RTK vs Network RTK: Comparing the Differences Between a Standalone Base Station and VRS

On construction and surveying sites, a position offset of only a few centimeters (a few in) can have serious consequences. In situations such as highway and railway construction, land boundary surveys, and building construction management—where precise positioning determines quality and safety—the typical GPS positioning error of a few meters (a few ft) is insufficient. The technology that emerged to eliminate that error is RTK positioning (Real Time Kinematic, real-time kinematic positioning). RTK corrects GPS errors in real time to deliver immediate centimeter-level high-precision positioning (half-inch accuracy).

In this article, we compare and explain the differences between the traditional RTK method (single base-station method) and the network RTK method (VRS method), which has recently become mainstream. We will clearly describe each system’s mechanisms, positioning accuracy, operating conditions, advantages and disadvantages, cost structure, and application scenarios in a way that is easy to understand for beginners in surveying and site supervisors.

Table of Contents

• What is RTK (single reference-station method)

• What is Network RTK (VRS method)

• Comparison of RTK and VRS: mechanisms, accuracy, and operational conditions

• Advantages and disadvantages of RTK and VRS

• Differences in cost structure

• Use cases and selection points

• Summary

• FAQ

• Introduction to LRTK products

What is RTK (Single Base Station Method)

RTK stands for Real Time Kinematic (real-time kinematic) and is a type of relative positioning that uses two GNSS receivers. In Japanese it is also called "real-time dynamic positioning." Specifically, first a receiver that will serve as the reference station (base station) is installed at a known point whose coordinates are accurately known. Then another receiver is placed at the point to be surveyed (an unknown point), and this is called the rover (mobile station). Both the reference station and the rover simultaneously receive GNSS signals from four or more satellites, and each independently computes its own position.

The reference station knows its precise position (coordinates of a known point), so it can calculate in real time the error between its known position and the position obtained by GNSS. The reference station sequentially transmits that error (correction information) to the rover by radio or similar means. The rover applies the received correction information to its own positioning data, canceling in real time the errors contained in the satellite signals. Through this mechanism, positioning errors that are normally several meters (several ft) are canceled out, allowing the current position to be determined with centimeter-level accuracy (half-inch accuracy). When operated properly, RTK positioning yields planar position errors of about 1–2 cm (0.4–0.8 in), and vertical errors are also within a few centimeters to around a dozen centimeters (a few in to around a dozen in).

It should be noted that the accuracy of RTK positioning is strongly affected by the distance between the base station and the rover (baseline length). When the two stations are close, many error sources—such as signal delay errors from the ionosphere and troposphere—are common and can be canceled out, but as the distance increases, differences in atmospheric conditions cause errors that cannot be fully corrected. Therefore, in typical RTK the base station is installed as close as possible to the survey work area (ideally within a few km (a few mi)), and correction information is transmitted to the rover using low-power UHF radios or long-range radio units. If operated correctly, RTK can achieve on-site high accuracy that clearly outperforms traditional standalone positioning (positioning with a single GNSS receiver). However, this method has the constraint that you must set up a base station on-site each time, which is a drawback because transporting equipment and installing it on known points is time-consuming.

What is Network RTK (VRS method)?

A method that eliminates the effort and constraints of placing a reference station on site is network RTK. Its representative method is called VRS (Virtual Reference Station, the virtual reference-point method). In network RTK, a network is built by installing a large number of fixed reference stations (electronic reference points) across a country or region in advance, and correction data are generated as if a virtual reference station existed near the user. The outline of the mechanism is that the network's operation server obtains the user's (rover's) approximate current position and integrates and analyzes the observation data from multiple fixed reference stations (electronic reference points) around it. It then simulates "what satellite signals would be observed if a reference station were placed very close to the user" and creates correction information corresponding to that location.

