Definition
A low-power non-directional beacon (NDB) installed at the middle marker of an instrument landing system (ILS) approach. It transmits a continuous signal that an aircraft's automatic direction finder (ADF) can home on, providing a backup means of identifying and tracking to the middle marker position on the approach.
Plain English
A small radio beacon located at the middle marker of an ILS approach. A pilot's ADF needle points to it, giving an extra way to find and follow the approach path.
Context Anchor
Seen on some instrument approach charts and older instrument landing system descriptions, especially where marker beacons and compass locaters are still published.
Derivation
Locater' comes from 'locate' — to find or fix a position. The beacon helps the pilot 'locate' the middle marker on the approach. 'Compass' refers to the ADF (which works like a radio compass), not a magnetic compass.
Why Pilots Care
It marks a key altitude checkpoint on the approach where the pilot confirms position, checks altitude, and prepares for the next segment or possible missed approach.
Intuition Check
Do not read “compass” here as the magnetic compass on the panel. In this term, it means a radio beacon that direction-finding equipment can point toward.
Example Sentence 1
The approach chart showed an MCL co-located with the middle marker, giving us an ADF backup if we lost the localizer.
Example Sentence 2
Crossing the MCL, the pilot noted the altitude and began timing to the missed approach point.