How to perform RTK surveying without a local base station: Summary of options

Table of Contents

• What is RTK (Overview of Real-Time Kinematic Positioning)

• Methods Using Network RTK

• Methods Using Satellite Augmentation (PPP/CLAS)

• Simple Surveying Using LRTK

• FAQ

What is RTK (Overview of Real-Time Kinematic Positioning)

RTK is a technology for performing high-precision satellite positioning in real time. Standalone GPS (GNSS) positioning typically has errors of around 5-10 m (16.4-32.8 ft), but by using RTK (Real-Time Kinematic) you can determine positions with errors within a few cm (a few in). RTK is being used across a wide range of fields such as civil surveying, construction sites, agriculture, and drone positioning, and has become an indispensable technology for achieving efficient and accurate positioning.

RTK positioning uses two GNSS receivers (a reference station and a rover). A reference station (base station) with known precise coordinates is established, and based on the satellite data it observes, it corrects the rover's positioning errors in real time. Specifically, the difference between the satellite signals received at the reference station and those received at the rover is calculated, and that error information is sent to the rover via radio or communications lines. The rover applies this correction information in its positioning calculations, allowing it to obtain highly accurate position coordinates immediately, which cannot be achieved by standalone positioning.

A major advantage of RTK is that it can achieve both "immediacy" and "high accuracy." Conventionally, measuring positions with centimeter-level accuracy (half-inch accuracy) required long periods of static observation (static surveying) or precise surveying using optical distance measurement. However, with RTK you can bring a receiver to the point you want to measure and obtain results in a short time. Because it provides immediate high-precision positioning on site, it is extremely effective for applications that demand real-time performance, such as as-built surveys and stakeout work at construction sites, guidance for autonomous tractors in agriculture, and automatic navigation of drones.

Method Using Network RTK

One representative method for performing RTK positioning without installing a local base station is to use network RTK. Network RTK is a system in which you access correction information services generated from multiple reference station networks (control point networks) maintained by governments or companies via the Internet, enabling RTK positioning with just a single rover. By using a pre-established external reference station network, users no longer need to place a base station on-site.

Specifically, the GNSS receiver on the rover is connected to the Internet via a cellular network or a mobile router and accesses the server of the correction information distribution service it subscribes to (Ntrip caster). On the service side, correction information for a virtual reference station (VRS: Virtual Reference Station) is generated using data from existing reference stations around the user and delivered in real time. The user's receiver receives correction data as if it had its own nearby base station, allowing it to obtain a high-precision positioning solution on the spot.

To use network RTK, you need an RTK-capable GNSS receiver (mobile station) and an Internet connection method (such as a built-in SIM or smartphone tethering). You must also sign a contract with a correction-information service provider in advance and obtain the connection ID and password, as well as the coordinate system settings to be used. On site, if you power on the receiver and the communication device and connect to the service, the solution will converge from a "float solution" to a "fixed solution" (Fix) within tens of seconds, and centimeter-level positioning (half-inch accuracy) will begin. Because there is no need to set up a base station, the preparation work before starting surveying is greatly shortened, which is an attractive point.

メリット:

• 初期導入が容易: 自前の基地局機材を用意せず、移動局用の受信機一台と通信端末（スマホやタブレット）さえあれば始められます。現場ごとに基準局を据え付ける必要がないため、測量開始までの準備時間を短くできます。サービスから提供される設定情報に従って接続するだけで良く、専門知識が少なくても導入しやすい方式です。

• 広範囲で高精度を維持: 複数の基準点データを組み合わせて補正情報が作られるため、基準局から遠い場所でも精度低下が緩和されます。一般に数km以上離れると単一基準局RTKでは誤差が増えますが、ネットワークRTKではVRS方式などにより長距離でも数cmの精度を保ちやすくなっています。携帯通信網さえ届けば全国どこでも安定してcm級測位が可能で、移動しながら広範囲を測量する場合にも適しています。