Correction information for the generated virtual reference point is delivered to the rover via an Internet connection such as a mobile phone (typically using the protocol called Ntrip). On the rover side, RTK calculations can be performed under the same conditions as if a reference station were located right next to it, resulting in centimeter-level accuracy (half-inch accuracy) equivalent to the single-base-station method. With the advent of the VRS method, the convenience of RTK positioning has improved dramatically. On-site, only a single rover receiver is required, and because the complicated task of setting up a reference station is unnecessary, surveying can begin immediately upon arrival. Because a virtual reference point is always set close to the measurement location, accuracy degradation due to baseline length is almost eliminated, enabling uniform high-precision positioning even when moving over a wide area.

In Japan, real-time correction services using the network of electronic reference stations established by the Geospatial Information Authority of Japan at approximately 1,300 locations nationwide (GEONET) are provided. By using these services, users can obtain high-precision coordinates in the Japanese geodetic system (World Geodetic System) on-site without having to set up their own reference station. Network RTK services offered by private companies have also become widespread in recent years. By using such wide-area reference-station networks, stable centimeter-level positioning is now possible virtually anywhere in Japan where there is mobile phone coverage.

Comparison of the Mechanisms, Accuracy, and Operational Conditions of RTK and VRS

Both RTK (single-base-station method) and network RTK (VRS method) can achieve centimeter-level accuracy with appropriate measures. However, there are major differences in the required equipment and operational procedures. The main differences are summarized below.

• Required equipment: Conventional RTK requires two GNSS receivers, one for the reference station and one for the rover, plus radio equipment (transceivers) to link the two stations. By contrast, the VRS method can perform positioning with only a single receiver on the rover side (a communication terminal for receiving correction data is required).

• Initial setup and effort: With the RTK method you must install a reference station at each site, set up a tripod, level the equipment, and place it to match known point coordinates, so preparing to start work takes time and effort. With the VRS method, because no reference station installation is necessary, you can start surveying as soon as you power on the receiver and connect to the network. The waiting time from arriving on site to beginning work is greatly reduced.

• Positioning coverage: In single-base RTK, accuracy decreases as you move farther from the reference station, so the practically coverable range is limited. Also, when correction information is sent by radio, the maximum positioning area is constrained by the radio range (on the order of several km to a dozen or so km). With VRS, as long as the mobile communications network is available you can cover an entire region, and because a virtual reference point close to the user is always generated according to the user’s position, high accuracy is maintained even when moving over a wide area.

• Operating cost: With RTK you must provide your own reference station equipment, but once the equipment is in place there is no running cost for use itself (if you use unlicensed radios, you also avoid radio usage fees). However, the purchase cost of high-performance GNSS receivers and radios is high. With VRS, instead of having your own reference station you need to subscribe to a network RTK service provided by government or private operators to receive correction information (service fees apply). Recently these services have become cheaper, and fees that used to be tens of thousands of yen per month have fallen to about a few thousand yen per month. There is also an increasing number of cases where users reduce equipment purchase costs and subscribe only for the required period.

• Positioning accuracy: When used appropriately, the positioning accuracy obtainable is almost equivalent for both methods. Horizontal positions are within a few centimeters (a few in), and vertical errors are within several tens of centimeters (several tens of in). However, in RTK accuracy gradually degrades with distance from the reference station, whereas VRS has the advantage of uniformity of positioning accuracy across a wide area because a virtual reference point is always nearby. Conversely, if the communications environment is poor and you cannot use a network RTK service, VRS cannot be used and you must switch to a conventional, self-operated RTK system in such cases.

• Coordinate datum: In single-base RTK the positioning is referenced to the reference station you installed yourself, so unless you align the reference station position to a known public coordinate, the obtained coordinates will be in an arbitrary local coordinate system (local coordinates). You may then need to convert them later to the national datum (World Geodetic System), which is an extra step. With VRS, the electronic reference-station network that underlies the correction information is tied to the national coordinate system, so the positioning results you obtain are from the start in a public coordinate system such as the Japan Geodetic Datum 2011. This avoids extra work when directly comparing survey points with public survey results or integrating with GIS data.

