What is GNSS? The Difference from GPS

Let's first clarify the difference between GNSS and GPS. GNSS (Global Navigation Satellite System) refers to the collective group of satellite positioning systems that use artificial satellites to measure location. The most well-known example is the U.S. GPS (Global Positioning System), but other systems include Russia's GLONASS, the EU's Galileo, China's BeiDou, and Japan's QZSS (Quasi-Zenith Satellite System, also known as "Michibiki"). Each country or region operates its own GNSS system. GPS is one of the GNSS developed by the United States, while GNSS encompasses these multiple satellite positioning systems.

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One Point: Historically, in Japan, the use of GPS has been more widespread, so satellite positioning systems are sometimes referred to simply as "GPS." However, strictly speaking, GPS is just one system within the broader GNSS.
 

The difference between GNSS and GPS lies in the "number of available satellites" and the resulting differences in positioning accuracy. By using not just a single GPS satellite constellation but also multiple GNSS (multiple satellite constellations), you can receive signals from a greater number of satellites, reducing the impact of errors caused by satellite positioning.

In other words, as the number of available satellites increases, positioning accuracy improves. In practice, combining multiple GNSS systems like GPS and GLONASS provides more stable accuracy compared to using GPS alone and reduces areas where positioning may be unavailable (such as in building shadows). By using GNSS-compatible receivers, high-precision and stable positioning can be achieved, which would not be possible with GPS alone.

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Why is GPS Alone Prone to Larger Errors?

In the GPS standalone positioning method (using a single receiver), we typically encounter errors of several meters. There are several main causes for these errors.

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• Satellite Orbit and Clock Errors: Errors due to forecast inaccuracies in the satellite's orbital information and deviations in the satellite's internal clock. Even slight discrepancies in atomic clocks, when accumulated, can affect positioning.

• Atmospheric Errors: These are the effects of signal propagation through Earth's atmosphere. Delays in the ionosphere (about 50-1000 km above the Earth) and refraction in the troposphere (from the surface to about 10 km) cause the signal travel time to increase, resulting in distance measurement errors.

• Multipath Errors: When GPS signals are reflected off buildings, the ground, or other surfaces (multipath), they mix with direct signals, causing inaccuracies in distance measurements. This effect is particularly pronounced in urban areas.

• Receiver Errors: Errors due to slight inaccuracies in the receiver's internal clock or noise, and poor geometry (satellite configuration) when there are not enough satellites, can also lead to decreased accuracy.

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As a result of the factors mentioned above, GPS standalone positioning inevitably leads to errors on the meter scale. For example, if your smartphone's GPS shows your location on a road next to a building, it is due to these error factors. While a few meters of discrepancy may be acceptable in standard car navigation or pedestrian navigation, for fields that require high precision, such as infrastructure maintenance or construction surveying, GPS alone is insufficient.
 

How RTK Corrects Errors

This is where RTK (Real-Time Kinematic) positioning technology comes into play. RTK, short for "Real-Time Kinematic," is a type of relative positioning system. Specifically, it uses two GNSS receivers: a base station (fixed station) and a rover (mobile station), which simultaneously receive signals from at least four satellites. The distance measurements between the two receivers are exchanged, and by correcting the discrepancies (errors), RTK calculates a more accurate position than standalone GPS positioning. The key feature of this method is that it can reduce the remaining errors to just a few centimeters.

Now, let’s walk through how RTK corrects the errors in GPS positioning.

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1. 基準局を設置: まず既知の正確な座標値を持つ地点（既知点）に基準局となるGNSS受信機を設置します。基準局は自分の真の位置を知っている受信機です。

2. 両局で同時に衛星測位: 基準局と移動局の双方が、同じタイミングで複数のGPS/GNSS衛星からの信号を受信し、それぞれ自分の位置を計算します。各受信機は通常のGPS測位と同様に衛星からの距離を測定します。

3. 基準局で誤差を算出: 基準局は自ら計算した測位結果と、自身の既知の正確な座標を比較することで、その時刻における測位誤差（衛星からの距離測定のずれ）を求めます。例えば基準局の計算上の位置が真の位置から東に10cmずれていれば、「現在、GPSには東方向に10cmの誤差が含まれている」と判断できます。

4. 補正情報の送信: 基準局が算出した誤差（補正量）を無線やインターネット回線でリアルタイムに移動局へ送信します。移動局は常に基準局からのリアルタイム補正情報（RTCMと呼ばれるフォーマットのデータなど）を受け取れる通信環境を用意します。

5. 移動局で補正適用: 移動局では、自身が測定した生の衛星距離データに対して、基準局から受信した補正量を適用します。こうすることで、大気の影響や衛星時計のずれなど両受信機間で共通の誤差要因が相殺され、移動局の位置をセンチメートル単位の精度で求めることができます​。

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This is the process of real-time error correction using RTK. In essence, it corrects the "discrepancy" in GPS positioning in real-time, providing high-precision relative positioning. Specifically, RTK uses precise distance measurement by analyzing the phase shift of the carrier wave (with a wavelength of approximately 20 cm) in the radio signals, and by resolving the integer ambiguity (the uncertainty of integer cycles), it achieves accuracy within a few centimeters. This level of precision cannot be reached with standard code-based positioning (pseudo-distance measurement).

