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Aviation History
1960
1960 - 1984.PDF
470 . ; - FLIGHT, 16 September I960 Round the Stands . . . RADIO AND ELECTRONICS ALTHOUGH still in the speculative stage, future systems of civil airtraffic control made their mark on the electronic exhibits at this year's Show. Not only was new equipment for such systems to be seen butevidence of the growing realization of the need for basic research and rethinking on certain aspects of systems operations could be discerned.At present, tenders are in for air traffic control equipment for two years experimental assessment by the Ministry of Aviation; the Apollo com-puter for Oceanic Control is expected to start experimental operation this winter. Holding some promise as a new long-range navigation aid, the VLFpropagation studies being carried out by RAE Farnboroogh Radio Department aim at discovering the potential accuracy that could beexpected of a VLF global navigation system. Two fixed stations at Rome and Malta monitor GBR Rugby on 16.0kc/s continually, recording thereceived signal. A third set of apparatus is carried in a Comet. For measurement purposes the phase of the received signal is com-pared with that of a high-stability local oscillator. Care has to be taken that the portable version of the oscillator, used in the aircraft, doesnot change in phase or frequency when transferred, for example, from the laboratory to the aircraft. The crystal-oven heater supply has to becontinuous and batteries for this have to be fined into any vehicle carrying the oscillator. In the aircraft, twin inverters supply the ovenfrom the bus-bars; and should the bus-bar supply fail the oven is switched over to a stand-by battery. Twin ovens are used and the totaloscillator drift amounts to 1 part in 10' per week. At the wavelength being investigated, the ionosphere and the earth'ssurface form a waveguide with a width of about two wavelengths. Varia- tions in the height of the ionosphere cause variations in the transmissiontimes of reflected waves. In Malta the transmission time varies by about 18 microsec between day and night. This would correspond to aposition error of about three nautical miles. As the aircraft oscillator is flown away from Rugby, the phase changebetween the received signal and the high-stability oscillator signal is measured continually. There is about 36° phase shift per nautical mile.The present transmission from Rugby is a keyed carrier transmission carrying Admiralty messages and the fact that the key may not bedepressed for long periods can cause a miscount in phase change. A practical navigation system would require a continuous wave. It isenvisaged that a comprehensive system would require six transmitters: four spaced around the equator and two close to the poles. The receiver in the aircraft employs a twin IF amplifier operatingat 525c/s. The bandwidth is lc/s. A common local oscillator is used to produce the twin intermediate frequencies and the twin amplifiersare used to cancel out phase changes in the receiver. It is also possible to use the high stability source as the local oscillator. The high stability oscillator is equipped with double ovens operatingat 90°C, controlled to within millidegrees. From the fundamental frequency of SMc/s, frequency division is carried out to yield lOOc/spulses, which act as a universal reference for the system. Multipliers provide the various harmonies for comparison with the received signal.In addition the lOOc/s pulses are divided by two and shaped to drive the synchrous motor operating the chart recorder. The narrow bandwidth of the receiver yields a reasonable signal/noiseratio in this noisy part of the radio spectrum. The effect of the proximity of whistler paths to the line of propagation is to be investigated later.The line of magnetic force along which whistlers are propagated from the Rugby area has been calculated to return to earth somewhere nearCape Town. Flights will be made in this region to discover whether any navigational difficulties arise. Various aerial systems are carried by the Comet. A long wire (it isonly a fraction of a wavelength at VLF) extends along the top of the fuselage; a balanced dipole (also a fraction of a wavelength) extends itsarms either side of the fuselage; and a loop aerial is carried in the aircraft structure. The aircraft normally flies at its economical cruising heightof 40,000ft. At present, navigation with the VLF system is carried out in retrospectin the laboratory, after the flight. Little accuracy could be expected with the initial Rugby-Griggion base line used, as the two transmitters arconly 50 miles apart. To test the system more realistically as a naviga- tional aid, the transmitter at Balbao in the Panama Canal zone will beused as one end of a base line. As a global navigation system, VLF offers certain advantages oversatellite systems. It provides a continuous means of determining position fairly cheaply, whereas a satellite system would require several expensivesatellites if time gaps between transits are to be avoided. Such gaps would be embarrassing during very high-speed flight. The strategicadvantages of one system over the other are debatable. The key to automatic flare-out, automatic landing, automatic datalrnks, and automatic air traffic control is reliability. The problem under discussion at present is how much reliability is required and how muchcan be expected of a given system. From their exhibits it is apparent that Elliott Brothers and Smiths Aviation Division are working on thesequestions as they apply to automatic flare-out and landing and Cossor arc studying the reliability of air-to-ground data links—which are likelyto be an essential part of air traffic control systems using digital computers. Above, Ferranti Airpass I and, below, Airpass II attack radar, which contains more transistors The Elliott approach to reliability in their autopilot and automaticflare-out equipment for :he VC10 is to arrange for components likely to fail to do so dormantly. This is rather like putting redundant strutsand ties into a bridge instead of building a second bridge alongside it. If a component within a circuit fails then the rest of the circuit isdesigned so that it can bear the increased burden imposed on it and still function within its operational limits. Certain components notamenable to this treatment, such as microswitches, are duplicated or triplicated. In addition the company are using other reliability techniques. Highintegrity engage circuits are incorporated. Engage mechanisms in the autopilot, for example, derive their power supply from within" the auto-pilot at a voltage, source impedance, and isolation chosen to be quite different to any other voltages in the aircraft. In this way the risk ofmaloperation due to leaks from other circuits giving spurious signals is reduced considerably. One point becoming increasingly clear as workon reliability circuits develops is that rigorous quality control and testing of components will be necessary in future systems. The double-channel philosophy could be seen in the new range ofairborne communication and navigation equipment exhibited by Marconi's Wireless & Telegraph. The type 6404 glide slope receiver,for example, is made up of four sealed modules: r.f. oscillator, r.f. amplifier, i.f. amplifier and filter unit. It is contained in a short J ATRcase and provision has been made for the addition of a second filter unit into the case should the receiver be required for a duplex automaticcontrol system. Only the filter is duplicated because it is the only module that would produce a misleading output if a failure occurredwithin it. Half of the units in the new Marconi range are made up of sealedmodules. They are pressurized with a dry inert gas and the seal can be easily made or broken. Magnetrons in sealed modules have broughtsomething of a bad name to this type of packaging, but as the new range does not have such energy dissipating elements within the sealedmodules, a gain in reliability is likely to be achieved by the method. The only relatively untried components in the system are the VHF transistors,particularly in the higher frequency region of the glide-path receiver. Life tests by the company indicate that little trouble is to be expectedfrom these, and the units will themselves be well tested by the time they come into service in the VC10. Cossor are assessing the reliability of a data transmission system inthe frequency range 3-22Mc/s to discover the limitations imposed by Search display of Airpass II with, visible on the left, the left-handed ergonomic controller which incorporates two knobs and seven switches and a rocking palm rest
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