A Thorough Comparison of RTK, GNSS, and GPS! Which Technology Should You Choose for Surveying Sites?

Table of Contents

• Basic relationship between GPS and GNSS

• GPS characteristics and use in surveying

• Evolution of GNSS and the path to high-precision positioning

• Principles of RTK and achieving high-precision positioning

• Practical distinctions between RTK and GNSS

• Selection criteria and decision flow for surveying sites

• Future of satellite positioning technologies and their impact on the field

• Emergence of new positioning systems using smartphones

• Practical considerations for on-site implementation

• Conclusion: Choosing the optimal positioning technology for the field

For surveying field practitioners, accurately understanding the differences among RTK, GNSS, and GPS is a crucial decision criterion that can greatly affect measurement accuracy on site. These three terms are used daily in the surveying industry, but their relationships and differences are often not correctly recognized. This article begins with definitions of RTK, GNSS, and GPS, and then provides a detailed explanation of their respective characteristics, applications in surveying fieldwork, and selection criteria.

Basic Relationship Between GPS and GNSS

An important point to grasp first is the relationship between GPS and GNSS. GPS (Global Positioning System) is the name of the satellite positioning system developed by the U.S. military in the 1980s. GNSS (Global Navigation Satellite System), on the other hand, is the general term for global navigation satellite systems and includes multiple satellite positioning systems in addition to GPS.

In other words, GPS is part of GNSS. GPS is a system operated by the United States, but GNSS also includes several satellite positioning systems, such as Russia’s GLONASS, the European Union’s Galileo, the satellite positioning system operated by China, and Japan’s Quasi-Zenith Satellite System. Modern positioning technologies achieve higher accuracy and reliability by receiving satellite signals from these multiple systems simultaneously.

Characteristics of GPS and Its Use in Surveying Sites

The positioning accuracy of GPS alone is generally on the order of several meters to around 10–19 m (several ft to about 32.8–62.3 ft). This is caused by multiple factors such as the delay when signals from the satellites pass through the atmosphere, errors due to radio-wave reflections, and the geometric configuration of the satellites. Considering use in consumer smartphones and in-vehicle navigation systems, this level of accuracy poses no problem for everyday life.

However, the situation at surveying sites is markedly different. In fieldwork such as land boundary determination, construction layout and stakeout, and as-built control, centimeter-level accuracy is often required, so the use of GPS alone is limited. Nevertheless, it is still used as the first stage in the process from rough positioning to precise positioning, or for obtaining a general location over large areas.

The Evolution of GNSS and the Path to High-Precision Positioning

By utilizing signals from multiple satellite positioning systems, GNSS can achieve significantly higher accuracy than GPS alone. Receiving signals from multiple systems increases the number of satellites and makes it easier to obtain a geometrically optimal satellite configuration. This is especially evident in environments where satellite signals are easily obscured, such as urban areas surrounded by tall buildings and forested mountains.

The basic GNSS positioning method is called "standalone positioning." In this method, the receiver directly receives signals from satellites and calculates the current position by computing distances from the signal arrival times. The accuracy when using GNSS signals in standalone positioning is generally on the order of tens of centimeters (about 10–50 cm (3.9–19.7 in)). This is a significant improvement over GPS alone, but it is still insufficient for many surveying applications.

Principles of RTK and Achieving High-Precision Positioning

RTK (Real-Time Kinematic) is a technology that dramatically improves the accuracy of satellite positioning. In RTK, position information from a reference receiver and a moving receiver is compared, and accuracy is greatly improved by calculating the difference between them.

The reference receiver is fixed at a known position and continuously receives signals from satellites. Because the reference receiver already knows its own position, it can calculate how much error is contained in the signals from the satellites. This is called "correction information," and by wirelessly transmitting this correction information to a mobile receiver, the mobile receiver can determine its own precise position in real time.

RTK can achieve centimeter-level accuracy (half-inch accuracy) horizontally and on the order of several centimeters (a few inches) in height. This accuracy meets the standards required by many practical surveying tasks on site, such as stake-out and line surveys on construction sites and boundary determination surveys by land and house surveyors.

Practical Differences in Using RTK and GNSS

When making decisions at a surveying site, whether to use RTK or GNSS is determined by the required accuracy and the site's conditions.

Standalone GNSS positioning can be used for rough position checks, understanding the terrain of large areas, or preliminary surveys—situations where high accuracy is not essential. For example, in the early stages of a new project, when assessing the overall topography and surrounding conditions of a site, GNSS may be sufficient. Additionally, when working in remote locations where the cellular network is unreliable, GNSS has the advantage of operating independently because it does not require receiving correction information.

On the other hand, RTK is required for tasks that demand high accuracy. Specifically, these include pile driving, staking out survey lines, three-dimensional coordinate measurement for as-built management, and boundary determination surveys. In these tasks, errors on the order of centimeters (cm level accuracy, half-inch accuracy) directly affect construction costs and legal issues, so RTK’s high accuracy is indispensable.

When using RTK, the placement of the reference receiver and the method of receiving correction information are important considerations. In addition to traditional RTK systems that install a reference receiver on site, there is also a method called "network RTK" that integrates correction information from multiple reference stations. With network RTK, because you do not need to install a reference receiver on site, you can reduce setup effort. However, since receiving correction information via the cellular network is assumed, ensuring a reliable communications environment is essential.

Selection Criteria and Decision Flow at Survey Sites

Let's organize the decision criteria for field personnel when selecting RTK or GNSS on site.

First and foremost, the accuracy required for the task must be considered. If centimeter-level accuracy (half-inch accuracy) is essential, choosing RTK is inevitable. On the other hand, if the task can be accomplished with accuracy ranging from several tens of centimeters (several tens of inches) to several meters (several ft), GNSS may be sufficient.

