What is VRS in RTK? Why it's mainstream in the United States and how it works

Table of contents:

• Introduction

• What is RTK?

• What is VRS?

• Relationship between GNSS positioning accuracy and RTK

• Why VRS is mainstream in the United States

• Comparison with conventional methods

• Issues of RTK/VRS

• Practical operation of RTK/VRS

• Future prospects of RTK/VRS

• Simple surveying using LRTK

• FAQ

Introduction

RTK (Real-Time Kinematic) is a technique that achieves real-time, centimeter-level (cm level accuracy, half-inch accuracy) high-precision positioning by applying differential corrections to positioning data from GNSS satellites, including GPS. Traditionally, a reference station had to be set up at each survey site, but the recently introduced VRS (Virtual Reference Station, virtual reference point) method allows uniform accuracy over wide areas without placing a reference station on site. In particular, network RTK has become mainstream in the United States, driven by the development of large-scale reference station networks and the expanded application across many fields. This article explains in detail the basics and mechanisms of RTK and VRS, and the reasons why the VRS method has become widespread in the United States. It also covers comparisons with conventional technologies, challenges, field use cases, and future prospects, and concludes by introducing LRTK, which is attracting attention in Japan as a solution for easily achieving high-precision positioning.

What is RTK?

RTK (Real Time Kinematic, real-time kinematic) is a relative positioning method using two GNSS receivers that determines high-precision position coordinates in real time. Typically, one receiver is installed at a point with known accurate coordinates (a reference point) and designated as the reference station (base station), and the other is placed at the point to be surveyed and called the mobile station (rover). Both receivers simultaneously receive signals from multiple GPS satellites (GNSS satellites), and each independently computes its own position.

Here, because the reference station knows its true position, it can calculate the positioning error by finding the difference between the satellite positioning result it receives and its actual position. The reference station then transmits that real-time error (correction information) successively to rovers via radio or communication links. The rover uses the received correction information to immediately correct the error components contained in its own GNSS positioning results. Through such differential corrections, effects that could not be avoided with standalone GNSS positioning—such as satellite orbit and atomic clock deviations, and ionospheric and tropospheric delays—are canceled out, dramatically improving positioning accuracy. Normally, GPS in smartphones and car navigation systems has errors of several meters (several ft), but with RTK the horizontal position can be reduced to about 1–2 cm (0.4–0.8 in), and the vertical direction to a few centimeters (a few in). In other words, RTK theoretically enables centimeter-level (sub-inch) positioning almost instantaneously.

RTK positioning has been widely used in situations requiring precise positioning, such as highway and railway construction, land boundary surveying, and construction equipment operation control. However, this method has the constraint that a reference station must be set up on-site. To establish a reference station, it is necessary to first obtain the coordinates of a known point and then mount expensive GNSS receivers and antennas on a tripod, setting them up along with batteries and radio equipment. Also, if the distance between the reference station and the rover (the baseline length) is too great, errors that cannot be fully corrected due to differences in atmospheric conditions at the two locations increase, degrading accuracy. Therefore, in practical operations, the reference station is placed near the work area (ideally within a few kilometers (a few miles)), and when the area is large the reference station is moved as needed or multiple reference stations are established. Setting up and managing reference stations takes time and effort and requires specialized knowledge, so this aspect has been one of the hurdles to the widespread adoption of RTK positioning.

What is VRS?

To eliminate the effort of installing a reference station at every site, a method called network RTK was developed. A representative example is the VRS (Virtual Reference Station, virtual reference point) method, which provides correction data as if a reference station were located near the user. Specifically, it uses a network of many reference stations (electronic reference points) deployed over a wide area by governments or companies; the rover's (user's) approximate position is sent to a server, which integrates and analyzes data from multiple surrounding reference stations. A virtual reference point is then established near the user, and the positioning errors that would have been received at that point are calculated in real time. The correction information from the generated virtual reference station is sent to the rover via a mobile network (the Internet), and on the rover side RTK processing is performed as if a reference station were "right next door," enabling a centimeter-level positioning solution (half-inch accuracy).

By using this VRS method, surveyors can achieve high-precision positioning with only a single receiver (rover) without preparing their own base station. The preparation time and personnel previously required to set up a base station are no longer necessary, leading to significant efficiency gains. Also, because virtual reference points are always set near each positioning location, there is the advantage that you can obtain uniform centimeter-level accuracy over a wide area (cm level accuracy (half-inch accuracy)) without worrying about accuracy degradation due to baseline length. For example, even when a single person surveys points while moving around a large development site, they can work with confidence while maintaining the same accuracy across the entire area. With conventional single-base-station RTK there used to be concerns like “beyond this point the accuracy might be unreliable,” but with VRS those concerns are almost unnecessary.

