Patent GB-660631-A (GB)
Title: Improvements in radio-location and detection arrangements
Inventor: RUST NOEL MEYER
IPC: G01S13/58
Filed: 1946-01-28
Abstract: 660,631. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 28, 1947 [Jan. 28, 1946], No. 27426/49. Divided out of 660,555. Class 40 (vii). In a frequency-modulated radar system wherein means are provided for indicating the radial velocity of reflecting objects by means of the Doppler effect, the modulating waveform comprises linearly increasing and linearly decreasing portions, means are provided for applying the beat-frequencies to two bandpass devices which are rendered effective respectively during the intervals that the radio-frequency is increasing and decreasing, and further means are provided which are responsive to outputs from the two band-pass devices to produce two indications which by a characteristic difference between them, indicate sense of motion and velocity of a moving target. The modulating waveform employed also comprises a constant amplitude portion and a further band-pass device passes the beat frequency signal derived in respect of this portion of the waveform to a further indicator measuring velocity. In the arrangement illustrated, the transmitter 3 is frequency-modulated by the truncated triangular wave 23a which is derived from the sine wave 20a from generator 20, by means of the steps indicated at 21a, 22a performed in limiter 21 and integrator 22 and final clipping at 23. Reflected waves picked up by the continuously rotating common aerial system 25 are fed through the hybrid-balance-unit 24 to the receiver mixer 8 where they combine with directly received transmitter energy from the control unit 9. The Dopplerbeat frequencies produced by moving targets during the intervals of unchanging transmitted frequency are filtered off by the band-pass amplifier 26 and operate any suitable frequencymeter 27 calibrated in terms of relative speed. The distancedependent beat frequencies are fed to band-pass amplifier and mixer units 28, 29 which are rendered operative alternately during the intervals of increasing and decreasing transmitter frequency by the controlling action of the output wave from the limiter 21 which is fed through switch 30. Mixers 28, 29 are also supplied with a heterodyning voltage from oscillator 31, the frequency of which is varied between, say, 210 and 300 kc/s. under the control of the saw-tooth wave 32a from generator 32. The mixer outputs which pass through band-pass amplifiers 33, 34 (200 kc/s. Œ 1 kc/s.) are fed independently to deflect the two beams of a double-beam type A oscilloscope 39, the time-base for which is provided by the frequency analysing saw-tooth wave 32a. The deflections due to moving targets will not be opposite one another due to the difference in the beat-frequency on the two halves of the modulating cycle and the direction of the displacement is indicative of the sense of the relative velocity. The P.P.I. tube 35 has its time-base controlled by the wave 32a in range and in angular position by the link 36 with the rotating aerial system. A normal P.P.I. display is produced by opening switches 30, 38, when the output from amplifier 33 only is fed to modulate the intensity of the tube trace. To observe moving targets switches 30, 38 are closed and the outputs from both units 33, 34 are fed to intensity-modulate the trace which is adjusted to the half-bright condition in the absence of signals by a control ganged with switches 30, 38. Indications due to stationary targets will be alternately lightened and darkened on the up and down strokes of the modulating wave, since the units 33 and 34 produce positive and negative outputs respectively, and will cancel out due to their superposition. Indications from moving targets will however not superimpose due to the Doppler frequency difference and will produce a resultant decrease or increase of brightness. Undesired targets at a particular velocity may be relegated to the background on the P.P.I. display by " warping " the transmitter modulating wave as at 23b by adjusting the bias supplied to the limiter 21 and so modifying the waveform generation as indicated at 20b, 21b, 22b. The rates of change of the transmitted frequency on the up and down strokes may thus be so modified that the beat frequencies produced during the up and down strokes of frequencychange by a particular target-velocity are equal. This adjustment of the slopes of varying-frequency portions of the modulating waveform, may be utilized to determine the relative speeds by observing the change of slope required to make the two beat frequencies coincide. The Provisional Specification also describes a feature which may be employed with the two beat-frequency channels. Oscillators may be locked to the intermittent beat frequencies from the two channels. The oscillator outputs may then be mixed to produce a beat note, indicative of the relative target speed, the frequency of which may be measured or reduced to zero by " warping " of the modulation waveform.