How RTK GPS Accuracy Is Different! Comparison with Conventional GPS Surveying and Benefits

Table of Contents

• What is RTK GPS?

• Differences from conventional GPS surveying (mechanism, accuracy, convenience)

• Applications of RTK GPS (construction, disaster surveys, infrastructure inspections, etc.)

• Benefits of RTK GPS and barriers to adoption

• RTK as a trend in future surveying technology

• Summary and introduction to simple surveying with LRTK

• FAQ (frequently asked questions and answers)

What is RTK GPS?

RTK GPS stands for "Real Time Kinematic GPS" and uses GNSS (Global Navigation Satellite System) to perform positioning in real time with centimeter-level accuracy (cm level accuracy (half-inch accuracy)). Whereas standard GPS (standalone positioning) determines its position relying only on satellite signals, RTK achieves higher accuracy through relative positioning using two or more receivers (a base station and a rover).

In the RTK method, a GNSS receiver called the reference station (base station) is first installed at a precisely known coordinate point. The base station receives signals from each GPS satellite and calculates in real time the difference between the position it computes and the known precise coordinates. This difference becomes the positioning error (correction value) and is transmitted via radio or the Internet to the mobile station (rover) operating in the field. The rover uses the received correction data to adjust its own positioning results and can immediately obtain a high-precision current position with the errors canceled out.

In this way, because RTK applies corrections simultaneously with satellite positioning, there is no need to process data afterward and it is characterized by the ability to confirm and record position coordinates on the spot. It can be seen as an advancement of conventional differential GPS (DGPS), and the secret to its high accuracy is that it uses the satellite signal's carrier phase (phase shift of the carrier wave). Because the carrier has a short wavelength on the order of several tens of centimeters (several tens of inches), analyzing those phase differences allows positioning to be pursued to an error level of a few centimeters (a few inches). With these technologies, RTK GPS enables real-time and high-precision positioning in surveying and civil engineering work, and has been attracting increasing attention in recent years.

Differences from conventional GPS surveying (mechanism, accuracy, usability)

Difference in mechanism: Conventional GPS positioning (standalone positioning) is performed with only one receiver, so it cannot correct satellite signal errors and simply computes the position as is. Signal delays in the ionosphere and troposphere, satellite clock and orbit errors, multipath (reflections) at the ground, and other various error factors accumulate, inevitably limiting positioning accuracy. This is why the current position shown in a smartphone map app can be off by several meters. In contrast, RTK surveying cancels common errors by relative positioning to a reference station. Because the reference station and the rover receive signals from the same satellites, error components common to both (such as satellite and atmospheric effects) are eliminated by differencing, and a highly accurate position is calculated from the remaining minute differences. In other words, the principle of RTK is "a mechanism that dramatically improves accuracy by mutually correcting each other with another GPS unit."

Difference in accuracy: Single-point positioning accuracy is generally around 5～10 m (16.4～32.8 ft) horizontally, and under some conditions the deviation can be larger. Errors in the vertical (elevation) direction are particularly large, and ordinary GPS has difficulty measuring altitude accurately. On the other hand, using RTK positioning, errors are contained within a few cm (within a few in). Planar positions can achieve ± 2～3 cm (± 0.8～1.2 in), and in elevation also about ± a few cm (± a few in); in good conditions, accuracies in the 1 cm range (cm level accuracy (half-inch accuracy)) are also achievable. In other words, with RTK GPS you can pinpoint a position not to within several meters on a map but to a fingertip-sized area. This difference is why people say "positioning accuracy is in a different league," and, for example, it enables obtaining the precise surveying accuracy required for establishing control points and as-built/construction quality management directly with GPS.

