Patent GB-1305197-A (GB)
IPC: F41G7/30
Filed: 1969-10-17
Abstract: 1305197 Guided missiles HUGHES AIRCRAFT CO 17 Oct 1969 51204/69 Heading F3A [Also in Division H4 A guidance system for automatically steering a missile 22 along a line-of-sight 25 to a target 21, comprises a portable guidance unit 20 including a telescope 23 for establishing the line-of-sight and an infrared telescope 26, aligned with the telescope 23, for receiving pulsating infrared radiation from a source mounted on the missile. The telescope 26 includes means for generating signals indicative of deviations of the missile from the line-of-sight and these signals are sent via a cable 30 to a signal processing box 31, the processed signals being sent as course correcting signals to the missile 22 via wires 32 which trail out from bobbins carried by the missile as it advances. The pulsating infrared source 50, Fig. 5, is mounted on the rear end of the missile (Figs. 3, 4, not shown) and comprises a housing 52 filled with a pyrotechnic composition 53 having an electrically operated firing squib 54. When the source 50 is ignited a heavy metal bar 56 is heated which in turn heats a molybdenum disc 55 having a zirconium carbide coating constituting a radiating surface. The radiant energy from the disc 55 is interrupted by a disc 58 rotated at 100 revolutions per sec. by a motor 57 whereby the infrared radiation pulsates at 200 cycle per sec. A battery supplies the electric energy to the source 50 and is connected in circuit when the missile is fired. The telescope 26, Fig. 7, comprises lenses 63, 64 for focusing the radiant energy on to a disc-shaped reticle 65, Fig. 9. A condenser lens 66 focuses the energy transmitted by the reticle 65 on to a large detector 67, this detector receiving energy from practically the whole field of view of the telescope which is 40 degrees. A small detector 70, which receives energy only from the central 6 degree portion of the whole field of view, is disposed in the hollow truncated cone of a light pipe 68 mounted in the centre of the lens 66, the internal surface of the light pipe being highly polished to reflect energy on to the detector 70. The detectors 67, 70 may be of lead sulfide. The reticle 66 is transparent to radiation but has an opaque modulating pattern formed thereon. To produce the modulation, the reticle is mounted in an annular holder 72 rotated at 18 revolutions per sec. by a motor 76 through gearing 75, 78. Phase reference signals to give the instantaneous angular position of the reticle 65 relative to the casing 60 are derived from two magnetic pickups (Fig. 8, not shown) spaced apart at 90 degrees adjacent the outer surface of the holder 72, only one such pick-up having, the reference 79, being shown in Fg. 7. The pick-ups respond to a band 80 of magnetic material disposed on the holder 72, the band width tapering from a maximum at 81 to a minimum at 82, the points 81, 82 being diametrically opposite. Thus two phase reference signals displaced at 90 degrees are generated. The modulation pattern consists of concentric bands as shown in Fig. 9, each band having a circular transparent portion and a circular opaque portion, these portions being so arranged that as the width of the transparent portion gradually increases the width of the opaque portion gradually decreases, the maximum part of the transparent portion being diametrically opposite the maximum part of the opaque portion. The size of the image focussed on to the reticle 65 is larger than the width of a band whereby the radiant energy transmitted by the reticle is amplitude modulated in sinusoidal manner as the reticle rotates. The concentric bands are divided into three groups. The centre group receives energy falling within the field of view of the telescope 26 extending 1 degree from the optical axis, the bands of this group producing modulation over the range 0-50% as measured in the radial direction from the centre of the reticle to the circumference of the group. The second group receives energy falling within the field of view extending between 1 and 6 degrees from the optical axis and modulate this energy over the 50%-100% as measured outwardly in the radial direction. The third, outer, group has 100% modulation throughout its radial extent. As indicated in Fig. 10 the % modulation of the radiant energy received from the source 50 is a function of the angular distance from the optical axis of the image thereof and hence is indicative of the angular deviation of the missile 22 from the line-of sight 25. Further, due to the modulation characteristics of the three groups of the concentric bands, the curve has its steepest section about the zero position and consequently subsequent control circuits do not require an extremely high gain. The processing of the signals received by the detectors 67, 70 is performed by the apparatus shown in Fig. 11. This apparatus comprises preamplifiers 100, 101 having a band-width of 150-300 c.p.s. A double throw relay switch 102 has its contact 105 normally connected to a contact 104 so that the signal from the large detector 67 is passed to the narrow band amplifier 112; however, should the amplitude of a signal from the small detector 70 exceed a certain value the relay-coil 2 moves the contact 105 to engage a contact 103 and the signal from the small detector passes to the amplifier 112. Thus when the missile 22 comes within the 6 degree field of view the relay is switched to the small detector and the relay returns to the large detector when the missile leaves that field. An amplifier 107 having a bandwidth of 175-225 c.p.a. ensures that the relay 102 only responds to radiation from the missile. After a time delay calculated to ensure that the missile is close to the target a voltage from source 111 locks the relay 102 so that it remains connected to the small detector 70. A demodulator 114 demodulates the tracking signal to recover the 18 c.p.a. modulating wave or error signal introduced by the reticle 65. The error signal passes to pitch and yaw phase applied from the magnetic pick-ups 79, 87 (see Fig. 7). The signals from the detectors 117, 118 are transmitted by the wires 32 to the missile as course correction signals via compensation networks 122, 123 and circuit 124.