Definition
A compass locator beacon installed at the site of an ILS middle marker. It transmits a low-frequency, low-power non-directional radio signal that an aircraft's automatic direction finder (ADF) can home on, allowing the pilot to navigate to the middle marker location and supporting the final segment of an instrument approach.
Plain English
A small radio beacon placed at the middle marker of an ILS approach. Its signal can be tracked by the aircraft's ADF, helping the pilot find and fly to that point on the approach.
Context Anchor
Seen on some instrument approach charts and in instrument landing system discussions.
Derivation
Locator' because it helps the pilot locate the approach course using an ADF; 'middle marker' because the beacon is co-located with the middle marker of an ILS, the marker positioned partway along the final approach (typically near the missed approach point on a Category I ILS).
Why Pilots Care
It marks a key position for timing, altitude checks, and deciding whether to land or go around.
Intuition Check
Do not read this as just the middle marker tone or light. The locator middle marker is a separate radio beacon placed at the same location as the middle marker.
Example Sentence 1
After identifying the LMM on the chart, the pilot tuned the ADF to its frequency to track inbound on the final approach course.
Example Sentence 2
At the LMM the crew cross-checked altitude and prepared for the missed approach point.