Multipath in Surveying
Definition
Multipath refers to the propagation of electromagnetic signals through multiple paths from a transmitter to a receiver. In surveying, this occurs when signals from satellite systems (GPS/GNSS), total stations, or other electronic instruments reflect off nearby surfaces before reaching the receiver, creating multiple signal arrivals at different times.
How Multipath Occurs
In GPS surveying, multipath errors develop when satellite signals bounce off reflective surfaces such as:
Each reflected signal travels a longer path than the direct signal, arriving at the receiver with a time delay. The receiver may correlate with a reflected signal rather than the direct signal, causing position errors.
Impact on Surveying Accuracy
Multipath is one of the most challenging sources of error in surveying because it:
Manifestations in Different Instruments
GPS/GNSS Systems: Multipath creates range errors by introducing time delays in signal reception. Single-frequency receivers are more susceptible than dual-frequency receivers, which can partially mitigate multipath effects.
Total Stations and EDM: Reflective surfaces near prisms can cause signal distortion, affecting distance measurements and angular accuracy.
RTK Surveying: Real-time kinematic systems experience multipath-induced phase variations that degrade positioning reliability.
Mitigation Strategies
Surveyors employ several techniques to minimize multipath effects:
1. Site Selection: Avoid surveying near reflective surfaces. Choose open sky locations with minimal obstructions.
2. Equipment Positioning: Place receivers and prisms away from metal structures, buildings, and other reflective objects.
3. Antenna Design: Use multipath-suppressing antennas with ground planes and choke rings that reject off-zenith signals.
4. Signal Filtering: Modern receivers incorporate multipath rejection algorithms that identify and reject signals with excessive delay.
5. Observation Techniques: - Observe during times with good satellite geometry - Use multiple frequencies when available - Increase observation duration for statistical filtering - Occupy multiple epochs and average results
6. Quality Control: Monitor dilution of precision (DOP) values and signal-to-noise ratios to identify multipath-contaminated measurements.
Detection and Analysis
Surveyors can detect multipath through:
Professional Best Practices
Experienced surveyors:
Conclusion
Multipath remains a significant challenge in modern surveying, particularly with GNSS-based methods. Understanding its causes, effects, and mitigation techniques is essential for achieving required accuracy standards. While complete elimination is impossible, proper planning, equipment selection, and survey procedures can substantially reduce multipath errors and improve overall survey quality.