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Aviation History
1939
1939 - 0296.PDF
io8 FLIGHT. FEBRUARY 2, 1939 o In order to reduce the congestion and the amount of work being done by ground D/F stations, there is a tendency nowadays to make use of automatic omni directional beacons for naviga tional purposes. At the moment transport crews flying between London and the Continent have the use of three of these—at Mitcham, Le Bourget and Schiphol (Amsterdam)—for posi tion-finding purposes. This is an example of code-sending omni directional beacon produced by Standard Telephones. receiver provides the pick-up, so that any number of machines can use the track without radio inter ference. Marker beacons can give " spot " positions, and speech can be superimposed on the transmission if required. In spite of these good features, at least two serious drawbacks have rather upset the faith of the system's warmest adherents. First, once a pilot gets off the equi-signal track, whether voluntarily, because of storms ahead, or by drift miscalcu lation, there is nothing to tell him how far off he is from his desired path. The proximity of other beacons on different frequencies, and the fact that practically all beacons give four courses, makes positive location quite a compli cated business.. Secondly, there is no doubt that the beams "swing" and produce, queer kinks and false courses, especially in mountainous country. An interesting innovation is now being tried in Australia and discussed in the U.S.A. This is the use of V.H.F. equi-signal beacons on the Lorenz principle. It has been known for some time that the distance of reception of the V.H.F. beacon is practically proportional to the height of the machine, and the fact that operating heights are increasing may contribute to the success of the experiment. The radiators of the long-range beacons are more elevated than those of the original Lorenz landing beacons, so it is hoped that the influence of ground conditions will not be so great. From 33 megacycles upwards a large number of beacons can be operated without interference troubles, even if their back radiation is utilised, with comparatively small spacing of carriers. Irrespective of whether frequency and constancy troubles are overcome, one objection to the track system remains, and that is the risk of losing the beam. Admittedly, a standby D/F receiver on the machine will assist the crew, but in the barren parts of Australia, at least, suitable medium-frequency transmitters are few and far between. On the other hand, there are not likely to be many parallel or crossing courses—at least for some years to come—so o that errors in identification are less likely. It may be, therefore, that the V.H.F. track beacon will prove to be the best for one given set of conditions, whilst high-frequency D/F ground systems are best for the others. Omni-directional beacons of the long-range or localiser type, work ing on medium frequencies (circa 263 kc/s), are a recent innovation, and their success depends more on the aircraft receiving equipment than on the transmitter as such. The triangulated system has been very carefully worked out so as to cover the needs of the major air routes. Speed of obtaining results is vitally necessary when trying to get a "fix," and the present length of transmission of one station and the interval before obtaining the result of the next one are thought by some people to be too long in view of modern cruising speeds. Again, it appears that the use of two systems is likely to be the ulti mate answer. For instance, a bear ing from the airport ahead can be obtained through the ground D/F system in order to check the track, whilst a '' cut'' can be got from a beacon (or a broadcaster) to one side of the track. For a given route a map can be specially prepared with compass roses marked at D/F or beacon stations, with bearing lines drawn off to intersect the track at convenient intervals. There is no doubt that the advent of the beacons has relieved some of the load on the ground D/F organisation, but so far no equivalent of the Adcock aerial system is available for aircraft fitting, so that "night error" and similar effects have still to be contended with. Possibly some experiments with a vertical type of '' anti-fading aerial at the beacon may be worth while in view of the promising results from this type at some of the major broadcasters. In any case, since night flying is now a regular institution, the elimination of errors at the trans mitting or receiving end is a necessity and should receive immediate attention. (5) Close-approach and Landing Aids.—At the present moment aircraft can take off in fog, fly in or above fog, but cannot land with any degree of certainty when the visibility is less than 100 yards or so. Until such a time as aircraft are designed with a speed range of the order of ten to one, radio development must proceed for the assistance of the conventional type of machine. The ultra-short-wave approach beacon has been stan dardised in Europe, and the various interpretations of it by Lorenz, Telefunken, and other firms give the same result so far as the pilot is concerned. Since the introduction of By way of combating night and other errors, the Adcock type of aerial system (shown on the left) is now used at the majority of aero dromes for direction finding other than that involved merely in homing or making UZZ" approaches. Above is the receiver concerned.
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