AIM 8/15/19  1/30/20 12/31/20 NOTE − The TCH for a runway is established based on several factors including the largest aircraft category that normally uses the runway, how airport layout affects the glide slope antenna placement, and terrain. A higher than optimum TCH, with the same glide path angle, may cause the aircraft to touch down further from the threshold if the trajectory of the approach is maintained until the flare. Pilots should consider the effect of a high TCH on the runway available for stopping the aircraft. 1. When installed with the ILS and specified in the approach procedure, DME may be used: (a) In lieu of the OM; (b) As a back course (BC) final approach fix (FAF); and (c) To establish other fixes on the localizer course. 2. In some cases, DME from a separate facility may be used within Terminal Instrument Procedures (TERPS) limitations: (a) To provide ARC initial approach seg- ments; (b) As a FAF for BC approaches; and (c) As a substitute for the OM. (b) The MM indicates a position approxi- mately 3,500 feet from the landing threshold. This is also the position where an aircraft on the glide path will be at an altitude of approximately 200 feet above the elevation of the touchdown zone. A MM is no longer operationally required. There are some MMs still in use, but there are no MMs being installed at new ILS sites by the FAA; and (c) An IM, where installed, indicates the point at which an aircraft is at decision height on the glide path during a Category II ILS approach. An IM is only required for CAT II operations that do not have a published radio altitude (RA) minimum. 1. ILS marker beacons have a rated power output of 3 watts or less and an antenna array designed to produce an elliptical pattern with dimensions, at 1,000 feet above the antenna, of approximately 2,400 feet in width and 4,200 feet in length. Airborne marker beacon receivers with a selective sensitivity feature should always be operated in the “low” sensitivity position for proper reception of ILS marker beacons. 2. ILS systems may have an associated OM. An MM is no longer required. Locations with a Category II ILS also have an Inner Marker (IM). Due to advances in both ground navigation equipment and airborne avionics, as well as the numerous means that may be used as a substitute for a marker beacon, the current requirements for the use of marker beacons are: (a) An OM or suitable substitute identifies the Final Approach Fix (FAF) for nonprecision approach (NPA) operations (for example, localizer only); and 2. Compass locators transmit two letter identi- fication groups. The outer locator transmits the first two letters of the localizer identification group, and the middle locator transmits the last two letters of the localizer identification group. 1. Compass locator transmitters are often situated at the MM and OM sites. The transmitters have a power of less than 25 watts, a range of at least 15 miles and operate between 190 and 535 kHz. At some locations, higher powered radio beacons, up to 400 watts, are used as OM compass locators. 3. A back course marker normally indicates the ILS back course final approach fix where approach descent is commenced. e. Distance Measuring Equipment (DME) f. Marker Beacon h. ILS Frequency  (See TBL 1 − 1 − 4.) g. Compass Locator TBL 1 − 1 − 3 Marker Passage Indications TBL 1 − 1 − 4 Frequency Pairs Allocated for ILS Marker Code Light OM GLYPH<C0042> GLYPH<C0042>  GLYPH<C0042> BLUE MM GLYPH<C0068> GLYPH<C0042> GLYPH<C0068>  GLYPH<C0042> AMBER IM GLYPH<C0068>  GLYPH<C0068>  GLYPH<C0068>  GLYPH<C0068> WHITE BC GLYPH<C0068>  GLYPH<C0068>  GLYPH<C0068>  GLYPH<C0068> WHITE Localizer MHz Glide Slope 108.10 334.70 108.15 334.55 108.3 334.10 108.35 333.95 108.5 329.90 108.55 329.75 108.7 330.50 Navigation Aids 1 − 1 − 13