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Aviation History
1955
1955 - 1088.PDF
200 FLIGHT, 5 August 1955 IN MEMORY OF MF AND MF/DF . . . bearings in order to determine a position. Much less time wasavailable in aviation; aircraft speeds demanded an instantaneous reply to the request for a bearing or a position. If, in a ship, ittakes five minutes to determine a radio-bearing from a beacon, the vessel will in this time have moved one kilometre; in a com-parable length of time an aircraft (at pre-war speeds) would move twenty kilometres. In order to satisfy the requirements for a quick reply to a re-quest for a bearing, watch on the air-ground communication channels was carried out with the direction-finding apparatusitself. The direction finder was equipped with a scale divided into 360 degrees, over which a pointer could be moved. Todetermine the bearing of an aircraft (which for this purpose transmitted a "dash" of approximately ten to fifteen secondsduration), the pointer was moved to a position where the signal- strength was minimum; the reading on the scale would then in-dicate the direction of die aircraft relative to the DF station. The main communication stations worked in close co-operationwith each other, and with secondary DF stations under their con- trol. The system of a main station with a number of auxiliarystations made it possible that the position of aircraft could be accurately determined. When an aircraft requested a QTF, allstations within the network took radio bearings and transmitted their results to the main station. There, the bearings were plottedon a map and the position of the aircraft determined. This pro- cedure sounds cumbersome when described on paper; actually,only one minute was required. The success of the communications and direction-findingsystem on medium frequencies was to no small measure due to the great enthusiasm with which the radio operators carried outtheir tasks, and to the pride they put in operating their stations to the best of their ability. The work required presence of mind,a cool head and great skill in handling of radio communications. How stringent the_ requirements were is borne out by the factthat of all the radio operators employed at a radio station, only about 15 per cent were able to cope with the traffic on thefrequency 333 kc/s. The standard of operating was about the same for all stationsin Europe, and the operators jealously guarded the good names of their stations; these must not be jeopardized by errors orclumsy working-methods or by a colleague not yet up to the required standard. I cannot speak too highly of this period, andI always proudly think of the time when I was one of those permitted to conduct radio communications on 333 kc/s; I willalways regard that time as the one which gave me the greatest satisfaction in the performance of my assigned task. Manners on the Air There was a certain code of good behaviour and politeness inradio-communications in those days. This code required, for in- stance, the exchange of signals such as GM ("good morning"),BJR VX ("bonjour, mon vieux"), TU ("thank you") and PSE ("please"). Failure to observe the rules of elementary politenessin radio communications was very strongly disapproved of. Although the radio operators of the various ground stations inEurope did not know each other personally, they nevertheless knew one another from the way in which they handled the morse-key. The manipulation of a morse-key, the rhythm and the specific characteristics of the signals, identified an operator, justas a person can be recognized by his voice. As a rule, an aircraft did not have to call a ground station morethan once. If it did happen that a first call passed unnoticed, the ground-station operator would feel greatly ashamed; and if, forinstance, he heard the signal "PHA de GED QRZ FNABC" ("Amsterdam from Croydon, you are being called by the aircraftFNABC") the operator at Amsterdam would be feeling really embarrassed. The speed with which information could be passed betweenskilled operators was much faster than the layman usually be- lieves. A simple message from an aircraft—indicating its altitude,including establishment of the communication, transmission of the message and acknowledgement by the ground station, wouldnot take more than about ten seconds. I have mentioned already the necessity for speed. As air traffic increased the radio frequen-cies in use became more and more crowded until, just before World War II, overloading of the channels began to be apparent,calling for greater experience on the part of the operators. More channels were required, but the normal process of gradual ex-pansion of services had to be interrupted on account of the war. It is obvious that the amount of communications traffic wasgreatly increased during periods of bad weather, especially when the visibility was poor. Aircraft had to be kept informed of theweather situation, and numerous messages had to be exchanged for air-traffic control purposes. In addition, aircraft required morefrequent radio bearings than during the clear weather. The practice of requesting QDMs developed into a valuablesystem of radio-aid-to-landing; but before enlarging on this de- velopment it would seem useful to give some figures on theaccuracy of radio bearings in general. During