Federal Aviation Administration Twentieth Edition
7.4.3 The glide path projection angle is normally adjusted to 3 degrees above horizontal so that it intersects the middle marker at about 200 feet and the outer marker at about 1,400 feet above the runway elevation. The glide slope is normally usable to the distance of 10 NM. However, at some locations, the glide slope has been certified for an extended service volume which exceeds 10 NM.
7.4.4 Pilots must be alert when approaching glidepath interception. False courses and reverse sensing will occur at angles considerably greater than the published path.
7.4.5 Make every effort to remain on the indicated glide path. Exercise caution: avoid flying below the glide path to assure obstacle/terrain clearance is maintained.
REFERENCE.
14 CFR Section 91.129(e).
7.4.6 A glide slope facility provides descent information for navigation down to the lowest authorized decision height (DH) specified in the approved ILS approach procedure. The glidepath may not be suitable for navigation below the lowest authorized DH and any reference to glidepath indications below that height must be supplemented by visual reference to the runway environment. Glide slopes with no published DH are usable to runway threshold.
7.4.7 The published glide slope threshold crossing height (TCH) DOES NOT represent the height of the actual glide slope on course indication above the runway threshold. It is used as a reference for planning purposes which represents the height above the runway threshold that an aircraft’s glide slope antenna should be, if that aircraft remains on a trajectory formed by the four.mile.to.middle marker glidepath segment.
7.4.8 Pilots must be aware of the vertical height between the aircraft’s glide slope antenna and the main gear in the landing configuration and, at the DH, plan to adjust the descent angle accordingly if the published TCH indicates the wheel crossing height over the runway threshold may be satisfactory. Tests indicate a comfortable wheel crossing height is approximately 20 to 30 feet, depending on the type of aircraft.
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 effects 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.
7.5 Distance Measuring Equipment (DME)
7.5.1 When installed with an ILS and specified in the approach procedure, DME may be used:
7.5.1.1 In lieu of the outer marker.
7.5.1.2 As a back course final approach fix.
7.5.1.3 To establish other fixes on the localizer course.
7.5.2 In some cases, DME from a separate facility may be used within Terminal Instrument Procedures (TERPS) limitations:
7.5.2.1 To provide ARC initial approach segments.
7.5.2.2 As a final approach fix for back course approaches.
7.5.2.3 As a substitute for the outer marker.
7.6 Marker Beacon
7.6.1 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.
7.6.2 Ordinarily, there are two marker beacons associated with an ILS, the outer marker (OM) and the middle marker (MM). Locations with a Category II or III ILS also have an inner marker (IM). When an aircraft passes over a marker, the pilot will receive the following indications:
7.6.3 The OM normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path.

7.6.4 The MM indicates a position approximately 3,500 feet from the landing threshold. This will also be 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.
7.6.5 The IM indicates a point at which an aircraft is at a designated decision height (DH) on the glide path between the middle marker and landing threshold.
7.6.6 A back course marker, normally indicates the ILS back course final approach fix where approach descent is commenced.
TBL ENR 4.1.1
Marker Passage Indications
Marker  Code  Light 
OM  . . .  BLUE 
MM   .  .  AMBER 
IM       WHITE 
　
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