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?

• GNSS positioning accuracy and its relationship to RTK

• Why VRS is mainstream in the United States

• Comparison with conventional methods

• Challenges of RTK/VRS

• Practical operation of RTK/VRS

• Future prospects of RTK/VRS

• Simplified surveying with LRTK

• FAQ

Introduction

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

What is RTK?

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

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 its real-time error (correction information) to the rover via radio or communication lines. The rover uses the received correction information to immediately correct the error components contained in its own GNSS positioning results. By such differential corrections, effects that could not be avoided with standalone GNSS positioning—such as satellite orbit and atomic clock offsets, and ionospheric and tropospheric delays—are canceled out, and positioning accuracy improves dramatically. Typically, GPS in smartphones and car navigation systems has errors on the order of several meters (several ft), but with RTK the planar position can be reduced to about 1-2 cm (0.4-0.8 in), and vertical errors can be reduced to a few centimeters (a few inches). In other words, RTK can theoretically enable near-instantaneous centimeter-level (half-inch-level) positioning.

RTK positioning has been widely used in situations that require precise positioning, such as highway and railway construction, land boundary surveying, and construction machinery operation management. However, this method has the constraint that a reference station must be installed on site. To establish a reference station, you need to first obtain the coordinates of known points and set up expensive GNSS receivers and antennas on tripods, along with batteries and radio equipment. Also, if the distance between the reference station and the rover (the baseline length) becomes too great, errors that cannot be corrected due to differences in atmospheric conditions at the two locations increase, degrading accuracy. Therefore, in actual operations, measures have been taken to place the reference station near the work area (ideally within a few km (a few mi)), and if the area is large, to move the reference station as needed or to install multiple reference stations. Setting up and managing reference stations takes time and effort and requires specialized knowledge, so this has been one hurdle to the widespread adoption of RTK positioning.

What is VRS?

The method that emerged to eliminate the effort of installing a reference station at each site is called network RTK. A typical 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 wide areas by national agencies or companies; the rover (user) sends approximate position information 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 observed at that point are calculated in real time. The correction information generated by the virtual reference station is sent to the rover via the mobile network (internet), and the rover performs RTK computations as if a reference station were right next to it, enabling centimeter-level positioning solutions.

Using this VRS method, surveyors can achieve high-precision positioning with only a single receiver (rover) and without having to set up their own base station. It eliminates the preparation time and personnel previously required to establish a base station, resulting in significant efficiency gains. Also, because virtual reference points are always set nearby for each positioning location, there is the advantage of obtaining uniform centimeter accuracy (cm level accuracy (half-inch accuracy)) over wide areas without worrying about accuracy degradation due to baseline length. For example, even when measuring points alone while moving across a large development site, you can work with confidence while maintaining the same accuracy across the entire area. With conventional single-base RTK there used to be worries like “accuracy beyond this point might be unreliable,” but with VRS such concerns are largely unnecessary.

In recent years, because of this convenience, network RTK has been becoming the global mainstream. In Japan, the Geospatial Information Authority of Japan’s network of continuously operating reference stations (approximately 1,300 stations, GEONET) is used to provide real-time correction services, enabling high-precision coordinates in the World Geodetic System to be obtained anywhere nationwide without installing a base station. In addition, private telecommunications companies and surveying equipment manufacturers have developed their own network RTK services. For example, "ichimill" has deployed more than 3,300 reference stations nationwide, allowing subscribers to achieve centimeter-level positioning (half-inch-level positioning) instantly anywhere in Japan simply by turning on their receiver in the field. Furthermore, Japan’s Quasi-Zenith Satellite System "Michibiki" has provided a centimeter-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 areas outside cellular coverage, such as mountainous regions. 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 U.S. GPS, Russia's GLONASS, Europe's Galileo, and Japan's Michibiki (QZSS). When "GPS" is commonly used it refers to the U.S. system among these GNSS, 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) inevitably incurs errors of several meters (several ft) due to various factors accompanying signal propagation from satellites. For example, slight offsets in satellite clocks and orbits, signal delays caused by the ionosphere and troposphere, and radio wave reflections from buildings and terrain (multipath) can combine to produce phenomena such as a smartphone map showing your current location off by several meters (several ft) from where you actually are. While an error of several meters (several ft) may be acceptable for many applications, even errors of a few centimeters (a few in) are unacceptable in civil surveying and construction sites. Differential correction techniques such as RTK were developed for this reason: by canceling GNSS errors in real time, they achieve centimeter-level accuracy (half-inch accuracy).

