Definition 1 of 2
Definition
A navigation signal error caused when a VHF radio wave passes through the spinning blades of a propeller or helicopter rotor, which periodically interrupt and reflect the signal. This produces small, rapid fluctuations in the received signal that can cause unsteady or inaccurate indications on VOR or other VHF-based navigation receivers.
Plain English
Spinning blades chop up the radio signal slightly as it reaches the antenna. That can make the navigation needle wobble or read a little off until the propeller or rotor RPM is changed.
Context Anchor
Seen in instrument navigation discussions of space-wave radio signals, especially when explaining why VHF and UHF navigation signals can be affected by the aircraft itself.
Derivation
Modulation here means a regular variation imposed on the signal -- in this case, by the blades passing in front of the antenna many times per second. The word comes from Latin modulari, 'to measure or regulate.'
Why Pilots Care
Unrecognized propeller or rotor modulation can produce misleading navigation information that leads to course deviations or missed approaches if the pilot does not cross-check with other sources.
Analogy
It is like watching a steady light through a spinning fan. The light source has not moved, but the blades can make what you see appear to flicker.
Grounding Statement
Picture the radio signal reaching the aircraft while the propeller or rotor repeatedly passes near the path to the antenna, causing a slight pulsing effect in what the receiver hears.
Intuition Check
Do not read this as a mechanical propeller or rotor problem. The error is in the received radio signal, not necessarily in the propeller, rotor, or engine.
Example Sentence 1
The instructor adjusted RPM slightly to eliminate the propeller modulation error that was causing the VOR needle to oscillate.
Example Sentence 2
In the helicopter, rotor modulation error caused the ADF bearing to swing erratically during the instrument approach.