Patent GB-614336-A (GB)
Title: Improvements in and relating to pulse radar systems
IPC: G01S7/20
Filed: 1946-05-22
Abstract: 614,336. Radiolocation. ELLIOTT, W. S. May 22, 1946, No. 15499. [Class 40 (vii)] In a radar system wherein echo signals are displayed as intensity modulations on the screen of a cathode-ray tube presenting azimuth and range information, such as a P.P.I or a B-type display, and wherein the aerial system is adapted to scan both in azimuth and elevation, the indications due to reflecting objects are made to appear with intensities proportional to their respective heights. The directional aerial, which produces a narrow beam in the vertical plane, is rotated about the shaft 2 at a constant rate, and is also raised and lowered in elevation at a lower rate by the motor 5 so that the aerial performs a tight helical scan. The video signal output from the transmitter-receiver 3 is amplified at 7 and is then passed through a " slicer " circuit 8 and further amplifier 10 to the modulating electrode of the P.P.I tube 11. The " slicer operates to limit the video signals at a lower limit just above the general noise level and at an upper limit which depends at any instant upon the height from which an echo signal is then being received, and is controlled by a signal fed from the unit 18, and generated as follows: The unit 14, which is synchronized from 3, produces a balanced sawtooth output which is applied across the input terminals of the sine-cosine potentiometer 22, the wipers of which are driven from the shaft 6 which is coupled to the driving means 5 so that outputs proportional to the sine and cosine of the angle of elevation are obtained. The sawtooth output proportional to the cosine of the angle of elevation measures ground range and is taken to the display unit 11 to produce the time base scan. The output proportional to the sine of the angle of elevation, which measures the height of a reflecting object at any instant, is applied to the " slicer " unit 8 through the circuits 16, 17, 18, 19 which suitably distort the waveform to compensate for the non-linear relationship between modulating voltage and intensity at the display unit 11. The intensities of the indications produced at 11 are consequently approximately linearly proportional to height. To enable the operator to judge heights an illuminated " brightness-height " scale may be provided. Since any one reflecting object produces an indication during. only one of a sequence of successive complete scans in azimuth, the display tube should be of the long afterglow type. According to a modification, particularly for use when the azimuthal scan covers only a limited arc, the aerial is made to " nod " vertically at a rate which is rapid compared to the rate of azimuthal scan. In the embodiment described in the Provisional Specification, the echoes are all limited to the same amplitude and applied to the intensity modulating electrode of the cathode-ray tube, and a further intensity modulating voltage, proportional to height, is applied to the cathode of the display tube. A limiting circuit is inserted between the cathode of the display tube and the circuit generating the control voltage to maintain the voltage between predetermined limits so that the beam should not be defocussed by very high targets or extinguished by very low ones.