Gentle explanation of RTK corrections: RTCM / NTRIP / VRS / CORS in brief

Table of Contents

• What is RTK?

• What is RTCM?

• What is NTRIP?

• What is VRS?

• What is CORS?

• Simple surveying with LRTK

• FAQ

The GPS position information used in everyday devices like smartphones and car navigation systems can sometimes be displayed several meters off from the actual position. Many people have experienced their current location shown off the road in a map app. Usually an error of several meters is not a major problem, but on surveying and civil engineering sites a deviation of even a few centimeters may be unacceptable. In situations where accurate position information determines quality and safety—such as highway or railway construction, land boundary surveys, and construction management of buildings—meter-level errors are insufficient. Real-time correction of positioning errors using RTK (Real-Time Kinematic) is powerful for these high-precision applications.

By using RTK you can instantly correct errors associated with satellite positioning and achieve high-precision positioning within a few centimeters (cm level accuracy (half-inch accuracy)). Today, centimeter-level high-precision positioning by RTK greatly contributes to improved efficiency in surveying work and advanced construction management, and it is attracting attention across a wide range of fields such as drone photogrammetry, precision agriculture with unmanned agricultural machines, machine guidance for construction equipment, and even autonomous driving technologies. This article explains the basics of RTK correction and, in an easy-to-understand way for beginners, organizes and explains related technical terms such as RTCM, NTRIP, VRS, and CORS.

What is RTK?

RTK (Real-Time Kinematic) is a positioning method that corrects GNSS (such as GPS) errors in real time to obtain centimeter-level accuracy. RTK positioning uses "relative positioning" with two GNSS receivers to derive highly accurate positions. One receiver is installed at a known accurate coordinate as the reference station (base station), and the other is carried to the point to be positioned as the rover (mobile) station. Because the reference station knows its exact position, it can determine the positioning error from the difference between the pseudo-range calculated from the received satellite signals and the known coordinates. This error is sent as correction information to the rover, which applies it to its position calculation in real time to cancel out the error. As a result, the error that was several meters in single-receiver positioning can be reduced to a few centimeters. This mechanism can cancel common error sources between both receivers such as satellite orbit errors, clock errors, and atmospheric delays (ionosphere and troposphere).

In theory, RTK can provide centimeter-level accuracy immediately, but actual positioning accuracy is affected by the surrounding environment and operational conditions. First, the satellite signal reception environment is important. Both the reference and rover stations should ideally be placed where the sky is open without tall buildings or trees nearby. In urban canyons or forests, signals can be blocked or reflected by structures causing multipath (reflected signal interference), which can increase positioning errors or delay obtaining a high-precision (FIX) solution. The number of available satellites and supported frequency bands also affect accuracy. By using a multi-GNSS receiver that receives not only GPS but also GLONASS, Galileo, and QZSS and supports multiple frequencies such as L1 and L2, you can reduce errors caused by uneven satellite geometry and ionospheric effects, enabling more stable positioning. Next, the distance between the reference and rover (the baseline length) is another important factor. The farther apart the two stations are, the greater the differences in local error sources, increasing residual errors that cannot be fully corrected. Therefore, in conventional RTK the reference station is typically set up as close to the survey site as possible (ideally within a few km) and operated while sending correction information via radio communication. With this method, planar positioning accuracy of a few centimeters and vertical accuracy from a few centimeters to several tens of centimeters can be achieved, enabling high-precision positioning on site that differs markedly from single-receiver GPS. On the other hand, this approach requires setting up a reference station on site each time, which involves time and effort to prepare and set up the equipment.

What is RTCM?

RTCM (Radio Technical Commission for Maritime Services) was originally the name of an international committee concerning maritime radio technology, but it has come to refer to the standard format for correction data used in GNSS positioning. The correction information sent from a reference station to a rover in RTK is usually distributed in RTCM message format. Simply put, RTCM is a "common language for conveying positioning corrections." Even receivers from different manufacturers maintain compatibility when exchanging data in RTCM format. RTCM messages include correction amounts for distances to each satellite and raw observation data observed at the reference station, and the rover receives and uses this information in its position calculations. Generally, "RTK correction data" refers to data in RTCM format. There were times when some manufacturers used proprietary formats (e.g., CMR), but for interoperability RTCM has become the de facto standard.

What is NTRIP?