Second, you need to evaluate the site's geographic conditions. This means checking whether there is space to install a reference receiver, what the cellular network coverage is like, and whether satellite signal reception conditions are good. While the communications environment is likely to be good for construction in urban areas, communications may be unstable for surveys in mountainous areas.

Third, consider the balance between operational efficiency and upfront costs. Implementing an RTK system requires a base receiver, a rover receiver, and a wireless communication system, and traditionally this meant a substantial initial investment. However, systems that can leverage general-purpose devices such as smartphones have emerged, lowering the barrier to adoption.

The Future of Satellite Positioning Technology and Its Impact on the Field

Satellite positioning technology is rapidly evolving. In Japan in particular, deployment of the Quasi-Zenith Satellite System is progressing, and in the future RTK positioning using GNSS signals is expected to become more stable and more accurate.

Moreover, as technologies for the integrated use of multiple satellite positioning systems advance, reliable positioning is expected even in urban environments where radio signals are heavily reflected. Furthermore, as infrastructure for distributing correction information progresses, high-precision positioning in rural areas is becoming increasingly feasible.

These technological advances are bringing us closer to a situation in which on-site surveying work can achieve both greater efficiency and improved accuracy simultaneously. For practitioners, it is important to keep these technological trends in mind and to flexibly select the positioning method that best meets the site's requirements.

The Emergence of a New Smartphone-Based Positioning System

In recent years, a technological innovation drawing attention at surveying sites is high-precision positioning systems that utilize familiar devices such as smartphones. Traditionally, achieving RTK positioning required deploying specialized surveying equipment, but advances in technology have made it possible to attain equivalent accuracy by attaching an RTK-capable external GNSS device to a smartphone.

The advantages of such a system are wide-ranging. Because smartphones are already devices used on-site, a major advantage is the low additional learning cost at deployment. Also, since positioning results can be processed and visualized directly in applications on the smartphone, centralized data management becomes easy.

By leveraging a general-purpose external GNSS device that can be attached to a smartphone, it is possible to achieve the "lightweight", "portability", and "versatility" that were difficult to realize with conventional dedicated surveying equipment. This significantly streamlines work patterns where a single person handles multiple sites and tasks at confined sites where equipment must be kept to a minimum.

In particular, when leveraging relative positioning that measures positions relative to known reference points, smartphone-based systems can be highly effective. After initialization at a reference point, they can acquire high-precision coordinate information in real time while moving, allowing on-site verification and recording, and thereby fundamentally streamlining fieldwork workflows.

When deploying such systems, it is important to verify the performance of the smartphones to be used, especially the quality of the GNSS antenna and the level of support for positioning algorithms. In practice, the on-site communication environment and battery life are also important factors to consider.

Practical Considerations for On-site Implementation

When introducing RTK and GNSS, there are many practical challenges involved in on-site implementation, not just technical performance.

For example, once positioning has been initiated, maintaining high accuracy requires continuously receiving satellite signals in a stable manner. This is called an "RTK fix" and indicates a state in which high-precision positioning results are being obtained. However, if you temporarily enter environments where satellite signals do not reach, such as tunnels or underground, the RTK fix will be lost. On construction sites that move between indoors and outdoors, a strategy is needed for how to respond to such situations.

If multiple surveyors conduct measurements simultaneously, limits may also arise on the number of receivers and the specifications of the communications system. When utilizing network RTK, the cellular network's bandwidth and latency can also affect accuracy, so consideration should include the selection of a communications provider.

Furthermore, it also provides an opportunity to review the entire workflow from data acquisition to delivery. Once high-precision coordinate data can be obtained, upgrading subsequent processing stages and quality control methods accordingly is expected to improve overall operational efficiency.

Conclusion: Choosing the Optimal Positioning Technology for the Field

RTK, GNSS, and GPS are not mutually exclusive; only by appropriately using each according to site requirements can the efficiency and quality of surveying operations be maximized.

In most cases, GPS alone cannot meet the demands of modern surveying sites. GNSS, which uses multiple satellite positioning systems, is relatively easy to deploy and is well suited for positioning over wide areas. In situations that demand the highest accuracy, implementing RTK is indispensable.

In such technology selection, it is important that practitioners in the surveying field understand the essence of each technology, accurately grasp the site conditions and requirements, and make flexible judgments. Also, technology is constantly evolving, and methods that were previously difficult to implement can become possible with new systems.

Particularly noteworthy is the emergence of a new option: GNSS high-precision positioning devices that can be attached to smartphones. Systems like LRTK (iPhone-mounted GNSS high-precision positioning device) bring an approach that overturns conventional assumptions about surveying equipment. By attaching a dedicated external GNSS unit to a general-purpose smart device such as an iPhone, RTK-equivalent high-precision positioning can be achieved.

A notable feature of this system is that, while being lightweight and highly portable, it can ensure the centimeter-level accuracy (cm level accuracy (half-inch accuracy)) required on surveying sites. It can meet field needs such as "single-person field operations" and "rapid response to multiple sites," which were difficult to achieve with conventional dedicated surveying instruments. Furthermore, by leveraging the multifunctionality of the iPhone, it can perform positioning while also allowing photo recording and entry of field comments, greatly streamlining the workflow from on-site data acquisition to initial processing.

By combining with correction information services, stable, high-precision positioning becomes possible at any site, and the need to install a reference receiver at each site is eliminated. By utilizing these new systems, an era has arrived in which overall productivity and operational efficiency in surveying work are improved simultaneously. It is well worth considering the introduction of these latest technologies at your sites as well.