• 公共座標系で測位可能: 配信される補正情報はあらかじめ公式の測地系（日本ではJGD2011やJGD2020等）に基づいているため、得られる測位結果は常に全球測地系に整合した座標値となります。自前基地局方式のように後でローカル座標への変換を行う必要が基本的にありません。複数の現場間でデータを突合する際も、共通の座標基盤上で比較ができます。

Disadvantages:

• Dependence on the communication environment: Because an internet connection is required, in areas outside coverage or with poor signal you cannot receive correction data and RTK positioning cannot be established. It is difficult to use in sites where mobile phones are out of range, such as mountainous areas or underground spaces, and users cannot remedy service-side communication failures or server maintenance. Compared with a privately operated base station, it should be noted that there is a risk of dependence on communication infrastructure.

• Ongoing usage costs: Using a network RTK service requires a contract, and recurring fees such as monthly charges or annual contract fees are incurred. If it is used frequently over long periods, total costs tend to become high. Also, when operating multiple receivers, license contracts may be required for each unit, so the cost burden increases as the number of people or units grows.

• Service area and standards limitations: The development status of reference station networks differs by country and region, so available services may be region-limited. In Japan, nationwide coverage has been achieved through VRS-type services that utilize the Geospatial Information Authority of Japan’s GEONET, but correction services available on remote islands or overseas may be limited. In addition, different providers may adopt different geodetic datums, requiring conversion to international geodetic systems or geoid height corrections.

In Japan, a representative network RTK service is ichimill (Ichimiru), a network RTK correction service provided by telecommunications carriers. ichimill has established more than 3,300 proprietary GNSS reference points nationwide at mobile phone base stations and other locations, allowing users to receive correction information anywhere in the country without having to set up their own base stations. There are also GNSS correction information distribution services provided by private companies, and contracting such services according to the application enables high-precision positioning over wide areas. The Geospatial Information Authority of Japan is also conducting experiments to provide real-time correction information using data from approximately 1,300 electronic reference points nationwide, and methods for using network-type RTK-GNSS in public surveying are being developed.

Methods Utilizing Satellite Augmentation (PPP and CLAS)

Using augmentation signals from satellites rather than via the Internet — the method of using augmentation signals from satellites — is also an option to achieve high-precision positioning without base stations. Representative examples include the PPP (Precise Point Positioning) method and CLAS (Centimeter-Level Augmentation Service) provided by Japan’s Quasi-Zenith Satellite System. These techniques, instead of directly using nearby reference station data, leverage correction data that provide globally high-precision estimates of GNSS satellite orbit and clock errors, as well as ionospheric and tropospheric delays, enabling a single receiver to achieve centimeter-level positioning accuracy.

CLAS (Centimeter Level Augmentation Service, シーラス) is a satellite positioning augmentation service provided by Japan's quasi-zenith satellite "Michibiki" (QZSS). Correction information generated from the Geospatial Information Authority of Japan's electronic reference station network is continuously transmitted on the L6 band from QZSS satellites in geostationary orbit. With a compatible GNSS receiver, users can receive the correction data directly from the Michibiki satellites overhead without an Internet connection, achieving positioning accuracy of several centimeters (cm level accuracy, half-inch accuracy). A major advantage is that even in areas without cellular coverage, such as mountainous regions or at sea, as long as the sky is open and the L6 signal can be received, high-precision positioning can be maintained. Also, because it is delivered via satellite, it is efficient for many mobile stations to use it over a wide area simultaneously, allowing multiple users to perform positioning at the same time without additional cost. Currently CLAS signals are provided so as to cover almost the entire country of Japan, and further use in surveying, construction, agriculture, and other fields is expected to increase in the future.

