Instrument
Landing Systems (ILS)
An Instrument Landing System (ILS) is used to guide an aircraft in night or low visibility conditions during landing.
This radio navigation system directs the aircraft down a slope to the runway's touch down position. With an ILS, multiple radio broadcasts are used to provide a precise approach to landing. One of the radio transmissions is a localizer. Its purpose is to provide horizontal guidance to the runway's center line. The Aircraft is guided vertically down the proper slope to the touch down spot by a separate glideslope broadcast. The pilot can interrupt the approach navigational aid system by using compass locator transmissions for outer and center approach marker beacons. Marker beacons indicate the distance from the runway. All of these radio signals work together to make an ILS a very accurate and reliable means of landing aircraft.
Localizer
frequencies (108 MHz–111.95 MHz). Beyond the far end of the approach runway, a horizontally polarised antenna complex generates two modulated signals. They built a 212° wide (1,500 foot) extending field 5 miles from the runway. Near the landing threshold. the field tapers to runway width. A VHF carrier wave modified with a 90 Hz signal fills the left part of the approach area. A 150 MHz modulated signal can be found on the right side of the approach. The localizer VHF frequency, which can be found on published approach plates and aviation charts, is tuned into the aircraft's VOR
receiver.
The
receiver uses localizer circuitry and components common to both, while the
circuitry particular to regular VOR
reception is turned off. The received
signals are filtered and rectified into DC before being used to drive the course
deviation indicator. The CDI of the VOR/ILS display deflects to the left if the
aircraft gets a 150 Hz signal. The runway is to the left, as indicated by this
symbol. With a left turn, the pilot must correct his course. This aligns the
course deviation indicator on the display and the aircraft with the runway centerline. When the VOR receiver receives a 90 Hz signal, the CDI deflects to
the right. To align the CDI and the aircraft with the runway centre line, the
pilot must turn to the right.
Glideslope
The
glideslope transmitter antenna is polarized horizontally. The transmitting
frequency range is 329.3 MHz to 335.0 MHz in UHF. The frequency is matched to
the ILS's localizer frequency. The glideslope receiver is automatically tuned
when the VOR/ILS receiver is tuned for the approach. The glideslope, like the
localizer, sends out two signals, one modulated at 90 Hz and the other at 150
Hz. The glideslope receiver decodes the signals in the same way that the
localizer receiver does. It controls the glideslope indicator, which is a
vertical course deviation indicator. The glideslope indicator works in the same
way as the localizer CDI, but at a 90-degree angle. Regardless of the type of
instrumentation in the aircraft, the VOR/ILS localizer CDI and glideslope are
displayed together. The glideslope antenna for aircraft reception comes in a variety
of shapes and sizes. A single dipole antenna mounted inside the aircraft's nose
is a popular choice. Glideslope reception has been incorporated into the same
dipole antenna used for VHS VOR
/ILS localizer reception by antenna
manufacturers. Antennas with blades are also used.
Compass Locators
It is critical for a pilot to be able to intercept the ILS in order to use it. A compass locator is a transmitter that has been designed specifically for this purpose. One is usually found 4–7 miles from the runway threshold, at the outer marker beacon. Another could be found about 3,500 feet from the threshold, at the middle marker beacon. The compass locator on the outer marker is a 25 watt NDB with a range of about 15 miles. It sends out omnidirectional LF radio waves (190 Hz to 535 Hz) that are keyed with the ILS identifier's first two letters. The locator is intercepted using the ADF receiver; therefore, no further equipment is necessary. If a middle marker compass locator is present, it functions similarly but is identified by the ILS identifier's last two letters. The pilot directs the aircraft to fly down the glidepath to the runway once it has been spotted.
Marker Beacons
There is also a middle marker beacon. It is roughly 3,500 feet from the runway on approach. It also broadcasts at a frequency of 75 MHz. To avoid being confused with the all-dash tone of the outer marker, the middle marker broadcast is modulated with a 1300 Hz tone that is a series of dots and dashes. When the signal is received, an amber-colored light on the instrument panel is illuminated by the receiver.
An inner marker beacon is used in some ILS approaches, and it emits a signal modulated at 3000 Hz in a series of dots only. It's near the runway threshold, near the land-or-go-around decision point of the approach. If the signal is present, it is utilized to illuminate a white light on the instrument panel when it is received. The three marker beacon lights are normally included in the audio panel of a general aviation aircraft, but bigger aircraft may have them separately. Marker lights or indications are frequently located near the glideslope display near the attitude director indicator on electronic display aircraft.
Also Read
Flight Director System Operations | Auto Flight Control Mode
VOR (VHF Omnidirectional Range) Navigation System Overview
D
istance Measuring equipment (DME) Overview & functions
ATC Transponder (ATCRBS) | Aircraft Radar Beacon Transponder
Altitude Encoder | Encoding Altimeter Function & uses
Radio Altimeter | Overview of Radio Altimeter
Hot Start & Hung Start | Abnormal start of Turbine Engine
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