Patent GB-1023927-A (GB)
Title: Improvements in or relating to radar systems
Inventor: WATT JAMES
IPC: G01S7/24
Filed: 1961-07-10
Abstract: 1,023,927. Radar. MARCONI CO. Ltd. June 22, 1962 [July 10, 1961], No. 24875/61. Heading H4D. In a marine pulse radar with a continuously rotating aerial (rotation period T secs., e.g. T=2) a bright display of the current situation and the situations that existed at times nT, 2nT and 3nT (e.g. n =60) in the past is produced by storing the echo signals on a moving magnetic tape 119 with three displaced pick-up heads 123, 124, 125 producing signals delayed by nT, 2nT and 3nT respectively, substantially undelayed signals and the three sets of delayed signals being applied sequentially to a P.P.I. CRT 134 by a rotating switch 138 to produce four superimposed or interlaced component PPI displays corresponding to the situations existing at the different times. In order to produce a true motion display, shift voltages corresponding to the positions of " own ship " (i.e. the ship carrying the radar) at the different times are applied to the display tube 134 through switches 144, 145 synchronized with the switch 138 and the bandwidth and recurrence rate of the echo signals is reduced before recording on the tape 119 by a storage CRT 112 with separate writing and reading beams scanning at different rates. As described, the switch 138 operates so that (1) the undelayed signals are applied to the display tube 134 during two consecutive rotations of the aerial and the nT, 2nT and 3nT delayed signals are applied for one rotation period each during the next successive three rotation periods, i.e. field sequential operation, or (2) the undelayed signals are applied to the display tube 134 during two consecutive radial line scans of the tube, the three delayed signals being applied respectively during the next three successive line scan periods, i.e. line sequential operation. Means are described for selectively producing (1) a true-motion display, (2) a relative motion display and (3) a display showing the future situation arising from a selectable course for " own ship "; in all cases the display is azimuth stabilized, e.g. north up. As shown the radar aerial 101 is rotated by motor 103 at 30 r.p.m. relative to true North under the control of compass motor 105, and radiates 1000 pulses/sec., i.e. 2000 lines per field scan, from trigger unit 110 and transmitter 107. The received echo pulses, produced every 1000 Á sec. by a single transmitted pulse are passed to a storage tube 112 wherein they are written on to a target 112T by an electron beam 112W, and read off therefrom by an electron beam 112R every 2000 Á sec., the reading and writing scans taking 1600 Á sec. and 120 Á sec. respectively for a 10 mile range display, such that resulting echo signals passing out through the amplifier 116 are reduced in bandwidth by a factor 120/1000 or 3/40 and the number of lines per scan is reduced by half to 1000. The echo signals are then recorded on the tape loop 119 by head 118 and, almost immediately, are read off by head 122. They then pass along the moving tape for 2 mins. until they are read off by head 123, and for a further 2 mins. and 4 mins. until they are read off by heads 124 and 125 respectively. The signals are then erased by head 121. The signals picked off by the four heads 122, 123, 124, 125 are passed to four contacts 1138, 2138, 3138, 4138 respectively of a rotary switch 138, having respective angular extents of 144 degrees, 72 degrees, 72 degrees and 72 degrees, such that the long persistence CRT 134, connected via video amplifier 139 to the moving switch contact 138R, rotating at 6 r.p.m., displays the " now " scan for 2/30 min. or two scans and then displays for one scan each displays delayed by substantially 2, 4 and 6 mins., the deflection of the electron beam by the rotating CRT time base coils 135 being put in synchronism with the pulse signals by means of 500 c/s. sync. pulses generated by generator 115 and separated out in the amplifier 139. The four component images so produced take up 10 sees. and the cycle is then repeated. As so far described the four component images would give a relative " north up " display with " own ship " at the centre of display, and to convert to a true motion display two deflector coils 141 are provided to deflect the three delayed images in N/S and E/W directions respectively. The two coils are respectively fed via switch 100 from resolvers producing voltage analogues of the ship movements in the two directions. One resolver (the E/W), comprises a rotary switch RSE including 36 R/C integrators charged by a voltage analogue of the ship's E/W speed component from a resolver R fed with outputs from a log L, compass C and tide drift indicator T1. Each R/C integrator is discharged in turn to earth by a contact W1, rotating anticlockwise with three others at 1 rev. in 6 mins. The three other contacts W2, W3 and W4 are spaced by 120 degrees and respectively pick off from the respective R/C integrators for 10 sees., voltage analogues of the E/W movement of the ship during the previous 2, 4 and 6 mins. The four contacts are connected to a rotary contact connected to the E/W deflection coil 141, such that the three delayed displays are shifted in the E/W direction by the requisite amounts. Similar rotary switches RSN and 145 are provided for the N/S movement. In a modification the switches 138, 144 and 145 may revolve at 6000 r.p.m. such that each radial scan line of the CRT 134 may be produced alternatively from the four pick off heads. In another modification the " now" display may be derived directly from head 118. Means, Fig. 2 (not shown), may be provided whereby an artificial " own ship " movement may be introduced into the display to test the navigational result of such a movement.