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{{technical}}
The '''difference in the depth of modulation''' or '''DDM''' is used by [[instrument landing system]]s in conjunction with the associated airborne receiving equipment to define a position in airspace.<ref>[http://www.answers.com/topic/difference-in-depth-modulation McGraw-Hill Dictionary of Scientific and Technical Terms.] "Difference in depth modulation: In directive systems employing overlapping lobes with modulated signals, a ratio obtained by subtracting from the percentage of modulation of the larger signal the percentage of modulation of the smaller signal and dividing by 100."</ref> DDM is usually expressed in percentage but 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 0.145% per metre {{Citation needed|date=August 2009}}. The two individual audio modulation frequencies and their associated sidebands are 150[[Hertz|Hz]] and 90Hz. The DDM for a localizer at the outer extremity of the course sector is 15.5% or an electric current equivalent of 150 microamperes half-scale deflection.
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