Definition
A low-power non-directional beacon (NDB) installed at the site of an ILS middle marker, transmitting a homing signal that pilots can track with their ADF receiver. It serves both as the middle marker for the ILS approach and as a standalone navigation aid for orienting and approaching the runway.
Plain English
A small radio beacon placed at the middle marker position on an ILS approach. It does two jobs at once: it acts as the middle marker, and it gives a signal pilots can home in on with their ADF.
Context Anchor
Seen in instrument approach equipment discussions and on some approach charts for older instrument landing system procedures.
Derivation
Called a 'locator' because it helps pilots locate themselves relative to the runway, and 'middle marker' because of its position along the ILS approach -- roughly half a mile from the runway threshold.
Why Pilots Care
Passage over the locator middle marker typically coincides with reaching decision altitude on a Category I ILS approach, prompting the pilot to decide whether to land or go around.
Grounding Statement
Picture it as a radio checkpoint near the runway that the aircraft can sense as it flies the final part of the approach.
Intuition Check
Do not read “marker” as a painted sign or physical runway marker. Here it means a radio-based point on an instrument approach path.
Example Sentence 1
The approach plate showed a locator middle marker, so we tuned the ADF to use it as a backup if the glideslope failed.
Example Sentence 2
The pilot used the locator middle marker signals to confirm position and maintain alignment during the final segment of the approach.