The differences above can be briefly summarized in tabular form as follows.

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Advantages and Disadvantages of RTK and VRS

Advantages of RTK (standalone base-station method)

• It does not depend on communications networks, so it can operate in areas outside mobile phone coverage or at sites without internet access.

• Once the base-station equipment is purchased, there are no running costs, allowing lower expenses over the long term.

• Since the base station can be installed at any location to suit local conditions, you can establish an independent positioning system that does not rely on other services.

Disadvantages of RTK (Standalone Base Station Method)

• The initial implementation cost for base station equipment (high-precision GNSS receivers, mounting tripods, radios, etc.) can be significant.

• Each job requires the effort of setting up and dismantling the base station, so it can take time from arriving on site to starting positioning.

• If radio communication with the base station is interrupted, positioning becomes impossible, so attention must be paid to radio conditions and maintaining line of sight. Also, the measurement range is limited to the area around the base station.

Benefits of Network RTK (VRS method)

• The equipment setup is simple, and you only need to carry one GNSS receiver. There is no need to carry heavy base station equipment or radios, nor to perform equipment installation on site.

• The convenience of being able to start measuring immediately upon arrival at the site directly improves field work efficiency. Because complex setup or coordinate-matching procedures are not required, even beginners can start work without hesitation.

• You get consistently stable accuracy even when moving across a wide area. For example, even when one person is surveying points across a large development site, you can achieve the same accuracy anywhere. There is no need to worry that “accuracy might be questionable from here…”.

• Conversion of reference points (coordinates) to a public coordinate system is unnecessary, and the obtained positioning results can be directly compared with coordinates on drawings or in GIS.

• No personnel are required for base station setup or management, allowing one person to work efficiently. Also, using a network service instead of acquiring expensive equipment reduces the initial investment burden.

Disadvantages of Network RTK (VRS method)

• An internet connection is essential, so it cannot be used in areas without cellular coverage. Also, if signal conditions are unstable, reception of correction data may be interrupted and positioning may be halted.

• A subscription or usage fee for the correction information service is required. Over the long term, costs may exceed those of purchasing equipment, so it is necessary to take measures such as selecting a plan based on usage frequency.

• Because it depends on the systems of the network RTK service provider, there is a risk that the service will be unavailable during outages or maintenance (although such events are rare).

Differences in Cost Structure

RTK and network RTK (VRS) differ in the structure of required equipment costs and running costs. First, in terms of initial setup expenses, the RTK approach requires purchasing two high-precision GNSS receivers for a base station and a rover, so the initial investment is large. It is not uncommon for a high-performance GNSS surveying instrument to cost several hundred thousand to over one million yen per unit, and when accessories such as radios and tripods are included, a conventional RTK kit can sometimes cost on the order of several million yen. In contrast, with the VRS approach you only need to prepare a single unit for the rover, which can significantly reduce equipment costs. Recently, low-cost GNSS receivers that can be used in conjunction with smartphones have appeared, making it possible to start centimeter-level positioning at a cost below several hundred thousand yen.

Next, in terms of running costs, with the RTK method, because you operate your own base station, there are basically no monthly fees. If you use license-exempt low-power radio or specified low-power radio, radio usage fees are also not required. On the other hand, the VRS method requires fees for correction information distribution services, such as those provided by private services. Service prices vary depending on the provider and contract, but competition in recent years has driven prices down, and plans available at a monthly cost of a few thousand yen to around 10,000 yen have become more common. If usage is frequent or long-term, total costs can increase, but it is also possible to keep costs down by taking advantage of short-term plans that can be contracted only for the required period or by using any regionally offered free services.

Overall, in terms of keeping initial investment low and being able to start immediately, the VRS method is easier to adopt, while if you will use it frequently over a long period, the RTK method tends to be more advantageous in terms of running costs. However, in recent years the falling cost of equipment and reductions in service fees have made it increasingly easier for small businesses and beginners to adopt high-precision positioning.

