What is a GNSS Terminal? Thorough Explanation of How It Works and the Benefits of On-site Use

What is a GNSS terminal (Basic structure and functions)

GNSS (Global Navigation Satellite System) is a general term for positioning systems that use multiple artificial satellites, including GPS. A GNSS terminal refers to a device that receives the radio signals transmitted from these satellites and calculates its current position (latitude, longitude, altitude). Familiar examples include car navigation systems and smartphones, which also have GNSS receivers and determine their current location using signals from satellites. However, when the term "GNSS terminal" is used in general, it often refers to dedicated high-precision positioning equipment used especially in surveying and civil engineering.

The basic structure of a GNSS terminal begins with an antenna to capture the weak radio signals from satellites, and a receiver connected to it that analyzes the signals to perform distance measurements and coordinate calculations. The receiver processes information from multiple satellites simultaneously to determine its position in real time. By using four or more satellites, the system can measure position anywhere on Earth. In addition, many GNSS terminals are equipped with functions to output the measured position information to external controllers or smartphone apps, or to record it in internal memory.

A major feature of GNSS terminals is that they can perform positioning anytime and anywhere in outdoor locations with a clear view of the sky. Unlike conventional optical surveying instruments (such as total stations), there is no need to set up a tripod and sight; as long as the sky is open you can obtain position coordinates simply by installing the antenna. For this reason they are highly effective for wide-area surveying and topographic surveys, and in recent years the use of GNSS terminals has rapidly expanded, particularly at surveying and construction sites.

How satellite positioning works (GPS/GNSS, differences between standalone positioning and relative positioning)

Satellite positioning using GNSS is a system that determines one’s position by receiving radio signals from artificial satellites in space. Specifically, each satellite continuously transmits radio signals that include time information about when the signal was sent, and a GNSS receiver picks these up. By calculating the difference between the reception time and the transmission time, the distance to the satellite is computed, and by combining distance information from multiple satellites using the principle of triangulation, the receiver’s current location is obtained. In principle, if signals from four or more satellites are available, latitude, longitude, and altitude can be calculated, enabling position measurements almost anywhere on Earth where radio signals can reach.

Also be sure to understand the difference between "GPS" and "GNSS." GPS (Global Positioning System) is a satellite positioning system operated by the United States and is one of the most widely used GNSSs in the world. GNSS, on the other hand, is a term that refers to the various satellite positioning systems including GPS, and includes Russia's GLONASS, Europe's Galileo, China's BeiDou, and Japan's Michibiki (QZSS). Recent GNSS receivers are predominantly multi-GNSS capable, able to receive signals simultaneously from these multiple satellite constellations, and because they can use more satellites at any given time compared with GPS alone, they make positioning more stable even under obstructions.

There are two main methods of satellite positioning: standalone positioning and relative positioning. Standalone positioning is performed using a single GNSS device and calculates your position based only on the received satellite signals. Positioning in smartphones and car navigation systems uses this standalone positioning, but because signal delay errors caused by the ionosphere and the atmosphere and clock errors of the satellites and receivers accumulate, errors of typically several meters (several ft) occur. On the other hand, relative positioning is a method that uses two or more GNSS receivers simultaneously to determine positional relationships. One receiver is installed at a known location (a reference point), and by using the distance difference and differences in observation data with the other receiver (the rover), many common errors can be canceled out. Relative positioning includes several methods, from differential GNSS (DGPS/DGNSS) with an accuracy on the order of tens of centimeters (several in) to RTK positioning, which can achieve high accuracy of a few centimeters (cm level accuracy, half-inch accuracy). The next chapter will explain in detail the principles and benefits of RTK, which in particular achieves centimeter-level accuracy (cm level accuracy, half-inch accuracy).

Principles and Benefits of RTK Positioning

RTK (Real Time Kinematic) positioning is a type of relative positioning that achieves high accuracy in real time. Its principle lies in operating two GNSS receivers simultaneously: a reference station (base station) and a rover (mobile station). The reference station is installed at a location whose exact coordinates are known in advance, and it calculates the difference (error) between the positioning results obtained from the satellite signals it receives and the true position. It then sends that error information to the rover in real time via radio or internet connection, and the rover applies those corrections to its own positioning data to compute its position. This cancels out errors such as ionospheric and tropospheric delays, and satellite clock and orbit errors, allowing the rover to achieve extremely high positioning accuracy at the centimeter level (half-inch accuracy).

A major advantage of RTK is that, by applying such corrections, it can reduce errors that used to be on the order of several meters (several ft) down to a few centimeters (a few in). In fact, under good conditions, high-precision positioning is possible, with accuracies of ±1–2 cm (±0.4–0.8 in) horizontally and about ±3 cm (±1.2 in) vertically. Also, because measurement results are obtained in real time, you can verify accurate coordinates on the spot and move on to the next task without waiting for post-processing, which improves efficiency. In particular, in civil engineering surveying, RTK's ability to measure height (elevation) directly has led to an increase in cases where leveling surveys are no longer necessary. Because precise vertical control, which was difficult with GNSS alone, can now be performed immediately on-site, this leads to significant time savings for tasks such as as-built (construction completion shape) confirmation.

