Patent GB-660555-A (GB)
Title: Improvements in radio-location and detection arrangements
Inventor: RUST NOEL MEYER
IPC: G01S13/50
Filed: 1946-01-28
Abstract: 660,555. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 28 1947 [Jan. 28, 1946], No. 2768/46. Class 40 (vii). A frequency-modulated radar system wherein the modulating waveform is of symmetrical triangular form, comprises means for measuring the ranges of reflecting objects and means for determining their velocities. When measuring range, the beat-frequency signals obtained by mixing the transmitted and received waves together are mixed with locally-generated oscillations which are frequency-modulated according to the same triangular law to derive for each reflecting object an intermediatefrequency signal of varying frequency which is passed through a fixed-frequency filter to a cathode-ray tube, the beam of which is swept in accordance with the same triangular law and which accordingly provides a range display. When measuring velocity, the beat frequencies obtained, from a selected reflecting object, when the frequency of the transmitted wave is increasing and decreasing are applied to a frequency-discriminating circuit the output from which is applied to a cathode-ray tube display arrangement which indicates the difference between the beat frequencies and hence the radial velocity of the selected object. In the embodiment illustrated, the transmitter 3 is frequency-modulated by the symmetrical triangular wave 4a from the generator 4 which is controlled by the-saw-tooth wave 5a of the same frequency from the generator 5. Reflected waves are mixed at 8 with a controllable amplitude of direct feed from the transmitter via 9 to produce beat frequencies which after amplification at 10 are fed to a mixer 11. This mixer receives a second input from a local oscillator 12, the frequency of which is adjustable manually or automatically to cause the beat-frequency corresponding to any particular range to be heterodyned to a value such that it passes the frequency selective amplifier 16. With switches 15, 17 to the left, the frequency of oscillator 12 is swept under the control of the triangular wave from 4 and the amplifier 16 is arranged to have a narrow bandwidth. The output from amplifier 16 is fed to produce vertical deflection of the range-indicating oscilloscope 7, the time-base for which is provided by the triangular wave. Stationary reflecting objects, which produce equal beat frequencies during the increasing and decreasing-frequency portions of the transmitter frequency modulation cycle, thus produce " blips " on the go and return portions of the time-base trace which superimpose to produce a single indication as at O. Moving targets are identified by a double " blip " as at M, due to the difference in the beat-frequencies produced during the two halves of the frequency-modulation cycle. The range display may be modified to provide a P.P.I. display in place of the type-A display described. When a target has been located switches 15, 16 are moved to the positions shown and the manual frequency control 13, 14 is set to the appropriate range to cause the heterodyned beat frequency to pass through amplifier 16 which is then in the wide-band condition. A frequency discriminator 19 is fed with the output of amplifier 16 and produces a display as shown on the velocityindicating oscilloscope 6, the time-base of which is provided by the saw-tooth wave 5a. The trace consists of two portions, displaced above and below the horizontal axis, corresponding to the differing beat-frequencies produced during the intervals of transmitter-frequency increase and decrease. The extent of the frequency-difference depends upon the relative target-speed which can be determined from the scale on the screen and the sense of the relative velocity will be apparent from the relative directions of displacement of the two portions of the trace. By suitable switching the same oscilloscope may be used for both range and velocity indications and also the saw-tooth and triangular waves 4a, 5a may be so synchronized that there is one saw-tooth for each frequency excursion up or down; additional means must then be provided to indicate sense of relative velocity by displacing the up and down stroke patterns horizontally in the velocity indication. Specification 621,846 is referred to.