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Aviation History
1946
1946 - 2173.PDF
OCTOBER 31ST, 1946 FLIGHT 473 BLIND APPROACH PROBLEMS energy might be' reflected from buildings and trees. This v.-ouM produce an incorrect difference between the two series of signals. Results of tests suggested that along the runway direction the position of the aircraft was defined to within about 30 yards—which was amply accurate for the completion of most of the approach. It was even found that deviations of the order of 30 yards might be present lor a period of some weeks over a par- ticular stretch, and would then disap- pear for a time. It appeared that on sites reasonably free from reflecting objects the system gave plan-position _ji jformation which was good enough for ^SJ approach but not sufficiently so for a blind landing. As far as the glide path beam was concerned, no quantitative measure- ments had yet been made in this country. The radiation patterns de- pended fundamentally on ground reflection, and uniformity was necessary since variation would alter the angle of the glide path. Here again, the more important disturbances were those aris- ing from reflecting objects. It should be noted that the height range covered by the glide-path indicator was, with the aircraft close to the runway, ex- tremely narrow; at a height of 150ft, for example, full-scale deflection theoretically occurred when the aircraft was only 15ft below the glide path. Another disadvantage was that continuous indication of distance from the runway was not provided. The importance of this was a matter of opinion, but it was probably easier to control an aircraft automatically if range information was continuously available. In his paper, the lecturer then described the Ground Con- trolled Approach system, in which operators watch the move- ments of the approaching aircraft on radar screens, and the approach controller " talks down " the pilot. Included in this is a search system, which displays on a cathode-ray tube the plan position of all aircraft within a range of 15 miles and below a height of 4,000ft. This G.C.A. system was described iu detail, with a complete example of approach "patter," in Flight of July nth, so the description will not be repeated. G.C.A. Advantages Mr. Pritchard said that the G.C.A. system provided some outstanding advantages. It required no airborne equipment other than VHF or HF communication sets, and relatively little training was required by the pilot. Unfortunately, the present equipment was expensive. Furthermore, since only one aircraft could be handled at a time on the approach path, the system introduced a limitation which would become more serious as traffic increased. It was restricted to approach guidance only, because, when an aircraft was about to touch- down, the elevation beam and/its side lobes illuminate ground which was at the same raj»^e, and the echo from the former became submerged in^ground returns. In addition, the 9O CYCLE PATTERN OF LOWER AERIAL of the runway, An aircraft cross-pointer indicator used in conjunction with the SCS.51. accuracy in determination of aircraft height, required for the actual landing, was unlikely to be achieved by a radar system which had necessarily to be placed a considerable distance from the touch-down point. The beam approach beacon system (B.A.B.S.) was the third to be described in the paper. In this a technique known as "secondary" radar is used. The transmission from the air- crafty is received by a ground station located at the remote and re-radiated to the aircraft. The aircraft transmits pulses which are received^ by the ground station and each pulse triggers a transmitter on a slightly different radio frequency after a known short-time interval; the radia- tion comes from one or other of two slots at the corners of a cavity resonator. These slots are switched at about eight cycles per second by mechanical short-circuiting at the centre of the slot. From one of them a pulse of a length of 5 microseconds is radiated; when the other is opened, a longer pulse, of 12 microseconds, is transmitted. Thus the radiation pat- terns are obtained and a system is achieved in which the relative ampli- tude of the short and long pulses should remain constant. The radiation is vertically polarized, the radio fre- quency is about 20.0 Kc/s, and the recurrence frequency' of the pulse is about 500 per second. The signals received in the aircraft thus comprise a short and a long pulse whose relative amplitude is determined by the angular position of the aircraft relative to the runway rtntre-line. The pulse amplitudes are displayed on a cathode- ray tube as shown in the drawings on page 474. The distance B-C on the tube is proportional to the range of the aircraft from touch-down, the indication at B being caused by the RUNWAY CLIDE PATH ECHO ECHO 150 CYCLE PATTERN UPPER AERIAL The vertical radiation patterns made by the SCS.51 gHde-path localizer, and the respective indications on the instrument. Diagrammatic representations of the displays on the cathode- ray tubes of the G.C.A. radar equipment. On the left is that for range-elevation and, on the right, that for range-azimuth. pulses transmitted from the aircraft, and those at C by the return pulses. They are displayed along the cathode-ray tube time-base with corrections to allow for the time delay at the ground station and for its distance from an assumed touch- down point. The accuracy of the range measurement is of the order of 500ft. No glide path is provided with the B.A.B.S. system, reliance being placed on the barometric altimeter and on continuous range indication to give vertical guidance. The weight added to the aircraft by the installation is about 120 lb, but it should be added that the equipment can also be used for radar beacon navigation and that this weight is based on a wartime design which could be miniaturized. The " presentation "is normally given to the navigator, who transmits the information to the pilot over the aircraft intercommunication system. Mr. Pritchard said that the airborne equipment for B.A.B.S. was complex, and that its weight—coupled with the fact that the information was presented to a navigator—rendered the system, in its present form, suitable only for large aircraft. It was not anticipated that it would be superior to SCS.ji in the matter of reflection errors. The difficulties of aircraft control under blind approach con- ditions, he continued, were now well appreciated. With modern aircraft, and particularly in cross-wind conditions, ev«n the most highly skilled pilot was severely taxed—and
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