What is a GNSS receiver? The latest technologies supporting high-precision positioning

Overview and Necessity of GNSS Receivers

A GNSS receiver is a device that receives radio signals from GPS and other GNSS (*Global Navigation Satellite System*, global navigation satellite systems) to calculate its current position with high accuracy. Simply put, it is a "positioning receiver" that captures signals from multiple positioning satellites to determine where on Earth you are, and it is sometimes called a "high-precision GPS receiver." In recent years, as positioning accuracy has improved and costs have fallen, GNSS receivers have become widely adopted and indispensable tools in surveying and construction.

If you ask why GNSS receivers are necessary, it is because tasks requiring centimeter-level positional accuracy (half-inch accuracy) are increasing. Traditionally, ordinary GPS (standalone positioning) has errors of several meters (several ft), so while it is sufficient for map apps and car navigation, it was not accurate enough for civil engineering surveys and construction management. For example, in laying out building foundations or measuring boundaries, errors of several meters (several ft) are unacceptable, and even deviations of several centimeters (several in) cannot be tolerated. To meet such needs for high-precision positioning, precise position measurement using GNSS receivers is being utilized. Recently, fields where higher positioning accuracy creates value—such as automatic control of construction machinery, drone surveying, and autonomous farming—are expanding, further increasing the importance of GNSS receivers.

Importance of Accuracy in Surveying and Construction

In the surveying and construction industries, positional accuracy is directly linked to work quality and safety. When surveyors determine land boundaries or measure elevations, very small errors can have major impacts later on. For example, in road construction, achieving the design-specified heights and slopes requires surveying with no deviation of even a few centimeters (a few inches). If construction proceeds with low accuracy, the finished structures may suffer distortions or steps, and there is a risk of additional rework.

Especially in recent years, digitization of construction sites has advanced, and methods that manage quality by comparing 3D design data with on-site survey data have become commonplace (the so-called "as-built control" and ICT construction). In this process, if the coordinates measured on site are not accurate, the digital technology is rendered meaningless. Therefore, GNSS receivers that can determine positions with centimeter-level accuracy (cm level accuracy (half-inch accuracy)) have become indispensable tools in surveying and civil engineering. In addition, high-precision measurements were traditionally performed with optical total stations and levels, but these instruments are constrained by weather and visibility and require skilled operation. With GNSS positioning, since you only need to receive radio waves from satellites, you can survey wide areas in a short time, and because it can sometimes be used in bad weather or in locations with poor visibility, it greatly contributes to improving site efficiency and reducing manpower.

Basic GNSS Technologies (How Satellites Work, Differences from RTK, Multi-frequency Support, etc.)

The basic principle of positioning using GNSS is that a GNSS receiver receives radio signals transmitted from multiple satellites, measures the distance to each satellite, and derives the position in three dimensions. By simultaneously capturing signals from four or more satellites, the receiver's longitude, latitude, and altitude are calculated. This method is called standalone positioning and is the technique used in common GPS-equipped smartphones and car navigation systems. However, with standalone positioning, effects such as signal delays in the atmosphere and satellite clock errors, as mentioned above, can result in positioning errors of several meters.

In surveying, GNSS's high-precision positioning technology known as RTK (*Real Time Kinematic*, real-time kinematic) is utilized. RTK is a type of relative positioning using two or more receivers, and it is a system that corrects in real time the errors that occur between a reference receiver (base station) and a receiver that determines position while moving (rover). Specifically, the base station calculates the error components from its own precise position and the satellite data it receives, and sequentially transmits that correction information to the rover. Because the rover applies the corrections when performing position calculations, centimeter-level positioning with errors of only a few centimeters or less becomes possible. Whereas conventional GPS completes positioning using only information from satellites, a major difference with RTK is that it uses data from ground-installed base stations or virtual reference stations (VRS) provided via a network.

Furthermore, the latest GNSS receivers dramatically improve positioning accuracy and stability through multi-frequency, multi-GNSS support. Multi-GNSS, as the name implies, means using multiple satellite positioning systems simultaneously in addition to GPS, such as Russia’s GLONASS, Europe’s Galileo, China’s BeiDou, and Japan’s QZSS (Michibiki). By increasing the number of satellites, it becomes easier to ensure satellites can be received even in the gaps between high-rise buildings or in mountainous areas, enabling positioning that is less likely to be interrupted. Multi-frequency refers to the capability to receive and process multiple radio frequencies (L1/L2/L5, etc.) transmitted by a single satellite. By combining signals at different frequencies, delay errors that occur in the atmosphere (particularly the ionosphere) can be canceled out, and the resolution of the ambiguity in distance calculations between the satellite and the receiver (integer ambiguity) becomes faster and more reliable. Simply put, dual-frequency and triple-frequency GNSS receivers achieve initial position fixing faster and more stably, and enable more precise positioning compared with single-frequency devices.