NTRIP (Networked Transport of RTCM via Internet Protocol) is a communication protocol for distributing correction data such as RTCM over the Internet. Traditionally, RTK correction information was mainly sent from the reference station by radio (such as UHF professional radio or specific low-power radio). However, with the spread of internet communication, distribution by the NTRIP method is widely used. In NTRIP systems, the reference station sends real-time correction data to a server (an NTRIP caster) via an internet connection, and the rover user receives that data by connecting to the internet with a smartphone or mobile router. The image is like an "internet radio station" that streams correction information and an "internet radio receiver" that listens. Because there is no constraint of radio range, correction information can be received anywhere within cellular coverage, allowing RTK positioning over a wide area. Another advantage of NTRIP is that one reference station's data can be distributed to many users simultaneously. Currently, many public agencies and private companies provide GNSS services using NTRIP to deliver correction information; RTK users log in to the service with a dedicated ID and password to obtain data (for example, regional services using electronic reference points operated by local governments or high-precision positioning services provided by communication companies).

What is VRS?

VRS (Virtual Reference Station) is one of the representative correction methods in networked RTK. As mentioned above, conventional RTK requires the user to place a reference station near the site, but VRS achieves this virtually. Specifically, a network of multiple GNSS reference stations spread over a wide area (such as distributed electronic reference points) is used to generate correction data as if a reference station existed near the user. The server on the service provider side receives approximate position information from the rover user, integrates and analyzes observations from multiple nearby fixed reference stations, and predicts "what signals would be received from each satellite if a virtual reference station were placed right next to the user." It then creates correction information corresponding to that virtual reference station.

The generated virtual reference station data is usually distributed to the rover via NTRIP. For the rover, it becomes equivalent to having a reference station right next to it, so high-precision RTK positioning is possible without worrying about error increases due to distance. The advantages of the VRS method include that the user does not need to physically set up a reference station on site. Because only a single receiver (rover) needs to be brought to the site, equipment configuration is simple and setup and teardown labor are greatly reduced. With fewer devices to carry and lower failure risk, the work efficiency of RTK surveying has dramatically improved. Also, within a reference station network a wide area can be covered, and even locations tens of kilometers away can achieve stable centimeter-level accuracy. Today, VRS-type services using reference station networks maintained by national and local governments and private companies are available in many areas, and anyone with a subscription can easily use centimeter-level positioning services.

What is CORS?

CORS (Continuously Operating Reference Station) refers to permanently installed GNSS reference stations and the networks they form. It denotes a continuous observation system consisting of multiple reference stations, and nationwide CORS networks are used for RTK and geodetic observation. CORS stations operate 24 hours a day, 365 days a year, continuously accumulating and distributing high-precision observation data. For example, in Japan the Geospatial Information Authority of Japan has installed about 1,300 GNSS observation stations called "continuous operating reference stations." These form a huge CORS network covering the whole country, used for real-time correction information distribution and crustal deformation monitoring and research. Similar large-scale reference station networks function as positioning infrastructure overseas as well, such as the CORS systems in the United States and permanent GNSS station networks in European countries. CORS networks are the foundation of networked RTK services such as VRS, and users can receive data from the nearest reference station or virtual reference station over the internet. With expanded CORS deployment, environments are being established in which individual users can achieve high-precision positioning without installing their own reference stations.

Simple surveying with LRTK

The RTK correction technologies explained so far are very useful, but historically they required considerable expertise and preparation to operate. In recent years, solutions that combine these technologies to enable anyone to easily use high-precision positioning have emerged. One example is the compact RTK-GNSS system "LRTK" provided by our company. The LRTK receiver is compact and easy to handle and pairs with a dedicated smartphone app, enabling centimeter-level positioning without specialized equipment operation or complex settings. For example, by mounting the LRTK receiver on a dedicated pole (monopod) and wirelessly connecting it to a smartphone, a single person can easily perform surveying tasks. There is no need to carry heavy base station equipment or radio units, and onsite complicated setup is unnecessary. Internally, the positioning data processing and NTRIP communication required for RTK corrections are automated, and the user simply sets the device at the point to be measured and presses a button in the app to obtain high-precision coordinates. Because LRTK works in combination with networked RTK services, centimeter-level positioning (cm level accuracy (half-inch accuracy)) is possible stably anywhere in Japan. Centimeter-precision positioning (cm level accuracy (half-inch accuracy)), which used to be expensive and specialized, has become more accessible and lower-cost with the advent of LRTK. There are actual use cases such as local governments introducing smartphone RTK systems for surveying disaster recovery sites. With lower initial adoption hurdles compared to conventional equipment, LRTK aims to "democratize" high-precision positioning as a tool that contributes to improved field productivity. For those who want to start using RTK, such all-in-one solutions can be a reliable ally.

