Aircraft Radio System J POWELL

ORDER NUMBER EA-356

AIRCRAFT RADIO SYSTEMS By J. Powell BA, C Eng, MIERE, GradIMA

Reprinted in India by

HIMALAYAN BOOKS °

New Delhi-110001. Distributed by: The English Book Store The Aviation People

17-L, Connaught Circus New Delhi-110001 (India)

Contents

5

Preface 1

Historical, technical and legal context 1 Introduction 1 Historical background 1 Basic principles of radio 2 Digital systems 8 Categorization of airborne radio equipments Navigation nomenclature 13 Interference 13 Maintenance 17 Regulating and advisory bodies 18

2

3

4

Communication systems 20 Introduction 20 V.h.f. communications 20 H.f. communications 29 Selcal 35 Audio integrating systems - (Intercom) 37 Testing and trouble shooting the audio systems 43 Automatic direction finding 45 Introduction 45 Basic principles 45 Simplified block diagram operation 4.7 Block diagram detail 47 Sources of system error 49 Installation 52 Controls and operation 54 Characteristics 55 Calibration and testing on the ramp 55 V.h.f. omnidirectional range (VOR) 58 Introduction 58 Basic principles 58 Doppler VOR (DVOR) Aircraft installation

61 63

Controls and operation 65 Simplified block diagram operation 65 Characteristics 65 Ramp testing 67

Instrument landing system Introduction 69 Basic principles 69

69

Simplified block diagram operation Installation 74 Controls and operation 76 Characteristics 77 Ramp testing 78 6

Hyperbolic navigation systems General principles

79

79

Omega navigation system 7

72

Distance measuring equipment Introduction 105 Basic principles 105 X and Y channel arrangements

83 105

110

The link with v.h.f. navigation 110 Installation 111 Controls and operation 112 Simplified block diagram operation 112 Range measuring and mode control 114 8 ATC transponder 121 Introduction 121 Basic principles 121 Installation 126 Controls and operation 127 128 Simplified block diagram operation Block diagram details 128 Characteristics 135 Ramp testing 136 9 Weather avoidance 139 Introduction 139 Weather radar 140 Choice of characteristics and features 140 Installation 146 Controls 147 Operation 148

Block diagram operation Scanner stabilization

150

Aircraft installation delay Interface 198

157

Other applications for weather radar 160 Weather radar characteristics 162 Maintenance and testing 164 Ryan storniscope 168 Appendix Factors affecting weather radar performance 169 10

11

197

Multiple installations 199 Characteristics '00 Ramp testing and maintenance 20 0 Appendix Sinusoidal frequency modulation 201 12 Area navigation 202 Development of airspace organization 202 Generalized area navigation system 202 VOR/DME based RNAV. principles 204 Bendix nav. computer programmer NP-2041A 206 King KDE 566 210 Standardization 213 Testing RNAV 215

Doppler navigation 172 Introduction 172 Doppler effect 173 Antenna mechanization 174 Doppler spectrum 175 ` Beam geometry 175 Transmitter frequency 176 Modulation 177 Over-water errors 178 Navigation calculations 179 Block diagram operation 179 Installation 182 Controls and operation 184 Characteristics 185 Testing 185 Appendix Relationships between aircraft and earth co-ordinates 186 The Doppler shifts for a four-beam Janus configuration 187 The aircraft velocity in earth co-ordinates expressed in terms of Doppler shifts 188

13 Current and future developments 216 Introduction 216 The state of the art 216 The flight deck 217 Multi-systern packages 219 Data link 219 ADSEL/DABS 2 21 Satcom and satnav 223 Microwave landing system 2 24 Microwave aircraft digital guidance equipment 225 Collision avoidance 2 29 The current generation of ARINC characteristics 230 Concluding remarks 231

Radio altimeter 189

Recommended reading 232

Introduction 189 Basic principles 189 Factors affecting performance Block diagram operation Monitoring and self-test Indicat or 196 Installation 196

191 192 195

Glossary

234

Exercises

246

Index 252

Preface

The cockpit and equipment racks of modern aircraft, large and small, are becoming filled with ever more sophisticated systems . This book attempts to describe a certain class of such systems, namely those which rely for their operation on electromagnetic radiation. The subject matter is complex and wide-ranging, hence not all aspects can be covered in one volume In deciding where the treatment should be light or perhaps non-existent, I have asked myself two questions: (1) which aspects can most usefully be covered in a book; and ( 2 ) at which group of people involved in aviation should a book covering such aspects be aimed? The answer to (1) must be `describe the theory'. One can, and indeed must, read or be told about how to operate the systems: how to navigate using the systems; how to solder, crimp and change items; how to use test equipment, etc. but proficiency is impossible without practice. On the other hand gaining an understanding of how a particular system works is more of a mental exercise which can be guided in a book such as this. This is not to say that more practical matters are neglected, since it would not help one's understanding of the theory of operation not to see, at least in words and pictures, how a particular system is controlled, presents its information, reacts to the environment, etc. Having decided the main line of attack the more difficult question of depth of treatment must be answered: in other words which group should be satisfied'.' Pilots need a superficial knowledge of how all the systems work; maintenance engineers on the ramp and in the hangar a more detailed knowledge; workshop engineers must have an understanding of the circuitry for perhaps a limited range of equipments; while designers should have the greatest depth of knowledge of a11. It is virtually impossible to draw dividing lines, but it is hoped that if enough theory is given to satisfy the aircraft radio maintenance engineer then the book might be useful to all groups mentioned. The depth of treatment varies, it being impossible to cover everything, or indeed anything, to the depth I would have liked. In particular few details of

circuitry are given since I feel most readers will be more interested in the operation of the system as a whole. Nevertheless, some circuits are given purely as examples. Should the reader need circuit knowledge, the equipment maintenance manual is the best place to find it, assuming he knows the system and he has a basic knowledge of electronics. The state of the art of the equipment described is also varied. 1 did not see the point of describing only equipment containing microprocessors, since the vast majority of systems in service do not use them as yet. On the other hand if the life of this book is not to be too severely restricted, the latest techniques must be described. Within the pages that follow, analogue, analogue/digital, hardwired digital and programmable digital equipments all find a place. As stated previously, the book is aimed primarily at the maintenance engineer. However, I hope several groups might be interested. This poses problems concerning the background knowledge required. For what I hope is a fairly substantial part of the book, any reasonably intelligent technically minded person with a basic knowledge of mathematics and a familiarity with aircraft will have no difficulty that two or perhaps three readings will not overcome. There are parts, however, where some knowledge of electronics, radio theory or more sophisticated mathematics is needed. In three chapters where the going gets a bit tough, I have relegated the offending material to an appendix. Some background material is covered in Chapter 1, in particular, basic radio theory and a discussion of digital systems in so far as coding and computers are concerned. If you are one of the few people who plough all the way through the Preface to a book, you may have decided by now that this book is concerned with theory and little else. That this is not so may be clear if I outline briefly the contents of each chapter. An introduction saying a few words about the history and function of the system is followed by a fairly thorough coverage of the basic principles. In some chapters the next item is a discussion of the installation, i.e. the units, how they are interconnected, which other systems they interface

with and any special considerations such as cooling, positioning, type of antennas and feeders, etc. This, together with a description of controls and operation, puts some practical meat on to the bare bones of the theory which continues with a consideration of the block diagram operation. In certain chapters the order: installation - controls and operation -- block diagram, is reversed where 1 thought it was perhaps to the reader's disadvantage to break up the flow of the more theoretical aspects. A brief look at characteristics, in practically all cases based on ARINC publications, and -testing / maintenance concludes each chapter. Most chapters deal with one system; none of them is exclusively military. The exceptions are, in reverse order, Chapter 13 where I look at the current scene and review some systems we should see in the next few years; Chapter 12 which is a bringing-together of some of the previously covered systems: Chapter 6 covering Omega, Decca Navigator and Loran C'; Chapter 2 which covers both radio and non -radio communications; and Chapter 1 where some chosen background material is given. I should point out that this is not a textbook in the sense that everything is examinable in accordance with some syllabus. The reader will take from the book however big a chunk he desires, depend ing on his background knowledge, his profession, the examinations he hopes to take and, of course, his inclination. Some will have, or end up with, an understanding of all that is included herein, in which case I hope the book may be seen as a source of reference.

Acknowledgements A number of manufacturers have given valuable assistance including the supplying of material and granting permission to reproduce data and illustrations. Without the generosity of the following, this book would have been of very limited use. Bendix Avionics Division Boeing Commercial Aero plane Company British Aerospace

Communications Components Corporation The Decca Navigator Company Limited Field Tech Limited Hazeltine Corporation IFR Electronics Inc King Radio Corporation Litton Systems International Inc., Aero Products Division

Marconi avionics Limited MEL Equipment Company Limited RCA Limited Rockwell-Collins (UK) Limited Ryan Storm scope Tel-Instrument Electronics Corporation (TIC) Although I am grateful to all the above, I must reserve a special word of thanks to Mr Wayne Brown of Bendix, Mr. A. E. Crawford of King and Mr. T. C. Wood of RCA, who arranged for the dispatch of several expensive and heavy maintenance manuals in reply to my request for information. These manuals, and indeed all other information received, were used in the preparation of this book and continue to be used in the training of students at Brunel Technical College, Bristol, England. I also wish to thank all my colleagues at Br unel who have helped, often unwittingly, in conversation. In particular my thanks go to John Stokes, Clive Stratton and Peter Kemp for proof-reading some of the chapters and also Leighton Fletcher for helping with the illustrations. May I add that, although I received technical assistance from the above, any mistakes which remain are obviously mine. I would be grateful to any reader who might take the trouble to point out any errors. Finally, my thanks to Pauline Rickards, whose fingers must be sore from typing; to the publishers who displayed great patience as the deadline for the submission of the typescript came and went; and, most of all, to my wife Pat and son Adam who showed even more patience and understanding than Pitman’s. Bristol, England

J. P.

I Historical, technical and legal context Introduction

This book deals with airborne systems that depend for their operation on the generation and detection of that intangible discovery the radio wave. Such systems split naturally into two parts: communications and navigation. The former provide two-way radio contact between air and ground, while the latter enables an aircraft to be flown safely from A to B along a prescribed route with a landing safely executed at B. An understanding of such systems requires a working knowledge of basic electronics, radio, computer systems and other topics. A book of this length cannot provide all that is necessary but it was thought that some readers might appreciate a review of selected background material. This is the objective of Chapter 1. It may be that on consulting the list of contents, the reader will decide to omit all or part of this chapter. On the other hand, some readers may decide that more basic information is needed, in which case, the list of recommended books will help point the way to sources of such material. Historical Background

In 1864 James Clerk Maxwell, Professor of Experimental Physics at Cambridge, proved mathematically that any electrical disturbance co uld produce an effect at a considerable distance from the point at which it occurred. He predicted that electromagnetic (e.m.) energy would travel outward from a source as waves moving at the speed of light. In 1888 Hertz, a German physicist, demonstrated ,hat Maxwell's theory was correct, at least over distances within the confines of a laboratory. It was left to the Italian physicist Marconi to generate e.m. waves and detect them at a remote receiver, as he did by bridging the Atlantic in 1901. Other notable landmarks in the development of radio include: 1897

First commercial company incorporated for the manufacture of radio apparatus: the

1936

Wireless Telegraph and Signal Company Limited (England), later the Marconi Wireless Telegraph Company Limited. Fleming's (British) discovery of the thermionic valve - the diode. First patent for a radar-like system to a German engineer, Hulsmeyer. Workable but not accepted. De Forest's (American) invention of an amplifying thermionic valve (triode). Direction-finding properties of radio waves investigated. Discovery of the oscillating properties of De Forest's valve. The first workable pulse radar.

1939

Invention of the magnetron in Britain.

1904 1904 1906 1911 1912

1948

Invention of the transistor by Bardeen, Brattain and Shockley (Bell Telephone Laboratories, USA). To bring us up to date, in the early 1970s the first microprocessor appeared from Intel (USA) leading directly to present-day microcomputers. Paralleling the progress of radio was the second of the three great developments of the twentieth century, i.e. powered flight in heavier-than-air machines. (The other two developments referred to are electronics and applications of nuclear physics; the reader is concerned with two out of three.) There can be few people who have not heard of Wilbur and Orville Wright; who designed and built the first successful powered aircraft which Orville flew for the first time at 10.35 on 17 December 1903, making a landing without damage after 12 seconds airborne. Since then landmarks in aviation, with particular reference to civil aviation, include: 1907 First fatality: Lieut. T. E. Selfridge, a passenger in a Wright Fly er. 1909 Bleriot (French) flies the English Channel. 1912 Sikorsky (Russian) builds first multi-engined (four), passenger (sixteen) aircraft. 1914 World War I. The years 1914-18 saw

advances in performance and a vast increase in number of aircraft, engines and pilots. 1919 Sustained daily scheduled flights begin in Europe. 1928 Whittle (British) publishes thesis on jet engine. 1929 First blind landing by Doolittle (American) using only aircraft instruments. 1937 Flying-boat service inaugurated from Britain to the Far East. Britain to Australia took 8 days in 1938, either by KLM or Imperial Airways. 1939 First jet -powered flight by He 178 (German). 1939 Inaugural air-mail service between Britain 1939 World War II. The years 1939 -45 saw the growth of world-wide military air transport services, and the USA established as the postwar leader in civil aviation. 1944 1945

1952 1953 1954

1958

1965 1970 1970

International Civil Aviation Organisation formed at Chicago conference. American Overseas Airlines operate scheduled flights over North Atlantic with landplane (DC 4). First civil jet aircraft, the Comet 1, goes into service with BOAC. First civil turboprop aircraft, the Viscount, goes into service with BEA. Previously unknown problem of metal fatigue discovered in Comet 1. Withdrawn. 1956 Tu 104 first jet aircraft to commence sustained commercial service. First transatlantic jet service by BOAC with the Comet 4. (PAA's Boeing 707 -120 follows three weeks later.) First short -haul jet to enter service, the BAC 1 -11. Boeing 747 introduced; the first of the Jumbo Jets. First civil aircraft supersonic flights, Concorde and the Tu 144.

From the time of the Wright brothers to the present day, the non -commercial side of civil aviation, known as general aviation (business and private) has grown with less spectacular firsts than its big brother, so that now by far the largest number of civil aircraft are in this category. It was inevitable that the new toys of radio and aircraft should be married early on in their history. Later the vast increase in air traffic made it essential that radio aids, in both communication and navigation, should be made full use of, to cope safely

with the crowded skies. In 1910 the first transmission of e.m. waves from air to ground occurred. Speech was conveyed to an aircraft flying near Brook lands Airfield (England) by means of an e.m. wave in 1916. By the 1920s, radio was being used for aircraft navigation by employing rudimentary direction -finding techniques (Chapter 3). The introduction of four -course low-frequency range equipment in 1929 provided the pilot with directional guidance without the need for a direction -finder on the aircraft. Steady progress was made up to 1939, but it was

world War I I which gave the impetus to airborne Radio innovations . Apart fr om very high frequency (v.h.f.) communications, introduced during the war, a number of radio navigation aids saw the first light of day in the period since 1939. These systems are described in the following chapters.

