Table of Contents

• About RTK high-precision positioning and stability

• Baseline length: how distance to the base station affects accuracy

• Ensuring satellite visibility: creating an environment that does not block signals

• Preparing communications: reliably receiving correction data

• Recommendation for simple surveying with LRTK

• FAQ

In recent years, centimeter-level GNSS positioning using RTK (Real Time Kinematic) has been spreading across surveying and construction sites. The adoption of such advanced technology can greatly streamline field surveying for civil works managers, surveyors, and infrastructure maintenance personnel. With RTK positioning, locations that would be off by several meters with standalone GPS can be determined in real time within a few centimeters of error, allowing a dramatic improvement in both accuracy and speed of surveying work.

About RTK high-precision positioning and stability

On the other hand, you sometimes hear complaints such as “RTK accuracy is not stable on site” or “positioning results fluctuate and cannot be trusted.” In practice, to reliably achieve centimeter-level accuracy with RTK, it is essential to properly configure the site environment and operational conditions. Three factors in particular strongly affect RTK accuracy: the baseline length (distance to the base station), satellite signal visibility, and the communication environment for correction data. If any one of these conditions is lacking, even the most advanced equipment cannot deliver RTK’s true accuracy. Conversely, by sufficiently securing these three points, you can consistently obtain centimeter-class positioning results. For example, even if the baseline is short, you may not obtain a fixed solution if the surroundings are enclosed by tall buildings and satellites are not adequately visible; and even if many satellites are visible, accuracy cannot be maintained if communication for sending correction information is interrupted. Only when all three conditions are met can RTK performance be fully realized. Below, we explain each of these three elements in detail.

Baseline length: how distance to the base station affects accuracy

The baseline length is the distance between the reference station (base) and the rover in RTK positioning. This distance to the base station is one of the fundamental factors that determines RTK accuracy. In general, the shorter the baseline, the more similar the error sources affecting GNSS satellites received by both stations are, so differential error elimination works more effectively and yields higher accuracy. Conversely, as the baseline increases, differences in error sources between the base and the rover (for example, ionospheric or tropospheric effects) grow, leaving residual errors that cannot be fully corrected and lowering positioning accuracy.

As a guideline, with short baselines on the order of a few km, RTK can deliver high accuracy with horizontal errors within a few centimeters. However, as baselines extend to 10 km or 20 km, errors gradually increase and in some cases may exceed several centimeters and reach about 10 cm (3.9 in). Note that the degradation in vertical accuracy is particularly noticeable as baseline length increases. Also, longer baselines make it harder to obtain a stable fixed solution, and solutions may revert to an unstable float solution due to slight noise or environmental variations in GNSS observation data. Single-frequency (L1-only) RTK receivers, in particular, cannot sufficiently correct ionospheric errors, making it difficult to maintain accuracy over long distances. Even with multi-frequency receivers, RTK with a single standalone base station is generally considered stable for high-precision positioning up to roughly 20 km.

So what should you do if the survey area is far from the reference station? One countermeasure is to set up your own base station near the survey site if possible. Since shorter baselines improve accuracy and stability, placing a base station within or close to the site is ideal if feasible. Another option is to use public or commercial network RTK services (such as VRS). Network RTK can generate a virtual base station near the user from data of multiple nearby reference stations, so even if the nearest physical reference stations are tens of kilometers away, you can receive correction information as if a base station were right next to you. This greatly mitigates accuracy degradation due to long baselines and enables centimeter-level positioning stably across wide areas. In Japan, several network RTK correction information services using the Geospatial Information Authority of Japan’s continuously operating reference station data are available. By using these services, you can receive high-precision correction information without installing your own base station and achieve stable RTK positioning over a wide area. Note also that if the coordinate values entered for the reference station contain errors, the resulting positioning will be shifted by the same amount; ensuring accurate coordinates for the reference station is therefore important in RTK operations.

Ensuring satellite visibility: creating an environment that does not block signals

RTK positioning requires both the base and the rover to receive radio signals from GNSS satellites, so having an open sky view is extremely important. In urban canyons surrounded by buildings or in dense forests, satellite signals may be blocked or reflected (multipath), making positioning impossible or severely degrading accuracy. In RTK, a fixed solution can only be obtained when both the base and the rover simultaneously track five or more common satellites, so if either station has poor visibility, high-precision positioning is impossible. Practically, the more common satellites available the better; if eight or more satellites can be tracked simultaneously, positioning will be more stable.

