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 meter.{{Citation needed|date=AugustMarch 20092023}} The two individual audio [[modulation]] frequencies and their associated sidebands are 90 and 150 [[Hertz|Hz]]. The DDM for a localizer at the outer extremity of the course sector is 15.5% or an electric current equivalent of 150 microamperes [[full scale deflection]].
