Patent GB-1344803-A (GB)
Title: Method and system of pulse coded vehicle guidance
IPC: G01S1/02
Filed: 1972-02-08
Abstract: 1344803 Optical guidance system McDONNELL DOUGLAS CORP 8 Feb 1972 5902/72 Heading H4D [Also in Division G4]  A vehicle guidance system which operates to provide a guidance light beam, employs means for producing pulse coding of a cross-sectional portion of the beam along its length so as to provide an indication of any deviation from correct following of the beam by a vehicle. The system is particularly described as an aircraft landing aid in which the beam defines a landing glide path, but may be employed in other contexts, as described hereinbelow. A constructional embodiment of a rig incorporating beam projection and modulation devices is described (Figs. 1-6, not shown). A pair of projectors supply respective beams which are intercepted by respective endless, moving sawtooth modulation assemblies (74). In the embodiment of Fig. 7.-Sawteeth sets 100, 102 mounted on endless roller chains 96, 98 create images 112c, 112d such that as an approaching aircraft 116 deviates from the optimum glide path, it ceases receiving the steady beam which demarcates that glide path and commences to receive a pulsating beam, the duration of the pulses of which reduces as greater deviations are reached. In order to enable the pilot to differentiate between deviations to one side of the glide path and to the other side of the glide path, one chain may be moved more rapidly than the other so that the images 112c, 112d have different pulse-repetition frequencies. Alternatively, or in combination with this procedure, a filter 108 having respective differently-coloured portions 108a, 108b may be employed to render the images 112c, 112d in different colours. In modifccations, the roller chains are replaced by a continuous film loop (118) (Fig. 8, not shown) embodying opaque sawtooth patterns thereon; or by a rotary disc (144) (Fig. 10, not shown) having opaque sawtooth patterns thereon; or by a rotary drum (156) (Fig. 11, not shown); or by a stationary template (252) (Fig. 17, not shown) embodying sawtooth formations from which the beam passes to a rotary, faceted drum (254). In the embodiment of Fig. 2 a verticallyreciprocable shutter 136 can partially intercept the light beam to produce pulse coding of one portion thereof to one side of the optimum glide path. The shutter is driven by a cam-and-camfollower arrangement embodying a heart-shaped cam 138 which gives linear rise and fall ratio of the shutter. The cam may have other profiles if non-linear shutter control is required. Other reciprocating mechanisms are described. In modifications two shutters (272, 274) (Fig. 18, not shown) may be used, and a lenticular lens (288) may be provided to increase the angular horizontal spread of the beam; a single shutter, movable on a parallelogram linkage (Fig. 19, not shown) and in combination with a colour filter (306) may be used; a pair of shutters (316, 318) (Fig. 20, not shown) connected at different points on a parallelogram linkage may be used to give different pulse repetition frequencies; and a shutter in combination with two different colour filters (348, 350) (Fig. 21, not shown) can be arranged to provide a beam whose coloured, pulsed regions lie on respective horizontal sides of a centre beam so as to be useful in defining the correct approach, for shipping, to a harbour or to define a safe passage in shallow water or between underwater obstructions, or to guide land vehicles such as tanks. In the embodiment of Fig. 12 an arrangement of movable sawtooth sets is combined with a known form of Fresnel lens optical landing system embodying a vertical Fresnel lens stack 168 and a horizontal row of lights 170 aligned with a centre lens cell 168a. In modifications blanking off masks and colour filters are provided (Figs. 14 and 15, not shown). The application of the pulse coded system in combination with a Fresnel system to an aircraft carrier 174 is described with reference to Fig. 13. The pulse code system equipment is gyro-stabilized. In another embodiment (Fig. 16, not shown) employing movable sawtooth sets, the sawteeth have complex-triangular shapes. In addition a colour filter arrangement (234) provides a central, clear vertical strip (234c) to each side of which a differently-coloured filter (234a, 234b) is provided. This construction thus also indicates lateral deviation from the runway centreline. Arrangements are described (Figs. 22, 23, not shown) for increasing the angular horizontal spread of the beam. In one arrangement, a plurality of light modules (364) are provided to offset loss of intensity occasioned by the increased spread. In the other arrangement the light source is a lantern (374) having a mantel (376). A condenser lens (378) which is curved in horizontal section and segmental (plane-convex) in vertical section is used to pass the beam to a suitable modulator (380), the beam then passing to a number of lenses (382) which can extend up to 360 degrees around the lantern. As applied to an aircraft carrier, the pulse code system may embody gyro-control as described with reference to Fig. 25. Here, respective prisms 386, 388 are rotated by servomotors 394, 400 controlled by a gyro-reference 396 to correct for roll and pitch, respectively. Prism 388 may be replaced by a mirror. Corrections for yaw and heave may also be provided. Where necessary, the provision of a shadow bar (such as at 244, Fig. 16, not shown) reduces intensity of the beam in the glide path. When the pulsed beam has a large vertical span with a relatively low glide path angle, such as for use in assisting landing of helicopters and VTOL aircraft a horizontal ground signal mask can be provided to cut off reflections from the ground. Alternatively, the vertical height of the sawteeth for coding the lower beam portion may be reduced relative to that for coding the upper beam portion. Where shutters are used, the range of movement of the lower beam portion shutter is restricted. For automatic guidance of vehicles equipped with photoelectric sensors and control circuits, additional modulation, at higher-than-visual frequencies can be provided. Such a system may be used in missile guidance. The system may employ infra-red or ultraviolet filters so as to render the beam non- visible to an observer not equipped with a suitable intensifying device.