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What is measured in order to establish aircraft position in relation to the localiser beam on an ILS?
• A
The difference in depth between the 90 Hz modulation and the 150 Hz modulation.
• B
The difference in time between the 90 Hz modulation and the 150 Hz modulation.
• C
The difference in phase between the 90 Hz modulation and the 150 Hz modulation.
• D
The bearing to the localiser antenna found by means of a loop antenna.

Refer to figure.

The Difference in Depth Modulation (DDM) is a principle used by the ILS to define a position in an airspace. The deviation detection of the aircraft from the desired track is based on two different overlapping lobes. These lobes are radiated by both the localiser and the glide path antennas. These consist of a 90 Hz lobe and a 150 Hz lobe.

For the glide path:

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

For the localiser, the same method is used:

• When the airborne receiver receives a stronger signal from the 90 Hz lobe than the 150 Hz lobe, it means that the aircraft is to the left of the localiser and needs to fly right.
• When the airborne receiver receives a stronger signal from the 150 Hz lobe than the 90 Hz lobe, the aircraft will be to the right of the localiser and needs to fly left.
• When both signals are received at the same strength it means the aircraft will be on the centreline and the localiser needle will indicate zero.

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• Greece
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