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Aviation History
1953
1953 - 0294.PDF
292 FLIGHT, 6 March 1953 AIRLINE AVIONICS . . . eight miles apart are used, ideally straddling the airfield. The Decca Navigator itself is similar in conception to Gee, but operates on the continuous-wave principle. Position- determining with Decca is accomplished by measuring the phase difference between the transmissions from the ground stations, accuracy being extremely high. Further, the system can be utilized at all altitudes down to sea level. Standard Decca presentation is likely to be the company's Flight Log, in which the aircraft track is continuously plotted on a simplified roll map mounted on the pilot's instrument panel. Admittedly, Decca requires the provision of separate airborne receivers, but the excellence of the aid's presentation, its accuracy and the small capital outlay required to provide coverage over large ground areas, far outweigh this disadvantage. Decca is also, of course, used by ships. Quite a different war-time development was H2S—the blind- bombing aid. This was an extremely powerful emission of centimerric waves from the aircraft in the form of a thin sheet of waves rotating about a vertical axis so as to sweep over the ground below. The reflected waves were then used to paint a picture on a cathode-ray tube face (television-wise) within the aircraft, showing the terrain below; on this, water appeared black, land areas were lighter, and large buildings and towns produced a bright reflection. From H2S has been developed modern cloud/collision warning radar carried in the nose of some modern transports. The tube- face picture of these sets shows the position of high ground and storm clouds in the path of the aircraft. American sets are carried in their latest airliners, and one of the best-known British manu facturers is Ekco. The types E.31 and E.38 of this company are excellent examples of commercially available airborne search radar. Operating at a 3 cm wavelength, the Ekco system is very flexible in operation and, in addition to the aforementioned tasks, can also be tilted downwards to provide position-fixing by map painting of the ground below. Distance -measuring Yet another British war-time development was a system of interrogator beacons which respond in a characteristic manner to a signal sent out from an aircraft. The airborne interrogator was named Rebecca and the ground beacon Eureka, the joint equip ment being irital in the accurate supplying of resistance movements and the dropping of airborne forces. From Rebecca/Eureka has come modern D.M.E., or distance-measuring equipment, D.M.E. is very useful in conjunction with a V.O.R. network since, although the former can supply bearing information to any aircraft within the network, distances can only be provided by D.M.E. (unless a number of bearings are taken simultaneously to provide crossing position lines). The D.M.E. system is, of course, radar, since it depends on a pulse emitted by the aircraft. This pulse is reflected by the ground V.O.R. station on a different frequency, the resulting "R0"' fixing the aircraft position. To make matters easier, frequency pairing is possible, the aircraft channel selector being dialled to the V.O.R. frequency; then the D.M.E. trans mitter operates on the transponder ground receiver channel and the D.M.E. receiver on the transponder transmitter channel. A number of companies manufacture D.M.E. in the U.S.A.; in the U.K. Ferranti, Murphy and Marconi are prominent. At the last S.B.A.C. exhibition Marconi's showed D.M.E. which had been developed by their associated company, Amalgamated Wireless (Australasia). The equipment operates on two fre quencies in the 200/235 mc/s band and employs double pulses for the interrogation and a single pulse for the ground response. Coding is provided by variation in pulse width and spacing, and a total of 12 different beacons can be selected. Presentation is by instrument pointer over a scale graduated variously from 0-10 and 0-100 nautical miles. Auxiliary equipment provides homing facilities in the form of a centre heading line and an orbiting indicator adjustable to a radius of from 1-19 miles. The Murphy D.M.E. is a lightweight set, very flexible in operation. Single or double pulse interrogation can be provided and the range meter is scaled from 0-20 or 0-200 miles. The ground beacons associated with the Marconi and Murphy sets have a power of 400 and 800 W respectively. Ferranti D.M.E. has undergone considerable airline trials. In 1940 a research team at Bristol University developed the Skiatron, which was a special cathode-ray tube whose screen was coated with finely ground potassium chloride crystals. Impact of the electron beam produced a black mark on the tube face, which could thus be treated as an epidiascope slide and projected on to a large screen (this was very useful for strategic plotting and large- scale operations). From H2S, airborne interception (A.I.) radar, the Skiatron tube face and the general technology of the P.P.I.s, or plan position indicators (a form of H2S painting a picture of the ground surface about the emitting station) have come a whole range of air traffic control devices. These may best be discussed by considering what happens