Basic Principles of Radio Radiation of Electromagnetic (e.m.) Waves and Antennas If a wire is fed with an alternating current, some of the power will be radiated into space. A similar wire parallel to and remote from the first will intercept some of the radiated power and as a consequence an alternating current will be induced, so that using an appropriate detector, the characteristics of the original current may be measured. This is the basis of all radio systems. The above involves a transfer of energy from one point to another by means of an e.m. wave. The wave consists of two oscillating fields mutually perpendicular t o each other and to the direction of propagation. The electric field (E) will be parallel to the wire from which the wave was transmitted, while the magnetic field (H) will be at right angles. A 'snapshot' of such a wave is shown in Fig. 1.1 where the distance shown between successive peaks is known as the wavelength. The velocity and wavelength of an e.m. wave are

Fig. 1.1

An electromagnetic wave

directly related through the frequency of the alternating current generating the wave. The law is: c= Y`f where : c is the speed of light (3 X 108 m/s). Y` is the wavelength in metres. f is the frequency in Hertz (cycles/s). A radiating wire is most efficient when its length is equal to half a wavelength. Thus for a frequency of 100 MHz the wire should be (3 X 10 8)/(2 X 100 X 106 ) = 1.5m long, in which case it is known as a dipole. In practice many airborne radio systems do not make use of dipole antennas since their size is prohibitively large, except at very high frequencies, and the radiation patt ern is not suited to applications where energy needs to be transmitted in or received from a certain direction. A close relative of the dipole is the unipole antenna which is a X/4 length conductor mounted vertically on the metal fuselage which acts as a ground plane in which a reflection of the unipole is `seen' to form a dipole. Thus a v.h.f. communication (comm.) unipole would be less than 60 cm long (centre frequency of the band is 127 MHz). Two unipoles are sometimes mounted back to back on the vertical stabilizer to function as a dipole antenna for use with VOR (Chapter 4) or ILS (Chapter 5). At frequencies in the region of 2-30 MHz (h.f.) a dipole would be between 5 and 75 m. Since the dimensions of aircraft fall, roughly speaking, within this range of lengths it is possible to use the aircraft as the radiating or receiving element. A notch or slot cut in a suitable part of the airframe (e.g. base of vertical stabilizer) has a large oscillating voltage applied across it, so driving current through the fuselage which in turn radiates. The notch/airframe load must be `tuned' to the correct frequency for efficient transmission. Without tuning, little energy would be radiated and a large standing wave would be set up on the connector feeding the notch. This is due to the interaction of incident and reflected energy to and from the antenna. An alternative type of antenna for this band of frequencies is a long length of wire similarly tuned, i.e. with variable reactive components. F or frequencies within the range 10-100 kHz the maximum dimension of even large aircraft is only a small fraction of a wavelength. At these frequencies capacitive type antennas maybe used. One plate of the capacitor is the airframe; the other a horizont al tube, vertical blade or a mesh (sometimes a solid plate). The aircraft causes the field to become

intensified over a limited region near its surface. The resulting comparatively strong oscillating E field between the capacitor's plates causes a current to flow in twin feeder or coaxial cable connected across the antenna. The airborne systems operating in the relevant frequency band are the receive-only systems covered in Chapter 6 (Omega, Decca and Loran C). Although ADF (Chapter 3) receives sign als in the band of frequencies immediately above those considered in this paragraph, one of its two antennas (sense) utilizes the principles discussed. An alternative to the capacitance antenna is the loop antenna which is basically a loop of wire which cuts the H field component of the e.m. wave. The field is intensified by use of a ferrite core on which several turns are wound. Use of two loops mounted at right angles provides a means of ascertaining the direction of arrival (ambiguous) of an e.m. wave. Such antennas are used for ADF (loop) and may also be used for Omega. At frequencies above, say, 3000 MHz the properties of waveguides may be used. A waveguide is a hollow metal tube, usually of rectangular cross section, along which an e.m. w ave can propagate. If the end of a waveguide is left open some energy will be radiated. To improve the efficiency, the walls of the waveguide are flared out, so providing matching to free space and hence little or no reflected energy back down the guide. Such an antenna is called a horn and may be used for radio altimeters (Chapter 11). Associated with the wave propagated along a waveguide are wall currents which flow in specific directions. A slot, about 1 em in length, cut in the waveguide so as to interrupt the current flow will act as a radiator. If several slots are cut the energy from them will combine several wavelengths from the antenna to form a directional beam. The direction depends on the spacing of the slots. Such antennas may be used for Doppler radar (Chapter 10) and weather radar (Chapter 9). The theory of some of the more esoteric antennas used on aircraft is a little sketchy and design is finalized, if not based, on empirical data. However the antenna is designed, it will only s ee service if it performs its function of transmitting and/or receiving e.m. waves in and/or from required directions. The directivity of an antenna, or the lack of directivity, is most clearly defined by means of a polar diagram. If we take a transmitting antenna and plot points of equal field strength (one value only) we have such a diagram. The same antenna used for receiving would, of course, have the same polar diagram. If the diagram is a circle centred on the antenna, as would be the case if the plot were in the plane perpendicular

to a dipole, then the antenna is said to be omnidirectional in the plane in which the measurements were made. A practical antenna cannot be omnidirectional in all planes, i.e. in three dimensions.

Table 1.3 Approximate bands for microwave frequencies Letter designation

Frequency range (GHz)

The e.m. Spectrum and Propagation

L

1-3

As can be seen from the previous paragraph, the frequency of the radio wave is an important consideration when considering antenna design. In addition the behavior of the wave as it propagates through the earth's atmosphere is also very much dependent on the frequency. However, before considering propagation, we will place radio waves in the spectrum of all e.m. waves (Table 1.1). In doing so we see that the range of frequencies we are concerned with is small when

S C X K Q

2-5-4

Table 1.1 The electro magnetic spectrum Hz Region 10 25

Cosmic rays

10 21

Gamma rays

10' 9 X rays 10 17 Ultraviolet 10 15 Visible 10 14 Infra-red 10 11 Radio waves

Abbreviation Frequency

Very low frequency Low frequency Medium frequency High frequency Very high frequency Ultrahigh frequency Superhigh frequency Extremely high frequency

VIE l.f. m.f. h.f. v.h.f. u.h.f. s.h.f. e.h.f.

3-30 30-300 300-3000 3-30 30-300 300-3000 3-30

Frequency band

Omega Decca Loran C ADF h.f. comm. Marker ILS (Localizer) VOR v.h.f. comm.

10-14 kHz

Weather radar (X) Doppler (K)

Radio frequency categorization

Name

System

ILS (Glideslope) DME SSR Radio altimeter Weather radar (C) Doppler (X)

compared with the complete spectrum. By general agreement radio frequencies are categorized as in Table 1.2. There is less agreement about the letter designations used for the higher radio frequencies which are tabulated with approximate frequency ranges in Table 1.3. Finally, Table 1.4 lists the frequencies used for airborne radio systems by international agreement. Table 1.2

3.5-7.5 6-12.5 12.5 -40 33-50 Table 1.4 Airborne radio frequency utilization (exact frequencies given in relevant chapters)

kHz kHz kHz MHz MHz MHz GHz

30-300 GHz

70-130 kHz 2-25kHz 100 MHz 75 MHz 108-112 MHz 108-118 MHz 118-136 MHz 320-340 MHz 960-1215 MHz 1030 and 1090 MHz 4.2-4.4 GHz

In free space, all radio waves travel in straight lines at the speed of light. Such a mode of propagation is known as the space wave. In addition, two other modes of propagation are used with airborne radio equipment: the ground wave and the sky wave. A fourth mode known as tropospheric scatter is used only for fixed ground stations since elaborate and expensive. equipment must be used at both ends of the link due to the poor transmission efficiency. The ground wave follows the surface of the earth partly because of diffraction, a phenomenon associated with all wave motion which causes the wave to bend around any obstacle it passes. In addition, the wave H field cuts the earth's surface, so causing currents to flow. The required power for these currents must come from the wave, thus a flow

of energy from wave to earth takes place causing bending and attenuation. The attenuation is a limiting factor on the range of frequencies which can be used. The higher the frequency the greater the rate of change of field strength, so more attenuation is experienced in maintaining the higher currents. Ground waves are used for v.l.f. and l.f. systems. Radio waves striking the ionosphere (a s et of ionized layers lying between 50 and 500 km above the earth's surface) are refracted by an amount depending on the frequency of the incident wave. Under favourable circumstances the wave will return to the earth. The distance between the transmit ter and point of return (one hop) is known as the skip distance. Multiple hops may occur giving a very long range. Above about 30 MHz there is no sky wave since insufficient refraction occurs. Sky wave propagation is useful for h.f. comm. but can caus e problems with l.f. and m.f. navigation aids since the sky wave and ground wave may combine at the receiver in such a way as to cause fading, false direction of arrival or false propagation time measurements. At v.l.f. the ionosphere reflects, rather than refracts, with little loss; thus v.l.f. navigation aids of extremely long range may be used. Above 30 MHz, space waves, sometimes called line of sight waves, are utilized. From about 100 MHz to 3 GHz the transmission path is highly predictable and reliable, and little atmospheric attenuation occurs. Above 3 GHz attenuation and scattering occur, which become limiting factors above about 10 GHz. The fact that space waves travel in a straight line at a known speed and, furthermore, are reflected from certain objects (including thunderstorms and aircraft) makes the detection and determination of range and bearing of such objects possible. Modulation Being able to receive a remotely transmitted e.m. wave and measure its characteristics is not in itself of much use. To form a useful link, information must

be superimposed on the e.m. wave carrier. There are several ways in which the wave can carry information and all of them involve varying some characteristic of the carrier (amplitude or frequency modulation) or interrupting the carrier (pulse modulation). The simplest, and earliest, way in which a radio wave is made to carry information is by use of Morse Code. Switching the transmitter on for a short time -interval, corresponding to a dot, o r a longer time -interval, corresponding to a dash, enables a message to be transmitted. Figure 1.2 illustrates the transmission of SOS, the time-intervals shown being typical. In radar the information which must be superimposed is simply the time of t ransmission. This can easily be achieved by switching on the transmitter for a very short time to produce a pulse of e.m. energy. When transmitting complex information, such as speech, we effectively have the problem of transmitting an extremely large number of sine waves. Since the effect of each modulating sine wave on the radio frequency (r.f.) carrier is similar, we need only consider a single sine wave modulating frequency. The characteristics of the modulating signal which must be transmitted are the frequency and amplitude. Figure 1.3 shows three ways in which a pulsed carrier may be modulated by a sine wave while Figs 1.4 and 1.5 show amplitude and frequency modulation of a continuous wave (c.w.) carrier. Both amplitude modulated (a.m.) and frequency modulated (f.m.) carriers are commonly used for airborne systems. With a.m. the amplitude of the carrier represents the amplitude of the modulating signal, while the rate of change of amplitude represents the frequency. With f.m. the amplitude and frequency of the modulating signal is represented by the frequency deviation and rate of change of frequency of the carrier respectively. Both a.m. and f.m. waves have informative parameters associated with them. With a.m. if the

, Time y

x = 0 .1s, y=0 -3 s Fig. 1.2 Morse code: SOS

Radio frequency transmitted

--, Time

Fig. 1.4 Amplitude modulation

Fig. 1.3 Pulse modulation - from top to bottom: unmodulated carrier, modulating waveform, pulse amplitude modulation, pulse width modulation and pulse position modulation

carrier amplitude is Y"e and the modulating signal amplitude is V„, then the modulation factor is Vt,-,, , 'C. This fraction can be expressed as a percentage, in which case it is known as the percentage modulation or depth of modulation (note sometimes depth of modulation is quoted as a decimal fraction). Figure 1.4 shows 100 per cent modulation. With f.m. the parameter is the deviation ratio which is given by the ratio of maximum frequency deviation (fd max) to maximum modulating frequency (fm max). The ratio fd/fr„ is called the modulation index and will only be constant and equal to the deviation ratio if the modulating signal is fixed in frequency and amplitude. In Figs 1.3, 1.4 and 1.5 the modulated signal is illustrated in the time domain, i.e. with time along the horizontal axis. It is instructive to look at the frequency domain representations as shown in

Fig. 1.5 Frequency modulation

Fig. 1.6 where a single sine wave of frequency fT , is the modulating signal. It can be seen that several frequencies are present, so giving rise to the idea of bandwidth of a radio information channel. The most significant difference between a.m. and f.m. is that the a.m. bandwidth is finite whereas, in theory, the f.m. bandwidth is infinite. In practice the f.m. bandwidth is regarded as finite, being limited by those extreme sidebands which are regarded as significant, say 10 per cent of amplitude of the largest frequency

frequency component is 3000 Hz we need only transmit a sample of the instantaneous amplitude every 1/6000 = 0-000 166 7 s (= 166.7 ps). Thus we have time-intervals during which we can transmit samples of other signals. The number of signals we can time multiplex on one carrier link depends on the duration and frequency of each sample. The shorter the sample duration the greater the bandwidth required, confirming the statement made earlier that more information requires wider bandwidths. Carrier (fc ) Fig. 1.6 Amplitude modulation and frequency modulation spectrums for a pure sine wave modulating signal of

Basic Receivers and Transmitters A much simplified transmitter block diagram is shown in Fig. 1.7. This could be called the all-purpose block diagram since it could easily be converted to a

frequency fm

component. The relative amplitudes of the carrier and sidebands depend on modulation factor and index for a.m. and f.m. respectively. In any information link there is a relationship between the bandwidth and the amount of information which can be carried, hence high-fidelity stereo broadcasts occupy a wide bandwidth. It is not, however, desirable to have as wide a bandwidth as possible since (a) the number of available channels is reduced; (b) electrical noise, generated at all frequencies by electrical equipment and components, and by atmospheric effects, will be present in the receiver channel at a greater power level the wider the bandwidth. The signal power to noise power ratio is a limiting factor in the performance of receiving equipment. The information in an a.m. wave is repeated in each of the sidebands; the carrier frequency component has no information content. As a consequence, at the expense of more complicated transmitting and receiving equipment, we need transmit only one sideband. Single sideband (s.s.b.) transmission conserves bandwidth, with attendant advantages, and is found in airborne h.f. comm. systems. Multiplexing In most airborne systems the required number of channels is obtained by allocating non-overlapping bands of frequencies centred on specified discrete carriers. This is known as frequency multiplexing. Shannon's sampling theory shows that a sine wave of frequency fm can be completely specified by a series of samples spaced at no more than 1/2 f m second (s). To transmit speech where the highest

Fig. 1.7

Basic radio transmitter block diagram

low-level a.m. transmitter, (little if any amplification of the carrier before modulation), a high-level a.m. transmitter (little if any amplification of the carrier after modulation), an s.s.b. transmitter (introduce a band pass filter after the modulator) or an f.m. transmitter (introduce a frequency multiplifer after the modulator). Obviously in the above examples the circuit details would vary greatly, particularly in the modulators, and if detailed block diagrams were drawn the underlying similarities in structure would be less obvious. The most basic type of receiver is a tuned radio frequency (t.r.f.), however this is rarely used. The standard receiver configuration is the superheterodyne (superhet) shown in Fig. 1.8. The desired r.f. is converted to a constant intermediate frequency by taking the difference frequency after mixing the received signal with the output from a local oscillator (Lo.). Since most of the amplification and selectivity is provided by constant frequency and bandwidth stages the design problem is eased. In both the transmitter and the receiver, r.f. oscillators have to b e tuned to different frequencies. In the transmitter it is the m.o. (master oscillator), while in the receiver it is the Lo. Modern practice is

Fig. 1.8 Basic superhetrodyne receiver block diagram

to usea frequencysynthesizerwith a singlecrystal to providestability dnd accuracy.

