Avionic standards
R&S
®
SMA100B
130User Manual 1178.3834.02 ─ 09
Figure 6-1: Approach navigation using instrument landing system (ILS) [1MA193]
An ILS system consists of three independent subsystems:
●
A glide slope for vertical guidance.
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A localizer for horizontal guidance.
●
(optional) marker beacons
Glide Slope
The glide slope transmitter is located near the end of the runway (nearest to the start of
the aircraft approach).
Typically, vertically aligned antennas transmit two intersecting main beams on top of
one another at carrier frequencies between 329 MHz and 335 MHz (see Table 6-2).
The top beam is usually modulated at 90 Hz and the beam below at 150 Hz [1MA193].
The information on position is provided after demodulation of the beam signals by eval-
uating the difference in depth of modulation (DDM). The following scenarios are possi-
ble:
●
Predominance of the 90 Hz beam: the aircraft is too high and must descend
●
Predominance of the 150 Hz beam: the aircraft is too low and needs to climb
●
The signal strength from both beams is equal: the aircraft is in the center, on the
right course.
If there is a predominance of the 90 Hz beam, then the aircraft is too high and must
descend. A predominant 150 Hz means that the aircraft is too low and needs to climb.
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