The ILS may contain three marker beacons: inner, middle and outer.
The marker beacons are located at specified intervals along the ILS approach and are identified by discrete audio and visual characteristics. Here’s a 3D visual.
- OM: Blue light, 400 Hz two dashes/second
- MM: Amber light, 1300 Hz Alternate dot and dash
- IM: White light, 3000 Hz only dots
The tone becomes quicker and higher as you get closer to the airport.
The OM, 4 to 7 NM from the runway threshold, normally indicates where an aircraft intercepts the glide path when at the published altitude.
The MM, 3500 feet from the runway threshold, is the Decision Height point for a normal ILS approach. On glide path at the MM an aircraft will be approximately 200 feet above the runway.
The IM. 1000 feet from the runway threshold, is the Decision Height point for a Category II approach.
References: “The Instrument Landing System”