In all of the abovethe logic may be reversed(positive and negativelogic). Thus we can representa binary digit (bit) by an electricalsignal,but if the number to be representedis largerthan l, we must combinebits into someintelligiblecode. DigitalSystems Binary code hasbeenmentioned;this is simply counting to the base2 rather than the basel0 Coding (decimalcode) aswe do normally. Unfortunately, Most of the airborne systemsin use are basically binary numberssoon becomevery large,for example analogue,i.e. they dealwith signalswhich represent 9 l r o = I 0 I I 0 I 1 2( t h e s u b s c r i p t s i n d i c a t i n g t h e variousquantitiescontinuouslyand smoothly. For = examplein DME a very smallincreasein rangeresults base),so octal (base8 23) and hexadecimal = 24) may be used. The machinemay still (base l6 in a correspondingincreasein time;we say time is an dealwith a 1/0 situationbut the numbersaremore analogueof distance. With a digital system, when written down, for example manageable information is representedby a numberencodedin 91 ro = l33s = 5816. Note, in the examplesgiven,if somesuitableway. Sinceit is difficult to detectmany different voltage we split the binary number into groupsof three from the right (leastsignificantbit, l.s.b.)we have or current levelsonly two are used,and this leads l , 0 l l , 0 1 1 2 = 1 , 3 , 3 s , i . e .e a c hg r o u pi s t h e b i n a r y naturallyto expressingnumbersto the base2 (binary code)wherethe only digits are 0 and l. It remainsto code for an octal digit. Similarly =5,816. defineelectronicrepresentations r 1 0 1 ,l 0 l l 2 of 0 and I in an and hexadecimalcodesare usedin digital Binary unambiguousway. Various methodsare usedwith code is usedfor the ATC octal computers, (a) beingby far the most common,in the (Chapter 8). The task of frequency transponder non-exhaustive list which follows. selectionis one which lendsitself to coding,and amongseveralwhich havebeenused,the two most - 0 commonarebinarycodeddecimal(b.c.d.)and two (a) Voltagelevel no voltage - t from five (2/5). Both of thesecodesretain the high voltage = l decimaldigit 'flavour' of the number to be encoded (b) Pulsepolarity positive = Q at the expenseof usingextra bits. To represent9l negative 16 (c) Pulseposition a time interval is split in we considerthe decimaldigits 9 and I separatelyto two halves: give: = | pulse in first half 9lro=1001 00016.9.6., pulsein secondhalf = 0 (d) Phasechange at specified read time a 9l ro=10001 11000275 sinewave: changesphase Equivalentsfor all the codesmentionedare givenfor = (180"C) | numbers0 to l5 in Table 1.5. decimal . doesnot change It can be seenfrom the abovethat more bits than = Q are absolutelynecessary phase are usedfor b.c.d. and2l5.

4. conversionfrom binarYto b.c.d.; 5 . b.c.d. fed to frequencysynthesizer; 6 . conversionfrom b.c.d. to specialcode; 7 . specialcode fed to readoutdevice.

Tabh 1.5 Variouscodeequivalents Code

Base

1 0 2

8 1 6

0 0000 0 0 I 000r I I 2 0010 2 2 3 00ll 3 3 4 0100 4 4 5 0l0l 5 5 6 0 1 1 06 6 ? 0lll 7 I 8 1000 l0 8 9 1 0 0 1I I 9 l0 l0l0 l2 A ll l0ll l3 E 12 I 100 14 c 13 ll01 15 D 14 lll0 l6 E 15 llll l7 F

2ls

BCD

0001 0001 0001 0001 0001 0001

0000 0001 0010 001I 0100 0l0l 0l 10 0lll 1000 l00l 0000 0001 0010 00lt 0100 0101

11000 11000 11000 11000 11000 u000

0l001 l 1000 10100 0l 100 0 l0 l q 0 0 1l 0 00101 0001I 10010 10001 01001 11000 10100 01100 01010 00110

If the bits are transmitted serially, one after the other in time down a line, then more time is neededfor the of a number than would be neededif transmission. binary codewere used. If the bits are transmittedin parallel,one bit per line, then more lines are needed. This hasa certain advantagein that the redundancy may be usedto detect transmissionerrors, for e x a m p l eI 0 I I O c o u l d n o t b e a 2 / 5 c o d e a n d I 0 I 0 could not be b.c.d. Error checking can also be used with binary codes. We will alwaysbe restrictedto a certainmaximum numberof bits, one of which can be designateda parity bit usedsolely for error detecting. Supposewe had eightbits available,eachgroup bf eight-bitswould be call-eda word of length 8 (commonly calleda byte). Ttrefirst sevenbits of the word would be used to encodethe decimal digit (0 to 127) while the eighthwould be the parity bit. For odd parity we set the parity bit to 0 or I so as to make the total numberof onesin the word odd; similarly for even parity. Thus61e= 00001l0l odd parity or j 3,o oooot 100 evenparity. Error correcting(as opposedto detecting)codesexist but do not find use in airborneequiPmentasYet. To considera practicalapplicationqf the above Srpposea particular frequencyis selectedon a control unit, we may havethe following sequenceof events: l. information from controller: 215 code', 2. conversionfrom2l5 to binary; binary data; 3. microcomputerprocesses

So far we haveonly discussedthe coding of numericaldata. The ISO (lnternationalStandards Organisation)alphabetNo. 5 is a seven-bitword code which can be usedto encodeupper and lower case letters,punctuationmarks,decimaldigits and various other charactersand control symbols' The full code may be found in most of the latest ARINC and will not be repeatedhere,however' characteristics examplesare A = I 0 0 0 0 0 I' O I 0 0 10,etc. Aparitybitmaybeadded n=i to give a byte. Wh"r. . limited numberof actualwords needto be 'distance','speed','heading',etc' special encoded,e.g. codesmay be designated'Suchcodesare describedin AR INC specification 429'2 digital in fo rm ation transfersystem(DITS) which is discussedin Chapter13. Microcomputers The microprocessorhas brought powerful computers on to aircraft to perform a number of functions, includingthe solution of navigationequations,in a more sophisticatedway than before' A microcomputerconsistsof a microprocessorand severalperipheralintegratedcircuits(chips),to help the microprocessorperform its function' There are four basicparts to computers,micro or otherwise:memory, arithmeticlogic unit (ALU)' control unit and the input/output unit (l/O)' In a microcomputerthe ALU and control unit are usually combinedon a singlechip, the microprocessoror centralprocessingunit (CPU)' Figure l '9 illustratesa basicsystem. The memory containsboth instructionsand data in the form of binary words' Memory is of two basic types, ,ead only (ROM) and random access(RAM)' The ROM doesnot rememberany previousstate which may haveexisted;it merely definesa functional relationshrpbetweenits input lines and its output lines. The RAM could be termed readand write memory; sincedata can be both readfrom memory urd written into memory, i.e. its statemay change' trnformation in RAM is usually lost when power is switched off. The ALU contains the necessarycircuitry to allow it to carry out arithmeticoperations,such asaddition and subtiaction,and logicalfunctions such as Boolean algebraoperations(combinationsof NANDs and NORsetc.).

f':

J

FA. t.9 Basicmicrocomputer organization The control unit providestiming instructionsand from memory, on the data bus, to the control unit synchronizationfor all other units. The control whereit is decoded. The programcounter signalscausethe other units to move data, manip_ulate automaticallyincrementsby one count, and after the numbers,input and output information. All this current instruction has been executedthe next instructionis fetched. This basiccvcle of: activity dependson a set of step-by-stepinstructions (known as the program)which residein memory. The l/O unit is the computer'sinterfacewith the fetch outsideworld. decode From Fig. 1.9 it can be seenthat the units are increment interconnectedby three main buses. A bus is several execute electricalconnectionsdedicatedto a particular task. A unidirectionalbus allowsdata flow in one direction only, unlike a bidirectionalbus where flow is two-way. is repeatedcontinuously. During the executionof an instructiondata may have to be fetched from ln a microcomputerwe usuallyhave: memory, for exampleto add two numbersthe instructionwill need to tell the CPU not only that an l. address bus: sixteenunidirectionallirfes; additionoperationis necessary, but the location.in 2. databus: eightor sixteenbidirectionallines; 3. control bus: the numberof linesvarieswith the the memory,of the numbersto be added. The rate at which instructionsareexecuted systemand may haveboth unidirectionaland dependson the complexityof the instrtrctionand the bidirectionallines. frequencyof the systernclock. Eachpulsefrom the .: clock initiatesthe next actionof the system;several To operate,eachstep-by-step instructionmust be actionsper instructionareneeded.Often the clock fetched,in order,from memoryand executedby the CPU. To keeptrack of the next stepin the program, circuitis on the CPU chip. the only external a programcounter is usedwhich incremelts eachtime componentbeinga crystal. The I/O data flows via logicalcircuits calledporrs. an hstruction is fetched. Before an instruction can be executed.it must be decodedin the CPU to Theseports may be openedin a similar way to that in determinehow it is to be accomplished. which memory is addressed.In somesystemsthe I/O On switch-on,the programcounter is set to the ports are treated as if they were RAM - an address first storedinstruction. The address(location) of this opensa particularport and data flows in or out of first instruction is placedon the addressbus by the that port dependingon whether a read or write signal pro$am counter causingthe instruction to be fetched is present. A variety of chips are used for l/O, some 10

of whi_chare very basic; others (programmable ports) more flexible. The program which is resident in ROM is srbdividedinto routines. Someroutineswill be runningcontinuouslyunlessstopped;others may only be called for when the need ariseJ. For example, a navigationcomputer will continuously compute the aircraft position by running the mainioufini (or bop) which instructs the ALU as to which calculationsmust be carried out using data available in memory. This data must be updated periodically by acceptinginformation from, siy, u ,"dio navigationsensor. When data is available from the extemalequipment,an interrupt sigral is generated and fed to the microcomputer on an interiupt line. Such a signalcausesthe computer to abandon the main routine and commencea serviceroutine which will supervisethe transfer of the new data into rrrmory. After transfer the main routine will rccommenceat the next step, rememberedby a CpU register. The topics discussedin the paragraphsabove can all be classifiedashardware or softwaie. The hardwareis the sum total of actual components up the computer: chips,active and passive T"king discretecomponents,and interwiring. Software comprises_programs, proceduresand the languagesor codesusedfor internal and external commu-nication. Softwaredeterminesthe stateof the hardwarear any particulartime. In an airbornecomputer both the software and hardware are fixed Uy itre designer. The operatordoesnot haveto program the computer in the sensethat he must write a routine; however, he plays his part in how the computer will function b1,,for example,selectinga switChposition which will causecertain data to be preseniedto him by the computer,insertingatard (hardware),on which codedinstructionsor data (software)havebeen *Titten, into a cardreader,etc. Examplesof the use of microcomputersare coruideredin someof the chaptersto follow. These applications,and the abovebrief discussion.should givethe readera basicidea on how computerswork; for detailsof circuitry and programminj consult the readily availablespecialistliteriture.

CaGgorization of Airborne Radio Eqripments Frcqucy and Modulation T* 9" techniquesinvolvedvary greatlywith the r.f. andtypeof modulationused,itls ofien usefulto crtegonze equipmentasto the bandof frequencies in

which it operates(seeTables1.2 and 1.3) and as being pulsed, a.m. or f.m. From both the desigr and maintenancepoint of view, the frequency at which equipmentoperatesis perhapsmore important than the modulation used, at least in so far ai the choice of componentsand test equipmentis concerned. The higher the frequency the greaterthe effect of stray capacitanceand inductance, sigral transit time and skin effect in conductors. In thi microwave region(s.h.f. and the high end of u.h.f.) wavezuide replacesco-axialcable,certainly above5 GHz] and specialcomponentswhosedimensionsplay a critical part in their operationareintroduced(klystrons, magnetrons,etc.). Analogue-Digital Theseterms have alreadybeen mentioned and certain aspectsof digital systemshavebeendiscussed.In modern airborne systemsthe information in the radio and intermediatefrequencystages,including the 'wireless' r.f. link, is usually in analogueform (the exception being secondarysurveillance rcdar (see Chapter8), to be joined in future by microwave gtail8 systems,data link and the replacementfor SSR(seeChapter l3)). In addition Commonlyused transducerssuch as synchros,potentiometers, microphones,telephonesand speakersare all analogue devices.Not all transducersarein the analogue Tlegory, a shaft angleencoderused in encoding altimetersis basicallyan analogueto digital converter. With the exception of the above almost everything . elsein current equipmentis digital, whereas previouslysystemswere all analogue.There is a further subdivisionwithin digitaliquipment into thoseusinga combinationof hardwareand software (computer-controlled)and thoseusingonly hardware (hardwiredlogic). The trend is towardsthl former. Function The two basic categorieswith regardto function are communicationsand navigation. If navigationis definedin its widest senseassafe,economicalpassage from A to B via selectedpoints (waypoints) then communicationssystemseould be consideredas belongingto the navigationcategory. If, however, communicationssystemsare regardedas those systemscapableof transmittingspeechover radio or wire links, and all other systemsas navigation,we iue obeyinga sensibleconvention. The introduction of data links will requiresomeamendmentto the definition of communicationssystems,since , non-navigationaldata will be transmittedbut not as a speechpattern. Navigationsystemsmay be subdividedinto radio

t1

category; landing-aids landingsystemsbelongto the category the of subdivisions these itfr*t?i',vpes of in will be considered Chapters systems lf nuuigution ui9: systems r'ndlnc, Position-fixing 111 "^.,. S. f f uiO 5 respectively' ffi; ;;rfieiiht-nndrng, d and the latter we nave ntaine For self'co into d environment-monitoring' I ^"' u.'i" t,ft.i subtlivide a]tim,etlrs radio dead while uses former systems' The -l.i-ghlri"olng weatheravoidance categorv andinstrument ilffi;;,i"t-tutta'

systemsconcemls andnon'radio,but only the radio posit i on-t txmg is on si i iv d ;;;":'-A;;ih.i possiblesub

fi "* ; ;it

(@er4''l.j$*

(VOR/DME/RNAV/ILS) SYSTEM NAVIGATION re{S80 IIITEGRATED

5750Hz mustbe attenuated Frequencies least20 dB. HarmonicDistortion [.essthan 7'5 percentwith 30 per centmodulation. Irss than2Opercentwith 90 per centmodulation. AGC No morethan 3 dB variationwith input signalsfrom 5 gV to 100mV. Transmitter Stability Carrierfrequencywithin t 0'005 per cent under prescribed conditions. PowerOutput 25-40W into a 52 O loadat theendof a 5 ft transmission line. Sidetone With90 per centa.m.at 1000Hz the sidetoneoutput strallbe at least100mWinto eithera 200 or 500O load. Mic. Input of Mic.audioinput circuitto havean impedance 150O for usewith a carbonmic.or a transistormic. from the (approx.)20 V d.c.carbonmic. operating supply. Antenna Verticallypolarizedandomnidirectional.

2A

,J

'i6

,$

$

To match 52 O with VSWR ( l'5 : l. Ramp Testing After checking for condition and assemblyand making availablethe appropriate power suppliesthe following (typical)'checksshouldbe madeat each stationusingeachv.h.f.

-

l. Disablesquelch,checkbackgroundnoiseand operationof volume control. 2. On an unusedchannelrotate squelchcontrol until squelchjust closes(no noise). Pressp.t.t. button, speakinto mic. and checksidetone. 3. Establishtwo-way communicationwith a remotestation usingboth setsof frequency control knobs,in conjunctionwith transfer switch,if appropriate.Checkstrengthand quality of signal.