To use RTK positioning in the field, a means of communication is required to deliver correction information from a base station to a rover. Common methods include direct communication between the base station and the rover using UHF-band radios, or receiving correction data over the Internet via cellular networks. In the latter case, by using network-type RTK services provided by the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations or by private reference-station networks, users can obtain correction information without having to set up their own base station. In Japan, using the Quasi-Zenith Satellite System Michibiki (QZSS)’s centimeter-level augmentation service (CLAS) (cm level accuracy (half-inch accuracy)) makes it possible to receive corrections via satellite even in mountainous areas with no Internet coverage. The development of these communication and augmentation methods has made it possible to use RTK positioning today with a simpler equipment configuration than in the past.

Types of GNSS Terminals and Selection Points (Accuracy, Size, Portability, Connectivity Features, Correction Signals)

GNSS terminals come in various types depending on their用途 and accuracy requirements. Traditionally, large tripod-mounted GNSS receivers and base station kits were mainstream, but in recent years portable handheld units and small devices that integrate with smartphones have emerged. Here we organize the main terminal types and the points to watch when selecting one.

• Positioning Accuracy and Supported Satellites / Frequencies：The device you should choose depends on the required accuracy. For high-precision surveying, an RTK-capable device that can deliver centimeter-level accuracy (cm level accuracy; half-inch accuracy) is essential. To achieve this, it is desirable to have a receiver that supports not only GPS but multiple satellite systems (multi-GNSS) and is dual-frequency or higher, able to receive multiple frequency signals such as L1/L2. Multi-GNSS and multi-frequency capable devices offer the advantage of having more satellites available, which improves resistance to signal outages and the stability of positioning accuracy. Conversely, for applications where an accuracy of several meters (several ft) is sufficient, a simple GPS terminal dedicated to standalone positioning may be adequate.

• Size and Portability: Because these are devices carried to and used on-site, size and weight are important factors. Recent handheld GNSS terminals are small and lightweight enough to fit in a pocket, and receivers that fit in a shirt chest pocket have appeared. A lightweight unit puts less strain when carried for long periods and can be easily brought into sites where transporting equipment is difficult, such as mountainous areas. On the other hand, large stationary receivers, while offering high performance, require more effort to set up and transport, so you should consider the trade-off with the performance you need.

• Usability and connectivity features: Easy-to-use interfaces and connectivity features that do not require specialist technicians are also important points when choosing a device. For example, a device that connects to a smartphone or tablet via Bluetooth or Wi‑Fi and can be operated intuitively from a dedicated app makes it easy to check and record positioning results. Simple models that complete positioning with button operations alone, and devices equipped with clear menus displayed in Japanese, are also favored on site. In addition, you should check connectivity aspects such as whether the acquired positioning data can be uploaded directly to the cloud or exported in formats compatible with other surveying software.

• Support for correction signals: To achieve high-precision positioning, the use of correction information such as the aforementioned RTK and SBAS is indispensable. Because the correction methods supported vary by terminal, you need to choose one that matches your company's operational style. For example, if you use a network-type RTK (such as VRS), you need communication functionality capable of receiving Ntrip streams via the Internet. If you install your own base station and broadcast over radio, a terminal with a built-in UHF radio module is convenient. Also, a feature unique to Japan's positioning environment is that some terminals can directly receive the CLAS signal from Michibiki (QZSS). If you frequently operate in mountainous areas or outside cellular coverage, choosing a receiver that supports QZSS augmentation signals makes it possible to receive corrections from satellites and achieve centimeter-level positioning. Consider which correction sources to support based on the accuracy you require and your usage scenarios.

On-site Use Cases (civil surveying, as-built management, pile-driving guidance, point cloud acquisition, disaster response, etc.)

GNSS receivers are utilized in a wide range of field operations, achieving improved efficiency and reduced labor compared with traditional methods. Here are some representative use cases.

• Civil surveying: GNSS is widely used for as‑built and land‑acquisition surveys of roads, development sites, and similar areas. Even on large sites, simply walking with an antenna allows successive point coordinates to be obtained, enabling a single person to survey without the need to reposition equipment such as a total station. Tasks like installing triangulation points on distant mountain summits are no longer necessary, contributing to faster and simpler topographic surveying.

• As-built management: GNSS surveying is also used to verify the as-built condition (final shape) of structures and ground after construction. Checks such as whether the finished road's elevation and gradient match the design, or whether the volumes of embankment and excavation conform to specifications, can be determined quickly by measuring multiple points with GNSS receivers. Leveraging the high cm level accuracy (half-inch accuracy) of RTK, as-built inspections that previously required time-consuming use of levels and total stations can now be carried out more efficiently.

