Patent GB-909140-A (GB)
Title: Improvements in electronic terrain analog simulator for aircraft
IPC: G09B9/30
Filed: 1959-12-31
Abstract: 909,140. Aircraft instrument circuits. GENERAL ELECTRIC CO. Dec. 31, 1959 [Jan. 2, 1959], No. 44399/59. Class 40 (1). [Also in Groups XXXIII, XXXVI and XL (c)] A system for simulating on a cathode-ray tube the appearance of an imaginary grid of lines on the terrain beneath an aircraft comprises a source of repetitive voltage pulses and three circuits responsive thereto to produce (1) blanking pulses for the tube beam, (2) Y- deflection potentials obtained by sampling the pulses, and (3) two X-deflection potentials obtained by sampling the pulses, reducing their repetition rate and switching them electronically to present alternately signals representing lines in mutually orthogonal co-ordinates, circuits (2) and (3) including computers which modify the signals in accordance with the aircraft flight characteristics. In Fig. 2, the output of an oscillator 3 is shaped at 4 to form pulses for use in the subsequent circuits. A blanking pulse generator 5 receives the pulses directly. The Y-deflection potential Vy is obtained from a sawtooth generator 6 and a computer 7 in the form (KY+Kp tan #) where p is the distance of the observer's eye from the tube, # is the aircraft pitch angle and K is the tube sensitivity factor, to produce an instantaneous deflection Y. The X-deflection potential Vx is equal to where h is the aircraft altitude above ground, # is the aircraft heading angle with respect to the datum which the display is to provide, r is the line spacing and n is the number of a particular line (so that nr represents the horizontal distance from the aircraft to the particular imaginary line being displayed). The horizontal and vertical lines are traced by providing two switched sets of voltages in which the respective values of # differ by 90 degrees. Two identical channels are therefore provided, each taking the pulses from the unit 4 and comprising sawtooth generators 9A and 9B, computers 14A and 14B for deriving the first term in the above equation from inputs representing h, # and #, and further computers 15a and 15b for obtaining the complete expressions. The pulse signal from 4 (2400 p.p.s.) is reduced to 60 p.p.s. in a sealer 10 and used to trigger the sawtooth generators 9A and 9B via " n-generators " 13A and 13B which provide a " staircase " waveform. The 60 p.p.s. signal also triggers a 30 p.p.s. multivibrator 12 to act as a switch for presenting the two sets of signals alternately. The " KY " signal for computers 15A and 15B is obtained from the sawtooth generator 6. The movement of the pattern in correspondence with the movement of the aircraft is achieved by varying the value of n in units 13A and 13B according to the equation dn/dt=1/r (V s  cos 0 - v sin #), where v and v s  are respectively the forward and sideways components of the speed of the aircraft. The computed Vx and Vy signals are passed through a " co-ordinate rotator " stage 8 which modifies them to V<SP>1</SP>x and V<SP>1</SP>y to allow for roll of the aircraft through an angle p, such that V<SP>1</SP>x=Vx cos p - Vy sin p and V<SP>1</SP>y=Vx sin p + Vy cos p. Computer circuits.-The units 9A and 9B comprise integrators to give a stepped waveform of total period 1/60 sec. combined with a sawtooth generator comprising a potentiometer slider 22 (Fig. 4) driven by a motor 20 running at a speed proportional to (v s  cos #- v sin #). This motor is controlled by a multiplier amplifier 19 supplied with the algebraic sum of v s  cos # and v sin # from cosine and sine potentiometers 16 and 17 respectively. The potentiometer output is added at 25 with signals from an integrator 26 supplied with the 60 p.p.s. and 2400 p.p.s. signals. In computers 14A and 14B (Fig. 5) an input signal nKY is multiplied by r in amplifier 27 and by cos # in cosine potentiometer 28. It is then applied to a high gain amplifier 29 having further potentiometers 30 and 31 in a feedback loop to divide the output by h cos #. The final signal forms one of the inputs to computer 15A or 15B (Fig. 6) where it is added in amplifier 44 with the other necessary signal obtained as follows. A signal KY from the unit 6 is multiplied by sin # at potentiometer 32. Another signal representing Kp is applied to an amplifier 33 having a cosine potentiometer 34 in a feedback loop to give Kp/cos #. The two signals are added in amplifier 35, multiplied by sin # at a sine potentiometer 41 and divided by cos # in a further amplifier 42 having a cosine potentiometer 43 in a feedback loop. The computer 7 (Fig. 7) multiplies Kp by sin # in a sine potentiometer 144 and divides the output by cos # in a similar feedback amplifier 45. The KY component from unit 6 is then added in a further amplifier 47. In those units which derive a term having cos # in the denominator, the output is limited when # approaches 90 degrees by modifying the cosine potentiometer accordingly (Fig. 8, not shown). The potentiometer is joined at its ends to positive and negative supplies and earthed at its centre point, thus giving a 180 degrees range of movement. The positions corresponding to 89 degrees and 91 degrees are then bridged by a wire which engages the slider but is broken at the 90 degrees point, so that the output can never fall below the values represented by 89 degrees and 91 degrees.