The current and future norm is to use single sideband(s.s.b.)mode of operationfor h.f. communications,although setsin servicemay have provisionfor compatibleor normal a.m.,i.e. carrier and one or two sidebandsbeing transmitted respectively.This s.s.b.transmissionand reception hasbeen describedbriefly in ChapterI and extensivelyin many textbooks. A featureofaircraft h.f. systemsis that coverageof a wide band of r.f. and useof a resonantantennarequiresefficient antenna tuning arrangementswhich must operate automatically on changingchannelin order to reduce the VSWR to an acceptablelevel.

Installation A typical large aircraft h.f. installation consistsof two systems,eachof which comprisesa transceiver, controller, antennatuning unit and antenna. Eachof NB . Do not transmiton I 2l '5 MHz (Emergency). the transceivers are connectedto the AIS for mic.. tel. Do not transmitif refuellingin progress. and p.t.t. provision. In addition outputs to Selcal. Do not interrupt ATC-aircraftcommunications. decodersare provided. Suchan installationis shown i n F i g .2 . 1l . The transceivers contain the receiver,transmitter, H.F. Communicataons power amplifier and power supply circuitry. They are mounted on the radio rack and providedwith a flow BasicPrinciples of cooling air, possibly augmentedby a fan. A The useof h.f. (2-30 MHz) carriersfor communication transceiverrated at 200 W p.e.p.needsto dissipate purposesgreatly extends the rangeat which aircrew 300 W when operatedon s.s.b.while on a.m. this canestablishcontact with AeronauticalMobile figure risesto 500 W. Telephoneand microphone Servicestations. This beingso, we find that h.f. jacks may be providedon the front panel,asmight a comm.systemsare fitted to aircraft flying routes meter and associatedswitch which will provide a which are,for somepart of the flight, out of rangeof meansof monitoring variousvoltagesand currents. v}t.f. service.Such aircraft obviouslyinclude public Coupling to the antennais achievedvia the transportaircraft flying intercontinentalroutes,but antennatuning unit (ATU). Somesystemsmay thereis alsoa market for generalaviationaircraft. employ an antennacouplerand a separateantenna The long rangeis achievedby useof sky waves couplercontrol unit. The ATU provides, which arerefractedby the ionosphereto suchan automatically,a match from the antennato the 50 Q extent that they arebent sufficiently to return to transmissionline. Closed-loopcontrol of matching earth. The h.f. ground wavesuffersquite rapid elementsreducesthe standingwaveratio to l'3 : I attenuationwith distancefrom the transmitter. or less(ARINC 559A). Ionosphericattenuationalsotakesplace,being Since the match must be achievedbetween line and greatestat the lower h.f. frequencies.A significant antennathe ATU is invariablymountedadjacentto featureof long-rangeh.f. transmissionis that it is part of the the antennalead-in,in an unpressurized zubjectto selectivefadingovernarrow bandwidths airframe. For high-flyinga'ircraft(most jets) the ATU (tensof cycles). is pressurized,possiblywith nitrogen. Someunits The type of modulation used,and associated may contain a pressureswitch which will be closed detailssuchas channelspacingand frequency wheneverthe pressurizationwithin the tuner is channellingincrements,havebeenthe subjectof adequate. The pressureswitch may be used for many papersand ordersfrom users,both civil and ohmmeter checksor, providingswitch reliability is military, and regulatingbodies. ARINC Characteristic adequate,may be connectedin serieswith the key No. 559A makesinterestingreading,in that it reveals line thus preventing transmissionin the event of a how conflictingproposalsfrom variousauthorities leak. Altematively an attenuatormay be swit;hed in (in both the legaland expert opinion sense)can exist to reducepower. at the sametime. Light aircraft h.f. systemsin serviceare likely, for

A

Mic.

28V---2-

Tcl.

No. 1 Xmit

No. I

t.r.

No. I p.t.t. No. 2 interlock No. I interlock

28V

2av

No. 2 p.t.t.

Mic. -

Tel.

l

28vl ruoz

+

Fig 2.1I Typicaldualh.f. installatbn financial reasons,to have a fixed antennacoupler. Sucha systemoperateson a restrictednumber of channels(say twenty). As a particular channelis selected,appropriate switching takesplace in the coupler to ensurethe r.f. feed to the antennais via previously adjusted,reactivecomponents,which make the effective antennalength equal to a quarter of a wavelength,thus presentingan impedanceof approximately 50 O. The required final manual adjustmentmust be carried out by maintenance personnelon the aircraft. The antennaused variesgreatly, dependingon the type of aircraft. For low-speedaircraft a long wire antennais popular although whip antennasmay be found on somelight aircraft employing low-powered h.f. systems. The aerodynamicproblemsof wire

c,

antennason aircraft which fly faster than, say, 400 knots, haveled to the useofnotch and probe antennaswhich effectively excite the airframe so that it becomesa radiatingelement. Modernwire antennasare constructedof copper-cladsteelor phosphorbronze,givinga reduced comparedwith earlierstainless.steel r.f. resistance wires. A coveringofpolythene reducesthe effectsof precipitationstatic. Positioningis normally a single spanbetween forward fuselageand vertical stabilizer. I:rger aircraft will have twin antennaswhile a single 'V' configuration,is more installation,possiblyin a aircraft. The r.f. feed is usually for smaller common at the forward attachment via an antennamast. The rear tetheringis by meansof a tensioningunit. The aniennamaqtis subjectto pitting and erosion

of the leadingedge;a neoprpnecoveringwill provide someprotection,nevertheless regularinspectionsare called for. Protection againstcondensationwithin the mastmay be providedby containersof silicagel which shouldbe periodicallyinspectedfor a changein colour from blue to pink, indicatingsaturation. Hollow mastsare usuallyprovidedwith a water-drain path which shouldbe kept free from obstruction. The two most important featuresof the rear tetheringpoint are that the wire is kept under tension and that a weak link is providedso asto ensurethat any break occursat the rear,so preventingthe wire wrappingitself around the verticalstabilizerand rudder. On light aircraft a very simplearrangementof a spring,or rubber bungee,and hook may be used. The springmaintainsthe tensionbut if this becomes excessivethe hook will open and the wire will be free at the rearend. On largeraircraft a spring-tensioning unit will be usedto copewith the more severe conditionsencountereddue to higherspeedsand fuselageflexing. The unit loads the wire by meansof a metal spring,usuallyenclosedin a barrelhousing. A serratedtail rod is attachedto the tetheringpoint on the aircraft and insertedinto the barrelwhereit is securedby a springcollet, the grip of which increases with tension. The wire is attachedto a chuck unit which incorporatesa copperpin servingas a weaklink desigredto shearwhen the tensionexceedsabout 180 lbf. Someunits incorporatetwo-stageprotection againstoverload. Two pins of different strengthsare used;shouldthe first shear,a smallextension(3/ 16 in.) of overalllength results,thus reducingtensionand exposinga yellow warningband on the unit. Notch antennasconsistof a slot cut into the aircraft structure.often at the baseof the vertical o stabilizer. The inductanceof the notch is series-resonated by a high-voltagevariablecapacitor driven by a phase-sensing servo. Signalinjection is via matching circuitry driven by a SWR sensingservo. 'Q' Since the notch is high the input is transformed to a voltageacrossthe notch which is ofthe order of thousandsofvolts. This largevoltageprovides the driving force for current flow in the airframe which servesas the radiator. A probe antenna,which is aerodynamically acceptable,may be fitted at either of the wing-tipsor on top of the verticalstabilizer. Againseriestuning providesthe necessary driving force for radiation. The probe antenna,aswell as the wire antenna,is liable to suffer lightningstrikes,so protection in the form of a lightning arrester(sparkgap)is fitted. Any voltagein excessof approximatelyl6 kV on the antennawill causean arc acrossthe electrodesof the hydrogen-iilledsparkgap,thus preventingdischarge

through the h.f. equipment. Build-up of precipitation static on antennas,particularlyprobes,is dealt with by providinga high resistance static drain (about 6 MSl) path to earth connectedbetweenthe antenna feed point and the ATU. It is important in dual installationsthat only one h.f. systemcan transmit at any one time;this is achievedby meansof an interlock circuit. This basic requirementis illustratedin Fig. 2.11 whereit canbe seenthat the No. I p.t.t. line is routed via a contact of the No. 2 interlock relay, similarlywith No. 2 p.t.t. The interlock relayswill be externalto the transceivers often fitted in an h.f. accessory box. While one of the h.f. systemsis transmittingthe other systemmust be protectedagainstinducedvoltages from the keyedsystem.In addition,with some installations,we may havea probe usedas a transmitting antennafor both systemsand as a receivingantennafor, say,No. I system. The No. 2 receivingantennamight be a notch. It follows that on keying either systemwe will havea sequenceof eventswhich might proceedas follows. HF I keyed: l. HF 2 keyline broken by a contactof HF I interlock relay; 2. HF 2 antennagrounded; 3. HF 2ATU input and output feedsgounded and feed to receiverbroken. HF 2 keyed: l. HF I keyline broken by a contactof HF 2 interlock relay; 2. HF I probe antennatransferredfrom HF l; ATU to HF 2 ATU; 3. HF 2 notch antennafeed grounded; 4. HF I ATU input and output feedsgrounded and feed to receiverbroken. C,ontrols and Operation Separatecontrollers are employed in dual installations, eachhaving'in-use'frequencyselectiononly. Older systemsand some light aircraft systemshave limited channelselectionwhere dialling a particular channel number tunesthe system,includingATU, to a pre-assigned frequency,a channel/frequency chart is required in such cases.With modern sets,indication ofthe frequency selectedis given directly on the controller. The controlsshownin Fig. 2.1 I arethosereferred to in ARINC 559A; variationsare common and will be listed below. Mode Selector Switch. OFF-AM-SSB Thc'turn off' function may be a separateswitch or indeed may not

31

be crnployed at all; snritchingon and offbeing achievedwith the masterradio switch. The 'AM' positionmay be designated'AME'(AM equivalentor compatible)and is selectedwhenevertransmission and receptionis requiredusinga.m. or s.s.b.plus full carrier(a.m.e.). The 'SSB'position providesfor transmissionand reception of upper sidebandonly. Although useof the upper sidebandis the norm for aeronauticalh.f. communicationssomecontrollers 'USB' 'LSB'positions. have and In addition 'DATA' 'CW'modes and may be available.The former is for possiblefuture use of data links by h.f. using the uppersideband- the receiveris operatedat maximumgain. The latter is for c.w. transmissionand reception,morsecode,by 'key bashing',being the information-carryingmedium. Flequency SelectorsFrequency selecton consist of, typically, four controls which allow selectionof frequenciesbetween 2.8 and 24MHz in I kHz steps (ARINC 559A). Military requirementsare for a frequencycoverageof 2 to 30 MHz in 0.1 kHz steps, consequentlyone will find systemsoffering 280 000 'channels'meeting theserequirementsin full or 28 000 channelsmeetingthe extendedrangebut not the 0'l kHz steprequirement. When a new frequency is selectedthe ATU must adjustitselfsincethe antennacharacteristics will change.For this purposethe transmitteris keyed momentarily in order that SWR and phasecan be measuredand usedto drive the ATU servos.

Indicator A meter mounted on the front panel of the controller may be providedin order to give an indication of radiatedpower. Block Dhgram Operation Tlansceiver Figure 2.12 is a simplified block diagram of an a.m./s.s.b.transceiver.The operationwill be describedby function. Amplitude Mo dulated Transm issio n The frequency selectedon the controller determinesthe output from the frequencysynthesizerto the r.f. translatorwhich shifts the frequency up and provides sufficient drive for the power amplifier(p.a.). The mic. input, after amptfication, feedsthe modulator which produces high-levelamplitudemodulation of the r.f. amplified by the p.a. The r.f. signalis fed to the ATU via the antennatransferrelay contact. The PA output signalis sampledby the sidetone detectorwhich feedssidetoneaudio via the contact of the deenergizedsidetonerelay and the sidetone adjustpotentiometerto the audio output amplifier.

Single Sideband Transmission Low-level modulation is necessarysincethere is no carrierto modulateat the p.a. stage,hencethe mic. input, /n., is fed to a balancedmodulator togetherwith a fixed carrier frequency,/., from the frequencysynthesizer.The balancedmodulator output consistsof both sidebands f" + f^ andf" - f^, the carrierbeingsuppressed. The requiredsidebandis passedby a filter to the r.f. SquelchControl Normel control of squelch translatorafter further amplification. thresholdmay be provided. As an alternativean r.f. Ifwe consideran audio responsefrom 300 to sensitivitycontrol may be used,but where Selcalis 3000 Hz we seethat the separationbetweenthe utilizedit is important that the receiveroperatesat lowestEs.b. frequencyand the highestl.s.b. full sensitivityat all timeswith a squelchcircuit being frequencyis only 600 Hz. It follows that the filter employedonly for aural monitoring and not affecting usedmust havevery steepskirts and a flat bandpass. the output to the Selcaldecoder. A mechanicalfilter can be usedin which an input transducerconvertsthe electricalsignalinto Audio Volume Control Providesfor adjustment of mechanicalvibrations,theseare transmittedby audiolevel. Sucha control may be locatedelsewhere, mechanicallyresonantmetal discsand coupling rods suchas on an audio selectorpanel, part of the AIS. and finally convertedback to an electricalsignalby an output transducer. C:lanfter This control is to be found on some h.f. Frequencytranslationis by a mixing process controllers. With s.s.b.signalswhile the phaseof thc rather than a multiplicativeprocesssinceif the re-insertedcarrier is of little consequenceits u.s.b./, + /n' were multiplied by try'we would frequencyshouldbe accurate.Should the frequency radiatea frequencyof//(/c + /n,') rather than be inconect by, say,in excessof t 20 Hz ft + f " + /.. The amount by which the u.s.b.is deterioration of the quality of speechwill result. translated,fi, is determinedby the frequencyselected A clarifier allows for manual adjustment of the on the controller. Final amplification takes place in re-insertedcarrier frequency. Use of highly accurate the p.a. prior to feedingthe r.f. to the ATU. md stablefrequency synthesizersmake the provision To obtain sidetonefrom the p.a. stagea carrier of such a control unnecessary. would needto be re-inserted.A simplermethod, 32

Sidetonc relay

To r.f./i.f. stages

f"+ff"-

f.

