Multipath in GNSS Surveying

Definition and Overview

Multipath is a significant error source in Global Navigation Satellite System (GNSS) surveying that occurs when satellite signals reach a receiver through multiple paths. The primary signal travels directly from the satellite to the antenna (direct path), while secondary signals arrive after reflecting off nearby surfaces such as buildings, water bodies, metal structures, or the ground itself. These reflected signals take longer to arrive and create phase and code delays that degrade positioning accuracy.

How Multipath Occurs

When electromagnetic waves from GNSS satellites encounter reflective surfaces in the receiver's environment, they bounce off these materials before reaching the antenna. Common reflective surfaces include:

Metallic structures and frameworks

Building facades and walls

Water surfaces and bodies

Paved areas and roads

Vegetation and terrain features

The receiver correlates both the direct and reflected signals, creating constructive and destructive interference patterns. This signal combination produces measurements that deviate from the true satellite-receiver distance.

Types of Multipath Effects

Code Multipath affects pseudorange measurements, typically causing errors ranging from a few centimeters to several meters depending on signal strength and reflection geometry. Carrier Phase Multipath affects the measured phase of the GNSS carrier wave, generally producing smaller errors (millimeters to centimeters) due to the shorter wavelength of carrier signals compared to code signals.

Impact on Surveying Accuracy

Multipath errors become particularly problematic in:

Urban environments with tall buildings

Areas near large metallic structures

Proximity to water bodies

Surveying near steep terrain

Construction sites with temporary structures

These environments can produce errors of 10 centimeters to several meters, severely compromising survey accuracy and requiring careful mitigation strategies.

Mitigation Strategies

Site Selection: Surveyors should choose observation locations away from reflective surfaces whenever possible. Open sky conditions with minimal obstructions provide the best multipath environment.

Antenna Design: Specialized GNSS antennas with multipath rejection capabilities, such as choke-ring antennas, can significantly reduce reflected signal reception. These antennas use concentric grooves to suppress signals arriving at low angles where reflections typically originate.

Signal Processing: Advanced receivers employ multipath mitigation algorithms that identify and suppress reflected signals based on signal characteristics and arrival timing.

Observation Techniques:

Collecting observations over extended periods allows multipath errors to average out

Using dual-frequency receivers helps identify multipath-affected measurements

Measuring during periods with favorable satellite geometry improves accuracy

Baseline Analysis: In relative positioning, comparing results from multiple sessions helps identify and correct multipath-induced errors.

Monitoring and Detection

Surveyors can detect multipath effects through:

Signal-to-noise ratio (SNR) analysis

Observing inconsistencies in repeated measurements

Examining residuals in GNSS processing software

Comparing results with known reference values

Conclusion

Multipath remains a persistent challenge in GNSS surveying, particularly in non-ideal environments. Understanding multipath characteristics, selecting appropriate antennas, choosing optimal survey locations, and applying suitable processing techniques enables surveyors to minimize its effects and achieve required accuracy standards. Modern GNSS receivers and processing software continue to improve multipath handling, but awareness and careful site management remain essential components of professional surveying practice.