Whle reading about ILS system used for aircraft I came across the term "Difference in the Depth of Modulation".

Yet the 90 Hz and 150 Hz modulations are exactly equal, about 20%, when the aircraft is flying correctly.
The difference in the detected signals is close to zero.

I realized that the antennae arrangement woulds cause signal received by an aircraft that was slightly off course would cause ( say)the 90 Hz RF to be stronger than the
150 Hz RF . A good airborne receiver detects the 20% modulation of 90 Hz RF stronger than the 20% modulation of the 150Hz.

In spite of precisely the same degree of modulation in the
90 Hz and 150 hz RF signals , the RF signals become unbalanced when the airborne receiver is slightly off course.
This slight RF unbalance results in the 90 Hz demoduated signal to be stronger than the 150 Hz signal.

Correct English for the ILS would be "Difference in Detected Demodulation"
INCORRECT name used by all
"Difference in the Depth of Modulation".
There IS NO DIFFERENCE IN THE 20% AM of the RF !

martymarty

When I was struggling to understand the concept of DDM, I happened to view 'martymarty's' reply.
I two hundred percent agree with martymarty's explanation.
I find this technically sound.

Nanda

Sir, why is it that they are using 90Hz and 150Hz as audio signals rather than the other?

I am just guessing why 90 hz and 150 hz were chosen as the modulating frequencies for each of the two transmitting antennae.

90 Hz and 150 Hz are symmetrically spaced with regard to the
120 hz : the 2nd harmonic of 60 Hz the power line
frequency of the electrical system in the US.

Any unintended, incidental modulation of the RF by the 2nd harmonic of the 60 hz power line would be more easily balanced by the aircraft receivers filters and the RF demodulation circuits.


This is only my guess though.

martymarty

Does anyone knows why the the localizer signals use only 20 percent modulation depth while the glideslope uses 40 percent?