RTK uses carrier-phase signals from GNSS satellites and, as described above, removes errors through relative positioning with a reference station. In recent years, not only GPS but the use of multiple satellite systems in multi-GNSS has increased the number of available satellites, further improving positioning accuracy and stability. In particular, in urban and mountainous areas where the sky is obstructed and there tend to be few satellites in view, receiving GLONASS, Galileo, and Michibiki in addition to GPS makes RTK solutions less prone to interruption and also contributes to fast initialization (acquisition of a fixed solution). In this way, the evolution of GNSS itself has greatly contributed to improvements in RTK performance.

Why VRS Is Mainstream in the United States

Because RTK networking progressed early in the United States, the VRS method has become established as the mainstream for high-precision positioning. The background is that, in a country with a vast territory 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 every time.

Many state geodetic authorities and Departments of Transportation (DOTs) have built VRS networks (real-time reference station networks) that cover entire states and offer them to surveyors and engineers. For example, the Ohio Department of Transportation installed numerous reference stations across the state and launched a VRS service in the mid-2000s, making it freely available to survey professionals within the state.

These state-level networks have now spread across the United States, and subscribers can obtain high-precision correction information simply by activating a mobile station in the field. In addition, data from the nationwide CORS (Continuously Operating Reference Stations) network operated by the National Geodetic Survey (NGS) have been utilized by various services, and the fact that they were established early as infrastructure helped drive adoption.

Furthermore, demand across diverse fields such as precision agriculture and automatic control of construction machinery became the driving force behind VRS adoption. 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 developed their own RTK base station networks and offered correction services using satellite communications (PPP method).

Network RTK—which allows multiple heavy machines and survey crews to use high-precision coordinates simultaneously—dramatically improves site efficiency and was therefore 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. In some areas, networks provided by state governments are available free or at low cost, creating an environment where even small and medium survey businesses can readily use network RTK.

Thus, supported by early infrastructure development and broad industry demand, the VRS method 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 at many sites the standard workflow is to “first connect to the network” and then begin positioning. From surveying and agriculture to autonomous driving, much of the centimeter-level positioning in the U.S. is achieved via VRS networks.

Comparison with Conventional Methods

As noted above, the VRS approach outperforms conventional methods in ease of preparation and breadth of applicability, and there are increasing cases where it is easier to adopt in terms of initial cost. However, using the service entails running costs such as monthly subscription fees; even after accounting for these, the advantages from reduced labor costs and improved efficiency are regarded as substantial.

Challenges of RTK/VRS

• Dependence on communications: The VRS method depends on communication lines such as mobile phones to receive correction data in real time. If the rover leaves cellular coverage or the communication is interrupted, high-precision positioning cannot be maintained during that time (if using a CLAS-capable receiver that can receive correction information directly from the satellites, positioning can continue even outside communication coverage). The service is also affected if the provider’s servers or the communication network experience faults.

• Satellite visibility and environmental factors: GNSS positioning assumes that satellites are visible overhead. Positioning itself becomes difficult unless outdoors with an open sky, and even high-precision RTK cannot sufficiently track satellites under viaducts, inside tunnels, or within forests. Accuracy can also degrade due to reflections (multipath) from building facades or heavy machinery and due to radio interference. In urban areas surrounded by high-rise buildings, obtaining a fixed solution can be difficult and positioning may become unstable.

• Initialization and positioning stability: To obtain an RTK fixed solution (integer solution), the receiver must adequately track the carrier-phase from the satellites and correctly resolve the integer ambiguities. Under good conditions initialization can take several seconds to several tens of seconds, but reinitialization is required if satellites are lost. Accuracy may temporarily drop when moving into the shadow of a building. Large ionospheric disturbances can also prevent corrections from keeping up and affect accuracy.

• Operating costs: Using a network-type RTK service incurs running costs such as monthly usage fees or annual membership fees (in some regions services may be provided free by government). Conventional systems required an initial investment to procure base station equipment, but aside from communication charges there were no ongoing operational costs. Whichever option is chosen, it is necessary to compare the required expenses with the benefits of improved accuracy.

• Handling of positioning coordinates: Coordinates obtained with RTK/VRS are typically based on a global geodetic datum (in Japan, the World Geodetic System). In actual construction or surveying, conversion to a local planar coordinate system and correction of elevation values using geoid height may be required. Although usability has improved compared with traditional surveying instruments, high-precision positioning still requires basic knowledge of GNSS and geodetic datums.

While taking the above challenges into account, RTK/VRS is being used as a technology that greatly contributes to improving on-site productivity. Going forward, advances in satellite and communication technologies are expected to gradually resolve these issues.

