Warning: This material is to be used for Training Reference only.Always consult the Operations Maintenance Manual

Instrument Landing System (ILS) - Localizer


localiser-radiation-characteristics.jpg
Localizer Radiation Pattern (source: http://en.wikipedia.org/wiki/File:ILS_illustration.jpg)


Introduction:


The localizer is one the two main components of the instrument landing system (ILS), the other being the glideslope. The localizer system consists of 2 different modulated tones which are received and interpreted giving a reading on a VOR indicator which displays whether the course of the airplane needs to be corrected. When the bar is swung to the either side of the screen, it indicates that the plane must fly towards the indicated direction to align itself with its proper angle for the landing approach.

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Operation

Frequencies:


The localizer transmits in the VHF frequency range that is used by the VOR system (108-112 MHz.). The two systems are kept separate by the allocation of their relative frequencies within that spectrum.

LOC
VOR

108.000 MHz
108.100 MHz

108.150 MHz


108.200 MHz
108.300 MHz

108.350 MHz


108.400 MHz
108.500 MHz

108.550 MHz


108.600 MHz
108.700 MHz

108.750 MHz

Example
alignment
Two modulated signals are transmitted on any one of the ILS channels. The first is modulated at 90 Hz, while the other is modulated at 150 Hz and these are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.
The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 Hz and 150 Hz signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the position of the approaching aircraft from the centerline.

Equipment:

Air Equipment: Antennas for the Localizer are typically paired with the VOR antennas and are of the Di-Pole variety. They need to installed as such that they remain horizontal ensuring proper alignment with the transmitted radio signals.

external image commant_CI157P_antenna.jpg

Ground Equipment: Antennas for the ILS Localizer system are located directly in line with the runway and usually a 1000 feet behind it. A signal is developed behind the ILS transmitter that is often of use. This is known as the back-course. It can be used for a localizer approach but the signal will be inverted. Some Nav systems installed on planes will convert the signal automatically while others will not.


external image images?q=tbn:ANd9GcTrHR3ou73lgEJzssBWT4NnrbSW8ufYYgPjVRTKafQmavswtM1Hyw
A Localizer Antenna Array. Source: http://forum.flyprat.no/showthread.php?t=19475
Inputs: N/A
Outputs: Modulates 2 signals, one at 90 Hz. and the other at 150 Hz. on any one of the ILS Nav frequencies.

Testing


Ramp Test

Procedures:

1. Tune in the ILS Test Frequency (108.100 MHz) on the Nav receiver.

2. Tune the test box to the same test frequency.

3. Ensure that the power flag on the Nav indicator pulls out of sight.

4. Check for the aural ident tone by selecting the nav on the ACP.

5. Test deflection in both directions all the way past scale.

Equipment: NAV 401, or any appropriate Tick (sp?) box

Bench Test

Procedures/Parameters: No Bench Procedures could be found online. Always consult the manual for the individual systems test procedures.

Tone Difference Values



Course Deviation Indicator
90 and 150 Hz % Mod.
DDM
Decibels
Deflection
Microamps
20.00, 20.00
0
0
Centered
0
15.35, 24.65
.093
4.114
Standard
90
12.25, 27.75
.155
7.102
Full Scale
150
10.00, 30.00
.200
9.542
+ Full Scale
194
0.00, 40.00
.4
Infinity
Full tone delete other
387

Equipment: NAV 401, Ammeter, Oscilloscope, Power Supply


References:

1. http://en.wikipedia.org/wiki/Instrument_landing_system
2. http://www.navfltsm.addr.com/ils.htm
3. Assorted ILS handouts provided in-class by the Instructor.
Christopher