Applications and deployment examples of GNSS receivers

High-precision GNSS receivers are used across a variety of fields. In particular, in the surveying and construction sectors, they are being introduced for the following applications.

• Land surveying and site investigation: GNSS receivers are proving effective for creating topographic maps and measuring boundary points. Tasks that traditionally required multiple people using total stations can be carried out efficiently by a single person with GNSS. Even in forests or vast sites, as long as reference points are secured, many survey points can be collected in a short time.

• Construction site management: GNSS receivers are used for layout work that sets positions and elevations according to design drawings, and for measuring the as-built condition of structures after completion. For example, in road or bridge construction it is common to use GNSS to instantly establish reference benchmarks or survey posts, aiding heavy equipment operation and inspection. Because positioning results can be obtained in real time, construction deviations can be detected on site more easily, contributing to quality assurance and reducing rework.

• Drone surveying and aerial photogrammetry: By obtaining high-precision coordinates of image capture positions with GNSS receivers mounted on drones, the accuracy of 3D models created from aerial photographs can be improved. Traditionally, it was necessary to place several known points in the aerial data (GCP: ground control point) for correction, but with high-precision GNSS it has become possible to obtain terrain data with an accuracy of several centimeters (several in) even without GCPs. The combination of drones and GNSS is being used for as-built management on construction sites and for mapping damage after disasters.

• Social infrastructure maintenance and disaster prevention: GNSS receivers are also used in infrastructure maintenance and disaster prevention, such as measuring settlement of tunnels and roads and monitoring displacement in landslide areas. By installing receivers at fixed points and continuously positioning them to monitor minute location changes, they help with early detection of anomalies and safety verification. There are also increasing cases where simple GNSS devices are brought into disaster sites immediately after an event to rapidly survey affected areas and support recovery planning.

GNSS receivers are thus being introduced across a wide range of applications, from civil engineering and construction sites to aerial surveying and even disaster prevention. Recently, centimeter-level positioning (cm level accuracy (half-inch accuracy)) has come to be required in non-surveying fields as well—such as autonomous vehicles and precision agriculture (self-driving tractors)—and demand for GNSS receivers is likely to continue growing.

Technical factors to consider when selecting a model (accuracy, portability, power supply, cloud support, etc.)

While GNSS receivers are broadly referred to by the same name, their performance and features vary by model. To select the appropriate equipment for field deployment, it is helpful to pay attention to the following technical factors.

• Positioning accuracy: Check whether the model can meet the required accuracy level. Receivers that advertise high accuracy typically support RTK and can achieve planar position accuracy of about ±1 cm (±0.4 in) and vertical accuracy of about ±2～3 cm (±0.8～1.2 in). Also check the datasheet for supported satellites (GPS, GLONASS, Galileo, QZSS, etc.) and frequency bands (L1/L2/L5, etc.). Models that support more satellites and frequencies generally provide more stable and higher-accuracy positioning.

• Portability and mobility: The device's size, weight, and ease of carrying are also important. In surveying work you often carry poles or tripods with the receiver mounted, so lighter and more compact equipment reduces the burden. Recently, GNSS receivers that integrate the antenna and battery into a single small unit that is easy to carry have appeared. Also, models that can connect to a smartphone or tablet via Bluetooth and be used as a controller are convenient because you don't have to carry a separate dedicated controller.

• Power and batteries: When used outdoors for long periods, battery runtime is an important selection factor. If you will survey all day on site, it's desirable to have at least a battery capacity that can run for several hours to a dozen or so hours. For built-in battery models, also check charging methods (whether USB power is supported, whether spare batteries can be swapped). If the model supports external power, you can also operate it while powering from a mobile battery pack or a vehicle power supply.

• Communication and cloud support: How you can utilize the positioning data obtained by the GNSS receiver is also important. For example, using network RTK requires receiving correction information over the Internet, so the device needs to support Wi‑Fi, 4G/5G communications, or Bluetooth tethering. It is also very efficient to have a system that uploads positioning results to the cloud in real time so they can be immediately shared and reviewed on office PCs. Recent models also include features such as syncing data to the cloud via a smartphone app and plotting the coordinates of surveyed points on a map for team sharing.

