RTK Positioning Mechanism: Base Stations, Correction Data, and the True Nature of Errors

Table of Contents

• What is RTK positioning?

• The true nature of GPS positioning errors

• How RTK achieves high precision

• The roles of base stations and correction data

• Use cases and benefits of RTK

• Simple surveying made possible by LRTK

• FAQ

What is RTK positioning?

RTK positioning is an abbreviation for Real Time Kinematic, a technique that dramatically improves the accuracy of satellite positioning. With ordinary GPS positioning, various factors can cause the reported position to be off by several meters. However, with RTK, using two GNSS receivers called a base station (also called a reference station) and a rover, errors can be corrected so that positions can be determined with centimeter-level accuracy. It is truly an advanced technology that enables real-time high-precision positioning. RTK positioning is also referred to as “RTK-GNSS” or “interferometric positioning methods.”

Because RTK uses two receivers, it is classified as a type of “relative positioning,” distinct from standalone GPS positioning (GNSS standalone positioning). The base station and rover simultaneously receive signals from the same satellites, and by canceling errors common to both, high-precision position computation is possible. This mechanism allows real-time positioning within a few centimeters of error, which was previously difficult with conventional GPS.

RTK originally developed in the surveying field, but today it is used widely in construction site management, autonomous agricultural machinery, drone aerial surveying, and many other fields. From surveying beginners to experienced technicians, RTK provides efficient and accurate position information, and demand continues to grow year by year.

The true nature of GPS positioning errors

To understand how RTK works, let’s first look at why ordinary GPS positioning can produce errors on the order of meters. In standalone GPS positioning, a receiver calculates its position based on radio signals sent from GPS satellites. However, this positioning includes the following error sources.

• Satellite orbit errors: The orbital information (ephemeris) of GPS satellites contains slight inaccuracies, which introduce errors into position calculations.

• Satellite clock errors: The atomic clocks onboard GPS satellites accumulate small errors that affect the measurement of signal arrival time.

• Ionospheric and tropospheric delays: GPS signals slow down as they pass through the Earth’s atmosphere, particularly the ionosphere and troposphere. Errors in estimating these delays affect positioning accuracy.

• Multipath: Signals reflected by buildings or terrain can interfere with the direct wave, causing arrival time shifts and thus errors.

• Receiver noise: The receiver’s own clock and circuitry have tiny noise and errors that affect position calculations.

Each factor alone can cause deviations on the order of tens of centimeters to several meters, and when combined the total error grows further. As a result of these accumulated factors, typical GPS standalone positioning can produce errors of about 5～10 m (16.4-32.8 ft). This is why, when using a smartphone map app or car navigation, your vehicle’s displayed position may sometimes be shifted relative to its true location. Meter-level errors are acceptable for everyday use, but they can be critical in civil surveying or setting out positions at construction sites. So how does RTK solve this error problem?

How RTK achieves high precision

RTK achieves its extraordinary precision through two main points: “relative positioning with a base station” and “use of carrier-phase measurements.” These allow RTK to correct errors in real time and boost positioning accuracy down to the millimeter level.

First, RTK performs relative positioning between two points, the base station and the rover. The base station is installed at a location with known accurate coordinates, while the rover is placed at the point to be measured. When both stations receive signals from the same satellites, both observations contain the common error sources mentioned earlier. Because the base station’s precise position is known, it can compute the “current errors contained in the received signals” in real time. It then transmits those error values to the rover, which applies the corrections to its own computed position. Conceptually, it is like “subtracting the error computed by the base station from the rover’s positioning result.” This cancels errors common to both stations—such as atmospheric effects and satellite orbit/clock errors—and reduces the remaining error to on the order of a few centimeters.

Next, RTK uses the carrier wave of the satellite signal to further improve precision. Ordinary GPS positioning uses the code (pseudo-range) embedded in the satellite signal, but code-based positioning alone is limited to meter-level accuracy. In contrast, the carrier wave transmitted by GPS satellites has a very short wavelength—about 20 cm (7.9 in)—and by precisely measuring the phase of this wave one can detect distance changes on the order of millimeters. In RTK, receivers track the carrier wave from the satellite and measure both “how many whole wavelengths arrived (the integer part)” and “how much fractional part remains (the fractional part).” However, using only the carrier wave, the number of whole wavelengths between the receiver and satellite (the integer ambiguity) is initially unknown. Therefore, code-based positioning is first used to compute a rough position (meter-level), and then comparison with the base station’s data resolves this integer ambiguity. Because the base station and rover observe the same satellite carrier-phase and taking their difference cancels out common integer multiples of the wavelength, the previously unknown integer number of wavelengths can be correctly estimated.

