RTK in Urban Canyons: How Buildings Affect GNSS Accuracy

For field engineers and surveyors involved in infrastructure inspection and construction management in urban areas, accurate positioning is indispensable. However, have you ever struggled with GNSS equipment in city centers lined with high-rise buildings, thinking "I can't get a reliable fix..."? Environments where buildings disturb satellite signals form artificial valleys—so-called "urban canyons." In such places, not only standalone GPS but also high-precision Real-Time Kinematic positioning (RTK) can sometimes produce unexpected errors.

RTK is a technology that can usually achieve positioning accuracy on the order of several centimeters (several in) in open outdoor environments, but in the gaps between buildings it is often reported that a fixed solution cannot be obtained and the solution remains a float. This article explains the challenges of RTK positioning in urban canyons and the specific ways buildings impact GNSS accuracy. It also introduces countermeasures for accuracy degradation and the latest technologies, and finally touches on a new, easy high-precision positioning solution called "LRTK." First, let’s look at what RTK is, its basic principle, and the conditions that determine positioning accuracy.

Table of Contents

• What is RTK? Basics of high-precision positioning

• Urban canyons and GNSS positioning

• Challenges of RTK positioning in city streets

• Points to maintain and improve RTK accuracy in urban areas

• Simple surveying with LRTK

• Frequently Asked Questions (FAQ)

What is RTK? Basics of high-precision positioning

RTK (Real-Time Kinematic) is a technique that corrects GNSS positioning errors in real time to obtain high-precision positions. By having two receivers—a base station (a receiver with known coordinates) and a rover (the receiver measuring the target)—simultaneously receive satellite signals and sending correction information from the base to the rover, RTK cancels error sources that cannot be removed by single-receiver positioning. Concretely, it corrects in real time by differential processing errors such as satellite orbit errors, atomic clock offsets, atmospheric delays (ionosphere and troposphere), and radio signal reflections (multipath interference), improving positional accuracy to very small errors on the order of several centimeters (several in).

Standard GPS in smartphones or car navigation systems produces horizontal errors of at least several meters (several ft). However, with RTK positioning, under favorable conditions you can obtain highly accurate position information with horizontal errors on the order of about 1–3 cm (0.4–1.2 in) and vertical errors of about 2–5 cm (0.8–2.0 in). For this reason, RTK is used in a wide range of applications such as civil engineering surveying, construction site as-built control, and automated guidance in agriculture. A key feature of RTK is that, unlike conventional total stations, it does not require line-of-sight (the straight-line view between the instrument and the target) and a single operator can complete the survey. Once a base station is set up, the rover can instantly obtain global coordinates anywhere, dramatically improving work efficiency.

That said, there are conditions to achieve "centimeter accuracy" with RTK. It requires receiving a sufficient number of satellites simultaneously, good satellite geometry with a low DOP (dilution of precision), and a communication environment that can stably receive correction information. Under open skies, you can lock on to more than 10 satellites and reliably obtain a fixed solution (Fix解), but in environments with many obstructions these conditions may not be met and maintaining accuracy becomes difficult. The next chapter examines why GNSS accuracy degrades in environments surrounded by buildings in urban areas.

Urban canyons and GNSS positioning

Roads and blocks in city centers lined with buildings often have most of the sky covered by concrete structures overhead, creating a narrow sky like a "canyon." In such urban canyon environments, satellite signals from GNSS are blocked or reflected by buildings, which greatly affects positioning accuracy.

The first problem is the restriction of satellite visibility. RTK positioning typically requires receiving five or more satellites (ideally 6–8 or more) stably. However, on streets between high-rise buildings you often see only a narrow patch of sky, and in extreme cases you may capture fewer than four satellites. Even if you barely receive four satellites, if those satellites are biased in the same direction the satellite geometry is poor and the positioning solution becomes unstable. In urban areas, a lack of satellites or degraded geometry often prevents RTK calculations from converging or prevents obtaining a fixed solution, leaving a float solution (Float解) and poor accuracy.

More serious is multipath interference. This refers to errors caused when satellite radio signals reflected off concrete walls or glass surfaces arrive at the receiver delayed. In urban areas, strong reflections from high-rise buildings can enter the GNSS receiver and introduce errors into the pseudorange (range measurement). In particular, in building shadows direct satellite signals are blocked and the receiver may pick up only reflected signals, a condition known as NLOS (non-line-of-sight). As a result, the positioning solution can be heavily disturbed and sometimes produce mispositioning offsets of several meters (several ft) from the true location. In urban canyons, satellite blockage and multipath are the main causes of significant GNSS positioning degradation.

