Difference in the depth of modulation: Difference between revisions

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The '''difference in the depth of modulation''' ('''DDM''') is used by the [[instrument landing system]]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://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.}}</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 landing 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 the 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 course line, the difference in the depths of the modulation between the two frequencies is zero.

This difference is conventionally displayed by the deflection of a moving coil indicator or needle on an instrument known as a [[horizontal situation indicator]] (HSI).