By exploiting carrier-phase differences in this way, distances can be measured not only to centimeter accuracy but, in some cases, to millimeter accuracy, enabling extremely precise position coordinates. In short, RTK positioning achieves a breakthrough in precision through a two-pronged approach: (1) high-precision distance measurement using short-wavelength carrier waves, and (2) real-time differential correction with a base station to remove errors.

The roles of base stations and correction data

Now let’s organize exactly what roles the base station and correction data play in RTK positioning. A base station (base station) is a GNSS receiver installed at a location whose accurate coordinates are known in advance. The base station continuously receives multiple satellite signals and computes in real time “how much error is contained in its observations.” Specifically, by calculating the difference between the “true accurate position” and the “position observed by GNSS,” it estimates the positioning error at that time.

The error information computed at the base station is formatted as correction data and transmitted to the rover. Correction data include corrections to the pseudo-range for each satellite direction and the carrier-phase observations recorded by the base station. When the rover receives this correction data, it applies the correction values to its own observed satellite data and computes a high-precision position. Exchange of such correction data is typically done in real time via radio communication or the Internet. Methods include transmitting corrections by radio from the base station using dedicated radios or using Internet-based correction services over mobile networks (for example, distribution via NTRIP).

Using correction data allows the rover to cancel error sources it cannot correct on its own. However, if the distance between the base station and rover is too great, differences in error sources between the two locations become significant; therefore, typical RTK operation requires that the rover be within a few tens of km of the base station. To cover wide areas, a network of multiple base stations—network RTK (such as VRS methods)—is used. In Japan, the Quasi-Zenith Satellite System “Michibiki” provides a centimeter-class augmentation service (CLAS), allowing some receivers to obtain high-precision positioning by receiving correction signals directly from satellites. Thus, RTK correction data are provided by various means and are used according to the operating environment.

In summary, the base station plays the role of a “teacher who knows the error,” and by using the correction data received from that teacher, the rover can correctly “check its answers.” RTK positioning is built on this cooperation between base stations and correction data.

Use cases and benefits of RTK

Now that you understand how RTK works and its effects, let’s see what benefits it brings in real-world applications. In short, RTK’s characteristics can be summarized as “fast,” “simple,” and “high-precision.” These advantages improve work efficiency and result quality across a variety of fields, including surveying and construction.

First, RTK streamlines surveying work. With RTK you can measure many points over a wide area in a short time. Traditionally, surveying a large site required measuring each point one by one with a total station, needing significant manpower and time. With RTK positioning, you can simply carry the receiver and obtain coordinates for each point instantly, enabling efficient single-person surveying. For example, even when measuring many points for cadastral surveys or land surveys, RTK can dramatically reduce work time.

Next, RTK improves quality by increasing accuracy. With centimeter-level positioning from RTK, construction and quality control accuracy is greatly enhanced. Civil works require structures to be built to the correct positions and heights per the design, and RTK positioning allows machinery to be guided precisely without extensive manual referencing from control points. In practice, ICT construction examples in which a bulldozer’s blade is equipped with an RTK receiver to automatically grade the ground report finishing within a few centimeters of error with almost no human intervention. In other words, RTK implementation enables both construction accuracy and productivity to be achieved.

RTK is also a core technology for industry digitalization initiatives such as i-Construction and ICT construction. Efforts like reducing labor in as-built management, automating heavy equipment, and using drones for progress monitoring are increasingly possible thanks to RTK’s high-precision positioning. Local government infrastructure inspections and disaster response also see cases where RTK is used to quickly and accurately record conditions at disaster sites. RTK is turning previously impossible on-site digital transformation into reality. The traditional effort and uncertainty inherent in surveying sites are being dramatically improved by RTK’s adoption, and the style of surveying work is beginning to be transformed.

Simple surveying made possible by LRTK

Although RTK positioning is extremely useful, it traditionally required specialized, expensive surveying equipment and advanced knowledge, and was not something everyone could easily handle. Recently, however, technologies and services have emerged to make RTK easier to use. A representative example is a cutting-edge solution called LRTK.

