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Aviation History
1937
1937 - 0150.PDF
54 FLIGHT. JANUARY 21, 1937. RADIO AIR NAVIGATION The Trend of British Development By RODERICK DENMAN, M.A., A.M.I.R.E., A.F.R.Ae.S. The author ot this article. AWELL-KNOWN indoor game assigns to unknown pairs of players the name of some animal, requiring that the male and lemale of each species identify themselves one to another by means of their charac teristic utterance. In the general hubbub that ensues the prize is awarded to the pair who first succeed in recog nising each other's cry. While it is on such lines that British aeronautical radio communication has necessarily to be conducted at the present time, the possibilities of selective reception are even scantier than have been suggested in the example given above. The actual conditions are fairly closely identifiable with those of the parrot house at the Zoo. This lack of clear channels for communication and direc tion-finding purposes arises from the nature of wireless itself, and from the inability of the early pioneers either to foresee or to make adequate provision for the growth of radio aids to aviation. To-day as is well known, we have barely sufficient wavelengths with which to maintain the present European system of direction-finding by ground stations This system is evidently wasteful in so far as it can be replaced by aircraft observations on radio beacons, but is important for traffic control operations and (at pre sent) unique in respect of its immunity from night error. We may therefore expect to see shortly not the abandonment ot the elaborate D.F network which now covers Europe, but the beginnings of an auxiliary beacon system which will enable pilots to locate themselves without transmitting requests tor a bearing or position. This might be done in several ways, some less economical than others. Medium- wave navigational beacons ("radio ranges"), as used in America, for instance, would require more wavelengths than we can hope to provide, having regard to the number of aii routes flown. A more efficient plan is to have tri angular groups of medium-wave stations possessing omni directional characteristics. In this system, which is bor rowed from marine practice, all three stations in a group work on a shared frequency, and each in its turn for a period of two minutes, thus forming a total cycle of six minutes per triangle. The conference of aeronautical radio experts which met in Paris last June decided to proceed on these lines, and contemplated the following as initial triangles: — Oslo Konigsberg A msterdam Hanover Berlin Warsaw Zurich Milan Toulouse Belgrade Madrid Naples Brindisi Stockholm Breslau Paris Stuttgart Munich Vienna Genoa Venice Marseilles Bucharest Palma Syracuse Benghazi Copenhagen Hamburg London Brussels Cologne Prague Lyons Rome Barcelona Sofia Seville Elmas Tripoli With the object of minimising night effect, the frequen cies were to be kept as low as possible. However, it is not going to be easy to find enough of these. It is probable that the existing type of aircraft direction finder will in due course be superseded by a form of cathode-ray direction finder, giving rapid and automatic bearing indications. Not only has one such device already been tested in America, but the subject is one in which British radio engineers have taken a leading part. It is important to realise the extreme rapidity with which bearings can be taken by means of the cathode-ray direc tion finder. The signal need not last longer than a few milliseconds, consequently we have a large potential in crease in the handling capacity of existing ground D.F. stations. Very rapid position finding is performed by one European group of stations where cathode-ray direction finders are already in use, but the necessity for collecting and signalling the results from the co-operating stations brings the operating speed far below the theoretical limit. Bearings are therefore to be preferred, and are growing rapidly in number. In Great Britain in 1932 the propor tion of bearings to fixes was 2 to 1 ; in 1936 it was 55 to 1. The total number of fixes in 1932. with three stations in operation, was 1,032. In 1936, with 25 D.F. sets work ing, it was only 988, or less than one per station per week. Ultrct'Short'Wave Beacons Although the wavelength shortage precludes the use oi medium-wave navigational beacons on a large scale in Eng land this is not necessarily true of the ultra-short-wave type of beacon. Of such are the Lorenz, Telefunken, Mar coni and Plessey R.T .E. beacons. Their characteristics are now fairly well known. At any given distance there is a large increase in signal strength with height, the useful range (at the lowest height of an approaching aircraft) being in the neighbourhood of 30 km. The question then arises as to the range of such beacons to aeroplanes flying at greater distances and greater heights. The diagram of Fig. 1, while only accurate for a par ticular set of conditions, will serve to indicate the general character of the re- , lationship between height and range 4 when the ultra-short wave transmitting • aerial is located a half wavelength above ground. By placing the transmit- J|L ting aerial several wavelengths above W ground the range may be increased; 11 it has been stated to be between 150 U and 200 km. for an aerial radiating 120 »-*5-U watts from the
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