• Pile-driving guidance：GNSS terminals can also be used for the work of driving piles to align with reference points (stakeout). If you enter the coordinate values from the design drawings, the receiver will guide you in real time on the distance and direction from your current position to the target point on site. For example, you can align while looking at a display such as "10 cm (3.9 in) to the east, 5 cm (2.0 in) to the north", and even a newcomer can install piles within an error range of a few centimeters (a few inches). Tasks that experienced surveyors used to perform carefully with tape measures and transits can be greatly simplified with GNSS navigation.

• Point cloud acquisition: GNSS is indispensable for obtaining 3D point cloud data via laser scanners or drone aerial photography. In drone photogrammetry, GNSS is used to geotag (capture coordinates of) each aerial image, and the resulting point cloud is aligned to a map coordinate system. Recently, solutions have appeared that combine smartphone-mounted LiDAR or 360° cameras with GNSS units to scan ground structures while walking and convert them into point clouds. High-density point clouds given global coordinates by GNSS are useful for a variety of purposes, including as-built management, deformation measurement, and recording buried utilities.

• Disaster response: At disaster sites such as earthquakes, landslides, and floods, portable GNSS terminals are highly effective for rapid situational assessment. They can measure and map the shape of collapsed slopes and the locations of damaged areas on site, helping to identify the extent of damage and to formulate recovery plans. Moreover, because they can obtain positioning information independently via satellite positioning even when communication networks are disrupted, they are also used to select drop points for relief supplies and to plan evacuation routes.

Operational improvements achievable through implementation (labor reduction, time savings, cloud integration)

Finally, we summarize what kinds of improvements to field operations can be achieved by introducing GNSS terminals. The main points are three: "labor reduction," "time savings," and "data digitization (cloud integration)".

• Labor reduction (improvement in personnel efficiency): GNSS surveying can basically be completed by a single person. Traditionally, two or more people were required to operate surveying instruments and hold targets, but with a GNSS terminal you can obtain positions simply by setting up the antenna and measuring by yourself. This reduces the effort needed to arrange personnel, allowing surveying work to proceed smoothly even at sites suffering from chronic labor shortages. In addition, there are more devices that can be used without specialized surveying skills, and cases are emerging where surveying that was previously outsourced to external contractors can now be handled solely by in-house staff.

• Reduced work time: The introduction of high-precision GNSS greatly shortens the time required for surveying and measurement. For example, in topographic surveys of large development sites, walking with a GNSS device allows you to acquire data far more quickly than measuring while moving and setting up a total station. Because coordinates can be obtained in real time on site, there is no wasted time waiting for results. Even in pile-driving operations, GNSS guidance can dramatically reduce the time needed for positioning, leading to an overall shortening of the schedule. Overall, GNSS devices streamline on-site measurement and surveying tasks and reduce lead times.

• Data sharing and cloud integration: Position data acquired by GNSS can be recorded and utilized instantly in digital form. If measurement results are uploaded from the field to the cloud, progress can be monitored in real time from the office, and measurement data can be immediately reflected in drawings. There is no longer a need to handwrite records in a paper field book and bring them back, preventing transcription errors. Furthermore, by linking acquired data with GIS and CAD software for analysis and design, centralized information management and smooth information sharing between the field and the office become possible. The use of GNSS terminals will also serve as a foundation for promoting on-site ICT and DX (digital transformation).

Finally: Examples of Introducing Simplified Surveying with LRTK and Its Benefits

Recently, smartphone-linked simple surveying tools that condense the above advantages have also appeared. A representative example is the new GNSS surveying system "LRTK", which is easy for beginners to use. LRTK is a solution that combines a smartphone and a compact high-precision GNSS receiver to achieve centimeter-level positioning (cm-level positioning; half-inch-level positioning). Previously, expensive stationary RTK equipment and complicated setup were required, but with LRTK, simply attaching a dedicated compact receiving terminal (LRTK Phone) to a smartphone and launching the app enables real-time centimeter-level accuracy (cm level accuracy; half-inch accuracy).

For example, when LRTK was introduced at a construction site, on-site staff without specialized surveying skills became able to perform as-built verification measurements themselves on the spot. By always carrying a pocket-sized device and being able to measure immediately with a smartphone and share data to the cloud when needed, waiting time for surveying was reduced and on-site decision-making became faster, according to reports. User-friendly surveying tools like LRTK are expected to greatly transform tasks that had previously been bottlenecked by expertise and personnel for surveying equipment, contributing to labor savings, faster operations, and digitalization at the site. As high-precision GNSS becomes increasingly accessible, opportunities for operational improvements are likely to expand to many more sites in the future.