Definition and Overview
GNSS multipath refers to the phenomenon where satellite signals received by a GNSS (Global Navigation Satellite System) receiver take multiple paths to reach the antenna. The primary signal travels directly from the satellite to the receiver, while secondary signals reflect off nearby surfaces such as buildings, water bodies, metal structures, or terrain before reaching the antenna. These reflected signals arrive slightly delayed, causing phase and code distortion that degrades positioning accuracy.
In surveying applications, multipath is one of the most significant sources of GNSS error, particularly in challenging environments. Unlike atmospheric errors such as ionospheric delay, which can often be corrected through modeling or dual-frequency processing, multipath errors are site-specific and difficult to eliminate completely.
How GNSS Multipath Occurs
Signal Path Mechanisms
When a satellite signal is transmitted, it travels through space toward the receiver antenna. In ideal conditions—such as open sky environments—the signal arrives directly. However, when objects are present in the signal path, reflection and diffraction occur:
The delayed reflected signals interfere with the direct signal, creating measurement errors in both code and phase observations. The error magnitude depends on signal strength, delay time, and the geometry of reflecting surfaces.
Technical Impact on Measurements
Code and Phase Errors
Multipath affects two fundamental GNSS measurements:
Code Pseudorange Errors: Reflections create a secondary signal peak that can shift the receiver's code correlation point. This typically causes errors ranging from 1 to 10 meters, depending on signal-to-noise ratio (SNR) and reflection characteristics.
Carrier Phase Errors: Phase multipath is generally smaller (millimeters to centimeters) than code multipath, but can significantly impact high-precision surveying when combined with other error sources.
The relationship between multipath error and reflection characteristics is nonlinear and depends on the reflection coefficient, signal strength, and the time difference between direct and reflected arrivals.
Mitigation Strategies in Surveying Practice
Hardware Solutions
Antenna Selection: Choke ring antennas and other specialized designs suppress reflected signals through electromagnetic shielding. These antennas are standard for high-precision surveying work and can reduce multipath effects by 70-90%.
Signal Processing: Modern GNSS receivers employ advanced correlation techniques, including narrow correlators and peak detection algorithms that minimize the influence of delayed signals.
Site Selection and Positioning
Surveying professionals minimize multipath through strategic planning:
Data Processing Techniques
Multipath Stacking: Observing stations over extended periods allows multipath patterns to average out, as satellite geometry continuously changes.
Sidereal Day Filtering: Since multipath characteristics repeat approximately every sidereal day (23 hours 56 minutes), surveyors can separate multipath from other errors through temporal analysis.
Double Difference Processing: Differencing observations between two receivers cancels site-specific multipath components, particularly effective for baseline surveys.
Applications in Surveying
Control Survey Establishment
When establishing geodetic control networks, surveying teams must account for multipath errors. High-order control surveys typically require centimeter-level accuracy, necessitating multipath mitigation strategies and extended observation sessions to achieve the required precision.
Construction and Engineering Surveys
In urban construction environments with numerous reflecting surfaces, multipath represents a major challenge. Machine control and building alignment applications require millimeter-level precision, making multipath reduction essential through proper antenna placement and real-time correction techniques.
Monitoring and Deformation Studies
Structural monitoring projects using GNSS benefit from careful site configuration to minimize multipath effects. Reflections from building elements can mask genuine deformation signals, requiring dedicated multipath characterization and removal strategies.
Related Surveying Concepts
Understanding multipath requires familiarity with GNSS error sources, atmospheric delay correction methods, and receiver tracking technologies. Surveyors should also understand signal-to-noise ratio (SNR) measurements, which indicate multipath intensity, and differential GNSS techniques, which effectively reduce site-specific errors.
Practical Monitoring and Diagnosis
Modern surveying software provides multipath indicators through SNR analysis and code-minus-carrier observables. Surveyors can diagnose multipath problems by observing:
By recognizing these indicators, surveyors can adjust field procedures and post-processing strategies to achieve design accuracy requirements.
Conclusion
GNSS multipath remains a critical consideration in professional surveying. While impossible to eliminate completely, understanding multipath characteristics and implementing proven mitigation strategies enables surveyors to achieve project specifications consistently and reliably.