Definition
A three-letter identifier assigned to a navigation facility, such as the localizer of an Instrument Landing System (ILS), used to uniquely identify that facility on charts, in flight plans, and in its Morse code audio identification. For an ILS localizer, the designator is preceded by the letter 'I' (transmitted as the Morse 'I-' prefix) when broadcast as the station identifier.
Plain English
A short three-letter code that names a specific navigation aid, like a localizer, so pilots can recognize it on charts and confirm by Morse code that they have tuned the right station.
Context Anchor
Seen when tuning and identifying the localizer for an instrument landing system approach.
Derivation
Designator comes from a Latin root meaning “to mark out” or “to point out.” That fits this use: the three letters point out exactly which localizer signal is being received.
Why Pilots Care
Ensures the pilot tunes and identifies the correct localizer frequency and course during an instrument approach.
Intuition Check
Do not read designator as a description, runway number, or clearance. Here it means the assigned letter code used to identify one specific localizer signal.
Example Sentence 1
After tuning the ILS frequency, the pilot listened for the Morse code three-letter designator to confirm the localizer was the correct one for the approach.
Example Sentence 2
Each ILS localizer has its own three-letter designator shown beside the frequency on the chart.