Locator Outer Marker (LOM) is a low/medium frequency NDB navigation aid that combines a marker beacon with a non-directional beacon to support ILS approaches. The beacon, traditionally termed a compass locator, transmits on 365 kHz and carries identification: the outer station sounds the first two letters of the localizer group, while the middle station sounds the last two letters. The collocated marker beacon, operating at 75 MHz, radiates 400 Hz tones in a pattern of two dashes per second, signalling to the pilot the glideslope intercept - or, equivalently, the Final Approach Fix - while the aircraft's marker receiver simultaneously delivers 3,000 Hz audio ‘dots’ to the headset.
What is a locator outer marker (LOM) in aviation?
The locator outer marker (LOM) is a navigation aid used as part of an instrument landing system (ILS) instrument approach for aircraft. In the United States, the outer marker is often combined with a non-directional beacon (NDB) to make a locator outer marker (LOM). LOMs are represented on aeronautical charts by a combined locator/marker symbol, and the system gives the pilot a visual blinking blue outer marker light.
The Locator Outer Marker (LOM) is a NAVAID used as part of the instrument landing system (ILS) instrument approach for aircraft. It denotes a specific point placed along the flight path of the final approach, where the aircraft is supposed to transition from en-route to approach phase while receiving both azimuth guidance from the localizer and range protection from the linked locator beacon. Functionally, the LOM comprises a marker beacon oriented perpendicularly to the flight path and a colocated non-directional beacon (NDB) that transmits on 365 kHz; this pairing supplies the dual principle data required by the category approach.
Approaching crew are alerted when they fly over it by the combination of cues that certify the fix. The first two letters of the localizer identification group in Morse. The LOM gives the pilot a visual (blinking blue outer marker light) indication on the cockpit marker panel together with an aural continuous series of audio tone dashes at 400 Hz precisely heard over the audio panel. These signals, together with the NDB that can be used to navigate directly to the location at distances of approximately 15 miles (24.1 km), confirm passage through the Final Approach Fix and support the pilot's check of crossing altitude. Because the outer beacon provides range information and fulfills a marker requirement, it remains a legal part of 14 CFR 91.175, even though newer RNAV systems with GPS, VOR intersections, DME or radar now substitute for it.
What is a compass locator at the outer marker?
A compass locator is a low-powered NDB station installed at the site of the outer marker of an ILS. It is a low- or medium-frequency radio beacon that transmits two-letter identification groups audibly in Morse code. The outer locator transmits the first two letters of the localizer identification group, and the beacon operates between 190 and 535 kHz with a range of at least 15 miles (24.1 km). Compass locators typically transmit at 25 watts or less and provide the same basic information to the pilot as a marker beacon.
What is LOM frequency aviation?
LOM frequency is the point where the 75 MHz marker beacon, modulated with two-letter Morse code at 1020 Hz, lies 4-7 NM from the runway threshold. The marker itself always transmits on 75 MHz, and its identity is announced by the same two-letter Morse code modulated at 1020 Hz. When the aircraft passes directly above, the receiver lights a three-lamp indicator and sounds a toggle-able aural warning, confirming that the outer marker has been crossed.
What is the difference between an outer marker and a locator outer marker?
The differences between an outer marker and a locator outer marker is that an outer marker is one of the Marker beacons (optional) that radiate only the 75 MHz carrier. Its installer plots the point at which NavaidComponent.markerPosition allows OUTER and connects a transmitter. When the facility radiates a carrier plus an associated low-power NDB bearing the same identifier, that combining facility is called a Locator Outer Marker (LOM). The co-sited NDB is identified by two or three letters, while the 75 MHz marker adds the usual dash-dash tone at 400 Hz. Therefore, the outer marker differs from the LOM solely in that it lacks the NDB: the presence of the low-power Non-Directional Beacon alongside the VHF marker transmitter forms the LOM, so both the range cue (600 m - 200 m) and the directional bearing can be obtained in one over-flight, whereas the marker-only device furnishes no bearing information and conveys only dasher identification.
What is the difference between a LOM and an LMM in aviation?
The difference between an LOM and an LMM lies in site and function. An LOM is an outer compass locator sited at the ILS outer marker' point, normally four to seven miles from the threshold. Its identifier is the two letters ‘RT'. An LMM is a middle compass locator co-installed with the middle marker, placed about 3,500 ft (1,067 m) from the runway end. Its identifier is two letters chosen to match any en-route fix with which it is co-located, for example ‘TH'. Both low-power low/medium frequency radio-beacons provide visual and audible cues to pilots and must each be assigned a name and code under FAA regulations, yet only the LMM doubles as the middle marker that indicates where the glide slope meets decision height, usually 200 ft (61 m) above touchdown zone elevation.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