the day-time the range of the medium frequency communications was about 300km, which was, in Europe, sufficient for all practical purposes— because of the composition of the networks of stations and thedelineation of the control areas of the main communication stations. Accuracy of the radio bearings was usually plus or minusone degree, often greater, and an accuracy of less than three degrees was not considered satisfactory. For the determinationof a position, bearings from three different ground stations were considered necessary, and the result should show as a very smalltriangle or, at ranges less than approximately 200 km, the bear- ings should go through one point on the map. At night, how-ever, the normal DF apparatus was subject to so-called "night error" which made bearings unreliable. Fortunately, with aspecially constructed aerial it was possible to eliminate this night error; but the antennae system required careful construction, ithad to be large, and erected away from buildings, power lines, etc. Therefore, night-error-free systems could be installed only at themain stations. At short distance, night error was not pronounced. The problem of effecting landings under poor visibility con-ditions has, of course, been a headache ever since aviation started. It would bring us too far if we attempted here to review thevarious endeavours to find a solution to this problem. The history of the development of radio aids to landing makes interestingreading, but we have to confine ourselves to the role which MF/DF played in this field. The QDM procedure was soonfound to be an excellent method of being directed to an airport. If the aircraft continued to request a QDM at frequent intervals,and if it followed the course supplied to it by the ground station, it inevitably had to arrive over the aerial system of the DFstation. This procedure was used extensively and developed into a system whereby a separate DF station, constructed speciallyfor this purpose, was installed in the approach path for one or more runways and, usually as close to the threshold of therunway as was compatible with the height of DF aerial (four to five metres high and of light construction). When an aircraftwished to use the system for landing, it changed to a slightly different frequency and started a series of requests for QDMs.The practice was that the aircraft operator would press down his key for about five or six seconds, then listen for the result of thebearing, which was given to him in the form of a course to steer; he would repeat this process until the aircraft passed over thedirection-finding station and on to the runway. This method worked fast and smoothly, but, obviously, it required considerableskill from the operators, both on the ground and in the aircraft. The system was later perfected by the installation of anotherdirection-finding station which could take "cross bearings" of the aircraft while it was nearing the runway on the approachpath and while being directed by QDMs. With this system it became possible to tell the pilot not only the course to steer butalso his distance to the runway. A further improvement was that the operator in the aircraft continuously transmitted a dash onthe medium frequency and the pilot told by the ground station, by radio telephony, the course and distance. Thus, during theapproach, when fast direct communication between the ground and the pilot was a necessity, the intermediate stage of the air-craft radio operator was eliminated. There were still other developments, all using the same basicprinciples. There was, for example, the ZZ procedure, used ex- tensively in Germany, by which the pilot was advised when hecould descend below the clouds. In all these systems, in communications with the aids to airnavigation, the human element played a great role, and each per- son participating in a major or minor capacity did so with greatenthusiasm and pride in the profession. The radio operator, who had assisted in the safe landing of an aircraft under sometimesextremely poor weather conditions, felt a satisfaction which he will always remember. Medium-frequency communications and direction finding con-tributed greatly to the regularity of civil aviation in the Eastern Hemisphere; without MF/DF it would not have been possiblefor the airlines to adhere as closely to schedule as has proved possible; certainly not in a region so invested with bad weatheras is Western Europe. And now, MF and MF/DF are dead. They have been re-placed by other systems of communication and radio aids to navigation, using other frequency bands. Radio telegraphy hasagain been replaced by radio telephony for short and medium distances and is being replaced by radio telephony for long dis-tance. This is not the place, nor would it be appropriate for me, to evaluate the two systems; we are in a different era, differentconsiderations count, and different, better, techniques can be applied. The natural development of an industry must followits course, or it would become incapable of further growth. Still, we should at this point pay tribute to the services ren-dered by MF and MF/DF during a period when aviation became increasingly dependent upon reliable systems of communicationand radio aids to navigation.
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