Reflections can combine with direct satellite signals and shift a GNSS coordinate estimate even when the antenna remains stationary. Compare signal diagnostics, reference behavior and site changes with the movement record to distinguish multipath from structural motion and other faults.
Table of Contents
How can a stationary antenna report movement?
A GNSS receiver estimates position from received satellite signals. Reflected paths can alter those measurements even when the antenna has not moved. The resulting coordinate variation is an observation problem, not proof of physical displacement.
NovAtel’s explanation of multipath distinguishes local reflections from errors that differential methods can mitigate. It also separates multipath with a direct signal present from non-line-of-sight reception, where the direct path is blocked.
This matters at fixed stations near metal cladding, water, retaining structures or moving equipment. RTK or other corrections do not automatically cancel a reflection that exists only at one antenna. Conversely, finding a reflective surface does not establish that a reported movement is spurious.
Keep the site’s approved monitoring response in force while investigating. This article provides a diagnostic evidence framework, not asset-specific alarm thresholds or authority to dismiss a warning.
Investigate the surroundings and the observations together
Start with the event timeline. Record when the apparent change began, which components changed and whether nearby stations show a related pattern. Preserve raw observations, processing versions, antenna metadata, solution-quality indicators and the reference-station record.
Then review the physical surroundings without moving the sensor. Compare installation photographs with the current scene: parked vehicles, scaffolding, new cladding, water level and changed obstructions. A convenient relocation may reduce reflections but also break the station’s measurement continuity.
The original Bilich and Larson study on GPS multipath mapping uses signal-to-noise-ratio time series and satellite-direction information to investigate reflecting environments. Its method supports looking beyond a coordinate plot. It does not make a single low signal-strength sample a conclusive multipath detector.
Request the indicators actually available from the installed receiver and processing chain. Avoid inventing a universal “multipath score” or a threshold that has not been validated for that equipment.
A competing-hypothesis review matrix
Use several hypotheses at once so the investigation does not become a search for confirmation of the first explanation.
| Hypothesis | Evidence worth checking | Reason not to conclude too early |
|---|---|---|
| Local reflected signals | Satellite-direction patterns, observation residuals, changed surroundings | Similar symptoms can have other causes |
| Physical antenna or mount movement | Mount inspection, maintenance history, independent survey | Small movement may not be visually obvious |
| Reference-station change | Reference coordinates, health and other station trends | Shared motion can also be physically real |
| Processing or configuration change | Software, weighting, antenna model and filter history | Timing alone does not prove causation |
| Structural environmental response | Temperature, loading, water level and independent sensors | Correlation is not permission to subtract motion |
| Data delivery or timing fault | Event time, receipt time, gaps and replay records | A smooth dashboard may conceal stale data |
The table is an editorial investigation aid. Record the evidence for and against each hypothesis, the remaining uncertainty and the operating decision. Do not relabel an unresolved event as “noise” simply because a cause has not been found.
The GNSS monitoring-system guide covers the reference architecture and quality-state design. This incident review should feed that architecture rather than replace it.
Why not subtract the repeating pattern?
A repeated pattern can be informative, but subtracting it assumes the pattern is an error and that its behavior remains applicable. Real deformation or thermal response may also repeat. A filter can remove part of the signal the system was installed to observe.
If a correction is proposed, retain raw and corrected series, the fitting interval, parameters and a validation interval that was not used to tune it. Explain its effect on event timing and uncertainty. Compare against an independent measurement suited to the movement question.
An optical monitoring check may help, but it has its own reference and environmental limitations. Agreement or disagreement between instruments is evidence to interpret, not an automatic vote on which device is correct.
Close the incident without rewriting history
A useful closure record states what was observed, what changed, which explanation is supported and what remains unresolved. If the antenna or monument is altered, document the discontinuity and the relationship between pre-change and post-change records.
Diagnostic access and configuration history make later trend changes interpretable. Retain the raw observations, quality indicators and environmental changes alongside the accepted movement series.
FAQs
Can RTK corrections remove all multipath error?
No. Multipath is strongly influenced by the local antenna environment. Differential corrections address shared errors but do not automatically cancel reflections that differ between the reference and monitoring stations.
Does a repeating coordinate pattern prove multipath?
No. Repetition is a clue to investigate, not a diagnosis. Structural temperature response, reference behavior and other periodic effects may also contribute. Review satellite observations, the surroundings and independent measurements.
Should suspected multipath be filtered out of the displacement record?
Not automatically. Preserve original observations and document the hypothesis, correction method and validation. An unvalidated filter can hide real motion or create a misleadingly continuous record.
