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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~~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[[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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