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>[http://www.answers.com/topic/difference-in-depth-modulation McGraw-Hill Dictionary of Scientific and Technical Terms.] "Difference in depth modulation: 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,howeverbut may also be expressed in microAmperesmicroamperes. Instrument Landinglanding system ground stations provide [[radio frequency]] signals that vary linearly in the depth of [[modulation]] from the centre or Coursecourse Lineline at a rate of 0.145% per metre {{Citation needed|date=August 2009}}. The two individual audio modulation frequencies and their associated sidebands are 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 half-scale deflection.
==Method==
A Modulationmodulation Depthdepth Comparisoncomparison Navigationalnavigational Aidaid (MDCNA), also known as an Instrument''instrument Landinglanding Systemsystem'' 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 [[Localizerlocalizer]] and [[Glidepathglidepath]] 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 between the two frequencies is zero percent (0%).
This difference is traditionally displayed by the deflection of a moving coil indicator or needle/s on an instrument known as ana [[horizontal situation indicator]], HSI.