FAQ

Q: What is the difference between RTK and ordinary GPS positioning? A: Ordinary GPS (GNSS) positioning uses a single receiver, so it accepts satellite signal errors as-is and results in position deviations of several meters. RTK uses two receivers, a reference station and a rover; the reference station calculates the positioning error and applies corrections to the rover in real time to cancel that error. As a result, centimeter-level accuracy can be obtained in real time (for example, ordinary GPS typically has errors of about 5–10 m (16.4–32.8 ft), whereas RTK can improve accuracy to about 1–2 cm (0.4–0.8 in)).

Q: What equipment and preparations are needed to start RTK surveying? A: Basically, you need a GNSS receiver (rover) capable of centimeter-level positioning and a reference station to provide correction information. If you operate your own system, you must install a reference receiver at a point with known accurate coordinates and connect it to the rover via radio communication or NTRIP. Accurate prior measurement of the reference station coordinates is also essential. If you use a public or private RTK service, you can start with a rover GNSS receiver and a mobile communication environment. Since correction data (reference station data) is provided by the service contract, there is no need to prepare your own base station.

Q: What if I cannot provide my own reference station? A: If there is an existing reference station network nearby, you can use a networked RTK service. By subscribing to electronic reference point networks operated by governments or to paid high-precision positioning services from service providers, you can obtain real-time correction data via the internet. This enables RTK positioning without installing your own base station. For example, using a VRS service you can receive high-precision correction information from a virtual reference point in real time and perform surveying tasks easily even alone.

Q: How far from the reference station can RTK maintain its accuracy? A: With conventional single-reference RTK, if the reference station is within a few km the accuracy of a few centimeters is easier to maintain, but as the distance increases to 10 km or 20 km, it becomes harder to obtain a fixed solution and accuracy tends to become unstable. This is because the satellite signal error environment at the reference and rover locations diverges with distance. Using networked RTK services (such as VRS), errors are corrected based on multiple reference stations, reducing accuracy degradation over wide areas, so similar centimeter-level accuracy can be practically achieved even at distant locations.

Q: What is the difference between VRS operation and conventional single-reference RTK operation? A: In conventional RTK the user must set up a reference station near the survey site and send correction information by radio. In VRS, a network of multiple permanent reference stations is used to generate virtual reference station data near the user. The user does not need to place their own reference station, and a single receiver can cover a wide area. Also, VRS virtually ensures a reference station is always nearby to compensate for distance-related accuracy degradation, enabling stable centimeter-level positioning even at remote locations.

Q: Can RTK positioning be used where the internet is unavailable? A: Even in sites without cellular coverage, RTK positioning is possible by directly connecting the reference and rover stations via radio communication. Radio range is limited, but correction information can be sent using specific low-power radio in ranges on the order of several hundred meters, and there are examples of RTK surveying in mountainous areas. If real-time communication is impossible, recording the reference station observations and later combining them with the rover data using PPK (Post-Processed Kinematic) is also effective. With PPK, RTK-equivalent accuracy can be achieved through post-processing even without onsite communications.

Q: Are there positioning devices that are easy for RTK beginners to use? A: Recently, RTK-capable devices designed for beginners have appeared. A representative example is the aforementioned LRTK system. The LRTK receiver works with a smartphone app and is designed for ease of use without complex settings. Even without specialist knowledge, following the app guidance yields high-precision positioning results. Products like LRTK are strong allies for beginners, making RTK surveying more accessible.

Q: What causes RTK to fail to produce a "FIX solution" (high-precision solution) for a long time? A: First, check the antenna installation environment. If the site is surrounded by buildings or trees or the sky view is narrow, satellite signal reception may be insufficient and RTK solutions can be unstable or fail to fix. Also check whether the rover is correctly receiving correction data (NTRIP connection status or radio communication). Errors in the reference station coordinate input or an excessively long baseline between the reference and rover can also lead to degraded accuracy or non-fixed solutions. Receiver settings (satellites and frequencies in use) and nearby radio interference can also be causes. Reviewing these points will often resolve most RTK positioning issues.