Patent GB-1260519-A (GB)
Title: Aircraft landing control system
IPC: G01S13/91
Filed: 1969-09-05
Abstract: 1,260,519. Aircraft landing control systems. C. A. LOVELL, I. D. NEHAMA, and J. E. FLEMING, [trading as AIR LAND SYSTEMS CO.]. 5 Sept., 1969 [5 Sept., 1968 (2)], No. 44012/69. Heading H4D. An aircraft landing control system based on a co-ordinate origin (0, 0, 0) situate at the centre of the midline CL of a runway 10 so that X, Y, Z represent deviations laterally, longitudinally, and vertically of the runway (Fig. 1) comprises perpendicular antennµ systems 14, 16 displaced a from the centreline and extending 2b vertically. Each system comprises Rx and or Tx units 18, 20; 22, 24 all located in a plane perpendicular to the runway; tranceivers 18, 22 being located at s 1  (a, 0, 0<SP>1</SP>) and s 3  (- a, 0, 0) with receivers located at s 2  (a, 0, 2b) and s 4  (- a, 0, 2b). Aircraft 26 at point A (x, y, z) has a coherent transponder 28 interrogated by coded signals from 18 or 24 and transmitting the received code to receivers 18, 20, 22, 24 with signal paths r 1 , r 2 , r 3 , r 4  respectively. The aircraft co-ordinate position outside the antennµ plane is determinable by measurement of any three of the paths from the time from transmission of an appropriate signal from 18, 22; retransmission in the aircraft; to receipt at 18, 20, 22, 24. It is shown that for signals from 18; where V=velocity of radio waves, and dt 1 , dt 2 , dt 3 , dt 4  are respective system delays; the times being measurable in known manner e.g. by triggering a timing circuit at 18, 20, 22, 24 on transmission from 18 and arresting the same on signal arrival. Alternatively, digital PN ranging may be employed wherein a pseudo random noise signal employing a sequence of "1" and "0" pulses wherein over a time interval the number of "1" 's differs from the number of "0" 's by #unity. This is continuously modulated on Tx 18, retransmitted at 28 on different carrier, and received at 18, 20, 22, 24. Each Rx has a phase or correlation detector generating the same code sequence as that transmitted in arbitrary time phase, which is matched, bit by bit, with the received signal until perfect match is achieved; the number of shifts in the locally generated sequence being a measure of the length of time necessary to achieve a match, and the elapsed transmission time. It is shown by geometry that so that by digital computing techniques questions (1) to (4), are soluble for r 1  to r 4  and thereafter equations (5) to (8) are soluble for x, y, z, whereby the aircraft position is known, and if approach path 30 is to be followed where m = slope and c is y-coordinate of touchdown; which are computer soluble to produce error signals for guidance, transmissible to the aircraft orally; or applied to operate a visual display, or to effect closed loop autopilot control. Ground equipment delay time components are balanced out and it is shown by mathematics that transponder delays impose only insignificant errors. It is also shown by mathematics that the positioned errors of the aircraft diminish as it approaches touchdown. The redundant measurement capacity inherent in measurement of four signal paths whereas three only are required for determination of aircraft position enables failure of one transmitter or one receiver to be tolerated by provision of an automatic takeover system. Plural aircraft may be controlled by provision of a number of frequency multiplex channels for each transponder and receiver, dictated from the control tuner for successive aircraft. The ground antennae structures may control approaches to plural runways. In a modification (Fig. 2, not shown) the aircraft is controlled to follow a glide path to an intersection with the z axis aligned with and above the runway centre line, after which it performs a flare-out approach to touchdown at a point displaced further along the runway axis; and the glide path slope may be varied by angular adjustment of the antennµ units, or by switching to other phased arrays. In an alternative system (Fig. 3, not shown) the antennµ units are disposed in a plane inclined to the vertical containing 3 stations S 1 , S 2 , S 3 ; and the aircraft is guided along a glide path which is the locus of a point equidistant from the three stations to a point lying in the plane, after which it may flare out to touchdown; the fourth station being provided for redundancy. Horizontal and vertical deviations from the predetermined glide path are calculated and used for control of the aircraft. The computer equipment may be located at a ground station, or in the aircraft or both. A number of operational techniques are described) with mathematical expositions. Fig. 4 shows an electronic arrangement applicable to the system of Fig. 1 wherein Rx and/or Tx ground stations 18, 20, 22, 24 comprise antennae 40, 42, 44, 46 located at points S 1 , S 2 , S 3 , S 4 . Parabolic or dipole antenna 40 is energized from Tx 48 radiating a square waveform carrier squarewave modulated; the modulator 50 comprising exclusive OR logic (Fig. 5, not shown) arranged so that a change of modulation from "1" to "0" or vice versa produces 180 degrees shift in phase of modulated carrier. The coded transmission signal interrogates transponder 28 of the aircraft, which retransmits in similar phase but at different carrier frequency to similar receiving antennµ 40, 42, 44, 46, whose signals are demodulated in arrays of demodulators 1, 2 ... "n" associated with each antenna; each selecting a particular carrier frequency. The appropriate demodulator e.g. 1 receives the signal at e.g. f 1  whose phase is shifted with respect to that of the modulation imposed by 50 to represent the transmission path length e.g. (r 1  + r 1 ), and comparison in range unit 56 develops coded signal t 1  representing the length. Similar signals t 2 , t 3 , t 4  are produced by phase comparison of signals e.g. f 1  received by antennae 42, 44, 46 to represent the path lengths e.g. (r 1 +r 2 ), (r 1 +r 3 ), (r 1 +r 4 ) and such signals t 1  to t 4  are supplied to digital computer unit 58 programmed to determine coordinates x, y, z, of the aircraft, from which a further computer (not shown) or a further programme of computer 58, determines guidance signals. Additional approaching aircraft are assigned different retransmission frequencies f i =f 2 ...f n  for its transponder, to which appropriate demodulators and range units respond to produce respective coordinates x 2 , y 2 , z 2  ... x n , y n , z n .