Definition
A low-power non-directional radio beacon (NDB) installed at the site of the outer marker of an Instrument Landing System (ILS), used to help aircraft find and intercept the ILS final approach course. It transmits a continuous signal that an aircraft's Automatic Direction Finder (ADF) can home on, and is typically usable within about 15 nautical miles.
Plain English
A small radio beacon placed at the outer marker of an ILS approach. The aircraft's direction-finding receiver can point at it, helping the pilot navigate to and line up with the runway's instrument approach.
Context Anchor
Seen on instrument approach charts and used during some instrument approaches to a runway.
Derivation
"Compass" here refers to the ADF, an older bearing instrument sometimes called the radio compass. "Locator" means it helps the aircraft locate the approach course. "Outer" places it at the outer marker — the farther of the two marker beacons along the ILS approach path.
Why Pilots Care
It marks the final approach fix and supplies directional guidance so the pilot can confirm position and begin the final descent segment safely.
Intuition Check
Do not read “compass” here as the magnetic compass in the cockpit. An Outer Compass Locator is a ground-based radio beacon; “outer” means it is associated with the outer marker area of the approach.
Example Sentence 1
The pilot tuned the ADF to the outer compass locator to help intercept the ILS final approach course.
Example Sentence 2
Passing the outer compass locator confirmed the aircraft was five miles from the runway.