Fq.2.l2 Typicalh.f. a.m./s.s.b. trmsceiverblockdiagram

which neverthelessconfirms that a sigral has reached the p.a.,is to usethe rectified r.f. to operatea sidetonerelay. When energizedthe contact of this relay connectsthe amplified mic. audio to the output audio amplifier. Amplitude Modulated Reception The receivedsignal passesfrom the ATU via the de+nergized antenna transferrelay contact to an r.f. amplifier and thence to the r.f. translator. After the translatornormal.a.m. detection takes place, the audio so obtained being fed to the output stage. A variety ofa.g.c. and squelch circuitsmay be employed.

t

Single Sideband Reception The circrit action on s.s.b.is similarto that on a.m. until after the translatorwhen the translated r.f. is fed t6 the product detector along with the re-inserted'carrier' /". The output ofthe product detector is the required audio

signal,which is dealt with in the sameway as before. Antenna Tuning Unit Figure 2.13 illustrates an automatic ATU simplified block diagram. On selectinga new frequency a retune sigrralis sent to the ATU control circuits which then: l. keys the transmitter; 2. insertsan attenuatorin transceiveroutput line (Fig.2.t2); 3. switcheson the tuning tone sigral generator (Fig.2.l2) and drivesa tune warninglamp (optional); 4. switcheson referencephasesfor servo motors. The r.f. signalon the input feed is monitored by a loading servosystem and a phasingservo system. If the load impedanceis high then the line current, /L, is low and the line voltage ZL is high. This is dctected by the loading s€rvodiscriminator which

El

I

Aru

Tune Tx Rctunc tone keY

Transceiver

p;g.2.13 Typicalh.f. a.t.u.blockdiagrarn

appL:: the appropriate amplitude and polarity d.c. sigral to a chopper/amplifier which in turn provides the control phasefor the loading servomotor. The auto transformertap is drivenuntil the load impedrnceis 50 O. Should Iy and Vynot be in phasethis is detected by the phasingservodiscriminator which appliesthe approp:iateamplitudeand polarity d.c. signalto a chopper/amplifier which in turn provides the control phasefor the phasingservomotor. The reactive elemenis,inductanceand capacitance, are adjusted untri.Il and Vy are in phase. As a result of the action of the two servo systemsa resistiveload of 50 O is presentedto the co-axial feed from the transceiver. When both servosreach their null positions the control circuits removethe signals listedpreviously. Ctaracteristics The following brief list of characteristicsare those of a systemwhich conformswith ARINC 559A. frequency Selection An r.f. nnge of 2'8-24 MHz coveredin I kHz increments. Method: reentrant frequency selectionsystem. Orannelling time lessthan I s. Mode of Operation Singlechannel simplex, upper singlesideband. g

Tlansmitter Poweroutput: 400 W p.e.p.(200 W p.e.p. operatiohal). Absolutemaximum power output: 650 W p.e.p. Mic. input circuit frequencyresponse:not more than I 6 dB variation from 1000 Hz levelthrough therange 350 Hz to 2500 Hz. Spectrumcontrol: componentsat or below /" -100 Hz and at or abovef" +29O0Hz shouldbe attenuatedby at least30 dB. Frequencystability: ! 2OHz. Shop adjustmentno more often than vearly. Pilot control (e.g.clarifier) not acceptable. lnterlock: only one transmitter in a dual system 'first-come, strouldoperateat a time on a first-served' basis,this includestransmittingfor tuning purposes. Receiver Sensitivity:4 pV max.; 30 per cent modulation a.m. (l pV s.s.b.)for l0 dB signaland noiseto noiseratio. A.g.c.: audio output increasenot more than 6 dB for input signalincreasefrom 5 to I 000 000 pV and no more than an additional 2 dB up to I V input signal level. Selectivity: s.s.b.,6d-Bpoints atf"+ 300 Hz and /. + 3100 Hz, t 35 dB pointsat f"andf" + 3500 Hz. A.m.: toensureproper receiveroperation(no adjacentch4nnelinterference)assumingoperationson 6 kHz spaceda.m. channels.

Overall response:compatible with selectivity but in addition no more than 3 dB variation between anv two frequenciesin the range300-1500 Hz (for satisfactorySelcaloperation). Audio output: two-wire circuit isolated from ground, 300 O (or less)output impedancesupplying 100 mW (0'5 Selcal)into a 600 O load.

w h e r e N= 1 2 ,1 3 . . . 2 7 ,

giving a total of sixteen tones betwecn 312'6 and 1479.1Hz. The tonesare desigratedby lettes A to S omitting I, N and O so a typical code might b,: AK-DM. The re ue 297Ocodesavailablefor assigrmentusing the first twelve tones, the addition of tonesP, Q, R and S (1976) bring the total to 10920. Codesor blocks ofcodes are assignedon Ramp Testing ard Maintenance Whilst regularinspection of all aircraft antennasis requestto air carrier organizationswho in turn assigt called for, it is particularly important in the caseof codesto their aircraft'either on a flight number or h.f. antennasand associatedcomponents. Any aircraft registration-related basis. maintenancescheduleshould require frequent Figure 2.14 illustrates a singleSelcalsystem inspection ofantenna tensioning units and tethering large passengertransport aircraft would norma,ly' points in the caseof wire antennas,while for both carry two identical systems. The decoder will probe and wire antennasthe spark gap should be recognizea receivedcombination of tones on rny of in3pectedfor signsof lightning strikes (cracking five channelswhich correspondsto that combrnation . and/or discolouring). selectedon the code selectand annunciator pa:;el. A functional test is similar to that for vh.f. in that When the correct code is recogrized the chime switch two-way communication should be establishedwith a and appropriate lamp switch is made. The lamp .witch remote station: all controls should be checked for supply is by way of an interrupter circuit so that the lamp will flash. A constant supply to the chime satisfactoryoperation and meter indications, if any, svitch causesthe chimes to sound once. Each lanrp strouldbe within limits. Safety precautions are particularly important sincevery high voltagesare holder, designatedHF I , HF I I etc. incorporatesa reset presenton the antenna systemwith the resulting switch which when depressedwill releasethe latched lamp switch and chime switch. The tone filters in the dangerofelectric shock or arcing. No personnel decoderwill typically be mechanicallyresonant should be in the vicinity of the antenna when devices. transmitting, nor should fuelling operations be in Variations in the arrangementshown and progress.Rememberwith many h.f. systemsa change describedare possible. Mechanicallythe control and of frequency could result in transmissionto allow annunciator panel may be separateunits. Should the automaticantennatuning. operatorrequireaircraft registration-related codes there will be no need for code selectswitches.the appropriatecodebeingselectedby jumper leadson Selcal the rear connectorofthe decoder. Although five resetleadswill be provided they The selectivecalling (Selcal.) system allows a ground may be connectedindividually, all in parallel to a group of aircraft using station to call an aircraft or singleresetswitch or to the p.t.t. circuit of the h.f. or vir.f. commswithout the flight crew having transmitter. In this latter caseisolation associated continuously to monitor the station frequency. (within the decoder)prevent'sneak'circuits, diodes A coded sigral is transmitted from the ground and keying one transmittercausingone or more i.e. to the tuned the v.h.f. or h.f. receiver rcceivedby othersto be keyed. appropriate frequency. The output code is fed to a The lamp and chime srrppliesshown can be Selcaldecoderwhich activatesaural and visual alerts at the operator'soption. Possibilitiesare to changed if and only if the receivedcode'correspondsto the reverse the situation and havesteady lights and the aircraft. code selectedin multi-stroke chimes,or havesteady lights and Each transmitted code is made up of two r.f. single-strokechime, in which casethe interrgpt bursts(pulses)eachof-l t 0'25 s separatedby a circuit is not used. period of 0.2 t 0'l s. During eachpulsethe The Selcalsystemswhich do not comply with per with two 90 modulated transmitted carrier is cent ARINC 596 may not providefacilitiesfor decodingof tones, thus there are a total of four tones per call; . five channelssimultaneously. A switch is provided on the frequenciesof the tones determine the code. the control panel with which the singledesired The tones available are given by the formula channelcan be selected;in this caseonly Selcalcodes receivedon the correspondingreceiverwill be fed to = antilog (0'054(/V- t) + 2O), [y

35

b L*-ntgs

Sslf test

[amp drive (5 wiresl

v.H.F.1

v.H.F.2

V.H.F.3

H.F.2

Fig. 2.f4 Typical Selcalblock diagram

the decodcr. Only one annunciator lamp is required. Codeselectionin an ARINC 596 systemis achieved by meansof a 'b.c.d.' format. Eachof the four tone selectorshas four wires associatedwith it; for any particular tone an appropriate combination of the wires will be open circuit, the rest grounded. If the

3G

tones A to S arc numbered I to 16 (0) the open wires will be as given by the correspondingbinary number; e.g.tone M-12-l l(X), so with the wires designated 8,4,2 and I we see8 and 4 will be open. Note this is termed so. not really b.c.d. but is nevertheless Testing of Selcalis quite straightforward. If

possiblea test rig,consistingofa tone generatorin conjunctionwith a v.h.f. and h.f. transmittershould he used,otherwisepermissionto utilize a Selcalequippedground station shouldbe sought. G

Audio IntegratingSystems(AlS) - lntercom Introduction All the systemsin this book exhibit a variety of characteristics but none more so than AIS. In a light. aircraft the function of the audio systemis to provide an interfacebetweenthe pilot's mic. and tel. and the selectedreceiverand transmitter;sucha 'system' might be little more than a locally manufactured junction box with a built-in audio panel-mounted amplifier and appropriateswitching. ln contraSta largemulti-crewpassenger aircrafthasseveral

sub-systems making up the total audio system. The remainderof this chapterwill be concernedwith the AIS on aBoeing747. It is unusual to considerall the systemsand sub-systems which follow as part of AIS, a term which should perhapsbe restricted to the system which provides for the selectionof radio system audio outputs and inpuis and crew intercommunications. Howevera brief descriptionof all systemswhich generate,processor recordaudio signalswill be given. The following servicescomprisethe complete audio system: l. flight interphone: allows flight deck crew to communicatewith eachother or with ground stations; 2. cabin interphone: allows flight deck and cabin crew to communicate:

Attendant's chime call

PA bverride VOR/ILSNAV systom Markerbsacon systom Low range radioaltimeter systom

Visual Pass.ent. audio (motionpic.) system

ATC system DMEsystom ADFsystem HF comnr un ication system

lSatcom I sysrem

I I

Headsets and microph,

tjgg'"q"rlj Fig. 2.15 Boeing747: typical communicationsfit (courtesyBoeingCommdrcialAeroplaneCo.)

37

3. serviceinterphone: allows ground staff to communicatewith each other and also with the flight crew; 4. passengeraddress(PA): allows announcements to be made by the crew to the passengers; 5. passengerentertainment system: allows the showing of movies and the piping of music; 6. gound crew call system: allows flight and ground crew to attract each other's attention; 7. cockpit voice recorder: meets regulatory requirementsfor the recording of flight crew audio for subsequentaccident investigation if necessary. It *rould be noted that the aboveare not completely separatesysremsasillustratedin Fig. 2.15 and describedbelow. The dividing lines between zub-systemsof the total audio system are somewhat arbitrary, and terminologl is varied; however the facilities describedare commonplace. Flight Interphone This is really the basic and most esential part of the audio system. All radio equipments having mic. inputs or tel. outputs, aswell asvirtually all other audio systems,interface with the flight interphone which may, in itself, be termed the AIS. A large number of units and componentsmake up the totd systemas in Table 2.1 with abbreviated termsaslisted in Table 2.2. Figuie 2.16 showsthe flight interphone block diagram,simplified to the extent that only one audio selectionpanel(ASP), jack panel etc. is shown. An ASP is shown in F i g .2 . 1 7 . A crew member selectsthe tel. and mic. signals required by useof the appropriate controls/switches on an ASP. The various audio signalsentering an ASp a.ie'selected by twelve combined push selectand volume controls. Each ASP has an audio bus feeding a built-in isolationamplifier. The v.h.f. and h.f. comm. ADF, interphone and marker audio signalsare fed to the bus via the appropriate selectbuttons and -volume controls. The vh.f. nav. and DME audio is fed to the bus when voice and rangeare selectedwith the Voice pushbutton; with voice only selectedthe DME audio is disconnectedwhile the vJr.f. nav. audio is pased through a sharp 1020 Hz bandstop filter (FLl) before feeding the bus. With the flii-normal switch in the fail position only one audio channel can be selected(bypassingthe amplificr) and the pA audio is fed direct to the audioout lines. Radio altimeter audio is fed direct tb the audio-out lincs. The above audio switching arrangementsare illustrated h Fig. 2.18. Note the sericsresistorsin the input

38

Table 2.1

Flight interphone facilities UPT

FlO

FIE

ASP

x

x

x

x

x

Jack panel

x

x

i

x

x

Int - R-T p.Lt.

x

Handheld mic.

x

X

llcrdrct

Jack

Jack

Boom mic. headset

x

Oxygen mask mic.

Jack

lnterphone speaker

X

OBSI

OBS2

M.E.

x

x

Jack Jack

feck

Jack

Jrct

hck

Jac*

Iack

x trck

lack Jrck

J.ct

A'X'indicates the particular unit or component is fitted at that station (column), 'Jack' indicatesa jack plug b fitted to enableuseof the appropriatemic. and/or tel.

Table2.2 Abbreviations CAPT F/O OBS m.e. -

Captain First Officer Observer Main Equipment Centre mic. - Microphone

a.s.p.int. rlt p.t.t. tel.

Audio SelectorPanel Interphone Radiotelephone hes to Transmit

- Telephone

a\dio lineswhich,togetherwith loadingresistors in the interphoneaccessory box, form an anti-cross talk network;if onecrewmemberhas,say,h.f.l selected on his ASPthen the resistivenetworkwill greatly attenuatesayh.f.2 whidr would otherwisebe audible shouldanothercrewmemberhaveselectedh.f.l and h.f.2. Six mic. selectbuttonsareprovidedon an ASP; threevJr.f. @mm.,two h.f. cornm.andPA. Additiond sritchesasociatedwith mic. selectand transmission aretheboom-mask andr.t.-int.p.t.t. on eachASPand alsop.t.t. buttonson the hand-heldmicrophones. jack panelsandthe captain'scontrolwheel (R/T-int.). To speakorrerinterphonea crewmembershould selectinterphoneusingthe r.t..int. switchon the a.s.p.which will connectmic. high(boom or mask)

c.E@{r-8..|& r^.&rEr.@*ccrd 'Eg-B

E] lr. s lucu O'El

or',a..,.r

D+-i

I

i I

ll ir r t

aaaaa

- - - - l

l l l

:

l!lri4

t t u g rrr,r l:JJ

n ti !r

!

+= :l---+i

F.--di

#--i il'F.'Li"ri-i il--r---l

t

|I

iI + Il s # - . "

|i 1,. i,..*. L-re'g!'e$!sret-l :i

i---r'"* T------1 ij-#S-.i i I

i

i

r

i Fi& 2.lt Ardb signelselectbn(courtcsyBoeiru Aeroplane Commercial Co.)

Fig. 2.16 Boeirg ?4?: night intqphonc (courtcry Boeirg CommercialAeroplaneCo.)

tr trtr trtrtr

i_p-

l l t l l

EOOM

ooooo. l-oo'l l-"i"_.] s o o--fio,, o CI

rE

,r'i

El

I t

-Gql#

rNr

rrr

o

**"

atr@

xrt

.3

r' l l -

F .

Fig.l.l7 Audioselection panel(c€urtesy Boeing Commercial Aeroplane Co.) to the interphone mic. high output feeding the flight interphoneamplifier in the interphoneaccessorybox. Alternativelythe captaincan seiectinterphoneon his control wheel p.t.t. switch which will energizerelay K2 thus making the mic. higft connectionas before. Note that the ASP r.t.-int.p.t.t. switchdoesnot rely on power reachingthe ASP for relay operation (see

f'

rrcw

rs

s*

*rcx

r|rrrftsa ro rLrcir

E r.Mro

IC'Off

{rcaet{d rrtat,da

hIGN

rg

E

Fi& 2.19 Microphone signalselection(courtesy Bocing CommercialAeroplaneCo.)

