Why is RTK GPS so accurate? An easy-to-understand explanation from how it works to the benefits of implementation

On job sites such as construction, surveying, agriculture, and drone operations, the term "RTK GPS" has become frequently heard. Although many people understand that it is a high-precision positioning technology, few field practitioners fully understand the mechanisms behind why it is so accurate. In this article, we explain from the basics why RTK GPS achieves high accuracy, and carefully describe the benefits and precautions for introducing it on site.

Table of Contents

• Why standard GPS positioning has errors

• RTK positioning's "differential technique" that cancels errors

• Understanding the differences between FIX, FLOAT, and SBAS solutions

• Two operational modes of RTK

• How implementing RTK positioning changes on-site work efficiency

• Site conditions to check before implementation

• Cost considerations: comparison between conventional equipment and smartphone-linked systems

• Start using RTK positioning in the field with LRTK

Reasons why standard GPS positioning produces errors

First, understanding why standard GPS (GNSS-only) positioning has errors is a prerequisite to understanding how RTK works.

The basic principle of satellite positioning is to measure the time it takes for radio waves transmitted from satellites to reach the receiver and calculate the distance. Since multiplying that time by the speed of light (approximately 300,000 kilometers per second) yields the distance, the distances to multiple satellites are combined to determine one's position using the principle of triangulation.

However, in reality, radio waves change speed as they pass through the Earth's atmosphere (the ionosphere and troposphere), causing errors in arrival time. Small deviations in satellite orbital information, clock errors inside the receiver, and reflections of radio waves from buildings and terrain (multipath) also amplify these errors. These factors accumulate, making it unavoidable for ordinary standalone GPS positioning to have errors of 3–5 m (9.8–16.4 ft).

The "Differential Technique" by Which RTK Positioning Cancels Out Errors

The core reason RTK positioning can achieve high accuracy lies in two techniques: "differential positioning" and "the use of carrier-phase measurements".

The concept of differential positioning is very simple. A reference station (a receiver installed at a known coordinate) receives satellite signals and determines the discrepancy between the "calculated position" and the "true known coordinate." This discrepancy includes ionospheric, tropospheric, and orbital errors, and the same errors occur at a mobile station located nearby. The reference station sends its discrepancy information (correction data) to the mobile station in real time, and when the mobile station applies this correction, the error components common to both stations are canceled.

In RTK positioning, not only the satellite signal code (C/A code) but also the carrier-phase (carrier phase) is used. The carrier wavelength is approximately 19 cm (7.5 in) for GPS L1 and is very short; by counting its phase you can obtain distance resolution orders of magnitude finer than code-based positioning. When the integer carrier-phase ambiguities are correctly resolved (the "FIX" solution), measurement accuracies of 1-2 cm (0.4-0.8 in) horizontally and 2-3 cm (0.8-1.2 in) vertically are obtained.

Understanding the differences between FIX, FLOAT, and SBAS solutions

The outputs of RTK receivers include multiple types of solutions.

To manage data quality in the field, it is important to accurately understand what each of them means.

A FIX solution is the highest-precision state in which the integer values of the carrier phase have been fixed, and horizontal errors of 1-2 cm (0.4-0.8 in) can be expected. A FLOAT solution is a transient state in which the integer values are not yet fixed, and accuracy is unstable, ranging from a few centimeters to several tens of centimeters (a few in to several tens of in). An SBAS solution (or standalone positioning) is a code-based positioning state that does not use the carrier phase, with an accuracy of about 1-5 m (3.3-16.4 ft).

In surveying and construction management, you must always confirm a FIX solution before taking measurements. If you continue measuring while the solution remains FLOAT, you may later find that the accuracy is insufficient and need to re-measure.

Two RTK operating modes

There are two types of operational modes for RTK positioning: "single-base-station RTK" and "network RTK".

Single-base-station RTK is a method in which a reference-station receiver is installed at a known point on or near the site, and correction data are sent and received with the rover via radio communication (UHF radio or the 920 MHz band). The working area is the range covered by the radio signal (roughly several hundred meters (several hundred ft) to several kilometers (several thousand ft)). It has the advantage that it can be used outside mobile phone service areas, but installing and managing the reference station requires effort and cost.

Network RTK is a method that uses a service delivering correction data from an electronic reference station network maintained by national or private entities via the Internet (3G/4G/5G). Because there is no need to set up a base station, work can be carried out with a single rover. In Japan, stable services are provided over wide areas, and this method is currently the mainstream.

How Introducing RTK Positioning Changes On-Site Work Efficiency

The greatest benefit of introducing RTK positioning is a major improvement in the efficiency of surveying and position-verification work. Traditionally, confirming the coordinates of each point on site required setting up a total station and a level and performing sighting and measurement procedures. With RTK positioning, simply placing the receiver antenna at the measurement point allows coordinates to be obtained in real time, greatly reducing the measurement time per point.

Also, because coordinate data are recorded directly as numerical values, mistakes in handwritten field notebooks and numerical transcription errors are greatly reduced. By managing data digitally, verification with design data, automatic generation of reports, and the creation of as-built documentation can also be streamlined.

On-site Conditions to Check Before Installation

There are site conditions that should be checked before implementing RTK positioning.

First, you need to check the communication environment in the work area. If you use network RTK, the service cannot be used in areas where mobile phone signals do not reach. In mountainous areas, remote islands, and inside underground structures, single-base-station RTK is required.

Next, check the satellite visibility environment. In urban areas with densely clustered high-rise buildings or in deep valleys, obstacles that cover the sky reduce the number of satellites that can be received, making it difficult to obtain a FIX solution. Before starting work, it is advisable to check reception conditions with satellite prediction software.

The waterproof and dustproof performance of equipment is also an important factor to check. Selecting equipment with a waterproof rating of IP65 or higher that can withstand on-site use ensures reliability for work in rainy conditions and in dusty environments.

Cost considerations: Comparison of conventional equipment and smartphone-integrated systems

Traditional RTK equipment, when equipped with a rover receiver, a base station receiver, and a data controller, can cost over ¥1,000,000 for a complete set in some cases. This cost has been a barrier to adoption for small-scale operators and for deployment to many users.

In recent years, compact RTK receiver modules that attach to smartphones have appeared, making RTK positioning possible at a fraction of the cost of traditional equipment. The app-based user interface is also intuitive, allowing field personnel without specialized surveying training to become proficient in a short time.

Field Applications of RTK Positioning Starting with LRTK

LRTK (iPhone-mounted GNSS high-precision positioning device) is a device that enables network RTK positioning simply by attaching it to your everyday iPhone. It supports the domestic Continuously Operating Reference Station (CORS) network and can obtain a stable FIX solution over a wide area.

Because dedicated controllers and large antennas are not required, it is easy to carry on-site, and a wide range of staff—from surveyors to construction managers and inspectors—can use it effectively. If you want to easily bring the high accuracy of RTK GPS to the field, please consider it as an option.

The reason RTK GPS achieves high accuracy lies in the use of differential techniques and the exploitation of carrier-phase measurements. Accurately understanding that mechanism and choosing an operational method suited to site conditions is the key to maximizing RTK positioning’s potential.