Patent GB-937847-A (GB)
Title: Improvements in or relating to radar apparatus
Inventor: KNOWLES PHILIP NEVILLE GEORGE
IPC: G01S13/72
Filed: 1960-03-29
Abstract: 937,847. Radar tracking. DECCA Ltd. June 28, 1961 [March 29, 1960], No. 11077/60. Class 40 (7). Relates to a plan position tracking system for a pulse radar having a beam aerial rotating in azimuth in which the echoes from a tracked target are selected by a video gate 45, Fig. 1, controlled by a gating pulse, Fig. 4b, which is produced when the amplitudes of two D.C. voltages representing the predicted cartesian coordinates x and y of the target are simultaneously equal, within predetermined limits, to the instantaneous amplitudes of X and Y sawtooth timebase voltages from generators 25, 26 synchronized with the pulse transmission and having slopes proportional respectively to sin 0 and cos 0, where # is the instantaneous bearing of the aerial, means being provided for periodically correcting the predicted co-ordinate voltages by means of position error voltages representing the difference between the predicted co-ordinates and the actual co-ordinates as determined by the radar so that the gate tracks the target. According to the present invention, each predicted co-ordinate voltage, e.g. the x voltage, Fig. 4h, as produced by the X tracker 16, represents. the integrated value of a corresponding velocity component signal and is given by the output from a velocity signal integrator 20 comprising a high-gain amplifier 21, a feed-back capacitor 22 and input resistor 23, the position error voltage being applied to make corrections to the velocity signal source and to the integrated value of the velocity component signal. The velocity components may be set in manually during the initial stage but thereafter the tracking is fully automatic, the velocity components being derived by measuring the rates of change of the integrated position error signal components between successive aerial scans in which the directional beam sweeps through the target. As shown, a pulse, Fig. 4d, representing in magnitude and polarity the x co-ordinate error is generated during each pulse recurrence period as the radar beam sweeps through the target and these error pulses are applied to a position error integrator 58 whose output is applied (1) via a capacitor 63 to the grid of the amplifier 21 in the velocity integrator 20 during the sweep-through period by means of a switch RL1 controlled by a control pulse, Fig. 4c, of duration substantially equal to the sweepthrough period to reduce the co-ordinate error, and (2) to a potentiometer 65 having a movable tap 66 which is moved with respect to time, as shown in Fig. 4l, so that the output voltage from the potentiometer is inversely proportional to time and represents the target velocity, said velocity voltage being applied via a velocity store 67, Fig. 3 (not shown), and long time constant circuit 85 to the input resistor 23 of the velocity integrator 20 to control the rate of change of the output position voltage. The feed-back capacitor 62 in the position error integrator 58 is normally discharged by a switch RL2 which is momentarily closed; Fig. 4e, after the aerial has swept through the target. When a substantial number, e.g. 6, of position error pulses are produced as the aerial sweeps through. the target, the effects of random noise pulses are reduced by choosing the relative magnitudes of the capacitors 22 and 63 and the time constant of the integrator 58 so that the position correction produced by the integrator 58 for each error pulse corresponds to a predetermined fraction, e.g. 1/3, of the determined error so that by the end of the sweep-through period about 90% of the position error is corrected. The predicted y co-ordinate voltage is produced by a corresponding Y tracker 17 and and y velocity and position signals are obtained from read-out circuits 88, 89 and 86, 87. The position signals may be used to position a marker trace on a C.R.T. P.P.I. radar display, the length and direction of the trace being controlled by the velocity signals to indicate the magnitude and direction of the target velocity. The predicted and y co-ordinate voltages from the X and Y trackers 16, 17, the X and Y timebases from the generators 25, 26 and echo pulses passed by the video gate 45 and shaped by a regenerator 46 are applied to a comparator 15, Fig. 2 (not shown), similar to that disclosed in Specification 937,845 to produce the x and y pulse position error signals, Fig. 4d, and the gating pulses, Fig. 4b, for the video gate 45. Outputs from the video gate 45 are also applied to decision circuits 90 and 91 of the type described in Specification 937,846 which (1) reject the position error signals and inhibit the application of the output from the time dividing potentiometers to the velocity stores if the amplitude of the gated signal suddenly increases due, for example, to multiple targets, noise or clutter, and (2) increase the duration of the gating pulses if the target is lost.