Practical Operation of RTK/VRS

RTK and VRS dramatically improve the efficiency and productivity of surveying work in the field. For example, tasks such as installing batter boards and surveying boundary points that were traditionally carried out by two-person teams using a total station can be completed by a single person in a short time when using a high-precision GNSS rover. On large development sites there is no need to worry about the base station’s radio range, and because workers can measure points anywhere simply by turning on the receiver on site, on-the-move surveying and as-built management are significantly sped up.

Moreover, by using the coordinate guidance (navigation) feature, you can input the coordinates of pile-driving positions and reference points shown on the plans into the receiver and easily have it guide you to those locations on site. Because the direction and distance to the destination are displayed in real time on the screens of tablets and smartphones equipped with GNSS receivers, even less experienced workers can drive piles to the exact position without getting lost. If the system includes AR functionality, intuitive methods such as overlaying a marker for the placement location on the camera feed to guide workers are also possible.

Furthermore, when combined with machine guidance・machine control, which mount GNSS receivers and control units on heavy equipment, construction automation progresses significantly. Bulldozer and excavator operators can concentrate on driving the vehicle while the machine itself determines its position with GNSS and automatically adjusts the blade's height and tilt, enabling the designed finish to be achieved without relying on manual work. Surveyors spend less time measuring and checking as-built conditions after construction, and because accurate as-built data can be obtained immediately, rework can be prevented.

By introducing GNSS positioning in the field in this way, high-precision surveying and construction management can be performed with a small crew, allowing the construction industry—despite labor shortages—to maintain high productivity. In addition, coordinate data acquired by GNSS can be shared instantly with stakeholders via the cloud, making post-survey data organization and communication smooth. For example, at a disaster site a single technician can survey damaged areas and immediately send point clouds of the terrain and precise position coordinates of damaged facilities to headquarters via the cloud. Even in situations that traditionally required time for initial response, network RTK–compatible equipment can speed up on-site surveys, and the precise data obtained can be applied immediately to disaster prevention plans and restoration design.

Future of RTK/VRS

High-precision GNSS positioning technology is expected to evolve further. First, full utilization of multi-GNSS such as GPS, GLONASS, Galileo, BeiDou, and QZSS, and the use of multiple frequencies including the L5 band in addition to L1/L2, will reduce positioning outages caused by insufficient satellite numbers or signal strength, enabling more stable maintenance of centimeter-level accuracy (half-inch accuracy) even in urban canyons or under tree cover. In addition, augmentation of the positioning satellites themselves and new technologies are progressing. Japan’s quasi-zenith satellite Michibiki is expected to be expanded into a seven-satellite constellation, further improving availability. Europe’s Galileo has also begun offering the High Accuracy Service (HAS), and an era in which decimeter-level (3.9 in) correction information 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. Orbital and clock errors are covered by wide-area common corrections (SSR), and by correcting only local atmospheric errors the system can swiftly achieve centimeter-level accuracy (half-inch accuracy) with fewer reference stations than before. In fact, Japan’s Michibiki CLAS is transmitting SSR-type augmentation information from satellites, and going forward hybrid correction services combining satellites + ground networks may become mainstream. This will enable high-precision positioning even in remote areas or at sea where mobile communications do not reach, further expanding the range of applications.

Devices and software are also moving toward greater miniaturization and simplification. GNSS receivers are becoming ever smaller and more power-efficient, and high-precision models that can perform positioning using only the chip built into a smartphone have begun to appear (some Android smartphones, for example, support L1/L5 dual-frequency and achieve accuracy on the order of tens of centimeters (tens of in)). In the future, even mobile devices in the hands of general users will be able to perform centimeter-level positioning (cm level accuracy (half-inch accuracy)) easily by connecting to the appropriate correction services. In addition, features that make field work easier—such as tilt correction using an IMU (inertial measurement unit) built into the receiver—are being put into practical use. They can obtain the accurate coordinate directly beneath the tip even if the pole is slightly tilted, reducing the workload for measuring points in confined spaces and on slopes.

Furthermore, as high-precision positioning information becomes commonplace, the range of applications will also expand. GNSS-based positioning is expected to be utilized in a variety of scenarios—from determining vehicle positions for autonomous driving, to autonomous drone navigation and aerial surveying, to visualization of buried utilities and precision navigation using AR glasses—spanning social infrastructure to everyday services. 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.

Simplified surveying using LRTK

Finally, we introduce LRTK (pronounced "el-arr-tee-kay"), a solution gaining attention in Japan for enabling easy use of RTK/VRS positioning. LRTK is a product series developed by Lefixea, a startup originating from Tokyo Institute of Technology, and embraces 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 and dramatically streamlines simple on-site surveying.

