5 Instrument landing system

Introduction

In order to be able to land the aircraft safely undervisual flight rules (VFR), i.e. without any indicationfrom instruments as to the aircraft's position relativeto the desired approach path, the pilot must have atleast 3 miles horizontal visibility with a ceiling notless than 1000 ft. Although most landings are carriedout under these conditions a significant number arenot; consequently, were it not for instrument aids tolanding a considerable amount of revenue would belost due to flight cancellations and diversions.

One method of aiding the pilot in the approach toan airport is to use a precision approach radar (PAR)system whereby the air traffic controller, having theaircraft 'on radar', can give guidance over thev.h.f.-r.t. The alternative method is to provideinstrumentation in the cockpit giving steeringinformation to the pilot which, if obeyed, will causethe aircraft to make an accurate and safe descent andtouchdown. The latter, which may becomplemented/monitored by PAR, is the methodwhich concerns us here.

Early ILS date back to before World War II; theGerman Lorentz being an example. During the warthe current ILS was developed and standardized inthe United States. The basic system has remainedunchanged ever since but increased accuracy andreliability have resulted in landing-minimumvisibility conditions being reduced.

The ICAO have defined three categories ofvisibility, the third of which is subdivided. Allcategories are defined in terms of runway visual range(RVR) (see ICAO Annex 14) and, except CategoryIII, decision height (DH), below which the pilot musthave visual contact with the runway or abort thelanding (see ICAO PANS-OPS). The variouscategories are defined in Table 5.1 where thestandards are given in metres with approximateequivalents in feet (in parentheses). Sometimescategories IIIA and B are called 'see to land' and`see to taxi'.

The ILS equipment is categorized using the sameRoman numerals and letters according to its

Table 5.1 ICAO visibility categories

Category d.h. r.v.r.

60 m (200 ft)

800 m (2600 ft)II 30 m (100 ft)

400 m (1200 ft)

IIIA

200 m ( 700 ft)IIIB

30 m ( 150 ft)IIIC

Zero

operational capabilities. Thus if the ILS facility iscategory II, the pilot would be able to land theaircraft in conditions which corresponded to thosequoted in Table 5.1. An obvious extension of theidea of a pilot manually guiding the aircraft with noexternal visual reference is to have an autopilot which`flies' the aircraft in accordance with signals from theILS (and other sensors including radio altimeter) i.e.automatic landing.

Basic Principles

Directional radio beams, modulated so as to enableairborne equipment to identify the beam centres,define the correct approach path to a particularrunway. In addition vertical directional beamsprovide spot checks of distance to go on the approach.The total system comprises three parts, each with atransmitter on the ground and receiver and signalprocessor in the aircraft. Lateral steering is providedby the localizer for both front-course andback-course approaches; the glideslope providesvertical steering for the front course only whilemarker beacons give the distance checks.

LocalizerForty channels are allocated at 50 kHz spacing in theband 108 . 10-111 .95 MHz using only those frequencieswhere the tenths of a megacycle count is odd; so, forexample 108 . 10 and 108 . 15 MHz are localizerchannels while 108 . 20 and 108 . 25 MHz are not.Those channels in the band not used for localizer are

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ILS datum point 0 . 155 DDM

150 Hz > 90 Hz

0.155 DDM

• Course line

Course sector < 6°Beacon

105 m

105 m

150 Hz < 90 Hz

Fig. 5.2 Localizer course selector

allocated to VOR. The coverage of the beacon willnormally be as shown by the hatched parts of Fig.5.1, but topographical features may dictate arestricted coverage whereby the ± 10° sector may bereduced to 18 nautical miles range.

Azimuth

Fig. 5.1 Localizer front beam coverage

The horizontally polarized radiated carrier ismodulated by tones of 90 and 150 Hz such that anaircraft to the left of the extended centre line will bein a region where the 90 Hz modulation predominates.Along the centre line an airborne localizer receiverwill receive the carrier modulated to a depth of 20per cent by both 90 and 150 Hz tones. Deviationfrom the centre line is given in d.d.m. (difference indepth of modulation), i.e. the percentage modulationof the larger signal minus the percentage modulationof the smaller signal divided by 100.

