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Aviation History
1952
1952 - 0355.PDF
8 February 1952 161 demand on the pilot can be very greatly reduced by instruments of the Zero Reader type. Surface Movement.—The size of large airports makes it impracticable to control the flow of aircraft between the runways and dispersal and loading points on the airport by visual means from a central point. In the United Kingdom a system has been developed known as the ground traffic indicator which is being tried out at London Airport. Actuated by magnetic detector coils in the taxi-ways and runways, it provides the surface- movement controller with information on a "mimic" illuminated display of the airport system and enables him to clear aircraft on the block signalling system. His instructions are passed to the aircraft by illuminated traffic signals sited alongside strategic points on the taxi-tracks. An alternative line of approach, also being pursued in the U.K. and U.S.A., is to apply radar surveillance to this problem. Experiments are being carried out this summer at London Airport, using a Marine Radar Equipment operating on three centimetres. Final consideration of the relative merits of the two systems cannot be made until investigations have proceeded further. Equipment is needed for guiding aircraft with sufficient accuracy for them to find and follow taxi-ways rapidly, and with ease in very low visibility. Not much can be said on this point except that experiments have been conducted for several years with leader cables buried below the surface of a taxi-track or runway, and that no means has yet been produced that would be acceptable and practicable for civil aviation. Requirements for the future So far [continued Mr. Stallibrass] this paper has dwelt more on problems arising from the present situation and the limitations of existing equipment. It would be a mistake to infer from this either that the existing facilities are bad in themselves or that the overall situation is very unsatisfactory. In fact, although much more needs to be done, particularly in certain parts of Europe, the airways system recently introduced in the U.K. works very much better than anything available here before and landing rates in bad visibility at the more important airports are increasing steadily. The first basic requirement [in solving en route problems] is for a navigational system that will provide aircraft with continuous information of their position, presented in such a form that they can make good any track laid down without the need for careful calculation and without requiring the presence of a specialist navigator among the crew. The two most likely contenders for selection as the new navigational system are the V.H.F. omni-directional range (V.O.R.) on 112-118 mc/s, together with distance measuring equipment (D.M.E.) on 1,000 mc/s; and the Decca system of hyperbolic navigation incorporating the automatic flight log and operating on low frequencies (100 kc/s). V.O.R. has been recommended by I.C.A.O. as a standard aid "in localities where conditions of traffic density and low visibility necessitate a short distance aid to navigation for the efficient exercise of air traffic control." The United Kingdom dissented from this recommendation as it doubted the adequacy of V.O.R. for this purpose, and these doubts have since been confirmed by the inaccuracies disclosed by extensive trials in the United States and by examination of the costs of providing V.O.R. /D.M.E. on the scale required in Europe. For these reasons at the recent meeting of the I.C.A.O., C.O.M. division, the U.K. took a leading part in discussing the advances that have recently been made in the development and application of Decca, and this resulted in a recommendation that the system should be further examined and evaluated operationally, preferably in the European region when so much cover already existed. Broadly speaking, the advantages offered by Decca are greater accuracy and cover, pictorial presentation, about two-thirds of the cost to authorities supplying ground installations and availability for shipping purposes. On the airborne side it may be about half the jost and a little over half the weight of corresponding equipment for V.O.R./D.M.E. The first need is for the navigational aid to enable aircraft either to be given separate tracks for a good deal of the flight or to be put on to a separate track while they are changing altitude, after which they can come back to the "centre-line" track again. This, however, only solves a part of the problem. At some point aircraft bound for the same destination have to be re-concentrated, and, even with a good navigational aid, the constant position reporting needed for this and for dealing with aircraft crossing or joining the airways throws a heavy load on the R/T. com munications. This leads naturally to a consideration of the role of surveillance radar. The first limitation is that the basic display on present types of radar—a plan position indicator—displays the positions of the aircraft as blips, without identificaion. Thus, the long-term requirement is for aircraft to carry radar transponders that transmit their identity in code, either continuously or when interrogated. In practice, this is likely to prove very difficult. One