In recent years, because of this convenience, network RTK has become globally mainstream. In Japan, the Geospatial Information Authority of Japan's network of continuously operating reference stations (approximately 1,300 stations, GEONET) provides a real-time correction service, allowing high-precision coordinates in the global geodetic reference frame to be obtained anywhere nationwide without installing a base station. In addition, private telecommunications companies and survey-equipment manufacturers are also deploying their own network RTK services. For example, "ichimill" has deployed more than 3,300 reference stations nationwide, and subscribers can simply power on a receiver in the field to obtain centimeter-level (half-inch-level) positioning anywhere in Japan instantly. Also, Japan's Quasi-Zenith Satellite System "Michibiki" has provided a centimeter-level (half-inch-level) augmentation service (CLAS) since 2018; with a compatible receiver, users can receive correction information directly from the satellites and continue high-precision positioning even in mountainous or other areas outside cellular coverage. These technological advances have made RTK surveying markedly easier and more flexible.

Relationship between GNSS Positioning Accuracy and RTK

GNSS (Global Navigation Satellite System) is a collective term for satellite positioning systems operated by various countries and regions, such as the United States' GPS, Russia's GLONASS, Europe's Galileo, and Japan's Michibiki (QZSS). When people commonly say "GPS," they usually mean the U.S. system among these GNSSs, but in this article, for convenience, we use "GPS" or "satellite positioning" to refer broadly to positioning by GNSS satellites.

Standalone GNSS positioning (positioning performed with a single receiver) suffers errors from various factors associated with satellite signal propagation, inevitably causing position offsets of several meters (several ft). For example, slight errors in satellite clocks and orbits, signal delays from the ionosphere and troposphere, and radio reflections (multipath) from buildings and terrain can combine, producing phenomena such as a smartphone map showing the current location off by several m (several ft). In many applications an error of several meters (several ft) may be acceptable, but in civil surveying and construction even deviations of a few centimeters (a few in) cannot be tolerated. That is where differential correction techniques such as RTK come in, canceling GNSS errors in real time to achieve centimeter-level accuracy (half-inch accuracy).

RTK uses carrier-phase signals from GNSS satellites to remove errors through relative positioning with a reference station, as described above. In recent years, not only GPS but the combination of multiple satellite systems—known as multi-GNSS—has increased the number of available satellites, further improving positioning accuracy and stability. In particular, in urban and mountainous areas the sky is often obstructed and the number of satellites in view tends to decrease, but by receiving GLONASS, Galileo, and Michibiki in addition to GPS, RTK solutions are less prone to interruption and this also contributes to faster initialization (obtaining a fixed solution). In this way, the evolution of GNSS itself has also greatly contributed to improvements in RTK performance.

Reasons VRS Is Mainstream in the United States

In the United States, because RTK networking developed early, the VRS approach has become established as the mainstream for high-precision positioning. The reason is that, in a country as vast as the United States where individual sites tend to be large, it was more efficient to obtain corrections from a wide-area network than to set up one’s own reference station each time.

There are many examples of state geodetic agencies and Departments of Transportation (DOTs) building statewide VRS networks (real-time reference station networks) and providing them to surveyors and engineers. For example, the Ohio Department of Transportation installed numerous reference stations throughout the state and launched VRS services in the mid-2000s, making them freely available to surveyors in the state.

These state-level networks now span the United States, and subscribers can obtain high-precision correction information simply by activating a rover in the field. In addition, data from the nationwide CORS (Continuously Operating Reference Stations) network operated by the National Geodetic Survey (NGS) are used by various services, and the fact that they were put in place early as infrastructure helped drive their adoption.

Furthermore, demand across diverse fields, such as precision agriculture and the automated control of construction machinery, has been a driving force behind the spread of VRS. Because autonomous operation of tractors over vast farmland requires consistent centimeter-level accuracy (cm level accuracy, half-inch accuracy) over wide areas, agricultural machinery manufacturers have been establishing their own RTK base station networks and providing correction services via satellite communications (PPP method).

In the construction industry, network RTK—which allows multiple pieces of heavy equipment and surveying crews to use high-precision coordinates simultaneously—dramatically improves on-site efficiency, so it was adopted early on, particularly for large-scale infrastructure projects. For example, major U.S. surveying-equipment manufacturers Trimble and Leica offer paid VRS services available nationwide (such as Trimble VRS Now and Leica SmartNet), and many surveying firms and construction companies subscribe to these services. There are also regions where state-government–provided networks can be used free of charge or at low cost, creating an environment in which small and medium-sized surveying contractors can readily use network RTK.

