Practical Guide for Construction Surveyors

Table of Contents

• What is RTK?

• Why the distance between the base station and the rover matters

• What happens if you are too far from the base station?

• Distance constraints depending on communication method

• How to achieve long-range RTK

• Conclusion: Simple surveying with LRTK

• FAQ

What is RTK?

RTK (short for Real Time Kinematic) is a technique that uses satellite positioning such as GPS to determine positions with high accuracy in real time. Standalone positioning typically yields errors of several meters, but RTK combines a receiver installed at a known coordinate called a "base station" with a positioning receiver called a "mobile unit (rover)". The base station compares its known accurate coordinate with signals from GNSS satellites to compute errors (corrections) and sends that correction information to the rover. By applying the received correction information to its own GNSS measurements, the rover cancels out errors and can determine its current position with very high accuracy, within a few centimeters. Because positioning results are updated in real time every second, RTK is used widely in fields that require immediate high-precision positioning such as civil surveying and construction, agriculture, and autonomous mobile robots.

Why the distance between the base station and the rover matters

RTK positioning assumes that the base station and the rover are within a certain proximity. This is because error factors contained in the satellite signals received by both stations change with distance. For example, signal delays due to the ionosphere and troposphere differ more between the two sites as the distance increases.

In distant locations, satellite visibility can differ and the commonality of satellite clock and orbit errors also diminishes. The correction information calculated by the base station basically represents errors near the base station. Therefore, when the rover is far from the base station, the errors measured at the base station and the actual error conditions at the rover diverge, and the effectiveness of the corrections diminishes.

What happens if you are too far from the base station?

If the baseline length—the distance between the base station and the rover—becomes too long, the accuracy and stability of RTK positioning gradually degrade. As a general guideline, a distance within 10 km is desirable. Up to about 10 km the base station and rover are mostly under the same error environment, making it easier to maintain centimeter-level accuracy; beyond that, error growth and longer initialization (time to obtain a FIX solution) occur.

As a rough rule of thumb, it is often said that for every 10 km distance from the base station, an additional horizontal error of about 1 cm (0.4 in) occurs. Therefore, for applications requiring high accuracy, use within a 20 km radius of the base station is recommended. That said, under certain conditions it can be practical even at around 20 km with errors within 3 cm (1.2 in). There are also reports of rovers obtaining FIX solutions even when more than 50 km away under favorable conditions. However, the farther the distance, the more unstable the positioning becomes, and there is a higher risk of reverting to a Float solution with only a slight deterioration in satellite signal conditions. When performing RTK positioning far from the base station, it is important to fully consider the risks of degraded accuracy and reduced FIX rate.

Distance constraints depending on communication method

The practical operating distance is also constrained by how correction information is transmitted from the base station to the rover. The main communication methods are local radio on site, distribution over the Internet (cellular networks), and network RTK using multiple base stations. Below are the characteristics and distance-related pros and cons of each.

In the case of radio communication

When correction data is sent directly from the base station to the rover using local radio, the radio coverage becomes the practical distance limit. It is common to mount transmitters such as UHF low-power radio in the 920 MHz band or LoRa on the base station and receive correction information with a corresponding radio receiver on the rover. In this case, even in clear line-of-sight conditions the base station’s signal typically reaches a radius of several km to about 5 km. In urban areas with many obstructions or inside forests, radio attenuation and shielding further reduce the communication range. Radio waves are highly directional, so ideally there should be no line-of-sight obstructions between the rover and the base station; in tunnels or shadowed mountain valleys the radio may not reach and RTK corrections cannot be received. If multiple rovers are used at once, all rovers must be within the range where they can receive the base station’s signal.

Measures to extend radio communication distance include installing the base station antenna at a higher location for line-of-sight and deploying repeaters as relays. However, fundamentally there are limits due to radio output and frequency bands, so for large sites where you need to operate more than 10 km from the base station you should consider methods other than local radio.

When using the Internet (Ntrip)

Connecting the base station to the Internet and distributing correction data via a dedicated server called an Ntrip server greatly relaxes distance constraints. The rover connects to the cellular network via a smartphone or mobile router and receives the base station’s data in real time over the Internet. Unlike local radio that directly transmits signals, rovers far from the base station can receive corrections as long as there is cellular coverage. In extreme cases, the base station and rover could be on opposite sides of Japan and still communicate via the Internet.

However, the problem remains that RTK correction data distributed from a single base station loses accuracy if the distance is too great. In practice, it is difficult to maintain high-precision RTK positioning unless the base station and the rover are kept within roughly 10–20 km. Therefore, even with Ntrip the location of the base station (or which nearby base station data you use) is important.

Ntrip distribution also allows multiple rovers to connect simultaneously and share a single base station’s data. Depending on server and service contracts, there may be a limit to the number of simultaneous connections, but it is generally possible to connect on the order of dozens of rovers. For large sites with many rovers, the Internet method’s advantage is that a single base station can provide correction information to multiple mobile units as long as cellular coverage is available.

Network RTK (VRS)

Using a network RTK service almost eliminates the distance problem with a single base station. This method uses a network of multiple fixed base stations to generate a virtual reference station (VRS) near the user and provide correction data for that location. The rover sends its approximate position to the network, and the server computes the local error information around the rover from the surrounding base stations’ data. As a result, the rover receives correction data as if a base station were located very close by, allowing centimeter-level accuracy to be maintained over long distances. In Japan, the Geospatial Information Authority’s electronic reference point network and private RTK correction services delivered via cellular networks (so-called “network RTK” services) provide high-accuracy corrections near mobile units nationwide.

