GB1359581A - Optical trackers - Google Patents

Abstract

1359581 Phuto-electric detector BRITISH AIRCRAFT CORP Ltd 5 Jan 1973 [5 Jan 1972] 498/72 Heading G1A [Also in Division H4] An optical tracker for tracking a distant light source, such as a flare carried by a missile, comprises a cylindrical lens which projects an image of the source, in the form of a focused line of light, upon the screen of a photo-electric detector. The screen has at least one pair of distinct light-sensitive areas, which are so arranged that their respective electrical outputs vary continuously and differentially as the line of light moves transversely to the direction of its length. Sum and difference signals are obtained from the detector outputs, and are used to derive error signals for tracking. The arrangement enables false error signals due to atmospheric shimmer to be reduced. In one form of screen a rectangular light-sensitive area 12, Fig. 2, is split by a diagonal line 13 to provide two triangular areas 12A, 12B. The focused line of light 11 normally lies centrally between the longer edges of the rectangle, producing equal outputs from the two areas. When it moves transversely, the output from one area is reduced, and the output from the other area is correspondingly increased. In another form of screen a rectangular light-sensitive area is divided into two areas each comprising a plurality of triangles, as shown in Fig.4. A form of screen is also described Fig.8 (not shown) in which a rectangular light-sensitive area is split into two by a single diagonal line as in Fig.2 and has a narrow central strip removed, in which is provided a multi-diagonal arrangement similar to that of Fig.4. An azimuthal tracker unit comprises a cylindrical lens 10, Fig.7, with a vertical axis 21 about which a support 20, in the form of a sector of a cylinder, may be rotated by a servomotor through a shaft 22 and worm gearing 23. A potentiometer pick-off 24 enables the position of the support 20 to be monitored. A detector 12, of the kind described with reference to Fig. 8, is symmetrically mounted within the support 20, and two infrared sources 25 are provided which are used intermittently during a tracking operation to illuminate the detector 12, to enable overall balance to be maintained. A similar orthogonally disposed unit may be provided for tracking in elevation. Circuitry for a tracker unit is described with reference to Fig.9 (not shown). This includes provision for switching to the inner areas of the detector 12 as the tracker nears the line of sight to the source, normalizing the difference signal by division by the sum signal, and periodic balancing making use of the pilot sources. In another embodiment, the detectors are fixed relative to their respective cylindrical lenses Fig.10 (not shown).