Patent GB-899629-A (GB)
Title: Mirror landing system
IPC: B64F1/18
Filed: 1960-05-06
Abstract: 899,629. Visual landing aids for aircraft; control systems. GENERAL PRECISION Inc. May 6, 1960 [Sept. 10, 1959], No. 16104/60. Classes 40 (1) and 118. [Also in Groups XXXVI and XL (c)] In a system for landing an aircraft on a carrier along a predetermined glide path by means of light reflected from a mirror, the latter is continuously and automatically tilted about its centre to compensate for pitch, roll and heave of the carrier, and an infra-red tracker is provided to lock on to the aircraft and determine its apparent angle of elevation and so control the tilt of the mirror. It is shown that the required angle of tilt is equal to ¢ (#+γ-#) where is the angle of elevation (with respect to the horizontal) of the light path from the mirror to the aircraft, γis the angle of pitch of the carrier and # is the angle between the light beam incident on the mirror and the horizontal. The angle # is not readily determinable and means are provided for computing it from the equation by using an infra-red tracker to determine the angle of elevation α of the aircraft relative to the deck; a is the linear distance through which the centre of the mirror has been displaced by motion of the carrier, b is the linear distance between the infra-red tracker and the mirror, #<SP>1</SP> is the required glide path, e is the angle between the vertical and the path by which the centre of the mirror has been displaced, and γ is the angle between the line b and the deck. Since the angles #<SP>1</SP> and e are small, the above expression can be simplified to where #h is the vertical displacement of the centre of the mirror. A computer (Fig. 4) has coarse and fine pitch signals from the ship's gyro fed via control transformers 404, 406 and suitable gearing to a servo amplifier 410 and motor-generator 414 which rotates an output shaft 430 (again through suitable gearing) to a position representing #. The elevation angle signal α from the infra-red tracker (see below) is mixed with the # signal in a differential synchro 432 and fed to a control transformer 434 whose output shaft 444 is set to a position representing (α- #) by a similar  servo loop. A signal #h from another computer (see below) is applied to a resolver 446 whose rotor is set by shaft 444 so that outputs representing #h sin (α - 8) and #h cos (α - #) are applied to summing networks 448 and 450 respectively. A further signal b sin (α=#) from the infra-red tracker (see below) is fed via a linear potentiometer 452 and a cosine potentiometer 454 as second inputs to these summing networks, the potentiometer sliders being manually set in accordance with #<SP>1</SP>. The outputs of the summing networks are therefore #h sin (α-#)+b sin (α+#)#<SP>1</SP> and #h cos (α- γ)+ b sin (α+γ) cos #<SP>1</SP> respectively, and these are applied as inputs to a tangent resolver 456. By a similar servo loop the output shaft of this resolver is rotated to a position representing arc tan (ratio of its inputs) which is the required function for #. This is combined with # in a differential 468 and subtracted from a manually set angle # in a further differential 472 to give a 2Á signal represented by the position of the output shaft 474. Coarse and fine synchros 476, 480 pass this signal to a servo system (not shown) for tilting the mirror. Infra-red tracker (Fig. 5).-Radiation from the aircraft is focused on to a detector 506 through a constantly-rotating reticle 508. Azimuth and elevation pick-offs 512 and 514 generate signals of frequency determined by the rotational speed of the reticle, and of phase depending on the angular positions of the pick-offs with respect to the direction of the incident radiation. These signals are mixed with that from the detector in phase discriminators 516, 518 and applied to servo systems 524, 532 to rotate the receiver until the phase differences are zero and the device is pointing directly at the aircraft. The output shaft 536 of the elevation servo system rotates a synchro 537 to generate the elevation signal α required for the computer of Fig. 4, and also drives a resolver 535 to produce the other signal b sin (α+γ) in conjunction with a fixed input voltage (not shown) representing b; γ is introduced by offsetting the rotor and stator by this angle. Height change computer (Fig. 6).-A vertical accelerometer 602 is stabilized against pitch and roll by signals γ and # supplied to gimbals 604, 606 from synchros driven by the ship's gyro. A pick-off 610 is supplied with A.C. and gives a modulated output which is demodulated at 614 to provide a D.C. signal representing vertical acceleration. This is fed to the torquer 608 and also to a capacitance-servo integrator 622, 632 to move a potentiometer slider 630 in accordance with vertical velocity. This signal is passed to a second capacitance-servo integrator 638, 648 to move a further potentiometer slider 646 to represent vertical displacement, and a coupled slider 650 passes an A.C. signal representing #h to the computer of Fig. 4. In order to maintain the accuracy of the integrators over long periods of time, the output signal is mixed with a reference signal at 652, and the difference passed through a demodulator 654 and mixed with the output from a servo integrator 658 driving a potentiometer slider 664. The mixed proportional and integral signals across load resistor 668 are applied as feed-back to the integrators 622 and 638. The accelerometer 602 is mounted close to the mirror so that the Ah signal will accurately represent the motion of the latter due to pitch, roll and heave of the vessel.