Patent GB-1391277-A (GB)
Title: Apparatus for tracking a luminous object
IPC: G01S3/786
Filed: 1972-04-10
Abstract: 1391277 Photo-electric position finding AUSTRALIA COMMONWEALTH OF 10 April 1972 [8 April 1971] 16448/72 Heading H4D Tracking apparatus, e.g. for tracking a landing aircraft and checking the accuracy of an instrument landing system, comprises a photo-sensitive target screen, means for scanning (e.g. with an electron beam) the target, means for determining the displacement in one dimension of the image of an object (e.g. the aircraft), from a reference on the target and means for causing the target to track the object in a direction orthogonal to the said one dimension. An oscillator 20, Fig. 3, feeds counter 24 which operates frame scan generator or digital to analogue converter 26 feeding frame coil 16. A line scan generator 22 feeds a line coil 18. Target 14 is therefore scanned conventionally along parallel lines by an electron beam controlled by the coils. The output from target 14 is passed to video amplifier 34, discriminator 36 and device 160. Whenever the object image is sensed device 160 opens gates 32 (only one shown), fed by counter 24. The gates pass the existing count to a store 90. The store therefore contains the count corresponding to the scan line containing the image. This is converted to Gray code by device 92 and transmitted to the tracked aircraft receiver 96. The count can be used to indicate the displacement of the image from a reference line and, if the reference line is aligned with, e.g. a predetermined flight path, indicates aircraft deviation. The deviation indicated by an I.L.S. receiver 100 is fed to a difference amplifier 98 also receiving the observed deviation. The output of amplifier 98 is used to operate chart recorder 102 which therefore indicates the accuracy of the I.L.S. system. Unit 80 controls the sensitivity of target 14 to maintain the output image signal constant. Line scan generator 22 feeds a store 85 gated by device 160. When the image is first located therefore the output of generator 22 is stored. As the output of 22 is a sawtooth its value stored in 85 represents the position of the image with respect to the ends of the particular scan line. The output of store 85 is fed to servo amplifier 87 and motor 88 to maintain the image at the line centre and hence cause object tracking. Potentiometer 142 and amplifier 140 are used to select the reference line. The output of a counter 104 is gated to an indicator 108 whenever the object image is first scanned and indicator 108 therefore provides a visual indication of the line containing the image. This may therefore be called over a normal radio link to the pilot. Fig. 5 illustrates how the apparatus (see Fig 4, not shown), is situated. If the azimuth accuracy of the I.L.S. is required the apparatus is set up at B and points towards ideal glide path 84. The scan lines are then vertical and the reference line coincides with a vertical line through the path 84. The aircraft is then tracked with the apparatus tracking in elevation. If the elevation accuracy is required the apparatus is set up at A with its optical axis pointing up along a plane through glide path 84. The scan lines are now horizontal with the reference line coinciding with the plane. The apparatus now tracks in azimuth as the aircraft moves down its glide path.