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Aviation History
1960
1960 - 0244.PDF
244 FLIGHT, 19 February 1960 SYSTEM SURVEY. PYE ILS AND AUTOMATIC LANDING By B. Williams* The new slotted aerial system for the Pye ILS glide-pathinstalled near the seaward end at the new Hong Kong runway THE standard ICAO non-visual aid to final approach andlanding is ILS; and this aid will be provided at all inter- national airfields until at least 1970—and probably very muchlonger. Practically all airliners are fined with ILS receivers and most modern types also have autopilot ILS couplers for auto-matic approaches. For these reasons, ILS will naturally be used, at least in the initial landing phase, in all automatic landingsystems likely to become operational in the 1960s. Until the advent of the latest Pye equipment, the extent towhich ILS could be used for automatic landing was limited by the wide tolerances permitted in the localizer and glide-path by thepresent ICAO specification, which is framed to include systems, such as SCS51, developed during World War 2. The permissiblevariations in the course line of ± 0.3 ° will allow a lateral displace- ment of ± 5bft in the touchdown area of a modern instrumentrunway—about 10,000ft from the localizer aerials. This is about five times greater than that thought permissible for automaticlanding. Again, the permissible variation in the glide angle of ±0.1° will allow variations in height above a fixed reference pointon the course line which are not acceptable for a direct change- over to an automatic flare-out system. Most glide-paths have useddipole aerials which are extremely difficult to adjust because of mutual impedances, with the result that glide-path angle is notcorrect or constant with distance along the path. For these reasons, the RAE Autoland system used ILS for azi-muth information only at distances greater than 4,500ft from touchdown and for elevation information down to heights on theglide-path greater than 150ft. Azimuth information down to touchdown is provided by Leader Cable laid on both sides of therunway to a distance of 5,000ft from touchdown. In most cases this takes it outside the airfield boundaries. Consequent problemsin land purchase and technical maintenance at most civil airfields are nearly insoluble. The elevation information necessary forautomatic flare-out is provided from a height of 60ft downwards by a radio altimeter; and the gap in positive guidance from 150ftto 60ft on the descent is covered by stored elevation information gathered from the glide-path in the preceding phase down to 150ft. There is thus a very real need for improvement in ILS to allowit to be used with the existing British system, in azimuth down to touchdown if possible, so as to dispense with the Leader Cablesystem, (or at least down to 60ft so that Leader Cable may be con- fined within the aerodrome boundary) and in elevation down to theheight of 60ft where the radio altimeter takes over. The present Pye ILS provides these improvements. In azimuth,an aerial system consisting of two dipoles spaced symmetrically about the focus of a 70ft parabolic reflector provides a course linestable to better dian 0.1 ° with only two electrical adjustments, one of carrier amplitude and the other of phase equality. The inherentstability of this simple aerial system is further increased by the addition of an automatic beam-control system which takes careof minor variations in carrier amplitude and modulation depth which may result from valve ageing. Long-term measurements at amonitor 500ft from the aerial have shown a course line stability of better than 0.06° with unattended equipment. Similar stabilitieshave been obtained with a monitor placed in the approach area short of the touchdown point. The very considerably improvedperformance of the present Pye localizer has been utilized in making more than 200 successful automatic landings without theaid of Leader Cables. In elevation, the completely new glide-path aerial system usingslot aerials, developed by Pye in conjunction with RAE, is free from mutual impedances inherent in the dipole type and, withsimple adjustments, the glide-path angle may be very precisely set. Glide-path angle is maintained within very narrow limits at allheights from 20ft upwards on the course line. As with the localizer, minor aberrations are corrected by an automatic beam-controlsystem. Precise glide-path information can now be supplied to * Adviser on Navigation Aids, Pye Telecommunications Ltd. an automatic landing system down to the height of 60ft at whichradio altimeter control takes over. The possibility of providing an automatic flare-out from the glide-path system alone is beingactively investigated. A decision to eliminate Leader Cable operation from the RAEautomatic landing system has not yet been taken, mainly because of the possibility of interference with localizer information atlow altitudes from localizer signals re-radiated from other aircraft flying in the vicinity. Such effects in fact have been observed, buiprecise information correlating them with aircraft tracks and heights is not yet available. The problem has been investigatedtheoretically by Pye and the maximum aberration of the course line between middle marker and touchdown has been deducedfrom the measured radiation patterns of the Pye localizer from aircraft of assumed echoing characteristics flying specific tracksin the localizer field. Experiments are now being conducted by Pye to determine practical values for aircraft flying these specifictracks. From these tests it should be possible to specify safe orbital radii and heights at which interference with automaticlandings using Pye localizer alone would be negligible. If such orbits are operationally acceptable, there is no reason why LeaderCable should be necessary, except at those few places where the localizer cannot be properly sited on the runway centre-line. The effect of illumination of other aircraft on the accuracy andstability of the glide-path has not yet been studied, because glide- path information has not hitherto been used below 150ft in theBritish automatic landing system. Investigation of possible effects may become necessary if the glide-path information is to be usedbelow 150ft or if automatic flare-out using only glide-path infor- mation is seriously considered. Future developments in the use of Pye ILS with an automaticlanding system will be mainly directed towards perfecting the present system by reducing the time for changeover to the stand-byequipment in case of valve or other component failure during an approach. In this connection, the possibilities of continuous high-frequency switching are being investigated. Provision of con- tinuous information to the controller on the precise instantaneousstate of readiness of the ILS, from monitors in the approach area, is also being considered. Pye feel that their present localizer aerial gives the minimumbeam-width consistent with the accuracy and stability of the course- line necessary for automatic landing, and no major modificationsare contemplated. Long-term development will be concentrated on the application of microwave techniques to the provision ofthe necessary information in azimuth and elevation for automatic landing. The parabolic localizer aerial of the Pye ILS recently installed on the new runway at Hong Kong Airport. This system has been used in Britain for automatic landing without Leader Cable
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