What is Standalone Positioning (Standalone GNSS Positioning)?

Standalone positioning is a method where a single GNSS receiver calculates its position by itself. The receiver receives orbital and time information sent by GNSS satellites (such as GPS, GLONASS, and Michibiki) and calculates the distance by measuring the time it takes for the radio signals to reach the receiver. The position is determined by measuring the distances to multiple satellites (at least four), and the intersection of these distances gives the receiver's location.

However, standalone positioning includes various error factors. These can include satellite orbit and clock errors, signal delays caused by passing through the ionosphere and troposphere (atmospheric errors), and the effects of multipath (reflected signal paths) from buildings or terrain. Since the receiver alone cannot correct these errors, the position information obtained will have some degree of deviation. Typically, the accuracy of standalone GNSS positioning is around several meters, and in some cases, errors of approximately 10 meters may occur.

For example, you may have experienced positional inaccuracies with car navigation systems or smartphone GPS maps. Despite this, standalone positioning does not require a dedicated base station and can be widely and easily used, making it common for applications such as maritime navigation, aviation, and automotive navigation, where high precision is not required.

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What is Relative Positioning (RTK Positioning)?

Relative positioning refers to a positioning method where multiple GNSS receivers, including a base station and a rover, conduct simultaneous GNSS observations to accurately determine the relative position (vector) between the two. Specifically, a base station (with known coordinates) and a rover (the point whose position is to be determined) are set up, and both receivers simultaneously receive signals from the same satellite. By comparing the time difference in signal arrival between the two receivers, the relative position of the rover as observed from the base station is calculated.

The key feature of this method is that by observing two points simultaneously, common error factors that could not be eliminated in standalone positioning are effectively canceled out. For example, if the base station and rover are in close proximity, they will both experience nearly the same satellite errors and atmospheric errors. By taking the difference between the observation data (using methods like double differencing), satellite orbit errors, ionospheric delays, and tropospheric delays can be canceled out, significantly reducing the errors.

This method is used in DGPS (Differential GPS) and RTK (Real-Time Kinematic) positioning. In DGPS, the base station calculates the position offset (typically a few meters) and sends this correction data to the rover, which then applies the correction to the standalone positioning result.

In RTK positioning, both the base station and rover observe the GNSS carrier phase (the carrier wave) and send the raw data from the base station to the rover in real time, where the relative positioning solution is derived. Since the carrier wave has a wavelength of several tens of centimeters and a high resolution, distance differences can be calculated with high precision from the phase shift. While the uncertainty of integer wavelengths (integer ambiguity) needs to be resolved, if the solution is stable, positioning can be determined in real-time with centimeter-level accuracy.

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To perform RTK positioning, a communication method is required to transmit observation data and correction information from the base station to the rover for error correction. Typically, data is exchanged in real-time via radio or the internet (mobile networks). Recently, network-based RTK (such as VRS) has become more widespread, allowing users to receive correction data from local electronic reference points or analysis services, eliminating the need to set up their own base station.

While this method involves additional steps, the benefit is that RTK achieves high precision that cannot be obtained through standalone positioning. Depending on the distance from the base station, the general accuracy of RTK positioning is within a few centimeters horizontally and vertically. In practice, positions are determined with errors of around 2-3 cm horizontally and 3-4 cm vertically, which is significantly more accurate than standalone positioning.

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Comparison of Accuracy Between Standalone Positioning and Relative Positioning

There is a significant difference in positioning accuracy between standalone positioning and relative positioning using RTK. Standalone positioning does not apply corrections, causing errors to accumulate, and typical GPS positioning results in position deviations of around 3 to 10 meters. In contrast, RTK positioning improves accuracy to within a few centimeters by canceling out errors through relative observations with the base station.

In other words, a position that could only be narrowed down to an area the size of a soccer ball in standalone positioning can be reduced to an error as small as a fingertip with RTK. While standalone positioning is easy to use, as shown in the table above, it falls short of relative positioning (RTK) in terms of accuracy.

RTK significantly reduces the unavoidable errors of standalone positioning by reflecting real-time correction data from the base station. Of course, with RTK, if satellite signals are lost or communication with the base station becomes unstable, the accuracy may decrease (resulting in a float solution or DGPS-level precision, instead of a fixed solution). However, under good conditions, the difference between standalone and RTK positioning is clear, and this is why positioning accuracy improves drastically with RTK.