Therefore, when conducting RTK surveying, selecting an open location with good surrounding visibility is a fundamental prerequisite. During positioning, ensure nothing blocks the sky above the antenna and take care not to lose satellites even temporarily. For example, when moving with the rover, stop in places where branches or building eaves do not obstruct overhead view, and position the antenna where it can see sufficient sky. Mounting the antenna on a tall pole (monopod or tripod) to keep it away from surrounding obstructions is also effective. Large reflectors such as metal fences or vehicle bodies near the antenna can cause multipath errors, so keep the antenna’s surroundings as open as possible. If necessary, laying a ground plane (radome) beneath the antenna to reduce reflections is an effective countermeasure. Modern RTK receivers often support multiple GNSS constellations such as GPS, GLONASS, Galileo, and QZSS (Michibiki), so using a multi-GNSS receiver to secure more visible satellites is recommended. Nevertheless, RTK is impossible in environments where satellite signals cannot reach, such as inside tunnels, in deep building shadows, or in dense forest interiors. In such sites, it may be appropriate to combine other surveying methods such as total stations rather than forcing GNSS.

Preparing communications: reliably receiving correction data

For RTK to work, error correction information generated at the base station must be delivered in real time to the rover. It is therefore essential to secure a communication means on site and ensure stable bidirectional data exchange. Main communication methods are radio links that transmit signals directly (specific low-power radios, UHF radios, etc.) and obtaining correction data via internet-based NTRIP services. If communication is interrupted, the rover cannot receive corrections and cannot maintain accuracy. (The communication volume for correction data itself is very small, on the order of several hundred bytes per second, but even a momentary disconnection has immediate effects.)

When using radio, pay attention to the radio range between the base and rover. In urban areas, line-of-sight ranges of a few km are typical; in open suburban areas, ranges of several to a dozen kilometers are common communication limits. Buildings and terrain will further reduce range. To maintain a high-quality fixed solution, it is desirable to receive correction data continuously at least once per second; unstable communications will quickly cause the positioning solution to revert to an uncertain float solution. Once a solution has reverted to float, it can take tens of seconds to minutes to regain a fixed solution, resulting in major loss of measurement time. When using radios, install the base station antenna as high as possible and avoid obstacles between it and the rover. In mountainous sites where cellular reception is unavailable, radio communication may be the only option, so prepare high-output radios or repeaters as needed. Also be aware that nearby radio systems operating on the same frequency band or strong noise sources can cause interference; check that radio communication will operate well on site before starting work.

When using network RTK via cellular networks, it is important to check the site’s mobile reception in advance. In locations where cellular networks are out of coverage—underground, inside buildings, or in mountain valleys—you will not be able to receive correction data and RTK will not function. In such areas, consider preparing GNSS equipment that can operate in base station mode to set up an on-site reference station. Choosing a SIM card from a carrier with good coverage at the site, or using a multi-network SIM service to reduce the risk of being out of coverage, is also effective. Don’t arrive at the site and panic because “no communication, RTK won’t work!”—make sure to perform pre-checks and prepare your communications.

In summary, to obtain stable centimeter-level accuracy with RTK, it is important to satisfy the following conditions:

• Keep the distance to the reference station (baseline length) as short as possible

• Ensure sufficient satellite visibility overhead and avoid obstructions and multipath

• Prevent interruptions to correction data communications from the reference station

With appropriate preparation and operation based on these points, RTK positioning can demonstrate its true value on site. Some may feel that assembling all these conditions for high-precision positioning is burdensome. Indeed, conventional RTK operations can require expensive dedicated equipment, base station installation, and communications setup. That is where LRTK comes in—an easy-to-use solution that removes much of this complexity and enables anyone to perform high-precision surveying.

Recommendation for simple surveying with LRTK

LRTK is our company’s high-precision RTK positioning system designed so that anyone on site can easily perform centimeter-level surveying. It is used by combining a dedicated compact GNSS receiver (LRTK unit) with a smartphone, enabling one-touch acquisition of high-precision absolute coordinates that standalone positioning cannot provide. For example, by mounting the LRTK unit on the included monopod and surveying, a single person can easily measure point positions. The smartphone app automatically calculates antenna height (the height at which the device is set), so accurate ground coordinates can be recorded without specialized knowledge.