Difference in convenience: The advantage of conventional GPS is the ease of determining position with just a smartphone, without dedicated equipment, but in reality its low accuracy means it cannot be used as-is for surveying purposes. To achieve high accuracy, careful observations with a total station (optical surveying instrument) or post-processing long-duration GPS observation data are required, making it difficult to obtain immediate results. RTK methods require preparing dedicated GNSS receivers and communication means, but have the advantage of yielding centimeter-level (about 0.4 in) results on the spot. In recent years, network RTK (a method that receives corrections via communication from government or private reference station networks) has also been developed, making high-precision positioning possible without installing your own base station. In addition, miniaturization and cost reduction of equipment have lowered the barriers to RTK use compared to before. In short, the difference is that "RTK requires a bit more preparation but offers overwhelmingly higher accuracy," while "conventional GPS surveying is easy but less accurate." Nowadays RTK has become more user-friendly, and systems that combine ease of use with high precision have emerged.

Use Cases of RTK GPS (construction, disaster surveys, infrastructure inspections, etc.)

RTK GPS, leveraging its high accuracy and immediacy, has begun to serve as an alternative to traditional surveying methods across a variety of field settings.

The main application scenarios include the following examples.

• Construction and civil engineering: In surveying for development sites and construction sites, using RTK enables rapid as-built management and stakeout (positioning). Even on large sites, workers can walk around carrying a rover receiver and instantly obtain the coordinates and elevations of each point. Tasks that previously required a surveyor to set up a total station and sight and record each point are greatly streamlined. Also, under the Ministry of Land, Infrastructure, Transport and Tourism's promoted ICT construction (smart construction), machine guidance and machine control, which equip construction machinery such as bulldozers and excavators with GNSS receivers for automatic control, have been put into practical use. This technology likewise depends on RTK's high-precision positioning and contributes to improved finish accuracy in earthworks and reduced labor requirements.

• Disaster investigation and disaster prevention: RTK helps quickly assess conditions at sites affected by earthquakes or heavy rainfall. For example, if the locations of ground fissures or landslide areas are recorded with high precision, they become indispensable materials for planning restoration work and sharing damage information. In particular, RTK devices that can perform positioning even in offline environments make surveying possible even when communication infrastructure is cut off by a disaster. In fact, during recent large earthquakes, pocket-sized RTK receivers proved useful in disaster areas where mobile phone networks were disrupted, contributing to recording and cloud-sharing damage sites with position information at centimeter-level accuracy (cm level accuracy (half-inch accuracy)). Even at sites where large-scale equipment cannot be brought in, a single small RTK unit allows personnel to walk the site themselves and perform surveys, greatly aiding rapid situation assessment and information sharing.

• Infrastructure inspection and maintenance: The use of RTK is advancing in inspection work for infrastructure such as roads, bridges, and tunnels. For example, when examining cracks in a bridge or a road collapse, recording the location as high-precision coordinates allows you to accurately reflect it on a map in the office later and plan repairs. Using RTK-enabled smartphone apps also makes it easy to upload photos to the cloud with accurate latitude, longitude, and orientation information attached. Because inspectors can share data from the field with one tap, the need to handwrite on paper drawings and carry them back is eliminated, speeding up report preparation and information sharing among stakeholders. Furthermore, the high-precision data obtained can be used to facilitate advanced initiatives such as projecting design drawings and piping networks onto the site with AR (augmented reality) or visualizing repair locations on site.

In addition, there are cases in drone-based aerial surveying where RTK is used to reduce the number of ground control points, and smart agriculture, where RTK-GNSS is used for autonomous tractor operation; centimeter-level positioning is being used across a wide range of fields.

Benefits and Implementation Hurdles of RTK GPS

While introducing RTK GPS brings many benefits, there are also issues (hurdles) that should be considered for its operation. Here we organize the main benefits and the hurdles at the time of implementation.

