How RTK Improves Accuracy: The Core in 5 Minutes

Table of Contents

• What is RTK?

• Accuracy of ordinary GPS positioning and error factors

• How RTK improves accuracy

• What is needed for RTK positioning

• Use cases of RTK

• Simple surveying with LRTK

• FAQ

What is RTK?

RTK (Real Time Kinematic) is a positioning technique that corrects errors in satellite positioning (GNSS) in real time to measure positions with centimeter-level accuracy (half-inch accuracy). (Kinematic means “motion,” indicating that it can position moving objects in real time.) It uses two GNSS receivers: a base station (fixed station) whose precise position is known in advance, and a rover (mobile station) placed at the point to be surveyed. By combining data from both, high-precision position information is computed. The base station calculates positioning errors from the satellite signals it observes and sends those error corrections to the rover in real time; applying these corrections at the rover greatly improves its position accuracy. Ordinary GPS positioning typically has errors on the order of several meters, but with RTK those errors can be reduced to a few centimeters. For this reason, RTK has gained significant attention in recent years in fields that require high-precision positioning, such as surveying and civil engineering. RTK is also positioned as a core technology supporting DX (digital transformation) in the construction industry, and its adoption is accelerating.

*(GPS is the name of the U.S. satellite positioning system; the collective term for satellites from various countries including Russia’s GLONASS and Japan’s QZSS (“Michibiki”) is GNSS. For convenience, this article uses “GPS.”)*

Accuracy of ordinary GPS positioning and error factors

Ordinary GPS positioning used in car navigation and smartphones generally has an accuracy that can deviate by a few meters. For example, commercial smartphone GPS can show errors of about 5–10 m (16.4–32.8 ft) even under good conditions. Such meter-level deviations are acceptable for checking your current location in a map app. However, when measuring land boundaries or determining the position of structures on a construction site, even errors of a few centimeters may be unacceptable. Conventional single-receiver GPS positioning (positioning performed with only one receiver) therefore has clear limitations for applications that require such high precision.

Why do ordinary GPS positions have errors on the order of meters? There are several causes; the main factors include the following:

• Signal delay in the atmosphere – Signals from satellites slow slightly as they pass through the ionosphere and troposphere, causing errors in distance measurement.

• Satellite and receiver clock errors – Small offsets in satellite orbits and clocks, and clock errors inside receivers, introduce errors into distance calculations.

• Multipath errors – If signals reflect off buildings or the ground and arrive via indirect paths, the measured distance is longer than the true path.

• Effects of satellite geometry – Accuracy varies with the number and geometric arrangement of satellites in view. When fewer satellites are available or they are clustered in one direction, errors increase.

These error factors cannot be fully corrected by a single receiver, so in practice real-time positioning accuracy is limited to a few meters. Achieving more precise measurements requires additional measures to cancel out these errors, which is the role RTK plays in enabling high-precision positioning.

How RTK improves accuracy

RTK achieves dramatic improvements in accuracy because relative positioning using two receivers can cancel out many errors. When the base station and rover receive signals from the same satellites at the same times, their observations contain common error components caused by weather and satellite effects. The base station compares its known accurate coordinates with the position calculated from satellite observations to determine the instantaneous error. The base station then transmits this correction information to the rover, which applies the corrections to its own measurements, canceling out the errors and obtaining a high-precision position. In simple terms, “measuring with two receivers instead of one allows common errors to be canceled, resulting in much higher accuracy.”

RTK also increases ranging precision by using the satellite signal’s carrier wave phase. Ordinary GPS measures distance from the timing of pseudo-random codes embedded in the signal, but RTK also measures the phase difference of the radio wave itself. The wavelength of signals from satellites is about 20 cm (7.9 in), so measuring phase shifts of that wave precisely can detect changes at the millimeter scale. Combining this advanced measurement technique with error correction enables RTK to achieve centimeter-level accuracy (half-inch accuracy).

The overall RTK positioning process is roughly as follows:

• The base station and rover simultaneously receive satellite signals. Both receivers observe signals from the same satellites.

• The base station calculates position error. The base station compares its known coordinates with the position computed from satellite observations and derives the error.

