12.9. Instrument Landing System (ils).

ILS is a radio beam system designed to provide the pilot with precise guidance from an elevated point in space down to a selected runway.

The ILS consists of three distinct elements, each of which requires ground transmitters and airborne receivers:

1. Localiser Provides lateral steering signals for front-course and back-

course approaches to the runway.

2. Glide Path Provides vertical steering signals for landings in one

direction (the front course) on the runway.

3. Marker Provide spot-checks of position at pre-determined distances Beacons from the threshold of the runway.

A typical ILS pattern is shown in figure 12.34

Fig. 12.34. INSTRUMENT LANDING SYSTEM

The system is allocated 40 VHF localiser channels, spaced 50 kHz apart, in the frequency range of 108,10 to 111.95 MHz and 40 corresponding UHF glidepath channels in the frequency range of 329.15 to 335.00 MHz, spaced 150 kHz apart.

The glidepath frequencies are paired with the localiser frequencies as shown in table 1.

Localiser	Glidepath	Localiser	Glidepath
Frequency	Frequency	Frequency	Frequency
108.10	334.70	110.10	334.40
108.15	334.55	110.15	334.25
108.30	334.10	110.30	335.00
108.35	333.95	110.35	334.85
108.50	329.90	110.50	329.60
108.55	329.75	110.55	329.45
108.70	330.50	110.70	330.20
108.75	330.35	110.75	330.05
108.90	329.30	110.90	330.80
108.95	329.15	110.95	330.65
109.10	331.40	111.10	331.70
109.15	331.25	111.15	331.55
109.30	332.00	111.30	332.30
109.35	331.85	111.35	332.15
109.50	332.60	111.50	332.90
109.55	332.45	111.55	332.75
109.70	333.20	111,70	333.50
109.75	333.05	111.75	333.35
109.90	333.80	111.90	331.10
109.95	333.65	111.95	330.95

TABLE 1 LOCALISER AND GLIDEPATH PAIRED FREQUENCIES

When a localiser frequency is selected on the Nav control panel, the corresponding glidepath frequency is also selected.

Localiser

The localiser is the lateral guidance portion of the ILS. 40 channels are provided at the odd tenth MHz from 108.1 to 111.9 MHz. Each localiser frequency being paired with a glidepath frequency.

The localiser transmitter is situated at the end of the runway with its aerial aligned with the centreline. The transmitter radiates a horizontally polarised signal, amplitude modulated, to the left of the centreline by a 90 Hz tone and to the right by a 150 Hz tone. The percentage of modulation by each tone is 20%.

The coverage limit within 10° of the front course line is 25 nautical miles at a height of 2000 feet. The coverage limit for the back course line is about 10 nautical miles.

There is a third amplitude modulation of 1020 Hz (or voice) for identification.

The amplitude modulated output at 90 Hz and 150 Hz is fed to a complex aerial array. The resultant RF field pattern is such that, at all positions along the extended centre line of the runway, an aircraft receiver observes a RF carrier modulated to an equal depth (of 20%) of the 90 Hz and 150 Hz tones. Along this line the difference in depth of modulation is zero.

On either side of the centre line the difference in depth of modulation between the two tones is proportional to the angular displacement. During a normal front course approach 90 Hz modulation is predominate to the left and 150 Hz predominate to the right.

Fig. 12.36. LOCALISER DIFFERENCE IN DEPTH OF MODULATION

The ILS localiser course sector is nominally 700 feet wide at the threshold of the runway. The ILS localiser is adjusted so that the difference in depth of modulation is 15.5% at points 350 feet each side of the centreline at the runway threshold.

A difference in depth of modulation of 15.5% detected by an aircraft localiser receiver produces an output of 150 microamps to each course indicator, which then shows a full scale deflection left or right. The variation or difference in depth of modulation with respect to angular displacement from the course line is linear from 0 to 18% DDM, corresponding to angles 0 to 3° off course. At greater angles the difference in depth of modulation is maintained above 15.5% so that the aircraft course indicator never indicates less than full scale deflection outside the course sector.

The back course, at 180°, allows the localiser to be used for landing in a direction opposite to the front course, although without a glideslope and with reversed needle indications.

The ICAO requires that for Category II landings, within the last 3500 feet preceding the threshold, the localiser beam bends less than 0.005 difference in depth of modulation, corresponding to 15 feet, 0.07° off course and 5 microamps at the indicator.

Glide Path

The glide path is the vertical guidance portion of the ILS. Forty glide path channels from 329.3 to 335.0 MHz are spaced 150 kHz apart, and each is paired with a localiser frequency.

The glide path transmitter is situated at the side of the runway approximately adjacent to the touch down point. The transmitter radiates a horizontally polarised signal amplitude modulated above the glide path by a 90 Hz tone and below by a 150 Hz tone. The percentage of modulation by each tone is 40 %.

The coverage limit is 10 nautical miles and the glide path signal defines a straight line approach path at an angle between 2.5 and 3.5° above the horizontal.

Two or more aerials are used to produce the composite RF field pattern. This pattern provides a descent path along which the RF carrier is modulated to an equal depth, of 40%, by the 90 Hz and 150 Hz tones. Along this glide path the difference in depth of modulation is zero..

On either side of the glide path, the difference in depth of modulation between the two tones is proportional to angular displacement.

Fig. 12.37. GLIDE PATH RADIATION PATTERN

A difference in depth of modulation of 17.5% detected by a receiver produces an output of 150 microamps corresponding to a full scale deflection of 0.75° above or below the glide path.

