Patent GB-1400704-A (GB)
Title: Radar display systems
IPC: G01S7/04
Filed: 1971-04-23
Abstract: 1400704 Radar; cathode-ray tube displays ELLIOTT BROS (LONDON) Ltd 6 June 1972 [23 April 1971] 11100/71 Headings H4D and H4T In an azimuth-scanning pulse radar with an azimuth-range cathode-ray tube (CRT) display, analogue deflection signals for said display are derived from a digital representation of the instantaneous bearing of the radar aerial and a separately generated time varying analogue range signal and means are provided for periodically producing on the CRT (in place of the radar display) an electronic target marker, e.g. an open centred cross, whose slant range R MO  and bearing # MD  are set by corresponding manually controlled digital signals, a D.C. analogue marker range signal being derived by sampling said time varying range signal at a time determined by said digital marker range signal. The display may be a type B display but the invention is described in detail as applied to an airborne mapping radar with a height-compensated resolved PPI display whose vertical axis corresponds to a reference axis Y of selectable orientation which may correspond to the ground track of the aircraft or to true North, means being also provided for offsetting the centre 0<SP>1</SP>, Fig. 3, of the display to display an area A immediately ahead of the aircraft position O, which area may shift with movement of the aircraft to be stabilized to survey a fixed area of the ground, i.e. true motion display. As shown in Fig. 2, the output slant range R RO  timebase signal from a sawtooth generator 9 is converted by a unit 13 (controlled by an altitude signal h) into a corresponding ground range R RG  timebase signal which is multiplied in devices 15 and 16 by respective digital signals cos # BD  and sin # BD  from registers 17, 18 (where # BD  is the aerial bearing relative to the axis Y) to give corresponding resolved analogue X and Y timebases at terminals 6 and 7. The digital co-ordinates R MO , cos # MD , sin # MD  of the target marker are stored in respective registers 31, 19, 20 and during each marker mode a control unit 41 (synchronized by the radar trigger via frequency dividers 39, 40) enables the marker waveform generator 28 and actuates switches 21, 22, 25 to couple the registers 19, 20 to the multipliers 15, 16 to apply a marker bright-up signal to the CRT bright-up terminal 27. A clock pulse counter 33 is reset by each radar trigger pulse and when the count reaches that stored in register 31 a comparator 34 activates a sample and hold circuit 14 responsive to the ground range timebase generator 13 to produce a D.C. voltage corresponding to the ground range of the marker, said voltage being converted by the multipliers 16 and 15 into D.C. X and Y marker position voltages. In order to offset the display, digital offset signals R DG  cos (# 9 -# DS ) and R DG  sin (# 9 -# DS ), see Fig. 3, stored in registers 45 and 46 are converted to analogue 53, 52 and then combined differentially with the outputs from the multipliers 15 and 16 is respective units 51, and 50, the offset signals being continuously updated for a true motion display; the display may be expanded about the offset position O<SP>1</SP> by increasing the gain of the deflection signal amplifiers 54, 55. When operating in the expanded offset mode, the output from each differential unit 50, 51 comprises a small difference between two large inputs and any differential variation in the characteristics of the D/A converters 16, 52 and 15, 53 feeding said units 50, 51 will result in jitter of the display which is prevented as described with reference to Fig. 4 (not shown) by periodically applying the digital slant range R DO  and bearing co-ordinates cos (# G -# DS ), sin (# G -# DS ) of the offset point O<SP>1</SP> to the registers 33, 19 and 20 in place of the corresponding marker co-ordinate signals R MO , cos # MD  and sin # MD  so that during such a test mode any differential errors in said D/A converters results in corresponding D.C. error outputs from the differential units 50, 51 which automatically modify the digital offset signals in the registers 46 and 45 to reduce said errors to zero. Fig. 4 also includes means for correcting the offset signals for any displacement, in the azimuth plane, between the aerial scanning axis and the projected position of the aircraft in said plane. The D/A multipliers 15, 16 may be replaced by corresponding non-linear converters responsive to digital angle values and a type B display may be produced by supplying digital angle values to registers 18 and 20, supplying the factor "one" to registers 17, 19 and supplying multiplier 16 with a constant reference voltage instead of the sweep waveform.