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Aviation History
1952
1952 - 1590.PDF
694 FLIGHT, 6 June 1952 I.A.T.A. RADIO SYMPOSIUM . . . (4) A halving of the probability of jamming by a single strong inter ference signal within the broadcast band. (5) The ability to pack more channels into a given frequency space. The disadvantages of S.S.B., as revealed so far, seem to lie in the necessity for accurate crystal control, using a close synchronizing tolerance of ±300 cycles, as a matter of course; and the increasing complexity of the receiver due to the need for automatic frequency control. Airborne equipment of S.S.B. would be generally equal in weight and bulk to that of D.S.B., would create a lighter drain on power supply, but have a slightly more complicated transmitter and receiver circuit. It was also pointed out that the S.S.B. ground receiver will accommodate D.S.B. reception as well without any alteration, but with some loss. French technicians suggested that a slight variation of the basic S.S.B., using an unsuppressed carrier, would eliminate some of the troubles stemming from the need to have very stable crystal-controlled radio carriers. While this would sacrifice a substantial amount of the theoretical advantage of S.S.B., it was said to be more adaptable to airline use, to make still closer cramming possible, and to improve intelligibility. What would be the application problems associated with the introduction of S.S.B. ? To introduce S.S.B. to practical airline operations, it would first be necessary to have international agreement through I.C.A.O. on type specifications and to provide some sort of interim equipment which would accommodate both S.S.B. and D.S.B. It was not considered particularly difficult to provide the latter system. However, it was felt that the airlines are not yet ready to recommend any set type specifications to I.C.A.O., because they are not yet sufficiently certain of circumstances under which it must be applied, or of the relative importance of other proposed solutions. At the same time, one of the present pressing problems of the airlines is the blotting out of North Atlantic communi cations by magnetic storms, to which S.S.B. would be as susceptible. VMJF. and U.HJF. What advantages can be derived from V.H.F.jU.H.F. multi-carrier area coverage schemes, and what effects do such schemes have on airborne equipment design ? The British "Climax" system—developed on the basis of a Home Office system of police ground communication—is now in general operation for air activity over the south of England and Wales. The most advanced system now in use consists of five stations on one nominally common channel, each in fact separated by 10 kc/s. Two of these stations share a single frequency, so that, in practice, the five stations cover a range of 36 kc/s. The system employs conventional transmitters, but with especially high carrier frequency stability. The advantage of "Climax" in operational control was held to be its concentration on a single channel. In the aircraft, multi-carrier area coverage avoids the necessity for continuous change of channels by the pilot as the aircraft moves through the various phases of flight to termination. It was therefore held to reduce pilot fatigue and to achieve a considerable economy in the number of frequencies necessary to cover a given area, as compared with V.H.F. communication by individual channels. Some American airlines which have experimented with variations on multi-carrier coverage since 1947 reported encouraging results and stated that they could confirm the need for frequency separations of the order of 10 kc/s as in the United Kingdom. What are the true channel requirements for future V.H.F. equipments (bearing in mind operational requirements, availability of frequencies and techniques in equipment design) ? It was estimated that a seven-fold increase in channel requirements for future V.H.F. equipment could be forecast within the next eight years. The present maximum number recognized in any region by I.C.A.O. is 19, and the next increase is scheduled for 1953, when the number will be 31, plus 4 for operational control, making a total of 35. It was reckoned that the requirement would be 70 channels by 1956 and 140 channels by i960. The development of equipment was felt to be keeping pace with the increase of demand. Nineteen to 50 channels are already accom modated in A.R.C. 1 equipment, as modified. Other equipment providing larger numbers of channels and already well on the way was discussed. What operational advantages can be derived from "private line" communications and other selective and/or high capacity means of com munications ? "Private line" in V.H.F. communications was defined as a system of automatic communications to provide a high percentage of routine communications for en route and terminal area traffic control messages, retaining a presentation of data in the aircraft until the air traffic control instruction is changed. As projected in the United States, it might use a special television technique with a high brilliance storage tube in the cockpit and which is capable of storing data in pictorial presentation for the intervals between transmission periods. To do its job properly, it was felt that "private line" must be backed up with a voice circuit for non-routine data not suitable for presentation on the pictorial display; and it was felt that such voice facilities might also be transmitted on V.O.R. and I.L.S. voice channels. It was asserted that "private line" will make possible a greater utilization of communications channels, since it was held to be able to handle up to 60 aircraft on a given set of frequencies. Presentation of Navigational Information What studies have been made of the optimum system resolution of a visual presentation in the light of its cockpit location and the speed, frequency and accuracy with which observations must be made ? The basic cockpit space available to the instruments most used for in-flight and landing, it was said, is confined to a panel area about 11 in deep and 22in wide. This follows from a requirement that the pilot needs a 20 deg angle of vision downward over the nose of his aircraft and from the fact that the depth of the space is limited by the height