Definition
A low-power, low- or medium-frequency non-directional radio beacon installed at the site of the outer marker of an Instrument Landing System (ILS). It transmits a continuous signal that pilots can home to using an Automatic Direction Finder (ADF), allowing the aircraft to be guided to the outer marker as part of the ILS approach.
Plain English
A small radio beacon located at the outer marker of an ILS approach. Pilots tune their ADF to it and use it to find and track inbound to the outer marker on the way to the runway.
Context Anchor
Seen on instrument approach charts, especially older instrument landing system procedures that include an outer marker or locator beacon.
Derivation
Locator' here means a beacon that helps the pilot locate a fixed point — in this case, the outer marker of the ILS. 'Outer' refers to its position farther from the runway than the middle marker. 'Compass' is historical: ADF needles point toward the beacon much like a compass needle, so beacons used this way were called compass locators.
Why Pilots Care
Gives a positive position fix so the pilot can confirm altitude and begin the descent on the glideslope with the correct timing.
Intuition Check
Do not read “compass” here as the airplane’s magnetic compass. An outer compass locator is a radio beacon that direction-finding equipment can point to; it is not the outer marker signal itself.
Example Sentence 1
The pilot tuned the ADF to the LOM frequency to track inbound to the outer marker during the ILS approach.
Example Sentence 2
At the LOM the aircraft was leveled at the published altitude before intercepting the localizer and glideslope.