Application Scenarios and Key Points for Selection

RTK and network RTK methods each have their own suitable application scenarios. In general, if mobile communications are available, the VRS method is recommended for its convenience and efficiency. For wide-area surveys or when moving between multiple sites, the advantages of VRS—no need to repeatedly set up a base station—become particularly apparent. Also, on sites without specialist surveying personnel, the VRS method’s simple setup makes it easy to handle and allows even beginners to operate it.

On the other hand, in mountainous and remote areas where communications infrastructure is not developed, even if you want to use VRS the signal may not reach, so there are cases where positioning can only be done with conventional RTK (a private base station). In such situations, a portable GNSS base station unit is set up on site and operated. Also, if the area to be surveyed is very limited and measurements are repeatedly carried out at the same location, permanently installing a base station can eliminate the need for preparation each time, so the RTK method may not be that troublesome. For example, if repeated surveying is required over a long period at a large construction site, you could consider permanently installing a base station on the roof of the site office and always using your own RTK within the site (providing stable corrections without incurring communication fees).

When making a selection, it is a good idea to consider the following points.

• Communication environment: Whether the work site is inside or outside cellular coverage. If it’s outside coverage, self-owned RTK becomes the only option.

• Measurement range and movement: Whether survey points span a wide area or consist of multiple scattered sites. For wide areas, VRS is advantageous; for a single, confined site, RTK can also be sufficient.

• Frequency and duration of use: Whether the project is temporary/short-term or will be used frequently over a long period. For short-term use, using a service rather than buying equipment is lower cost; over the long term, purchasing equipment can be more cost-effective.

• Budget and personnel: If initial funds are limited, it’s realistic to start with VRS plus a low-cost receiver. Conversely, if you have the budget and want to hold your surveying equipment as an asset, purchasing RTK equipment is an option. Also consider whether you have personnel skilled in device setup—VRS is easier to operate without specialized knowledge.

The optimal solution depends on site conditions and the desired mode of operation, but in recent years, with the spread of network RTK services, the VRS method has been adopted in many surveying operations. However, to prepare for contingencies, it is also common to operate by using both methods as appropriate—for example, by preparing an in-house simple reference station that can be switched to in the event of a signal outage.

Summary

RTK and network-based RTK (VRS) are both technologies that correct satellite positioning errors to achieve centimeter-level accuracy (half-inch accuracy). The single-reference-station RTK method is an established technique that has long been used in surveying and has the advantage of enabling high-precision positioning without relying on communications infrastructure. On the other hand, the VRS method, by utilizing a network of reference stations, dramatically increases convenience and especially proves its worth for wide-area surveys and tasks involving frequent movement. There is no significant difference in positioning accuracy between the two, but in terms of operating costs and ease of use, surveying using the VRS method is becoming the mainstream approach.

That said, the optimal method varies depending on the site environment and application. As explained in this article, it is important to choose the approach that fits your needs by taking into account conditions such as the communications environment, cost, work area, and frequency. Advances in technology have recently produced compact, low-cost equipment and inexpensive correction services. High-precision positioning is becoming accessible not only to surveying professionals but also to site supervisors and construction managers. Utilizing RTK and network RTK will increasingly enable efficient, high-quality surveying and construction.

FAQ

Q: Which is more accurate, the RTK method or the VRS method? A: Basic positioning accuracy (several centimeters (a few in)) is equivalent for both methods when properly operated. Both RTK and VRS can achieve errors within a few cm (a few in) when stationary. However, whereas RTK’s accuracy degrades when far from the base station, VRS always has a virtual reference point nearby, so its accuracy remains stable even at long distances. Also, if environmental factors are the same, stand-alone RTK is not inherently less accurate; any accuracy difference is due to differences in operational conditions.

