Common misunderstandings in RTK articles: correctly understanding the “conditions” for centimeter-level accuracy

Table of Contents

• What is RTK? The background behind the focus on centimeter accuracy

• Conditions required to achieve centimeter accuracy with RTK positioning

• Common misunderstandings about RTK accuracy and the truth

• Latest trends in simplified RTK surveying: high-precision positioning with LRTK

• FAQ

What is RTK? The background behind the focus on centimeter accuracy

In recent years, on-site needs in construction, civil engineering, and surveying have increasingly favored positioning with the smallest possible errors. Conventional GPS-based positioning (GNSS standalone positioning) is convenient to use, but its errors are generally on the order of several meters. For simply checking your current position in a map app, an error of a few meters is acceptable, but when positioning structures on an actual construction site or detecting slight displacements in infrastructure inspections, centimeter-level accuracy is required.

This is where RTK positioning (Real Time Kinematic positioning), a high-precision positioning technology, has attracted attention. RTK uses two GNSS receivers—a base station and a rover—to perform real-time relative positioning, canceling various errors inherent in satellite positioning and enabling high-precision positioning that cannot be achieved by standalone positioning. In practice, when conditions are met, RTK can reduce positioning errors to several centimeters horizontally and several centimeters vertically (height). In other words, the GPS positioning errors that normally deviate by 5–10 m (16.4–32.8 ft) can be reduced by roughly a factor of 100 with RTK.

Such centimeter-level accuracy is extremely important for Japan’s promoted construction DX (i-Construction). For example, in road or bridge construction, large surveying errors can cause the position or elevation of the completed structure to be off. However, with RTK, accurate stakeout and as-built measurement according to design drawings can be performed, enabling high-quality construction. Centimeter-level position information also dramatically improves data reliability for drone surveying and equipment monitoring. As a result, RTK technology is being adopted across a wide range of sites, from major general contractors to small and medium-sized construction companies, surveying firms, and infrastructure maintenance personnel.

Conditions required to achieve centimeter accuracy with RTK positioning

To achieve “errors of several centimeters” with RTK, several conditions must be met. RTK is indeed a powerful technology, but it is not something that will always yield 1 cm accuracy simply by using it; appropriate environments and procedures are required to extract that level of accuracy. Let’s look at the main points in order.

• Correction information from a base station: In RTK, a base station installed at a known accurate position is indispensable. The base station calculates error components from the satellite signals it receives and sends that correction information to the rover in real time. Thanks to this correction information, the rover (the point you want to measure) can cancel errors that it could not correct on its own and compute a high-precision position. Nowadays, correction information can be obtained without preparing your own base station by using public Continuously Operating Reference Station networks or private correction services (network RTK using mobile networks). In any case, receiving accurate correction data in real time is a prerequisite for achieving centimeter-level accuracy.

• Distance from the base station: The closer the base station (or virtual reference point) is to the rover, the more similar the error factors affecting both (such as atmospheric signal delays and satellite orbit errors), and thus the higher the achievable accuracy. Generally, if the rover is within several kilometers of the base station, it is easier to maintain centimeter-level accuracy with RTK. Conversely, as distance increases, uncorrectable errors grow and positioning accuracy gradually degrades (as a rule of thumb, the error increases by a few millimeters for every 1 km of distance). Therefore, for high-precision surveying, it is effective to place a base station near the survey area or to use network RTK based on public reference stations distributed nationwide.

• Satellite positioning environment: To obtain centimeter-level accuracy with RTK, it is also important to be in an environment where radio signals from GNSS satellites can be received stably. In an open area with a clear view of the sky and sufficient visible satellites, dense observation data can be obtained and RTK solutions—especially the high-precision “fixed solution”—can be stably achieved. On the other hand, in urban canyons, forests, or when satellite geometry is poor, the number of receivable satellites may decrease and RTK accuracy will be affected. In particular, in multipath environments where satellite signals are reflected by buildings or terrain, errors increase and obtaining an accurate solution becomes difficult. To achieve centimeter-level accuracy, it is necessary to position yourself in as unobstructed a location as possible or choose time periods with fewer obstructions.

• GNSS receiver and frequency bands: The performance of the GNSS receiver used in RTK positioning also affects accuracy. For high precision, a receiver that can track carrier-phase (the phase of the radio wave) level signals from satellites is required, allowing handling of raw data with millimeter-level detail. Receivers that support multi-GNSS and multi-frequency signals are even more advantageous. Being able to use multiple satellite systems (not just GPS but also GLONASS, Galileo, QZSS “Michibiki”, etc.) increases the number of satellites visible overhead and improves positioning stability. Using multiple frequency bands such as L1/L2 helps remove ionospheric errors and shortens the time to initial convergence of the fixed solution. Recently, inexpensive multi-band RTK-GNSS modules have appeared, making high-precision positioning easier to achieve than before.

