Definition
A low-power non-directional beacon (NDB) installed at the same site as the outer marker of an Instrument Landing System (ILS). It transmits a continuous radio signal that the aircraft's Automatic Direction Finder (ADF) can home to, helping the pilot intercept and track the localizer course inbound to the runway. The outer marker itself is a 75 MHz beacon that signals passage over a fixed point on the final approach course, typically 4–7 nautical miles from the runway threshold.
Plain English
A small radio beacon placed at the outer marker of an ILS approach. It gives the aircraft a signal to follow so the pilot can find and line up with the final approach path before reaching the runway.
Context Anchor
Seen on instrument procedure charts, chart legends, and route descriptions where the outer marker point is used as a named navigation fix.
Derivation
"Compass locator" describes its purpose — it helps the aircraft's direction-finding equipment locate a fixed point. "Outer marker" comes from its position: the outermost of the marker beacons along the ILS final approach course. Together, an LOM is the radio beacon that locates the outer marker for the pilot.
Why Pilots Care
It gives pilots a reliable way to confirm they have reached the outer marker so they can start timing segments of the approach or begin descent.
Intuition Check
Do not read “compass” here as the magnetic compass on the panel. In this term, it refers to a ground radio beacon that aircraft navigation equipment can point toward.
Example Sentence 1
Cleared direct to the LOM, the pilot tuned the ADF, identified the Morse code, and began tracking inbound on the localizer.
Example Sentence 2
On the approach plate the LOM was shown right at the outer marker for the ILS to runway 22.