What is RTK GNSS? Basics of High-Precision Positioning Technology in Construction and Surveying

RTK stands for Real Time Kinematic, a technology that uses satellite positioning systems (GNSS) to perform centimeter-level high-precision positioning in real time. GNSS is the mechanism for obtaining position information from satellites such as GPS, GLONASS, and Michibiki (QZSS), but standalone positioning typically yields errors of several meters. RTK uses two receivers—a reference station and a rover—simultaneously to correct such positioning errors, enabling a moving positioning device to determine its location with an accuracy of a few centimeters.

In construction sites and surveying, the immediate and high-precision position measurements provided by RTK are essential for improving work efficiency and ensuring quality. In recent years, the cost and size of RTK equipment have decreased, making centimeter-level positioning, which once required specialists and expensive equipment, more accessible. For example, small GNSS receivers that can be used with smartphones have appeared, ushering in an era in which anyone can easily utilize high-precision positioning. This article explains the basic knowledge of RTK-GNSS, how it differs from ordinary GPS positioning, and specific applications and benefits for construction and surveying.

Table of Contents

• Definition and basic principles of RTK-GNSS

• Differences from conventional GNSS positioning (error sources and RTK corrections)

• Use of RTK-GNSS in surveying

• Use of RTK-GNSS on construction sites

• Benefits of introducing RTK-GNSS

• Realizing simple surveying with LRTK

• Frequently Asked Questions (FAQ)

Definition and basic principles of RTK-GNSS

RTK-GNSS is a type of positioning method called “relative positioning,” which uses at least two GNSS receivers (a reference station and a rover) simultaneously to determine position. Unlike ordinary GNSS positioning with a single receiver, RTK achieves high precision by canceling out common errors between the two receivers through relative positioning between two points.

Main flow of RTK positioning:

• Reference station – A reference station installed at a point with known accurate coordinates receives signals from multiple GNSS satellites. The reference station compares the position it calculates by positioning with the pre-entered accurate known coordinates and computes the difference (positioning error) in real time.

• Transmission of correction data – The reference station sends the calculated error information to the rover via radio communication or the Internet. This error information is the “correction data.”

• Rover – The rover at the point to be surveyed applies the correction data received from the reference station to its own positioning results and calculates highly accurate coordinates with the errors removed. This allows the rover to know its precise position in real time.

As the name implies, corrections are performed in real time, so RTK positioning provides immediate positioning results on site. There is no need to wait for post-processing, which is a major advantage because positions can be confirmed instantly in the field. RTK can also be considered an evolution of the conventional Differential GPS (DGPS). RTK focuses on the carrier phase of GNSS signals and analyzes minute phase differences to achieve extremely precise distance measurements. Because the carrier wavelengths are relatively short—on the order of several tens of centimeters—using these phase differences makes it possible to pursue positional accuracy down to a few centimeters or less.

Note that for high-precision corrections it is important that the distance between the reference station and rover not be too large. Generally, the baseline length from the reference station to the rover should be kept within about 10 km (6.2 mi). If they are too far apart, error factors assumed to be common to both stations (discussed later) may change and correction accuracy will decrease. However, in recent years network RTK systems (Ntrip methods, etc.) that use the Geospatial Information Authority of Japan’s continuously operating reference station data have become widespread, and networks of multiple reference stations now enable high-precision corrections even at sites tens of kilometers away. First, understand the basic RTK principle using a single reference station, and keep in mind that using such services makes broader-area use possible.

Differences from conventional GNSS positioning (error sources and RTK corrections)

Normally, GNSS positioning performed with a single receiver (standalone positioning) has position errors of several meters to more than ten meters. When a map app on a smartphone shows your current location offset from the actual location, this is due to such errors. On the other hand, RTK positioning can reduce errors to within a few centimeters. The reason RTK achieves such high accuracy is that it cancels out various error sources that affect conventional GNSS positioning by using relative positioning with a reference station.

