Patent GB-869770-A (GB)
Title: A target identification system by radar
IPC: G01S7/06
Filed: 1959-03-17
Abstract: 869,770. Radar. KABUSHIKI KAISHA TOKYO KEIKI SEIZOJO. March 17, 1959 [March 24, 1958], No. 9257/59. Class 40(7) [Also in Group XL(b) ] In a radar system transmitting waves polarized in a particular mode, e.g. circular polarized, horizontally linear polarized or vertically linear polarized, and capable of receiving only echo waves polarized in the same mode, the amplitudes of echoes from different types of targets vary in different ways if the polarization mode is changed. According to the present invention this property is utilized in a scanning pulse radar having an intensity-modulated range-angle display, e.g. P.P.I. or type B, to produce coloured images, the colour of any one image being characteristic of the type of target producing the image, by transmitting sequentially n different polarization modes, e.g. n = 3, and producing a coloured image composed of n different primary component colours, the amplitude of each primary component being proportional to the amplitude of the echo produced during the transmission of the corresponding polarization mode. The recurrence period of the polarization modes is preferably such that each mode is produced a plurality of times, e.g. nine times, as the beam sweeps through a target. Each image primary colour component may be produced during its corresponding polarization mode period, in sequential colour components, or alternatively the n sets of echoes may be stored and the colour components produced simultaneously. The final display may be produced by a known type of colour television C.R.T., e.g. a shadow-mask or Lawrence tube, or by the optical superimposition of the images of n separate C.R.T.s having differentially coloured phosphors or filters. The different polarization modes may. be produced of separate aerials sequentially energized but as described they are produced by a single aerial, the polarization mode of the transmitted waves being changed abruptly, Fig. 8, or continuously, Fig. 7; in the latter case echoes corresponding to the desired n different modes are selected by gating pulses. As shown in Fig. 7, TE 01  waves in an input rectangularly waveguide are converted by the section 1 into circular guide TE 11  waves which are applied to a circular guide section 2 containing a metal fin producing circular polarized waves which are applied to a continuously rotating section 4 of similar construction which produces linearly polarized waves whose plane of polarization rotates with the section 4. In a modification, Fig. 3 (not shown), the components 2 and 4 are replaced by a single twistable section of circular waveguide having a longitudinally twisted partition which rotates the plane of polarization as the section is twisted. The output from the section 4 is applied to a horn aerial 8 through a similar rotatable section 6 so that the polarization of the transmitted waves changes progressively, Fig. 5 (not shown), from circular to elliptical, plane polarized in a reference direction determined by the setting of the section 6 as shown by an indicator 23, elliptical, circular, plane polarized in a direction orthogonal to the reference direction, elliptical and back to circular. Echoes corresponding to three desired modes are selected by gating pulses 25, 26, 27 generated by switches 21, 20, 19 actuated by contacts 18, 17, 16 on cams 15, 14, 13 driven synchronously with the waveguide section 4 by synchros 9, 11; the gating pulses may be applied directly to the cathode ray tube or tubes. As shown in Fig. 8 circular polarized waves are transmitted from a horn aerial 4 through a rotating disc 5 comprising sectors 6 covered by metal grids which convert the circular polarized waves into linear polarized waves and sectors 7 of polystrene foam which allow the passage of the circular polarized waves without modification. The direction of the linear polarization depends upon the direction of the aerial 4 from the rotational axis of the disc 5. The polarization mode may also be changed by varying the spacing and depth of the metal plates of a parallel plate free-space polarizer Fig. 1 (not shown).