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Aviation History
1952
1952 - 1589.PDF
6 June, 1952 693 I.A.T.A. RADIO SYMPOSIUM Representatives of Governments, Airlines and Industry Discuss Airborne Equipment FOLLOWING the recent three-day symposium on airborne radio equipment convened in Copenhagen during the I.A.T.A. technical conference, the Association has issued a summary of the discussions, which is published here in some what abbreviated form. Though not to be considered as an official and final record of the proceedings, it does provide an interesting cross-section of opinions on an important aspect of civil air transport equipment and an indication of future trends. Repre sented in the discussions, which were presided over by Mr. Edgar Post (superintendent of navigational aids, United Air Lines), were 20 airlines, 30 manufacturers, six other international organizations and agencies, the civil aviation department of seven states and the military air forces of three. It will be seen that the discussions are summarized in convenient question-and-answer form. R/T Communications To what extent can V.H.F. and/or U.H.F. meet the operational requirements for all aircraft communications ? There was no support for the idea that H.F. might be completely replaced by V.H.F. or U.H.F. for all aircraft communications. The use of V.H.F. over large water areas is considered to be impracticable at present. Complete dependence on V.H.F. over land is technically possible, but its use would to a large extent depend upon the economics of changing over as well as the availability of land lines in certain sparsely inhabited areas of the world. The feeling was expressed that a greater differentiation may eventually be made between air traffic control messages and other communications messages, and that there may be a corresponding diversion of equipment as well. What are the effects of audio wave form distortion in RIT communica-t ons; what are the factors governing intelligibility; and what criteria can be developed to provide maximum intelligibility ? The crux of this question was the search for greater intelligibility of messages on present R/T. communications. A certain amount of built-in distortion is necessary and even desirable in order to make messages more understandable and to reduce the weight of airborne equipment, but there are no satisfactory methods of determining the safe limits of such distortion. Careful selection and limitation of the R/T. vocabulary was first advanced as a means of improvement. It was noted that, while a range between 200 and 6,200 cycles is necessary to cover all speech sounds, the top limit of this range can be reduced to between 2,500 and 3,200 cycles without too great a loss of understanding—so long as the transmission is devoted to a reasonably familiar subject. Cockpit noise, either entering into transmission through the microphone or obscuring reception through receivers, was held to be a major factor in limiting intelligibility. It was suggested that manufacturers might pay more attention to cockpit soundproofing. As an ideal situation, cockpit noise could be kept out of the microphone if pilots could use either mask or boom-type microphones, but neither of these was considered feasible for normal operations. There was wide agreement, however, that intelligibility could be substantially improved and the input of cockpit noise reduced if the R/T. speech practices of both flight crew and ground controllers could be improved. Attention was recommended for diction, voice placement and proper microphone positioning. An effective train ing method was said to be the recording of normal messages by personnel, who are then given an opportunity to hear their performance on a "play-back." "Speech clipping"—a form of modulation which reduces the limits of voice range in order to emphasize the more important speech sounds—was also stressed as a means of increasing the power of vocal transmission; R/T. should require as little deviation as possible from the normal voice level. Reception conditions would improve with the introduction of cockpit loudspeakers, but first of all there was a need for a quiet cockpit. Difficulties in maintaining the transmission qualities of both microphones and headsets were mentioned. Deterioration and variation of quality and performance was said to be much greater in carbon microphones than in the magnetic or dynamic type. It was felt that, of these last, the former have reached the limit of their capabilities and that future improvements were to be found with the latter. How efficient from the viewpoint of band width as related to intelligence, and power as related to intelligence, are present RjT. communications? It was recalled that the band width required for maximum intelligibility is from 300 to 6,200 cycles, and that this quality does not seriously drop when the top range is limited to 3,000. There is a rapid deterioration, however, if it is pushed below 2,400 cycles. An upper limit in the region of 3,000 cycles was therefore held to be sufficient for practical purposes. Manufacturers asserted that airlines should indicate to them the general environmental conditions in which they wish the equipment to operate, rather than specify exact band widths from the beginning. Overall power efficiency of airborne radio equipment was held to be low. In particular, low antenna efficiency is a drain on the whole system: with an ideal of 100 per cent, it presently varies from 2 to 20 per cent. Both manufacturers and airlines were urged to concentrate on improve ments to present systems, which must in any case remain in use for some years, rather than to spend all of their effort on development of new systems and equipment items. High Frequency What are the true channel requirements for future H.F. airborne equip ment bearing in mind operational requirements, availability of frequencies, and techniques in equipment design) ? Generally speaking, the number of channels to be built into a piece of equipment was felt to provide no particular problem; difficulties were foreseen largely in complexities of control. It was felt that flexibility would be a fundamental requirement; while 10 channels might be adequate in an aircraft flying a single short route, as many as 100 channels would be necessary on longer routes. The view was expressed that it might be possible to begin with a comparatively simple form and add on groups of channels by a "building block" process. An indication of a possible upward limitation was found in the experience of the United States, where 144 to 156 frequencies are considered necessary for H.F. use. Several warnings were sounded that the forthcoming transition from presently-used frequencies to those allotted to air transportation will increase the need for flexibility in H.F. equipment, and that it might be advisable to provide extra channel space for as many as 200 frequencies during the transition period in view of the anticipated need to carry both old and new frequencies in certain cases. What improvements are to be expected in D.S.B. H.F. communications by the introduction of new or improved techniques, for example, in antenna and modulators, the use of space diversity systems, etc. ? Reports were presented on the results of experimentation with antennae which make it possible to increase the efficiency of H.F. transmissions. These have included shorted wire or grounded antenna;, tail and wing cap antennae, and open wire types. While 85 per cent of transmissions from aircraft in flight are directed to stations fore and aft, signals which may be sent to ground stations to the port and starboard during the remainder of the time may be of vital importance and should be taken into account when aircraft antennae are being designed. Space diversity systems using two antennae and receivers connected by an electronic switch are now being applied experimentally to aviation after having originated in point-to-point ground communications. Air- to-ground tests of the system indicate definite advantages for long-range R/T. The elimination of radio noise between messages was considered desirable as a means of reducing pilot fatigue due to what was termed "continuous ear battering." It was considered that this might be obtained by selective signalling, the use of CODAN circuits and of "squelch" in high-frequency receivers. The latter means was held to be operationally simple, but technically difficult, because an additional pilot-operated control would probably be necessary. It was pointed out, however, that some American equipment now uses a coupled volume and sensitivity control, which seems to work effectively. What would be the operational advantage of "single side band" S.S.B.) over "double side band" D.S.B.) in H.F. communications? The S.S.B. system consists basically of transmission of messages on a carrier tone and one side band, rather than with the two side bands presently customary. A form of S.S.B. using a voice-suppressed carrier system is now under experimental test in Britain and was reported to have shown these advantages:— (1) The large improvement of odb. in signal strength. (2) Better intelligibility in the face of interference. (3) Almost complete absence of distortion due to selective fading. Seen here during the Airborne Radio Equipment Symposium of the I.A.T.A. Technical Conference at Copenhagen are a group of representa tives of various international organizations, including L M. Layzell, R. C. Lawes and M. I. Forsythe-Grant of International Aeradio, Ltd.; R. P. Dutton, Collins Radio Corporation; L W. D. Sharp, Plessey Co., Ltd.; H. T. Blaker, Pan American World Airways; J. G. Adam, B.E.A.; and C. P. Edwards, Ministry of Supply.
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