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Difference in the depth of modulation

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Difference in the Depth of Modulation or DDM is a method used by the Instrument Landing System in conjunction with the associated airborne receiving equipment to define a position in airspace. DDM is usually expressed in percentage, however may also be expressed in microAmperes. Instrument Landing system ground stations provide radio frequency signals that vary linearly in the depth of modulation from the centre or Course Line at a rate of 0.00145% per metre. The two individual audio modulation frequencies and their associated sidebends are 150Hz and 90Hz.

Method

A Carrier and Sideband CSB, and Sideband Only SBO signal, transmitted from Localizer and Glidepath antennas produce a space modulated signal resulting from the vectorial addition of two or more audio signals that vary according to position of the receiving aircraft.

The difference between the two modulation depths produce an error current signal in the airborne receiver. When an aircraft follows the courseline, the difference between the two frequencies is zero percent (0%). The DDM for a Localizer at the outer extremity of the course sector is 15.5% or a current equivalent of 150 microAmperes.

This difference is traditionally displayed by the deflection of a moving coil indicator or needle/s on an instrument known as an horizontal situation indicator, HSI.

See also

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