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Use Cases of RTK

RTK, which provides high-precision positioning information, is used in various fields. Especially in civil engineering, construction, and infrastructure-related sites, RTK positioning has become indispensable as a means to streamline measurement tasks that previously required manual labor and effort, while also improving accuracy. Below are some representative fields of application:

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• Civil Surveying: RTK is used for establishing reference points in terrain and land surveys and for map creation and as-built management. By using RTK-GNSS, a wide range of points can be measured in a short time, achieving coordinates with errors within a few centimeters. Compared to traditional total stations, this method increases work efficiency and contributes to labor-saving in surveying tasks.

• Construction Management and Staking (Positioning): RTK is also used in positioning tasks on construction sites. The accuracy of marking the location and elevation of structures based on design drawing coordinates, known as "staking," is critical. Using RTK-compatible equipment, a single person can accurately determine the measurement points. Additionally, RTK-GNSS is installed in heavy machinery for machine guidance and machine control, enabling millimeter-level construction accuracy through operator assistance or automated construction.

• Infrastructure Inspections and Maintenance: RTK positioning demonstrates its power in infrastructure inspections, such as roads, railways, and bridges. For example, RTK enables monitoring of rail track distortions or road subsidence with point observations over time, achieving centimeter-level accuracy. In highway infrastructure installations or identifying repair sites, work efficiency improves by using pre-measured high-precision coordinates to pinpoint the locations on-site.

• UAV Surveying (Drone Aerial Photography): More and more examples are emerging of RTK-equipped drones for photogrammetry. While positioning accuracy of aerial photos using standalone positioning can deviate by several meters, RTK-equipped drones can record the position of each photo within a few centimeters. As a result, high-precision 3D surveying models can be created with fewer ground control points, leading to improvements in the accuracy and efficiency of soil volume calculations and as-built management.

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In this way, RTK's high-precision positioning is playing a vital role across a wide range of sites, from surveying and construction to infrastructure maintenance. Traditionally, it was a technology used only by surveying specialists, but recently, with the miniaturization and reduction in cost of equipment, cases where on-site technicians directly handle RTK devices have been increasing.

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The Evolution of RTK Positioning with LRTK

With the growing demand for RTK positioning, recent years have seen the introduction of products that have further evolved traditional RTK devices to be more user-friendly. A prime example of this is the compact RTK-GNSS system called LRTK. Traditional RTK equipment required bulky gear, including a stationary base station, a rover mounted on a pole, as well as radios and external batteries. The high initial setup costs also posed a barrier for small to medium-sized businesses. However, LRTK has been designed to allow anyone, anywhere, at any time, to utilize RTK technology, significantly improving convenience through technological innovations.

One of the main features of LRTK is the miniaturization and lightweight design of the equipment. For example, the LRTK Phone, a smartphone-integrated product, has a receiver weighing just 125g and measuring only 13mm in thickness, making it pocket-sized. The antenna, GNSS receiver, battery, and communication module are all integrated into this single device, allowing it to serve as a versatile surveying tool capable of centimeter-level positioning. Unlike traditional equipment, no cables are required, and wireless integration with smartphones makes it incredibly easy to handle on-site. Positioning results and point identification can be viewed directly on the smartphone screen, and measurement data can be shared immediately through the cloud. The price is also much more affordable compared to traditional models, making it more accessible for widespread on-site adoption, with the expectation of one device per person in the future.

Furthermore, LRTK incorporates the latest GNSS solutions from a technological standpoint. For instance, the receiver is compatible with three-frequency signals, making it more resistant to multipath and ionospheric errors, and making it easier to achieve stable fixed solutions. Additionally, it is compatible with Japan's Quasi-Zenith Satellite System (Michibiki), which provides centimeter-level correction services (CLAS), enabling high-precision positioning using satellite correction signals even in mountainous areas or offshore, where mobile communication networks are unavailable.

This means that even in environments where VRS corrections over the internet are not available, RTK-level accuracy can be maintained. With this system that balances on-site mobility and positioning accuracy, LRTK has evolved into a practical RTK device that can be effectively used on construction sites.

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The LRTK Phone transforms a smartphone into a versatile surveying tool with centimeter-level accuracy. Its pocket-sized design makes it easy to carry and use whenever needed.

In traditional RTK systems, specialized knowledge and complex equipment setups were obstacles, but LRTK has made positioning tasks significantly more accessible. As an RTK-GNSS device that on-site technicians can operate with the ease of using a smartphone, its applications will continue to expand. The advent of LRTK, which can be seen as the democratization of high-precision positioning, marks a major step forward in supporting construction ICT and infrastructure DX.