Difference Between GPS Standalone Positioning and Multi-GNSS Positioning

First, let's clarify the differences between GPS standalone positioning and Multi-GNSS positioning. GPS standalone positioning, as the name suggests, uses signals solely from US GPS satellites. GPS provides global positioning services with approximately 30 satellites, and with the RTK method, high precision can be achieved in flat environments with no obstructions. However, when positioning is only based on GPS, the number of available satellites decreases in areas such as in the shade of buildings or under trees, which can lead to reduced positioning accuracy or, in the worst case, the inability to determine the position.

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On the other hand, Multi-GNSS positioning uses multiple satellite systems, such as GPS, GLONASS, Galileo, BeiDou, and QZSS, in addition to GPS. For example, around Japan, in addition to GPS satellites, numerous GLONASS and BeiDou satellites can be received, and signals from QZSS satellites, which stay for long periods near the zenith, are also available. The benefits of Multi-GNSS can be broadly summarized as follows:

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• Improved Accuracy Due to Increased Number of Satellites: With more satellites available for positioning calculations, improvements in geometric configuration (DOP values) can be expected, resulting in improved positioning accuracy. Using multiple frequencies and multiple satellites, as opposed to only using single-frequency GPS, also helps cancel out ionospheric errors and reduce multipath errors.

• Shorter Initialization Time for Positioning: The more satellites and signals available, the shorter the time required for RTK positioning to achieve a fixed position (initialization time). Compared to when there are fewer satellites, Multi-GNSS enables quicker integer ambiguity resolution, reducing wait times for tasks.

• Reduced Risk of Positioning Failure Due to Satellite Obstruction: If a particular satellite system experiences issues or signal blockage, other systems can cover, allowing positioning to continue smoothly. For example, if there is a temporary disruption in GPS satellites, GLONASS and Galileo can be used to provide coverage, minimizing the impact.

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As seen, compared to standalone GPS, Multi-GNSS offers a significant advantage in terms of positioning accuracy and stability. Now, let's take a look at how Multi-GNSS impacts RTK positioning in particular.

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Impact of Multi-GNSS on RTK Positioning

The introduction of Multi-GNSS significantly improves the accuracy and stability of RTK positioning. By using multiple satellite systems, the reliability of model calculations for error correction data transmitted from the base station to the rover increases, which further reduces positioning errors. Additionally, there have been reports that the necessary observation time to achieve the same accuracy can be shortened. Here, we will explain the main effects of Multi-GNSS from three perspectives.

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1. ​Improved Error Correction Accuracy: In Multi-GNSS-enabled RTK, observation data from not only GPS but also other satellite systems is used simultaneously, creating redundancy in the positioning calculations. For example, the estimation of delay errors caused by the ionosphere or troposphere can be more accurately performed by combining multi-satellite and multi-frequency data. As a result, the quality of RTK solutions improves, and positioning errors become more stable, staying within a few centimeters. In fact, increasing the number of available satellites and signals has been reported to shorten RTK initialization time, improve positioning success rates, and ultimately stabilize accuracy.In other words, Multi-GNSS accelerates the convergence of RTK solutions, making it easier to maintain high-precision solutions once obtained.

2. Improved Positioning Stability (Availability): The biggest advantage of an increased number of satellites is that "high-precision solutions are less likely to be interrupted." Data shows that using a quad-constellation of GPS, GLONASS, Galileo, and BeiDou, compared to a dual constellation of GPS and GLONASS, improves RTK solution availability by more than 40%. Particularly in difficult environments (such as urban areas with buildings or valleys in mountainous regions), having as many usable satellites as possible greatly affects the stability of RTK. With Multi-GNSS, enough satellites can be maintained at all times, making it easier to maintain stable solutions even while moving during surveying tasks.Moreover, Multi-GNSS also enhances the ability to maintain positioning in challenging signal environments. Multi-GNSS improves positioning continuity under signal obstruction. For example, in locations like forests or under overpasses, where GPS signals tend to be interrupted, signals from other satellites can still be received, reducing the likelihood of position loss.The image above shows the use of a GNSS receiver in an environment surrounded by trees. With a Multi-GNSS-enabled device, the chances of continuously capturing multiple satellites in such environments are significantly higher. In the past, there was frustration when "positioning would not recover until moving to an open area," but with Multi-GNSS, RTK positioning can be maintained with significantly improved stability, even in environments with many obstructions.

