Difference in the Depth of Modulation
Encyclopedia
The difference in the depth of modulation or DDM is used by instrument landing system
Instrument Landing System
An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

s in conjunction with the associated airborne receiving equipment to define a position in airspace. DDM is usually expressed in percentage but may also be expressed in microamperes. Instrument landing system ground stations provide radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 signals that vary linearly in the depth of modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 from the centre or course line at a rate of 0.145% per metre . The two individual audio modulation frequencies and their associated sidebands are 150Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 and 90Hz. The DDM for a localizer at the outer extremity of the course sector is 15.5% or an electric current equivalent of 150 microamperes half-scale deflection.

Method

A modulation depth comparison navigational aid (MDCNA), also known as an instrument landing system uses the concept of space modulation
Space modulation
Space modulation is a radio Amplitude Modulation technique used in Instrument Landing Systems that incorporates the use of multiple antennas fed with various radio frequency powers and phases to create different depths of modulation within various volumes of three-dimensional airspace...

 to provide guidance to aircraft when on final approach.

A carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...

 and sideband
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, containing power as a result of the modulation process. The sidebands consist of all the Fourier components of the modulated signal except the carrier...

 CSB, and sideband only
Double-sideband suppressed-carrier transmission
Double-sideband suppressed-carrier transmission : transmission in which frequencies produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally completely suppressed.In the double-sideband...

 (SBO) signal, transmitted from localizer
Localizer
In aviation, a localizer is one of the components of an Instrument Landing System , and it provides runway centerline guidance to aircraft. In some cases, a course projected by localizer is at an angle to the runway . It is then called a Localizer Type Directional Aid...

 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
Horizontal Situation Indicator
The horizontal situation indicator is an aircraft instrument normally mounted below the artificial horizon in place of a conventional heading indicator. It combines a heading indicator with a VOR/ILS display, reducing pilot workload by lessening the number of elements in the pilot's instrument...

, HSI.

See also

  • Instrument landing system
    Instrument Landing System
    An instrument landing system is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument...

  • Amplitude modulation
    Amplitude modulation
    Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...

  • Space modulation
    Space modulation
    Space modulation is a radio Amplitude Modulation technique used in Instrument Landing Systems that incorporates the use of multiple antennas fed with various radio frequency powers and phases to create different depths of modulation within various volumes of three-dimensional airspace...

  • Radio navigation
    Radio navigation
    Radio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...

  • ICAO
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