The '''difference in the depth of modulation''' ('''DDM''') is used by [[instrument landing system]]s in conjunction with the associated airborne receiving equipment to define a position in airspace.<ref>[{{cite encyclopedia |url=http://www.answers.com/topic/difference-in-depth-modulation |dictionary=McGraw-Hill Dictionary of Scientific and Technical Terms.]"|entry=Difference in depth modulation: |quote=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.{{Citation needed|date=March 2023}} 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]].
