Patent GB-1499376-A (GB)
Title: Target acquisition apparatus
IPC: G01S3/78
Filed: 1975-04-08
Abstract: 1499376 Detecting object photo-electrically HONEYWELL Inc 8 April 1975 [8 April 1974] 14302/75 Heading H4D A target acquisition apparatus 10 comprises a direction sensitive detector 12 for producing electrical signals in response to periodic electromagnetic pulse signals of known repetition rate and duration arriving from a target within a field of view, an indicator for providing an indication when the detector is directed so that the target is located within a predetermined area of the field of view, and signal discriminating circuitry including gating means for passing the electrical signals from the detector to the indicator only in response to gating signals, a window pulse generator running at a frequency substantially the same as the repetition rate of the electromagnetic pulses and generating window pulses having a duration greater than that of the electromagnetic pulses, and a synchronization circuitry responsive to the position of the electromagnetic pulses in the windows to adjust the frequency of the generator so that the electrical signals occur during a predetermined portion of the windows. The detector 12, Figs. 1 and 2, has a predetermined field of view 64 and a bandpass optical filter 58 which rejects electromagnetic energy outside a narrow band of frequencies. The field of view 64 is imaged by lens 62 on to a sensor 60 having a number of discrete sensing elements. If a target is illuminated with a pulsed laser beam having an optical frequency which will be passed by the filter and if the target is within the field of view, its image will fall on the sensing elements in the detector. Light sensitive diodes of the sensor 60 are supplied with a bias voltage by a detector bias supply 14. Electrical signals from the detector 12 are supplied through preamplifiers and summing amplifiers 16. The summing amplifiers sum the signals from the detector 12 so as to produce signals indicating that movement of the detector to the right, left, up or down is necessary to centre the target in the field of view. These signals, identified in Fig. 1 as RIGHT, LEFT, UP and DOWN, are supplied to level detectors 18, 18', 18" and 18<SP>111</SP>. Summing amplifiers 16 also provide a separate signal to a level detector 20 any time a properly illuminated target is within the field of view. Level detectors 18, 20 pass their input signals to gates 22, 24 respectively only when the input signals exceed a predetermined magnitude. Gates 22, 24 are controlled by gating signals supplied on a conductor 26 from window logic 28 which produces a gating signal only during the time pulsed laser energy reflected from an intended target is expected to arrive at detector 12. Timing of the gating signals is primarily controlled by a clock 30 which supplies a signal having the same repetition rate as the pulse rate of the laser beam. The clock 30 supplies a higher frequency clock signal to synchronization circuitry 32 which adjusts the clock input to window logic 28 to keep the windows centred around the arrival times of laser energy from a valid target. Window logic 28 is also controlled by lock-on logic 34 which produces a signal when it is determined that the windows are timed to pass energy from a valid target. The signals from gates 22 are supplied to last pulse logic 36 which also receives an input from gate 24. Last pulse logic 36 rejects laser energy from objects between the laser transmitter and the intended target. The signal from gate 24 determines the last pulse. Passage of the proper number of pulses through gate 24 permits last pulse logic 36 to pass concurrent signals received from gates 22. The signals from lock-on logic 34 and last pulse logic 36 are supplied to lamp drive circuits 38, 40 respectively which convert short duration pulses into longer duration signals for illuminating lamps in a display device 42. Target acquisition apparatus which is incorporated into a pilot's helmet 52, Fig. 2, has a parabolic visor 54. The helmet has a housing 56 which carries the detector 12. The visor 54 has a display projector 66 including a housing 68 in which a number of lamps and film for projecting the cuing indicators and reticle are mounted. A display configuration is shown in Fig. 7 wherein RIGHT, LEFT, UP and DOWN cuing indicators are respectively identified by reference numerals 70, 72, 74 and 76 and the reticle is identified by reference numeral 78. The projector 66, Fig. 2, has a mirror 80 for projecting the display on to the visor 54. The display pattern is a collimated virtual image which appears to be superimposed at infinity on the scene outside of the visor. The circuitry for combining signals from the detector to produce UP, DOWN, RIGHT and LEFT cuing controls, the supplying of these signals to level detectors and gating circuitry and the window logic, lock-on logic and synchronization means are more fully described in the Specification.