What is Single-Frequency RTK (L1)?

Single-frequency RTK is a positioning method that uses only the L1 frequency band (about 1575 MHz) of the signals received from GPS or GNSS satellites for RTK positioning. In RTK, the carrier phase of the satellite signals received by both the reference station (known point) and the rover (positioning point) are compared, and real-time corrections are applied to calculate a highly accurate position. Since single-frequency RTK uses only the L1 signal for this calculation, the system is simple and low-cost. However, it has the limitation that frequency-dependent error factors, such as ionospheric delay, cannot be directly corrected. As a result, depending on the distance to the reference station and the surrounding environment, positioning accuracy and stability may be more susceptible to interference.

​

Advantages of Single-Frequency RTK

• Low Cost and Compact: GNSS receivers that support single-frequency are often simple in structure and are available at a lower price, making them compact. This allows for the adoption of RTK positioning with affordable GNSS modules, making it easier to implement in budget-constrained sites.

• Low Power Consumption: Since only the L1 signal is processed, the computational load is low, and generally, the power consumption is less compared to dual-frequency devices. This makes it advantageous for small devices that prioritize battery life.

• Simple System: Due to being single-frequency, the setup and operation are relatively simple, and complex calibrations for each frequency are not required. The system configuration is also simple, making troubleshooting easier.

​

Disadvantages of Single-Frequency RTK

• Longer Initialization Time: Single-frequency RTK tends to take longer to achieve a Fix solution (integer fixed solution). It can take anywhere from a few minutes to tens of minutes, leading to delays at the beginning of the work. In fact, receivers that only support L1 have been reported to take 4 to 10 minutes for initialization.

• Limitations in Positioning Accuracy and Stability: Single-frequency L1 is unable to fully correct signal delays caused by the ionosphere and other factors, leaving residual errors. Particularly when the distance from the base station increases, the differences in ionospheric errors grow larger, leading to degraded accuracy or unstable solutions. Typically, maintaining accuracy is difficult beyond a baseline of a few kilometers, and achieving a Fix solution becomes more difficult over long distances.

• Susceptibility to Environmental Factors: With only a single frequency, there is less satellite signal data available, and positioning becomes unstable in environments with many obstructions or multipath effects. In areas with poor sky visibility, such as forests or under viaducts, the L1 signal from satellites is more likely to be interrupted, leading to accuracy degradation or positioning failure. This is a notable disadvantage.

​

What is Dual-Frequency RTK (L1 + L2)?

Dual-frequency RTK is an RTK positioning method that simultaneously uses two frequencies, the L1 band and the L2 band (approximately 1227 MHz), from GPS/GNSS satellites. The principle is the same as single-frequency RTK, where the carrier phase difference between the base station and rover is utilized. However, by combining signals from different frequencies, dual-frequency RTK can cancel out the effects of ionospheric delay, which is a major advantage.

By combining observation data from the L1 and L2 bands, ionospheric errors can be offset, allowing for high-precision positioning solutions. This technical difference allows dual-frequency RTK to converge faster and be more stable, making it easier to obtain high-precision position information even in complex environments. Many current high-precision GNSS receivers (e.g., u-blox ZED-F9P) support multi-band, and this dual-frequency RTK enables centimeter-level positioning.

​

Advantages of Dual-Frequency RTK

• Faster Convergence Speed: By using both L1 and L2 frequencies, the resolution of carrier phase integer ambiguity becomes easier, significantly reducing the time required to achieve an initial Fix solution. Experiments have shown that dual-frequency receivers can achieve a Fix in just a few seconds to a few tens of seconds, dramatically reducing initialization wait times, which previously took several minutes with single-frequency RTK.

• Improved Positioning Accuracy and Stability: The use of two frequencies effectively removes ionospheric errors, allowing for stable centimeter-level accuracy over long durations and long distances. This is especially advantageous for high-precision positioning over long baselines (over 10 km) and in environments such as urban areas and mountainous regions, where single-frequency RTK would struggle to maintain stable results.

• Strength in Harsh Environments: Positioning with multiple frequencies can reduce the effects of signal degradation caused by obstructions or multipath interference. For instance, in environments like forests or under bridges, where signals tend to weaken, the L2 signal can complement the L1 signal, reducing interruptions and enabling high-precision real-time positioning even in challenging environments.

• Support for Wide Area Corrections: Dual-frequency receivers are easily compatible with wide-area correction services such as VRS (Virtual Reference Station) using the national electronic reference point network and the CLAS service from the quasi-zenith satellite system. This enables centimeter-level positioning over large areas without the need to set up a local reference station, making positioning operations efficient even for large infrastructure projects.

​

Disadvantages of Dual-Frequency RTK

• Higher Equipment Cost: Dual-frequency GNSS receivers and antennas are generally more expensive than single-frequency models. High-performance RTK systems require advanced receiver circuits and antenna designs, which leads to higher initial setup costs and operational expenses compared to single-frequency systems. However, in recent years, low-cost dual-frequency modules have become available, narrowing the cost gap.

• Larger Receiver and Antenna Size: Since both L1 and L2 signals need to be received, antennas must support multiple frequency bands, which tend to make them physically larger and heavier. Additionally, the complexity of the receiver circuits makes miniaturization more challenging compared to single-frequency systems. In scenarios requiring portability or drone integration, dual-frequency equipment can be bulkier compared to single-frequency systems.