If the above conditions are met, RTK can achieve astonishing accuracy within several centimeters both horizontally and vertically. However, these are “ideal conditions,” and in real surveying environments, accuracy may vary due to environmental or operational factors. The next section explains common misunderstandings about RTK accuracy encountered in the field and the truth behind them.

Common misunderstandings about RTK accuracy and the truth

RTK is an excellent technology that can achieve centimeter-level accuracy, but without a sufficient understanding of its mechanisms and required conditions, it can lead to incorrect expectations or operational mistakes. Here, we整理 common misunderstandings often seen in RTK articles and promotions, along with the correct understanding.

Misunderstanding 1: “Using RTK always guarantees within 1 cm error”

Some people mistakenly believe that introducing RTK will always produce positioning errors less than 1 cm in any situation. In reality, RTK accuracy is influenced by surrounding environmental conditions. As mentioned earlier, if satellite reception conditions are poor, RTK may not achieve a fixed solution and temporary errors of about 10 cm can occur. In particular, in urban areas surrounded by high-rise buildings or in heavily forested mountain areas, while average horizontal and vertical accuracy may be within several centimeters, instances of deviations exceeding 10 cm have been observed. Therefore, rather than believing “RTK = always 1 cm accuracy,” it is important to correctly understand that “in good environments it achieves several centimeters, and at worst several tens of centimeters”. In other words, RTK is a means to greatly reduce errors, but one should keep in mind that environmental conditions can impose limits on achievable accuracy.

Misunderstanding 2: “If centimeter accuracy is achieved, absolute positions are also accurate”

Because of the term “centimeter-level accuracy,” people may assume “the measured coordinate itself has only a few centimeters of error in a global geodetic frame,” but caution is required here as well. The coordinates obtained by RTK primarily mean centimeter-level relative accuracy with respect to the base station, assuming the base station’s position is accurately known. If the base station’s coordinates contain errors or biases, the coordinates measured by the rover will be biased by the same amount. For example, if you use a self-installed base station that is actually off by several meters as the reference for RTK surveying, distances within the site may be highly precise, but overlaying the obtained coordinates on national maps could show several meters of offset. Understand that centimeter accuracy does not necessarily equal absolute accuracy; it indicates high relative accuracy tied to the reference point.

In this regard, using the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations (CORS) network for network RTK or correction services provided by mobile carriers is reassuring, because these reference networks are calibrated in public coordinate systems, and the positioning results have high absolute accuracy in public geodetic frames. Conversely, when using a locally installed temporary base station on site, you need to calibrate it against known points beforehand or perform post-processing coordinate transformation.

Misunderstanding 3: “RTK surveying requires expensive, specialized equipment”

Some may think RTK requires special surveying equipment and that its introduction is prohibitively expensive. Indeed, until not long ago RTK-GNSS receivers were expensive, costing several million yen, and required specialized knowledge to operate. However, thanks to technological progress and wider adoption, RTK equipment has become smaller and more affordable. For example, compact GNSS receivers that fit in the palm of your hand and devices that achieve centimeter-level positioning in conjunction with a smartphone have begun to appear. As a result, high-precision positioning, once limited to specialist surveyors, is becoming more accessible to a wider range of users.

Additionally, the spread of network RTK means that you often don’t need to carry expensive radios or base stations to each site. Correction information can be received via a smartphone’s mobile connection, so only a rover receiver is required. Furthermore, as a service that complements satellite positioning, Japan’s QZSS “Michibiki” offers the CLAS (Centimeter Level Augmentation Service); by using CLAS you can obtain high-precision augmentation signals directly from satellites even in mountainous areas without internet connectivity. In short, current RTK surveying does not necessarily require bulky equipment and can be achieved with relatively simple setups.

Misunderstanding 4: “The latest smartphones can achieve centimeter accuracy on their own”

High-performance GNSS chips have been incorporated into smartphones, and some are advertised as supporting “multi-frequency” and “positioning augmentation.” This may lead to the idea that “the latest smartphone alone can achieve centimeter accuracy without dedicated GNSS equipment.” However, at present it is extremely difficult for a smartphone alone to achieve RTK-level 1–2 cm accuracy. One reason is antenna performance: the small built-in antennas in smartphones suffer greater signal noise, making it hard to obtain a stable fixed solution. The augmentation information a smartphone can receive is limited, and standalone performance typically improves accuracy only to the order of several tens of centimeters (the realm of SBAS or multi-frequency standalone positioning). Real-time centimeter-level accuracy still requires a dedicated RTK-capable receiver and appropriate correction information.

That said, in recent years there are easy-to-use RTK devices that work in conjunction with smartphones. By using the smartphone’s display and communications while pairing it with a high-performance GNSS receiver, you can now achieve accuracy comparable to traditional fixed units. A representative example of such devices is the LRTK series.