39

Fig. 2.19). Interphone mic. signalsfrom all ASh are fed to the flight interphone amplifier which combines them and feedsthe amplified interphone audio to all ASPsfor selectionas required. Pressing a mic. selectbutton on the ASP will connectthe correspondingsystemmic. input lines to relayK2 and to contactson the ASP r.t.-int. p.t.t. switch. Thus when a p.t.t. switch is pressed,the mic. lineswill be madeby either the contactsof K2 or by the ASPp.t.t. switchin the r.t. position. In Fig. 2.19 the h.f.2 selectswitch is shown as typical of all comni. selectswitches.Whenthe PA selectswitch is pressed the flight interphonemic. circuit is interruptedand PA audio is applied to the fail-normal switch; in additionthe mic. linesto the PA systemare made. Operationof any p.t.t. switch mutesboth interphone speakers to preventacousticfeedback. Cabin Interphonc The cabin interphoneis a miniatureautomatic telephoneexchangeservicingseveralsubscribers: the cabin attendantsand the captain. In addition the systeminterfaceswith the PA to allow to be made. announcements Numbersaredialledby pushbuttonson the telephonetype handsetsor on the pilot's control unit. Eleventwo-figurenumbersare allocatedto the plus additionalnumbersfor PA in subscribers, 'all-attendants' variousor all compartments,an call 'all-call'. and an Two dialling codesconsistof letters: P-Pis usedby an attendantto alert the pilot (call light flasheson control unit and chime soundsonce) while PA-PAis usedby the pilot to gain absolute priority over all other usersof the PA system. The directory is listed on the push-to-talkswitch incorporatedin eachhandsetto minimizeambient noise. All diallingcode decodingand the necessarytrunk switchingis carriedout in the centralswitchingunit, CSU(automaticexchange).The CSU also contains three amplifiers,one of which is permanently allocatedto the pilot on what is effectivelya private trunk. Of the five other availabletrunks, two are allocatedto the attendants,two to the PA systemand -onefor dialling. (Note a trunk is simply a circuit which can connecttwo subscribers.) The cabin interphoneand serviceinterphone qystemsmay be combinedinto a common network by appropriateselectionon the flight engineer's interphoneswitch panel,captain'sASP and cabin interphonecontrol unit. Any handsetmay then be lifted and connectedinto the network (dial'all-call'). In a similarway the flight interphonecircuits may be usedto make specificcallsover.thecabin interphone system.

n

Crll light

Attondrnt's .t tion3 (typrcrl)

c.:to!:S"hin! Itntcrphonc I lrudio acccrl lbox I Ft. 2.20 Boeing 7rt7: cabin interphonc (courtcsy Boeitg Commercial Aeroplane Co.) .** --o;*

The systemis more complex than has been suggestedabovebut a basic description has been given, zupportedby Fi1.2.20. ServiceInterphone A total of twenty-two handsetjacks arelocatedin variousparts of the airframein order that ground crew can communicatewith one anotherusingthe serviceinterphonesystem. The systemis rather simplerthan thoseconsideredabove. Mic. audio from 'pressto talk' depressed, are all handsets,with combinedin and amplifiedby the serviceinterphone amplifier in the interphoneaudio accessorybox. The amplified signalis fed to all handsettels. Volume control adjustmentis providedby a preset potentiometer. With the flight engineer'sinterphoneswitch selectedto ON the input summingnetworks for both serviceand flight interphonesystemsare combined. All mic. inputs from either systemare amplified and fed to both systems. Address' PassengQr The systemcomprisesthree PA amplifiers,tape deck, annunciatorpanel,attendant'spanel,PA accessory speakerswitch paneland box, control assemblies, fifty-three loudspeakers.The variousPA messages havean order of priority assigredto them: pilot's ts, attendant'sannouncements, announcemen prerecordedannouncementsand finally boarding music. All PA audio is broadcastover the speaker systemand also,except for boardingmusic,overrides

L--------

r----- -T----1

,:.*Q I

*.*A i

L-

l-*.,^f-J, .*,,"."

L__------.J fr

aLaclioir6

cricull

r!r6xr ,i[ma

I ,".",*,.1-liii',T:f..

' i''' *llF^*.; ^*' l;n:'6--'"'"" ; *,,*,* ,"*

f

euo'o

?il6

Fig,2.2l Boeing?47: serviceinterphone(courtesyBoeing CommercialAeroPlaneCo.)

stethoscope entertainmentaudio fed to the passenger emergencyannouncement headsets.A prerecorded may be initiated by the pilot or an attendant,or automaticallyin the event of cabin decompression' 'fasten A chimeis generatedwhen the pilot tums on 'no smoking' siglts. seat-belt'or addressamplifiersare fed via the The passenger flight or cabininterphone systemsfor pilot oratiendantannouncementsrespectively.Distribution of audio from the amplifiersto the speakersin various zonesdependson the classconfiguration,sincesome *noun.itntnts may be intended for only a certain classof passengers. The necessaiydistribution is achievedby meansof switcheson the speakerswitchingpanel. Audio is also fed to the flight interphonesystemfor sidetone purposes. Number 2 and number 3 ampliliers ere slavedto number I for all'classannouncements.Should be requiredthe parallel separateclassannouncements control relay is energized,so separatingthe number I audio from that of number 2 and 3. The control assembliesin the PA accessorybox contain potentiometersused to set the gain of the PA

lf,b__^-rc,

L____r (courtesy Boeing address F1g.2.22Boeing?4?:passenger Co') AeroPlane Commercial

amplifiers. When the aircraft is on the ground with ;;;i;g gearlocked down and ground powgl applied the lev-efofspeakeraudio is reducedby 6 dB' The tape deck containsup to five tape cartridges apart from the necessarytape'{rive mechanism' piaybackhead and a pre-amplifier' Boarding musicis Ltectea at an attendint'spanelwhile prerecorded imnouncementsare selectedby meansof twelve pushbuttonson the annunciatorpanel' PassengerEntertainment SYstem entertainmentsystemof the Boeing The pa-ssenger 747 andan! other modern largeairliner is perhaps also the mmt complex of 3ll airbome systems'lt is and, the systemliklly to caus€most trouble the fortunatelv, teait litcelyto affect the safety of or a fire to leads aircraft unlessbad servicing ^ loose-articlehazard. Evenon the sametype ol aircraft a variety of serviceswill be availablesince different op.r"iott will offer different entertainment the in a bid to capturemore customers' In view of is description following abovecomments, the particularlybrief and doesnot do justice to the complexitY involved.

41

Movie audio

P.A. override

Other submultiplexers Seats 1 2 3 Channel select Other seat demultiolexers

Other seat colurnns

1

2 3 Seats

Fig.2.23 Boeing747: simplified passengerentertainment system

'system', Both moviesand music are provided,the movie as can be seenfrom the schematicdiagram audio being fed to individual seatsvia the music n Fig.2.?4. The horn and flight-deckcall button are portion of the system.Ten tape-deckchannels,four locatedin the nosewheel bay while tl'reground-crew movieaudio channelsand one p.a.channel(total call(with illumination)and auralwarningbox areon fifteen) are provided usingtime multiplexing. A time the flight deck. Operation is self-explanatoryfrom interval,ternreda liame, is divided into fifteen the diagram. Should horn or chinte sound, the ground channeltimesduringwhich the signalamplitudeof crew, or flight crew respectively,will contact each eachchannelis sampled.The audiosigrralamplitudes other usingone of the interphonesystems. arebinary coded(twelve bits) and transmitted, togetherwith channelidentification, clock and sync. pulses,over a co-axialcablerunning throughout the aircraft. The music channels(five stereo,ten monauralor a mixture)aremultiplexedin the main multiplexer,the resultingdigitalsignalbeingfed to six submultiplexers CFO CiCW CALL +. in series,the final one being terminatedwith a suitable load resistor. Movie and PA audio are multiplexed with the musicchannelsin the zonesubmuliiplexers, Fig.2.24 Boeing747:groundcrewcall(courtesy Boeing eachof which feedsthree or four columnsof seat Aeroplane Conrmercial Co.) demultiplexers.Channelselectionis madeby the passenger who hearsthe appropriateaudio over his Cockpit Voice Recorder - stethoscopeheadsetafter digital to analogue conversionin the demultiplexer. Alternate zone An endlesstape provides30 niin recordingtime for submultiplexersare usedasback-upin the event of audio signalsinput on four separatechannels.The prime submultiplexerfailure (classpriorities exist if channelinputs are captain's,first officer's and flight failuresmean somepassengers must havethe engineer'stransmitted and receivedaudio and cockpit entertainmentservicediscontinued). areaconversation.Passenger addressaudio may be The controls necessaryfor activationof the substitutedfor the flight engineer'saudio in an entertainments systemarelocatedon attendants' aircraft certified to fly with two crew members. control panels. The microphoneinputs should be from so-called 'hot mics', i.e. microphoneswhich are permanently Ground Crew Call Syrtem live regardless of the setting of ASP or control Ground crew call is hardly worthy of the title column switches. The areamicrophone(which may

42

Flt. eng. hot mic. tel. Record head

lst. off. hot mic. tel.

Area Mic.

lo Q Playback I head

Pre-amp-

Erase

Test Jack

4, Landing

parking

P"::;

ffif

Essontirl flt. inst. bus bar

Ft1.2.25 Typicalcockpitvoicerccorderblockdiagram

be s.eparate from the control panel) is strategically situatedso that it can pick up night crew speechand generalcockpit sounds. While the control panelis situatedin the cockpit, ., the recorderunit (CVR) is locatedat .:heother end of the aircraft where it is leastlikely to suffer damagein the event of an accident. The CVR is constructedso asto withstand shock and fire damage,and additionally is paintedin a fire-resistantorangepaint to assistin recoveryfrom a wreck. The recorded audio may be erasedproviding the landinggearand parking brake interloik relav' contactsare closed. As a further safeguardaiainst accidentalerasurea delay is incorporited in the bulk erasecircuit which requiresthp operator to depress the 'erase'switch for two secondibefore "r"a*" commences. Test facilities are provided for all four channels,

separatelyor all together. A playbackhead and monitor amplifier allowsa satisfactorytest to be observedon metersor heardover a headsetviajack plug sockets. Pressingthe test button on the control panel or the all-testbutton on the CVR causesthe channelsto be monitored sequentially. The power supply for the system should be from a sourcewhich provideqmaximum reliabilitv. Sincethe tape is subjectto wear and thus has a limiied life, the CVR should be switchedoff when nqt in use. A suitable method would be to remove power to the CVR wheneverexternalground power is connected.

Testingand Trouble Shootingthe Audio Systems Variousself-test facilitiesmaybeprovided by which tl:l

tonesmay be generatedand heardover headsets. However,to testproperlyall switchesshouldbe operatedand all mic. and tel.jacks,aswell as speakers,shouldbe checkedfor the requiredaudio. This shouldbe sufficiently loud, clearand noise-free. Amplifier gainpresetsin accessoryboxesmay needto be adjusted. A full functional test is best done by two men, althoughit is not impossiblefor one man with two headsetsand an extensionlead to establish two-way contact betweenvariousstations. Faults can be quite difficult to find owing to the complicatedswitchingarrangements.Howeverthe wide rangeof switchingcan be usedto advantagein order to isolatesuspectunits or interconnections. Disconnectingunits providesa good method of

4

finding short circuits or howls due to coffee-induced tel.-mic.feedback(i.e. spilt liquid providinga conductingpath betweentel. and mic. circuits). Whereone has a number of units in series,e.g. demultiplexersin an entertainmentsystem, disconnectingcan be a particularly rapid method of fault-finding;it is usually.best to split the run in half, then in half again,and so on until the faulty unit or connectionis found. Continuity checkson very long cablescan be achievedby shorting to earth at one end and then measuringthe resistanceto earth at the other. The resistanceto earth should also be measuredwith the short removedin casea natural short exists.

3 Automaticdirectionfinding

Introduction Most readerswill havecome acrossthe principle on which ADF is basedwhen listeningto a transistor radio. As the radio is rotated the signalbecomes weakeror stronger,dependingon its orientation with respectto the distant transmitter. Of courseit is the antennawhich is directionaland this fact has been known sincethe early days of radio. In the 1920sa simpleloop antennawas usedwhich could be rotated by hand. The pilot would position the loop so that there was a null in the signal from the station to which he was tuned. The bearingof the stationcould then be readoff a scaleon the loop. Tuning into anotherstation gaverise to another bearingand consequentlya fix. Apart from position-fixingthe direction-findingloop could be usedfor homing on to a particularstation. This primitive equipmentrepresentedthe first use of radio for navigationpurposesand came to be known as the radio compass. The systemhas been much developedsince those early daysand in particularits operationhasbeen simplified. Within the band 100-2000kHz (I.f./m.f.) thereare many broadcaststationsand non-directional beacons(NDB). An aircraft today would have twin

Athwartships loop

receiverswhich, when tuned to two distinct stations or beacons,would automaticallydrive two pointerson an instrumentcalleda radio magneticindicator (RMI) so that eachpointer gavethe bearingof the correspondingstation. The aircraft position is where the two directionsintersect. Sincesucha system requiresthe minimum of pilot involvementthe name radio compasshascome to be replacedby automatic direction finder (ADF).

BasicPrinciples TheLoop Antenna

'l

he first requirementof any ADF is a directional antenna. Early loop antennaswere able to be rotated first by hand and subsequentlyby motor, automatically. The obviousadvantageof havingno moving partsin the aircraft skin-mountedantennahas led to the universaluseof a fixed loop and goniometer in modern equipments,althoughsomeolder types are still in service. The loop antennaconsistsof an orthogonalpair of coils wound on a single flat ferrite core which concentratesthe magnetic(H) field componentof the e.m. waveradiatedfrom a distant station. The plane

Rotot (sctrch coil)

Forc and aft loop Fb.3.l

Loop entcnnaand goniometcr

45

ol one coil is alignedwith the aircraftlongitudinal axiswhile the other is alignedwith the lateralaxis. The currentinducedin eachcoil will dependon the directidnof the nragneticfield. Whenthe plane of the loop is perpendicular to the directionof propagation, no voltageis inducedin the loop since the linesof flux do not link with it. lt canbe seen that if one loop doesnot link with the magneticfield the other will havemaximumlinkage. Figure3.1 showsthat the loop currentsflow through the stator (resolver)where.providing windingof a gonionreter the characteristics o1'eachcircuitareidentical,the magneticfield detectedby the loop will be recreated in so far asdirectionis concerned.We now effectivelyhavea rotating loop antennain the form of the eoniometerrotor or searchcoil. As the rotor turnstf,rough360otherewill be two peaksand two nullsof the voltageinducedin it. The output of the rotor is the input to the ADF receiverwhich thus sees is the rotor asthe antenna.Suchan arrangement known asa Bellini-Tosisystem. Sincewe areeffectivelybackwith a rotatingloop situationwe shouldconsiderthe polardiagramof suchan antennaaswe areinterestedin its directional properties. In Fig. 3.2 we havea verticallypolarizedt.e.m. wavefrom the direction shown. Tl.ratcomponentof the H field linking with the loop will be H sin 0, so a plot of the loop current againstI producesa sine curveasshown. The polar diagramof such an antennawill be asin Fig. 3.3. It canbe seenthat natureof the plot the nulls of the sinusoidal because arefar more sharplydefinedthan the peaks. The abovehasassumeda verticallypolarizedwave which is in fact the casewith NDBs and most

