How are GNSS and GPS different? An explanation of the basic knowledge and selection points needed for RTK surveying

Table of Contents

• Basic differences between GNSS and GPS

• What is RTK surveying?

• Advantages of RTK surveying using GNSS

• Accuracy comparison between GPS alone and GNSS

• Key points for selecting an RTK surveying system

• Methods of providing correction information and their importance

• Practical challenges and countermeasures in RTK surveying

• Operational efficiency gains from high-precision positioning

• Developments in the latest GNSS positioning technologies

• Understand GNSS, GPS, and RTK surveying correctly and proceed with implementation

Surveying practitioners often hear the terms GNSS and GPS. These terms are frequently used interchangeably, but they are actually different concepts. Correctly understanding this distinction is especially important when implementing RTK surveying. In this article, we explain the differences between GNSS and GPS in detail and describe the key points to consider when selecting RTK surveying.

Basic differences between GNSS and GPS

To understand the relationship between GNSS and GPS, let’s first confirm their respective definitions. GPS (Global Positioning System) is the name of the satellite positioning system operated by the United States. GNSS (Global Navigation Satellite System), on the other hand, refers to the collective term for multiple satellite positioning systems. In other words, GPS is one system included in GNSS, and GNSS is a broader concept.

Currently, multiple satellite positioning systems are in operation worldwide. The U.S. GPS is the oldest and is widely used around the world. Meanwhile, there are several other systems, such as Russia's GLONASS, Europe's Galileo, and China's BeiDou satellite positioning system. In addition, Japan operates its own system called the Quasi-Zenith Satellite System, and by combining these systems it is possible to measure position information with higher accuracy.

The term GNSS is used to encompass all of these satellite positioning systems. In the surveying industry, using the term GNSS rather than simply GPS clearly indicates that multiple systems are being utilized. This is a very important characteristic from the perspectives of improving accuracy and ensuring reliability.

What is RTK surveying?

RTK (Real-Time Kinematic) surveying is a surveying method that acquires high-precision positional information in real time. In conventional GNSS surveying, errors on the order of several meters (several ft) were common, but RTK surveying can acquire positional information with accuracies of several centimeters (several in), and even at the millimeter level (millimeter-level in).

The mechanism of RTK surveying is based on installing a receiver at a fixed position called a reference station and transmitting correction information from that reference station to the rover in real time. The reference station is placed at a known coordinate location and receives signals from satellites. It calculates the errors that occur and sends that correction information to the rover via radio or a communication network. By receiving this correction information, the rover can calculate its own position more accurately.

Thanks to this mechanism, RTK surveying achieves rapid and accurate measurements compared with conventional surveying methods. It is increasingly used on construction sites in particular, and is a system that makes a major contribution to improving construction accuracy and shortening construction schedules.

Advantages of RTK Surveying Using GNSS

GNSS-based RTK surveying has a significant advantage: the ability to utilize multiple satellite systems simultaneously. When relying on a single system, satellite signals can be blocked by obstructions such as buildings or trees. However, by using multiple systems in parallel, you can receive signals from another system even if one system's signals cannot be received.

By integrating multiple satellite positioning systems in this way, the number of observable satellites increases and more stable position computations are achieved. As a result, high-precision positioning becomes possible even in complex environments—such as urban areas, construction sites, and forested land—where GPS alone would struggle.

In addition, using multiple systems also has the advantage of shortening the initialization time (the time required for ambiguity resolution). In traditional systems, determining the correct integer value when beginning measurements required tens of seconds to several minutes. However, by using GNSS, this initialization time is greatly reduced, allowing high-precision positioning to be started much more quickly.

Furthermore, by combining information from multiple satellites, it becomes possible to detect and correct error factors such as atmospheric delay more accurately. This improves measurement accuracy and yields more reliable surveying results.

Accuracy Comparison Between Standalone GPS and GNSS

There is a marked difference in real-world accuracy between positioning using GPS alone and positioning using GNSS, which combines multiple satellite systems. GPS-only solutions are more susceptible to effects such as radio signal delays in the atmosphere, multipath reflections, and satellite geometry, which tend to degrade measurement accuracy. In particular, in environments with many buildings or trees, signal blockage can even make obtaining measurements difficult.

On the other hand, GNSS systems can more effectively compensate for these error factors by processing signals from multiple satellite positioning systems simultaneously. Moreover, because satellite geometry is more likely to be kept in an optimal state, more stable positioning is achieved. In actual surveying sites, many reports indicate that using GNSS enables high-precision measurements even in locations where positioning with GPS alone is difficult.

When implementing RTK surveying, it is important to consider adopting GNSS-capable receivers rather than relying on GPS alone. Although initial implementation costs may be somewhat higher, selecting equipment that supports multiple systems will be advantageous in the long term in terms of practical accuracy and reliability.

Key Points for Selecting an RTK Surveying System

When introducing an RTK surveying system, you need to consider several important points. First and foremost, check the number of satellite systems supported. By selecting a receiver that supports as many systems as possible—GPS, GLONASS, Galileo, the Quasi-Zenith Satellite System, etc.—you can expect higher accuracy and greater stability.