• Correction data is sent to the rover. The base station transmits the error information to the rover in real time via radio or the Internet.

• The rover applies corrections and obtains a high-precision position. The rover incorporates the received corrections into its positioning to compute a corrected high-precision current position.

Because this process runs in real time, the rover can maintain centimeter-level accuracy even while moving. Note that the closer the base and rover are, the better the errors cancel; generally, centimeter-level accuracy is easier to maintain within a few km. As distance increases, the commonality of error factors decreases and accuracy gradually degrades. To address this, network RTK systems that distribute corrections over the Internet from networks of base stations (e.g., VRS using Continuously Operating Reference Stations) have become widespread. These services allow users to achieve high-precision positioning over wide areas with a single rover, without deploying their own base station.

Also note that long-duration static GNSS surveying (static positioning) that observes for several hours can, under certain conditions, achieve millimeter-level accuracy superior to RTK. However, because static methods require post-processing of results, RTK is widely used in applications that require immediate, real-time positioning.

What is needed for RTK positioning

Traditionally, performing RTK positioning required several specialized pieces of equipment. The basic components are as follows:

• GNSS receiver for the base station – A receiver installed at a point whose accurate coordinates are known. Also called a reference station, it continuously observes satellite signals and computes errors.

• GNSS receiver for the rover – A receiver installed at the point or on the object to be located. Also called a rover, it receives corrections and computes its own position with high accuracy.

• Communication link between base and rover – A communications link is required to send corrections from the base to the rover. When using radio, low-power radio systems are used, covering several km from the base. Recently, methods that distribute correction data over the Internet (such as NTRIP) have become common.

In addition, an environment with a clear view of the sky is necessary to receive satellite signals well (accuracy deteriorates in canyons of tall buildings or inside forests). If you do not provide your own base station, you need to be able to receive correction data from public reference stations such as the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations or from commercial correction services.

Traditional RTK surveying had three main challenges. First, the high cost of specialized equipment. Conventional RTK GNSS receivers and base station equipment could be very expensive, sometimes requiring investments on the order of millions of yen. Second, operational complexity. Setting up a base station at a known point, establishing communication with the rover, and starting measurements required specialized knowledge. Radio communication requires maintaining line of sight, selecting appropriate frequency bands, and sometimes obtaining radio licenses; even with network RTK (VRS), subscribing to and configuring the corresponding service is necessary. Troubleshooting in the field often required experienced personnel, limiting RTK use to a relatively small number of surveying specialists. Third, the effort required to utilize data. Traditional equipment recorded survey data in the receiver, which then had to be transferred to a PC and imported into CAD software. Tagging photos with position information required matching coordinates to photos afterward. Real-time sharing of field-collected information was not easy, and analog practices such as writing numbers in field notebooks were common. These issues made RTK attractive for accuracy but not something that every field technician could routinely use.

Use cases of RTK

Real-time, high-precision position information provided by RTK is used in many fields. Major use cases include:

• Surveying and mapping – RTK is used for measuring land boundaries and creating topographic maps. Tasks that previously took a long time can be performed quickly with RTK while maintaining accuracy.

• Construction site management – On civil engineering sites, RTK is indispensable for quality control of as-built placement and for machine control (machine guidance). Accurate stakeout and piling work improve quality and reduce rework.

• Agriculture (precision farming) – RTK is used for tractor automation and other precision agriculture tasks. Autonomous farm machinery that drives straight across fields for uniform tillage or applies fertilizer at precise points is enabled by RTK’s centimeter-level positioning.

• Drone surveying – RTK is powerful for aerial photogrammetry and aerial imaging. Drones equipped with RTK can tag images with precise coordinates, dramatically improving the accuracy of mapping and 3D modeling. This reduces the need to place many ground control points.

Beyond these, RTK is increasingly used for monitoring ground subsidence for road and bridge maintenance, recording positions during railway and power line inspections, and more. GNSS-RTK is also beginning to be applied for precise self-localization of autonomous vehicles and mobile robots, expanding its scope. The ability to know high-precision positions in real time directly contributes to productivity improvements and labor savings, so adoption of RTK technology is rapidly expanding among large corporations, small and medium enterprises, and local governments.

