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Aviation History
1959
1959 - 0520.PDF
20 February 1959 255 The back-to-back aerials of the Decca Radar DASR.l 10 cm surveillance radar. They were designed and, in this particular instance, made by the French company, Societe Nouvelle d'Electronique Marconi microwave link with remote control-facilities is installedin New Zealand and a number of Collins links are being set up in the U.S.A. Decca Radar have also produced link equipment.The projected British airways surveillance system will probably also incorporate microwave link systems. Link frequencies maybe allocated in the telecommunications band of 3,800 to 4,200 Mc/s, the multi-channel and television band of 6,000to 6,400 Mc/s or the civilian and Service link band of 7,300 to 7,600 Mc/s. The Use of Radar Information In the normal air traffic control procedure, the controllers obtaininformation on the position, progress and intentions of an aircraft by means of telephone, teleprinter and radio communications.The information is noted on flight progress strips which are inserted in holders and shifted from point to point on special racksto form an approximate picture of the traffic pattern over a par- ticular route or sector. It is therefore largely up to the controllerto visualize the situation in the air, although the actual situation as opposed to reported location of aircraft may be affected bynavigational accuracy, communications time-lag and adherence of the pilot to procedures. The separation required between aircraft in a particular areais accordingly based on a knowledge of the navigational accuracy and degree of control which can be achieved. Over the NorthAtlantic, for instance, separations of 30 min along track and 120 miles across track are required because the facilities now generallyused do not permit more accurate control either from the ground or in the air. Dectra and Doppler can considerably improveNorth Atlantic navigation. Much closer separations apply on airways over Europe and the U.S.A. In the British Southern Air Traffic Control Centre at LondonAirport, surveillance radar was adopted some years ago as a means of locating aircraft in the London terminal area and of guidingthem from stacks to the runway centreline. Although it provided a raw radar presentation which had to be viewed by an operatorisolated in a darkened cubicle, the radar information was par- ticularly useful in establishing the positions of aircraft in relationto each other. The actual radar used was an American MEW S-band unit with a range of something over 100 miles.In due course M.T.C.A. sponsored development of the Cossor ACR.6 airfield control radar and later a 50 cm Marconi S.232 wasobtained. The ACR.6 was used for procedural control in guiding aircraft from the Watford and Epsom stacks on to the I.L.S.centreline. The S.232 could pick up aircraft very soon after take- off and was applied to the control of outbound aircraft. Thelonger range MEW equipment is now being replaced by two Marconi S.14 radars, separately mounted but synchronized torotate back-to-back. One is set to cover the high sector and the other the low. The latest equipment to be ordered for S.A.T.C.C. is a pair of Marconi S.264A 50 cm radars of 500 kW power—incidentally representing the first use of power klystrons in civil radars. The first S.264A is to be installed by June this year. The raw radar picture is full of confusion. In addition (and thisis often considered more important than the point just mentioned), the screen must be viewed in a darkened room. From the con-troller's point of view both these factors are of major importance. A man can only concentrate on a radar screen in a darkened roomfor a few hours before eye-strain begins to tell and, in any case, darkened surroundings are uncongenial. A higher level of lightcan be tolerated if "white minus amber" lighting is used. Radar must be combined with information from all other sourcesbefore it achieves full usefulness, but radar and flight progress strips cannot be viewed together in either full darkness or fulldaylight. Many air traffic controllers consider that the first step towards making radar a really useful air traffic control tool is torender it usable in daylight. Bright displays are discussed in greater detail later in this account. The main difficulty with raw or near-raw radar lies in identifyingaircraft. Conventional D.F. is often used to assist in this, but is obviously ineffective where two or more aircraft lie on or near thesame bearing. The controller can order an identifying turn, but this uses valuable communications time and may take the aircraftundesirably off course. The ideal presentation would be in the form of a three-dimen-sional "goldfish bowl" in which all aircraft were clearly identified and could be seen at their actual heights. No radar at present inoperation can produce a true three-dimensional picture although those using a volumetric scan are capable of presenting plan-position and height on separate tubes. But progress towards the complete three-dimensional picture is following the rather simplertechnique of producing an unequivocal plan-position picture in which each aircraft indication bears a height reference. Based to a large extent on military defence-system work, a civilaviation data handling system is being developed by Marconi, and Decca Radar are also working on civil equipment to the same end.The first elements of Marconi's system were exhibited at the S.B.A.C. display last year and their military equivalent was tosome extent revealed at R.R.E. and at the Marconi test site at Rivenhall last year (Flight for May 2 and October 10). It is basedon a store into which full information on each aircraft can be fed and from which such information can be selectively withdrawn.In this way, both flight-plan information and actual observations can be combined and computed for the progress controller. To this end a considerable variety of additional features havebeen added to the normal raw radar display in order to produce as the working medium a completely synthetic picture bearingonly relevant and identifiable information. A control system based on this type of surveillance radar will therefore consist of threemajor blocks. The first of these will be devoted to gathering information on aircraft movements from a radar presentationincorporating the various clutter-removing systems. On this it is possible to superimpose a map of the area being viewed either asa transparent engraved overlay or in the form of a video map electronically superimposed on the tube face. To produce a videomap, a radial beam is set to rotate on a cathode ray tube in synchronism with the rotation of the aerial. The beam is scannedthrough a photographically marked glass slide and converted into video signals which are mixed with the incoming radar signals.Video map eliminates parallax errors and always remains syn- chronized with the radar picture when different range-settings areselected or when the display is off-centred in order to concentrate on one sector of an area. A transparent mask engraved withdirection lines is often mounted over the tube face so that bearings can be read against a bearing scale round the periphery of the tube.Instantaneous automatic gain control (I.A.G.C.) is often applied to even out responses of varying strengths. Cathode ray tube displays can be divided basically into twotypes, moving- and fixed-coil. The former have deflecting coils at the neck of the tube which rotate in synchronism with the aerial.They are simple to operate and offer a high degree of bearing accuracy; but only a limited amount of additional information canbe incorporated in display without affecting quality. On fixed-coil displays, the fly-back or idle time of the scanning spot can be usedto provide something like a dozen additional patterns and traces, which makes them most useful for traffic-control work. As mentioned above, an aircraft can be identified in a raw radarpicture either after performing some distinctive manoeuvre which will make its track on the screen obvious or by taking D.F. bearingson it and deciding from these where it lies. For this purpose it is possible to superimpose directly on the radar displav the bearingtrace or traces produced by one of the automatic D.F. or fixing systems. The D.F. traces orieinate on the radar display at thegeographical location of the D.F. ground stations. Finally, controls can be provided with which the origin and bearing of a movableline can be positioned anywhere on the radar screen so that additional bearing or vector information can be derived. With the features described above it is possib'e to modify theraw radar to the extent of partially or wholly eliminating clutter.
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