Definition
A built-in self-checking function within a ground-based navigation facility (such as a VOR, ILS, or localizer) that continuously verifies the accuracy and integrity of the signal it transmits. If the signal drifts outside published tolerances, the internal monitoring feature either removes the identification, shuts the facility down, or switches to a standby transmitter so pilots are not misled by faulty guidance.
Plain English
A self-watching system inside a navigation station on the ground that constantly checks its own signal. If something goes wrong, it warns pilots by turning off the station's identifier or shutting the station down so no one flies on bad information.
Context Anchor
Seen in discussions of how ground-based navigation facilities are monitored for safe use during instrument flight.
Derivation
Internal' meaning inside the facility itself, and 'monitoring' from the Latin monere, meaning 'to warn.' The name reflects the function: the facility warns itself (and therefore pilots) when its own signal becomes unreliable.
Why Pilots Care
It gives pilots confidence that a navigation signal is usable or that it will be withdrawn if faulty, reducing the risk of following inaccurate guidance.
Intuition Check
Do not read “internal monitoring” as the pilot monitoring something inside the airplane. Here it means the navigation facility checks itself from within its own equipment.
Example Sentence 1
When the localizer signal drifted out of tolerance, the internal monitoring feature removed the identifier, alerting pilots that the approach was no longer usable.
Example Sentence 2
Before using an ILS approach, pilots know the internal monitoring feature keeps the signal reliable.