GB951571A - Star angle sensor - Google Patents

Abstract

951,571. Star trackers. UNITED AIRCRAFT CORPORATION. Sept. 27, 1962, No. 36708/62. Heading H4D. Apparatus for determining the position of a radiant energy source comprises a reticle of a material opaque to the energy of the source and having apertures therein transparent to the energy and extending from a common point, means for focusing an image of the source at a point in the plane of the reticle, means for causing relative rotation between the image and the apertures so that the image falls on each aperture in turn, and means responsive to light from the image passing through the apertures for registering the difference in the time intervals between the light passing through two of the apertures during different parts of one cycle of relative rotation. In the apparatus of Fig. 1, light from a star passes through a rotating optical wedge 16 into a telescope system of the Maksutov-Cassegrain type, comprising correcting lenses 10, apertured primary mirror 12 and secondary mirror 14. The star image, caused to rotate in a circle by wedge 16 is focused and directed on to a reticle 22 by prisms 18 and 20. This reticle, Fig. 3, is formed with four slits A, B, C and D, symmetrically arranged as shown with respect to the yaw and pitch axes. In one arrangement a separate light detector 26a, 26b, 26c and 26d, Fig. 5, is placed behind each slit, and emits a pulse each time the star image passes through the respective slit. Detector 26a gives out a pulse as the image sweeps past the slit A, and switches on gate 28. This lets through oscillations from oscillator 30 to an adding circuit 32. The passage of the image past slit B produces a pulse from detector 26b which closes gate 28. The adding circuit 32 thus holds an oscillation count proportional to the time of travel of the star image from slit A to slit B. The pulse for detector 26b also opens gate 34, starting the count in the adding circuit 36 which is stopped by a pulse from detector 26c, closing gate 34 and opening gate 38. The adding circuit 36 then holds an oscillation count proportional to the time of travel of the star image from slit B to C. Similarly gate 38 passes a number of oscillations to adding circuit 32 proportional to the time of travel of the star image from slits C to D. However, in this case the said number is subtracted from that already in adding circuit 32 so that the final count in the adding circuit represents the difference between the two times of travel of the star image, which in turn is proportional to the amount by which the star is above or below the pitch axis (i.e. its Y co-ordinate). The adding circuit 36 similarly ends up with a count proportional to the X coordinate of the star. The counts in the two adding circuits are transferred into pitch and yaw valves by shaping networks 40 and 44, respectively. In a second embodiment using the reticle of Fig. 6, the position of the star in pitch is measured from the difference in the times taken for the image to pass from slit F to slit H and from slit H back to slit F. Similarly the position of the star in yaw is measured from the difference in the times taken for the image to pass from slit G to slit E and from slit E back to slit G. A circuit similar to that shown in Fig. 5 is used, accepting that the pulse from detector 26a switches off gate 38, that from detector 26b switches off gate 42, that from detector 26c switches off gate 28 and that from detector 26d switches off gate 34. If the modification of Fig. 9 is used only one large detector 64 is necessary for all four slits. A pick-up device, for example a magnetic type, is placed on the rim of the optical wedge 16 at its thickest point and generates pulses in detectors 63, 65, 67 and 69 in turn. These are fed to four normally closed gates which are thus opened in turn. The signals from the detector 64 are fed to all four gates but only pass through that which is open, the gate passing any particular pulse being then automatically re-shut. The outputs of the four gates are fed respectively to the four channels of the arrangements shown in Fig. 5. In another similar arrangement (Fig. 11) the output of each gate is used to open the gate next in line, only one detector 63 thus being needed.