→Method: added link to https://en.wikipedia.org/wiki/Instrument_landing_system
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The '''Differencedifference in the Depthdepth of Modulationmodulation''' or ('''DDM''') is a method used by the [[Instrumentinstrument Landinglanding Systemsystem]]s in conjunction with the associated airborne receiving equipment to define a position in airspace.<ref>{{cite DDMencyclopedia is|title=Answers usually- expressedthe inMost percentage,Trusted howeverPlace mayfor alsoAnswering beLife's expressedQuestions |entry=Difference in microAmperesthe depth of modulation |dictionary=McGraw-Hill Dictionary of Scientific and Technical Terms |url=http://www.answers.com/topic/difference-in-depth-modulation Instrument|quote=In Landingdirective systemsystems groundemploying stationsoverlapping providelobes [[radiowith frequency]]modulated signals, thata varyratio linearlyobtained inby subtracting from the depthpercentage of [[modulation]]fromof the centrelarger orsignal Coursethe Linepercentage atof a ratemodulation of 0the smaller signal and dividing by 100.145%}}</ref> perDDM metreis usually expressed in percentage but may also be expressed in microamperes.{{Citation needed|date=AugustMarch 20092023}}. The two individual audio [[modulation]] frequencies and their associated sidebands are 90 and 150 [[Hertz|Hz]] and 90Hz. The DDM for a Localizerlocalizer at the outer extremity of the course sector is 15.5% or an electric current equivalent of 150 microAmperesmicroamperes [[full half-scale deflection]].
==Method==
A Modulationmodulation Depthdepth Comparisoncomparison Navigationalnavigational Aidaid (MDCNA), also known as an [[Instrument_landing_system|Instrument Landing System]], uses the concept of [[space modulation]] to provide guidance to aircraft when on final approach.
A [[Carrier wave|Carriercarrier]] and [[Sidebandsideband]] (CSB), and Sideband Only [[Double-sideband suppressed-carrier transmission|SBOsideband only]] (SBO) signal, transmitted from [[LocalizerInstrument landing system localizer|localizer]] and [[GlidepathInstrument 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.
The difference between the two modulation depths produce an error current signal in the airborne receiver. When an aircraft follows the courselinecourse line, the difference in the depths of the modulation between the two frequencies is zero percent (0%).
This difference is traditionallyconventionally displayed by the deflection of a moving coil indicator or needle/s on an instrument known as ana [[horizontal situation indicator]], (HSI).
