A locator outer marker, or LOM, is a navigation aid used as part of an instrument landing system (ILS) instrument approach for aircraft in the United States and other countries. An LOM is an non-directional beacon (NDB) co-located with the outer marker beacon in the ILS approach, so that aircraft can navigate directly to the location using the NDB as well as be alerted when they fly over it by the beacon. The LOM is becoming less important now that GPS navigation is well established in the aviation community. Canada has taken the process further and abandoned marker beacons altogether, replacing the outer marker in ILS approaches entirely with the NDB.
In the US, LOMs are identified by two-letter Morse code modulated at 1020 Hz. LOMs use the first two letters of the parent ILS's identification. For example, at New York's JFK runway 31R the ILS identifier is I-RTH and the LOM ident is RT. If this facility were a locator middle marker (LMM) its identifier would be the last two letters, TH.