Benefits

• Dramatic improvement in positioning accuracy: To reiterate, the centimeter-level positioning accuracy provided by RTK is the greatest advantage. This allows delicate alignment tasks that traditionally relied on a surveyor's intuition and manual labor to be replaced by machine-based positioning. For example, leveling surveys and the installation of batter boards can also be handled by GNSS, reducing rework and the accumulation of errors and contributing to quality assurance. Because precise coordinates, including elevation, can be obtained, comparison with design values for as-built management and on-site verification of construction results become possible, contributing to improved construction quality.

• Improved Work Efficiency and Productivity: By enabling high-precision positioning in real time, the speed of surveying work increases dramatically. A single operator carrying an RTK receiver can acquire a large number of survey points in a short time, allowing labor savings even for wide-area surveys. Because multiple people can carry their own GNSS units and conduct surveys concurrently, there is no longer any waiting time in line. As a result, cost advantages such as reduced personnel and shortened project duration are realized. In addition, because the obtained data can be stored and shared electronically, time spent handwriting in field notebooks and transcribing data is also reduced.

• Safety and reduced burden: GNSS surveying can acquire positions even in locations where ensuring a clear line of sight is difficult, so, for example, there is no need to take the risk of installing surveying instruments beneath cliffs with poor visibility or at disaster sites. It also reduces the burden of workers having to carry equipment for long periods while moving between survey points. Recently, products that integrate GNSS antennas into helmets have appeared, enabling workers to survey simply by walking, and the use of RTK is also contributing to improved on-site safety.

• Support for digitalization: Coordinate data acquired by RTK can be directly imported into CAD drawings and GIS maps or shared via the cloud, making digital data integration easy. By replacing paper-based workflows with digital ones, you can promote DX (digital transformation) of operations. The Ministry of Land, Infrastructure, Transport and Tourism's *i-Construction* initiative also calls for electronic delivery using high-precision as-built management data and the use of 3D design data, and RTK can be said to be the foundational technology supporting this trend.

Barriers to Implementation

• Hardware acquisition cost: In traditional systems that require a set of RTK-GNSS receivers and base stations, initial costs could run to several million yen, posing a burden for small businesses. However, in recent years cheaper GNSS chips and compact terminals have appeared, and products available from around a few hundred thousand yen have become more common. Depending on the required accuracy and functionality, it is increasingly possible to choose affordable equipment, and cost barriers have been falling — for example, smartphone-compatible RTK terminals mentioned below.

• Technical training and operation: To perform high-precision positioning correctly, a certain level of knowledge is required, such as reference station coordinate setting, handling of geodetic datums, and communication settings. Also, to use survey results officially, it may be necessary to have the equipment certified and to perform work under the supervision of a licensed surveyor (regulations for public surveys, etc.). On the operational side, there is also the burden of ensuring a communication environment to receive satellite correction information and of setting up equipment on site. These represent new skills to master for technicians accustomed to handling conventional optical surveying instruments; however, recently software automation and UI improvements have simplified setup tasks, and there are an increasing number of products that non-specialists can operate after a short period of training.

• Dependence on the positioning environment: GNSS positioning fundamentally requires an open sky to achieve good accuracy. In the canyons of high-rise buildings or inside forests, satellites cannot be observed sufficiently, causing positioning to become unstable or accuracy to deteriorate. Also, because the correction effect weakens as you move too far from the reference station, single base station operation is generally limited to approximately within 10 km (32808.4 ft) (errors increase beyond that). Due to these constraints, measures are needed such as installing base stations on buildings in urban areas or using equipment on the mobile station that can receive augmentation signals from the Quasi-Zenith Satellite (Michibiki) in mountainous areas. However, by using network RTK services that combine data from multiple reference stations to cover a wide area, the distance issue has been considerably mitigated today. As for environmental factors, the latest receivers are multi-GNSS capable (using not only GPS but also GLONASS, Galileo, Michibiki, etc. simultaneously), increasing the number of positioning satellites and thus improving the reliability in maintaining accuracy.