as a modern airliner approaches an international airport in bad weather. At a range of some 60-200 miles, micro-wave early-warning (MEW) will pick up the incoming airliner as a small "blip" on *the traffic controller's tube face. The airliner will be interrogated by the VHF/RT and asked to execute a manoeuvre; this will show up on the screen and provide a means of identification. Knowing what each blip represents, the controller will then be able to bring in his traffic in a co-ordinated sequence. Bad-weather Landings When the airliner is some 30 miles distant the pilot will be directed to change to a different frequency, namely that used by the terminal area controller. The latter sits at an airport surveil lance radar set (A.S.R.), which shows the same picture as the centre of the MEW screen, but on a considerably larger scale. Adjacent to the surveillance radar set will be an installation known in Great Britain as a G.C.A. installation and in the United States as a precision approach radar (P.A.R.). The signal transmitted to the cathode-ray tube of this latter equipment gives the approach controller precise knowledge of the position of the incoming air craft once it has turned on to its final approach. If the pilot requests a G.C.A. landing the following sequence of operations takes place : the G.C.A. hut's rotating UHF surveillance aerial puts out a steady sweeping emission—like a lighthouse— which is reflected from the aircraft concerned each time the beam is pointing in the appropriate direction. The position of the air craft is thus clearly indicated on the P.P.I, scope within the hut. The approach radar uses an even higher frequency—known as super high-frequency, or S.H.F.—in order to provide the most precise definition of the aircraft on the tube face. Two approach radar aerials are used, one of which scans through a 20 deg arc facing downwind along the runway in use, while the other is beamed over the runway centre line and scans through a vertical arc of between 5 and 8 deg to provide glide-path information. As the aircraft approaches the runway any deviations from the ideal glide-path are transmitted back to the pilot over the R/T. Alterna tively, the pilot can be ordered by the G.C.A. controller to fly his aircraft in a certain manner and so follow the correct glide-path. At the same time as position-finding aids were being deve loped, landing aids were also rapidly being brought to the point where they could ensure a successful landing in all but the very worst conditions. The I.L.S. (instrument landing system) consists of a V.H.F. transmitter, known as a localizer, at the upwind end of the runway in use, emitting a split radio beam whose consecutive halves are keyed together by a Morse transmission of E and T, or A and N, the central zone being a continuous note. Alternatively, the beam may be modulated to 90 cyc/sec on one side and 150 on the other, the notes being continuous. A separate transmission consists of a similar beam turned over on its side to provide glide- path indication. The I.L.S. receiver on board the aircraft is sensitive to the localizer and glide-path beams, and, according to the position of the aircraft on its approach, feeds signals to crossed needles on one of the pilot's instruments. The needles will cross in the centre of the dial when the aircraft is correctly aligned on the landing run. As the aircraft progresses down the runway it passes over an outer marker about 4J miles out from touch-down and a middle marker about 4,000ft out; these are short-range fan markers, giving precise indication of distance-to-go. It will be appreciated that D.M.E. can be integrated with I.L.S., just as it can with V.O.R., the frequencies being paired, using the airfield V.O.R. station as a target. The aircraft is thus provided with continuous range indication throughout its approach. The latest development is a completely automatic bad-weather instru ment approach in which the localizer and glide-path signals are fed through an approach coupler to the aircraft autopilot. And P.A.R., previously described, can be used to monitor I.L.S. landings. Finally, attention may be turned to "Consol," which is an M/F long-distance navigational aid. Three aligned ground station aerials broadcast overlapping 220 deg sectors, the signal consisting of Es and Ts by Morse in groups of a fixed number; according to the position of the aircraft, so will the number of Es and Ts heard vary. Over-water range is up to 800 nautical miles day and night. The radio compass provides a unidirectional bearing indication, a sense aerial and a receiver detecting the direction of the signal received from a non-directional ground marker beacon. The signal from the compass receiver is fed into one of the pilot's instruments, the needle of which gives the bearing of the ground station from the aircraft. Radio altimeters consist of an ultra- high-frequency transmitter and receiver emitting frequency- modulated continuous waves which are reflected from the ground directly beneath the aircraft. The frequency difference between the outgoing and incoming wave is indicated on the altimeter scale, which, it will be noted, actually measures the height of the aircraft above the ground.
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