• Increased Power Consumption and Data Volume: Since dual-frequency systems process multiple signals, they can lead to higher power consumption compared to single-frequency systems. Also, the amount of correction data (such as RTCM) transmitted for RTK operations increases with dual frequencies, which slightly raises the load on wireless and communication networks. These factors can affect battery life and communication costs.

​

Use Cases in the Construction and Surveying Industry
RTK positioning, with its high positioning accuracy and immediacy, has been increasingly utilized across various scenes in the construction and surveying industries. Each system, whether single-frequency or dual-frequency, has its own optimal use, but with the recent widespread adoption of dual-frequency RTK, the efficiency of on-site operations has greatly improved. Here, we will introduce some specific use cases.

​

Application Examples in Civil Surveying
In civil surveying fields such as road and land development, land boundary determination, and as-built management, rapid coordinate acquisition through RTK is essential. Traditionally, total station surveying was the mainstream method, but by using RTK-GNSS, wide areas can be measured in a short amount of time, significantly reducing manual labor. Even with a single-frequency RTK receiver, centimeter-level positioning can be achieved in open sites, but currently, dual-frequency RTK is increasingly being used for better stability. With dual-frequency, it is possible to maintain stable centimeter-level accuracy across the entire surveying site, reducing re-measurement and waiting time, which ultimately leads to improved efficiency in construction management. Particularly in the Ministry of Land, Infrastructure, Transport and Tourism’s promotion of i-Construction, the use of RTK positioning is encouraged, and dual-frequency RTK is applied in high-precision as-built management and 3D surveying.

​

Utilization in Infrastructure Maintenance
The demand for high-precision GNSS positioning is also increasing in infrastructure maintenance, such as for railways and highways. For example, in track subsidence measurements and road displacement monitoring, it is essential to quickly and accurately capture changes between reference points. By using RTK, correction information from a network of reference stations or electronic reference points (CORS) installed over a wide area enables immediate acquisition of position coordinates on-site. In environments where satellite visibility is limited, such as in mountainous railways or roads under viaducts, single-frequency RTK may result in unstable solutions. However, with dual-frequency RTK, it is easier to maintain accuracy even in complex terrain.
As a result, reliable dual-frequency RTK receivers are used in infrastructure inspection sites, supporting positioning work near tunnel exits or in areas with dense trees. With high-precision position information, repair planning and anomaly detection can be carried out quickly and accurately.

​

Application in Automated Construction (Machine Guidance)
RTK-GNSS plays a crucial role in automated construction systems such as machine guidance and machine control, which have been widely adopted in recent years. By equipping heavy machinery like bulldozers and excavators with GNSS receivers and sensors, the position and height of the machines can be controlled in real-time, eliminating the need for manual surveying or setting up reference points. These GNSS receivers, mounted on construction machinery, need to perform high-precision positioning while constantly moving, which is why dual-frequency RTK receivers are almost always used. With dual-frequency RTK, the machinery can quickly re-converge even if there is some instability or temporary satellite obstruction, minimizing interruptions to the work. For example, during excavation work, GNSS can continuously adjust the blade height, directly improving finish precision and reducing the need for rework. In automated construction sites, the stable operation of RTK is directly linked to productivity, making reliable dual-frequency RTK systems indispensable.

​

Utilization and Benefits of LRTK
LRTK is an all-in-one RTK-GNSS receiver that integrates the antenna, battery, and radio into a single compact design. On-site, RTK positioning can be initiated with just a smartphone and the LRTK device, significantly simplifying the complex equipment connections and wiring previously required. Its rugged and compact housing provides excellent portability and durability, making it reliable for use even in harsh outdoor environments. Of course, it supports L1/L2 dual frequencies, enabling fast initialization and stable centimeter-level positioning. Additionally, it is compatible with the CLAS (Centimeter Level Augmentation Service) reinforcement signal from Japan’s Quasi-Zenith Satellite System, allowing real-time positioning with correction data from satellites, even in mountainous areas or remote islands without internet access. While traditional RTK equipment required setting up a base station and ensuring communication infrastructure, LRTK greatly alleviates these constraints.

​

The main benefits of implementing LRTK are summarized as follows:

• Dual Frequency RTK Support – By receiving both L1 and L2 frequencies, LRTK offers significantly faster Fix acquisition and enhanced positioning stability compared to traditional systems. It can be confidently applied to precision-focused surveying and automated construction, directly improving on-site productivity.

• All-in-One Integrated System – By integrating the antenna, receiver, battery, and wireless communication into a single device, LRTK simplifies the equipment configuration. This reduces the time spent on bringing equipment to the site and setting it up, allowing anyone to easily start RTK positioning. The wireless design eliminates the need for cable connections, preventing issues such as disconnection during operations.

• Augmentation Service Compatibility – LRTK can operate without the need for a dedicated base station, as it is compatible with satellite augmentation signals like CLAS and VRS networks. It can achieve centimeter-level accuracy independently in areas with no communication coverage and is useful for continuous wide-area positioning or as a backup in emergencies.

• Increased On-Site Efficiency – LRTK is easy to use with an intuitive smartphone app, requiring no specialized knowledge. Positioning data can be shared and utilized instantly through cloud integration. It also includes tilt correction functionality, allowing accurate point coordinates to be obtained even with tilted poles, making surveying over obstacles smoother. Overall, LRTK significantly improves the workflow in surveying and construction sites, helping to address manpower shortages and reduce work time.