To land an aircraft safely, the three axis have to be controlled, the horizontal, vertical and the aircraft speed. That means, the approach angle to the runway must be right, it had to be at the center of the runway at the suitable speed. The ILS systems provide the signals for the glide angle and the center of the runway.
The subsystems are:
a. The localizer:The signal provides the guide to enable the aircraft to follow a straight path towards the center of the runway.It is a expanding signal sent from the end of the runway creating a field of 2.5 degrees ( about 1500 feet wide ate about 5 miles from the runway.
Two sets of transmitter send different frequency signals, 150Mhz on the right and 90Mhz on the left. Both the signal overlap in the middle at the angle of expanding 2.5 degrees. If the aircraft is on the left of the center line, it will receive the 90Mhz signal and the indicator will show that the localizer is on the right.If the aircraft is receiving both, then the system knows that it is on the right path.
The antenna sends a controlled horizontal signals of 2 frequencies, the upper portion is 90Hz and the lower is 150 hz. Both the signal meets to give a center line (chord) of 3 degrees from the end of the runway.
The aircraft is fitted with a antenna, receiver blackbox and indicators.
Indicator: In glass cockpit, the glideslope and localizer are shown on the EADI and EHSI. Sample as below..
C. Marker Beacons
D. Compass Locator
The localizer and glide slope signals cover a very small angle. It is difficult for an aircraft to locate and intercept the signals. A more wide spread signal is required to provide the location where to intercept the ILS. This is provided by the compass locator signal normally situated about 4-7 miles from the runway. It is like a radio station of ADF, where the radio signal is sent 360 signal at 190Hz to 535hz with the range of about 15 miles.
I hope the short article on How ILS system work will give the readers some idea of the system.
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