Patent GB-1086487-A (GB)
Title: Improvements in or relating to radar display apparatus
Inventor: PARKER ERIC
IPC: G01S7/22
Filed: 1965-07-21
Abstract: 1,086,487. Radar displays. DECCA Ltd. July 15, 1966 [July 21, 1965], No. 31072/65. Heading H4D. The invention relates to the provision on a PPI radar display of markers originating at chosen targets and directed towards the position in the display of the radar carrying craft. Thus, when a target has moved after the initial positioning of the respective marker, the possibility of collision with the radar carrying craft is determinable by observing the target's position with respect of the marker. The invention works with both relative motion and true motion displays. The apparatus shown in Fig. 1 provides an azimuth stabilized, relative motion display and has provision for the interscan display of three markers. The scan coils 15 are rotated in synchronism with the antenna 10, with a compass 19 providing the azimuth stabilization correction. The markers are produced by means of orthogonal deflection coils 27 fed from current generators 26. A position signal generator 20 is provided for each marker having two controls 21 and 22 which may control the X and Y deflection signal directly (Fig. 2), or may control the range and angle inputs of a sine/cosine resolver to thereby produce the X and Y deflection signals (Fig. 3). The X and Y signals from the generators 20 are fed to a selector switch 24, which may comprise a permanent magnet which, by being spun rapidly past a circularly arranged array of read switches connected to respective generators, can feed the X and Y signals from the generators 20 in rapid succession to the current generators 27 via an interscan opening gate 25. The X and Y signals are amplitude modulated (e.g. at 10,000 c/s.) by a modulator 23, such that each marker, as produced by its respective X and Y signals, is a straight line extending from the target towards the radar carrying craft image. In a true motion display, the craft movement signals are applied to the X and Y signals, as well as to the target signals. In the case of a ship's head up display, compass 19 is omitted and correction is made in the position signal generators, which in this case advantageously comprises sine/ cosine resolver, with the correction being applied to the shaft of the resolver (Fig. 4). In the position signal generator shown in Fig. 2, positive or negative voltages are provided by centre earthed source 30. The voltage is amplitude modulated by source 35 and transformer 36 before being applied across X and Y potentiometers 31 and 32. In this generator the length of the marker reduces as X and Y reduce. If said length is desired constant, the generator of Fig. 3 may be used, wherein sine/cosine resolvers 40 are used, fed with a constant amplitude modulation signal from transformer 44 and a variable D.C. voltage from source 41, the variation being caused by potentiometer 43. Instead of a straight time marker, a V- or X-shaped marker can be used with the apex of the V or the centre of the X positioned on the target and the arms of the V or X pointing either side of the radar carrying crafts image by equal amounts and defining the minimum allowable approach distance of the target to said craft, i.e. if the target moves and keeps within the arms of the V or X, there is a collision danger. Such a V or X marker can be produced by adding a small amount of the output of one generator 26 to the output of the other generator 26 and similarly for the other deflection coil. If it is desired to investigate the result of a proposed change of course, an arrangement such as shown in Fig. 5 may be used to produce X and Y voltages which are added to those produced by the current generators 26. A sine/cosing resolver 72 is used and has its shaft rotated to represent the direction of the vector difference between the craft's actual course and the proposed course and a centre earthed (not shown) potentiometer 74 placed across its input and set to represent the length of said vector difference (i.e. change of velocity). A source 70 feeds a potentiometer having a wiper 71 driven upwards by motor drive means and feeding potentiometer 74. With wiper 71 at the bottom of its travel, the X and Y signals are both zero, and the marker can be set on the target as normal. The motor drive is now started and the generated X and Y signals move the marker such that it points to the position the craft would have if it adopted the proposed new course. If a target is seen to travel along this moved marker, then the new course is liable to collision.