Definition
The guaranteed usable range of a navigation aid's signal, within which the FAA ensures the signal is free from harmful interference by other stations transmitting on the same or adjacent frequencies. Outside this protected service volume, the same frequency may be reused by another facility, and signal reliability is no longer assured.
Plain English
Every ground-based navigation station is guaranteed a clean, interference-free signal out to a certain distance and altitude. Beyond that distance, another station somewhere else may be using the same frequency, so the signal can no longer be trusted.
Context Anchor
Seen in en route and substitute airway procedures when the FAA describes whether a route can safely use ground-based navigation signals.
Derivation
"Protection" here is used in the engineering sense of "shielded from interference," not "kept safe from harm." The frequency is protected — meaning reserved for that station's exclusive use within a defined volume of airspace.
Why Pilots Care
Ensures navigation signals remain usable so substitute routes can be flown safely without loss of course guidance.
Analogy
It is like radio stations being assigned far enough apart so one station does not bleed into another while you are trying to listen.
Intuition Check
Do not read protection here as ATC protecting your altitude or separating traffic. It means the navigation frequency itself is protected from radio interference along the route.
Example Sentence 1
When planning the off-airway leg, the pilot checked that both VORs remained within their frequency-protected service volumes for the planned altitude.
Example Sentence 2
Controllers confirmed frequency protection before clearing the aircraft to use the backup route.