The localizer course sector is defined as that sectorin the horizontal plane containing the course line

(extended centre line) and limited by the lines onwhich there is a d.d.m. of 0 . 155. The change ind.d.m. is linear for ± 105 m along the lineperpendicular to the course line and passing throughthe ILS datum point on the runway threshold; thesepoints 105 m from the course line lie on the 0.155d.d.m. lines, as shown in Fig. 5.2. The beacon issituated such that the above criterion is met and thecourse sector is less than 6°. Outside the coursesector the d.d.m. is not less than 0.155.

The ICAO Annex 10 specification for thelocalizer-radiated pattern is more complicated thanthe description above indicates, in particular in thevarious tolerances for category I, II and III facilities;however we have covered the essential points for ourpurposes.

The airborne equipment detects the 90 and 150 Hztones and hence causes a deviation indicator to showa fly-left or fly-right command. Full-scale deflectionis achieved when the d.d.m. is 0 . 155, i.e. the aircraftis 2-3° off course. Figure 5.3 shows a mechanical andelectronic deviation indicator both showing slightlyover half-scale deflection of a fly-right command.Provided the pilot flies to keep the command bar atzero, or the autopilot flies to keep the d.d.m. zero,the aircraft will approach the runway threshold alongthe course line.

In addition to the 90 and 150 Hz tones thelocalizer carrier is modulated with an identificationtone of 1020 Hz and possibly (exceptionally categoryIII) voice modulation for ground-to-aircommunication. The identification of a beaconconsists of two or three letters transmitted by keyingthe 1020 Hz tone so as to give a Morse coderepresentation. The identification is transmitted notless than six times per minute when the localizer isoperational.

GlideslopeGlideslope channels are in the u.h.f. band,

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Fig. 5.3 Electromechanical and electronic course deviationindicators (courtesy Bendix Avionics Division)

specifically 328 .6-335 .4 MHz at 150 kHz spacing.Each of the forty frequencies allocated to theglideslope system is paired with a localizer frequency,the arrangement being that localizer and glideslopebeacons serving the same runway will have frequenciestaken from Table 5.2. Pilot selection of the requiredlocalizer frequency on the controller will cause bothlocalizer and glideslope receivers to tune to theappropriate paired frequencies.

Table 5.2 Localizer/glideslopefrequency pairing (MHz)

Localizer Glidepath

108 . 10 334.70108 . 15 334.55108 . 30 334.10108 . 35 333.95108 . 50 329.90108 . 55 329.75108 . 70 33050108 . 75 330.35108 . 90 329.30108 . 95 329.15109 . 10 331.40109 . 15 331.25109 . 30 332.0010935 331.85

Localizer Glidepath

109 . 50 332.60109 . 55 332.35109 . 70 333.20109 . 75 33305109 . 90 333.80109 .95 333.65110 . 10 334.40110 . 15 334.25110 . 30 335.00110 . 35 33485110 . 50 329.60110 . 55 329.45110 . 70 330.20110 . 75 33005110 . 90 330.80110 .95 330.65111 . 10 331.70111 . 15 331.55111 . 30 332.30111 . 35 332.15111 . 50 332.90111 . 55 332.75111 . 70 33350111 . 75 333.35111 .90 331.10111 . 95 330.95

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Elevation

0 . 450 or 0 . 22 DDM

0.120 DDM= 090 Hz > 150 Hz

DDM = 0.0875

150 Hz > 90 Hz

Beacon

\Runway

DDM = 0.0875

0.12 ►

The principle of glideslope operation is similar tothat of localizer in that the carrier is modulated with90 and 150 Hz tones. Above the correct glidepaththe 90 Hz modulation predominates while on thecorrect glidepath the d.d.m. is zero, both tones givinga 40 per cent depth of modulation. The coverage andbeam characteristics shown in Figs 5.4 and 5.5 aregiven in terms of the glidepath angle, typically24-3°. Category I facilities may have asymmetricalupper and lower sectors, the figure of 0 .0875 d.d.m.corresponding to an angular displacement of between0 .070 and 0 . 140 0. By contrast a category III facilityis as shown in Fig. 5.5 with a tolerance of ± 0 . 02 0 onthe 0 . 12 0 lines.

