GNSS Multipath vs Spoofing: Evidence and Limits

[Pavel]

Short answer: Multipath reflects authentic signals and often depends on geometry and surroundings; spoofing introduces counterfeit signal structure or navigation data. One position jump or distorted correlation peak is not enough—use spatial, temporal and receiver-level evidence plus trusted references.

Before you start​

• Keep GNSS and drone work passive unless the experiment has explicit authorization and an RF-controlled environment; do not jam, spoof or interfere with navigation or aircraft.
• Record receiver clock, antenna, gain, sample rate, observation location precision and ground truth before interpreting anomalies.

Step-by-step method​

1. Step 1: Record satellite C/N0, pseudorange residuals, Doppler, clock bias, position and raw correlator or I/Q data where available.
2. Step 2: Check whether anomalies correlate with low-elevation satellites, buildings, vehicle movement or repeat at the same physical geometry, which supports multipath.
3. Step 3: Compare independent antennas, receivers, inertial data or trusted timing. Common coherent changes across satellites can justify deeper spoofing analysis.
4. Step 4: State alternative explanations and confidence. Escalate only through authorized safety or spectrum channels; never transmit a counter-signal.

Concrete example​

A position error that disappears when moving the antenna away from a reflective wall is strong multipath evidence. Simultaneous identical Doppler behavior across satellites and implausible clock motion may be suspicious but still requires corroboration.

How to judge the result​

The analysis is responsible when raw evidence supports the conclusion, uncertainty is explicit and ordinary antenna, clock, ephemeris and interference faults have been tested.

What to record​

• Separate frequency-reference accuracy from absolute time alignment: 10 MHz disciplines frequency while PPS marks a time boundary.
• Use multiple independent observables for GNSS interference or spoofing claims, such as spectrum, AGC behavior, correlation, navigation consistency and trusted reference data.
• For direction finding or RF classification, report uncertainty and test across channels, distances, devices and temperatures.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Calling any GNSS anomaly spoofing can mislead users and distract from common antenna, multipath and receiver problems.

Final check​

Repeat the passive observation against an independent trusted reference and state uncertainty. Do not make spoofing, interference or aircraft claims from a single receiver screenshot.