Instrument Landing SystemsILS

An Australian Technology

You should be able to: describe the principles of operation of ILS identify the components of an ILS explain the operation of an ILS to block

diagram level describe the operation of controls and

interpretation of indicators in an ILS identify specialised test equipment used to

test an ILS

ILS – Instrument Landing System

Instrument landing systems used in modern aircraft including:

the airborne receiving equipment the instruments pilots use to navigate along

the required flight path the ground based radio transmitting

equipment.

ILS – Instrument Landing System

ILS enables a suitably equipped aeroplane to make a precision approach to a particular runway (non-visual)

The three main elements of ILS are:– The localizer– The glideslope– Marker beacons

Operation Principles

Ground equipment

a localiser (LOC) transmitter and antenna located at the far end of the runway

a glideslope (GS) transmitter and antenna located near the runway threshold

an outer marker antenna located approximately 8 kilometres from the runway threshold

a middle marker antenna located approximately 1 kilometre from the runway threshold

an inner marker beacon antenna located approximately 150 metres from the runway threshold.

ILS ground antennas

Glideslope

Glideslope channels are in the UHF band specifically between 328.6Mhz & 335.4 MHz

The glideslope aerial is usually situated about 300 metres in from the runway threshold.

Aerial is situated some 100 to 200 metres to the side of the runway.

Glideslope Signals

A large 90 Hz lobe overlaps a 150 Hz lobe in the vertical plane.

Above correct glide path the 90 Hz modulation predominates while on the correct glide path the d.d.m is zero, both tone giving 40 percent modulation.

A particular glideslope transmission frequency is always paired with the same localizer frequency.

Glideslope propagation

The glideslope signal is radiated to produce two intersecting lobes, one above the other. The upper lobe is modulated by a 90 Hz signal, the lower lobe by a 150 Hz signal

Localizer Localizer: The localizer provides directional

guidance along the extended centre line of the landing runway.

Localizer transmits on frequency in the VHF band between 108.10 MHz & 111.95 MHz.

Its transmitting aerial (may be 20m wide and 3 m high) is positioned at the far end of the runway (stopping end), and typically 300 m beyond the end.

Localizer Signals

Localizer transmits two overlapping lobes of RF energy on the localizer’s carrier frequency as it did for the vertical signal.

Left hand side of approach is modulated at 90 Hz (yellow sector)

Right hand side of the approach path is modulated at 150 Hz (blue sector)

Signal transmitted along the approach path is sometimes called the localizer front beam.

Localizer Signals

Localizer back beam

Marker Beacons

ILS marker beacons transmit a vertical elliptical pattern, (fan-shaped).

Outer markers: located 8 Klm from the runway threshold-aural series of low pitched(400 Hz) dashes transmitted at 2 per sec -a flashing blue light

Marker Beacons

Middle Markers:– located approximately 1 Klm (0.6 nm) from the

runway threshold.

– Aural series of medium-pitched(1.3 KHz) dots and dashes transmitted six per sec.

Inner Markers:– Close to threshold 150metres

– indicated by high-pitched (3KHz) dots six per sec

– flashing white light.

Marker beacon antenna installation

Marker beacon antenna installation

ILS equipment

ILS antenna locations

ILS Categories

Cat 1 - DH - 200ft Cat 2 - DH - 100ft Cat 3 - DH - 0 ft.

– all of which are well below the decision heights allowed for an IMC rated pilot carrying out an ILS approach (an absolute minimum of 500ft aal, or higher)

– DH decision height

ILS frequency allocations

Question

The ILS frequency for runway 35 at Canberra airport is 109.35 MHz. What is the glideslope frequency?

Localiser Freq Glideslope Freq109.35MHz 331.85Mhz

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