The LRTK receiver is housed in a compact enclosure weighing approximately 125 g and about 1.3 cm (0.5 in) thick, containing an antenna, GNSS module, battery, and communication module, 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 uploading of positioning data to the cloud are carried out through the smartphone. No complicated wiring is required, and centimeter-level positioning (cm level accuracy (half-inch accuracy)) can begin immediately once it is attached to the smartphone. 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 or surveying pole for precise positioning, or by placing the pole tip on the ground to obtain the coordinates of a point. In addition, LRTK supports CLAS of Japan’s Quasi-Zenith Satellite System Michibiki, allowing centimeter-level positioning (cm level accuracy, half-inch accuracy) to continue via augmentation signals from the satellite even in areas where mobile communications do not reach, such as mountainous regions. 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 integrates with a dedicated smartphone app and cloud service to provide a variety of all-in-one functions. For example, it offers the following versatile surveying and recording features.

• Centimeter-level point positioning (cm level accuracy (half-inch accuracy)): You can obtain high-precision coordinate values with just a few seconds of observation. Averaging or improving accuracy through multiple observations is also possible with a single tap.

• High-precision photo recording: When you take a photo with your smartphone camera, the image is automatically tagged with the precise coordinates and azimuth of the shooting location. It makes it easy to compare photos of the same point over time or to visualize photo-tagged points on a map for management.

• 3D scanning (point cloud acquisition): Combined with the iPhone’s LiDAR scanner, you can record surrounding structures and terrain as 3D point clouds. Because the acquired point cloud data is assigned global coordinates, you can immediately view the measured point clouds on a map and use them for volume calculations and creating drawings.

• Coordinate guidance (stakeout assistance): 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 tasks to locate designated coordinates in the field can be performed without getting lost with LRTK (also supports visual navigation via AR display).

• Cloud integration: Positioning results and recorded data are automatically synchronized to the cloud on site. There is no need to bring data back to the office for manual transfer, and results can be shared immediately from the field.

By leveraging LRTK in this way, you can perform simple surveying in 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 barrier to surveying is lowered, and site supervisors and construction managers themselves can carry out accurate surveys whenever needed. It is also easier to adopt from a cost perspective compared with conventional high-precision GNSS equipment, and more municipalities and companies are beginning trial implementations. Even those who thought "RTK seems difficult" or "it's too expensive" 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 reference station at a single location and applies corrections via relative positioning with the rover, while VRS is a method that sets a virtual reference station near the user from a network of multiple reference stations to provide corrections. Simply put, RTK uses a private reference station, whereas VRS uses a network of reference stations provided as a service. With conventional methods a reference station needed to be set up for each site, but with VRS a single receiver can cover a wide area and achieve uniform accuracy anywhere.

Q: Can RTK positioning really achieve accuracy of a few centimeters (a few in)? A: Under the right conditions, planar errors are typically on the order of 1–2 cm (0.4–0.8 in). In fact, RTK-GNSS surveying is recognized as a method that meets accuracy requirements within 3 cm (1.2 in) in public surveying. However, in some environments accuracy can degrade due to multipath or signal blockage, 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 up to at most about 5 cm (2.0 in).

Q: What is required to use the VRS method? A: Basically, you need a GNSS receiver (rover) capable of connecting to a VRS distribution service and a communications link (such as mobile data). In Japan, it is common to subscribe to a correction information service that uses the Geospatial Information Authority of Japan’s electronic reference station data or to a private VRS service (e.g., SoftBank’s ichimill) and connect using the NTRIP protocol. With a compatible receiver, you can also receive CLAS signals from Michibiki (QZSS) to obtain corrections without a communications link. Recently, there are products that simplify service connection by linking the receiver and a smartphone app, and some will automatically connect to the optimal reference station just by logging in to the positioning app.

Q: Can RTK positioning be done with only a smartphone? A: At present, it is difficult to complete RTK positioning with centimeter-level accuracy (cm level accuracy (half-inch accuracy)) using a commercially available smartphone alone. Built-in GNSS chips are becoming capable of reaching sub-meter-level performance, but to obtain truly centimeter-level accuracy (cm level accuracy (half-inch accuracy)) a dedicated high-precision GNSS antenna and receiver are still required. However, by using products that combine a smartphone with an external receiver (for example, the aforementioned LRTK), you can easily achieve centimeter-level positioning with the smartphone serving as the interface. While smartphone GNSS may become higher-precision in the future, at this time the use of auxiliary devices is the practical option.