of the pilot's knees. Instruments to be concentrated within that space, it was felt, are A.S.I., machmeter, clock, attitude indicator, pitch, roll, azimuth, beacon angle, pressure height, radio height, rate of climb, approach information, slip, power and rate of turn. To cram this amount of instrumentation into 242 sq in, it was pointed out, requires the utmost possible ingenuity. A determining factor in presenting information on these instruments is the degree of accuracy that is required in each case. A second factor is readability and lack of ambiguity; and, in the latter sense, it was warned that instruments using spiral scales, or one long and one short pointer, could be misread under conditions of stress. It was also considered undesirable to present on the same instrument data relating to things normally twinned in an aircraft: as an example, the inclusion of rev. counters for two engines on a single dial face has, on occasion, led to feathering the wrong engine. It was recommended that markings on the face of instruments be held to the absolute minimum necessary to give information and that non linear scales should be avoided; and that, for optimum interpolation, the angular distance between markings should be 5 deg. It was stressed that information should be displayed in the expected sense and that, because virtually all personnel are conditioned to clock reading, all instrument values should increase in a clockwise direction. At the same time, when an indicator is displaced from its zero by an alteration in aircraft attitude, it should be put back by an operation of the control which works in the same sense. The grouping of information is limited, to some extent, by the fact that the cone of attention of the eye— within which each pointer movement can be noted without eye movement —is approximately 8 deg. To what extent is the increasing multiplicity of indications likely to cause confusion and so prevent full appreciation of the available information ? (By multiplicity of indications is meant the use of indicators performing more than one function, instruments combining information on one unit from various sources, the integration of information from various sources into one presenta tion, and the availability of the same information on more than one instru ment.) Pilots participating in the symposium asserted that their requirements could best be met if engineers gave them the illusion of actual visual flight, so that there would be no change of concept between instrument and visual flight conditions. At the same time, they pointed out that the pilot does not have a great deal of time for comparing and figuring, and that they therefore prefer instruments which can be glanced at rather than those which must be read. It was stated that different presentation patterns are demanded for various purposes, such as direction finding, tracking, instrument approach, and the like. Instrumentation, it was said, has two purposes—to aid the pilot in doing what he should do, and to monitor him in the process. These are not entirely compatible and, it was felt, can never be entirely reconciled. The principal effect of multiple indication in instruments is to render the coupling between navigation aids and aircraft performance semi automatic and less dependent on the exertions of the pilot himself. The suggestion was made that it might be advisable to make the co-pilot the coupling agent actually handling the controls according to the communi cations given him by instruments, and to have the pilot act as the monitor. However, airlines actually using multiple instruments such as the Zero Reader, asserted that the new instrumentation has relieved the pilot to the point where he can act as the coupler himself and still do a better monitoring job. What are the relative merits of pictorial, graphic, and instrument (sym bolic through dials, needles, etc.) presentations, either in general or for specific applications ? This discussion drew to a large extent on the answers to the preceding questions as regards instrument presentations already in use, and intro duction of new material centered largely on pictorial presentation. It was pointed out that, while one type of pictorial presentation instrument, based on V.O.R. and D.M.E., has been put to limited experimental use, general international application is still a matter for long-term considera tion because there is no satisfactory airborne D.M.E. for international use at the present time. It was pointed out that many pictorial presenta tions make it hard for the pilot to discriminate within limits closer than one-half mile, due to the physical limitations of the picture. This fact, it was suggested, might hold pictorial instruments to a purely monitoring function. It was noted that the amount of data presented in any kind of compo site pictorial display should be limited strictly to that actually needed. Experience with cockpit lighting indicated that black and grey colouring may be the most clear and effective for pictorial presentation in view of the general use of red cockpit lighting. Pictorial presentation, in facsimile Or other similar form, may possibly have an application to communica tions as well as to navigation. Antenna Installation Problems To what extent can aircraft antenna requirements be standardized to permit their incorporation in all new aircraft at the initial design stage ? The use of suppressed antenna; makes the incorporation of antenna requirements in all new aircraft at the initial design stage of major importance. Suppressed "package" antenna; have so far been developed successfully for glide slope, marker beacon (75 mc/s), radio altimeter (400 mc/s), V.O.R., and low-frequency equipment. Suppressed H.F. antenna; have been a somewhat greater problem. H.F. antennae have been embedded in aileron areas and main fuselage sections, as well as in isolated tail sections, with considerable success. The suggestion was advanced that, if antenna matching units employ ing valves were mounted in the fuselage, considerable maintenance difficulty would result. It was also felt that these valves, if mounted in the tail, might be affected by vibration unless entirely new mounting techniques were to be used.
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