Multipath Error in GNSS/GPS Surveying: Detailed Glossary & Technical Explainer

Multipath error is a persistent and complex phenomenon affecting the accuracy of Global Navigation Satellite System (GNSS) and Global Positioning System (GPS) surveying. In the context of high-precision positioning, understanding, recognizing, and mitigating multipath is essential for reliable results across geodesy, construction, cadastral, and navigation applications. This technical guide explores the underlying science, practical impacts, field recognition, and state-of-the-art mitigation strategies for multipath error.

What is Multipath Error?

Multipath error in GNSS/GPS occurs when satellite signals reach the receiver via two or more paths: the intended direct (line-of-sight) path and one or more indirect paths due to reflections from surfaces such as buildings, water, vehicles, or the ground. The receiver cannot always distinguish between these signals, leading to errors in the calculated position because reflected signals arrive later than the direct signal. The additional distance traveled by the reflected signal increases the measured range, causing both pseudorange and carrier phase errors.

Multipath is especially problematic in environments with abundant reflective surfaces (urban areas, near water, inside forests with wet leaves, etc.), and its impact can range from negligible to several meters, depending on the environment, receiver quality, and antenna design.

The Science of Signal Propagation and Reflection

Direct vs. Reflected Signal Paths

Satellite signals are designed to travel in a straight line from satellite to receiver (Line-of-Sight, LOS). In reality, many signals encounter obstacles, resulting in:

• Reflection: The signal bounces off a surface—such as glass, metal, water, or the ground—creating one or more Non-Line-of-Sight (NLOS) paths.
• Scattering: When a signal strikes a rough surface (e.g., leafy trees, rocky ground), it scatters in multiple directions.
• Diffraction: Bending of signals around obstacles, less pronounced at GNSS frequencies but can still contribute.

Reflected signals travel farther than the direct path, arriving later and with changed phase and amplitude. The receiver’s correlator, which decodes the timing of the incoming signal, may interpret the combination as a single, delayed signal, resulting in position errors.

Analogy

Imagine shouting in a canyon: the direct sound wave reaches your friend, but echoes from the canyon walls arrive slightly later. If your friend tried to estimate your distance based on the timing of all sounds, the echoes would confuse the calculation—just as multipath confuses a GNSS receiver.

Causes of Multipath Error

Multipath arises from a mix of environmental and technical factors:

Man-Made Sources

• Buildings/Urban Canyons: Glass, steel, and concrete are highly reflective to GNSS signals. Dense cities create environments where direct LOS is often blocked and only reflected NLOS signals are present.
• Vehicles and Metal Objects: Moving or stationary, these surfaces dynamically reflect signals.
• Infrastructure: Towers, fences, and signs can act as reflectors.

Natural Sources

• Vegetation: Wet leaves and dense canopy reflect and scatter signals.
• Water Bodies: Still water acts as a near-perfect mirror for low-elevation satellite signals.
• Snow, Ice, Terrain: Reflect or scatter signals, sometimes unpredictably.

Atmospheric Influences

Atmosphere does not directly cause multipath but can amplify it by altering the signal’s propagation path through refraction, especially at low elevation angles.

Effects of Multipath Error on Surveying Accuracy

Multipath can degrade GNSS accuracy in both pseudorange and carrier phase measurements:

• Pseudorange Measurements: Most affected; errors can be several meters in consumer-grade devices under heavy multipath, but are usually centimeters to decimeters with survey-grade equipment and good conditions.
• Carrier Phase Measurements: Errors in the millimeter to centimeter range. Especially problematic for high-precision RTK and network RTK, where a single multipath event can cause loss of integer ambiguity resolution or solution jumps.
• Survey Manifestations: Position “jumps,” “drift,” long RTK fix times, and inconsistent results on repeated occupations of the same point.

Multipath is particularly dangerous in safety-critical applications (e.g., aviation, autonomous vehicles) and is a leading factor in the performance requirements set by agencies such as the International Civil Aviation Organization (ICAO).