Fig,3.3 Loopaerialpolardiagram broadcaststations.Howevera verticallypolarized earth and signaltravellingovernon-homogeneous strikingreflectingobjects,includingthe ionosphere, can arriveat the loop with an appreciable horizontallypolarizedcomponent.The currentin the loop will then be due to two sources,the vertical and horizontal cornponents,which will in generalgive in the a non-zeroresultarrtnull, not necessarily direction of the plane of the antenna. This polarizationerrordictatesthat ADF ihould only be usedwith groundwavesignalswhich in the l.f./m.f. bandsare usefulfor severalhundredmiles. However. polarizedsky they arecontaminated by non-vertically wavesbeyond,say,200m at 200 kHz and 50 m at 1600 kHz, the effect beingmuch worseat night (night effect) $*o read , The SenseAntenna The polar diagramof the loop (Fig. 3.3) showsthat the bearingof the NDB will be givenas one of twtr

Plane of looP

l

tl

Direction of propagatron -------------+

/

l,/

(l \

i

E fieldO

|

.-0,,

F8. 3.2 To illustrate degrccof coupling of loop acrbl

tt6

H field

figures,l80o apart, sincethere are two nulls. In althoughnot as clearlydefinedas the nulls for the order to determinethe correctbearingfurther figure-of-eight(Fig. 3.4). information is neededand this is providedby an omnidirectionalsenseantenna. In a verticaliv polarizedfield an antennawhich is omnidireitional in Simplified Block Diagram Operation the horizontal plarreshouldbe of a type which is excited by the electric(E) field of the t.e.m. wave Automatic direction finding (ADF) is achievedby i.e. a capacitanceantenna. The output of suchan meansof a servoloop. The searchcoil is driven ro a antennawill vary with the instantaneousfield stablenull position,a secondnull beingunstable. strengthwhile the output of a loop antennavariesas the instantaneousrate of changeof field strength - The searchcoil o-utput,after amplification,is phase-shifted by 90" so as to be either in phaseor out (Faraday'slaw of inducede.m.f.). As a of phasewith the senseantennaoutput, dipending on consequence, regardless of the direction of the t.e.m. the direction of the NDB. prior to addingto the wave,the senseantennar.f. output will be in phase sensesignalthe phase-shifted loop signalis switched quadraturewith respectto the searchcoil r.f.butput. in phasein a balancedmodulator at a rate determined In order to sensethe direction of the NDB the two by a switchingoscillator,usuallysomewherebetween antennaoutputs must be combinedin such a way as a 50 Hz and 250 Hz rate. Whenthe compositesignal either to cancelor reinforce,and so either the sense is formed in a summingamplifier it will be or the loop signalmust be phaseshifted by 90.. amplitude-modulatedat the switchingfrequencysince A compositesignalmadeup of the searchcoil for one half period the two input signalswill be in output phaseshifted by 90' and the senseantenna phasewhile for the next half period they will be in output would appearas if it camefrom an antenna antiphase(seeFigure3.6). the polar diagramof which was the sum of thosefor The amplitudemodulationis,detected in the last the individual antennas.Now the figure-of-eightpolar stageof a superhetreceiver.The detectedoutput will diagramfor the loop can be thought of asbeing be either in phase,or in antiphase,with the switching generatedaswe considerthe output of a fixed search oscillatoroutput and so a further 90" phase-shiftis coil for variousn.d.b.bearings or the output of a requiredin orderto providea suitablecontrolphase rotatingsearchcoil for a fixed n.d.b.beaiing,either for the servomotor. The motor will drive either separatehalvesof the figure-of+ightwill be clockwiseor anticlockwisetowardsthe stablenull. 1v%the rdu out ot phase.As a consequence the sense When the null is reachedtherewill be no searchcoil antennapolar diagramwill add to the loop polar output henceno amplitudemodulationof the diagramfor somebearings,and subtractfoiothers. compositesignalso the referencephasedrive will be The resultantdiagramis a cardiodwith only one null, zero and the motor will stop. Should the servomotor Le in sucha position that the searchcoil is at the unstablenull the sliehtest -a--\. disturbancewill causethe motor to drive aiay fiom / t / \ \ \ \ , / l \ \ this position towardsthe stablenull. The senieof the / t l \ connectionsthroughoutthe systemmust be correct / t l \ for the stablenull to give the bearing. / \ t . A synchrotorquetransmitter(STTx),mountedon the searchcoil shaft, transmitsthe bearineto a remote indicator. '.

r \

'. \-

'l

\

l /

/ \

-\-__-i--

./

z'

Fig. 3.4 Compositepolar diagram

Block Diagram Detail Tuning Modern ADFs employ so-calleddigital tuning wherebyspot frequenciesare selected,as opposedto older setswherecontinuoustuning *.s usurl. A conventionalfrequencysynthesizeris usedto generatethe local oscillator(first l.o. if double superhet)frequency. The tuning voltagefed to the v.c.o.in the phaselock loop is alsousedfor varicap

47

Loop antenne Synchro.torque Tx o l P

S u m m i n ga m P .

Fig.3.5 An ADF simplifiedblockdiagram

tuning in the r.f. stages.Remoteselectionis by b.c'd. (ARINC 570) or someother codesuchas 215. BalancedModulator Figure3.7 showsthe balancedmodulator usedin the King KR 85. DiodesCR 1 l3 and CR I l4 areturned on and off by the switchingoscillator(Q 3l I and Q 312) so alternatelyswitchingthe loop signalto one of two sidesof the balancedtransformerT I 16. The output of Tl l6 is thus the loop sigral with its phase swiichedbetween0o and 180" at the oscillatorrate. Receiver A conventionalsuperhet receiveris usedwith an i.f. frequencyof 14l kHz in the caseof the KR 85 ; i.f. andr.f. gain may be manuallycontrolledbut in any casea.g.c.is used. An audio amp, with normal gain control, amplifiesthe detectsdsignaland feedsthe AIS for identificationpurposes.A beat frequency oscillator (b.f.o.) can be switched in to facilitate the identificationof NDBs transmittingkeyed c.w- The /t8

b.f.o. output is mixed with the i.f. so asto produce an audio differencefrequency. Good sensitivityis requiredsincethe effectiveheight of modern low-dragantennasgivesa low levelof signalpick-up' Good selectivityis requiredto avoid adjacentchannel interferencein the crowdedI'f./m.f. band. Indication of Bearing In all indicatorsthe pointer is alignedin the direction of the NDB. The angleof rotation clockwisefrom a lubber line at the top of the indicator givesthe relative bearingof the NDB. If the instrumenthas a fixed scaleii is known as a relativebearingindicator (RBD' More common is a radio magneticindicator (RMI) which hasa rotating scaleslavedto the compass heading. An RMI will give the magretic bearingof the NDB on the scaleaswell as the relativebearingby the amount of rotation of the pointer from the lubber line. Figure3.8 illustratesthe readingson RBI and RMI for a givenNDB relativebearingand aircraft heading. An RMI normally providesfor indication of

A

A

NDB 1

NDB 2

two magneticheadingsfrom a combinationof two ADF receiversand two VOR receivers.Figure 3.9 showsa typical RMI while Fig.3.l0 showsthe RMI which may circuit and typical switchingarrangements be internal or external to the RMIs.

Assumesearchcoil aligned with zero bearing

Sourcesof SystemError

NDB 2 to right

NDB 1 to left

Loop

r.t. Switching voltage

I

Automatic direction finding is subjectto a number of sourcesof error, asbriefly outlined below.

\A/\A I

ra_

hdg.

I

Balanced mod. O/P Sense r.f.

Composite signal

M,|AA

N.D.B.

A

I

AAA -iAA I

Detected Rx out Reference phase

I

l-.-1

r i r -

N.8. Waveshapes and relative time scalesare not exactly as shown. Fig. 3.6 DiagramshowingADF phaserelationships

R.M.l.

R.B.I

Fig. 3.8 Diagramof RMI and RBI readings

T116 Astable multivibrator o311-312

Fig. 3.7 King KR 85 balancedmodulator - simplified

49

Night Effect This is the polarizationerror mentio,ned previouslyunder the headingof the loop antenna. The effect is most noticeableat sunriseclr sunset when the ionosphereis changingmost rapidly. Bearingerrorsand instabilityareleastwhen tunedto an NDB at the low end of the frequencyrangeof the ADF. CoastalRefraction The differing propertiesof land and waterwith regardto e.m.groundwaveabsorption leadsto refractionof the NDB transmission.The effectis to changethe directionof traveland so give riseto an indicatedbearingdifferent from the actual bearingof the transmitter. Mountain Effect If the wave is reflected by mountains,hills or largestructures,the ADF may measurethe direction of arrivalof the reflectedwave. The nearerthe reflectingobject is to the aircraft the greaterthe error by the geometryof the situation. Stotic Interference Static build-up on the airframe

Fig.3.9 KNI 581 RMI (courtesyKing RadioCorp.) ADF No. 1

VOR No.l

ADF No

No. 2.

No.1.

cto

cto

No. 2. R.M.l

26V 4OO Hz Ref.

t ---/ / Red

No'1 R'M'l' ,/tr\ (w E) \9/ A_-A

Fig. 3.10 Radio magneticindicator: simplified circuit

50

VOR No. 2.

and the consequentdischargereducesthe effective rangeand accuracyof an ADF. Thunderstormsare alsoa sourceof static interferencewhich may give rise to largebearingerrors. The ability of ADF to pick up thunderstormshasbeenusedby one manufacturerto give directionalwarning of storm activity (Ryan Stormscope). Vertical or Antenna Effect The vertical limbs of the crossedloopshavevoltagesinducedin them by the electriccomponentof the e.m.wave. If the planeof a loop is perpendicularto the direction of arrivalof the signalthere will be no H field coupling and the E field will induce equalvoltagesin both vertical limbs so we will havea null as required. Should, however, the two halvesof the loop be unbalanced,the current inducedby the E field will not sum to zero and so the direction of arrival to give a null will not be perpendicularto the plane of the loop. An imbalancemay be due to unequalstray capacitanceto earth either sideof the loop; howeverin a well-designed Bellini-Tosisystem,where eachloop is balancedby a centretap to earth, this is not a severe problem. Station Interference When a number of NDBs and broadcaststationsare operatingin a given areaat closelyspacedfrequenciesstation interferencemay

result. As previouslymentionedhigh selectivityis requiredfor adequateadjacentchannelrejection. Quadrantal Enor (QE) It is obvious that the two fixed loops must be identicalin electrical characteristics, asmust the stator coils of the goniometer. If the signalarrivesat an angle0 to the planeof loop A in Fig. 3.1I the voltageinducedin loop A will be proportionalto cos0 and in loop B to cos(90-d)= sin0. If now the searchcoil makesan angled with the stator P then the voltageinducedin the searchcoil will be proportionalto (cosOX cos@)- (sinOX sin@)providedthere is no mutual couplingbetweenthe interconnectingleads. So when the searchcoil voltageis zero: cosOXcos@=sinOXsin@ or: cot0 = tan0 and: 0=6+ 90+ifX 180 wherey'y'is 0 or any integer. This is simply a mathematicalmodel of the situationpreviously ddscribedunder the headingof the loop antenna. Now considerthe two loopsnot electrically identicalso that the ratio of the maximum voltages inducpd in the two loops by a given signalis r. The condition for zero voltagein the searchcoil is now:

Direction of arrival

Fig. 3. I I Diagram showingsearchcoil signalas a function of direction of arrival

51

cotO=rXtan@' when 0=O

cotp=o

circuits and the loop connectionswill lead to errorsin the searchcoil Position.

lnstallation

therefore A typical transport aircraft ADF installationis shown i n F i g . 3 . l 2 ; N o . 1 s y s t e mo n l y i s s h o w n ,N o ' 2 b e i n g sirnilir except that different power bus barswill be when used. Main power is 28 V d.c', the 26 V, 400 Hz cot0=0 0=90 being usedto supply the synchros' lt is vital that the 26 V 400 Hz fed to the ADF receiveris from the therefore samesourceas that fed to the RMI' tan6'=0 so 0'=0+NXl80 The loop antennaand its connectingcableform g = part of the input circuit of the receiverand so must 180 or 270) we (alsowhen In thesetwo cases p = n o ' s t t iravea fixed known capacitance(C) and inductance 0 h a v et h e s a m es i t u a t i o na sb c l o r el . e . (L). This being so the length and type of loop cableis error. the so error' an ipecified by the manufacturerof the loop. The will be there angles At intermediate not be exceeded,but it can be bearingindicatedby the searchcoil will be incorrect' length specifiedmust compensatingC and L are provided in tlnee shorter value made Sincethis type of errorhasa maximum the circuit. in placed correctly error' quadrantal eachquadrantit is called equalizercontainsthe loop corrector r'f' The will cause QE NDB the from wave t.e.m. Now the to compensatefor a components reactive currentsto flow in the metalstructureof the aircraft' necessary provide to and QE correction' A cable loop the from short Eachof the loops will receivesignalsdirect 3.13. Cl,C2,Ll,L2 Fig. given in is ciicuit typical airframe' the trom signals NDB and alsore-radiated (loop una C:, C4,L3, L4 providecompensation Sincethe aspectratio of the aircraft fuselageand correction provide L6,L7 L5, QE while equalization) energy wingsis not I : I the effect of the re-radiated stator of appropriate the in current the attenuating by equivalent is on th. t*o loopswill be different:this is equalizer loop the goniometer.The QE corrector to makingtwo physicallyidenticalloopselectrically to the IooP. dissimilar.The resultingquadrantalerror could be up mounted close Similar considerationsapply to the senseantenna to 20" maximum. to can be nradeby usinga which is required to presenta specifiedcapacitance Fortunately,compensation of cable given length a we have Again receiver. the possibly and QE QE correctorloop equalizer the combined which must not be exceededbut can be madeshorter correctionbuilt into the loop. Nt>rrnaliy an iqualizeris fitted. Often both an r.f. field producesa gleatervoltagein the longitudinal proviclecl are usedto achieve and a suscepti-former equalizer identical' loop than in the lateralloop if the loopsare receiver'The the to capacitance input statetl the more have antennas This beingthe casesomeloop devicewhich matching passive susceptlformeris a turnson the lateralloop than the longitudinallocp, the effective to increase transformer auto utilizesan typical correctionUelngt Z|' in the middle of the antenna.Typicalunitsare sense the of capacitance quadrants. shbwnin Fig. 3.14. As an alternativethe necessary may be achievedin a single Loop Alignment Error If the longitudinalloop plane matchingand equalization matching/equalizing The coupler. senseantenna is not parallelto the aircraft longttudinalaxis then a the antenna' to close mounted are unit(s) constantloop alignmenterrorwill exist. Tire loop antennawill consistof the crossedcoils wound on a ferrite slab and encapsulatedin a Field Alignment Error If the loop antennais offset low-draghousing. On high-speedaircraft the loop will from the aircraft centreline the maxima of the zeros' be flush with the skin but on slower aircraft the will the as quadrintal error will be shifted, housingmay protrude slightly, givingbetter signal Consequentlythe situation wherethe NDB is at a pick-up. relativebearingof 0, 90, 180pr 2?0o will not give The senseantenna can take many forms' On large zeroerror. is c-apacitiveplate Ft transport aircraft a suppressed 'towel rail' a aircraft comrnon,whereason slower Loop Connector Stmy Coupling Reactive coupling type of antennamay be used. Generalaviation external between or connections the loop between t a n @ ' = es o

52

Q'=90+NXl80

N o .1 2 8 V d . c .

