Definition
A low-power, low- or medium-frequency non-directional radio beacon (NDB) installed at the site of an ILS outer marker. It transmits an identifier consisting of the first two letters of the ILS localizer identification, allowing pilots to home, track, or hold using an automatic direction finder (ADF) as part of the published instrument approach.
Plain English
A small radio beacon placed at the outer marker of an ILS approach. Pilots can use it with their ADF to find the start of the approach, fly inbound, or hold there.
Context Anchor
Seen on instrument approach charts and in instrument approach procedure information for some ILS approaches.
Derivation
"Compass locator" reflects its original purpose: helping pilots locate and orient (like using a compass) to the outer marker fix using ADF. "LO" is simply the chart shorthand for that beacon.
Why Pilots Care
It provides reliable navigation guidance to establish the aircraft on the proper ILS descent path at the outer marker.
Intuition Check
Do not read LO as a general word meaning “low.” In this context, LO names a specific radio locator associated with the outer marker of an ILS approach.
Example Sentence 1
The approach plate showed an LO at the outer marker, so the pilot tuned the ADF to its frequency and identified it before commencing the approach.
Example Sentence 2
LO signals allow the crew to cross-check position before intercepting the glideslope.