Q: How should positioning be performed in mountainous areas or other locations without network connectivity? A: In areas where mobile signals do not reach, unfortunately the VRS method cannot be used. In that case you must set up your own reference station on site and perform positioning using the conventional RTK method. Also, within Japan you can use a system called CLAS of Michibiki (QZSS) that allows reception of correction signals directly from satellites even without network communication (a compatible receiver is required). For example, a CLAS-capable device such as the LRTK Pro2 can continue centimeter-level positioning via satellite augmentation even outside communication coverage.

Q: What do you need to start RTK positioning? Can it be done with only a smartphone? A: To perform centimeter-level RTK positioning (cm (in)), you need a compatible GNSS receiver. The GPS built into a typical smartphone does not offer sufficient accuracy and cannot obtain the positioning data required for RTK (such as carrier phase). Therefore, first prepare an RTK-capable GNSS receiver (rover). For the RTK method, you must set up another receiver as a base station and arrange a way to transmit correction information wirelessly. For the VRS method, since corrections are received via the Internet, you need to subscribe to a network RTK service and have a means of communication (such as smartphone tethering or an embedded SIM). Recently, various compact GNSS receivers that can connect to smartphones have been released, and by using them you can connect to correction services via a smartphone app and easily achieve high-precision positioning.

Q: If we already operate our own GNSS reference stations, are there still benefits to using VRS? A: Yes, even if you already have your own reference stations, there are benefits to using a VRS service. For example, when surveying multiple distant sites, VRS is efficient because you can measure at each site immediately with just one receiver without having to relocate your own reference stations each time. VRS is also suitable for long-distance surveys that your own reference stations cannot fully cover, or for continuous positioning while in motion (e.g., vehicle or drone surveys), since it can cover a wide area. Some companies keep their existing reference stations mainly as backups or for use when outside communication coverage, and switch to VRS for routine operations to improve work efficiency. However, if you already own reference stations, a strength is that you can still use your in-house RTK in locations without communication service. Ideally, you should use both depending on the situation.

Introduction to LRTK Products

Conventional RTK equipment was expensive and required specialized knowledge, but the recently introduced LRTK (an RTK positioning solution developed by Refixia) has greatly lowered that barrier. LRTK is an ultra-compact GNSS receiver series conceived as a “pocket-sized RTK surveying device anyone can use on site.” The antenna, GNSS receiver, battery, and wireless communication module are integrated into a device weighing approximately 125 g with a thickness of just 13 mm (0.51 in), and by attaching this single unit to a smartphone and launching the app, centimeter-level positioning can be started easily. Communication links with the smartphone via Bluetooth or Wi-Fi, and since reception of correction data and cloud transmission of positioning results can also be done through the phone, cumbersome cable connections are unnecessary. Its portability for easy field carrying and the intuitive operation via a smartphone app make it simple enough to be used even by those who are not surveying professionals.

Compact yet the LRTK series delivers full-fledged performance. The higher-end model, LRTK Pro2, supports the centimeter-level augmentation service (CLAS) provided by Japan's Quasi-Zenith Satellite System 'Michibiki', enabling high-precision positioning using only augmentation signals from satellites even in mountainous areas outside mobile communication coverage. The LRTK series also features a tilt compensation function that automatically corrects for slight tilts of the antenna at the tip of a survey pole to obtain accurate coordinates directly beneath. This is an innovative feature that allows measurements without positional error even when the antenna must be angled to avoid obstacles. Furthermore, its dustproof and waterproof rugged design lets it be used with confidence in the harsh field environments of civil engineering construction sites.

By leveraging LRTK in this way, you can bring the advanced positioning technologies of RTK and network RTK to the field more easily than ever. LRTK is a solution that contributes to greater efficiency and sophistication in surveying and construction management by enabling everyone to utilize high-precision location information. If you’re interested, be sure to check the details of LRTK products. It will surely transform on-site positioning work.