For example, in experiments using a multi-GNSS RTK receiver in open areas, planar position errors averaged about 2 cm (0.8 in), vertical errors were about 3–4 cm (1.2–1.6 in), and even the maximum deviations were only around 5–7 cm (2.0–2.8 in). On the other hand, performing the same positioning in downtown areas surrounded by buildings produced average errors of around 5 cm (2.0 in), which is practically acceptable, but due to signal reflection and blockage, occasional errors exceeding 10 cm (3.9 in) occurred. Thus, even with the same RTK system, accuracy varies depending on the environment, and urban canyons are a harsh environment for RTK.

Challenges of RTK positioning in city streets

When performing RTK positioning in urban areas, note that the fix rate—the rate at which fixed solutions are obtained—drops significantly for the reasons described above. In building shadows, positioning may remain as a float solution and accuracy may be limited to tens of centimeters (several in), failing to meet the accuracy required for construction or surveying. Moreover, even when the receiver displays "FIX" and appears to have a fixed solution, there is a risk of a false fix if pseudorange errors from multipath have contaminated the data. In urban canyons, RTK solutions are often unstable, causing position wander or jumps, so it is essential on site to continuously monitor solution quality and proceed with caution.

Receiving correction information reliably is also a challenge. Although network RTK using cellular communication is available in many places, signal conditions in urban canyons can be unstable. Near subsurface road levels or in deep canyon areas between high-rise buildings, smartphone and mobile router signals can weaken, causing interruptions in receiving correction data (such as VRS data via NTRIP). Additionally, if you transmit corrections from your own base station on a specific frequency (UHF), buildings may block the radio and you may not be able to cover city areas adequately. For this reason, network RTK over the cellular internet (VRS method using the national Continuously Operating Reference Station network) is generally the mainstream for RTK surveying in urban areas. Nevertheless, urban radio environments require consideration of noise from surrounding Wi‑Fi and broadcast stations. GNSS reception may be affected near high-voltage power lines or communication antennas on buildings, so it is desirable to perform positioning as far away as possible from such strong-field sources.

Points to maintain and improve RTK accuracy in urban areas

To achieve the best possible RTK accuracy even in harsh urban environments, several countermeasures and measures are effective. When surveying on site, paying attention to the following points helps maintain accuracy.

• Ensure as much open sky as possible: Even in a canyon between buildings, choose intersections, plazas, or building setback spaces where the sky is relatively open. Points with a relatively large open sky overhead can receive more satellite signals and increase the likelihood of obtaining a fixed solution.

• Antenna setup techniques: Mount the GNSS antenna as high as possible and keep it away from surrounding obstructions. Using a telescopic pole to raise the antenna overhead or, if possible, placing it on a building rooftop or a high point expands satellite visibility. Also, attaching a ground plane (metal plate) to the antenna blocks reflections from below, and setting the antenna elevation cutoff angle to around 15° to exclude low-elevation satellites can reduce multipath to some extent (※ be careful not to block too many satellites—adjust according to site conditions).

• Timing selection: Satellite geometry changes with time. It is effective to check satellite visibility predictions with GNSS planner tools and choose times when more satellites are visible and DOP values are favorable. For example, rather than around noon when satellites cluster near the zenith, morning and evening when satellites are spread broadly east-west may allow satellites to appear from behind building shadows. Simply shifting the measurement time can improve conditions, so if possible consider adjusting survey schedules.

• Use high-performance receivers: For urban positioning it is desirable to use a GNSS receiver that supports multi-GNSS and multi-frequency reception. Tracking multiple constellations such as GLONASS, Galileo, and QZSS (Michibiki) in addition to GPS increases the total number of visible satellites when some are temporarily obscured. In particular, Japan’s Quasi-Zenith Satellite System (QZSS, Michibiki) tends to remain near the zenith for long periods, so it is often possible to capture at least one satellite overhead even in city streets, aiding positioning. Receivers that support multiple frequencies such as L1/L2/L5 improve ionospheric error removal and speed up ambiguity resolution for initial fixed solutions. Modern GNSS antennas with enhanced multipath rejection are also effective in obstructed environments. Additionally, receivers with an integrated IMU (inertial measurement unit) can bridge short satellite outages using inertial sensors and help maintain continuity of position.

• Use satellite-based augmentation services: Japan’s QZSS provides centimeter-level augmentation services such as CLAS. Using a CLAS-capable receiver allows you to obtain correction information directly from satellites, maintaining high-accuracy positioning even where internet from a base station is hard to reach. Similar wide-area augmentation services are progressing overseas, and taking advantage of such services can complement RTK weaknesses.

Even with the above measures, there will still be cases in urban areas where RTK positioning is difficult. In such cases, consider combining methods—for example, obtain a reference coordinate at a location where positioning is possible even in building shadows, then use a total station for detailed measurements. Alternatively, instead of insisting on real time, record GNSS observation data on site and later process it in the office using reference-station data in PPK (Post-Processed Kinematic) mode. Rather than forcing RTK in unsuitable conditions, flexibly switching positioning methods according to circumstances will lead to accurate positioning and safer work.