LRTK is a system developed by a venture born from the Tokyo Institute of Technology, consisting of a small high-precision GNSS receiver and a smartphone app. The notable device is the “LRTK Phone,” an RTK-capable receiver that can be attached directly to the back of a smartphone. Simply attach it to your phone and launch the dedicated app, and this palm-sized device can perform centimeter-level positioning—an innovative product. Because the antenna and battery are integrated, complex setup like that required for traditional bench-mounted GNSS surveying equipment is unnecessary, achieving the convenience of “your smartphone becomes a surveying instrument.”

By using LRTK, RTK positioning becomes even more accessible. For example, during infrastructure inspections of bridge cracks, photos taken with a smartphone can be automatically tagged with accurate coordinates obtained by LRTK and saved to the cloud. In disaster site records or public facility maintenance, attaching accurate position information to photos and notes greatly reduces the effort of organizing data later. LRTK also supports Michibiki’s centimeter-class augmentation service (CLAS), so it can continue high-precision positioning by receiving augmentation signals from satellites even in mountainous areas without mobile connectivity. The device’s small, lightweight form factor makes it easy to carry, and unique use cases—such as attaching it to a safety helmet to track workers’ positions in real time—are possible, offering potential applications in site safety management.

Thus, LRTK evolves high-precision RTK positioning into a solution anyone can use easily. Centimeter accuracy that once required specialized equipment costing millions of yen can now be achieved with a smartphone and an affordable small device. It is easy for surveying beginners to handle and is attracting attention as a new surveying method that contributes to on-site DX. If you want to use high-precision positioning more easily, consider adopting the latest technologies such as LRTK.

FAQ

Q. What does RTK stand for? A. RTK stands for Real Time Kinematic. It refers to a method in satellite positioning that applies correction data from a base station in real time to improve accuracy, enabling positioning accuracy on the order of centimeters.

Q. What equipment is required for RTK positioning? A. Fundamentally, RTK requires two GNSS receivers: a base-station GNSS receiver installed at a known point and a rover GNSS receiver used at the location to be positioned. Additionally, a communication method to transmit correction data from the base station to the rover (radio or Internet-based communication) is necessary. Nowadays, it is possible to perform RTK positioning without owning a base station by subscribing to private correction data services. Also, to achieve centimeter-level precision, it is common to use multi-frequency GNSS receivers such as L1 and L2.

Q. What level of accuracy can be achieved with RTK? A. Depending on the environment, RTK positioning can achieve horizontal accuracy on the order of a few centimeters and vertical accuracy from a few centimeters to at most a few tens of centimeters. This is significantly more precise than typical GPS standalone positioning errors (about 5～10 m (16.4-32.8 ft)). Under ideal conditions, cases exist where horizontal accuracy of 2-3 cm (0.8-1.2 in) and vertical accuracy within 5 cm (2.0 in) are obtained. However, accuracy may degrade in environments with tall buildings nearby or poor sky visibility where satellite signal reception is obstructed.

Q. Can RTK be used without a base station? A. Strictly speaking, RTK cannot be performed without base-station (reference point) data. However, you can use existing base-station networks without installing your own base station. For example, you can obtain correction data by subscribing to network RTK services (such as VRS) provided commercially or by using electronic reference point data from agencies like the Geospatial Information Authority of Japan. Some GNSS receivers can also receive CLAS augmentation signals transmitted from Michibiki (quasi-zenith satellites) to perform corrections.

Q. Are qualifications or licenses required to use RTK? A. No special qualifications are required merely to use RTK. However, if you transmit correction data from a base station to a rover by radio, certain types of radio equipment may require licenses or qualifications under the Radio Law (for example, maritime or land radio operator licenses), depending on the radio used. On the other hand, correction services using mobile networks or RTK equipment that uses very low-power radio may be operable without licenses. Check whether any licenses are required depending on your equipment and communication method.

Q. Can beginners in surveying use RTK effectively? A. Yes. Nowadays, more RTK-capable equipment is beginner-friendly. Traditional RTK equipment was specialized and expensive, but solutions like LRTK, which can be used with smartphones, allow intuitive app-based operation and greatly lower the barrier to entry. Basic GNSS knowledge is necessary, but with training, even those with little surveying experience can perform centimeter-level positioning in the field.