RTK Multipath: Why Your RTK Fix Drops Out in Cities and Forests

RTK multipath is a common reason an RTK fix drops out or becomes inaccurate in real-world jobs, especially in urban canyons, under tree canopy and in steep terrain. This guide explains why it happens, how to spot it and what helps in the field.

Introduction

RTKdata's accuracy guidance calls multipath interference the biggest accuracy killer after distance to the base station. Signals bounce off nearby surfaces or are blocked, and the receiver struggles to keep a reliable fix.

Often, the receiver isn't the problem; the environment is. And that gets expensive: wrong points create rework on construction sites, in surveying, or in machine control.

Key Takeaways

• Multipath means a satellite signal reaches the antenna along reflected paths as well as the direct one, for example off buildings, vehicles, metal or water.
• On the carrier phase, multipath is at most about 5 cm on GPS L1 or L2. On the code it is typically less than 2 to 3 m, which can delay a fix.
• A wrong fix is the dangerous case. NGS notes that incorrect fixes and noise have produced height errors of more than 2 dm.
• Corrections can't remove multipath, because it depends on the surroundings of each antenna and doesn't cancel between rover and reference station.
• Mitigation happens in the field: site choice, antenna, elevation mask and redundant checks.

Understanding RTK positioning: Why the environment matters

RTK reaches centimeter level by measuring the carrier phase of satellite signals and combining it with correction data from a reference station, delivered over NTRIP or radio. The carrier phase is precise but ambiguous, because the receiver must resolve a whole number of wavelengths to each satellite. Until it does, the solution is FLOAT. Once it does, it is FIX. Our guide to GNSS quality indicators explains the difference.

Corrections remove errors that the rover and a nearby reference station share, such as satellite clock, orbit and atmospheric errors. However, ESA's Navipedia notes that the main errors left without correction are multipath, interference and receiver noise. That's why mitigation starts at the measurement location, not in software.

RTK multipath: Errors from reflections, trees, and terrain

RTK multipath means the GNSS signal reaches the antenna not only directly, but also as reflected copies, for example off glass, concrete, vehicles, water or the ground. NGS guidelines list trees, buildings, tall vehicles, water and metal power poles as typical sources. According to Navipedia, the main cause is an antenna close to reflecting structures, and the effect is strongest for satellites low on the horizon.

How large is the error? On the carrier phase, multipath is at most a quarter of a wavelength, about 5 cm on GPS L1 or L2, and typically less than 1 cm. On the code measurements, it is typically less than 2 or 3 m, and NGS notes that this code noise limits the first step of ambiguity resolution. That is one reason a receiver can stay in FLOAT for a long time.

The worst case is a wrong fix. NGS warns that multipath can cause an incorrect ambiguity resolution and gross position errors, particularly in height, and notes that height errors of more than 2 dm have been seen. Signal blockage adds to this, because fewer satellites mean weaker geometry.

Common symptoms:

• no RTK fix
• a FLOAT solution that won't resolve
• sudden position jumps
• a fix that "looks good" but is actually wrong

Urban canyon: What goes wrong in cities, forests, and mountains

Cities are where RTK multipath is most obvious. In an urban canyon, building facades block part of the sky and reflect signals. Meanwhile, vehicles and cranes keep changing the reflections around you.

In forests, the canopy blocks and reflects signals. NGS notes that when trees or buildings obstruct a rover, its signals can be interrupted and it has to re-initialize, and RTKdata's accuracy guidance rates dense forest as a poor environment.

In mountains, slopes hide part of the sky, while rock faces and water surfaces reflect signals.

Tracking more satellites helps. For example, NGS notes that using GPS and GLONASS together increases the speed and reliability of ambiguity resolution. It can also give fixes in traditionally bad conditions, such as urban canyons and between rows of tree canopy.

Mitigation strategies for multipath interference: Setup, checks, and corrections

Good mitigation of RTK multipath starts with site choice. NGS advises against using sites with probable multipath for control points, and especially for an RTK base station. These include sites:

• under or very near tree canopy
• within 30 m of structures taller than the antenna
• near vehicles, metal objects or signs
• next to large bodies of water

For the antenna, NGS recommends a ground plane, choke ring or another high-quality multipath-rejecting antenna at the base. In addition, an elevation mask of 10° to 15° blocks the low satellites whose signals add the most noise. NGS recommends 10°.

Quality checks

• Check the solution status. Only FIX gives centimeter-level results. Our troubleshooting guide says initial convergence can take up to 90 seconds, so investigate if FLOAT lasts beyond 2 to 3 minutes.
• Set a PDOP limit. For example, NGS uses PDOP 4.0 or lower for its RT3 accuracy class, and stricter limits of 3.0 and 2.0 for the more accurate RT2 and RT1 classes.
• Re-measure important points. NGS recommends redundant observations three to four hours apart, so that satellite geometry and multipath effects differ.
• Check known control points whenever you have them.

What corrections can and can't do

A correction service can't clean up a reflected signal at your rover. Instead, it rules out the other causes: distance to the reference station, stale corrections and connection problems. RTKdata's AUTO mountpoint serves the nearest reference station, and our guide to RTK accuracy over distance explains why that matters.

RTK multipath: A checklist for robust accuracy

Use these points as a quick guide in the field:

• Choose a spot with open sky above about 15°, and nothing within 30 m that rises above the antenna.
• Keep away from vehicles, metal, signs and large water surfaces.
• In forests, prefer clearings and edges, and expect a longer time to fix.
• Use a multi-constellation receiver and a good antenna.
• Wait for FIX, and investigate if FLOAT lasts beyond 2 to 3 minutes.
• Re-measure critical points at a different time and check known control points.
• Rule out distance and connection issues by using corrections from a nearby reference station.