Major error sources in GNSS positioning:

• Satellite orbit errors: Errors caused by slight deviations in the satellite’s orbital position

• Satellite clock errors: Errors caused by small differences between the satellite’s atomic clock and the receiver’s internal clock

• Ionospheric and tropospheric delays: Delay errors caused by the slowing and refraction of satellite signals in the ionosphere and troposphere

• Multipath errors: Errors caused when radio waves reflect off buildings or terrain and arrive via indirect paths (ghost signals)

• Receiver noise: Errors from internal circuit noise of the receiver or surrounding electromagnetic noise

In standalone positioning, all of the above errors accumulate, limiting accuracy. In RTK, because the reference station and rover receive signals from the same satellites simultaneously, common error components such as satellite orbit errors, satellite clock errors, and ionospheric and tropospheric delays can be almost completely canceled out. By subtracting the error information (correction data) obtained at the reference station on the rover side, coordinates close to the “true position” with these errors removed can be obtained. However, errors that differ by observation point, such as multipath from signal reflections and receiver-specific noise, cannot be completely removed.

Even so, because most of the major error sources can be corrected, RTK delivers vastly higher positioning accuracy. Previously, obtaining high-precision positions required long static observations or averaged measurements, but RTK enables real-time centimeter-level positioning even while moving. This technological innovation has made it possible to replace and streamline tasks that were difficult with conventional GPS accuracy through automated GNSS positioning.

Use of RTK-GNSS in surveying

RTK-GNSS is highly powerful in land and topographic surveying. Cases that used to require a surveyor to set up an optical surveying instrument called a total station (TS) and observe each point carefully can now be covered in a short time using an RTK-capable GNSS receiver. For example, when conducting topographic surveys of a large development site, one base station and one rover can be prepared, and personnel can walk the site to quickly obtain elevations and horizontal positions of successive points. Because laborious tasks such as setting up equipment and ensuring lines of sight can be reduced, required personnel can be drastically reduced and work becomes more efficient.

Coordinates obtained with RTK are recorded as geodetic coordinates based on known reference points (in Japan, for example, JGD2011). Therefore, it is easy to later import the data into CAD drawings or compare them with coordinates on design drawings. Obtaining survey points in a global coordinate system means you can handle data with a common standard from the start without building a local survey network for each site.

RTK is also used for control point surveying. For example, when establishing new control points (points used as the basis for stakes or benchmarks) at a construction site, using a network-type RTK survey (VRS, etc.) that receives correction information from the Geospatial Information Authority of Japan’s continuous reference station network allows the determination of new control points on the spot with centimeter accuracy even at remote sites. This greatly reduces the need for long-distance traverse surveys (extending a survey network from known points one by one) and enables quick creation of site control.

In recent years RTK-GNSS has also been mounted on drones (UAVs) for photogrammetry. Combining an aerial camera with RTK-GNSS eliminates the need to place many ground control points (ground targets) for accurate terrain models and orthophotos. There are cases where RTK-equipped drones completed topographic surveys of large development sites in a short period. Because aerial RTK allows real-time position correction during capture, the labor of installing numerous ground control points was significantly reduced.

Use of RTK-GNSS on construction sites

RTK plays an important role in site management for civil engineering and construction as well. Under the Ministry of Land, Infrastructure, Transport and Tourism’s promotion of ICT construction (smart construction), GNSS receivers are being mounted on construction machinery such as bulldozers and excavators (backhoes) to enable automatic or semi-automatic operations through machine guidance and machine control. For example, by equipping a bulldozer blade with RTK-GNSS and automatically controlling the blade’s vertical position based on height and slope from 3D design data, operators who are not experienced can still perform grading according to the design. Similarly, mounting GNSS on an excavator enables excavation along a pre-set dig line without installing stakes for elevation control.

There are also applications where a roller is equipped with an RTK receiver to manage compaction counts by location. Because the current position of heavy machinery and work history can be recorded with high accuracy, construction quality can be standardized without relying solely on operator intuition or experience. RTK-GNSS surveying instruments are also useful for as-built verification (measuring the completed shape). Tasks that used to require supervisors to walk and perform numerous leveling measurements can now be completed quickly by walking the site with an RTK rover to acquire area-wide elevation data. Recording measurement results as point cloud data allows comprehensive as-built inspection records without omissions.