Simple surveying with LRTK

Although RTK is extremely useful, until recently adopting it required expensive dedicated equipment and specialized knowledge, making it a high barrier for many practitioners. Recently, however, a more user-friendly RTK solution called "LRTK" has emerged. LRTK is a system developed by Reflexia that consists of a compact RTK‑GNSS receiver and a smartphone app; simply attaching it to a smartphone enables real-time centimeter-level positioning (centimeter-level (in)). It supports network RTK (VRS) using Japan’s Continuously Operating Reference Station network and augmentation signals from QZSS (CLAS), allowing stable positioning nationwide from urban to mountainous areas.

For example, tasks that traditionally required two people for as-built measurement on construction sites can now be done by a single worker walking the site with a pole fitted with the LRTK receiver while coordinates of each observation point are recorded in the cloud in real time. Measurement results can be checked on the spot and additional measurements taken immediately if needed. The smartphone app can display design drawings and control points in AR on the screen, enabling intuitive execution of complex stakeout and layout tasks. A tilt function that detects pole inclination and applies automatic correction allows accurate measurement of the pole tip position even when the pole cannot be held perfectly vertical due to obstructions. Because these features are designed for ease of use by non-experts, LRTK is reassuring for sites introducing RTK for the first time.

By utilizing simple surveying with LRTK, surveying work in urban areas can be dramatically streamlined and labor-saving. There is no need to bring heavy tripods or expensive optical survey instruments to the site, and anyone can perform high-precision positioning in a short time, changing the style of surveying. A free trial lending system is also available so you can test usability and accuracy in the actual field before introducing it. If you are interested in leveraging high-precision GNSS, consider trying simple surveying with LRTK. By adopting the latest technology, urban surveying sites can increasingly achieve both efficiency and high accuracy. Use RTK and LRTK to solve positioning challenges that have been difficult until now.

Frequently Asked Questions (FAQ)

Q: Why is it difficult to achieve RTK accuracy in urban areas (city streets)? A: When high-rise buildings surround the area, GNSS satellite signals are blocked and the number of receivable satellites decreases. Additionally, reflected signals from buildings and glass (multipath) enter the receiver and cause positioning errors. These effects of satellite blockage and multipath interference make it hard to obtain an RTK fixed solution in urban areas, causing unstable accuracy.

Q: What is the difference between a fixed solution (Fix) and a float solution (Float)? A: In RTK positioning, a fixed solution means the integer ambiguity of carrier-phase cycles has been correctly resolved, yielding the highest accuracy (errors within a few centimeters). A float solution means the integer component has not been resolved and the solution is computed with fractional ambiguities, resulting in lower accuracy (errors can be from several centimeters (several in) up to about 1 m (3.3 ft)). For high-precision surveying, always confirm that the solution shows "FIX" before recording.

Q: How far from the base station can I position? A: RTK accuracy gradually degrades as the distance from the reference station increases. Generally, centimeter accuracy can be maintained within roughly a dozen kilometers, but beyond 20 km it takes longer to obtain a fixed solution and errors tend to expand from a few centimeters to several tens of centimeters. In Japan, network RTK using the Continuously Operating Reference Station network (VRS method) can provide corrections as if a virtual base station is always nearby, allowing centimeter-level accuracy even in areas more than 50 km away.

Q: Can RTK surveying be done with a smartphone? A: A smartphone’s built-in GNSS alone cannot achieve centimeter accuracy, but combining it with an external RTK-capable receiver makes it possible. For example, using a compact device attachable to a smartphone, like LRTK, can convert a smartphone’s normally meter-level GPS into centimeter-level positioning after corrections. Since operations are done via a smartphone app, the workflow is simple and measurement results can be checked against maps or design drawings on site.

Q: Any tips to improve RTK accuracy in urban areas? A: First, choose locations where the sky is as open as possible. Use a multi-GNSS receiver to increase the number of tracked satellites, raise the antenna to reduce reflections and blockage, and select time windows with favorable satellite geometry. Receivers that can use QZSS (Michibiki) augmentation signals also improve stability in urban environments. These measures can help improve RTK accuracy even in city streets.

Q: Can RTK surveying be done outside cellular coverage? A: There are ways to perform RTK without cellular coverage. If you set up your own base station and transmit correction data wirelessly, you can perform real-time positioning without internet. In Japan, the QZSS CLAS centimeter-level augmentation service can be received directly by compatible receivers, enabling centimeter accuracy without a base station or communications. If you cannot obtain a real-time FIX on site, saving observation data and performing post-processing (PPK) later is also an effective solution.