[...] One is modulated at 90 Hz, the other at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.
The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.
If there is a predominance of either 90 Hz or 150 Hz modulation, the aircraft is off the centerline.
|Quoting 9VSIO (Reply 3):|
Would this be why you always intercept the GS from below? To avoid the false GS?
|Quoting horstroad (Reply 1):|
the transmitted frequencies have the shape of a lobe. there are 4 lobes, 2 for glide slope, 2 for localizer, each a pair of 90Hz and 150Hz modulated.
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