4OOHz

Panel lights supply

I

Corrector box

No. I VOR

From No. 2 ADF or No. 2 VOR

Compass hdg

Sense aerial Fig.3.l2 TypicalADF installation

o i -cc

Fig. 3.13 Quadrantalerror corector/loop equalizer (straight-through connectionsnot shown)

aircraft might usea wire antennaor, asan alternative, a whip antenna. Somemanufacturersnow producea combinedloop and senseantennafor the general aviationmarket. The position of both antennasis important. The loop shouldbe mounted on, and parallelto, thecenire line of the aircraft with nomore than 0'25" alignmenterror. While the loop may be on top or

Sense ae. cable equalizer lnsulated sense ae. terminal

lnner screen Fig; 3.t4 Senseaerial matchin!

53

bottom of the fuselage it shouldnot be mountednear the nose,tail, largeor movableprotuberances or near othersystemantennae.Similarconsiderations apply to the senseantenna,althoughbeingomnidirectional alignmentis not a problem. Ideallythe senseantenna will be mountedat the electricalcentreof the aircraft in orderto giveaccurateover-station turn-aroundof the bearingpointer. The interconnectionsin the systemmust take into Fig. 3.15 ARINC 570 control panel (typical) accountthat the phasingof voltagesproducedby senseand loop antennaswill be different for top and bottom mounting. The methodusedwill dependon the manufacturer but if the systemconformsto Functiort Switch. OFF-ANT-ADF In the antenna ARINC 570 the synchrorepeaterconnections will be position(ANT) the receiveroperatesfronr the sense asin Table3.1. If, asin somelight aircraft a n t e n n ao n l y , t h e b e a L i n p g o i n t e rb e i n gp a r k e da t 9 0 " r e l a t i v e p o s i r i o nm a y b e u s e dl b r b e a r i n g . T h i : Table3.1 Synchroconnectionsfor alternateaerial t u n i n g N D B , l s t l t i o n a n d i d e n t i f i c a t i o nI.n t h e A D F locations.Indicatorsynchroreceivercorrections positionsignalsfrorn both loop and senseantenna p r o v i d en o l n r a lA D F o p e r a t i o nt,h e R M I i n d i c a t i n g Aerial position Bottom Bottorn Top Top t h e b e a r i n go l ' t h e s t a t i o n . loop, loop, loop,

ottO'oo'

SI Synchro 52 transmitter S3 corrections Rl R2

bottorn sense

top scnse

top sense

sl s2 s3

sl s2

S3 S2

S3 R2 RI

SI RI R2

RI R2

loop, bottom sense S3

s2 sl R2 RI

Fretluenq,St,/ecl(rrobs Threeknobsareused;one is nrorrntedco-ariallywith the functionswitch,to s e l e c ft r e q u e n c yi n, 0 . 5 , l 0 a n d 1 0 0k H z i n c r e m e n t s . D i g i t r l t v p e t ' r e q u e ny ed i s p l a ys e g m e n ti sn d i c a t et h e selectedflr'qrrencv.The informationis passedto the r e c e i r eur sp l r l l k ' l b . c . t i .

Ilcat F-requt'rtct'Oscillatr'r Sx,itch Selectsthe BFO installations,the goniometeris in the indicator and lirr useu'henthe NDB selec:ted is identifiedby the bearingis presented directlyratherthan by synchro t ' r n - o lk- ie-r i n g o l ' t h e ea r ri e r . feed then the following correctionsare necessary: A nurrrberof other su,itches nraybe found on v a r i o u sc o r r t r o l l e r as s. b L i e l l yd c s c r i b ebde l o w . l. loop from top to bottom: longitudinalcoil connections to goniometerstatorreversed: Ftur 90 Hz

0.155DDM Bercon Course sector < 6o

0 ' 1 5 5D D M

ILS datum point

150 Hz < 9O Hz Fig. 52

70

Localizer ooursc,lelector

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

s p e c i f i c a l l3y2 8 . 6 - 3 3 5 .M 4 H z a t 1 5 0k H z s p a c i n g . Eachof the forty frequencies allocatedto ihe glideslope systemis pairedwith a localizerfrequency, the arrangement beingthat localizerand glidesiope beaconsservingthe sarnerunway.will haveliequencies takenfrom Table5.2. pilot selectionol'the required localizerfrequencyon the controllerwill cause'both localizerand glideslope receivers to tune to the appropriatepairedfrequencies. Table5.2 Localizer/glideslope frequencypriring (MHz) Lot'alizer

()li ll--

To transpondcr

tl-+

ll-*

t= \.,r n, sensitivc devices

Fi& E.lt

Encodingdisc

Tracks

Fig.8.l9 Segmentfor12300ft,code0l0ll0l0o. Encodingaltimcter range- l000-32 700 ft

Mode C load

f-

tl

U I

*-1 Encoding alttmeter

+v

A/R onloff

j

-L =

Fig. 8.20 Simplifiedaltitudeencodercircuit

134

Transponder

Segments

D2 R1

-v2

Ditch digger

t-__

circuit Fig.8.2l Sidelobe suppression

lnput

in, d

GI M2,O

M3, O

waveformsin Fig. 8.22. MonostableMl associated and AND gateGl separatethe Pl pulsefrom P2 and P3 so that M2 will be triggeredby Pl only and will not be triggereduntil the next interrogation. M2 and M3 provide a gating waveform about I ps wide in the P2 pulseposition. Th" input pulsesarealso appliedto a'ditch-digSer' circuit. Prior to Pl, Dl is forward biasedand the junction of Cl/R2 is low. Both inputs to G2 ate low. The leadingedgeof Pl causesDl to conduct chargingCl rapidly. The laggingedgeof Pl causes Dl to cut off, sincethe junction Cl/R2 falls by an amount equal to the amplitudeof Pl . Cl now through Rl and R2. WhenP2 arrivesDl discharges will conduct providingthe amplitudeof P2 is sufficient. The time constantClRlR2 and the bias voltagesVl and Y2 ue chosenso that if P2>Pl AND gate G2 will receivean input via D2 which will be coincidentwith the gatingwaveformfrom M3' Thus the SLS pulsegeneratorM4 will be triggeredif suppression and only if Y2> Pl, the subsequent pulsebeing usedto inhibit the receivervideo output to the spikeeliminator.

Characteristics Junction c1 R2

Fig. 8.22 Side lobe suppressionwaveforms

The followingsummaryis drawnfrom ARINC No.572.1for theMk 2 ATC Characteristic It is worthpointingout that several transponder. arenot requiredfor featuieson the Mk I transponder will find many engineer the the Mk 2, however whichhavesomeor all of still in service transponders the following: 135

l. two-pulseSLS; 2. SLS countdown - receiverdesensitized when the number of SLS pulsesexceedsa limiting figure; 3. low sensitivity selection; 4. receivervideo signaloutput socket; 5. remote automatickeying; 6. externaltransmittertriggeringposition; 7. audiomonitor: 8. transmissionof SPI pulsewheneverD4 is one bit ofthe altitudereportingcode. Receiver Minimum Ttiggering Level (MTL) -77 to -69 dBm at antennaor -80 to -72 dBm at transponder. Dynamic Range M T L t o 5 0 d B sa b o v em . t . l . Frequency and Bandwidth Centrefrequency1030MHz. - 3 d B p o i n t sa t 1 3 M H z . -60 dB pointsat ! 25 MHz. Decoding Facilities Decoderoutput lbr pulsesspaced8, I 7 and 2 I ps tolerancet 0.2 gs on spacing.Automaticmode C decodingregardless of modeselectionswitch. Spaceprovisionfor 25 ps decoding.

Reply Delay 3 t 0'5 ps. Reply RateCapabiliry 1200repliesper second. Reply Pulse Interval Tolerance t 0'l gs for spacingof any pulse,other than SPI.rvith respectto F I ; t 0'15 ps for spacingof any pulsc'with respectto any otherexceptFl: t 0.1 prsfor spacing of SPI with respectto F2. Mutual SuBpression P.ilse 25-331s duration. Manitor Lamp To light when five repliesaredctectedat a rategreater thar 150 repliesper second.To stayilluminatedfbr l 5 s a f t e rl a s tr e p l yd e t e c t e d . Antenna Polarization:vertical. v . s . w . r .b: e t t e rt h a n l . 4 l : I a t 1 0 3 0a n d 1 0 9 0M l t z .

Ramp Testing A transpondercan be testedril situ usingone of s e v e r apl o r t a b l et e s ts e t s .A s u i t a b l er a n t pt e s ts e t will testby radiatitrrr._bc. ceplbleof interrogatingon at leastmcldesA and ( . be capablerlf simulatinga sidelobe interroXptlon. displaythe transponder reply and providea ffeansof measuringthe transponder transmitterfrequency.

Side Lobe Suppression Facilities Pl > P2 + 6 dBs shouldgive90 per cent reply rate. 6 dBsratherthan ICAO 9 dBsensures ATC 5OOA adequate marginto allow for performancerundown in service. A popular test set is the IFR ATC 6004 illustratedin Figure8.23. A reasonfor its popularity is the fact SLSPulse Duration that it can testboth DME and ATC transponder with 25-45 ps. a comprehensive rangeof checks,making it suitable for functional testson the ramp or bench. Transmission Pl, P2 and P3 pulsesaregenefatedand usedto key a crystal-controlled 1030MHz oscillator.The interval Tlansmitter Frequency betweenPl and P3.isswitchedto simulatea mode 1 0 9 0 13 M H z . Ay'Cinterlace,two mode A interrogationsbeing transmittedfor eachmode C. The following Minimum Peak Power characteristics may be variedby front panelcontrols:

500w.

l. Pl-P3 intervd - to checkdecoder; Reply hIttlseCharacteristics 2. P2 amplitude- to checkSLS; Duration 0.45 1 0'l ps measuredbetween50 per cent 3. Transmitterpoweroutput - to checkMTL. amplitudepoints. 0'05-0'l ps risetime, l0-90 per cent. The reply is displayedby a bank of lamps,one for 0'054'2 ps delaytime,90-10per cent. eachcode pulseand oire for the SPI pulse. There is 136

#ffi "n[f*

x:

l

*

t

l

l

r

* * ) l

r* i I

# ' $'tl*$ i . ffi

t r

) poweroutput of transponder (1 50 per cent accuracy);

3 . frequencyof the'transpondertransmitter; 4 . percentage reply; 5 . invalidaltitudecode,i.e.no C pulsesor C I and C4 together;

t

';*'!,r,".t'-'.1;u

,:-:

t+

Fig.8.23 ATC600A(courtesy tFR Electronics lnc.)

6 . absenceof code pulsesin reply to nrode C interrogation. Supplyis by rechargeable batteryor a.c.,battery operationis limited by a timer. Further l'eatur.es are directconnectionto the transponder via an external 34 dB pad, self-testing of display,lampsand battery and direct connectionofencodingaltirneter.

alsoa numericalreadout which showseither the pilot codeor the altitude in thousandsof feet. In addition TIC T-33B and T-438 to this basicinformation the following can be checked: The TIC approachto ramp testingis to useseparate testsetsfor L bandequipments, the T-338 and l. F2 timing; T-43B beingthosefor ATC transponder.

Fig. 8.24 TIC T-438 (courtesyof Tel-InstrumentElectronics Corp.)

137

Specificationsfor the two test setsare identical exceptfor the addedfacility of direct connectionof an encoderwhich is availableon the T'438. of thesetest setsand the ATC The capabilities 6O0Aare similar in so far as ATC transponderramp testingis concerned,in that they both meet the FAA requirements.Differencesarelargelydue to the use of the ATC 6004 as a bench test, although it should be noted that a particularATC 600A is best usedas eithera benchtest setor a ramp test setbut not both. the TIC test setsdo not To detailthe differences, havefacilitiesfor continuouslyvaryingPl -P3spacing or strobingthe F2 pulse,and do not indicateinvalid 'no altitude'informationor transmitterpower. or

Featuresof the TIC test setsnot availableon the ATC 600A are provisionof all military and civil modesof interrogationand changeof scalefor percentage reply meter(0-10 per cent SLS on: 0-100per cent SLS off). Thereare other minor and one other major difference.in that differences, the TIC test setsare designedftlr use in the cockpit on the groundor in flight, the antennabeing mountedon the test set asopposedto the ATC 600A wherethe antennais mountedon a tripod nearthe l'or the aircraftantenna-The antennaarrangements the useol direci connection TIC test setsnecessitate to the tr;ulsponderfor receiversensitivitychecks.

:

.J

, t 2) after the sync.phasereversal.The maximumvalueof iy'is 57 or I l3 giving56 or I l2 databits transmittedat a 4 M bitls rate. The trailing edgeof the datablock has0.5 ps of r.f. addedto ensurethe demodulationof the lastbit in the dara block is completedwithout interference. An optionalP5 may be radiatedasan SLS control pulsein the sameway that p2 is radiatedfrom an ICAO interrogator.p5 will be transmitted0.4 ps

Preamble3.5us 2rs

t

Data Block 15'5or 29'5rs

I

0.5 0.5 0.25

H

0'8 rs

ffi

53 54 55 56 57

'

Sync. phase reversal

56 or 112 phase reversalpositions

o'l------{l* O'aj ".- o.r FS

ll

ll

FS

pS

rf = t03O MHz

-

Phasereversal 'l Positionbit :

Phase reversal position bit : 0

Fig. 13.4 ADSEL/DABS format interrogation before the sync.phasereversal.For an aircraft fitted with a DABS transponderthe receivedP5 amplitude will exceedthe amplitudeof the data block hencethe transponderwill not decodethe d.p.s.k.modulated sigral. An ICAO transponderequippedaircraft will not reply to a DABS interrogationsincethe P2 pulse will triggerthe SLS suppression circuit. A DABS transponderwill generateICAO replies (12 information pulses)in responseto ICAO interrogationsand DABS repliesin responseto all-call and DABS interrogations;DABS transpondersalso generatesquitterat randomintervalsto allow acquisitionwithout interrogation(similarto DME auto-standby, Chapter7). A DABS replyis only similarto an interrogationin so far asit containsa preamblefollowedby a data block of 56 or I l2 bits. The preambleconsistsof four 0'5 ps pulseswith the spacingbetweenthe first pulse andthe second,third and fourth pulsesbeing 1.0,3.5 from leadingedge and4'5 prsrespectively, measured to leadingedge. The data block begins8 prsafter the 222

leadingedgeof the first preamblepulseand usespulse positionmodulation(p.p.rn.)at a data rateof I M bit/s. In the I gs intervalallottedto eachdatabit a 0'5 gs pulseis transmittedin the first half il the data b i t i s a ' l ' a n d i n t h e s e c o n dh a l f i f a ' 0 ' . T h e d a t a block is thus 56 or I l2 ps long. The r.f. is 1090MHz asfor the ICAO SSR. Thereare four typesof interrogationfrom an ADSEL/DABSinterrogatorall of which havethe samepreamble.The all-calldatablock containsa of 28 onesin a 56-bitblock,the all-callreply sequence detailsof data containsthe aircraftaddress, equipmenton boardand parity bits for interchange purposes.A surveillance interrogation error-checking and parity bits and alsoa ol 56 bits containsaddress dn t h e r e p e aot f t h e h e i g h ti n f o r m a t i o nr e c e i v e o ground. An aircraftrecognizing the address in a interrogationwill replywith a 56-bitdata surveillance block containingthe altitudeor identity. The of dataarein I l2-bit blocks remaininginterchanges both ways,a comm.-Ainterrogationgivingriseto a

5 6 o r 1 1 2p s

Preamble

br o.5