The difference in the depth of modulation (DDM) is used by
instrument landing systems 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. The two individual audio
modulation
Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information.
The process encodes information in form of the modulation or message ...
frequencies and their associated sidebands are 90 and 150
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
Space modulation is a radio amplitude modulation technique used in instrument landing systems (ILS) that incorporates the use of multiple antennas fed with various radio frequency powers and phases to create different depths of modulation withi ...
to provide guidance to aircraft when on final approach.
A
carrier and
sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands c ...
(CSB), and
sideband only (SBO) signal, transmitted from
localizer and
glide path
In aviation, instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the ai ...
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 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
The horizontal situation indicator (commonly called the HSI) is an aircraft flight instrument normally mounted below the Attitude indicator, artificial horizon in place of a conventional heading indicator. It combines a heading indicator with a V ...
(HSI).
See also
*
Amplitude modulation
Amplitude modulation (AM) is a signal modulation technique used in electronic communication, most commonly for transmitting messages with a radio wave. In amplitude modulation, the instantaneous amplitude of the wave is varied in proportion t ...
*
International Civil Aviation Organization
The International Civil Aviation Organization (ICAO ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international sch ...
(ICAO)
*
Radio navigation
Radio navigation or radionavigation is the application of radio waves to geolocalization, determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of Radiodetermination-satellite servi ...
References
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Radio navigation