When using satellite augmentation systems, the receiver must be compatible. For example, to use CLAS you need a GNSS receiver that can decode CLAS signals on the L6 band; standard GPS receivers do not support this. Therefore, note that investment in new compatible equipment may be required. Also, positioning with PPP/CLAS can take slightly longer for initial convergence compared with RTK. It can take a few seconds to a few minutes from the start of receiving correction information until a high-precision fixed solution is obtained, and immediately after beginning positioning it is common to have a "float solution" with somewhat larger errors. However, once a fixed solution is obtained, centimeter-level accuracy (half-inch accuracy) is stably maintained.

On the other hand, because satellite-based augmentation methods are essentially based on global navigation satellite systems (GNSS), the coordinates obtained are values in the world geodetic reference frame (ITRF). When used within Japan, conversion to the Japanese geodetic system (JGD2020, etc.) may be necessary (in the case of CLAS, because correction information is generated based on Japan’s geodetic system, positioning can in practice be achieved almost directly at the accuracy of the Japanese geodetic system). Reception of satellite signals requires a clear view of the sky, and in environments with many surrounding obstructions accuracy degrades or positioning may fail, as with RTK positioning.

In addition to Japan's CLAS, overseas there have also emerged satellite communication–based high-precision positioning services offered by private companies, and high-precision positioning services from Europe's Galileo satellites (HAS: High Accuracy Service). These differ in accuracy and usage conditions depending on the region and service, but they are notable as a means of obtaining global positioning augmentation without having to carry a base station.

Benefits:

• No communication infrastructure required: Because correction data can be received directly from satellites, high-precision positioning is possible even in mountainous areas and remote islands where mobile phone signals do not reach. It eliminates the need to prepare an Internet connection at the site and incurs no communication costs.

• Well suited for wide-area and simultaneous use: Because a single satellite can broadcast correction information over a wide area, many mobile stations can use it simultaneously within the area. For example, CLAS signals cover all of Japan, so users can achieve uniform positioning accuracy no matter where they are, and performance does not degrade when multiple people use it at the same time (as long as the satellite can be received).

• No need to install reference stations: Because this method literally does not require base stations, on-site setup work and equipment transportation are unnecessary. It is particularly effective for temporary surveys and environments lacking communication infrastructure, and excels in ease of use.

Disadvantages:

• A compatible receiver is required: To receive satellite augmentation signals, a dedicated high-performance GNSS receiver is essential. CLAS-compatible and multi-band-capable equipment is generally more expensive than typical GNSS receivers, creating a hurdle in initial costs.

• Initial positioning takes time: Utilizing augmentation information requires some convergence time before high accuracy is achieved. In cases like network RTK, when radio conditions are good a fixed solution can be obtained in a few seconds, but in some situations it may take a few minutes to stabilize. If you need high-accuracy values immediately after starting work, you must account for the convergence wait time.

• Service coverage varies: CLAS is a service intended for use within Japan and cannot be used outside Japan (because Michibiki signals do not reach those areas). If you want to use an equivalent service overseas, you need to subscribe to the SBAS or PPP services for that region or consider other augmentation methods. Also, because the coordinate systems provided by services may differ, care is required when handling positioning results.

Simplified Surveying with LRTK

As described above, the conventional general methods for performing high-precision RTK positioning have been either "set up your own base station" or "use an external correction service". However, in recent years new approaches have emerged that further reduce the effort of these traditional methods and allow anyone to easily handle cm-level positioning. One of these is a system called LRTK.

LRTK (ell-ar-tee-kay) is a smartphone-integrated positioning solution that minimizes the need for specialized surveying equipment and expert configuration and aims to "enable anyone to easily achieve cm-precision (cm level accuracy (half-inch accuracy)) positioning with just a smartphone."

LRTK combines a dedicated compact GNSS receiver (a smartphone-mounted device) and a mobile app to achieve RTK-level high-precision positioning with a simple procedure. For example, by holding the receiver attached to your phone in one hand and simply pressing a button in the app at the point you want to measure, you can obtain high-precision coordinates for that point. It can also perform vertical positioning, which is difficult with ordinary GPS, achieving professional surveying-level accuracy of about ±1–2 cm (±0.4–0.8 in) horizontally and within a few cm (within a few in) vertically. Yet operation is intuitive and does not require the complex setup or equipment adjustments typical of conventional RTK systems.

