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According to the localiser principle of operation in an ILS system, the difference in depth of modulation (DDM):

1. decreases with respect to the angular displacement from the centreline.
2. increases with right displacement from the centreline.
3. decreases with left displacement from the centreline.
4. increases linearly with displacement from the centreline.

The combination regrouping all the correct statements is…

  • A
    2 and 3.
  • B
    2 and 4.
  • C
    1 and 4.
  • D
    1 and 3.

Refer to figure.

The Difference in Depth Modulation (DDM) is a principle used by the ILS to define a position in an airspace. As seen in the figure, the glide path is indicated by 2 different lobes. A 90 Hz lobe and a 150 Hz lobe.

  • When the airborne receiver receives a stronger signal from the 90 Hz lobe than the 150 Hz lobe, it means that the aircraft is above the ‘ideal’ glideslope.

  • When the airborne receiver receives a stronger signal from the 150 Hz lobe than the 90 Hz lobe, the aircraft will be below the ‘ideal’ glideslope.

  • When the both the signals received at the same strength it means the aircraft will be on the ‘ideal’ glideslope and the glideslope needle will indicate zero.

Assume that you are on the centreline, if you move away from the centrelline, however, still in the range of the localizer the DDM will increase.

DDM = AM (90 hz) - AM (150 hz) / 100

So once you go off centreline, either the 90 or 150 increases, causing DDM to increase (negative is also assumed to be an increase).

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