Instrument Landing System ( ILS)
- ILS is the only Navigational Aid for Landing and final Approach having two transmitters installed at far end of the Runway.This is system that uses VHF & UHF radio waves. ILS has been in usage since 1946.This is only because of ILS system that today the AFCS has been very efficient even during automatic landing.
- Unlike ILS the VOR,ADF and DME are the navigational aid for airway or air route.
- The two subsystems of ILS systems are (1) Localizer (LOC) (2) Glide Slope (GS)
Localiser
The horizontal navigational aid for landing of and air craft is called localiser that sets the plane aligned with the center line of the runway.
The localizer transmits VHF with frequency range of 108-112 MHz with increment of 0.5 MHz.
The Localiser transmits two lobs let & right with 90 Hz & 150 Hz.The lobs cut each other or extended center line of the runway.the localiser have also there morse code.the lobs as shown as under:
The vertical navigational aid for landing is called glide slope(GS).Glide Slope transmit UHF of 328.6 - 335 Mhz.The glide slope frequency is automatically set when the crew tune the frequency of localier.
Glide Slope
Marker Beacons
In Instruments landing system the visual and audible cues to the crew are provided by an other very important navigational aid that is called "MARKER BEACONS".The marker beacons are sites at two or three distances on the extended center lines of the runway and provides the NavAid to confirm the crew of progress.They transmit 75 Mhz with there unique morse code with 3-4 W of power. The marker beacons are divided in to three categories such as (1) Outer Marker (2) Middle Marker (3) Inner Marker
(1) Outer Marker (OM): the outer marker is sited at 4 - 7 miles from runway thresholds.It Transmit dash dash(- -) morse code by operating at frequency of 400 Hz.When the aircraft approaches at Outer marker the Cyan light in on on ILS instrument and the OM icons gets high on the display.
(2) Middle Marker (MM): the milddle marker is sited at 3500 ft from runway thresholds.It Transmit alternating dash dot(- .) morse code by operating at frequency of 1300 Hz.When the aircraft approaches at Outer marker the Yellow light in on on ILS instrument and the MM icons gets high on the display.
(3) Inner Marker (MM): the inner marker is sited at 2000 ft from runway thresholds.It Transmit dot dot(..) morse code by operating at frequency of 3000 Hz.When the aircraft approaches at Outer marker the White light in on on ILS instrument and the IM icons gets high on the display.