In addition, there are many other noteworthy factors to consider for reliable on-site use, such as dust and water protection (IP rating), guaranteed operation in extreme temperature environments, the presence or absence of built‑in memory, and compatibility of positioning data formats. Ultimately, it is important to choose a GNSS receiver that, in light of your company’s use cases, “meets the required accuracy, is easy to handle on site, and facilitates data utilization.”

Future technological trends and changes in the field

The technologies surrounding GNSS receivers are expected to advance further in the future, bringing significant changes to on-site operations. Here are some notable trends.

• Pursuit of miniaturization and ease of use: Advances in technology will further drive the miniaturization and weight reduction of receivers. In the future, high-precision GNSS may become standard not only in survey-specific instruments but also in commercially available smartphones and mobile devices. Indeed, RTK-capable receivers that work with smartphones have become more common in recent years, and products that are easy to use even for non-specialists have appeared. As a result, opportunities for field technicians themselves—not just experienced surveyors—to carry out surveys will increase.

• Advancement of satellite positioning networks: GNSS constellations around the world are being reinforced, improving positioning accuracy and service coverage. Japan’s Quasi-Zenith Satellite “Michibiki” is planned to be expanded from a four-satellite system to a seven-satellite system in the future, promising accuracy augmentation services that can be used continuously and in all weather. For example, the centimeter-class augmentation service (CLAS) provided in Japan enables high-precision positioning even in mountainous areas without an Internet connection when a dedicated receiver is used. Going forward, enhancements in augmentation signals from satellites and the expansion of private cloud-based reference station networks will create an environment where “anyone, anywhere can measure to the centimeter (to 0.4 in).”

• Integration with BIM/CIM and AR: As high-precision positioning becomes easier to obtain, real-time integration with design data and geospatial information will become more active. For example, it will become commonplace on sites to AR-display design drawings or the locations of buried utilities on a tablet screen based on the current position measured by GNSS. In some cases, systems are already being trialed that automatically generate 3D models from survey data and color-code differences between as-built and design on the spot. The fusion of GNSS receivers with other advanced technologies will drive greater efficiency and sophistication in construction management and inspection.

• Further labor reduction and safety improvements: With the spread of GNSS receivers and advances in automation, more processes—from heavy equipment operation to surveying—will be carried out without human intervention. ICT construction—where autonomously controlled bulldozers and graders use GNSS to determine their positions and grade according to design surfaces—is already at a practical stage, and in the future it will spread to small- and medium-scale projects and surveying tasks, standardizing construction based on consistently accurate coordinate information. Moreover, the ability to survey and monitor remotely without requiring people to enter hazardous areas is expected to dramatically improve on-site safety.

Thus, high-precision positioning technology centered on a GNSS receiver not only improves surveying efficiency but also contributes to digital transformation (DX) and workstyle reform at construction sites. It is important to keep abreast of the latest technological trends and consider how they can be applied to your company's operations.

Finally: Introducing GNSS Surveying Techniques Anyone Can Use and LRTK

With the advancement of GNSS receivers, the era in which surveying was only for specialist technicians is changing.

Nowadays, even field technicians and construction managers are increasingly able to perform high-precision positioning easily. As a representative example of such "surveying techniques anyone can use", we introduce a product called LRTK offered by our company.

LRTK is a smartphone-integrated RTK-GNSS positioning system designed so that even those who have never used specialized equipment can intuitively achieve centimeter-level positioning (cm-level accuracy, half-inch accuracy). Specifically, it consists of a small GNSS receiver (3-frequency compatible) attachable to smartphones and tablet devices and a dedicated app, and the entire process from starting positioning to sharing data can be completed with a single button. Because LRTK can continue high-precision positioning by receiving the CLAS signal from Japan’s Quasi-Zenith Satellite System Michibiki even in mountainous areas without internet connectivity, it can be used regardless of location. Furthermore, acquired position information and point cloud data are immediately uploaded to the cloud, making it easy to check on-site survey results or calculate distances and areas between measured points from the office. It also integrates with a smartphone’s built-in LiDAR (light detection sensor) and camera functions to enable 3D scanning and visualization via AR, so a single LRTK unit enables seamless surveying, drawing creation, and sharing.

In this way, LRTK is a solution that opens up a world of simplified surveying, allowing anyone to perform surveying work that until now required expensive equipment and specialist knowledge. High-precision positioning technology using GNSS receivers is advancing rapidly, and its range of applications on worksites is likely to expand further. Please keep an eye on the latest technological trends and consider applying them to your company's operations. I hope this article will be of help in the introduction of GNSS receivers.