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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 [[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 course 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 a [[horizontal situation indicator]], HSI.
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