Simple surveying with LRTK

Although RTK is highly useful, the need for specialized equipment and operational knowledge has historically limited its use to specialists. LRTK was developed to break this situation and make RTK surveying accessible to anyone. LRTK combines a smartphone with a compact RTK-capable device so that, with just a smartphone, a single person can perform centimeter-level surveying.

Specifically, by installing a dedicated app on a smartphone and connecting it to an LRTK device, users can achieve high-precision positioning with a single tap, without dealing with complex device settings or positioning calculations. For example, an LRTK antenna unit can be mounted on the top of a pole; simply press a button on the smartphone at the desired point to instantly record a high-precision coordinate for that point. Acquired position data can be viewed on the smartphone map and easily saved to the cloud for sharing. Functions are also provided to automatically tag photos with the measured coordinates, greatly streamlining field recording tasks.

Compared with conventional RTK equipment, LRTK is overwhelmingly lightweight and compact and is priced affordably, making it practical to carry and use one per person. It eliminates complicated radio setup and the need for specialized knowledge; beginners can start positioning by following the smartphone app’s prompts. This combination of ease of use and practical accuracy has made LRTK a new style of simple surveying on construction sites and in surveying work. For example, LRTK can enable surveying tasks that previously required two people to be completed by one person, significantly reducing on-site work time. If you are considering RTK adoption, try experiencing simple surveying with LRTK.

FAQ

Q: What does RTK stand for? A: RTK stands for “Real Time Kinematic.” It refers to a technology that enables centimeter-level positioning (half-inch accuracy) by correcting satellite positioning errors in real time—precision that ordinary GPS cannot achieve.

Q: How accurate is RTK? A: Typical RTK GNSS provides horizontal accuracy on the order of a few centimeters. Under good conditions, errors often fall within 1–2 cm (0.4–0.8 in), which is orders of magnitude more precise than ordinary GPS (errors of several meters). However, vertical (elevation) accuracy is generally slightly worse than horizontal and can be on the order of around 10 cm (3.9 in). In environments with tall buildings or trees that block satellite signals, RTK accuracy can degrade to tens of centimeters.

Q: What is needed to perform RTK positioning? A: Fundamentally, you need two GNSS receivers—one for the base station and one for the rover—and a communication method to connect them. The base station requires accurate coordinates, so it must be set at a known point or use public base station data such as the Geospatial Information Authority’s reference stations. Communication equipment or Internet access is needed to exchange correction information. Recently, methods that connect a single rover to public correction services without deploying your own base station have become common.

Q: Can RTK surveying be done by one person? A: Yes. In the past, deploying a base station and operating a rover sometimes required multiple people, but network RTK now allows single-receiver positioning. Smartphone-based systems (such as LRTK) also enable one person to perform RTK surveying easily.

Q: Does RTK positioning take time? A: Compared with single-receiver positioning, RTK does not take any special extra time to obtain results. As the name implies, RTK computes high-precision positions in real time by applying corrections. However, obtaining a high-precision fixed solution may require an initial convergence time of several tens of seconds to a few minutes for resolving integer ambiguities. Once centimeter-level positioning is fixed, the solution updates continuously with movement. If satellite signals are interrupted and positioning is lost, reacquiring signals typically restores a high-precision solution in a short time. Waiting times are rarely an issue in normal work.

Q: What is the difference between DGPS and RTK? A: DGPS (Differential GPS) improves accuracy by applying correction information from a base station to a single-receiver solution. DGPS typically reduces errors of several meters to around 1 m (3.3 ft), but cannot achieve the centimeter-level accuracy of RTK. RTK, broadly speaking, is a form of differential GPS, but it uses carrier-phase measurements and advanced relative positioning methods to achieve much higher accuracy than DGPS.

Q: Is it expensive to introduce RTK? A: Historically, a large initial investment in specialized equipment was required, but in recent years low-cost GNSS receivers and public correction services have lowered the barrier to entry. For example, smartphone-compatible LRTK systems can deliver centimeter-level positioning at a much lower cost than traditional professional equipment. Correction service subscriptions typically incur monthly fees, but many providers offer short-term plans so small projects can adopt RTK affordably.