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Aviation History
1960
1960 - 0245.PDF
FLIGHT, 19 February 1960 STANDARD TELEPHONES AND ILS A HIGH-ACCURACY instrument landing equipment embody-ing novel features, such as a dual-beam localizer, a double-sideband mechanical modulation system and a transistorized monitoring chain, is now being produced by Standard Tele- phones & Cables Ltd. Localizer and glide-path operate respectively in the 107.9-111.9Mc/s and 328.6-335.4Mc/s bands, both employing 90 and 150c/s tone modulations, and the system is therefore compatible with existing ILS, but in most respects it surpasses ICAO requirements and represents the latest development in this field. It has been fully evaluated by the Ministry of Aviation and initial results are extremely encouraging. Orders have been received from MoA, the Belgian Rigie des Voies Aeriennes and Radio Suisse, while a number of others are being negotiated. Quantity production has begun. The dual-beam localizer has been adopted because the very narrow energy lobes radiated by die course (directional) array will eliminate or at least substantially reduce siting problems, while the superimposed radiation pattern of the subsidiary (clear- ance) array will provide correct indications outside the directional localizer sector and at the same time mask any secondary lobes radiated by the directional array. Ideally, an aircraft approaching the touchdown end of the run- way should receive only the 90 and 150c/s tones radiated directly from the localizer array, so that the pilot can fly accurately along the extended runway centre-line by keeping the ILS cross-pointer vertical needle centred. Unfortunately, most airports are sur- rounded by buildings and other obstructions so that in many instances the aircraft receives signals re-radiated from objects near the beam as well as direct signals. According to their phase, reflected signals will add to the direct signals and, by upsetting the equality of the tone modulations, cause the cross-pointer needle to deviate to left or right even though the aircraft is precisely over the extended centre-line of the runway. Re-radiations in fact cause course-bending. As the aircraft approaches the runway, further complications arise because the path of the direct radiation decreases more rapidly than that of the reflected ray. Fluctuation of the cross- pointer needle is therefore aggravated, making the approach path extremely difficult to fly manually and totally unusable for auto- pilot-coupled approaches. The adverse effect of the reflected signals increases in direct proportion to the angle between the direct and reflected rays. If the reflecting objects are close to the centre-line (angle small) the phase-variation is slight and, as there is little difference in the relative paths, the resultant effect upon the cross-pointer at extreme range is small and causes only minor course changes as the aircraft nears the runway. If, on the other hand, the reflected object is some 500ft off the centre-line, phase-variation is considerable, the difference in path length between the direct and reflected signals is greater and course- bends occur which become progressively worse as the aircraft approaches touchdown. Obviously, the width of the radiated beam must be narrowed so that, at the worst, only those objects near the centre-line will be illuminated. A narrow beam directed along the approach path is in any case unlikely to encounter more than very small struc- 245 tures which would not be illuminated sufficiently to cause trouble. Experience indicates that larger objects generally lie outside the 30° sector covering the centre-line. These considerations have led S.T.C. to adopt a localizer aerial with both directional and all-round radiation patterns. At 12-14° either side of the course-line, the radiation is almost zero, while the peak level is at a maximum at about 2-3° off course. The ratio of on-course to off-course signals is such that reflecting objects more than about 8° off course are not illuminated suffi- ciently to give rise to any troublesome reflections, even if they are close to the localizer. Yet such a narrow-beam aerial gives rise to many minor lobes which, if not masked by the complementary clearance aerial radiation, would result in a number of false Dual transmitters for Standard Telephones and Cables ILS courses. The two aerials are energized by two identical trans- mitters whose frequencies are offset by about 9.5kc/s. On course, the ratio of directional to clearance aerial signals is approximately 3:1, while about 8° off course they become equal and outside this region the clearance signals predominate by a factor of about 3 :1. The band width of the airborne receiver exceeds the 9.5kc/s frequency difference between the two signals and it will therefore receive both, but the capture effect will result in the suppression of the weaker signal. In the region of the front (and back) course, therefore, reflections arising from the clearance radiations will be suppressed and exert no influence on the cross-pointer, while outside this area, radiation from the clearance antenna will pre- dominate and cause a full-scale needle deflection in the appropriate direction. The localizer directional aerial consists of a broadside array of 12 horizontal dipoles, backed by an open reflecter screen, which radiates an 18°-wide beam concentrated along the approach. The aerial system is carried on a metal framework about 85ft wide and 7ft high, as shown below. The clearance aerial is a broadside array of three horizontal dipoles mounted on a platform about 8ft high located some 50ft behind the main array. Apart from the advantages of freedom from course bends and increased accuracy of azimuth guidance, a narrow beam coupled with the high-gain aerial greatly increases range and makes the system particularly suitable both for medium-range approaches The localizer course array (right) and clearance array (left) of the S.T.C. ILS shown installed at Hurn Airport for MoA trials
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