Unlike traditional RTK methods, LRTK users do not need to provide their own base stations or contract external correction services. This is because the underlying technology employs proprietary algorithms that leverage cloud-based correction data and positioning information from multiple points to achieve high accuracy even with a single receiver. Furthermore, the acquired coordinate data can be converted on the spot into Japan Geodetic Datum coordinates and displayed on a map, so post-positioning coordinate transformations and data processing are also automated. In short, it is a revolutionary system that allows precise positioning immediately with nothing more than a smartphone and an LRTK device, even without a professional surveyor.

LRTK can be said to be a third option that eliminates the hassles that existed with both the self-owned base station method and the network RTK method. The equipment you need to bring to the site is minimal, you don't have to worry about the communication environment, and the ease of getting results simply by pressing a button in an app is an appeal not found in conventional approaches. Of course, the optimal method varies depending on required precision management and on-site conditions, but for those who want to try high-precision surveying more easily, LRTK is a promising solution. Currently, materials summarizing detailed implementation methods and use cases for LRTK have also been published, so those who are interested should definitely refer to them. With the power of cutting-edge technology, high-precision positioning should become more accessible.

FAQ

Q: What is the difference between RTK and ordinary GPS positioning? A: Standalone GPS (GNSS) positioning can have position errors of about 5-10 m (16.4-32.8 ft) due to satellite signal errors, whereas RTK positioning uses correction information from a base station to cancel those errors and can determine position with an accuracy of a few cm (a few in) both horizontally and vertically. In other words, the major difference is that RTK provides dramatically higher positioning accuracy compared with ordinary GPS.

Q: How accurate is RTK surveying? A: With properly operated RTK positioning, horizontal positions are generally within about ±1-3 cm (±0.4-1.2 in), and vertical (height) accuracy is about ±3-5 cm (±1.2-2.0 in). However, accuracy is affected by the distance to the reference station and satellite reception conditions, and errors tend to increase over longer distances. Accuracy can also decrease in environments with many obstructions. If operated at close range to the reference station in an open-sky environment, you can expect errors to be around 2 cm (0.8 in).

Q: What is required to use network RTK? A: To use network RTK, you need an RTK-capable GNSS receiver (the rover) and a means of Internet connectivity. Specifically, in addition to the GNSS receiver unit and antenna, prepare a communication device to connect to the correction data service (such as a built-in SIM or smartphone tethering). Prior to use, sign a contract with the service provider and obtain a login ID and connection server information, then configure these in the receiver or positioning app. At the site, simply power on the receiver and establish the communications link; correction data reception will begin and high-precision positioning will start within a few to several tens of seconds.

Q: Are there benefits to installing your own reference station (base station)? A: Yes — under certain conditions, installing your own reference station can be advantageous. For example, it can enable RTK positioning in locations without communication infrastructure; reduce long-term costs by avoiding service subscription fees; and allow your company to manage the station’s coordinates and operations so you can guarantee accuracy internally. However, because of the initial investment in station equipment and the operational effort required, it is not a solution to recommend for everyone. Consider the site environment and frequency of use, and consider adoption when those benefits outweigh the drawbacks.

Q: What kind of positioning method is LRTK? A: LRTK is a new system that, unlike conventional RTK, achieves centimeter-level (half-inch-level) high-precision positioning using only a compact GNSS receiver and a smartphone. Users do not need to provide a base station themselves, and they can use it without worrying about complicated communication settings. Its proprietary algorithms and cloud technology enable RTK-level accuracy with the simple operation of pressing a button on a smartphone. It can be thought of as an "RTK positioning service that anyone can easily use," greatly simplifying surveying work on site. Compared with traditional methods, the barriers to deployment and operation are lower, making it easier to handle even for non-specialists.