Although d.d.m. = 0 lines occur at 2 0, 3 0 and 4 0they are not stable in the sense that if the pilot obeys

the steering commands he will not maintain thecorresponding angles of descent. The first stable nulloccurs at 5 0 which for a glidepath of 3° is at 15°.This is sufficiently different from the desired descentangle to create few problems; however to avoidconfusion the glideslope beam should be 'captured'from below.

Once in the correct beam fly-up and fly-downsignals are indicated to the pilot in much the sameway as with the localizer. Figure 5.3 illustrates afly-up command of just over half-scale deflection.The glideslope output is more sensitive than localizerin that typically a 4° off the glidepath will givefull-scale deflection (about 0 . 175 d.d.m.) comparedwith about 22° off the course line for full-scaledeflection.

10 nm

Course

Azimuth

Fig. 5.4 Glideslope coverage

Marker BeaconsA marker beacon radiates directly upwards using acarrier frequency of 75 MHz. The modulating signaldepends on the function of the marker.

line An airways, fan or 'Z' marker is a position aid foren-route navigation located on airways or at holdingpoints. As such it is not part of ILS. The carrier ismodulated with a 3000 Hz signal which causes awhite lamp to flash in the aircraft while stationidentification in Morse code is fed to the AIS.

The outer marker is normally located 4+ milesfrom the runway threshold. The carrier isamplitude-modulated by 400 Hz keyed to give twodashes per second which can be heard via the AIS andcauses a blue (or purple) lamp to flash.

The middle marker is located 3500 ft from therunway threshold. The carrier is amplitude-modulatedby 1300 Hz keyed to give a dot-dash pair 95 times perminute which can be heard via the AIS and causes anamber lamp to flash.

The ILS marker beam widths are sufficiently widein the plane perpendicular to the course line to coverthe course sector.

Fig. 5.5 Glideslope beam characteristics

Simplified Block Diagram Operation

Since the localizer and VOR frequencies occupy thesame band it is normal to have a v.h.f. navigationreceiver which selects, amplifies and detects signalsfrom either aid, depending on the frequency selected.

Figure 5.6 illustrates the basic block diagram of alocalizer receiver. A conventional single or doublesuperhet is employed. A.g.c. is important since anincrease in the 90 and 150 Hz output signals by thesame factor would increase the magnitude of thedifference, so giving more deflection of the deviation

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Band passfilter

300-3000 Hz

Band passfilter

r To AIS

Tuning

Superhet

Flag

90 Hz

150 Hz

Band passfilter

Rectifier

Rectifier

Deviation indicator

Fig. 5.6 Localizer simplified block diagram

yin

ILS HI

out

Vref

Fig. 5.7 King KN 72 band pass filter, simplified

indicator for the same d.d.m. Signal separation isachieved by three filters: audio, 90 and 150 Hz. Theaudio signal, identification and possibly voice, ispassed via audio amplifiers (incorporating a noiselimiter) to the AIS. The 90 and 150 Hz signals arefull wave rectified, the difference between therectifier outputs driving the deviation indicationwhile the sum drives the flag out of view.

The 90 and 150 Hz filters, together with therectifiers and any associated circuitry, are often partof the so-called VOR/LOC converter which may bewithin the v.h.f. navigation receiver or a separate unit.A combined converter will usually employ activefilters which serve as either 30 Hz bandpass filters forVOR operation or 90/150 Hz band pass filters for

localizer operation. Figure 5.7 shows the circuit usedin the King KN 72 VOR/LOC converter. When aVOR frequency is selected the ILS Hi line is low, soturning off Q1 which effectively disconnects R2 fromthe circuit, the centre frequency of 30 Hz is set byR3. Selection of a localizer frequency causes the ILSHi line to go high, so turning on Q1 and placing R2 inparallel with R3. R2 is set to give a centre frequencyof 90 or 150 Hz as appropriate.

The glidepath receiver converter block diagram issimilar to that of the localizer except that the audiochannel is not required. A separate receiver may beused or all navigation circuitry may be within thesame unit. In any event separate antennas are usedfor localizer and glidepath.

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