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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 150[[Hertz|Hz]] and 90Hz.
==Method==
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