Patent GB-968125-A (GB)
Title: Improvements in or relating to radar display apparatus
Inventor: ROBINSON CHARLES FREEMAN
IPC: G01S7/06
Filed: 1962-02-22
Abstract: 968,125. True motion radar. DECCA Ltd. Feb. 18, 1963 [Feb. 22, 1962], No. 7002/62. Heading H4D. In a true motion radar display apparatus periodic relative motion information is given by storing relative motion position information and periodically superimposing it on the true motion display with the stored position of the radarcarrying craft coinciding with the present position of the said craft in the true motion display. In the simplest embodiment, Fig. 1, echo information from a pulse radar receiver 1 is normally displayed on a long persistence C.R.T. 3 as a P.P.I. true motion display under the control of deflection control means 4. The echo signals from the receiver 1 are also stored as relative motion signals in a storage means 5 which may comprise a storage tube. The storage means is periodically cleared and refilled and the relative motion information therefrom is displayed on the C.R.T. 3 during interscan periods, with the position of the radar carrying craft superimposed on the present position of the said craft in the true motion display. The resultant display is as shown in Fig. 2 where O 1  to O is the true motion path of the radarcarrying craft, A 1  to A and B 1  to B are the true motion paths of two distant craft, and A 2  and B 2  are the relative positions of the two said craft when the radar-carrying craft was at position O 1 , superimposed on the C.R.T. with O and not O 1  as the scan centre. Lines A 2  to A and B 2  to B therefore represent the relative motions of the two craft relative to point O during the time represented by the line O 1 O. If the result of a change of course is desired, e.g. O 3  to O, Fig. 3, instead of O 1  to O, then the relative motion display is superimposed with O 3  as the centre, A 3  to A and B 3  to B, now being the resulting relative motions. The screen of the display tube may be formed with phosphur dots alternately of long persistence and short persistence material, the true motion display using the former and the relative motion display the latter. In a more complicated embodiment, Fig. 4, the relative motion echo signals are fed to a true motion store 16 and a relative motion store 17. Each store may comprise a double-ended storage tube, Fig. 5, having a long persistence storage screen 23, a radial writing scan and a television type reading scan, wherein, for the true motion store, the writing scan is deflected under the control of signals on lines 53, Fig. 7, from a true motion shift control, comprising a mechanical sine/cosine resolver 52, having a rotatable input shaft rotated via motor 51 by an amount corresponding to the distance gone by the craft, as determined by the craft's log, and two output shafts respectively rotated by amounts proportional to the sine and cosine resolutions of the said distance in accordance with the heading of the craft as given by a signal on line 55 from the craft's compass. The said deflection converts the latent image on the screen 23 to a true motion form, and the reading scan is similarly shifted, but in reverse, to keep the present position of the radar-carrying craft in the centre of the read-out display, with the past positions of this craft streaming out behind it. Normal relative motion scanning is used for writing in the relative motion store, whilst other than normal relative motion scanning is only used for reading from the relative motion store, when the effect of a change of course is being investigated. Alternatively, the storage system may comprise, Fig. 6, a relative motion, radial scan P.P.I. display 30, viewed via an optical system 33 by a true motion television camera 31 and a relative motion camera 32. An optical system 34, controlled by the signals from control 52, is interposed between the optical system 33 and the camera 31, to produce the true motion shift. The read-out outputs of the two stores are fed to a coincidence gate 42, which only passes signals to the television type display 43, when signals are simultaneously read out from the two stores by the synchronized read-out scans. In order that the display 43 shall normally display the true motion output of store 16, a gating " on " signal is applied to the relative motion side of gate 42 with switch 45 in the position shown. Switch 45 is ganged with a switch 71 and both are closed when the relative motion display is to be superimposed. Closing switch 71 connects scan shift unit 70, which comprises, Fig. 7, a sine/cosine mechanical resolver 61, resolving a sawtooth signal produced by generator 10 into sine and cosine components according to the angular setting of shaft 66, forming the output shaft of a differential gear assembly 63 to which the compass reading is applied. The sine and cosine resolution signals from the resolver 52 and the resolver 61 are respectively subtracted in means 67 and the resulting sine and cosine signals varying in a sawtooth manner are fed to control the read-out scan from store 16 when switches 71 and 45 are closed. The result is that the centre of the read-out scan moves up and down the trail of the radar-carrying craft and that the display means 43 shows the past positions of the various craft at the time corresponding to the position of the scan centre on the trail of the radar-carrying craft, which craft is constantly shown at the centre of the display. The resultant display produced by the combination of the true and relative motion displays due to the persistence of the display means 43 is as shown in Fig. 9, the thin, un-arrowed lines being the normal true motion display, with the present positions of the craft O, A and B at the head of the said lines, and the thick lines being the superimposed relative motion of the craft A and B in the directions shown by-the arrows. The sharp rise to the sawtooth waves from generator 60 causes the thick lines to show a darting movement along the direction of the arrows. This darting can be removed, however, by increasing the frequency of the sawtooth wave. The differential gear assembly 63 also comprises an adjustable shaft 65, spring biased to zero, by means of which a desired test change of course can be added to the output of shaft 66. Sine and cosine resolutions of the sawtooth output of generator 60 are produced by the resolver 61 according to the test heading and are subtracted as before in subtractor 67 from the corresponding outputs of resolver 52 to produce read-out shift signals for the true motion store, sweeping the scan centre along the hypothetical past course of the radarcarrying craft. The outputs of the resolver 61 are also subtracted in subtractor 68 from signals produced by resolver 62 by resolving the sawtooth waves according to the actual compass reading as supplied over line 55. The sine and cosine resolution outputs from 68 are applied via switch 72, Fig. 4, to shift the read-out centre of the relative motion store. When no additional heading is applied by shaft 65 the outputs of subtracter 68 are zero and no shift of the read-out centre in the relative motion store occurs. The shifting of both read-out centres causes the trails of the craft to cross and pass a signal through the gate 42, such that the display 43 is as shown in Fig. 10, where the thin unarrowed lines are the true motion trails of the craft, headed by the present positions of the craft O, A and B, and the thick lines A 4  to A and B 4  to B are the relative motions of the craft A and B assuming the hypothetical true motion of craft O given by the thick line O 4  to O. To test the result of a proposed change of speed, the peak amplitude of the sawtooth waves applied to resolver 61 may be varied as desired, Fig. 8 (not shown). The display 43 may comprise a colour display tube on which the relative and true motion displays may be in different colours. The relative motion trails may be interrupted to appear as lines of dots. The storage area in the true motion store may be made twice as big as the final display to avoid blank edges in the true motion display as the centre of the read-out scan moves away from the centre of the storage area. The centre of the true motion read-out scan may be fixed and a reverse true motion shift may be applied to the relative motion read-out scan. This necessitates a larger relative motion storage area. A true motion shift may be applied to the final display. This, however, necessitates larger storage areas for both the true motion and the relative motion store. Both the storage areas may have diameters 1¢ times that of the final display and half scale true motion shifts may be used for the read-out of the true motion store and reversed for the readout of the relative motion store. Alternatively, the half scale true motion shift may be applied reversed to the final display tube, keeping craft O in the centre of the display, the display raster being 1¢ times the area of the display. The true motion may be automatically reset when the edge of the storage area is approached.