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Aviation History
1938
1938 - 0183.PDF
JANUARY 20, 1938. FLIGHT. Croydon's radio department withthe three two-way installations —two for normal work and thethird (on the right) for short- range controlled-zone operationsin QBI conditions. A telewriter is now used for rapid communi-cation between the operator and the control officer in charge. time is the Microray (18 centi- metre wavelength) radio link which connects the cables on the French and English side across the Channel between Croydon and Le Bourget. Owing to the electrical charac- teristics of these ultra-short waves they can only be used under certain conditions, the most important of which are that the two terminals must be within optical range t>f each other. For this reason their application is somewhat limited, and it seems probable that teleprinters and cables will continue to replace radio equip- ment for aerodrome ground services on the busier routes. (2) Meteorological Services.—The collection and dis- semination of weather reports is on an international basis. Radio transmission is employed both by secondary stations reporting to the head of their group and by the head in broadcasting the observations of the area under its control. The broadcasts are of interest to aircraft in flight, since the transmissions are at stated times and, being in the form of an international code, can be read by all. This is one argument for the retention of radio transmission for this class of work, since in many cases it avoids the necessity for a pilot to call up the ground station to ask for a weather report on the already overworked aircraft wavelength. Secondary reporting stations can, of course, use telephone or teleprinters for sending data to the area control. The wavelength used for these main transmissions is scheduled for about 1,200-1,300 metres, but Croydon uses the "aircraft" wave (900 metres) to broadcast the local report, so that the operators keeping watch on this wave need not transmit demands for this information. In this connection it is worth while remembering that a little in- telligent listening may often avoid the necessity for trans- mission. The wavelengths used in the future for this service are closelv bound up with the air-to-ground and ground-to-air communication problem which is dealt with below. With the advent of Transatlantic and Transpacific services, the aircraft can do work as valuable as can the ground organisation by reporting local weather condi- tions, thus two-way communication is necessary. Within the past few years picture transmission by radio has become a commercial achievement, and this opens up the prospect of complete weather maps being '' trans- mitted" to long-distance aircraft. In the case of airship work, in particular, there are many people who think that it is best to navigate by the weather map and not by straight line or great circle methods, and with increased aircraft speeds and range it is quite possible that the air mass chart may become as important as the orthodox route map or marine chart. One form of weather reporting which is likely to become of more all-round value is radio telephony broadcasting, as at present given by Borough Hill on 1,224 metres. This wavelength has had to be altered several times due to inter- ference from, or to, broadcasting stations, but the main idea of the service is good, and it could be carried on another wavelength if a further shift is inevitable. If each country had its own corresponding station, private flying would be aided considerably and the information would not be despised by the air lines. The Present Channel (3) Air to Ground and Ground to Air Services.—In Europe civil aviation radio communication started on 900 metres and, except for small extensions of the wavelength band from time to time, the majority of the traffic still passes at those frequencies. Apart from the fact that in 1920 the vagaries of short- wave transmission were not thoroughly understood, the adoption of 900 metres wavelength presented several attrac- tions for the aircraft services. First, it enabled reason- ably accurate direction finding to be done at the ground receiving point, and, secondly, transmissions on that wave- length were much more reliable in range and constancy over a period of time than those of the higher frequencies. The first reason is probably the one for which the present method has been retained as, with the advance of radio science, the trend for purely communication use is the employment of short wavelengths at the expense of medium and long waves, especially where long distances have to be covered. For two-wav communication alone, the United States civil air lines almost exclusively operate by telephony on 50 to 100 metres, and reserve the 1,000-metre band for their navigational beacons. With this improvement in radio technique it is probable
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