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), used by pilots as a navigation aid for tracking inbound to the final approach course and for identifying the outer marker position.
Plain English
A small radio beacon placed at the outer marker of an ILS approach. Pilots can home in on its signal to help line up with the runway and to confirm they are passing the outer marker.
Context Anchor
Seen on instrument approach charts and in instrument approach discussions, especially with approaches that use a compass locator near the outer marker.
Derivation
Locator' comes from the Latin locare, meaning 'to place.' The beacon helps the pilot locate themselves along the approach. 'Outer' refers to its position farther from the runway than the middle marker -- it is the first of the marker positions a pilot crosses on the way in.
Why Pilots Care
It marks the final approach fix, allowing the pilot to start timing approach segments and begin descent.
Intuition Check
“Outer” does not just mean somewhere outside the airport. Here it means the locator positioned farther out on the final approach path, near the outer marker area.
Example Sentence 1
Cleared for the ILS approach, the pilot tuned the ADF to the outer locator and tracked inbound to intercept the final approach course.
Example Sentence 2
After crossing the outer locator the crew began timing the approach segment and started descent.