Network RTK lets users perform RTK positioning over wide areas without setting up their own base station. However, using such services requires a subscription and per-rover usage fees, and they only work where cellular coverage exists; in mountain areas without cellular signal they cannot be used. In those cases, you need to combine your own base station plus radio, or use satellite augmentation services as described below.

How to achieve long-range RTK

If you need to do RTK positioning in distant locations, several approaches can be considered. Fundamentally, using the aforementioned network RTK service is the most reliable method. Within the service area, centimeter-accuracy positioning is possible anywhere without worrying about distance. Note, however, that service subscription fees and communication environment are required.

Another method is to move the base station closer and divide the survey area. For example, when surveying a vast area, you can maintain high accuracy by relocating the base station as needed and operating rovers within 10–20 km of the base for each sub-area. You can also consider extending the area by placing multiple base stations in a relay configuration, though this becomes operationally complex.

Furthermore, using wide-area augmentation systems is effective. In Japan, the Quasi-Zenith Satellite System “Michibiki” provides a centimeter-class augmentation service (CLAS) that, with a compatible receiver, can deliver a few-centimeter accuracy nationwide in real time. Because CLAS delivers correction signals from satellites, users do not need to install their own base stations. Overseas, PPP (Precise Point Positioning) services using L-band correction signals from geostationary satellites are becoming more common and can achieve centimeter-level positioning without base stations. However, PPP often requires several to tens of minutes for initial convergence, so in terms of immediacy RTK (differential methods) still has an advantage.

Thus, to perform high-precision positioning beyond the distance limits of a base station, it is important to combine network RTK, satellite augmentation, and other methods according to the environment. By choosing the optimal method based on site communication conditions, required accuracy, and cost, you can benefit from high-precision positioning even far from a base station.

Conclusion: Simple surveying with LRTK

LRTK is a simple surveying solution developed to address the operational challenges of RTK described above. By combining a smartphone with an ultra-compact high-precision GNSS receiver, anyone can easily achieve centimeter-level positioning. LRTK automatically uses network RTK services over the Internet and augmentation signals from Japan’s Quasi-Zenith Satellite “Michibiki” (CLAS), so users can obtain high-precision positions without installing their own base station. This makes stable positioning possible in remote areas such as mountain regions without worrying about “how far you can be from the base station.” No on-site equipment setup or complicated configuration is required; surveying is completed by following smartphone screen operations. As a simple surveying tool that can be used without RTK expertise, LRTK aims to become the new field standard. It will provide a smooth surveying experience not constrained by distance or communication environment to all sites that require high-precision positioning.

FAQ

Q1. What is RTK? A1. RTK (Real Time Kinematic) is a technique that uses two GNSS receivers—a base station and a mobile unit—to correct satellite positioning errors in real time and obtain high-precision positions. Standalone positioning typically has errors of several meters, but RTK can achieve positioning within a few centimeters horizontally and vertically.

Q2. What accuracy can be obtained with RTK? A2. In theory, accuracy of less than a few centimeters can be achieved. In real-world conditions, when conditions are good you can expect horizontal errors of about 2–3 cm (0.8–1.2 in) and vertical accuracy below 5 cm (2.0 in). However, accuracy varies with satellite geometry, surrounding obstructions, distance to the base station, and other factors. Short observation times can produce larger errors, but repeating measurements at the same point and averaging can stabilize accuracy.

Q3. How far can you position with RTK from the base station? A3. A general guideline is that the distance between the base station and the rover should be about 10 km or less. Within this range it is easier to maintain centimeter-level accuracy and initialize (obtain FIX) quickly. Positioning up to around 20 km has been achieved with errors of a few centimeters in many cases and may be acceptable operationally. Beyond that, errors tend to increase and initialization can take longer or become unstable. Except in special circumstances, positioning more than 30 km away becomes impractical.

Q4. Can RTK be used in sites outside cellular coverage? A4. In areas without cellular coverage, Ntrip-based RTK using the Internet cannot be used. However, as an alternative you can communicate directly between the base station and rover using local radio. With radio you can use RTK even in remote mountain areas without cellular coverage (limited to the radio’s reach). Also, in Japan you can use satellite-delivered correction information such as Michibiki CLAS to achieve high-precision positioning even without terrestrial communication infrastructure.

Q5. What is a network RTK service? A5. A network RTK service combines data from multiple base stations to provide local error information near the user for high-precision positioning. The representative VRS (Virtual Reference Station) method delivers correction data equivalent to having a base station near the user. This allows consistent centimeter-level positioning over wide areas such as nationwide in Japan without the user installing a base station. Network services require communications and a subscription, but they make it easy to achieve high-precision positioning over wide areas.

Q6. What is LRTK? A6. LRTK is a simple surveying platform centered on an ultra-compact RTK-GNSS receiver used together with a smartphone. Designed for one-touch operation without specialist knowledge, it enables centimeter-level positioning and greatly simplifies field surveying. LRTK uses not only radio from a base station but also Internet-delivered corrections and Japan’s satellite augmentation signals (CLAS), allowing high-precision positioning without worrying about distance from a base station or lack of cellular coverage. Compared with traditional large surveying equipment, LRTK offers portability while delivering both ease of use and high accuracy as the next-generation positioning tool.