The ILS Marker Beacons will be described separately.

Fig. 12.38. GLIDE PATH DIFFERENCE IN DEPTH OF MODULATION

Localiser Receiver

The localiser receiver is essentially a VHF receiver in the frequency band 108.10 to 111.95 MHz with 50 kHz spacing. In the receiver, the amplitude modulation of

the localiser carrier is detected and filtered into its 90 Hz, 150 Hz and audio frequency components.

The audio frequencies are amplified to allow identification of the transmitter by the Morse code ident at 1020 Hz.

The 90 Hz and 150 Hz tones are amplified, rectified and compared in amplitude by a comparator circuit which controls the left/right pointer of a deviation indicator.

Fig. 12.39. LOCALISER RECEIVER

The circuit is adjusted to give zero output for a receiver RF input which is modulated to a depth of 20% by each of the two tones. This corresponds to zero difference in depth of modulation, the on-course condition.

A flag alarm circuit is included in the receiver to warn the pilot of any malfunction. The flag is usually operated by a moving coil mechanism. The current which is obtained is the sum of the rectified 90 Hz and 150 Hz tones producing 250 microamps. The flag remains out of view as long as these currents are maintained.

Glide Path Receiver

The glide path receiver is essentially a UHF receiver in the frequency band 328.6 to 335.4 MHz with 150 kHz spacing between channels.

The 90 Hz and 150 Hz tones detected from the carrier are amplified, rectified and compared in amplitude by a comparator circuit which controls the up/down pointer of a deviation indicator. The circuit is adjusted to give zero output for a receiver RF input which is modulated to a depth of 40% by each of the two tones. This corresponds to zero difference in depth of modulation, the on-glide path condition and the pointer is centred on the scale.

The flag circuit operates the same as in the localiser receiver.

Fig. 12.40. GLIDE PATH RECEIVER

Amplitude Comparator

Since the localiser and glide path receivers are very similar, one general block diagram will be used (figure 12.41.).

With normal RF signal strength the receiver AGC maintains the carrier signal at a constant level. The detected 90 Hz and 150 Hz components are therefore directly proportional to the respective depths of modulation. The signal is amplified by an audio amplifier and applied to 90 Hz and 150 Hz pass filters that separate the frequencies. Each frequency is rectified by a bridge rectifier and a positive dc voltage is developed which is directly proportional to the depth of modulation of the input signal.

Fig. 12.41. AMPLITUDE COMPARATOR DEFLECTION CIRCUIT

An input signal that has equal depths of modulation for 90 Hz and 150 Hz modulations would develop a positive dc voltage out of each rectifier and the voltages would be equal.

The rectifier output voltages are applied to a meter movement which is a zero centre micro-ampere meter and requires 150 microamps for full scale deflection. When the voltages on both sides of the meter are of the same polarity and the same magnitude, there is no difference in potential across the meter and therefore no current flows through the meter and the meter will remain centre zero.

When one signal is greater than the other, one output voltage from the rectifiers is greater than the other and a potential difference exists across the meter. The magnitude of the potential difference depends on the difference between the 90 Hz and 150 Hz signals. The greater the potential difference, the greater the deflection of the meter pointer.

The resistor marked flag adjust is common to both rectifiers and the current through the resistor is the sum of the currents produced by the two rectifiers. Since the current from either rectifier is proportional to the audio voltage applied to the rectifier and the audio is proportional to the depth of modulation. The total current through this resistor is therefore proportional to the sum of the two currents and is labelled flag current.

The meter movement connected across the flag adjust resistor responds to changes in current through the flag adjust resistor. This is a 1000 ohm movement that actuates a flag which is in view until the movement reaches a predetermined value. A flag condition known as Peeping Flag, where the flag is almost hidden by a mask, occurs with 240 microamps of current. When the flag is in view, it indicates to the pilot that the RF field strength or total modulation is not sufficient to provide valid deflection readings.

A diagram of the rectifiers and meter movement is shown in figure 12.42.

Fig. 12.42. COMPARATOR AND INDICATOR MOVEMENT

The current resulting from the voltage developed across the 150 Hz rectifier flows through R_1? R₂ and the deviation indicator meter to deflect the deviation

bar to the left and produce a positive voltage at P₁.

Current resultant from the voltage developed across the 90 Hz rectifier flows through the deviation indicator meter to deflect the deviation bar to the right and produce a positive voltage at P2.

The failure warning flag is supplied with the sum of the deviation bar operating currents and remains out of view so long as these currents are maintained.

Localiser Aerial

The localiser aerial is designed to receive very high frequency radio signals between 108 and 112 MHz. A typical localiser aerial is shown in figure 12.43.

Fig. 12.43. LOCALISER AERIAL

The aerial is a horizontally polarised half-wave folded dipole and has a characteristic impedance of 50 ohms. It exhibits a VSWR of 2 : 1 or less over the localiser frequency range.

Glide Path Aerial

The glide path aerial is designed to receive ultra high frequency radio signals between 329 and 335 MHz. A typical glide path aerial is shown in figure 12.44.

Fig. 12.44. GLIDE PATH AERIAL

The glide path aerial is a horizontally polarised half wave folded dipole and has a characteristic impedance of 50 ohms. It exhibits a VSWR of 2 : 1 or less over the glide path frequency range.

Glide path aerials are smaller than localiser aerials since they operate in the UHF band.