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Line 1: '''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 in the depth of [[modulation]] between the two individual audio modulation frequencies of 150[[Hertz\|Hz]] and 90Hz. A [[Carrier]] and [[Sideband]] CSB, and Sideband Only [[Double-sideband_suppressed-carrier_transmission\|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.

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