ILS (INSTRUMENT LANDING SYSTEM)

ILS (Instument Landig System) is a ground-based instrument approach system that provides precision guidence 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 meteorological condition (IMC), suh as low ceillings or reduced visibility due to fog, rain, or blowing snow

ILS facilities are a high accurate and dependable means of navigating to runway in IFR condition. When using ILS, the pilot determines aircraft position primarily by reference to instrument. The system may be devided functionally into three parts:

a. LOCALIZER - Guidance information to centreline runway

b. GLDEPATH - Guidence information of landing angle

c. MARKER - Provide Range information From Threshold

A). LOCALIZER

The primary component of the ILS is the localizer, which provides lateral guidance/give information centreline runway to the aircraft. The localizer transmitter operates on VHF frequency within the frequency range of 108.10 to 111.95 MHz

The signal transmitteed by localizer consists of two vertical fan-shaped patterns that overlap, at the centre (see Localizer Signal Pattern). They are aligned with the extended centreline of the runway. The right side of pattern,as seen from ILS antene is modulated at 90 Hz and is called "yellow" area. The left side of the pattern i modulated at 150 Hz and is called the "blue" area. The overlap between two ares provide s the on-track signal. Modulation depth on-course 20 percent for each frequency code identification (1020Mhz, 5 percent) and radiate about 100 watts power ( Horisontal plarization)

B). GLIDEPATH
Glidepath transmitter operating in UHF frequency within frequency range 329.15 to 335.00 MHz radiates its signals in the direction of the localizer front course. The Glidepah give guidence information of landing angle to the aircraft.

Like the localizer, the glidepth signal consist of two overlapping beams modulated at 90 Hz and 150 Hz. Modulation depth on course 40 percent for each frequency and radiated about 5 watt power. Unlike the localizer, however, these signal are aligned above each other and are radiated primarily along the approach track. The thicness of the overlap area is 1.4 degree or 0.7 degree above and 0.7 degree below the optimum glidepath.

The Gladepath signal may be adjusted between 2 degree and 4.5 degree above a horisontal plane. A typical adjustment is 2.5 degree to 3 degree, depanding upon such factors as obstructions alng the approach path and the runway slope. The standar Glidepath angel is 3.0 degree.

C). MARKER BEACON

Marker beacons provide information on distance from runway by iditifiying predetermined pionts along the approach track. These because are low-power transmitter, that operate at a frequency of 75 MHz with 3 Watt or less rated powe output. They radiate an elliptical beam upward from the ground. At an altitude of 1000 ft, the beam dimensions are 2400ft long and 420ft wade. At higher altitudes, the dimensions increas significantly.

a) Outer Marker

Provide Final Approach Fix for Nonprecision Approach, modulation 400Hz, 95 percent. Keying: Two dashes per second. Blue Light. Located 4 to 7 miles from end runway.

b). Midle Marker

Indicates approximate Decision Height Point, modulation 1300 Hz, 95 percent. Keying 95 alternate dot and dash combination per minuts. Amber light. Located 3000 to 6000 feet from threshold.

c). Inner Maeker

Modulation 3000 Hz, 95 percent. Keying: dot, white light and located 1000 to 1500 feet from threshold.