Thus, supported by early infrastructure build-out and broad industrial needs, VRS has become the norm for high-precision GNSS positioning in the United States. If you have an RTK receiver, you can receive corrections from various services without setting up a dedicated base station, so in many field sites the standard workflow is to first connect to the network and then begin positioning. From surveying to agriculture and autonomous driving, much of centimeter-level positioning in the United States is achieved via VRS networks.

Comparison with Conventional Methods

As noted above, the VRS approach outperforms conventional methods in terms of simpler setup and broader applicability, and there is a growing number of cases where it is easier to adopt from an initial-cost standpoint. However, service use entails running costs such as monthly subscription fees, but even after accounting for these, the benefits from reduced labor costs and improved efficiency are considered substantial.

Challenges of RTK/VRS

• Dependence on communications: In the VRS method correction data are received in real time, so there is a dependence on communication lines such as mobile phone networks. If the rover moves out of cellular coverage or communication is interrupted, high-precision positioning cannot be maintained during that time (if a CLAS-compatible receiver can receive correction information directly from satellites, positioning can continue even outside communication coverage). Service provider servers or the communication network can also affect availability if they experience failures.

• Satellite visibility and environmental factors: GNSS positioning assumes that satellite visibility in the sky is available. If you are not outdoors with an open sky, positioning itself becomes difficult; even with high-precision RTK, satellites cannot be adequately tracked under overpasses, inside tunnels, or in forests. Reflections (multipath) from building facades or heavy machinery and radio interference can also degrade accuracy. In urban areas, when surrounded by tall buildings it can be difficult to obtain a fixed solution, and positioning may become unstable.

• Initialization and positioning stability: To obtain an RTK fixed solution (integer solution), the receiver must sufficiently track the carrier-phase from satellites and correctly resolve the integer biases. Under good environmental conditions, initialization can be achieved in several seconds to several tens of seconds, but if satellites are lost reinitialization is required. Accuracy can temporarily degrade when passing into the shadow of buildings while moving. Also, when ionospheric disturbances are large, corrections may not keep up and accuracy can be affected.

• Operational costs: When using a network-type RTK service, running costs such as monthly fees or annual membership fees are incurred (in some regions these may be provided free of charge by government authorities). On the other hand, with the conventional approach you needed initial investment to equip your own base station equipment, but the operation itself incurs no costs other than communication charges. Whichever you choose, it is necessary to compare and consider the required expenses and the benefits of improved accuracy.

• Handling of positioning coordinates: The position coordinates obtained by RTK/VRS are usually based on a global geodetic datum (in Japan, the World Geodetic System). In actual construction and surveying, it may be necessary to convert to a local plane coordinate system and to compute heights by applying geoid height corrections to the ellipsoidal heights. Also, although usability has improved compared to traditional surveying instruments, high-precision positioning still requires fundamental knowledge of GNSS and geodetic datums.

While mindful of the challenges described above, RTK/VRS is operated as a technology that greatly contributes to improving on-site productivity. In the future, advances in satellite and communication technologies are expected to gradually address these challenges.

Practical Operation of RTK/VRS

RTK and VRS dramatically improve the efficiency and productivity of surveying work in the field. For example, stakeout and boundary point surveys that were traditionally performed by two people using a total station can be completed by a single person in a short time if a high-precision GNSS rover is used. Even on large development sites there is no need to worry about the base station’s radio coverage, and because workers can take measurements anywhere simply by turning on the receiver on site, mobile surveying and as-built management are significantly sped up.

Also, by using the coordinate guidance (navigation) feature, you can input the coordinates of the stake locations and reference points shown on the plans into the receiver and easily have it guide you to those spots on site. Because the direction and distance to the destination are displayed in real time on the screen of a tablet or smartphone equipped with a GNSS receiver, even inexperienced workers can drive stakes to the correct positions without getting lost. With a system that includes AR capabilities, intuitive methods are also possible, such as displaying markers for the installation points on the camera feed to guide users.

Furthermore, when combined with machine guidance・machine control, which equip heavy machinery with GNSS receivers and control units, construction automation advances significantly. Bulldozer and excavator operators can focus on operating the vehicle while the machine itself determines its position using GNSS and automatically adjusts the blade’s height and tilt to achieve the design-specified finish without manual intervention. Surveyors’ effort to measure and check as-built conditions after construction is reduced, and immediate, accurate as-built data prevents rework.