Next, it is important to consider how to construct the communication network. In RTK surveying, correction information must be transmitted from the base station to the rover, and there are multiple options for this communication, such as radio links, mobile carrier networks, and publicly available correction information distribution services. The optimal communication method should be selected according to the site environment and the state of the communication infrastructure.

Additionally, the ruggedness of equipment appropriate to the operating environment is also an important consideration. When anticipating use on construction sites, features such as dustproofing, waterproofing, and shock resistance are required. Checking these performance specifications in advance and selecting equipment suited to the site environment is important from the perspective of long-term operation.

Compatibility with existing systems should also be taken into account. If you operate multiple surveying instruments, it is necessary to verify whether the newly introduced RTK surveying system can properly integrate with your existing software and workflows. This helps ensure smooth operation and improved efficiency after implementation.

How to Provide Correction Information and Its Importance

The success of RTK surveying hinges on how quickly and accurately correction information can be provided. There are three main approaches to providing correction information.

The first is the method of building your own reference station. With this method, you install a reference station at the survey site and transmit correction information from there to the roving receiver. The advantage of this method is that you can build a completely independent system and are not dependent on external communications infrastructure. On the other hand, building and maintaining a reference station requires a certain amount of effort and cost.

The second option is to use publicly available correction-information distribution services. In many regions, positioning correction information is distributed via wireless or communication networks. With this approach, you do not need to build your own base station, which can reduce initial investment. However, it increases dependence on communication infrastructure and may be difficult to use in areas where the communication environment is unstable.

The third is a hybrid approach that combines multiple methods. By normally using public broadcast services while flexibly responding—such as by utilizing your own reference station in situations where signal reception is difficult—it can achieve maximum accuracy and convenience.

When selecting the method for providing correction information, it is important to comprehensively evaluate the on-site environment, the condition of the communications infrastructure, and the operational budget.

Practical Challenges and Countermeasures in RTK Surveying

There can be a gap between the theoretical accuracy of RTK surveying and its practical operational usability.

For example, the time required for initialization can vary greatly depending on the field environment. In environments with good satellite geometry, initialization can be completed in a few seconds, but in complex environments such as around buildings, initialization can take longer.

In such cases, using a GNSS receiver that supports multiple systems can potentially shorten initialization time. Also, by checking satellite geometry and performing a preliminary survey of communications status before arriving at the site, on-site efficiency can be improved.

Furthermore, the placement and height of the receiver also have a significant impact on accuracy. Near buildings or at low elevations, satellite signals are more easily obstructed. Installing the receiver in as open and elevated a location as possible is essential for achieving stable positioning.

To operate RTK surveying effectively, it is important to understand these practical challenges in advance and to take measures to address them.

Operational Efficiency Brought by High-Precision Positioning

High-precision positioning information obtained from RTK surveying brings efficiency to various operations. On construction sites, measurement time is shortened and construction accuracy is improved, which reduces rework in subsequent processes. This leads to an overall shortening of project schedules and cost reductions.

Moreover, improving the reliability of survey data enables more appropriate decision-making during the design and construction phases. Planning based on high-precision data helps prevent on-site problems and also contributes to improved safety.

Furthermore, there is a growing trend toward automating surveying work. With high-precision positioning information, various types of construction machinery can be automatically controlled, greatly improving work efficiency. These technological advances are also attracting attention as measures to address labor shortages.

Deployment of the Latest GNSS Positioning Technologies

GNSS positioning technology continues to evolve. In addition to improvements in the performance of satellite positioning systems themselves, application technologies that utilize them are emerging one after another. In particular, the realization of high-precision positioning using mobile information devices is bringing significant changes to the surveying industry.

Recently, devices that add high-precision positioning capabilities to mobile devices such as the iPhone have been attracting attention. These devices support multiple satellite systems, and an increasing number of them can achieve accuracy equivalent to RTK surveying. By using iPhone-mounted GNSS high-precision positioning devices, existing mobile devices can be utilized as surveying instruments, enabling high-precision positioning while reducing the cost of purchasing new equipment.

These devices provide significant convenience, especially for simple surveying tasks and field surveys. By selecting an iPhone-mounted device that supports multiple systems and RTK corrections, you can perform surveying work more flexibly and efficiently with accuracy comparable to conventional surveying instruments.

Let's properly understand GNSS, GPS, and RTK surveying and proceed with implementation

We have provided a detailed explanation of the differences between GNSS and GPS, and the basic mechanisms and advantages of RTK surveying. As surveying operations become more advanced, correctly understanding these technical differences leads to appropriate equipment selection and effective operation.

When introducing RTK surveying, it is important to consider adopting a GNSS system that supports multiple satellite constellations rather than relying on GPS alone. By selecting equipment suited to the field environment and establishing appropriate methods for providing correction information, high-precision and stable positioning can be achieved.

In particular, leveraging cutting-edge technologies such as iPhone-mounted GNSS high-precision positioning devices has made it possible to introduce high-precision positioning capabilities without significantly changing existing operational frameworks. By selecting devices that support multiple systems and provide the functions required for RTK surveying, both improved efficiency and increased accuracy in surveying operations can be achieved simultaneously, leading to strengthened competitiveness.