→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 encyclopedia |title=Answers - 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 |url=http://stinetwww.dticanswers.milcom/oaitopic/oai?verbdifference-in-depth-modulation |quote=getRecord&metadataPrefix=html&identifier=ADA138301In 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,howeverbut may also be expressed in microAmperesmicroamperes.{{Citation Instrumentneeded|date=March Landing2023}} systemThe groundtwo stationsindividual provideaudio [[radio frequencymodulation]] signalsfrequencies thatand varytheir linearlyassociated insidebands theare depth90 ofand 150 [[modulationHertz|Hz]]. fromThe theDDM centrefor ora Course Linelocalizer at athe outer rateextremity of 0.00145%the percourse metre.sector Theis two15.5% individualor audioan modulationelectric frequenciescurrent andequivalent theirof associated150 sidebandsmicroamperes are 150[[Hertz|Hzfull scale deflection]] and 90Hz.
==Method==
A [[Modulationmodulation Depthdepth Comparisoncomparison Navigationalnavigational Aid]]aid (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]] and [[Sideband]] CSB, and Sideband Only [[Double-sideband_suppressed-carrier_transmission|SBO]] signal, transmitted from [[Localizer]] and [[Glidepath]] 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 courseline, the difference between the two frequencies is zero percent (0%). The DDM for a Localizer at the outer extremity of the course sector is 15.5% or a current equivalent of 150 microAmperes. ▼
This difference is traditionally displayed by the deflection of a moving coil indicator or needle/s on an instrument known as an [[horizontal situation indicator]], HSI.▼
▲A [[Carrier wave|carrier]] and [[Sidebandsideband]] (CSB), and Sideband Only [[Double-sideband_suppressedsideband suppressed-carrier_transmissioncarrier 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 betweenin the twodepths frequencies is zero percent (0%). The DDM for a Localizer atof the outermodulation extremity ofbetween the coursetwo sectorfrequencies is 15zero.5% or a current equivalent of 150 microAmperes.
▲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).