By deploying GNSS positioning in the field in this way, high-precision surveying and construction management can be carried out with a small number of personnel, allowing high productivity to be maintained even in the construction industry facing labor shortages. In addition, coordinate data obtained by GNSS can be shared immediately with stakeholders via the cloud, so post-survey data processing and communication are smooth. For example, at a disaster site a single technician can survey the damaged area and, on the spot, send point clouds of the terrain and the precise coordinates of damaged facilities to headquarters via the cloud. Even in situations that used to require a long initial response time, network-RTK-compatible equipment can speed up field surveys, and the precise data obtained can be used immediately for disaster prevention planning and restoration design.

Future prospects of RTK/VRS

High-precision GNSS positioning technology is expected to evolve further. First, fully utilizing multi-GNSS such as GPS, GLONASS, Galileo, BeiDou, and QZSS, and using multiple frequencies such as L5 in addition to L1/L2 will reduce positioning outages caused by an insufficient number of satellites or weak signal strength, enabling more stable centimeter-level accuracy (half-inch accuracy) even in urban canyons and under tree cover. In addition, augmentation of the positioning satellites themselves and new technologies are advancing. Japan’s quasi-zenith satellite system “Michibiki” is expected to be expanded to a seven-satellite configuration in the future, which should further improve availability. Europe’s Galileo has also started offering a High Accuracy Service (HAS), and an era in which decimeter-class correction information (0.1 m / 0.33 ft / 3.9 in) can be obtained directly from satellites worldwide is imminent.

Also, the development of PPP-RTK technology, which fuses RTK and PPP (Precise Point Positioning), is attracting attention. Orbit and clock errors are covered by wide-area common corrections (SSR), and by correcting only the local atmospheric errors the system can achieve centimeter-level positioning quickly with fewer reference stations than before. In fact, Japan’s Michibiki CLAS broadcasts SSR-type augmentation information from satellites, and hybrid correction services that combine satellites and ground networks may become mainstream going forward. This will enable high-precision positioning even in remote areas or at sea where cellular coverage does not reach, further expanding the range of applications.

Devices and software are also advancing toward miniaturization and simplification. GNSS receivers are becoming ever smaller and lower power, and high-precision models that can perform positioning using only chips built into smartphones have begun to appear (among Android smartphones there are models that support L1/L5 dual-frequency and achieve accuracies of tens of centimeters (several inches)). In the future, mobile devices in the hands of general users will also be able to perform centimeter-level positioning (half-inch-level positioning) easily by connecting to the appropriate correction services. In addition, features that make fieldwork easier, such as tilt compensation provided by receivers’ built-in IMUs (inertial measurement units), have been put into practical use. Even if a pole is somewhat tilted, the exact coordinates directly beneath the tip can be obtained, reducing the burden of taking survey points in confined spaces and on sloped ground.

Moreover, with high-precision positioning becoming commonplace, application areas will expand. GNSS-based positioning is expected to be utilized in a variety of scenarios—from social infrastructure to everyday services—such as determining the driving position of autonomous vehicles, autonomous navigation and aerial surveying by drones, and visualization of buried utilities and precision navigation using AR glasses. RTK and VRS, leveraging their high accuracy and real-time capabilities, will become one of the foundational technologies supporting the smart society of the future.

Simple Surveying with LRTK

Finally, I will introduce LRTK (LRTK), a solution gaining attention in Japan for easily utilizing RTK/VRS positioning. LRTK is a product series developed by Lefixea (Refixia), a startup spun out of Tokyo Institute of Technology, and is built on the concept of “a pocket-sized RTK surveying device that anyone can use.” It is an ultra-compact, high-precision GNSS receiver that works in conjunction with a smartphone, dramatically streamlining simple on-site surveying.

The LRTK receiver houses an antenna, a GNSS module, a battery, and a communication module in a compact enclosure weighing approximately 125 g and about 1.3 cm (0.5 in) thick, and is attached to the back of a smartphone such as an iPhone for use. It connects wirelessly to the smartphone via Bluetooth or Wi‑Fi, and reception of correction information and cloud transmission of positioning data are handled through the smartphone. No complicated wiring is required, and once attached to the smartphone, centimeter-accurate positioning (cm level accuracy (half-inch accuracy)) can begin immediately. There is no need to prepare a base station, and an LRTK app that supports network RTK (VRS) automatically obtains correction data, so little specialized operation is required.