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Use Cases of Multi-GNSS RTK Positioning in Construction and Surveying

High-precision RTK positioning with Multi-GNSS support is being increasingly applied in various construction and civil engineering sites. Here, we introduce some representative use cases and the benefits of implementing this technology.

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• Positioning in Construction Management: In construction site management, RTK positioning is used for machine location management and verifying the as-built conditions. With Multi-GNSS-enabled RTK, you can reliably determine your position with centimeter-level accuracy, regardless of where you are on the site. This allows a smaller surveying team to work more efficiently. The ability to maintain stable positioning regardless of weather or time of day also contributes to increased productivity on-site.

• Improved Precision in Pile Driving: In foundation piling, deviations in pile placement are not acceptable. In RTK-GNSS guidance and management for pile position, Multi-GNSS reduces positioning interruptions due to satellite switching, continuously providing accurate coordinates. This ensures smooth guidance for pile driving machine operators and prevents rework due to mistakes.

• As-Built Measurements and Inspections: GNSS surveying devices are often used for large-scale as-built measurements, such as for roads and embankments. With Multi-GNSS-enabled RTK surveying equipment, high-precision positioning is consistently available across the measurement area, allowing for faster collection of measurement points compared to traditional methods. As a result, the process of creating as-built management charts is streamlined, and the accuracy of quality management improves.

• Infrastructure Maintenance Inspections: High-precision GNSS is also gaining attention for regular inspections of infrastructure such as highways and railways. In areas where GPS signals may be obstructed, such as near tunnel exits or under elevated bridges, Multi-GNSS ensures uninterrupted position measurement by compensating with signals from other satellites. This allows for safe and high-precision measurements for tasks like monitoring bridge displacement and tracking rail subsidence.

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As shown above, Multi-GNSS-enabled RTK positioning brings significant benefits in various scenarios on-site. By improving the "reliability" and "continuity" of positioning, it ultimately leads to increased overall work efficiency and ensures quality control.

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Features of LRTK and the Use of Multi-GNSS

Now, let’s take a look at the features of the LRTK series as a practical RTK positioning system utilizing Multi-GNSS. LRTK, developed by Lefixea, a startup from the Tokyo Institute of Technology, is an RTK-GNSS solution that has gained attention as a "universal surveying tool" that anyone can use on construction sites. By simply attaching a pocket-sized receiver to a smartphone or tablet, real-time centimeter-level positioning is made possible, and the data obtained can be instantly shared on the cloud. This combination of ease of use and high accuracy has made LRTK a tool that field technicians can carry one per person, allowing them to perform positioning or inspections immediately when needed.

One of the major strengths of the LRTK series is its support for Multi-GNSS and multi-frequency. For example, the smartphone-attached "LRTK Phone" can simultaneously receive satellite signals from GPS, GLONASS, Galileo, BeiDou, and QZSS, even with its compact antenna, and it is equipped with a high-performance GNSS receiver that supports L1, L2, and L6 frequencies.

Additionally, it is compatible with Japan’s Quasi-Zenith Satellite System (QZSS) and its centimeter-level augmentation service (CLAS), enabling standalone centimeter-level positioning by receiving correction data directly from satellites, even in areas without mobile network coverage, such as mountainous regions or disaster sites. Of course, in areas with communication coverage, it is also compatible with standard network RTK, allowing positioning using correction data from a base station (electronic reference points). This dual augmentation method ensures stable, high-precision positioning in any field environment.

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The design of the LRTK system also takes on-site usability into consideration. For example, the LRTK Phone can be easily attached to a smartphone with a single touch and includes a built-in battery, allowing for easy centimeter-level positioning with one hand.

As shown in the image above, you can check positioning results and points on the smartphone screen while holding the surveying rod and walking to observe the measurement points. Compared to traditional stationary GNSS receivers, the LRTK system is much more portable, allowing one person to efficiently carry out surveying and staking tasks even in small sites or situations with a labor shortage.

Additionally, higher-end models like the LRTK Pro2 come equipped with an inclination correction function, which automatically corrects the position of the antenna at the top of the pole even when it is tilted, enabling measurements at locations where obstacles make it difficult to place the pole straight.

In this way, LRTK fully utilizes Multi-GNSS technology, providing a high-precision GNSS solution that meets the practical and convenient requirements on construction sites.