What's the Difference Between GNSS and GPS? Why RTK is Needed

GPS is familiar from car navigation systems and smartphone map apps, but haven't you ever been guided to a location slightly off your destination? This happens because GPS-based positioning has inherent accuracy limitations. In recent years, scenarios demanding higher-precision positioning—such as drone autonomous navigation and 3D surveying at construction sites—have increased. Consequently, a technology called “RTK” is gaining attention as a replacement for conventional GPS.

This article explains the difference between GNSS and GPS, why RTK with its smaller error margin is needed, and introduces its mechanisms and use cases.

What is GNSS? Differences from GPS

First, let's clarify the difference between GNSS and GPS. GNSS (Global Navigation Satellite System) is the collective term for all satellite positioning systems that use artificial satellites to determine location.

The most well-known is the U.S. GPS (Global Positioning System), but others include Russia's GLONASS, the EU's Galileo, China's BeiDou, and Japan's QZSS (Quasi-Zenith Satellite System: nicknamed Michibiki). GPS is one GNSS developed by the United States, and GNSS is the broader concept encompassing these multiple positioning satellite systems.

So what's the difference between GNSS and GPS? It's the “number of satellites available” and the resulting difference in positioning accuracy. Using multi-GNSS (multiple satellite constellations) instead of just the single GPS satellite group allows for the reception of signals from more satellites at any given time. This reduces the impact of errors caused by satellite positioning.

In other words, increasing the number of available satellites improves positioning accuracy.

In practice, combining multiple GNSS systems like GPS+GLONASS provides more stable accuracy compared to GPS alone and reduces environments where positioning is impossible (such as behind buildings). Using a GNSS-capable receiver enables high-precision, stable positioning that cannot be achieved with GPS alone.

Why Does GPS Alone Have Large Errors?

The GPS positioning we commonly use (positioning with a single receiver) typically has errors of several meters.

There are several main causes for this error.

• Satellite Orbit and Clock Errors Errors stemming from discrepancies in predicted satellite orbit information and drift in the satellites' internal clocks. Even atomic clocks accumulate slight deviations over time, affecting positioning accuracy.

• Atmospheric Errors Effects caused when radio waves propagate through Earth's atmosphere. Delays in the ionosphere (approximately 50–1000 km above ground) and refraction in the troposphere (ground level to about 10 km) increase signal propagation time, introducing errors in distance measurement.

• Multipath Errors When GPS signals reflected off buildings or the ground (multipath) reach the receiver, they interfere with the direct signal, causing distance measurements to be inaccurate. This multipath effect is particularly significant in urban areas.

• Receiver-Side Errors Minor inaccuracies in the receiver's internal clock, errors caused by noise, and poor satellite geometry (satellite configuration) when insufficient satellites are available also lead to reduced accuracy.

The combination of these factors inevitably results in meter-level errors when relying solely on GPS positioning. For example, the experience of a smartphone GPS showing your location on the road next to a building is caused by these error factors. While a few meters of deviation is acceptable for standard car navigation or pedestrian navigation, GPS alone is insufficient for fields requiring high precision, such as infrastructure maintenance and construction surveying.

How RTK Corrects Errors

This is where RTK (Real Time Kinematic), a high-precision positioning technology, comes into play. RTK stands for “Real Time Kinematic” and is a positioning method classified as relative positioning. Specifically, it uses two GNSS receivers—a base station (fixed station) and a rover station—to simultaneously receive signals from four or more satellites. By exchanging distance measurements between the two receivers and correcting the resulting discrepancies (errors), it calculates a more precise position than standalone positioning. The key feature of this method is its ability to reduce residual errors to within just a few centimeters.

Let's examine step-by-step how RTK corrects GPS positioning errors.

• Set Up the Base Station First, install a GNSS receiver as the base station at a location with known, precise coordinates (known point). The base station is a receiver that knows its true position.

• Simultaneous Satellite Positioning at Both Stations Both the base station and the rover station receive signals from multiple GPS/GNSS satellites at the same time and calculate their respective positions. Each receiver measures distances from satellites, similar to standard GPS positioning.

• Error Calculation at the Base Station The reference station compares its calculated positioning result with its own known, precise coordinates to determine the positioning error (the deviation in distance measurements from satellites) at that moment. For example, if the reference station's calculated position is 10 cm east of its true position, it can be determined that “GPS currently has a 10 cm error in the east direction.”

• Transmission of Correction Data The reference station transmits the calculated error (correction amount) to the mobile station in real time via radio or internet lines. The mobile station maintains a communication environment capable of constantly receiving real-time correction data from the reference station (such as data in the RTCM format).

• Correction Application at the Mobile Station The mobile station applies the correction values received from the base station to its own raw satellite distance measurements. This cancels out common error factors between both receivers, such as atmospheric effects and satellite clock drift, enabling the mobile station to determine its position with centimeter-level accuracy.