Depending on site conditions, it can be used flexibly—mounted on a monopod (monopod) or a survey pole for full-scale positioning, or by placing the pole tip on the ground to obtain point coordinates. In addition, LRTK supports the CLAS of Japan’s Quasi-Zenith Satellite System Michibiki, so even in mountainous areas where cellular communication does not reach, augmentation signals from the satellite make it possible to continue centimeter-level positioning. This provides the reliability to perform high-precision positioning independently even in situations where infrastructure has been severed, such as disaster sites.

The LRTK system works in conjunction with a dedicated smartphone app and cloud service to provide a variety of functions all-in-one. For example, it offers the following versatile surveying and recording features.

• Centimeter-level point positioning (cm level accuracy (half-inch accuracy)): With just a few seconds of observation you can obtain high-precision coordinate values. Position averaging and accuracy improvement through multiple observations are also possible with one tap.

• High-precision photo recording: When you take a photo with your smartphone’s camera, the photo is automatically tagged with the exact coordinates and azimuth of the capture location. It is easy to compare photos of the same location in a time series or to visualize photo-attached points on a map for management.

• 3D scanning (point cloud acquisition): In combination with the iPhone’s LiDAR scanner, you can record surrounding structures and terrain as 3D point clouds. The acquired point cloud data are assigned global coordinates, enabling immediate confirmation of measured point clouds on a map and use for volume calculations and drawing creation.

• Coordinate guidance (stakeout support): If you enter design coordinates or known point information into the app, the direction and distance to the target point are displayed in real time. Stakeout work to find specified coordinates on site can be performed without confusion using LRTK (visual navigation via AR display is also supported).

• Cloud integration: Positioning results and recorded data are automatically synced to the cloud on the spot. There is no need to bring data back to the office for manual transfer, allowing immediate sharing of results from the field.

By using LRTK in this way, you can perform simple surveying during everyday work without specialized surveying equipment. Because high-precision on-site positioning and recording can be completed with just a smartphone and a pocket-sized receiver, the barriers to surveying are lowered, allowing site supervisors and construction managers themselves to carry out accurate surveys whenever needed. It is also easier to introduce from a cost perspective compared with traditional high-precision GNSS equipment, and an increasing number of municipalities and companies are piloting it. Even those who thought "RTK seems difficult" or "it's too expensive to afford" can easily experience the latest RTK technology with LRTK.

FAQ

Q: What is the difference between RTK and VRS? A: RTK is a method that installs a single reference station at one location and applies corrections by relative positioning with a rover, while VRS is a method that sets up a virtual reference point near the user from a network of multiple reference stations and applies corrections. Simply put, RTK uses your own reference station, whereas VRS uses a network of reference stations provided as a service. With traditional methods, a reference station had to be installed at each site, but VRS can cover a wide area with a single receiver and offers the advantage of consistent accuracy anywhere.

Q: Does RTK positioning really achieve accuracy of a few centimeters? A: If conditions are met, it will be within about 1–2 cm (0.4–0.8 in) in the horizontal plane. In fact, RTK-GNSS surveying is recognized as a method that meets accuracy requirements within 3 cm (1.2 in) even for public surveying. However, depending on the environment, multipath or signal obstructions can degrade accuracy, so it is not always completely error-free. Empirically, in open areas horizontal positions are stable to a few centimeters (a few in), and elevations are stable to a few centimeters to at most 5 cm (2.0 in).

Q: What is required to use the VRS method? A: Basically, you need a GNSS receiver (rover) that can connect to a VRS correction service and a communications link (such as mobile data). In Japan, it is common to subscribe to a correction information service using the Geospatial Information Authority of Japan’s electronic reference station data or to a commercial VRS service (e.g., SoftBank’s ichimill) and connect via Ntrip. With a compatible receiver, you can also receive the CLAS signal from QZSS (Michibiki) and obtain corrections without communications. Recently, there are also products that simplify service connection by linking the receiver with a smartphone app so that simply logging into the positioning app automatically connects you to the optimal reference station.

Q: Can RTK positioning be done with just a smartphone? A: Currently, it is difficult to complete RTK positioning at centimeter-level accuracy (cm level accuracy, half-inch accuracy) using a commercial smartphone alone. Built-in GNSS chips are becoming capable of sub-meter-level positioning (under 1 m (3.3 ft)), but achieving truly cm-level accuracy (cm level accuracy, half-inch accuracy) still requires a dedicated high-precision GNSS antenna and receiver. However, products that combine a smartphone with an external receiver (for example, the aforementioned LRTK) allow you to use the smartphone as an interface to easily achieve centimeter-level positioning (cm level accuracy, half-inch accuracy). There is a possibility that smartphone GNSS will become higher-precision in the future, but at present using auxiliary devices is the realistic option.