RTK as the Future Trend in Surveying Technology

High-precision GNSS surveying technologies, such as RTK, are expected to become indispensable in future surveying and construction management. Behind this are the trends of miniaturization and cost reduction of equipment and the trend toward ICT adoption. RTK, which used to be a technology limited to specialized surveying firms, can now be used by construction companies, local governments, and contractors that carry out in-house construction. If anyone on site can immediately handle high-precision positional information, delays caused by waiting for surveys or layout will be reduced and overall productivity will improve. This change also contributes to the nationwide promotion of DX (digital transformation) in the construction industry.

On the technical side, the Quasi-Zenith Satellite System Michibiki and satellite positioning services from various countries are expected to provide sub-centimeter-level positioning. Initiatives to deliver wide-area correction information via satellite, such as Japan's CLAS and the United States' PPP-RTK services, have already begun, and in the future an era may come when "just turning on a receiver under the open sky yields immediate centimeter-level accuracy." RTK's integration with other technologies will also advance. For example, applications could include using drones plus RTK to automatically survey disaster sites inaccessible to people, or integrating RTK into AR glasses to display guidance information in a worker's field of view. If high-precision positioning is established as readily available infrastructure, measurement and position-checking processes that were previously time-consuming will be automated behind the scenes, and we can expect a future in which human effort can be directed toward more creative work. RTK can truly be said to be a trend-setting technology that will guide the future of surveying and construction.

Summary and Introduction to Simplified Surveying with LRTK

As we've seen so far, RTK GPS provides overwhelmingly higher accuracy and immediate on-site usability compared with conventional GPS surveying. Centimeter-level differences in accuracy elevate the quality control of surveying and construction to the next level and offer many benefits in terms of efficiency and safety. On the other hand, traditionally it required the preparation of specialized equipment and advanced knowledge, so it was not something everyone could handle easily. However, in recent years this situation has been changing significantly. Technological innovation has produced products and services that make RTK easier to use, making it easier for beginners and small-scale sites to take on high-precision surveying.

A representative example is the simple RTK surveying system for smartphones "LRTK". LRTK is a solution composed of a compact GNSS receiver that can be attached to an iPhone or similar device and a dedicated app, designed so that anyone can achieve centimeter-level positioning without complex configuration. By simply attaching the palm-sized, weighing approximately 160 g device to a smartphone and turning it on, it receives signals from satellites and can achieve high-precision positioning in about 20 seconds. Correction data from reference stations are automatically obtained via the internet, and when the status is Fix (integer fixed solution), positions can be measured with an accuracy of about horizontal ±2 cm (±0.8 in) and vertical ±4 cm (±1.6 in). On site, your current coordinates are displayed on the smartphone screen in real time, and then you simply record coordinates for each point in the same way you would use a map app. Saved data can be synced to the cloud, allowing office staff and partner companies to share and review them immediately.

By leveraging LRTK, tasks that previously required expensive surveying equipment and specialized skills can be replaced with a single smartphone. For example, by combining an iPhone’s camera and LiDAR capabilities you can perform 3D point cloud scans and assign high-precision coordinates to the acquired point clouds. It also supports AR (augmented reality) navigation that overlays construction drawings or the positions of underground utilities on the smartphone screen, and includes guidance functions that lead operators to staking/pile-driving positions, intuitively supporting tasks that traditionally relied on the intuition of experienced workers.

Furthermore, because LRTK is compatible with the high-precision augmentation signals (CLAS) broadcast by Japan’s Quasi-Zenith Satellite Michibiki, it can continue centimeter-level positioning (cm level accuracy (half-inch accuracy)) by satellite communication alone even in areas where mobile signals do not reach. Thanks to this, it remains effective in situations with unstable communications infrastructure, such as immediately after a disaster, and data recorded on site can be synced and shared later after moving to a location with network connectivity.

In fact, some municipalities, including Fukui City, have introduced smartphone surveying using LRTK for disaster response and infrastructure management, reporting that it can acquire field data more quickly and at lower cost compared with conventional methods using total stations. Private construction sites are also beginning trial implementations, adopting it as a simple positioning tool to assist with batter board installation and as-built inspections.