problem arises from the difficulty of supplying full coded information on a P.P.I, without cluttering up the display. Others lie in the choice of the interrogating/reply frequencies. The M.C.A. is beginning by experiments with a simple transponder, which could be operated by the pilot on request and would mark his particular blip distinctively on the P.P.I, for so long as he kept the transponder in operation. Another need is for the accurate determination of height. Radar height-finders currently in use rely on measurement of angles and distance. A slight error in the angular measurement becomes quantitatively more serious as the distance increases between aircraft and height-finder; and, except at fairly short ranges, the accuracies of present height-finders restrict their application mainly to assisting in identification and to preventing directions for unnecessary avoiding action by aircraft that are, in fact, separated by some thousands of feet. The answer to the accuracy problem is not easy, although one idea is for a trans ponder which, coupled to a barometric capsule, would transmit continuous identification of height as well as identity. However, this would be a very complicated apparatus if one considers that indication of 500-ft levels over present operating heights might require 70-80 height codes, in addition to the coding required for identification purposes. This problem is still under review, but a successful solution does not appear likely in the near future. Apart from the need for better height-finding and identification facilities, a full return from the employment of radar requires better methods of displaying and applying the information derived from it. At the moment information in the traffic control centre dealing, for example, with a stretch of airways, is derived from two sources. First, there is what is termed the "procedural" information, which is derived from a flight plan and an air traffic control clearance assigning particular altitudes and times at reporting points to each aircraft. These are checked and amended from time to time as position reports are received from the aircraft and are displayed by means of written flight progress strips on a flight progress board. The operation of this naturally requires a reasonable amount of light. Second, there is the radar information, at present confined to the P.P.I., which has to be viewed in obscurity since there is still no certainty of the long- promised daylight viewing tube. In the absence of an effective means of "marrying" the two types of information so that both are available to a single controller, the present emphasis in long-distance radar work has to be on dealing with a minority of situations involving closer separation and on monitoring traffic against untoward incidents, leaving the basic separation minima as required for a procedural system, rather than in speeding up the entire traffic flow by relying primarily on information "taken off the tube." This problem requires urgent attention, and it should be possible to learn a good deal more when the "procedural" air traffic control centre now at Uxbridge is trans ferred to London Airport and occupies the same accommodation as London Radar. The functions of surveillance radar for approach control may be defined as :— (1) To give aircraft navigational assistance in positioning themselves for final approach. (2) To monitor the terminal airspace against "intruders." (3) To marshal aircraft from holding patterns to final in order to secure a well-spaced traffic flow. (4) To monitor aircraft on their take-off paths in order to reduce the separation required between aircraft outbound from an airport and those In the holding pattern or on final approach. Judged by this, the present Federal G.C.A. equipment (as installed at London Airport) has serious limitations in that its vertical cover is cut off at about 4,000ft and it is subject to inter ference from permanent echoes. The I.C.A.O. specification for what is termed Surveillance Radar Element (S.R.E.) was based on the old conception that this should be primarily an adjunct to the radar approach system. The cover specified is not adequate for the application of this equipment to air traffic control, either in altitude or range. The specifications laid down for new British radar equipment (approach control radar) went further than this and provided for cover to extend up to io,oooft at about 20 miles. It also includes permanent echo cancellation, and there is little doubt that equipment similar to or better than this will be essential for improved landing rates in future. At the moment it is assumed that ultimately either some form of "daylight viewing" radar tube will be produced or that the intensities of lighting in different parts of the control room will be so varied that the approach, aerodrome and radar control will be integrated in the same accommodation. Since the new control accommodation to be erected at London Airport, for example, will obviously have to be usable for many years, these assumptions have serious repercussions. Nevertheless, it is pretty clear that these controls must be integrated for ultimate efficiency and that the display problems mentioned will have to be overcome. To be concluded)
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