Latest trends in simplified RTK surveying: high-precision positioning with LRTK

The barrier to utilizing RTK’s centimeter accuracy has steadily decreased, and at the forefront of this trend is a solution called LRTK. LRTK is an ultra-compact RTK-GNSS receiver series developed by Refixia, and when combined with a smartphone or tablet it realizes a “palm-sized surveying instrument.”

For example, the “LRTK Phone” weighs only about 165 g and, despite its pocketable compactness with a thickness of about 1 cm (0.4 in), supports GPS, GLONASS, Galileo, QZSS “Michibiki,” and other multi-GNSS systems and even features triple-frequency (L1/L2/L5) capability. By attaching this device to an iPhone or iPad and launching the dedicated LRTK app, the smartphone instantly becomes a centimeter-accurate surveying instrument.

The LRTK series’ strengths are not limited to hardware. Ease of use has been carefully considered: positioning data is transferred to the smartphone in real time via Bluetooth. Position information acquired on site can be uploaded to the cloud-based “LRTK Cloud” with a single tap, enabling instant data sharing with staff in the office. This allows remote verification of the latest survey results, measurement of distances or areas between survey points on the cloud, and other collaborative work to be performed seamlessly.

Moreover, LRTK supports not only network RTK but also Japan’s satellite augmentation signal CLAS from QZSS “Michibiki.” Thus, even in mountainous areas without mobile coverage, LRTK can obtain high-precision correction information directly from satellites and continue centimeter-level positioning. This overturns the conventional notion that “high-precision positioning depends on communications” and is a major advantage for stable RTK surveying anywhere.

In this way, using LRTK enables one person to easily achieve centimeter-level positioning. There is no need to lug around heavy tripods and fixed receivers; surveying can be completed with just a smartphone and a pocket-sized device. Acquired data are automatically tagged with coordinates such as latitude, longitude, and elevation, facilitating smooth comparison with design drawings and maps. Because photos, notes, and point cloud data can be managed together with positioning results on the smartphone or cloud, report creation and handover to subsequent processes are also streamlined.

By correctly understanding the conditions and mechanisms of RTK and incorporating these latest tools, the era when anyone can enjoy centimeter-level positioning on site is imminent. Why not experience the next-generation simplified surveying with LRTK while maximizing RTK’s potential?

FAQ

Q. Can RTK positioning really achieve about 1 cm accuracy? A. Yes—when conditions are met, high-precision positioning with horizontal/planar errors of about 1–3 cm (0.4–1.2 in) and vertical errors of about 3–6 cm (1.2–2.4 in) is possible. However, this is under open-sky conditions with good satellite visibility. In urban canyons or forests, temporary errors exceeding 10 cm can occur. The important point is that RTK is far more accurate than ordinary GPS, but you should understand that environmental factors can still cause some error.

Q. Can RTK positioning be done without preparing a base station? A. Yes. In Japan, the Geospatial Information Authority’s CORS network is established nationwide and provides network RTK services (such as VRS). If the rover receiver can receive correction information via the internet, RTK positioning is possible without installing your own base station. Additionally, Japan’s QZSS “Michibiki” CLAS signal can be used to receive direct satellite corrections in areas far from base stations, enabling centimeter-level accuracy.

Q. What is the difference between RTK and DGPS (Differential GPS)? A. DGPS also receives corrections from a base station like RTK, but the accuracy and methods differ. DGPS primarily corrects code-positioning errors and achieves accuracy on the order of several tens of centimeters to about 1 m. RTK uses carrier-phase measurements to reduce errors to several centimeters. RTK achieves stable high precision by obtaining a fixed solution, whereas DGPS only suppresses slowly drifting errors, so RTK is superior for long-term stable high-precision positioning.

Q. Can someone without surveying experience master LRTK? A. Yes. LRTK is designed to minimize specialized operations: connect to a smartphone and start the app, and you can follow the guide to begin positioning. Basic operations like observing a point and saving it are performed intuitively with button taps, and acquired data are automatically plotted on a map. You don’t need to deal with complicated settings or positioning calculations like with traditional surveying instruments. However, knowing RTK principles and precautions (such as the importance of clear sky view) will help you achieve reliable high accuracy.

Q. What advantages does LRTK have compared to conventional high-precision GNSS equipment? A. The biggest advantages are portability and ease of use. LRTK fits in your pocket, greatly reducing the burden of carrying equipment on site. Its smartphone integration enables real-time cloud sharing and data management. Accuracy has been experimentally confirmed within a few millimeters to a few centimeters, comparable to conventional multi-million-yen GNSS receivers. Moreover, with CLAS reception from Michibiki, positioning can continue even outside mobile coverage, making it effective for surveying in remote mountainous areas. Overall, LRTK offers a new surveying style: “accuracy comparable to high-end equipment, with the usability of a smartphone.”