These solutions that bring the benefits of RTK GPS closer to users are accelerating the democratization of surveying. If the on-site personnel themselves can perform surveys immediately when needed without having to call in a specialized survey team, not only will work speed and efficiency improve, but centralized management and sharing of data will also become smoother. RTK technology is expected to continue evolving and spreading, but the best way to appreciate its convenience is to try it on-site first. Why not leverage simple surveying systems like LRTK and take on the challenge of transforming your operations with high-precision positioning?

FAQ (Frequently Asked Questions and Answers)

What is the difference between RTK and regular GPS?

A: Regular GPS positioning is performed with a single receiver and cannot correct satellite signal errors, so it produces errors on the order of several meters. By contrast, RTK positioning uses two units, a base station and a rover, and by applying the correction data sent from the base station can reduce errors to a few centimeters. Simply put, RTK is "a system that improves accuracy by using another GPS unit so the two cancel out each other's errors."

What is required to perform RTK positioning?

A: As a basic configuration, three things are required: "a GNSS receiver for the base station", "a GNSS receiver for the rover", and "a means to connect the two via communications". In addition, if you install a base station you need to determine its precise coordinates in advance. If you do not provide your own base station, you can use existing base station networks such as the Geospatial Information Authority of Japan's Continuously Operating Reference Stations (CORS) or commercial VRS services. Recently, services that provide correction information over the Internet have become widespread, and the rover can automatically obtain base station data as long as it has a communications terminal.

How accurate can RTK be?

A: With properly operated RTK positioning, you can expect roughly 2–3 cm (0.8–1.2 in) of error in horizontal position, and vertical accuracy on the order of a few cm (a few in). Under good environmental conditions (clear sky view and a high number of satellites), even higher accuracy can be achieved—for example at the 1 cm level (around 0.4 in). However, in areas where tall buildings are clustered or in forests, satellite signals can be disrupted and accuracy may degrade. "Used in open-sky environments, you can get a few cm (a few in) accuracy; under poor conditions it can be tens of cm (several in) or more."

How far away can RTK positioning be used?

A: Generally, when using a single reference station it is desirable for the distance between the reference station and the rover to be within 10 km (6.2 mi). As the distance increases, the difference in atmospheric errors at the two sites grows, reducing the effectiveness of corrections and degrading accuracy. If you want to use it over a wider area, network RTK that combines data from multiple reference stations can achieve high-precision positioning even when the rover is tens of kilometers (tens of miles) away from any single reference station. In that case, techniques such as virtual reference station (VRS) are used to compensate for distance-dependent errors.

Can you use RTK positioning with a smartphone?

A: Yes. In recent years it has become possible to make use of RTK positioning with smartphones. By attaching a small RTK-capable GNSS receiver to your smartphone and using a dedicated app, you can enhance the position information obtained by the phone to centimeter-level accuracy (half-inch accuracy). For example, if you mount a device like the LRTK introduced in this article on your smartphone, you can perform easy high-precision positioning without traditional surveying equipment. This allows field workers to use their own smartphones to survey immediately when needed, increasing the number of situations where they don’t have to request the surveying specialist department.

Are RTK-capable devices expensive?

A: Traditionally, a complete RTK-GNSS surveying system could cost several million yen, making it very expensive. However, in recent years inexpensive GNSS modules and compact receivers have appeared, making them more accessible to individuals. Some affordable receivers can be purchased for under a few hundred thousand yen, and products that can be used in combination with smartphones have begun to become widespread. High-performance, survey-specific instruments are still costly, but by determining the required accuracy and features, it is becoming possible to choose RTK equipment in a more affordable price range. You can also try equipment by renting it or use subscription-based services, so it is worth considering an acquisition model that fits your company’s needs.