An (ILS) Instrument Landing System is a ground-based equipment that radiates guidance information to be received by an aircraft during final approach for correct and safe landing. It is composed of ground based equipments such as transmitter, combining/dividing network, antenna and monitors systems.
ILS have three subsytem: Localizer, Glidepath/Glideslope and Marker Beacons.
A.The vertical plane containing the runway centerline is defined by VHF transmitter called Localizer.
B.The horizontal plane of 2 to 4º of vertical angle above the horizontal plane is defined by a UHF transmitter called the Glide slope.
C.Distance information installed on a fixed distance reference points are provided by markers.
MARKER BEACONS
Beacons are AM transmitters operating at 75 MHz located at specific points along the landing path used to designate distance away from the threshold.
Operating Radio Frequency
75 MHz ± 0.01%
Modulating Frequency
Inner Marker (when installed) : 3000Hz
Middle Marker: 1300 Hz
Outer Marker: 400 Hz
Coverage
Inner Marker (when installed) 150 m ± 50 m
Middle Marker : 300 m ± 100 m
Outer Marker : 600 m ± 200 m
Identification Signal
Inner Marker (when installed): 6 dots per second continuously
Middle Marker: Series of dots and dashes
a. dashes keyed at a rate of 2 dashes/sec
b. dots keyed at a rate of 6 dots/sec
Outer Marker: 2 dashes/sec continuously
Monitoring
Automatic Monitoring System that transmit a warning signals to a control point during
a. Failure of modulation or keying
b. Reduction of Power output to less than 50%
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