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Aviation History
1956
1956 - 0496.PDF
496 FLIGHT, 27 April 1956 LONG-RANGE NAVAIDS . . . wave, and this may lead to errors of up to 2 deg in a zone over350 and under 500 n.m. from the station. The principle of Consol is a radiated pattern having many lobes,or sectors, rotated electronically. Each sector has either a dash or dot characteristic, such sectors being disposed alternately. Thespacings between the dots and between the dashes are such that if the two characteristics are brought together a steady tone results,and this takes place on the boundary lines between the sectors. The normal Consol transmission follows a complete cycle,which is repeated continuously; the cycle normally occupies 40 seconds. It consists of, firstly, the station identification signalfollowed by a two-second break. Then follows a succession of dots or dashes, a continuous tone, and then more dots or dashes.This cycle is caused by the rotation of the pattern, and the number of dots (or dashes) heard before the steady tone, or "equi-signal,"determines the bearing of the receiver in relation to the trans- mitter. Thus, a navigator wishing to obtain a position line fromthe Consol transmitter merely has to tune into the station on the aircraft A.D.F. (or communications) receiver. He then countsthe number of dots or dashes following the identification up to the receipt of the steady tone or equi-signal. This will indicate hisposition within the Consol sector, and by referring the received count to a chart overprinted with the Consol pattern a positionline can be obtained. It will be appreciated that as there are several sectors havingidentical polar diagrams a certain ambiguity exists. It is therefore necessary to have an approximate idea of the position of the air-craft so that the appropriate sector may be selected on the chart. However, these sectors are between 10 and 15 deg in width andin the unlikely event of the navigator being unable to decide with certainty which is the correct sector his doubts may be resolvedby use of the A.D.F. The process of obtaining a position line takes, with practice, about a minute. The Consol transmitter energizes an aerial system, the dis-position of which results in two sectors which are of no use for navigation. The antenna array consists of three aerials erected ina straight line, the aerials being spaced three wavelengths apart. By energizing one outer aerial with current 90 deg ahead of thecentre aerial in phase, and the other outer aerial 90 deg behind it, the whole of the radiated pattern is caused to rotate about thecentre aerial. If the line joining the three aerials is extended the sectors bisected by this line are unusable. Generally it may bestated that only those sectors falling within the areas ± 60 deg of the median line on either side of the aerial line are of use. Loran. A system that found considerable favour during theWar, and which has since been extended to civil use, is Loran, an aid based on similar principles to the well-known Gee but havinga considerably greater range. The name Loran is derived from the function of the aid, namely Long Range Navigation. WhileGee is a British system, credit for the development of Loran goes to the United States.Loran is, in its standard form, a medium-frequency aid, operat- ing in the frequency band from 1,800 kc/s to 2,000 kc/s. At these frequencies propagation and range are subject to the iono-sphere and to the question of whether the ground wave or the reflected wave is received. Generally speaking, the ground wavesmay be used at ranges of up to 900 n.m. by day, while at night the reflected sky waves permit reception at up to 1,400 n.m. The system operates on hyperbolic principles, a hyperbolabeing the locus of a point having a fixed difference in range from two other points. (This fundamental, by the way, holds good forall hyperbolic systems.) Let us consider now the method employed in Loran to propagate a series of hyperbolic position-lines. If two transmitters are sited at a given distance from each otherthey can be taken to represent the two points referred to in the previous paragraph. Now, irrespective of frequency, the speed ofradio waves through the atmosphere remains constant, and is known. Thus, if the time of origin of a pulse of radio-electricenergy at the transmitter is known and the time of receipt at the receiver is taken it will obviously be only a matter of calculationto establish the distance between the transmitter and receiver. And this is the precise principle upon which Loran operates. Known as a "chain," the two Loran transmitters are locatedsome hundreds of miles apart. One transmitter of the chain, the master, emits a number of uniformly spaced pulses eachsecond. The second, or slave, transmitter emits a corresponding series of pulses, the two transmissions being time-locked inrespect of each other. This highly accurate time-relationship is allowed for in the receiver, which is able to establish as a resultthe time taken for each pulse to arrive from each transmitter. An electronic computor included in the receiver measures the dif-ference between the times taken for the master and slave pulses to reach the receiver. It is a comparatively simple matter to plot a number ofhyperbola; for known difference-values on a chart; Loran charts are therefore prepared with the hyperbolic pattern overprinted,each hyperbolic position-line being identified by its difference- value. Accordingly, the operator of the Loran receiver merelynotes the difference-reading on his indicator (which is, by the way, a cathode-ray tube) and refers this reading to his chart to obtain aposition line. Unlike Gee, Loran provides only a single position-line. Thus,to obtain a fix it is necessary to take readings for two chains having reasonably interlocking patterns. It is usual when settingup the ground installations to try to select sites that will permit the two hyperbolic patterns to cut at a wide angle. The systemallows for one station to be made common to two chains, this usually being the master. Incidentally, unlike Consol, the accuracy of Loran does notsuffer as a result of comparable ground waves and sky waves, full allowance being made for this. The accuracy of the receivedposition-line largely depends upon the skill of the operator and other factors, but an average figure may be considered to be1.5 n.m. at 800 n.m., using the day ground waves; at night, on sky waves, this error increases to up to about 5 n.m. The cathode-ray-tube indication, together with "blinking," provides excellent fail-safe characteristics. To be concluded) THE 707 IN UNIFORM Taken within the last few weeks in the Boeing plant at Renton, near Seattle, this is one of the first photographs to show production of the KC-135 tanker for Strategic Air Command. In many ways similar to the 707 airliner, the tanker has already been ordered to a total of at least 375. The last piston-engined KC-97s are in the background. REPUBLIC/WESER AGREEMENT AN agreement has been signed between Republic Aviation and' the Finanz und Verwaltungs-Gesellschaft Weser of Bremen by which the West German aircraft firm will assemble and testF-84F and RF-84F aircraft before delivery to the new Luftwaffe. The American company will help in initial planning and sub-sequently assist in the transport, test flying and modification of the aircraft. Assembly installations, it is stated, will be plannedto receive large numbers of Thunderstreaks and Thunderflashes. c "FLIGHT" PHOTOGRAPHS MANY of the photographs reproduced in Flight, i.e., thosetaken by our own photographic staff (as indicated on pub- lication) are subsequently available in the form of high-qualityprints. Prices are as follows (dimensions given in inches): 5i X 3j, Is 4d (glossy or semi-matt, unmounted) or 2s (tonedsepia, unmounted); 6x4, 2s 6d or 4s; 8x6, 3s 6d or 5s 6d; 10X8, 5s or 7s lOd; 12x10, 6s or 9s 8d; 15x12, 9s 6d or13s; 20 x 16 (semi-matt only), 13s or 18s. Also available are a postcard series, totalling over 200, depict-ing both historic and modern aircraft of the R.F.C., R.N.A.S., R.A.F. and Fleet Air Arm. Complete lists of the series are obtain-able on application. Prices are 8d each card, or 71d eack for quantities of a dozen or more. One view only—in most cases agood three-quarter front or side—is available for each aircraft type- Also available as postcards, at the same prices, are reproductionsof general-arrangement drawings of some 20 British aircraft that have appeared in our "Aircraft Intelligence" pages.
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