35 °  10 °  10 °  10 NM  35 °  18 NM  Figure 9-34. Localizer coverage limits. its frequency for use by the associated ATC facility in issuing  approach and landing instructions. Unlike the localizer, the GS transmitter radiates signals only  in the direction of the final approach on the front course. The  system provides no vertical guidance for approaches on the back  course. The glidepath is normally 1.4° thick. At 10 NM from  the point of touchdown, this represents a vertical distance of  approximately 1,500 feet, narrowing to a few feet at touchdown. The localizer course is very narrow, normally 5°. This  results in high needle sensitivity. With this course width,  a full-scale deflection shows when the aircraft is 2.5° to  either side of the centerline. This sensitivity permits accurate  orientation to the landing runway. With no more than one- quarter scale deflection maintained, the aircraft will be  aligned with the runway. Two VHF marker beacons, outer and middle, are normally  used in the ILS system.  [Figure 9-35]  A third beacon, the  inner, is used where Category II operations are certified. A  marker beacon may also be installed to indicate the FAF on  the ILS back course. GS describes the systems that generate, receive, and indicate  the ground facility radiation pattern. The glidepath is the  straight, sloped line the aircraft should fly in its descent from  where the GS intersects the altitude used for approaching the  FAF to the runway touchdown zone. The GS equipment is  housed in a building approximately 750 to 1,250 feet down  the runway from the approach end of the runway and between  400 and 600 feet to one side of the centerline. The course projected by the GS equipment is essentially the  same as would be generated by a localizer operating on its  side. The GS projection angle is normally adjusted to 2.5°  to 3.5° above horizontal, so it intersects the MM at about  200 feet and the OM at about 1,400 feet above the runway  elevation. At locations where standard minimum obstruction  clearance cannot be obtained with the normal maximum GS  angle, the GS equipment is displaced farther from the approach  end of the runway if the length of the runway permits; or the GS  angle may be increased up to 4°. The OM is located on the localizer front course 4–7 miles  from the airport to indicate a position at which an aircraft, at  the appropriate altitude on the localizer course, will intercept  the glidepath. The MM is located approximately 3,500 feet  displacement.  Figure 9-35. Localizer receiver indications and aircraft displacement. 2.5°  2.5°  Outer Marker Beacons  Middle Marker Beacons  ´  ´  9-37 Glideslope (GS) Marker Beacons