters and in remote islands and mountainous areas.

Q: Can beginners use LRTK for surveying? A: Yes. LRTK is designed so that people without specialized knowledge can operate it. A user-friendly interface and guidance are provided in the smartphone app, and by following the steps you can start positioning. Because the complex equipment setup and difficult coordinate calculations that used to be necessary are automated, even those unfamiliar with surveying can carry out positioning with centimeter-level accuracy (cm level accuracy, half-inch accuracy) with confidence. Of course, when used by experienced surveyors, it is possible to achieve results more efficiently than before.