These applications enable both productivity improvements and quality assurance for the entire project, making RTK an increasingly indispensable technology for next-generation construction sites. RTK use is also expanding in infrastructure maintenance and management. For example, railway companies require millimeter-level displacement measurements for track and catenary inspections, so efforts are underway to mount RTK on measurement vehicles that combine GNSS with high-precision sensors to precisely measure rail and sleeper deformation. Although RTK cannot be used directly in environments where satellite signals do not reach, such as inside tunnels or under elevated structures, research and development are progressing on maintaining high-precision positioning by supplementing short periods without GNSS with augmentation signals from Japan’s quasi-zenith satellite Michibiki (CLAS) and fusion with IMUs (inertial measurement units). RTK and other high-precision positioning technologies are expected to play an increasingly broad role in the future.

Benefits of introducing RTK-GNSS

Introducing RTK technology to construction and surveying sites brings various benefits not found in traditional surveying and construction management. Here are some main advantages.

• Dramatic improvement in positioning accuracy and immediacy: With RTK, you can acquire coordinates in real time with centimeter-level accuracy. This greatly reduces the effort required to establish control points and perform leveling, and because survey results can be confirmed on site, rework is reduced. Because positioning is dynamic and immediate rather than static, as-built inspections and comparisons with design values can be performed rapidly. For example, using an RTK-capable receiver (such as an L1/L2-capable unit), smartphone GPS position information that normally has errors of about 5–10 m (16.4–32.8 ft) can be improved to approximately ±2 cm (±0.8 in), enabling field surveying that was previously difficult to perform instantly.

• Improved work efficiency and productivity: As RTK-GNSS equipment has become smaller and more mobile, workers can carry their own high-precision surveying tools. Surveys can be done immediately when needed with no waiting time, and multiple people can perform surveying and inspection tasks in parallel. Where previously one expensive GNSS unit per team led to waiting, having several low-cost devices allows concurrent operations. Pocket-sized devices now allow easy movement around a site, enabling access to measurement points at heights or in tight spaces.

• Cost reduction and lower barriers to adoption: In the past, a complete RTK-GNSS survey kit could cost millions of yen, but recently affordable GNSS modules and small receivers have appeared, bringing prices within reach of individuals and small companies. Products that work with existing smartphones or tablets are available, so you do not need to purchase large dedicated equipment. Dedicated software is offered as intuitive apps so that even non-experts can operate them after brief training. As a result, it is now easier to incorporate high-precision positioning into field operations without large investments or highly specialized personnel.

• Promotion of digitization and information sharing: Modern RTK-GNSS solutions can integrate with cloud services so that obtained positioning data can be instantly shared and utilized. For example, coordinates of points and photos taken with RTK-capable devices can be uploaded to the cloud on the spot and shared in real time with office staff. This eliminates the need to record notes on paper and manually transfer them later, and data are backed up automatically. In infrastructure inspection, photos of cracks can be saved to the cloud together with precise coordinates and orientation, enabling collaborative access by all stakeholders. Timely sharing of field and office information greatly contributes to efficient report creation and maintenance planning.

• Effectiveness even outside communication coverage or during disasters: RTK positioning typically requires radio communication or Internet connections between the base and rover, but in Japan, devices compatible with the Michibiki-provided Centimeter-Level Augmentation Service (CLAS) can obtain correction data via satellite even in mountainous areas without mobile coverage. In other words, centimeter-level standalone positioning is possible without Internet access. In the 2023 Noto Peninsula earthquake, CLAS-compatible RTK receivers were useful when communication networks were cut, contributing to damage recording and sharing. Even in disaster response situations where bringing large equipment is difficult, a pocket-sized GNSS terminal can quickly record site conditions with precise location data. Being able to secure positioning methods that do not rely on communication infrastructure provides great reassurance for infrastructure maintenance and disaster prevention.

Realizing simple surveying with LRTK

Introducing RTK-capable equipment that is easy for anyone to use is key to leveraging high-precision positioning on site. One noteworthy solution is our digital surveying system “LRTK.” LRTK is an integrated system combining an RTK-GNSS receiver, a dedicated app, and cloud services, designed so that even first-time RTK users can easily start centimeter-level positioning.