Recognizing Multipath in the Field

Field recognition of multipath requires careful observation of GNSS system status and environmental clues:

• High PDOP: Indicates poor satellite geometry, often due to NLOS signals.
• Long Fix Times: Prolonged ambiguity resolution in RTK or network RTK signals multipath interference.
• Solution Instability: Frequent toggling between float and fix status or sudden loss of fix.
• Position Drift/Jumps: Unexplained movement while stationary.
• Low or Fluctuating SNR: Reflected signals have lower SNR; rapid SNR changes suggest multipath.

Pro Tip: Always monitor PDOP, SNR, and solution status. Document environmental conditions (presence of reflective surfaces, vehicles, water, etc.) during survey setup.

Mitigation Strategies for Multipath Error

Antenna Technology

• Choke Ring Antennas: Specialized with concentric rings to absorb low-angle signals, reducing ground reflection multipath.
• Ground Planes: Metal discs beneath antennas block signals reflected from below.
• RHCP Antennas: Right Hand Circular Polarized antennas naturally attenuate multipath, as reflected signals reverse polarization.
• Elevated Placement: Mount antennas above ground and away from obvious reflectors.

Receiver Technology

• Multi-Constellation & Multi-Frequency: Track more satellites/signals to select those least affected by multipath.
• Advanced Algorithms: Modern receivers use SNR, phase, and code correlation metrics to detect and reject multipath.

Field Techniques

• Site Selection: Prefer open sky, minimize nearby reflective surfaces.
• Antenna Elevation: Place antennas on tripods or poles to minimize ground reflection.
• Observation Timing: Plan measurements when satellites are high in the sky (high elevation angles).

Post-Processing

• Multipath Filtering/Detection: Software identifies and downweights suspect measurements using statistical and signal quality analysis.
• Redundant Observations: Repeat measurements and compare to identify anomalies.

Advancements in Multipath Mitigation Technology

Modern GNSS surveying benefits from cutting-edge innovations:

• Multi-Frequency Tracking: Dual- and triple-frequency receivers use frequency diversity to distinguish multipath.
• AI & Machine Learning: Pattern recognition algorithms learn typical multipath at sites and adapt filtering in real time.
• Expanded Satellite Constellations: More satellites in view mean better geometry and more multipath-immune options.
• Enhanced Signal Modulation: New signals (e.g., BOC) have sharper autocorrelation, helping receivers better separate direct from reflected signals.
• Smart Antenna Arrays: Electronic beamforming favors skyward signals, suppressing ground/low-elevation multipath.

Manufacturers like Hemisphere, Trimble, and Leica integrate these advancements, ensuring their latest receivers deliver high-precision results even in challenging environments.

Key Takeaways

• Multipath error is caused by satellite signal reflections, leading to significant GNSS position inaccuracies.
• It is most severe in urban, water-adjacent, or metal-rich environments.
• Both pseudorange and carrier phase measurements are affected, compromising survey accuracy and RTK performance.
• Recognition involves monitoring PDOP, SNR, fix status, and coordinate consistency.
• Mitigation combines advanced antennas, multi-frequency receivers, smart field practices, and post-processing.
• Ongoing technological advances continue to improve multipath resistance, making accurate GNSS positioning feasible in more environments.

Glossary of Terms

• GNSS (Global Navigation Satellite System): A collective term for all satellite-based positioning systems, including GPS, GLONASS, Galileo, and BeiDou.
• GPS (Global Positioning System): The U.S.-operated segment of GNSS, widely used worldwide.
• Pseudorange: The measured distance between satellite and receiver, including all errors (multipath, clock, atmospheric).
• Carrier Phase: The precise measurement of the GNSS signal’s carrier wave phase, critical for high-precision techniques.
• RTK (Real-Time Kinematic): Technique utilizing carrier phase and real-time corrections for centimeter-level accuracy.
• PDOP (Position Dilution of Precision): Metric indicating satellite geometry quality; low values mean better potential accuracy.
• NLOS/LOS: Non-Line-of-Sight (reflected path) and Line-of-Sight (direct path) signal reception.
• Choke Ring Antenna: An antenna with concentric metal rings to reduce multipath.
• Ground Plane: A conductive plate beneath the antenna to block ground-reflected signals.
• RHCP: Right Hand Circular Polarized antennas, preferentially receiving direct GNSS signals.

For professional-grade GNSS/GPS solutions that minimize multipath error and ensure survey reliability, contact us or schedule a demo today!