T-6A INSTRUMENT NAVIGATION
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 of approximately 2400 feet in width and 4200
feet in length at 1000 feet above the antenna. 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 and the aural tones.
Ordinarily, there are two marker beacons associated with an ILS approach, the OM and MM.
Locations with a Category II ILS also have an Inner Marker (IM). When an aircraft passes over
a marker, the aircrew will receive the indications shown in Figure 1-2.
1. The OM normally indicates the position at which an aircraft at the appropriate altitude on
the LOC course will intercept the ILS glidepath.
2. The MM indicates a position approximately 3500 feet from the landing threshold. This is
also the position where an aircraft on the glidepath will be at an altitude of approximately 200
feet above the elevation of the touchdown zone.
3. The IM will indicate a point at which an aircraft is at a designated DH on the glidepath
between the MM and landing threshold.
Marker Passage Indications
Marker Code Light
●● ●● WHITE
Figure 1-2 Marker Passage Indications
The lowest authorized ILS minimums, with all required ground and airborne systems
components operative, are:
Category I - DH 200 feet and Runway Visual Range (RVR) 2400 feet (with touchdown
zone and centerline lighting, RVR 1800 feet).
Category II - DH 100 feet and RVR 1200 feet.
Special authorization and equipment required for Category II.
INTRODUCTION TO NAVIGATION SYSTEMS 1-9