For example, attaching the small device LRTK Phone, which can be retrofitted to a smartphone, turns your phone into a high-precision surveying instrument. By attaching a palm-sized receiver with an antenna and battery to the back of your smartphone and launching the dedicated app, you can record high-precision coordinates as casually as using a map app. You can also take photos while positioning and tag the photos with the point’s coordinates and orientation with a single tap. The big advantage is that anyone can use RTK positioning in field work without heavy instruments or specialized knowledge.

For more advanced surveying, the rugged, stationary LRTK Pro series is available, featuring dustproof/waterproof performance and long operating time. This integrated unit includes antenna, GNSS receiver, radio, and battery, and can receive Michibiki’s CLAS signal directly to perform standalone centimeter-level positioning even in mountainous areas with difficult Internet connectivity. It also features a tilt compensation function that automatically corrects the coordinates of the pole tip when surveying with a tilted pole, enabling efficient point acquisition in obstructed sites. A unique product in the lineup is the LRTK Helmet, with an antenna and receiver built into a worker’s helmet; simply walking the site with the helmet on allows hands-free surveying, dramatically improving safety and efficiency.

By using the LRTK series, construction, civil engineering, and surveying sites can easily achieve high-precision GNSS positioning, substantially reducing work time and improving productivity. LRTK meets the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* requirements and is expected to be an optimal solution supporting digital transformation in the construction industry.

Frequently Asked Questions (FAQ)

Q: What is the difference between RTK and ordinary GPS? A: Ordinary GPS positioning (GNSS standalone) uses a single receiver and typically has errors of about 5–10 m (16.4–32.8 ft). RTK positioning uses two receivers, a reference station and a rover, and achieves centimeter-level accuracy by applying error corrections sent from the reference station. Simply put, RTK is a system in which “one GPS corrects the other to improve accuracy.”

Q: What is needed to perform RTK positioning? A: Basically, you need three things: a “GNSS receiver for the reference station,” a “GNSS receiver for the rover (the target to be positioned),” and a “communication method connecting the two (radio or Internet).” Additionally, if you set up your own reference station, you need to determine its accurate coordinates (latitude, longitude, and elevation) beforehand. If you do not prepare your own reference station, you can obtain correction information via the Internet from national or private reference station networks (for example, the Geospatial Information Authority of Japan’s continuous reference stations or VRS services).

Q: What positioning accuracy can be achieved with RTK? A: When properly operated, RTK positioning can achieve horizontal position errors of about 2–3 cm (0.8–1.2 in) and vertical accuracy on the order of a few centimeters. Under favorable environmental conditions with ample satellite visibility, even higher accuracy (around 1 cm (0.4 in)) can be achieved. However, in areas with dense tall buildings or where the sky is not open, satellite reception may be poor and accuracy may degrade.

Q: How far apart can RTK be used? A: When using a single reference station, it is desirable for the distance between the reference station and rover to be roughly within about 10 km (6.2 mi). Correction accuracy decreases as distance increases due to differences in atmospheric conditions. If you want to cover a wider area, using network RTK with multiple reference stations such as continuously operating reference stations allows high-precision corrections for rovers tens of kilometers away.

Q: Is RTK used outside surveying and construction? A: Yes. For example, in agriculture, RTK-GNSS is used for automatic steering of tractors, enabling precision agriculture that performs seeding and pesticide application with centimeter-level accuracy. RTK is also used in drone mapping for high-precision map creation, in localization for automated guided vehicles and autonomous vehicles, and in creating high-precision maps—any field that requires highly accurate positioning benefits from RTK technology.

Q: Can RTK positioning be used with smartphones? A: Yes. In recent years it has become possible to utilize RTK correction information on smartphones. By combining a small RTK-capable GNSS receiver that can be attached to a smartphone with a dedicated app, you can enhance the GPS position data obtained by the phone to centimeter accuracy. For example, attaching our LRTK Phone device to a smartphone enables high-precision positioning without traditional surveying instruments.

Q: Are RTK-capable surveying instruments expensive? A: In the past, RTK-GNSS equipment could cost millions of yen, but recently affordable GNSS modules and small receivers have appeared, making them more accessible to individuals. Some inexpensive receivers can be purchased for under a few hundred thousand yen, and products that can be used with smartphones are becoming widespread. High-end professional RTK equipment remains relatively expensive, but you can now choose affordable equipment based on required accuracy and functionality.