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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 |urltitle=http://www.answers.com/topic/differenceAnswers - the Most Trusted Place for Answering Life's Questions |entry=Difference in- the depth- of modulation |dictionary=McGraw-Hill Dictionary of Scientific and Technical Terms |entryurl=Difference http://www.answers.com/topic/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. |title=Answers - the Most Trusted Place for Answering Life's Questions }}</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&nbsp;[[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]].
 
==Method==
A modulation depth comparison navigational aid (MDCNA), also known as an ''instrument[[Instrument_landing_system|Instrument landingLanding system''System]], uses the concept of [[space modulation]] to provide guidance to aircraft when on final approach.
 
A [[Carrier wave|carrier]] and [[sideband]] (CSB), and [[Double-sideband suppressed-carrier transmission|sideband only]] (SBO) signal, transmitted from [[Instrument landing system localizer|localizer]] and [[Instrument landing system glide path|glide path]] 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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