Although LRTK performs advanced GNSS correction processing internally, users do not need to worry about complex settings. The LRTK unit, acting as the rover, has built-in mobile communications and automatically acquires necessary correction information such as data from the Geospatial Information Authority of Japan’s continuously operating reference stations. Therefore, there is no need to install your own base station on site—just bring the LRTK unit and a smartphone and you can begin centimeter-class positioning immediately. The device has a built-in battery and is easy to carry; it is compact enough to fit in a pouch and will not get in the way even in tight sites.

The achievable positioning accuracy is comparable to professional high-end GNSS equipment. Under favorable conditions, LRTK can achieve horizontal errors of about ±1-2 cm (±0.4-0.8 in) and vertical errors of about ±3 cm (±1.2 in). It also has a function to average multiple quick measurements; for example, averaging 60 measurements at the same point can produce a remarkable accuracy of about 8 mm (0.32 in). Despite such precision, operation is extremely simple—just press a button in the smartphone app.

Thus, LRTK is a solution aimed at “making difficult RTK surveying accessible to anyone.” In addition to high-precision point surveying, optional functions enable acquisition of wide-area 3D survey data with absolute coordinates, making it a true next-generation surveying tool. RTK technology is no longer limited to specialists; the time is coming when anyone on site can use it. If you are considering introducing RTK to your site or want to make surveying more efficient, please consider trying LRTK. Even first-time users can perform high-precision positioning after a short lecture, and LRTK can bring new possibilities for high-precision positioning to your site.

FAQ

Q. What is RTK? A. RTK stands for “Real Time Kinematic,” a technique that achieves centimeter-level GNSS positioning by correcting GNSS errors in real time. One receiver is fixed as a base station, and by differencing observation data with the rover, errors are canceled to realize high-precision positioning.

Q. What is required for RTK positioning? A. RTK positioning basically requires a GNSS receiver for the reference station, a GNSS receiver for the rover, and a means of communication connecting the two. The reference station is installed at a point with known accurate coordinates, and correction data are sent to the rover via radio communication (specific low-power radios, UHF radios, etc.) or internet communication. Public and commercial reference station networks are increasingly used so that corrections can be obtained without providing your own reference station (network RTK), but in all cases the rover must have a communication link (radio or cellular) available.

Q. How far from the reference station can RTK positioning still be performed? A. For conventional single-base RTK, it is desirable that the distance to the reference station be within about 20 km. Beyond that, differences in ionospheric and tropospheric errors increase, making fixed-solution acquisition unstable and tending to reduce accuracy to several centimeters or more. However, using network RTK can provide similar accuracy even for areas tens of kilometers away due to the virtual reference point effect.

Q. Can RTK be used in any environment? A. RTK surveying generally needs an open sky overhead. In urban areas surrounded by buildings or in forests, satellite signals may not be receivable and positioning becomes difficult. Accuracy can also degrade when ionospheric effects are significant. Furthermore, if communication with the reference station is interrupted, corrections cannot be received, so network RTK is difficult to use in deep mountain locations without cellular coverage. RTK has environments where it does not perform well; in such cases, consider adjusting measurement points or temporarily combining other surveying methods.

Q. What is the difference between a fixed solution (Fix) and a float solution (Float)? A. In RTK positioning, when the carrier-phase integer ambiguities can be correctly resolved, the result is a fixed solution (Fix), yielding centimeter-level accuracy horizontally and vertically. If the ambiguities are unresolved, the result is a float solution (Float), with accuracy typically on the order of several tens of centimeters to about 1 m. RTK usually begins in a float state and converges to a fixed solution when sufficient satellites and good observation conditions are available. Only by maintaining a fixed solution can you reliably use RTK’s high-precision positioning results.

Q. Can you do RTK positioning with a smartphone? A. Ordinary smartphone built-in GPS alone cannot achieve RTK-level accuracy, but with an external RTK-capable GNSS receiver, centimeter-class positioning is possible using a smartphone. For example, our LRTK Phone is an RTK system that links a smartphone to a dedicated small GNSS unit; simply connect the unit to the phone and launch the app to obtain high-precision position information easily. Recently, other companies have also released small Bluetooth RTK receivers, and using smartphones or tablets for convenient RTK surveying is becoming more common. By leveraging smartphones as field tools, the benefits of RTK’s high-precision positioning—once accessible only to specialists—are becoming available to a wider range of users.