This outlines the flow of real-time error correction using RTK. In essence, it is a mechanism that achieves high-precision relative positioning by correcting GPS positioning “errors” in real time. Specifically, RTK performs precise ranging using the carrier phase of radio waves (analyzing phase differences in carrier waves with a wavelength of about 20 cm). By resolving integer ambiguity, it achieves accuracy within a few centimeters. This level of accuracy cannot be achieved using standard code-based positioning (pseudorange measurement) alone.

Network-Based RTK

Traditionally, users needed to set up their own reference stations on-site. However, network-based RTK services have become widespread in recent years, utilizing mobile communication networks to distribute reference station data nationwide.

For example, SoftBank's “ichimill” service maintains over 3,300 proprietary reference points nationwide, allowing users to receive correction data via communication without setting up their own reference stations.

Using such services enables easy centimeter-level positioning with just a single mobile receiver.

RTK Application Examples

High-precision positioning via RTK is expanding its use across various fields, including civil engineering surveying and infrastructure maintenance. Here are several representative examples.

Civil Engineering Surveying and Construction

In the construction industry, RTK-GNSS is beginning to be used in scenarios traditionally reliant on manual labor or optical surveying instruments. For example, combining the LiDAR (Light Detection and Ranging) function of tablets or smartphones with an RTK-GNSS receiver enables 3D surveying at job sites without expensive laser scanners—a method also recommended by the Ministry of Land, Infrastructure, Transport and Tourism. Using RTK in this way allows efficient, high-precision topographic surveying and as-built management, driving digital transformation (DX) from design through construction management. Furthermore, RTK is used for heavy machinery position control (machine guidance/machine control), contributing to improved construction accuracy and labor savings through ICT-based construction*.

Infrastructure Maintenance (Roads, Railways, Bridges, etc.)

The application of RTK is also anticipated in the maintenance and inspection of social infrastructure like highways and railways. For example, in railways, RTK-GNSS-integrated sensors can be used for precise periodic measurements of rail subsidence and distortion, or for monitoring the displacement of structures like overhead wires and tunnels, enabling automation and advancement of maintenance work. For roads and bridges, mobile units equipped with RTK-capable GPS, which has low positioning error, are used to inspect road surfaces and structures, accurately recording the locations requiring repairs. In fact, RTK is also effective for automating and unmanned operations in traffic infrastructure monitoring. New maintenance methods based on high-precision positioning data are being researched and implemented.

Drone Surveying and Inspection

Equipping drones with RTK-GNSS dramatically improves the accuracy of aerial surveying and facility inspections. For example, in construction site aerial surveys for as-built management or in bridge and power line inspections, attaching precise location data (geotags) to images captured by RTK-equipped drones yields centimeter-accurate survey results without post-processing. Furthermore, during autonomous flight, RTK minimizes deviation from pre-set flight paths, reducing the risk of collision with adjacent structures. Achieving stable flight with RTK enables drones to safely replace manual inspections in hazardous areas.

Other Fields

RTK applications span agriculture, logistics, disaster prevention, and more. In agriculture, RTK-corrected positioning enables autonomous operation of tractors and other farm machinery, as well as pinpoint pesticide spraying by drones. In logistics, demonstrations of RTK-enabled autonomous buses and delivery drones are progressing. In sports, there are applications for wearable devices that can measure athletes' movements with centimeter-level precision.

As such, RTK's high-precision positioning is anticipated as a foundational technology supporting the smart transformation of all industries, with its application scenarios expected to expand significantly in the future.

ICT Construction※ “ICT (Information and Communications Technology) Construction” refers to methods that utilize GNSS, 3D design data, machine control technology, and other tools to enhance the efficiency and sophistication of civil engineering construction. As part of the Ministry of Land, Infrastructure, Transport and Tourism's (MLIT) i-Construction initiative, advanced examples include automated grading using heavy machinery equipped with RTK-GNSS.

Introduction to LRTK and Requesting Materials

Finally, we introduce “LRTK”, a solution enabling easy RTK utilization. LRTK is a series of RTK-GNSS receivers provided by Refixia Inc. Its hallmark is an all-in-one design, featuring a palm-sized housing that integrates an antenna, GNSS receiver, radio, and battery.

It eliminates the need for complicated equipment connections or on-site installation work. Simply bring it to the field, turn it on, and start high-precision positioning. By connecting a single receiver to a network-based RTK service, centimeter-level positioning is immediately achievable.

Refixia also provides services like the LRTK smartphone app to assist field operations and LRTK Cloud for managing and sharing positioning data in the cloud, ensuring ease of use even for first-time RTK adopters. LRTK continues to be developed and refined as a solution addressing the challenge faced by the construction industry and surveyors: “We want to use high-precision positioning more easily.”