Network RTK: Traditionally, users had to set up their own base stations on-site, but recently, network RTK services that deliver correction data from base stations across the country via mobile communication networks have become more widespread. For example, "ichimill," provided by SoftBank, has established over 3,300 proprietary reference points nationwide, allowing users to receive correction data via communication without needing to set up their own base station. With such services, a single rover receiver can easily achieve centimeter-level positioning accuracy.

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Use Cases of RTK

High-precision positioning with RTK is increasingly being applied across various fields, starting with civil surveying and infrastructure maintenance. Here, we will introduce several representative use cases.

• Civil Surveying and Construction Industry: In the construction industry, RTK-GNSS is starting to be used in situations that previously relied on manual labor or optical instruments for surveying. For example, by combining the LiDAR (Lidar) functionality of tablets or smartphones with RTK-GNSS receivers, 3D surveying on-site can be performed without the need for expensive laser scanners, a method recommended by the Ministry of Land, Infrastructure, Transport, and Tourism. By using RTK, terrain surveying and as-built management can be done efficiently and with high precision, further promoting DX (Digital Transformation) from design to construction management. RTK is also used in machine position control (machine guidance/machine control), contributing to improved construction accuracy and labor-saving through ICT-based construction.

• Infrastructure Maintenance (Roads, Railways, Bridges, etc.): RTK’s application is also expected in the maintenance and inspection of social infrastructure such as highways and railways. For example, in the railway sector, RTK-GNSS sensors are used to measure rail subsidence and distortion regularly and to monitor structural deformations in overhead lines and tunnels, enabling automation and enhancement of maintenance work. For roads and bridges, RTK-compatible GPS devices mounted on moving vehicles are used to inspect road surfaces and structures, accurately recording the location of areas that need repairs. In practice, RTK is also effective in the automation and unmanning of transportation infrastructure monitoring, and new maintenance methods based on high-precision positioning data are being researched and implemented.

• Drone Surveying and Inspections: By equipping drones with RTK-GNSS, the accuracy of aerial surveying and equipment inspections has improved dramatically. For example, in construction site aerial imaging for as-built management or the inspection of bridges and power lines, RTK-equipped drones can provide centimeter-level accuracy without the need for post-processing by tagging images with precise location information (geotags). Furthermore, when flying autonomously, RTK minimizes deviations from pre-set flight paths, reducing the risk of collision with nearby structures. With RTK, stable flight becomes possible, allowing drones to safely replace manual inspections of dangerous areas that were traditionally conducted by people.

• Other Fields: RTK is also being utilized in various other sectors, including agriculture, logistics, and disaster prevention. In agriculture, RTK positioning correction enables autonomous operation of tractors and precise pesticide spraying by drones. In logistics, RTK-enabled autonomous buses and delivery drones are undergoing trials. In sports, there are examples of RTK applied to wearable devices that can measure athletes' movements with centimeter-level accuracy.

• As shown, RTK’s high-precision positioning is expected to support the smartification of various industries and its applications are expected to continue expanding in the future.

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ICT Construction: "ICT (Information and Communication Technology) construction" refers to methods that use GNSS, 3D design data, machine control technology, and other tools to improve and advance the efficiency of civil engineering construction. As part of the i-Construction initiative promoted by the Ministry of Land, Infrastructure, Transport, and Tourism, there are advanced examples such as heavy machinery equipped with RTK-GNSS performing automatic grading.

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Introduction to LRTK and Information on Requesting Materials

Finally, we would like to introduce LRTK, a solution that allows easy use of RTK technology. LRTK is an RTK-GNSS receiver series provided by Lefixea, featuring an all-in-one design that integrates the antenna, GNSS receiver, radio, and battery into a compact, palm-sized unit.

It eliminates the need for complex equipment connections and on-site setup, enabling high-precision positioning with just a simple power-on after bringing the device to the site. When connected to a network-based RTK service using a standalone receiver, centimeter-level positioning can be achieved instantly.

Lefixea also offers services such as the LRTK smartphone app to assist with on-site operation and LRTK Cloud for managing and sharing positioning data on the cloud, making it easy for even first-time RTK users to use with confidence. LRTK has been developed and improved as a solution to address the challenges faced by the construction industry and survey professionals who need "easy access to high-precision positioning."